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Polyribosomal complexes are selectively localized beneath postsynaptic sites on neuronal dendrites; this localization suggests that the translation of the mRNAs that are present in dendrites may be regulated by synaptic activity. The present study tests this hypothesis by evaluating whether synaptic activation alters the immunostaining pattern for two proteins whose mRNAs are present in dendrites: the dendrite-specific cytoskeletal protein MAP2 and the alpha-subunit of CAMKII. High-frequency stimulation of the perforant path projections to the dentate gyrus, which terminate in a discrete band on the dendrites of dentate granule cells, produced a two-stage alteration in immunostaining for MAP2 in the dendritic laminae. Five minutes of stimulation (30 trains) caused a decrease in MAP2 immunostaining in the lamina in which the activated synapses terminate. After more prolonged periods of stimulation (1-2 hr), there was an increase in immunostaining in the sideband laminae just proximal and distal to the activated band of synapses. The same stimulation paradigm produced a modest increase in immunostaining for alpha-CAMKII in the activated laminae, with no detectable changes in the sideband laminae. The alterations in immunostaining for MAP2 were diminished, but not eliminated, by inhibiting protein synthesis; the increases in CAMKII were not. These findings reveal that patterned synaptic activity can produce domain-specific alterations in the molecular composition of dendrites; these alterations may be caused in part by local protein synthesis and in part by other mechanisms.

In the rat dentate gyrus, the lateral perforant path, the medial perforant path, and the major part of the hilar projection to the molecular layer share the lamination domain, mainly in the outer one-third of the molecular layer, the middle one-third, and the inner one-third, respectively. To reveal the order of the afferent fiber lamination and to have an indication of how the synaptic sites on dendrites are determined, we investigated the ontogeny of afferent fiber lamination in the dorsal hippocampus by injecting 1, 1'-dioctodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) into the entorhinal cortex and hippocampus in vivo. Fibers from the contralateral hilar region were found under the pia mater of the infrapyramidal blade at postnatal day 3 (P3), whereas the entorhinal afferent fibers were absent in the infrapyramidal blade. Then the medial and the lateral perforant path appeared under the pia mater in the infrapyramidal blade as riding on top of the preexisting laminae by P7 and by P11, respectively. Based on the established knowledge that most entorhinal layer II neurons simultaneously innervate both the suprapyramidal blade and infrapyramidal blade by branching, it is assumed that the medial and lateral perforant path in the suprapyramidal blade await an appropriate timing for sprouting of interstitial branches into the infrapyramidal blade. The granule cells in the infrapyramidal blade had dendritic growth cones by P11. Calretinin immunohistochemistry revealed Cajal-Retzius cells in the infrapyramidal blade even at P14. Under the pia mater, axon growth cones of ingrowing afferent fibers may interact with the dendritic growth cones or the Cajal-Retzius cells, and determines the synaptic sites on the granule cell dendrites., (Copyright 1999 Wiley-Liss, Inc.)

Status epilepticus remains a life-threatening condition associated with a high mortality. In order to understand the pathophysiological mechanisms underlying sustained seizures, the identification of structures involved in seizure activity allowing to define epileptic networks may be important. Thus, local cerebral metabolic rate for glucose (LCMR(glc)) was measured in a rat model of self-sustaining status epilepticus (SSSE) induced by a brief intermittent perforant path stimulation of 30 min, using the quantitative [(14)C]2-deoxyglucose autoradiographic technique. SSSE induced a generalized bilateral increase in LCMR(glcs) affecting 27 of the 42 structures studied. Largest metabolic increases (>250%) were recorded in the hippocampus, amygdala, entorhinal and piriform cortices, and lateral septum. Marked metabolic activation was also seen in basal ganglia areas such as the substantia nigra, globus pallidus and accumbens nucleus. LCMR(glcs) in brainstem, some midbrain structures, and in the neocortex were not affected by SSSE. In conclusion, a brief stimulation of the hippocampus induced a reproducible limbic SSSE in 100% of the rats, characterized by the metabolic activation of limbic and extralimbic structures, known to be involved in this type of seizures. Therefore, this new model allowing the development of a well-defined SSSE, appears to be particularly suitable for further studies on the mechanisms involved in status epilepticus., (Copyright 1999 Elsevier Science B.V.)

We investigated the effects of basolateral amygdala (BLA) priming on long-term potentiation (LTP) in the dentate gyrus (DG). In the control animals, the induction of high-frequency stimulation (HFS) to the perforant path (PP) resulted in hippocampal LTP at all the time intervals tested. A priming stimulation to the BLA prior to the application of HFS to the PP resulted in the enhancement of the excitatory post-synaptic potential (EPSP)-LTP and population spike (PS)-LTP in the DG from 90-min post-HFS onwards. These findings suggest that the amygdala has a potential role in the modulation of some aspects of memory that are mediated by the hippocampus.

Phosphorylation of the transcription factor cAMP responsive element-binding protein (CREB) is thought to play a key role in synaptic plasticity and long-term memory. However, direct evidence for CREB phosphorylation during hippocampal long-term potentiation (LTP) in vivo is sparse. Here, we show that, in the intact animal, CREB is rapidly phosphorylated in response to high-frequency stimulation but not low-frequency stimulation of the perforant pathway. CREB phosphorylation occurred in a biphasic manner, with a first peak at 30 min and a second long-lasting peak beginning 2 hr after tetanic stimulation and lasting for at least 24 hr. Only stimuli that generated nondecremental LTP promoted a sustained hyperphosphorylation of CREB but not stimuli that produced decremental LTP. CREB phosphorylation was specifically triggered in the dentate gyrus, as well as the CA1, but not the CA3, hippocampal region. Pretreatment with the NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate completely prevented activation of CREB. Together, we have resolved the spatial and temporal dynamics of CREB phosphorylation during hippocampal LTP, showing that the transcription factor CREB is specifically recruited at two distinct time points in some forms of hippocampal synaptic plasticity in vivo.

In vitro, extracellular electrophysiological recording was used to test granule cell responses in P60 rats after neonatal PCA and 5, 7-DHT. Granule cell population EPSP and spike responses were in the normal range for both PCA and 5,7-DHT groups. However, the degree of paired pulse facilitation was reduced in both of these groups relative to control, reflecting a diminished synaptic drive. Synaptic potentiation in the 5,7-DHT group was not different from control, but was significantly reduced in slices from PCA-treated rats., (Copyright 1999 Elsevier Science B.V.)

The axons of the neurons in the medial and lateral components of the entorhinal cortex (MEC and LEC) form the medial and lateral perforant paths (MPP and LPP) which represent the major source of cortical input to the hippocampus. Anatomical, physiological, and pharmacological studies have shown that MPP and LPP are distinct. Unfortunately, assessment of the functional significance of damage to either of these pathways has not used tasks known to be sensitive to hippocampal function in the rodent. In this study, we performed dissociated lesions of MPP and LPP using a combined physiological and anatomical method. Rats with lesions of either the MPP or the LPP were tested on place learning in the water task and on a discriminative fear conditioning to context task. The results indicated that the MPP, but not LPP, lesions resulted in impaired place learning. The context discrimination data revealed an amygdala-like, reduced fear effect of MPP lesions and an enhanced discriminative fear conditioning to context effect of LPP lesions. Consistent with a two-stage model of spatial learning proposed by Buzsaki (Buzsaki G. Two-stage model of memory trace formation: a role for 'noisy' brain states. Neuroscience 1989;31(3):551-570), the impairment in the water task can be interpreted as reflecting the higher efficiency of the MPP synapses in activating hippocampal neurons. The context discrimination results can be explained by either a dissociation of sensory information that reaches the MEC and LEC, or alternatively, by a dissociation between the limbic nature of the MEC and the sensory nature of the LEC.

