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Objective: To determine whether muscle blood flow before and after exercise is abnormal in patients with diabetes mellitus., Research Design and Methods: Muscle blood flow (MBF) was measured with the 133Xe clearance technique in 15 nondiabetic subjects, 10 patients with insulin-dependent diabetes mellitus (IDDM), and 11 patients with non-insulin-dependent diabetes (NIDDM) at rest and after exercise. None of the patients had neuropathy., Results: The median resting MBF was similar in all three groups. The median postexercise MBF was significantly greater in nondiabetic subjects (40.1 ml.min-1.100 g-1 of tissue) than in patients with IDDM (25.7 ml.min-1.100 g-1 of tissue; P less than 0.01) or NIDDM (14 ml.min-1.100 g-1 of tissue; P less than 0.01). The difference between IDDM and NIDDM was not significant., Conclusions: Diabetic patients have abnormalities of MBF in response to exercise. This abnormality occurs in the absence of clinical diabetic neuropathy.

1. The present experiments investigated the effects of gamma-amino-butyric acidB (GABAB) receptor stimulation on the excitatory and inhibitory responses of neostriatal neurons evoked by stimulation of the subcortical white matter in a rat neostriatal slice preparation. 2. Intracellular recordings showed that single-impulse stimulation of the corpus callosum evoked monosynaptic, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-sensitive excitatory postsynaptic potentials (EPSPs) that were attenuated by the GABAB receptor agonist, p-chlorophenyl-GABA (baclofen, 0.5-10 microM) in a concentration-dependent manner. Baclofen also blocked the GABAA-mediated inhibition of neostriatal cell responses, which were revealed by paired-impulse stimulation of the subcortical white matter. Both of these effects persisted in slices in which the anterior cortex was removed, indicating that the site of action for baclofen was intrinsic to the neostriatum. The GABAB antagonist 3-amino-2-hydroxy-2-(4-chlorophenyl)-propanesulfonic acid (saclofen, 250-500 microM) reversed the depressant actions of baclofen on both the excitatory and inhibitory responses of neostriatal cells. 3. Concentrations of baclofen as high as 100 microM, which markedly attenuated EPSP amplitude, did not exert direct effects on resting membrane potential, current-voltage relationship, input resistance, or spike threshold and thus appeared to have no postsynaptic effect on the population of neurons recorded. 4. These results indicate that, in contrast to other regions of the CNS, the depressant effects of baclofen on glutamate-dependent EPSPs are mediated exclusively through GABAB receptors located presynaptically on the terminals of glutamatergic afferents.(ABSTRACT TRUNCATED AT 250 WORDS)

In the companion report [Nisenbaum and Berger (1992) Neuroscience 48, 561-578] the contrasting paired impulse responses to stimulation of the corticostriatal pathway which define the Type I and Type II subpopulations of striatal neurons were shown to reflect differential regulation by GABAergic and dopaminergic inputs. More specifically, the decreased probability of spike discharge (inhibition) to long interstimulus intervals (60-260 ms) characteristic of Type I neurons was found to be dependent on dopaminergic input via D1 receptor activation, whereas the inhibition to short interstimulus intervals (10-20 ms) distinctive of Type II neurons was found to be mediated by GABAergic input acting through GABAA receptor stimulation. The present experiments have further investigated the contribution of GABAergic and dopaminergic feedforward and/or feedback circuits to the functional identities of Type I and Type II neurons using an in vitro corticostriatal slice preparation. In this preparation, the cortical afferents to the striatum are preserved, allowing for activation of striatal cells in a manner similar to that used in vivo; however, all axons arising from midbrain and brainstem structures including the substantia nigra are transected, and intrastriatal GABAergic pathways are reduced. Consistent with the predicted effect of disrupting these two neurotransmitter pathways, the paired impulse responses of striatal neurons recorded in vitro were not similar to the responses of either Type I or Type II neurons recorded in vivo. Indeed, the paired impulse profiles of striatal neurons recorded in vitro were relatively homogeneous in that virtually all cells displayed an increased probability of spike discharge (facilitation) to the second impulse of all interstimulus intervals (10-500ms) tested. Low concentrations of allosteric agonists for the GABAA receptor, pregnanolone (5 microM) and pentobarbital (50 microM), selectively inhibited spike discharge in response to short interstimulus intervals (10-20 ms) for approximately 40% of the neurons sampled, but produced no change in facilitation to longer interstimulus intervals (30-500 ms). The agonist-induced inhibition to short interstimulus intervals was blocked by bicuculline (10-20 microM), and was not mimicked by the GABAB receptor agonist, baclofen (1-5 microM). In addition, application of dopamine (5-10 microM) or the D1 receptor agonist, SKF38393 (2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine; 5 microM), inhibited spike discharge to longer interstimulus intervals (40-500 ms) for approximately 10% of striatal cells recorded. The inhibition to longer interstimulus intervals was blocked by the D1 receptor antagonist, SCH23390 [R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin+ ++-7-ol], but not the D2 antagonist, sulpiride.(ABSTRACT TRUNCATED AT 400 WORDS)

