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The aim of this review is to explore the relationship between melatonin, free radicals, and non-excitatory amino acids, and their role in stroke and aging. Melatonin has garnered significant attention in recent years due to its diverse physiological functions and potential therapeutic benefits by reducing oxidative stress, inflammation, and apoptosis. Melatonin has been found to mitigate ischemic brain damage caused by stroke. By scavenging free radicals and reducing oxidative damage, melatonin may help slow down the aging process and protect against age-related cognitive decline. Additionally, non-excitatory amino acids have been shown to possess neuroprotective properties, including antioxidant and anti-inflammatory in stroke and aging-related conditions. They can attenuate oxidative stress, modulate calcium homeostasis, and inhibit apoptosis, thereby safeguarding neurons against damage induced by stroke and aging processes. The intracellular accumulation of certain non-excitatory amino acids could promote harmful effects during hypoxia-ischemia episodes and thus, the blockade of the amino acid transporters involved in the process could be an alternative therapeutic strategy to reduce ischemic damage. On the other hand, the accumulation of free radicals, specifically mitochondrial reactive oxygen and nitrogen species, accelerates cellular senescence and contributes to age-related decline. Recent research suggests a complex interplay between melatonin, free radicals, and non-excitatory amino acids in stroke and aging. The neuroprotective actions of melatonin and non-excitatory amino acids converge on multiple pathways, including the regulation of calcium homeostasis, modulation of apoptosis, and reduction of inflammation. These mechanisms collectively contribute to the preservation of neuronal integrity and functions, making them promising targets for therapeutic interventions in stroke and age-related disorders.

Dysregulation of cholesterol homeostasis is associated with several pathologies including cardiovascular diseases and neurological disorders such as Alzheimer's disease (AD). MicroRNAs (miRNAs) have emerged as key post-transcriptional regulators of cholesterol metabolism. We previously established the role of miR-7 in regulating insulin resistance and amyloidosis, which represents a common pathological feature between type 2 diabetes and AD. We show here an additional metabolic function of miR-7 in cholesterol biosynthesis. We found that miR-7 blocks the last steps of the cholesterol biosynthetic pathway in vitro by targeting relevant genes including DHCR24 and SC5D posttranscriptionally. Intracranial infusion of miR-7 on an adeno-associated viral vector reduced the expression of DHCR24 in the brain of wild-type mice, supporting in vivo miR-7 targeting. We also found that cholesterol regulates endogenous levels of miR-7 in vitro, correlating with transcriptional regulation through SREBP2 binding to its promoter region. In parallel to SREBP2 inhibition, the levels of miR-7 and hnRNPK (the host gene of miR-7) were concomitantly reduced in brain in a mouse model of Niemann Pick type C1 disease and in murine fatty liver, which are both characterized by intracellular cholesterol accumulation. Taken together, the results establish a novel regulatory feedback loop by which miR-7 modulates cholesterol homeostasis at the posttranscriptional level, an effect that could be exploited for therapeutic interventions against prevalent human diseases.

A reduction in taurine content accompanies the ageing process in many tissues. In fact, the decline of brain taurine levels has been associated with cognitive deficits whereas chronic administration of taurine seems to ameliorate age-related deficits such as memory acquisition and retention. In the present study, using rats of three age groups (young, adult and aged) we determined whether the content of taurine and other amino acids (glutamate, serine, glutamine, glycine, alanine and GABA) was altered during ageing in different brain areas (cerebellum, cortex and hippocampus) as well non-brain tissues (heart, kidney, liver and plasma). Moreover, using hippocampal slices we tested whether ageing affects synaptic function and plasticity. These parameters were also determined in aged rats fed with either taurine-devoid or taurine-supplemented diets. With age, we found heterogeneous changes in amino acid content depending on the amino acid type and the tissue. In the case of taurine, its content was reduced in the cerebellum of adult and aged rats, but it remained unchanged in the hippocampus, cortex, heart and liver. The synaptic response amplitude decreased in aged rats, although the late phase of long-term synaptic potentiation (late-LTP), a taurine-dependent process, was not altered. Our study highlights the stability of taurine content in the hippocampus during ageing regardless of whether taurine was present in the diet, which is consistent with the lack of changes detected in late-LTP. These results indicate that the beneficial effects of taurine supplementation might be independent of the replenishment of taurine stores.