Transcranial magnetic stimulation (TMS) has become a promising treatment of affective disorders in humans, yet the neuronal basis of its long-lasting effects in the brain is still unknown. We studied acute and lasting effects of TMS on reactivity of the rat hippocampus to stimulation of the perforant path. Application of TMS to the brain of the anesthetized rat caused a dose-dependent transient increase in population spike (PS) response of the dentate gyrus to perforant path stimulation. In addition, TMS caused a marked decrease in inhibition and an increase in paired-pulse potentiation of reactivity to stimulation of the perforant path. Also, TMS suppressed the ability of fenfluramine (FFA), a serotonin releaser, to potentiate PS response to perforant path stimulation. Chronic TMS did not affect single population spikes but caused an increase in paired-pulse potentiation, which was still evident 3 weeks after the last of seven daily TMS treatments. After chronic TMS, FFA was ineffective in enhancing reactivity to perforant path stimulation, probably because it lost the ability to release serotonin. In addition, the beta adrenergic receptor agonist isoproterenol, which caused an increase in PS in the control rats, failed to do so in the TMS-treated rats. These results indicate that TMS produces a long-term reduction in efficacy of central modulatory systems.

Long-term potentiation (LTP) of excitatory transmission is a likely candidate for the encoding and storage of information in the mammalian brain. There is a general agreement that LTP involves an increase in synaptic strength, but the mechanisms underlying this persistent change are unclear and controversial. Synaptic efficacy may be enhanced because more transmitter glutamate is released or because postsynaptic responsiveness increases or both. The purpose of this study was to examine whether increased extracellular glutamate concentration was associated with the robust and well-characterized LTP that can be induced in the rat dentate gyrus. To favor the detection of any putative change in extracellular glutamate associated with LTP, our experimental strategy included the following features. 1) Two separate series of experiments were carried out with animals under pentobarbital or urethan anesthesia; 2) changes in extracellular concentration of glutamate were monitored continuously by microdialysis coupled to enzyme amperometry; and 3) dialysate glutamate levels and changes in the slope of excitatory postsynaptic potential evoked by activation of the perforant path were recorded precisely at the same site. Tetanic stimulation of the perforant path increased persistently test-evoked responses in the dentate gyrus (by 19 and 14% in barbiturate and urethan group, respectively), but there was no glutamate change either during or after LTP induction and no indication of increased glutamate efflux when low-frequency stimulation was applied. The results do not rule out a possible contribution of enhanced glutamate exocytosis to LTP induction and/or maintenance because such a presynaptic change may not be detectable extracellularly. However, our findings and other data supporting the notion that neurotransmitter glutamate may hardly leak out of the synaptic cleft conflict with the hypothesis that LTP could also involve a broad synaptic spillover of glutamate.

Previous studies have demonstrated that remacemide and its desglycinyl metabolite, AR-R 2495AA, reduce neuronal damage in animal models of ischemia, subarachnoid hemorrhage, and traumatic brain injury. The aim of the present study was to investigate whether remacemide hydrochloride also alleviates seizure-induced neuronal damage in a model of status epilepticus induced by the stimulation of the perforant pathway (PP) in the rat. Chronic oral remacemide treatment (3 x 25 mg/kg/day) was started either 2 days before or 2 h after the beginning of PP stimulation (2 mA, 20 Hz, 0.1 ms pulse duration for 60 min). The effects of remacemide treatment on the severity of seizures, electroencephalogram (EEG) parameters, seizure-induced neuronal damage in the temporal lobe regions, and memory impairment were compared to unstimulated and stimulated vehicle-treated controls, and carbamazepine-pre-treated (3 x 40 mg/kg/day) rats. Both remacemide and carbamazepine pretreatments, but not remacemide posttreatment, decreased pyramidal cell damage in the CA3 and CA1 subregions of the hippocampus (P < 0.05). In addition, overall neuronal damage in the extrahippocampal temporal lobe regions (the piriform cortex, entorhinal cortex, and the amygdaloid complex) was milder in remacemide-pretreated rats compared to stimulated control rats (P < 0.01). The neuroprotective effect was most evident on the side contralateral to stimulation. Remacemide or carbamazepine pretreatment had no evident effect on the number or duration of behavioral seizures during PP stimulation. Neither drug altered the spectral parameters of the baseline EEG or prevented status epilepticus-induced EEG slowing observed 2 weeks after PP stimulation. Nor did remacemide or carbamazepine treatment alleviate spatial memory impairment determined in a Morris water-maze task 2 weeks after PP stimulation. Our data provide evidence that pretreatment with remacemide has a moderate neuroprotective effect against status epilepticus-induced neuronal damage.

A common feature of temporal lobe epilepsy and of animal models of epilepsy is the growth of hippocampal mossy fibers into the dentate molecular layer, where at least some of them innervate granule cells. Because the mossy fibers are axons of granule cells, the recurrent mossy fiber pathway provides monosynaptic excitatory feedback to these neurons that could facilitate seizure discharge. We used the pilocarpine model of temporal lobe epilepsy to study the synaptic responses evoked by activating this pathway. Whole cell patch-clamp recording demonstrated that antidromic stimulation of the mossy fibers evoked an excitatory postsynaptic current (EPSC) in approximately 74% of granule cells from rats that had survived >10 wk after pilocarpine-induced status epilepticus. Recurrent mossy fiber growth was demonstrated with the Timm stain in all instances. In contrast, antidromic stimulation of the mossy fibers evoked an EPSC in only 5% of granule cells studied 4-6 days after status epilepticus, before recurrent mossy fiber growth became detectable. Notably, antidromic mossy fiber stimulation also evoked an EPSC in many granule cells from control rats. Clusters of mossy fiber-like Timm staining normally were present in the inner third of the dentate molecular layer at the level of the hippocampal formation from which slices were prepared, and several considerations suggested that the recorded EPSCs depended mainly on activation of recurrent mossy fibers rather than associational fibers. In both status epilepticus and control groups, the antidromically evoked EPSC was glutamatergic and involved the activation of both AMPA/kainate and N-methyl-D-aspartate (NMDA) receptors. EPSCs recorded in granule cells from rats with recurrent mossy fiber growth differed in three respects from those recorded in control granule cells: they were much more frequently evoked, a number of them were unusually large, and the NMDA component of the response was generally much more prominent. In contrast to the antidromically evoked EPSC, the EPSC evoked by stimulation of the perforant path appeared to be unaffected by a prior episode of status epilepticus. These results support the hypothesis that recurrent mossy fiber growth and synapse formation increases the excitatory drive to dentate granule cells and thus facilitates repetitive synchronous discharge. Activation of NMDA receptors in the recurrent pathway may contribute to seizure propagation under depolarizing conditions. Mossy fiber-granule cell synapses also are present in normal rats, where they may contribute to repetitive granule cell discharge in regions of the dentate gyrus where their numbers are significant.