Antipsychotic drugs are known to block dopamine receptors soon after their administration, resulting in an increase in dopamine neuron firing and dopamine turnover. Nonetheless, antipsychotic drugs must be administered repeatedly to schizophrenics before therapeutic benefits are produced. Recordings from dopamine neurons in rats have revealed that chronic antipsychotic drug treatment results in the time-dependent inactivation of dopamine neuron firing via over-excitation, or depolarization block. Furthermore, the clinical profile of the response to antipsychotic drugs appears to correspond to the dopamine system affected: antipsychotic drugs that exert therapeutic actions in schizophrenics inactivate dopamine neuron firing in the limbic-related ventral tegmental area, whereas drugs that precipitate extrapyramidal side effects cause depolarization block of the motor-related substantia nigra dopamine cells. One factor that remains unresolved with regard to the actions of antipsychotic drugs is the relationship between dopamine turnover and depolarization block--i.e., why does a significant level of dopamine release or turnover remain after antipsychotic drug treatment if dopamine cells are no longer firing? We addressed this question using an acute model of neuroleptic-induced depolarization block. In this model, dopamine cells recorded in rats one month after partial dopamine lesions could be driven into depolarization block by the acute administration of moderate doses of haloperidol. However, similar doses of haloperidol, which were effective at increasing dopamine levels in the striatum of intact rats, failed to change dopamine levels in lesioned rats. This is consistent with a model in which neuroleptic drugs exert their therapeutic effects in schizophrenics by causing depolarization block in DA cells, thereby preventing further activation of dopamine neuron firing in response to external stimuli. Thus, attenuating the responsivity of the dopamine system to stimuli may be more relevant to the therapeutic actions of antipsychotic drugs than receptor blockade or decreases in absolute levels of dopamine, which could presumably be circumvented by homeostatic adaptations in this highly plastic system.

The electrophysiological, biochemical and behavioral responses produced by administration of haloperidol were studied in intact rats and in rats with 6-hydroxydopamine-induced partial lesions of the nigrostriatal dopamine pathway. In both control rats and rates tested four to 10 days postlesion, the electrophysiological response of nigral dopamine neurons to increasing doses of haloperidol consisted of either: (1) an increase in firing rate which reached a plateau at six to 10 spikes per second, or (2) no response (i.e., less than 20% change in firing rate). Administration of additional doses of haloperidol up to lethal levels did not elicit further changes in dopamine cell firing in these rats. In contrast, in 6-hydroxydopamine-treated rats tested four to six weeks postlesion, acute administration of haloperidol was not only more consistent in producing increases in dopamine cell firing rate, but also caused six out of seven dopamine neurons tested to cease firing upon entering a state of depolarization block. In all cases in which depolarization block was observed, dopamine cell firing was reinstated by either iontophoretic application of gamma-aminobutyric acid or intravenous administration of apomorphine. In parallel studies, haloperidol caused an increase in the extracellular dopamine levels measured by microdialysis in the striatum of control rats, whereas administration of the same dose of haloperidol to 6-hydroxydopamine-treated rats four to six weeks postlesion did not elicit any change in extracellular dopamine levels. In addition, administration of haloperidol at a dose which was ineffective in control rats produced gross motor deficits in the 6-hydroxydopamine-treated rats when tested four to six weeks postlesion. These results show that 6-hydroxydopamine-induced dopamine depletions produce a time-dependent change in the responsivity of the nigrostriatal dopamine system to acute haloperidol administration. In this altered system, the induction of depolarization block of spike activity in nigral dopamine neurons by haloperidol was not associated with a corresponding decrease in extracellular dopamine levels measured in the striatum. However, it appeared that depolarization block did prevent haloperidol-induced increases in extracellular dopamine levels. The occurrence of depolarization block in the dopamine-depleted animal may limit the capacity of this system to respond to additional compromise, in spite of the compensatory processes that contribute to maintaining motor function.

Extracellular recordings from identified dopamine neurons were used to assess the effect of 6-hydroxydopamine-induced partial lesions of the nigrostriatal dopamine system on the sensitivity of the residual dopamine neurons to the dopamine agonist apomorphine. This was done by testing the response of identified nigral dopamine neurons in control and lesioned rats to systemic apomorphine administration at two time points: 1) 6-10 days post-lesion, when the loss of dopamine cells is nearly complete, and 2) 4-8 weeks post-lesion, which should be sufficient time for changes in dopamine receptor density to occur. As reported previously, dopamine neurons in control rats were inhibited by systemic administration of apomorphine, with their sensitivity being inversely related to their initial firing rate. The sensitivity of the residual dopamine neurons to apomorphine was unaltered in rats tested 6-10 days after depletions of at least 60% of striatal dopamine. However, by 4-8 weeks post-lesion, there was a significant increase in the sensitivity to apomorphine; furthermore, sensitivity was no longer related to baseline firing rate, but instead was uniformly high in all dopamine neurons tested at this time. This enhanced sensitivity was not altered by hemisection of the striatonigral projection, suggesting that the increased sensitivity to apomorphine was most likely a result of a time-dependent up-regulation of somatodendritic autoreceptors on the residual dopamine neurons.

The pathogenesis of a variety of neurological and mental disorders has been attributed to the malfunctioning of central dopaminergic systems. Twenty years of investigation concerning the way that dopamine systems function in the brain has yielded a great deal of information about the control and behavior of these systems. This collection of seemingly disparate facts is reviewed. A hypothesis is then presented that attempts to synthesize from these facts a view of the electrophysiological functioning of midbrain dopamine systems that may be helpful in understanding brain mechanisms underlying the pathology of such disorders as Parkinson's disease and schizophrenia.

Reliable diagnosis of cardiac allograft rejection is at present only possible using endomyocardial biopsy. We have serially measured epicardial evoked T wave amplitude during ventricular pacing with an externalized QT driven rate responsive pacemaker telemetered to a TP2 analyzer in 13 patients (12 males) followed for 19 (14-26) days after transplantation. A total of 228 records were analyzed. Rejection was defined on endomyocardial biopsy. On 17 of the 31 occasions on which biopsy was performed during the study, specimens showed significant (moderate) rejection. In 11 patients the initial biopsy proven rejection episode was associated with a significant fall in the evoked T wave amplitude from 1.3 (0.7-2.3) mV to 0.6 (0.5-1.8) mV (P less than 0.005), which began 2 (1-4) days earlier. One patient with uncontrolled diabetes mellitus had no change in evoked T wave amplitude during rejection. The evoked T wave amplitude did not fall in the absence of histologic rejection. These results suggest a noninvasive method for detecting cardiac rejection, which appears both sensitive (92%) and specific (100%) in the first rejection episodes.