Taurine is especially abundant in rodent brain where it appears to be involved in osmoregulation and synaptic plasticity mechanisms. The demonstration of a physiological role for taurine has been hampered by the difficulty in modifying taurine levels in most tissues, including the brain. We used an experimental strategy to reduce taurine levels, involving treatment with guanidinoethyl sulfonate (GES), a structural analogue of taurine that, among other properties, acts as a competitive inhibitor of taurine transport. GES delivered in the drinking water of rats for 1 month effectively reduced taurine levels in brain structures (hippocampus, cerebellum and cortex) and outside the brain (heart, muscle, kidney, liver and plasma) by between 50 and 80 %, depending on the tissue. This partial taurine depletion did not affect either basal synaptic transmission or the late phase of long-term potentiation (late-LTP) in hippocampal slices. In vivo microdialysis studies in the hippocampus revealed that GES treatment reduced extracellular taurine levels and the magnitude of taurine released in response to the application of either N-methyl-D-aspartate (NMDA) or a hypoosmotic solution, without affecting release mechanisms. Finally, we demonstrated in hippocampal slices that a brief GES application can mimic taurine action on the conversion of a decremental LTP into a perdurable late-LTP, concluding that GES might replace taurine function in some mechanisms such as those implicated in synaptic plasticity.

Background L-3,4-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia is an incapacitating complication of L-DOPA therapy that affects most patients with Parkinson's disease. Previous work indicating that molecular sensitization to dopamine receptor D1 (D1R) stimulation is involved in dyskinesias prompted us to perform electrophysiological recordings of striatal projection >medium spiny neurons> (MSN). Moreover, because enhanced D1R signaling in drug abuse induces changes in spine density in striatum, we investigated whether the dyskinesia is related to morphological changes in MSNs. Methods Wild-type and bacterial artificial chromosome transgenic mice (D1R-tomato and D2R-green fluorescent protein) mice were lesioned with 6-hydroxydopamine and subsequently treated with L-DOPA to induce dyskinesia. Functional, molecular, and structural changes were assessed in corticostriatal slices. Individual MSNs injected with Lucifer-Yellow were detected by immunohistochemistry for three-dimensional reconstructions with Neurolucida software. Intracellular current-clamp recordings with high-resistance micropipettes were used to characterize electrophysiological parameters. Results Both D1R-MSNs and D2R-MSNs showed diminished spine density in totally denervated striatal regions in parkinsonian mice. Chronic L-DOPA treatment, which induced dyskinesia and aberrant FosB expression, restored spine density in D2R-MSNs but not in D1R-MSNs. In basal conditions, MSNs are more excitable in parkinsonian than in sham mice, and excitability decreases toward normal values after L-DOPA treatment. Despite this normalization of basal excitability, in dyskinetic mice, the selective D1R agonist SKF38393 increased the number of evoked action potentials in MSNs, compared with sham animals. Conclusions Chronic L-DOPA induces abnormal spine re-growth exclusively in D2R-MSNs and robust supersensitization to D1R-activated excitability in denervated striatal MSNs. These changes might constitute the anatomica

Co-activation of NMDA and dopamine receptors is required for the induction of the late phase of LTP (L-LTP) that is dependent on new protein synthesis. Other neuromodulatory substances may also contribute to this process. Here, we examined whether taurine is one of the neuromodulators contributing to L-LTP induction, since it is known that taurine uptake induces a long-lasting synaptic potentiation dependent on protein synthesis, and taurine uptake inhibition blocks L-LTP induced by tetanization. Experiments were conducted using rat hippocampal slices where field synaptic potentials were evoked and recorded in CA3-CA1 synapses. Taurine (1 mM) applied 10 min before a high frequency stimulation (HFS) train converted a transitory early-LTP (E-LTP) into an L-LTP dependent on protein synthesis. This taurine effect was blocked by a taurine uptake inhibitor. A facilitation of L-LTP induction was also obtained by pre-application of SKF38393, a D1/D5 dopamine receptor (D1R) agonist. In this case, LTP facilitation was not affected by the taurine uptake inhibitor. Nevertheless, when taurine and SKF38393 were simultaneously pre-applied at a concentration that individually did not modify E-LTP, they produced a synergistic mechanism that facilitated the induction of L-LTP with a sole HFS train. This facilitation of L-LTP was blocked by inhibiting either taurine uptake or D1R activation. Taurine and SKF38393 activated different signaling pathways to transform E-LTP into L-LTP. Taurine-induced L-LTP facilitation required MAPK activation, while D1R-agonist-induced facilitation depended mainly on PKA activation and partially on MAPK activation. On the other hand, the synergistic mechanisms induced by the cooperative action of taurine and SKF38393 were impaired by inhibitors against MAPK, PKA and PI3-K. This pharmacological profile resembles that displayed by L-LTP induced by three HFS trains at 10-min intervals. These results indicate that taurine uptake is necessary and cooperates w