Metabotropic glutamate receptors (mGluRs) provide a powerful control of synaptic transmission in the hippocampus and may serve as a target for drug development in human temporal lobe epilepsies. Agonists and antagonists at these receptors influence the development and propagation of seizures in some animal models of epilepsy. Experimental seizures can change the level of expression of mGluRs in the rat hippocampus. In the human dentate gyrus of patients suffering from temporal lobe epilepsy (TLE), group III mGluR mediated inhibition of synaptic transmission is almost lost in the sub-group with Ammon's horn sclerosis. We tested the modulation of synaptic transmission by the group III mGluR specific agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) in the dentate gyrus outer molecular layer in control and amygdala-kindled rats, a common model for TLE. Extracellular field potential recordings upon subthreshold stimulation of lateral perforant path fibers were measured simultaneously in the outer molecular layer and granule cell layer. Analysis of 'paired-pulse' characteristics in the absence and presence of L-AP4 and group III mGluR mediated inhibition of synaptic transmission in the lateral perforant path revealed no significant alterations in fully kindled rats. Since there is no evidence of altered L-AP4 responses, a loss of group III mGluR function, particularly that of subtype mGluR8, seems not necessary for the kindling epilepsy., (Copyright 1999 Elsevier Science B.V.)

Purpose: To determine the neuroprotective effects of halothane during constant stimulation of the perforant path., Methods: Male Sprague-Dawley rats had electrodes implanted into the perforant path and dentate granule cell layer under halothane anaesthesia (1-2% in oxygen). They were then divided into four groups. In group 1 (n = 9), the perforant path was stimulated at 20 Hz for 2 h under halothane anaesthesia (1-2%). In group 2 (n = 3), the animals were unstimulated but maintained under halothane anaesthesia (1-2%) for 2 h with the electrodes in place. Both groups 1 and 2 had the electrodes removed and were then allowed to recover fully from the anaesthetic. In groups 3 and 4, the electrodes were held in place with dental acrylic. Both of these groups were allowed to recover fully from anaesthesia. In group 3 (n = 3), 24-48 h after recovery from anaesthesia, the perforant path was stimulated at 20 Hz for 2 h. Group 4 (n = 3) received no stimulation. After 14-17 days, the rats were killed, and morphometry and cell counts were performed on the hippocampi from rats in groups 1 and 2., Results: Cell densities were not significantly different between control (group 2), unstimulated rats, and animals stimulated under halothane anaesthesia (group 1). Stimulation in the unanaesthetised rats resulted in severe neuronal loss in hilus, CA1, and CA3., Conclusions: Halothane protects hippocampal neurons against damage induced by constant stimulation of the perforant path.

Various forms of synaptic long-term potentiation and depression have been studied in detail in hippocampus and neocortex. Each are produced by specific patterns of synaptic activation and rely on electrical stimulation of afferents for their induction. Few studies have explored the ability of activity produced by cortical networks themselves to generate similar long-term changes in synaptic efficacy. Experiments have shown that periods of synchronized network bursting in both slices and dissociated cultures of hippocampus can lead to persistent network discharges. Conversely, one study has reported that network discharging can disrupt the induction of long-term potentiation in hippocampus. Whether long-term depression can be induced by synchronous network discharging is unknown. In experiments reported here, we examined the long-term effects of synchronized activity within hippocampal CA3 networks on synaptic potentials produced by the converging perforant path. Prior to induction of network bursting, slices were incubated in a perfusate containing picrotoxin and elevated (4 mM) Ca2+ and Mg2+. To induce network discharges, the concentration of both divalent cations were reduced to normal levels (1.5 mM). Following 20 min of network bursting, perforant path synapses, that did not participate in network discharging, underwent a 30% non-decrementing long-term depression. At the same time, synchronized network discharges, that were absent prior to induction, persisted upon return to preinduction conditions. The antagonist of the N-methyl-D-aspartate receptor, D(-)2-amino-5-phosphonovalerate, blocked both long-term depression of perforant path excitatory postsynaptic potentials and persistent network discharging. Results suggest that activity generated by hippocampal networks is able to produce long-term depression of non-coincidentally active synapses.

The perforant path input (pp) is a major direct source of specific sensory information for the CA1 hippocampal region. The termination area of this pathway, the stratum lacunosum-moleculare, has the highest concentration of dopamine receptors in the hippocampus. We have examined the properties of the pp input and its modulation by dopamine. The input is glutamatergic and has a larger NMDA component than the Schaffer collateral (sc) input. Dopamine strongly inhibits the response to pp stimulation (IC50 approximately 3 microM) but not the response to sc stimulation. Dopamine reduces both the NMDA and AMPA components of transmission at the pp and increases paired-pulse facilitation. In the sc, the NMDA component but not the AMPA component is decreased, and paired-pulse facilitation is not affected. The effect of dopamine on the pp does not depend on GABAA inhibition but is reduced by the antagonists of both D1 and D2 families of dopamine receptors. The effect is not completely blocked by the combination of D1 and D2 antagonists, but is completely blocked by the atypical neuroleptic clozapine. Our results provide the first evidence for strong dopaminergic control of transmission in the perforant path. By inhibiting this pathway, dopamine hyperfunction and/or NMDA hypofunction abnormalities implicated in schizophrenia may isolate CA1 from its main source of sensory information.

Single-electrode voltage-clamp techniques and bath application of the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovaleric acid (APV) were used to study the time course of seizure-induced alterations in NMDA-dependent synaptic currents in granule cells of the dentate gyrus in hippocampal slices from kindled and normal rats. In agreement with previous studies, granule cells from kindled rats examined within 1 wk after the last of 3 or 30-35 generalized tonic-clonic (class V) seizures demonstrated an increase in the NMDA receptor-dependent component of the perforant path-evoked synaptic current. Within 1 wk of the last kindled seizure, NMDA-dependent charge transfer underlying the perforant path-evoked current was increased by 63-111% at a holding potential of -30 mV. In contrast, the NMDA-dependent component of the perforant-evoked current in granule cells examined at 2.5-3 mo after the last of 3 or 90-120 class V seizures did not differ from age-matched controls. Because the seizure-induced increases in NMDA-dependent synaptic currents declined toward control values during a time course of 2.5-3 mo, increases in NMDA-dependent synaptic transmission cannot account for the permanent susceptibility to evoked and spontaneous seizures induced by kindling. The increase in NMDA receptor-dependent transmission was associated with the induction of kindling but was not responsible for the maintenance of the kindled state. The time course of alterations in NMDA-dependent synaptic current and the dependence of the progression of kindling and kindling-induced mossy fiber sprouting on repeated NMDA receptor activation are consistent with the possibility that the NMDA receptor is part of a transmembrane signaling pathway that induces long-term cellular alterations and circuit remodeling in response to repeated seizures, but is not required for permanent seizure susceptibility in circuitry altered by kindling.