A 53 year old woman with symptomatic pacemaker associated superior vena cava syndrome was treated successfully with balloon angioplasty. She was well six months after the procedure.

Intracellular recordings were obtained from identified dopamine (DA) neurons in rat midbrain slices maintained in vitro. DA neuron membranes exhibited pronounced instantaneous and time-dependent anomalous rectification that showed evidence of maximal activation at average membrane potentials of -63 and -78 mV, respectively. Action potentials were followed by prominent afterhyperpolarizations (AHP) that consisted of two components. The fast component showed evidence of inactivation at -63 mV independent of the initial membrane potential, whereas the longer-duration, later component increased in amplitude at hyperpolarized potentials. Unlike DA neurons recorded in vivo, there was no evidence of spike frequency adaptation or summation of AHPs with prolonged depolarization-induced spike trains. Spontaneous spike discharge occurred via an endogenous pacemaker potential that was dependent on both TTX-sensitive and cobalt-sensitive processes. Hyperpolarizing prepulses could activate rebound pacemaker discharge, but this rebound activity was progressively blocked with larger-amplitude hyperpolarizing prepulses. DA neurons recorded in the anesthetized animal, freely moving animal, and in vitro preparations have been shown to exist in two states of activity: 1) spontaneously discharging action potentials or 2) hyperpolarized, quiescent, and nonfiring. Furthermore, although it is rare to find DA neurons in the untreated animal in transitional states of activity, quiescent neurons can be activated by stimuli that place a demand on the DA system. The evidence presented here is consistent with the hypothesis that the special combination of membrane properties of DA neurons contribute to the segregation of their activity into active or inactive states.

We report here the presence of fast subthreshold oscillatory potentials recorded in vitro from neurons within layer 4 of the guinea pig frontal cortex. Two types of oscillatory neurons were recorded: (i) One type exhibited subthreshold oscillations whose frequency increased with membrane depolarization and encompassed a range of 10-45 Hz. Action potentials in this type of neuron demonstrated clear after-hyperpolarizations. (ii) The second type of neuron was characterized by narrow-frequency oscillations near 35-50 Hz. These oscillations often outlasted the initiating depolarizing stimulus. No calcium component could be identified in their action potential. In both types of cell the subthreshold oscillations were tetrodotoxin-sensitive, indicating that the depolarizing phase of the oscillation was generated by a voltage-dependent sodium conductance. The initial depolarizing phase was followed by a potassium conductance responsible for the falling phase of the oscillatory wave. In both types of cell, the subthreshold oscillation could trigger spikes at the oscillatory frequency, if the membrane was sufficiently depolarized. Combining intracellular recordings with Lucifer yellow staining showed that the narrow-frequency oscillatory activity was produced by a sparsely spinous interneuron located in layer 4 of the cortex. This neuron has extensive local axonal collaterals that ramify in layers 3 and 4 such that they may contribute to the columnar synchronization of activity in the 40- to 50-Hz range. Cortical activity in this frequency range has been proposed as the basis for the "conjunctive properties" of central nervous system networks.

A novel mechanism for regulating dopamine activity in subcortical sites and its possible relevance to schizophrenia is proposed. This hypothesis is based on the regulation of dopamine release into subcortical regions occurring via two independent mechanisms: (1) transient or phasic dopamine release caused by dopamine neuron firing, and (2) sustained, "background" tonic dopamine release regulated by prefrontal cortical afferents. Behaviorally relevant stimuli are proposed to cause short-term activation of dopamine cell firing to trigger the phasic component of dopamine release. In contrast, tonic dopamine release is proposed to regulate the intensity of the phasic dopamine response through its effect on extracellular dopamine levels. In this way, tonic dopamine release would set the background level of dopamine receptor stimulation (both autoreceptor and postsynaptic) and, through homeostatic mechanisms, the responsivity of the system to dopamine in these sites. In schizophrenics, a prolonged decrease in prefrontal cortical activity is proposed to reduce tonic dopamine release. Over time, this would elicit homeostatic compensations that would increase overall dopamine responsivity and thereby cause subsequent phasic dopamine release to elicit abnormally large responses.

Extensive damage to the nigrostriatal dopamine (DA) system initiates a number of compensatory changes that may be involved in counterbalancing the effects of the lesion. In this study, we examined whether changes in the electrophysiological activity of the remaining DA cells play a role in compensating for 6-hydroxydopamine (6-OHDA)-induced depletions of striatal DA. DA cell activity in lesioned rats was assessed along three dimensions: (1) the relative proportion of the remaining DA neurons firing spontaneously, (2) their firing rate, and (3) their firing pattern. Histofluorescence studies revealed a sparing of DA neurons in the midbrain of 6-OHDA-lesioned rats relative to the levels of DA remaining in the striatum. With respect to DA cell activity, depletions of up to 96% of striatal DA did not result in substantial alterations in the proportion of DA neurons active, their mean firing rate, or their firing pattern. Increases in these parameters only occurred when striatal DA depletions exceeded 96%. These results suggest that the biochemical and receptor compensations produced in the DA system in response to injury are of sufficient magnitude to allow the DA cells to maintain baseline levels of activity. In this way, the remaining DA neurons would maintain the wide dynamic range of electrophysiological responsivity that may be necessary for the normal function of the extrapyramidal motor system.