Cell adhesion molecules and downstream growth factor-dependent signaling are critical for brain development and synaptic plasticity, and they have been linked to cognitive function in adult animals. We have previously developed a mimetic peptide (FGL) from the neural cell adhesion molecule (NCAM) that enhances spatial learning and memory in rats. We have now investigated the cellular and molecular basis of this cognitive enhancement, using biochemical, morphological, electrophysiological, and behavioral analyses. We have found that FGL triggers a long-lasting enhancement of synaptic transmission in hippocampal CA1 neurons. This effect is mediated by a facilitated synaptic delivery of AMPA receptors, which is accompanied by enhanced NMDA receptor-dependent long-term potentiation (LTP). Both LTP and cognitive enhancement are mediated by an initial PKC activation, which is followed by persistent CaMKII activation. These results provide a mechanistic link between facilitation of AMPA receptor synaptic delivery and improved hippocampal-dependent learning, induced by a pharmacological cognitive enhancer.

Seizures have profound impact on synaptic function and plasticity. While kainic acid is a popular method to induce seizures and to potentially affect synaptic plasticity, it can also produce physiological-like oscillations and trigger some forms of long-term potentiation (LTP). Here, we examine whether induction of LTP is altered in hippocampal slices prepared from rats with different sensitivity to develop status epilepticus (SE) by systemic injection of kainic acid. Rats were treated with multiple low doses of kainic acid (5 mg/kg; i.p.) to develop SE in a majority of animals (72-85% rats). A group of rats were resistant to develop SE (15-28%) after several accumulated doses. Animals were subsequently tested using chronic recordings and object recognition tasks before brain slices were prepared for histological studies and to examine basic features of hippocampal synaptic function and plasticity, including input/output curves, paired-pulse facilitation and theta-burst induced LTP. Consistent with previous reports in kindling and pilocapine models, LTP was reduced in rats that developed SE after kainic acid injection. These animals exhibited signs of hippocampal sclerosis and developed spontaneous seizures. In contrast, resistant rats did not become epileptic and had no signs of cell loss and mossy fiber sprouting. In slices from resistant rats, theta-burst stimulation induced LTP of higher magnitude when compared with control and epileptic rats. Variations on LTP magnitude correlate with animals' performance in a hippocampal-dependent spatial memory task. Our results suggest dissociable long-term effects of treatment with kainic acid on synaptic function and plasticity depending on its epileptogenic efficiency. © 2012 Suárez et al.

The effects of different K- concentrations (3-100 mM) on both the extracellular amino acid levels and field potentials, evoked by perforant pathway stimulation, were studied 'in vivo" in the rat dentate gyrus by means of a brain dialysis device, formed by a hollow fiber plus a stainless-steel electrode. Perfusion with low K concentrations (3-12 raM; Krebs Ringer bicarbonate) specifically enhanced the dialysate levels of taurine and concomitantly increased the population spike amplitude. High K concentrations in pert'usate ( > 25 raM) did not further increase the levels of taurine but enhanced both glutamate and 7- aminobutyric acid levels, whereas the population spike diminished drastically. The absence of calcium ions in the perfusion liquid increased both basal and K+-enhanced taurine levels. The specific enhancement of cxtracellular taurine by physiological K- concentrations may represent an autoregulative mechanism of nervous tissue excitability.

The present work studied the neurotransmitter mediating the depressive effect of sensory stimulation on the Schaffer-CA1 transmission. Field responses of the CA1 region evoked by ipsilateral CA3 stimuli were recorded in paralyzed, locally anesthetized rats following the same experimental paradigm as in the previous work. The tissue zone under recording was perfused in vivo by an implanted hollow fiber (brain dialysis device) with either Krebs-Ringer bicarbonate (KRB), or KRB with penicillin, atropine, acetylcholine or eserine. Results were the following: (1) atropine-increased the field excitatory postsynaptic potential (EPSP) amplitude in a dose-dependent manner and totally abolished the modulatory action of sensory stimulation; (2) both the field EPSP and the modulatory action of sensory stimulation remained unaltered during the blockade of GABAergic activity by penicillin; (3) acetylcholine as well as eserine induced a great diminution of both field EPSP and population spike amplitudes, without altering the effect of sensory stimulation; (4) penicillin and atropine induced multiple population spikes, reversing the effect of sensory stimulation and increasing the cell excitability. These results demonstrate that the sensory modulation of information transfer through the Schaffer-CA1 synapse is mediated by a muscarinic cholinergic mechanism. The dose-dependent increase in the field EPSP by muscarinic blockade is evidence for the existence of a cholinergic presynaptic inhibition on the Schaffer collateral terminals.