The dual impairment of both long-term potentiation (LTP) in the dentate gyrus and spatial memory by N-methyl-D-aspartate (NMDA) blockers such as 2-aminophosphonovaleric acid (APV) or dizocilpine (MK-801) is considered supportive evidence for the hypothesis that LTP-like mechanisms are involved in spatial memory. However, several studies suggest that, at doses that affect aspects of behavior, LTP is not yet blocked. One possible explanation may be that the blockade of NMDA receptors affect processes other than LTP, which are required for learning. In the present study, we assessed in vivo the effects of the NMDA receptor antagonist MK-801 on LTP and on frequency-dependent inhibition, which has previously been shown to reflect activity of GABAergic interneurons in the rat dentate gyrus. We report here that NMDA receptors are instrumental in frequency-dependent inhibition. Furthermore, frequency-dependent inhibition was found to be more sensitive than LTP to the NMDA antagonist MK-801. Our findings indicate that, in addition to the blockade of LTP, the application of NMDA antagonists affects local circuit activity in the dentate gyrus. The results direct attention to the potential role of interneuronal activity in general and of frequency-dependent inhibition in particular in dentate gyrus related behaviors.

Induction of long-term potentiation within the hippocampal formation can be modulated by afferent influences from a number of subcortical structures known to be involved in hippocampal-dependent learning and memory. This study performed on freely moving rats investigated the effects of stimulation of the noradrenergic locus coeruleus nucleus and the serotonergic dorsal raphe nucleus on spontaneously decaying posttetanic long-term potentiation in the dentate gyrus and the hippocampal CA1 area, respectively. High-frequency electrical stimulation of the locus coeruleus or the dorsal raphe elicited a well-expressed behavioural reaction of exploratory or defensive type, respectively, but did not significantly alter transmission at perforant path-dentate gyrus or Schaffer collateral-CA synapses, when delivered either before tetanic stimulation of the perforant path or the Schaffer collaterals or long (hours and days) after previously induced long-term potentiation had completely decayed. However, when locus coeruleus or dorsal raphe stimulation was delivered with the same parameters during a limited time (minutes and hours) after marked or even complete decay of tetanus-induced long-term potentiation at perforant path-dentate gyrus or Schaffer collateral-CA1 synapses, the potentiation was partially or entirely restored but never increased beyond the initial level of potentiation. In CA1, stimulation of ipsilateral and contralateral Schaffer collaterals demonstrated that the restoration of previously existing long-term potentiation by dorsal raphe stimulation was input-specific, occurring, like tetanus-induced potentiation, only in the pathway which had previously been tetanized. These findings suggest that the noradrenergic locus coeruleus and the serotonergic dorsal raphe can influence not only induction, but also spontaneous decay of long-term potentiation in the hippocampal formation. Since hippocampal long-term potentiation is thought to play a role in certain kinds of learning and memory, and association of tetanic stimulation with activation of ascending neuromodulatory systems is required for full expression of long-term potentiation, the restoration of hippocampal long-term potentiation by activation of a neuromodulatory system alone may serve as a mechanism of associative reminder which may underlie facilitation of memory retrieval after a period of forgetting, as has been observed in trained rats under similar conditions.

Adult male Long-Evans rats were prepared with stimulating and recording electrodes in the perforant path and dentate gyrus, respectively. Urethane-anesthetized acute (ACU) preparations and chronically-implanted freely-moving (CHR) animals received moderate-intensity (50-75% of maximum) stimulation pulses every 15-20 s for 4-5 hr in order to assess the stability of evoked field potentials. Significant increases in both population spike amplitude (+6.3%/hr) and EPSP slope (+2.5%/hr) were seen over the course of testing in the ACU group as a whole, while the CHR group showed significant decreases in EPSP slope (-3.3%/hr) but not spike (-1.2%/hr). Thus, both preparations were unstable, though the group mean drift differed in direction. Field potential drift was also affected by body temperature and stimulation intensity; drift was significantly greater when temperature was not controlled, and responses to moderate-intensity stimulation tended to be less stable than responses to high-intensity pulses. Our results indicate that a drift of 4-6% per hour in individual subjects is common; in unheated acute preparations, drift can equal or exceed 20% per hour (3/7 cases). These findings show that response instability can pose significant problems for electrophysiological investigations of neural plasticity.

Transection of the entorhino-dentate perforant path is a well known model for lesion-induced axonal sprouting and glial reactions in the rat. In this study, we have characterized the microglial reaction in the dentate molecular layer of the SJL/J and C57Bl/6 mouse. The morphological transformation of the microglial cells and their densitometrically measured Mac-1 immunoreactivity were correlated with the density of silver-impregnated axonal and terminal degeneration and the myelination of the degenerating medial and lateral perforant pathways. Anterograde axonal and terminal degeneration leads to: (i) altered myelin basic protein immunoreactivity with the appearance of discrete myelin deposits preferentially in the denervated medial and significantly less so in the lateral perforant path zone from day 2 after lesioning; (ii) an increase in number and Mac-1 immunoreactivity of morphologically-changed microglial cells in the denervated perforant path zones with more pronounced morphological transformation of microglia in the medial than in the lateral perforant path zones at day 2 but not day 5 after lesioning; and (iii) a linear correlation between the density of microglial Mac-1 reactivity and axonal degeneration in the medial but not in the lateral perforant path zone at two days postlesion, and a linear correlation in both zones at five days postlesion. We propose that the differentiated microglial response is due to the different densities of axonal and terminal degeneration, as observed in the individual cases. The finding of a potentiated or accelerated microglial activation in the medial as compared to the lateral perforant path zone suggests different kinetics of microglial activation in areas with degenerating myelinated and unmyelinated fibers.

We compared the interaction between the metabotropic glutamate receptor agonist ACPD and arachidonic acid with the interaction between ACPD and nerve growth factor (NGF) on presynaptic function in hippocampus. ACPD interacted with both NGF and arachidonic acid to increase KCl-stimulated endogenous glutamate release and calcium concentration in synaptosomes prepared from whole hippocampus and synaptosomes prepared from untetanized dentate gyrus. The data indicate that prior induction of long-term potentiation (LTP) in perforant path granule cells synapses occluded the interaction between ACPD and both NGF and arachidonic acid, suggesting that these agents may play a role in the generation of LTP in dentate gyrus.

The Morris water maze (MWM) has been used to assess cognitive function in rats after a variety of lesions designed to model brain damage and to assess the effects of drugs, growth factors, and neural transplants on post-operative deficits. The present study examined recovery of spatial navigation in the MWM over time in order to model the spontaneous recovery of cognitive function seen in humans. Diffuse axonal injury, a neuropathology commonly associated with traumatic brain injury (TBI), was modelled by transecting the perforant path (PP) bilaterally, either caudal to the hippocampus or dorsal to it at the decussation of the dorsal hippocampal commissure. Both groups with PP cuts showed substantial deficits initially, but spatial performance recovered with time and training. Recovery of platform finding was nearly complete within 14 days of testing, but recovery of platform searching did not occur for 2 or 3 more weeks. When the platform was moved to a new location, a continuing deficit in learning rate was revealed. When the platform was moved to a new position every day, this deficit was even more evident. These results illustrate the multi-faceted nature of recovery after brain injury and provide a new model for assessing the effects of manipulations designed to modulate recovery.