The regulation of spike generation in rat midbrain dopamine (DA) neurons was investigated using in vitro intracellular recordings. DA neurons fired long (greater than 1.8 ms) action potentials that exhibited comparatively depolarized spike thresholds (approx. -35 to -45 mV). Depolarization of the DA neuron increased the duration and the threshold of subsequent action potentials. The action potential was composed of two distinct components, a fast (0.8-1.5 ms duration) initial segment (IS) spike which triggered a slow (1.5-3 ms duration) somatodendritic (SD) component. Cobalt application (2 mM) blocked the SD spike component and revealed fast TTX-sensitive spikes. These fast spikes were also observed in untreated neurons following large hyperpolarizing pulses, and showed consistent changes in threshold and amplitude during membrane depolarization. Administration of 4-aminopyridine decreased the threshold of this TTX-sensitive spike, whereas tetraethylammonium (TEA) had no effect. When the fast spike was blocked by TTX, depolarization was ineffective in triggering further spike activity. However, after the administration of TEA (but not 4-AP), high threshold cobalt-sensitive spike activity could be triggered by relatively small depolarizations. TEA increased the duration of the SD portion of the action potential without altering the action potential threshold. The effect of 4-AP on spike threshold and the increase in SD spike duration caused by TEA were similar in nature to the changes in action-potential waveforms produced by polarizing the DA neuron membrane. Drawing from evidence gathered here and in previous in vivo studies, the properties of the TTX-sensitive fast spike are consistent with those of the IS spike component of the action potential, whereas the SD component is similar in nature to the high threshold calcium spike. One hypothesis that can be drawn from these studies is that dendritic and axonal spiking regions may exist in different functional subcompartments of the DA neuron, and may be independently modulated by pharmacologically distinct conductances. Under these conditions, synaptic influences could exist to modulate dendritic excitability and thus regulate putative dendritic spike-dependent functions, such as neuronal activity state, electrical coupling, and dendritic DA synthesis and release.

Parkinson's disease is associated with degeneration of the dopaminergic component of the nigrostriatal pathway. However, the neurological symptoms of this disorder do not emerge until the degenerative process is almost complete. A comparable phenomenon can be observed in animal models of Parkinson's disease produced by the administration of the selective neurotoxin, 6-hydroxydopamine (6-OHDA). Studies using such models suggest that the extensive loss of dopaminergic neurons is compensated, in large part, by increased synthesis and release of dopamine (DA) from those DA neurons that remain, together with a reduced rate of DA inactivation. These findings may have important implications for the diagnosis and treatment of a variety of neurological and psychiatric diseases, as well as for our understanding of plasticity in monoaminergic systems.

A fatal case of Lyme carditis occurring in a Suffolk farmworker is reported. Post-mortem examination of the heart showed pericarditis, focal myocarditis and prominent endocardial and interstitial fibrosis. The additional finding of endodermal heterotopia ('mesothelioma') of the atrioventricular node raises the possibility that this could also be related to Lyme infection and account for the relatively frequent occurrence of atrioventricular block in this condition. Lyme disease should always be considered in a case of atrioventricular block, particularly in a young patient from a rural area. The heart block tends to improve and therefore only temporary pacing may be required.

Dopamine (DA) cells are known to be very sensitive to direct acting DA agonists, and inhibition of DA cell firing by low doses of DA agonists is generally considered to be an action of these agonists on DA cell autoreceptors. During intracellular recording from spontaneously discharging DA cells in vivo, intravenous administration of apomorphine (20 micrograms/kg i.v.) elicited a hyperpolarization and an increase in input resistance. The calculated reversal potential of the apomorphine effect was approximately -40 mV. However, in non-firing DA cells the reversal potential of these effects was significantly different (P less than 0.01), being close to the reversal potential of responses induced by stimulation of striatonigral pathways (i.e. -67 mV). In addition, haloperidol (0.01 mg/kg i.v.) reversed the hyperpolarization produced by apomorphine but not the increase in input resistance. Transection of striatonigral pathways eliminated most of the increase in input resistance accompanying apomorphine administration, and shifted the apomorphine reversal potential to a value positive to 0 mV. Low doses of apomorphine were also found to affect a class of zona reticulata (ZR) interneurons. Apomorphine caused decreases in ZR cell firing rate, which were abolished by striatonigral pathway transection. Thus, the following mechanism is proposed for the electrophysiological actions of autoreceptor-selective doses of apomorphine on DA cells: (1) apomorphine directly inhibits spontaneous DA cell discharge by inhibiting the slow depolarization preceding action potentials and thereby hyperpolarizes the DA cell, (2) decreased DA cell firing disinhibits GABAergic striatal cells, whose increased firing preferentially (3) inhibits GABAergic ZR interneurons, and thus (4) removes an inhibitory input to DA cells, resulting in an increase in input resistance.

Intracellular recordings were obtained from directly identified rat nigral dopamine cells in vivo. This identification was based on an increase in glyoxylic acid-induced catecholamine fluorescence in the impaled dopamine neurons. One of three compounds was injected intracellularly into each cell to produce the heightened fluorescence: (1) L-DOPA, to increase the intracellular dopamine content by precursor loading; (2) tetrahydrobiopterin, a cofactor for tyrosine hydroxylase, to increase intracellular dopamine concentration through activation of the rate-limiting enzyme for dopamine synthesis and (3) colchicine, to arrest intraneuronal transport and thus allow the build-up of dopamine synthesizing enzymes and dopamine in the soma. In addition, dopamine cells were antidromically activated from the caudate nucleus and collision with a directly elicited action potential was demonstrated. Identified dopamine neurons were shown to possess an input resistance of 31.2 +/- 7.4 M omega (means +/- SD) and a time constant of 12.1 +/- 3.2 ms. The action potentials were of long duration (2.75 +/- 0.5 ms) with a marked break between the initial segment and the somatodendritic spike components. The initial segment was the only component commonly elicited during antidromic activation. Spontaneously occurring action potentials were usually preceded by a slow, pacemaker-like depolarization. Burst firing by summation of depolarizing afterpotentials was observed to occur spontaneously, but could not be triggered by short depolarizing current pulses. Intravenously administered apomorphine demonstrated the same inhibitory effect on cell firing that was previously reported to occur when recording extracellularly from identified dopaminergic neurons. The determination of the electrophysiological characteristics of a population of cells directly identified as containing a specific neurotransmitter (in this case, dopamine) may allow one to construct better models of a system's functioning. Thus, the high input resistance and long time constant of dopamine-containing cells, combined with their burst/pause firing mode, may be important functionally with respect to a possible modulatory effect of dopamine in postsynaptic target areas.