In the adult brain, neurotrophin-3 (NT-3) is mainly localized in dentate granule cells, and its expression is decreased by various stimuli, e.g., seizure activity. We have examined the role of endogenous NT-3 for excitatory synaptic transmission at lateral perforant path-dentate granule cell synapses using hippocampal slices from NT-3 knock-out (+/-) and wild-type (+/+) mice. Paired-pulse facilitation (PPF) and also short-term synaptic plasticity induced by a brief, high-frequency train of afferent stimulation were reduced, but the expression of long-term potentiation was not affected in the NT-3+/- mice. Incubation of the slices with recombinant NT-3 reversed the deficit in PPF through a mechanism requiring de novo protein synthesis, implying that the impaired short-term plasticity does not result from a developmental alteration. No changes of overall presynaptic release probability, measured by the progressive block of NMDA receptor-mediated synaptic currents by MK-801, or desensitization of AMPA receptors were detected. Because NT-3 expression is reduced after focal seizures, impaired short-term facilitation may represent a protective response that limits the propagation of epileptiform activity from the entorhinal cortex to the hippocampus.

The loss of neurons in the dentate gyrus have been evaluated in rats in which kindling had been induced through stimulation of the hippocampal perforant path. The total number of granule cells, hilar neurons and the total number of neurons expressing the message for somatostatin (SS-mRNA) were estimated with stereological methods in five rats sacrificed 24 h after the last seizure and compared with estimates made in five control animals. The mean total number of granule cells was 981,000 in control rats and 943,000 in kindled rats, a non-significant difference at the 5% level of significance (P = 0.78, Student's t-test). The mean total number of hilar neurons was 52,100 compared with a mean of 58,520 in the control group, the difference being statistically significant (P = 0.04). The mean total number of SSergic hilar neurons was 16,630 compared with a mean of 15,430 in the control rats (non-significant, P = 0.12). These findings indicate that hilar neurons are lost after kindling but that these neurons are not somatostatinergic.

Kynurenic acid is an excitatory amino acid antagonist with preferential activity at the N-methyl-D-aspartate subtype of glutamate receptors. It is produced endogenously in the brain, but is synthesized more effectively in the periphery. The influence of peripheral kynurenic acid on brain function is unclear because kynurenic acid is likely to penetrate the blood-brain barrier poorly. To determine the potential central effects of peripheral kynurenic acid, we compared its effects in the hippocampus after peripheral or direct administration. The hippocampus of the rat was chosen as a test system because this region receives glutamatergic inputs, and because responses to stimulation of these inputs can be compared after peripheral drug administration in vivo, and after direct administration of drugs in vitro. Peripherally-administered kynurenic acid was injected via a catheter in the jugular vein. Bath-application to hippocampal slices was used to test effects of direct administration. Area CA1 pyramidal cells and dentate gyrus granule cells were examined by extracellular recording and stimulation of area CA3 or the perforant path, respectively. Pairs of identical stimuli were used to assess paired-pulse inhibition and paired-pulse facilitation. Kynurenic acid decreased evoked responses in area CA1 and the dentate gyrus, both in vivo and in vitro. Effective concentrations were in the low micromolar range, and therefore were likely to be mediated by antagonism of N-methyl-D-aspartate receptors. In both preparations, area CA1 was more sensitive than the dentate gyrus, and paired-pulse facilitation was affected, but not paired-pulse inhibition. Control solutions had no effect. We conclude that kynurenic acid can enter the brain after peripheral administration, and that peripheral and direct effects in the hippocampus are qualitatively similar. Therefore, we predict that effects of endogenous kynurenic acid that was synthesized peripherally or centrally would be similar. Furthermore, the results suggest that modulation of the glycine site of the N-methyl-D-aspartate receptor, for example by kynurenic acid, may vary considerably among different brain areas.

In order to identify genes that may underlie the maintenance of long-term potentiation (LTP) at perforant path synapses, complementary DNA libraries were synthesised from dentate gyrus total RNA extracts prepared 48 h after the induction of LTP and from control dentate gyrus extracts. Through differential screening of the LTP library we have identified the mitochondrial 12S rRNA (mt12SrRNA) as a transcript that was elevated at this late time. Northern blot analyses showed that the elevation in mt12SrRNA expression began around 8 h and persisted for at least 2 weeks post-tetanus. We then examined the expression patterns of other mitochondrially-encoded genes and demonstrated a similar elevation in their expression. mt12SrRNA levels were also elevated in other hippocampal regions, including areas CA3 and CA1 and were elevated following low-frequency stimulation or in the presence of an N-methyl-D-aspartate receptor antagonist where induction of LTP was precluded. Taken together, these observations suggest that a long-lasting up-regulation of energy production may be triggered by synaptic activity and this activity need not be of sufficient strength to induce LTP, but may be related to the induction of a metaplastic state., (Copyright 1998 Elsevier Science B.V.)

The perforant path forms a monosynaptic connection between the cells of layer II of the entorhinal cortex and the pyramidal cells in hippocampal area CA3. Although this projection is prominent anatomically, very little is known about the physiological properties of this input. The distal location of these synapses suggests that somatically recorded perforant-path excitatory postsynaptic potentials (EPSPs) may be influenced by the activation of voltage-dependent channels in CA3 cells. We observed that perforant-path EPSPs are reduced (by approximately 25%) by blockade of postsynaptic low-voltage-activated calcium and sodium channels, indicating that perforant-path EPSPs are amplified by the activation of these channels. These data suggest that the perforant path may represent an important and highly modifiable direct connection between the entorhinal cortex and area CA3.

Mossy fibre sprouting and re-organization in the inner molecular layer of the dentate gyrus is a characteristic of many models of temporal lobe epilepsy including that induced by perforant-path stimulation. However, neuroplastic changes on the dendrites of granule cells have been less-well studied. Basal dendrites are a transient morphological feature of rodent granule cells during development. The goal of the present study was to examine whether granule cell basal dendrites are generated in rats with epilepsy induced by perforant-path stimulation. Adult Wistar rats were stimulated for 24 h at 2 Hz and with intermittent (1/min) trains (10 s duration) of single stimuli at 20 Hz (20 V, 0.1 ms) delivered 1/min via an electrode placed in the angular bundle. The brains of these experimental rats and age- and litter-matched control animals were processed for the rapid Golgi method. All rats with perforant-path stimulation displayed basal dendrites on many Golgi-impregnated granule cells. These basal dendrites mainly originated from their somata at the hilar side and then extended into the hilus. Quantitative analysis of more than 800 granule cells in the experimental and matched control brains showed that 6-15% (mean=8.7%) of the impregnated granule cells have spiny basal dendrites on the stimulated side, as well as the contralateral side (mean=3.1%, range=2.9-3.9%) of experimental rats, whereas no basal dendrites were observed in the dentate gyrus from control animals. The formation of basal dendrites appears to be an adaptive morphological change for granule cells in addition to the previously described mossy fibre sprouting, as well as dendritic and somatic spine formation observed in the dentate gyrus of animal and human epileptic brains. The presence of these dendrites in the subgranular region of the hilus suggests that they may be postsynaptic targets of the mossy fibre collaterals.

One of the most prominent roles of metabotropic glutamate receptors (mGluRs) in the CNS is to serve as presynaptic receptors that inhibit transmission at glutamatergic synapses. Previous reports suggest that the presynaptic effect of group II mGluRs at corticostriatal synapses can be inhibited by activators of protein kinase C (PKC). We now report that activation of PKC inhibits the ability of group II and group III mGluRs to regulate transmission at three major synapses in the hippocampal formation. Thus, this effect may be a widespread phenomenon that occurs at glutamatergic synapses throughout the CNS. We also report that this response is not limited to PKC-activating phorbol esters but that activation of A3 adenosine receptors induces a PKC-dependent inhibition of group III mGluR function at the Schaffer collateral-CA1 synapse. In addition to inhibiting mGluR modulation of excitatory synaptic transmission, we found that activation of PKC reduces inhibition of forskolin-stimulated cAMP accumulation by group II and group III mGluRs, suggesting that the effect of PKC on mGluR signaling is not specific to their effects on neurotransmitter release. This led us to test the hypothesis that PKC acts upstream from effector proteins regulated by mGluRs and acts at the level of the receptor or GTP-binding protein. Interestingly, we found that PKC inhibited mGluR-induced increases in [35S]-GTPgammaS binding in cortical synaptosomes. These data suggest that PKC-induced inhibition of mGluR signaling may be mediated by the inhibition of coupling of mGluRs to GTP-binding proteins.

We examined the duration of intermittent perforant path stimulation (PPS) needed to induce self-sustaining status epilepticus (SSSE) in rats. Seven-minute PPS did not induce SSSE. Some rats receiving 15 min and all animals after 30 min PPS developed SSSE that continued for hours. The animals killed 3 days after SSSE showed extensive neuronal damage. Those which were allowed to survive for 6 weeks after SSSE displayed spontaneous seizures., (Copyright 1998 Elsevier Science B.V.)

Maintenance of long-term potentiation in perforant path-granule cell synapses is accompanied by increased glutamate release. Here we investigate the role of synaptophysin in release and in expression of long-term potentiation in dentate gyrus. We report that long-term potentiation was accompanied by increased endogenous glutamate release and increased tyrosine phosphorylation of synaptophysin, but these changes were attenuated when long-term potentiation was inhibited by the tyrosine kinase inhibitor tyrphostin AG879 or by the NMDA antagonist D-aminophosphonovalerate. In vitro analysis revealed that KCl-induced glutamate release was abolished in synaptosomes prepared in the presence of antisynaptophysin. The data suggest a role for synaptophysin in release and indicate that activation of tyrosine kinase and synaptophysin phosphorylation contribute to long-term potentiation perhaps by modulating glutamate release.

Granule cells in the rat hippocampal dentate gyrus contain intracellular receptors for the adrenal hormone corticosterone. Activation of these receptors seems essential for granule cell viability, since removal of the adrenal gland (adrenalectomy) results within three days in apoptotic-like degeneration of granule cells. In the present study we used extracellular in vitro recording methods to study the synaptic transmission in the dentate gyrus of adrenalectomized animals, in sham-operated controls and adrenalectomized rats treated with a low dose of corticosterone. We found that particularly three days after adrenalectomy orthodromic field responses in the dentate gyrus were reduced in amplitude. Corticosterone-treated rats did not show this impairment of synaptic transmission. Antidromically-evoked field responses were also reduced after adrenalectomy, which indicates that postsynaptic cell properties rather than signal transduction in the synapses are under steroid control. Responses to paired pulse stimulation were only marginally affected, suggesting that interneuronal networks may be less affected by the hormones than the principal cells. These electrophysiological data indicate that adrenalectomy induced apoptotic-like degeneration in the hippocampal dentate gyrus is clearly associated with impaired processing of incoming information.

Local modulation of hippocampal-evoked responses to perforant path stimulation was studied by leaking drugs from the recording pipette placed in the dentate gyrus of anesthetized young (3 months old), aging (17 months old) and old (28 months old) rats. In old rats, the excitatory postsynaptic potential (EPSP) slope was much reduced compared to young and aging rats. The population spike (PS) size was similar in all age groups. Bicuculline caused a marked increase in PS size relative to population EPSP, and reversed the response to the second pulse in a paired-pulse paradigm from inhibition to facilitation. The effect of bicuculline was only slightly reduced in old rats. The 5-HT1a agonist 8-OH-DPAT potentiated PSs in the dentate gyrus, while not affecting paired-pulse inhibition. The effect of 8-OH-DPAT was slightly reduced in old rats. Carbachol, a cholinergic agonist, reversed paired-pulse inhibition into facilitation in the young brain, but not in aging and old rats. These results demonstrate that age affects differentially the action of biogenic amines on hippocampal reactivity to afferent stimulation.

The functional projection of the medial perforant path (MPP) to different CA3 subfields was studied in urethan-anesthetized rats using current source density analysis. MPP stimulation resulted in an early-latency (presumed monosynaptic) sink with onset of 2-3 ms at the distal apical dendritic layer of CA3 (stratum lacunosum molecule) and a long-latency (presumed disynaptic, >7 ms) sink at stratum lucidum and radiatum of CA3. The population spike (onset 5. 3-6.1 ms), a sink at CA3 pyramidal cell layer, was observed 67% of the time (12 of 18 rats) in CA3a, 44% (8 of 18) in CA3b and 58% (7 of 12 rats) in CA3c following MPP stimulation. Population spike was not observed during presumed disynaptic excitation of CA3. Both early-latency sink (excitatory postsynaptic potential) and population spike in CA3 revealed robust paired-pulse facilitation (PPF). In contrast, little PPF was found for the MPP-evoked excitatory sink at the middle molecular layer of the dentate gyrus. The data suggested that the entorhinal cortex provides a strong monosynaptic excitation of different subfields of CA3. A direct entorhinal to CA3 input bypasses the dentate gyrus and may play a role in normal hippocampal signal processing and neural plasticity., (Copyright 1998 Elsevier Science B.V. All rights reserved.)

Dentate spikes (DSs) are positive-going field potential transients that occur intermittently in the hilar region of the dentate gyrus during alert wakefulness and slow-wave sleep. The function of dentate spikes is unknown; they have been suggested to be triggered by perforant path input and are associated with firing of hilar interneurons and inhibition of CA3 pyramidal cells. Here we investigated the effect of DSs on medial perforant path (MPP)-granule cell-evoked transmission in freely moving rats. The MPP was stimulated selectively in the angular bundle while evoked field potentials and the EEG were recorded with a vertical multielectrode array in the dentate gyrus. DSs were identified readily on the basis of their characteristic voltage-versus-depth profile, amplitude, duration, and state dependency. Using on-line detection of the DS peak, the timing of MPP stimulation relative to single DSs was controlled. DS-triggered evoked responses were compared with conventional, manually evoked responses in still-alert wakefulness (awake immobility) and, in some cases, slow-wave sleep. Input-output curves were obtained with stimulation on the positive DS peak (0 delay) and at delays of 50, 100, and 500 ms. Stimulation on the peak DS was associated with a significant increase in the population spike amplitude, a reduction in population spike latency, and a decrease in the field excitatory postsynaptic potential (fEPSP) slope, relative to manual stimulation. Granule cell excitability was enhanced markedly during DSs, as indicated by a mean 93% increase in the population spike amplitude and a leftward shift in the fEPSP-spike relation. Maximum effects occurred at the DS peak, and lasted between 50 and 100 ms. Paired-pulse inhibition of the population spike was unaffected, indicating intact recurrent inhibition during DSs. The results demonstrate enhancement of perforant path-evoked granule cell output time-locked to DSs. DSs therefore may function to intermittently boost excitatory transmission in the entorhinal cortex-dentate gyrus-CA3 circuit. Such a mechanism may be important in the natural induction of long-term potentiation in the dentate gyrus and CA3 regions.