Using three independent in vivo methods, we have obtained evidence for electrotonic coupling between sets of rat zona compacta dopaminergic neurons: (1) Lucifer yellow injection into single dopamine neurons resulted in labeling of two to five dopamine neurons in 18 out of 33 injections. Similar injections into reticular formation or nigral reticulata cells did not demonstrate multiple labeling. (2) Intracellular recording revealed spontaneously occurring small (3-15 mV) fast potentials that often triggered action potentials in dopamine neurons when the membrane potential was close to firing threshold. These fast potentials had a firing rate and pattern similar to that reported previously for extracellularly recorded dopamine neurons. Fast potentials were activated antidromically from the caudate nucleus at a latency similar to that reported for dopamine neurons, followed high frequency antidromic stimulation at a constant latency, and collided with spontaneously occurring fast potentials. However, directly elicited action potentials would not collide reliably with antidromically activated fast potentials. Intracellular injection of depolarizing or hyperpolarizing current increased and decreased, respectively, the rate of occurrence of these potentials. The firing rate of fast potentials could be increased and decreased by the intravenous administration of dopamine antagonists and agonists, respectively. (3) Simultaneous extracellular recording from pairs of DA neurons revealed numerous instances of synchronized action potentials. This was observed more frequently following intravenous haloperidol administration. Sets of burst firing dopamine neurons recorded simultaneously consistently demonstrated a decrease in the interspike interval as the burst progressed; a phenomenon commonly reported in other electrically coupled systems. Electrical coupling has been suggested to be present in sets of identified nigrostriatal dopamine neurons. Electrical communication between these neurons could be involved in modulating burst firing and in synchronizing dopamine release.

Intracerebral injections of the neurotoxin 6-hydroxydopamine (6-HDA) can produce selective, near-total destruction of the dopamine (DA)-containing neurons of the nigrostriatal bundle. The dysfunctions in animals with these lesions show many parallels with those present in Parkinsonian patients. Among these are the extensive loss of DA neurons in the basal ganglia, neurological impairments including akinesia, paradoxical kinesia in response to activating conditions, and improved sensory-motor function after the administration of DOPA. Moreover, as with patients with preclinical Parkinsonism, 6-HDA-treated rats with less extensive lesions show few or no behavioral dysfunctions, but are unusually sensitive to the akinesia-inducing effects of stress and dopaminergic antagonists. In this review, we summarize the behavioral effects of 6-HDA-induced depletion of striatal DA in the rat and then focus on the compensatory changes that may underlie the preclinical stage of the disorder. These compensations appear to include an increase in the number of active DA neurons, an increase in the release of DA per impulse from residual terminals, and a decrease in the amount of DA inactivated by high affinity uptake. Collectively, these alterations permit a few residual DA neurons to maintain a normal level of control over cellular activity in the striatum.

In order to observe the fine details of intracellularly stained neurons in brain slices the slices must be cleared of opaque matter. This clearing process involves dehydration of the slice, which typically results in significant shrinkage of the cleared tissue. However, how this shrinkage affects neuronal morphology has not been demonstrated to date. In this paper we detail the artifacts induced in the morphology of stained neurons by this clearing process. During dehydration-induced shrinkage of the brain slices, neurons stained with the water-soluble dye, Lucifer yellow, demonstrated a dramatic decrease in size to less than two-thirds of their original dimensions. In contrast neurons stained with the horseradish peroxidase/diaminobenzidine reaction-product did not shrink with the slice; instead the dendrites bent and curled during dehydration with no loss in cell-soma size. We have managed to circumvent these artifacts by using as a clearing agent the solvent dimethyl sulfoxide, which is miscible in both aqueous and organic phases. This solvent will clear tissue slices without inducing the concomitant artifacts caused by tissue shrinkage occurring with the alcohol-dehydration process.

The responses of dopamine cells in the substantia nigra to iontophoretically administered dopamine and intravenous apomorphine were compared to the responses of spontaneously active neurons in the caudate nucleus. Dopaminergic cells were six to ten times more sensitive to dopamine and intravenous apomorphine than 86 percent of the caudate cells tested. This differential sensitivity of dopamine auto- and postsynaptic receptors may explain the apparently paradoxical behavioral effects induced by small compared to large doses of some dopamine agonists and may provide a means of developing new types of drugs to antagonize dopaminergic influence in the central nervous system.

Dopamine (DA) neurons have been recorded in vivo in four states of activity: hyperpolarized, nonfiring; single spike firing; burst firing; and depolarization inactivation. Nonfiring DA neurons can be made to fire by iontophoretic application of the excitatory substances glutamate and cholecystokinin, or by depolarizing current injection. Spontaneously active DA cells typically fire in a slow (3 to 8 Hz) irregular pattern. In vivo intracellular recordings revealed that this pattern is sustained by the alternation of two currents: a spontaneously occurring slow depolarization (13 +/- 3 mV amplitude, 78 +/- 40 msec duration) which brings the membrane potential of the DA cell to spike threshold (-42 mV), and an afterhyperpolarization mediated by a calcium-activated potassium conductance (IK(Ca)). The slow depolarization is a pacemaker-like conductance, with a rate of rise proportional to the membrane potential. The regular pacemaker pattern of the spontaneously occurring slow depolarization is interrupted by the IK(Ca) which appears to be triggered by calcium entry during the action potential. Thus, intracellular injection of the calcium chelator EGTA will cause DA cells to fire in a regular, pacemaker pattern. The IK(Ca) is observed after single spikes and trains of spikes with the amplitude of the afterhyperpolarization being proportional to the number of spikes in a train. Both the afterhyperpolarization and the firing accommodation observed during depolarizing current injection can be blocked by intracellular injection of the calcium chelator EGTA.

The effects of the dopamine agonist bromocryptine on several measures dopaminergic function were assessed in the rat. Following inhibition of impulse flow with gamma-butyrolactone, and after dopa decarboxylase inhibition, dopa accumulation and its reversal by dopamine agonists is easily studied. In this model, bromocryptine (10 mg/kg, i.p.) caused a significant decrease in dopa accumulation in both the striatum and olfactory tubercle which was prevented, but not reversed, by the dopamine antagonist (+)-butaclamol (4 mg/mg, i.p.). The inactive isomer, (-)-butaclamol was without effect. Analysis of an vitro 3H-spiperone binding 2h after bromocryptine (10 mg/kg, i.p.) revealed a 30% decrease in the number of striatal dopamine receptors labelled (Bmax), with no change in receptor affinity for 3H-spiperone. No changes in binding were seen when animals were sacrificed 30 min or 48 h after bromocryptine. In extracellular single unit recording experiments, bromocryptine-induced depression of nigrostriatal dopamine cell firing was found to be largely reversible by the dopamine antagonist haloperidol when injected within 5 min of intravenous bromocryptine. However, when haloperidol was injected more than 20 min after bromocryptine, no reversal of bromocryptine-induced depression of cell firing was obtained. These results strongly suggest that bromocryptine interacts in an irreversible fashion with central dopaminergic receptors.

Electrophysiological and biochemical techniques were used to study midbrain dopamine systems. In the electrophysiological studies, projection areas of individual dopaminergic cells were identified by antidromic activation. Dopamine cells which innervate the piriform cortex and those that innervate the caudate nucleus demonstrated their usual dose-dependent inhibitory response to both the intravenous administration of the direct-acting dopamine agonist apomorphine and the microiontophoretic application of dopamine. In contrast, the firing rate of dopamine neurons which project to the prefrontal cortex and of those terminating in the cingulate cortex was not altered by either the intravenous administration of low to moderate doses of apomorphine or microiontophoretically applied dopamine. The mean basal discharge rate and degree of burst firing was also different between these subgroups of midbrain dopaminergic neurons. Mesoprefrontal and mesocingulate dopamine neurons had mean firing rates of 9.3 and 5.9 spikes/s respectively, and showed intense burst activity. Mesopiriform and nigrostriatal dopamine cells had discharge rates of 4.3 and 3.1 spikes/s and displayed only moderate bursting. The dopaminergic nature of those mesocortical neurons insensitive to apomorphine and dopamine was confirmed using combined intracellular recording and catecholamine histofluorescence techniques. Thus, after the intracellular injection of colchicine and subsequent processing for glyoxylic acid-induced histofluorescence, the injected cells could be identified by their brighter fluorescences compared to the surrounding, normally fluorescing, non-injected dopamine neurons. Using biochemical techniques, subgroups of midbrain dopaminergic systems were again found to differ. The administration of gamma-butyrolactone increased dopamine levels in all areas sampled (prefrontal, cingulate and piriform cortices as well as the caudate nucleus). However, although this effect was readily reversed in both the piriform cortex and caudate nucleus by pretreatment with apomorphine, this treatment had no effect on the increased dopamine levels observed in the prefrontal and cingulate cortices. In addition, the decline in dopamine levels after synthesis inhibition with alpha-methyltyrosine was significantly faster in the prefrontal and cingulate cortices relative to the caudate nucleus. The piriform cortex showed an intermediate decline which was not significantly different from that observed in any of the other regions.(ABSTRACT TRUNCATED AT 400 WORDS)

The existence of a striatonigral GABAergic pathway has been well established both anatomically and biochemically. During intracellular recording from identified DA neurons in vivo, stimulation of the striatum (100 microA, 50 microseconds pulses) elicits an inhibitory postsynaptic potential (IPSP) and a rebound depolarization. The IPSP is a short latency (1.8-2.2 ms) conductance increase to chloride, since: the reversal potential is near the chloride reversal potential reported for other cells (-68 mV); intracellular chloride injection progressively reverses the IPSP into a depolarization with a similar time course; and the response of DA cells to systemic injection of the chloride channel blocker, picrotoxin, also exhibits a similar reversal potential. In contrast, during extracellular recording, stimulation of the striatum at low levels of intensity (e.g. 20 microA at 10 Hz) increases the firing rate of DA cells. Stimulation of the striatum will, in addition, elicit IPSPs in a subclass of substantia nigra zona reticulata neurons at the same latency as the IPSPs triggered in DA cells. These IPSPs also reverse with intracellular chloride injection. However, their amplitude is larger and their duration longer than observed in DA cells, and there is no depolarizing rebound. The late component of the IPSP in the zona reticulata neurons corresponds temporally to the rebound depolarization seen in DA cells in response to striatal stimulation. In addition, when recorded extracellularly, striatal stimulation will inhibit the firing of this class of zona reticulata interneurons at the same stimulation parameters that will excite DA cells. These data suggest that striatal cells may send branched fast-conducting GABAergic projections to zona reticulata cells and DA cells. Furthermore, low levels of striatal stimulation can excite DA cells by preferentially inhibiting interneurons in the zona reticulata which are more sensitive to the inhibitory effects of GABA than are DA neurons.