The glutamate analog, L-2-amino-4-phosphonobutyric acid (L-AP4) is a selective agonist for several members of the metabotropic glutamate receptor (mGluR) family. Activation of presynaptic mGluRs by L-AP4 causes a suppression of synaptic transmission in the central nervous system. In this study, the role of 1 subtype of mGluR in the nervous system was investigated by analyzing mutant mice lacking the L-AP4-sensitive receptor, mGluR4. Experiments designed to probe hippocampal function showed no impairments in acquisition of spatial learning in the water maze task. However, in a spatial reversal learning task, the mutant mice exhibited significantly accelerated learning performance. Furthermore, in a probe trial administered 6 weeks posttraining, these mice showed impaired spatial accuracy. The results suggest that the mutant mice differed in their ability to learn and integrate new spatial information into previously formed memory traces and that their use of stored spatial information also was altered. Thus, the presynaptically expressed mGluR4 plays a role in the processing of spatial information.

Long-term potentiation (LTP) at the mossy fiber-->CA3 pyramidal cell synapse in the hippocampus is an NMDA-independent form of LTP that requires cAMP-dependent protein kinase (PKA) activity and can be induced by forskolin, a general activator of adenylyl cyclases. Presynaptic Ca2+ influx and elevated cAMP may be obligatory for mossy fiber LTP. Because the Ca2+-stimulated type 1 adenylyl cyclase (AC1) is expressed in the dentate gyrus and CA3 pyramidal cells, it is hypothesized that AC1 may be critical for mossy fiber LTP. To test this hypothesis, we examined several forms of hippocampal LTP in wild-type and AC1 mutant mice. Wild-type and AC1 mutant mice exhibited comparable perforant path LTP recorded in the dentate gyrus as well as decremental LTP at the Schaffer collateral-->CA1 pyramidal cell synapse. Although the mutant mice exhibited normal paired pulse facilitation, mossy fiber LTP was impaired significantly in AC1 mutants. High concentrations of forskolin induced mossy fiber LTP to comparable levels in wild-type and AC1 mutant mice, indicating that signaling components downstream from the adenylyl cyclase, including PKA, ion channels, and secretory machinery, were not affected by disruption of the AC1 gene. These data indicate that coupling of Ca2+ to activation of AC1 is crucial for mossy fiber LTP, most likely via activation of PKA and enhancement of excitatory amino acid secretion.

Long-term potentiation (LTP) of excitatory transmission is an important candidate cellular mechanism for the storage of memories in the mammalian brain. The subcellular phenomena that underlie the persistent increase in synaptic strength, however, are incompletely understood. A potentially powerful method to detect a presynaptic increase in glutamate release is to examine the effect of LTP induction on the rate at which the use-dependent blocker MK-801 attenuates successive N-methyl-D-aspartic acid (NMDA) receptor-mediated synaptic signals. This method, however, has given apparently contradictory results when applied in hippocampal CA1. The inconsistency could be explained if NMDA receptors were opened by glutamate not only released from local presynaptic terminals, but also diffusing from synapses on neighboring cells where LTP was not induced. Here we examine the effect of pairing-induced LTP on the MK-801 blocking rate in two afferent inputs to dentate granule cells. LTP in the medial perforant path is associated with a significant increase in the MK-801 blocking rate, implying a presynaptic increase in glutamate release probability. An enhanced MK-801 blocking rate is not seen, however, in the lateral perforant path. This result still could be compatible with a presynaptic contribution to LTP in the lateral perforant path if intersynaptic cross-talk occurred. In support of this hypothesis, we show that NMDA receptors consistently sense more quanta of glutamate than do alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors. In the medial perforant path, in contrast, there is no significant difference in the number of quanta mediated by the two receptors. These results support a presynaptic contribution to LTP and imply that differences in intersynaptic cross-talk can complicate the interpretation of experiments designed to detect changes in transmitter release.

Relatively little information exists regarding the spatial structure of synaptic neuropil in the brain. The present electron microscopic study employs unbiased stereological techniques and Monte Carlo simulations to characterise quantitatively the spatial organisation of synaptic circuitry in the dentate gyrus of the hippocampus, an area of particular importance in mechanisms of learning and the subject of a number of experimental neurobiological models of synaptic plasticity such as long-term potentiation. Firstly, tissue shrinkage/expansion resulting from embedding was assessed by imaging 300-microm thick hippocampal slices in the course of the entire embedding protocol, giving a value of 94.3 +/- 1.1% for distance measures and 84.3 +/- 2.8% for volumetric measures. Secondly, numeric synaptic density, Nv, was estimated using the disector. Thirdly, accumulated area of post-synaptic densities (PSDs) per tissue volume, Sv, and the overall cell membrane area per tissue volume, Sv*, were assessed using unbiased stereological rules coupled with image analysis of single sections. Finally, the mean area of individual PSDs was derived as a ratio Sv/Nv giving: 0.0394 microm2 for axo-spinous PSDs (thus representing approximately 1.3% of total cell membranes) and 0.0769 microm2 for dendritic shaft PSDs (approximately 0.25% of total cell membranes). From these data, the mean nearest neighbour distance between synapses was estimated using Monte Carlo simulations of a random 3D arrangement of synapses constrained by PSD sizes (a truncated Poisson process), giving a value of 0.48-0.51 microm. The physiological importance of the morphometric data obtained is discussed in terms of assessing (i) the role of synaptic environment in modifying synaptic efficacy and (ii) the plausibility of cross talk between synapses in relation to extrasynaptic neurotransmitter diffusion and transient depletion of extracellular Ca2+.

Monosynaptic perforant path responses evoked by subicular stimulation were recorded from CA3 pyramidal cells of rat hippocampal slices. These monosynaptic responses were isolated by using low intensities of stimulation and by placing a cut through the mossy fibers. Perforant path-evoked responses consisted of both excitatory and inhibitory components. Excitatory postsynaptic currents (EPSCs) were mediated by both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acidreceptors (AMPAR) and N-methyl--aspartate receptors (NMDAR). Inhibitory postsynaptic currents consisted of gamma-aminobutyric acid-A (GABAA-) and -B (GABAB)-receptor-mediated components. At membrane potentials more positive than -60 mV and at physiological [Ca2+]/[Mg2+] ratios, >30% of perforant path evoked EPSC was mediated by NMDARs. This value varied as a function of the membrane voltage and external [Mg2+]. Two types of responses were observed after low-intensity stimulation of the perforant path. The first type of response showed paired-pulse facilitation and was reduced by 2-amino-4-phosphonobutyric acid (AP4). The second type of response showed paired-pulse depression and was reduced by baclofen. Electrophysiological and pharmacological characteristics of these two types of responses are similar to the properties of lateral and medial perforant path-evoked EPSPs in the dentate gyrus.