The effect of the addition of phenoxybenzamine to a regular insulin preparation on the absorption of insulin and plasma glucose concentrations was investigated in five normal subjects. The addition of phenoxybenzamine (20 micrograms) to insulin (0.2 U/kg body wt) resulted in an acceleration of insulin absorption, with a higher and earlier peak insulin concentration at 45 min and higher insulin concentrations throughout the 180 min of investigation. With phenoxybenzamine, plasma glucose concentrations fell more rapidly; nadirs were lower and more rapidly achieved. Two subjects experienced hypoglycemic symptoms when phenoxybenzamine was added to insulin. It is concluded that the addition of phenoxybenzamine to subcutaneously injected insulin increases the bioavailability of insulin for at least 3 h, which leads to a greater fall in plasma glucose concentrations.

In vitro intracellular recordings were made from neurons in the rat midbrain slice. Two neuronal types could be distinguished in dopamine-containing (DA) midbrain regions based on electrophysiological criteria. One neuron type exhibited short duration action potentials (less than 1.5 msec), could fire at high frequencies (greater than 10 Hz), and exhibited either phasic or burst firing patterns. This neuron did not exhibit tyrosine hydroxylase immunoreactivity. A second neuronal type exhibited a unique set of electrophysiological properties, which included (1) a spontaneous pacemaker-like depolarizing potential, (2) a highly regular firing pattern, (3) long duration (greater than 2 msec) action potentials, and (4) a high (i.e., depolarized) spike threshold. This neuron was consistently double labeled using intracellular staining and immunocytochemical localization of the catecholamine-specific enzyme tyrosine hydroxylase, and thus represented the DA neuronal type. Midbrain DA neurons stained with Lucifer yellow could be separated into 3 classes based on their location and morphology: (1) fusiform neurons with laterally projecting dendrites in the dorsal substantia nigra zona compacta region, (2) multipolar cells with laterally and ventrally projecting dendrites in the ventral substantia nigra zona compacta, and (3) neurons with fusiform and multipolar somata and radially projecting dendrites in the ventral tegmental area. The dendrites also exhibited spine-like protrusions and ended with specialized forked processes. Spontaneously firing DA cells recorded in vitro had a number of distinguishing electrophysiological characteristics in common with those of DA neurons recorded in vivo, such as the presence of a slow depolarizing potential driving spike activity and a characteristic depolarized spike threshold (approximately-36 mV). However, in contrast to that found in vivo, the DA cells characterized here exhibited substantially higher input resistances and fired spontaneously in a very regular pacemaker pattern. Burst firing was not observed. Spike activity was apparently dependent on 4 depolarizing events: (1) a voltage-dependent TTX-sensitive slow depolarization, (2) a cobalt-sensitive low threshold depolarization that was activated during the rebound from brief membrane hyperpolarizations, (3) high threshold dendritic calcium spikes which gave rise to the spike afterhyperpolarization, and (4) a high threshold initial segment sodium spike. These depolarizations were modulated by several processes, including a 4-aminopyridine-insensitive delayed repolarization, an instantaneous and time-dependent anomalous rectifier, and an afterhyperpolarization. Although low threshold depolarizations and rebound action potentials could be triggered by the membrane repolarization following small membrane hyperpolarizations, comparatively larger hyperpolarizations attenuated this rebound activation, thereby suppressing anodal break excitation.(ABSTRACT TRUNCATED AT 400 WORDS)

The recent discovery of new methods for tracing brain pathways and studying their function has greatly expanded our knowledge of the areas of the brain involved in extrapyramidal disorders and has begun to change our concept of how these areas function. One such change has involved our understanding of the role of the dopaminergic system within the basal ganglia. Rather than dopamine being seen primarily as a major influence on striatal output it is now perhaps best conceptualized as one link in a set of circular pathways, one of whose functions is to modulate nondopaminergic output from the substantia nigra to other brain regions. In addition, new evidence suggests that this neurotransmitter system may play a role in the way sensory inputs are handled by the basal ganglia. These new roles for the nigral dopaminergic system raise important new questions about its function in extrapyramidal motor systems as well as provide possible answers to questions concerning the mechanisms underlying some of the symptoms seen in disorders involving the basal ganglia.

Intracellular recordings in vivo were obtained from dopamine-containing neurons of the rat substantia nigra. These neurons were identified electrophysiologically by antidromic activation and histochemically by L-dopa injection and subsequent fluorescence histochemistry. Extracellular spikes and antidromic conduction velocity of the neurons were identical to those previously described for putative dopaminergic neurons. Spontaneous intracellular fast potentials, slow depolarizations during burst firing, and spike prepotentials were observed.

The effects of the antipsychotic drug haloperidol (HAL) on the electrophysiological activity of dopamine (DA)-containing cells in the substantia nigra was assessed in rats 6 weeks after partial 6-hydroxydopamine (6-OHDA)-induced lesions of the nigrostriatal DA pathway. Depleting 75% or more of striatal DA altered the response of DA neurons to acute HAL administration. Whereas acute HAL administration generally accelerates DA neuron firing in control rats, similar HAL doses given to lesioned rats not only increased firing rate but induced depolarization block of DA neuron spike generation similar to that resulting from chronic neuroleptic administration. In contrast, acute administration of doses of HAL up to lethal levels typically could not induce depolarization block of DA neurons in non-lesioned rats. This preparation thus could be an effective model for investigating the exacerbation of behavioral deficits produced by an increased demand placed upon a compromised DA system, as may occur in Parkinson's disease or with antipsychotic drug treatment.