Tuberomammillary histamine neurons (TM) in the posterior hypothalamus project to extensive parts of the brain, including the hippocampal formation. The purpose of the present experiments was to investigate whether activation of the TM modulates signal transfer from the perforant pathway (PP) or ventral hippocampal commissure (VHC) to the dentate gyrus (DG) in freely moving rats. Paired pulses of electrical stimulation were delivered to PP or VHC, and evoked field potentials (fEPSPs and pop spikes) were recorded in the DG. Before activating PP or VHC, the TM was triggered by electrical stimulation. Experimentation was performed during four behavioral conditions: exploration, grooming, awake immobility, and slow-wave sleep. Electrical activation of the TM was found to modify dentate fEPSPs evoked by PP or VHC stimulation without generating a field potential by itself. Train stimulation of the TM (100 Hz, 500 ms) preceding paired pulses on the hippocampus by 50 ms decreased dentate fEPSPs in dependence of the ongoing behavior and the pathway stimulated. During exploration but not consummatory behavior, the PP signal was reduced when preceded by TM stimulation; during consummatory behavior but not exploration, the VHC signal was reduced. In contrast to other hippocampal afferents which increase pop spikes but leave fEPSPs unchanged, TM stimulation decreased dentate fEPSPs without affecting pop-spike activity. Thus, the TM-histaminergic system seems to modulate signal processing in the dentate gyrus in a specific way, exerting an inhibitory action on the entorhinal input only during learning-related exploratory behavior.

An inbred strain of Wistar rat (GH), which is deficient in nerve growth factor (NGF), was used to assess the possible role of NGF in the generation of long-term potentiation in perforant path-granule cell synapses. The data show that NGF was significantly decreased in the dentate gyrus of GH rats, that this deficit was accompanied by an impairment in long-term potentiation (LTP) and that intraventricular injection of NGF substantially reversed this impairment. Analysis of depolarization-induced glutamate release in synaptosomes prepared from dentate gyrus of control rats revealed that NGF alone was without effect, but in combination with the metabotropic glutamate receptor agonist, aminocyclopentane-1,3-dicarboxylic acid (ACPD), NGF induced a significant increase in release. This effect was occluded by prior induction of LTP, suggesting that the interaction between these agents may be required to enhance transmitter release which accompanies LTP in dentate gyrus. In contrast to the effect of NGF and ACPD on glutamate release in control rats, the combination of these agents had no effect on release in synaptosomes prepared from GH rats, which might be explained by the marked decrease in trk receptors in dentate gyrus of GH rats. It was concluded that the impaired ability of GH rats to sustain LTP is associated with a reduction in NGF concentration, a reduction in stimulated release of NGF and a decrease in trk receptors in dentate gyrus. It is proposed that these data indicate a role for NGF in the generation of long-term potentiation in perforant path-granule cell synapses.

In one computational model of hippocampal function, the entorhinal cortical input to CA1 is hypothesized to play a key role in the ability of CA1 to decode CA3 recodings. Here, we develop a modification of this CA1 decoder hypothesis that is applicable to several computational theories of hippocampal function, and then we electrophysiologically investigate one assumption of this new hypothesis. First, using biologically realistic estimates, we calculate that CA3-induced CA1 excitation is too high and that inhibition plausibly plays a role in this CA1 decoder model. Thus motivated, we turn to a physiological demonstration to substantiate the plausibility of the proposed mechanism. Using the rat hippocampal slice, we examine an interlaminar interaction between the distal perforant path input to hippocampal CA1 stratum moleculare and the more proximal Schaffer collateral input to stratum radiatum. Perforant path activation provides sufficient inhibition to block homosynaptic long-term potentiation elicited by a suitably strong stratum radiatum input. For this interlaminar interaction to be most effective, perforant path activation must both precede and follow Schaffer collateral activation. Perforant path-evoked inhibition in CA1 can thus serve as a viable mechanism in the learned decoder theory of hippocampal CA1.

Intracellular recordings were performed to examine the perforant path projection from layer III of the entorhinal cortex to the subiculum in rat combined hippocampal-entorhinal cortex slices. Electrical stimulation in the medial entorhinal cortex layer III caused short latency combined excitatory and inhibitory synaptic responses in subicular cells. In the presence of the GABA(A) antagonist bicuculline and the GABA(B) antagonist CGP-55845 A inhibition was blocked and isolated AMPA- or NMDA receptor-mediated EPSPs could be elicited. After application of the non-NMDA antagonist NBQX and the NMDA antagonist APV excitatory responses were completely blocked indicating a glutamatergic input from the neurons of the medial entorhinal cortex layer III. By stimulation from a close (< 0.2 mm) position in the presence of NBQX and APV and either CGP-55845 A or bicuculline we could record monosynaptic fast GABA(A) or slow GABA(B) receptor-mediated IPSPs, respectively. We compared synaptic responses in subicular cells induced by stimulation in the medial entorhinal cortex layer III with responses elicited by stimulation of afferent fibres in the alveus. The EPSPs of subicular cells induced by stimulation of alvear fibres could be significantly augmented by simultaneous activation of perforant path fibres originating in the medial entorhinal cortex layer III, while delayed activation of alvear fibres after stimulation of the perforant path resulted in a weak inhibition of the alveus evoked EPSPs. Thus, the perforant path projection activates monosynaptic excitation of subicular neurons. Therefore the entorhinal cortex does not only function as an important input structure of the hippocampal formation but is also able to modulate the hippocampal output via the entorhinal-subicular circuit.

Early stage Alzheimer's disease (AD) pathology is associated with neurodegeneration of systems within the temporal cortex, e.g. the entorhinal cortex, perforant pathway and hippocampus. The perforant pathway provides the major neuronal input to the hippocampus from the entorhinal cortex and thus relays multimodal sensory information derived from cortical zones into the hippocampus. The earliest symptoms of AD include cognitive impairments, e.g. deficits in short-term memory and attention. Consequently, we have investigated the effect of bilateral knife cut lesions to the perforant path on cognition in rats using models measuring primarily short-term memory (operant delayed match to position task), attention (serial five-choice reaction time task) and spatial learning (Morris water maze). Rats receiving bilateral perforant path lesions showed normal neurological function and a mild hyperactivity. The lesion produced little effect on attention assessed using the five-choice task. In contrast, animals with equivalent lesions showed a robust delay-dependent deficit in the delayed match to position task. Spatial learning in the water maze task was also severely impaired. The delay-dependent deficit in the match to position task was not reversed by tacrine (3 mg/kg) pretreatment. The present data support a selective impairment of cognitive function following perforant path lesions that was confined to mnemonic rather than attentional processing. These findings complement primate and human studies identifying a critical role of the perforant pathway and associated temporal lobe structures in declarative memory. Degeneration of the perforant pathway is likely to contribute to the mnemonic deficits characteristic of early AD. The failure of tacrine to ameliorate these deficits may be relevant to an emerging clinical literature suggesting that cholinomimetic therapies improve attentional rather than mnemonic function in AD.