Intracellular recordings from identified nigral dopamine neurons in the rat revealed that their potentials are composed of four components: (1) a slow depolarization, (2) an initial segment spike, (3) a somatodendritic spike, and (4) an afterhyperpolarization. By combining intracellular and extracellular recording techniques with anatomical studies using intracellular injections of Lucifer yellow, an attempt was made to localize each of these potentials to various neuronal compartments. Lucifer yellow injections demonstrated that the dopamine neurons recorded have a pyramidal or polygonal shaped soma, 12-30 microns in diameter, with 3-6 thick major dendrites which extend 10-50 microns from the soma before bifurcating. The axon appears to rise from a major dendrite 15-30 microns from the soma. Based on this anatomical configuration, results from the electrophysiological studies suggest that: (1) the slow depolarization is a pacemaker-like conductance most likely localized to the somatic region, (2) the initial segment spike is a low-threshold spike probably located at the axon hillock, (3) the somatodendritic spikes are long duration spikes that rapidly inactivate with depolarization, have a high threshold, and are localized to the dendritic regions. The action potential is then terminated by a long duration afterhyperpolarization. Our data further suggest that spike generation may be initiated by a slow depolarization at the soma triggering a spike in the low-threshold axon hillock which then spreads across the already-depolarized soma to trigger the dendritic spike. Based on the above findings, dopamine neurons can be compartmentalized electrophysiologically and morphologically into subcomponents, each associated with spikes and specific ionic currents. The high threshold dendritic component of the action potential demonstrates rapid inactivation with depolarization, and thus occurs over a rather narrow range of membrane polarization. This limited range of action potential generation may be important in control of dendritic dopamine release and/or modulation of electrical coupling between dopaminergic neurons.

Biochemical and electrophysiological and electrophysiological studies suggest that GABA agonists increase the activity of dopaminergic neurons in the zona compacta (ZC) of the substantia nigra despite a known GABAergic input to ZC cells. Using single-unit recording techniques we have investigated this "paradoxical" effect. One population of neurons located in the zona reticulata (ZR) of the substantia nigra was found to be 20 times more sensitive to iontophoretically applied GABA than ZC neurons. GABA introduced by means of microiontophoresis into the ZR caused an increase in ZC cell activity while glutamic acid introduced in the same manner produced an inhibition of ZC cells. The latter effect was blocked by low doses of picrotoxin. Muscimol (i.v.) caused a decrease in ZR cell activity at the same dose that caused a parallel increase in ZC cell firing rate. These data suggest that ZC cells receive an inhibitory GABAergic input from ZR cells that are in turn inhibited by low doses of GABA agonists. Such an anatomical arrangement would account for the "paradoxical" excitatory effect of GABA agonists on ZC neuron activity.

In addition to firing in a single spiking mode, dopamine (DA) cells have been observed to fire in a bursting pattern with consecutive spikes in a burst displaying progressively decreasing amplitude and increasing duration. In vivo intracellular recording demonstrated the bursts to typically ride on a depolarizing wave (5 to 15 mV amplitude). Although the burst-firing frequency of DA cells showed little correlation with the base line firing rate, increases in firing rate were usually associated with an increase in burst firing. Increases in burst firing could also be elicited by intracellular calcium injection and could be prevented by intracellular injection of EGTA, suggesting a calcium involvement in bursting. Blockade of potassium conductances with extracellular iontophoresis of barium or intracellular injection of tetraethylammonium bromide could also trigger an increased degree of burst firing in DA cells. These data suggest that the increased calcium influx accompanying an increased firing rate triggers burst firing, possibly by inactivating a potassium conductance. A switch from a single spiking mode to a burst-firing mode may be important in modulating striatal DA release, as shown for burst firing in other preparations.

In view of the reported increase in thromboembolic episodes following cyclosporine A (CyA) therapy, the effect of this drug on platelet aggregation and thromboxane A2 release was investigated. The addition of CyA, at therapeutic concentrations to platelet rich plasma from normal subjects in vitro was found to increase aggregation in response to adrenaline, collagen and ADP. Ingestion of CyA by healthy volunteers was also associated with enhanced platelet aggregation. The CyA-mediated enhancement of aggregation was further enhanced by the addition in vitro of therapeutic concentrations of heparin. Platelets from renal allograft recipients treated with CyA also showed hyperaggregability and increased thromboxane A2 release, which were most marked at "peak" plasma CyA concentration and less so at "trough" concentrations. Platelet hyperaggregability in renal allograft patients on long-term CyA therapy tended to revert towards normal following the replacement of CyA with azathioprine. Hypertensive patients with renal allografts on nifedipine therapy had normal platelet function and thromboxane release in spite of CyA therapy. These observations suggest that CyA-mediated platelet activation may contribute to the pathogenesis of the thromboembolic phenomena associated with the use of this drug. The increased release of thromboxane A2 (a vasoconstrictor) may also play a role in mediating CyA-related nephrotoxicity.

Acute administration of the antipsychotic drug haloperidol causes an increase in the firing rate and in the proportion of spontaneously active dopamine neurons in the substantia nigra. In contrast, repeated administration of haloperidol for 21 days results in a dramatic decrease in the number of dopamine neurons displaying spontaneous electrophysiological activity. In vivo intracellular recordings revealed that these nonfiring dopamine neurons have a membrane potential significantly more depolarized than dopamine neurons in control rats, although their input resistances were equivalent. Neurons displaying this depolarization block could not be activated by depolarizing current injection, but could be made to fire in response to hyperpolarization of the membrane. Furthermore, administration of autoreceptor-selective doses of the direct acting dopamine agonist apomorphine hyperpolarized inactivated dopamine neurons in treated rats, resulting in initiation of spontaneous activity. Thus, repeated administration of neuroleptics appears to cause a feedback excitatory drive to nigral dopamine neurons of sufficient magnitude to depolarize the membrane beyond the spike generating region. As a result, a tonic depolarization blockade of spontaneous spike activity is induced.