Extracellular amino acid levels and field potentials evoked by perforant pathway stimuli were studied in vivo by means of a dialysis device, perfusing the rat dentate gyrus with low chloride solutions. When balanced with acetate, these perfusions enhanced the granule cell population spike amplitude. A specific extracellular taurine enhancement occurred whenever Cl− was replaced by acetate solution, reaching an increase of 20-fold over the basal taurine levels when 125 mM Cl− was replaced, whereas other amino acids remained unchanged. A considerable degree of Cl− replacement with iodide was needed, however, to obtain significant increases of extracellular taurine. Perfusions with bromide instead of Cl− did not cause any change in levels of extracellular amino acids including taurine. Furosemide, an inhibitor of Cl− transport, greatly reduced the taurine increase evoked by the low extracellular concentration of permeant anions. This drug also inhibited the taurine release induced by perfusion with 9 mM K+. These findings indicate that the extracellular increase of taurine, evoked by low permeant anion concentrations, may result from the taurine release through a furosemide-sensitive process.

The effects of -aminobutyric acid (GABA) on the extracellular levels of taurine and on excitability in the dentate gyrus were studied in anesthetized rats by the dialytrode technique. The dentate gyrus was perfused by means of a dialytrode with Krebs-Ringer-bicarbonate or GAB A solutions. Amino acid contents in perfusates and dentate field potentials evoked by electrical stimulation of the perforant pathway were evaluated. GAB A drastically elevated the levels of extracellular taurine in a dose-dependent manner, decreasing the amplitude of the population spike. This result indicates that GABA stimulates taurine release, probably by a counter-transport process. It is suggested that in physiological conditions an increase in extracellular taurine may be produced by synaptically released GABA.

[EN] The relationships between theta rhythm and epileptic spikes, evoked by penicillin, were studied in the rat hippocampus. Records were taped and processed off-line, and autocorrelation functions, averages and power spectra of the EEG and frequency histograms of the epileptic spikes were calculated. Results showed that: (1) epileptic spikes tend to occur in a preferred phase of theta rhythm; (2) they provoked a reset of the phase of theta rhythm, acting as internal stimuli; (3) epileptic spikes decreased in frequency and were often abolished when theta rhythm was evoked; (4) these effects appeared to be dependent on the medial septal pathway. These findings indicate the existence of an antagonism between two hippocampal phenomena (epilepsy and theta rhythm). On the other hand, they also seem to be interrelated since the generation of one is accompanied by a reciprocal decrease in the other., [FR] Les relations entre le rythme thêta et les pointes épileptiques induites par la pénicilline furent étudiées dans l'hippocampe du rat. Les enregistrements furent faits sur bandes magnétiques puis traités ensuite afin d'évaluer les finctions d'autocorrélation, des moyennes, le spectra de puissance de l'EEG, les histogrammes de fréquence des pointes épileptiques. Les résultats montrèrent que: (1) les pointes épileptiques tendaient à apparaître dans certaines phases préférentielles du rythme thêta; (2) elles provoquaient un recalage de la phase du rythme thêta en agissant comme des stimulus internes; (3) la fréquence des pointes épileptiques décroissait et souvent les pointes furent abolies lorsque le rythme thêta était évoqué; (4) ces effets seraient dependant de la voie septal-médiale. Ces données indiquent l'existence d'un antagonisme entre deux phénomènes hippocampiques (épilepsie et rythme thêta). Il semblerait toutefois qu'il y ait interrelation entre eux puisque l'apparition de l'un s'accompagne d'une diminution de l'autre.

The effect of inhibition of the high-affinity uptake of glutamate on the extracellular concentration of amino acids and on neuronal excitability was studied in vivo in the hippocampus of the rat The dentate gyrus or CAl field were perfused through a dialytrode with Krebs-Ringer-bicarbonate or dihydrokainic acid solutions. The spontaneous electrical activity and evoked field potentials were recorded concomitantly at dendritic or somatic levels. The results showed that with dihydrokainic acid: (1) the extracellular concentrations of both glutamate and taurine were markedly increased in both areas of the hippocampus, the response of taurine being greater in CA1, while other amino acids were unaffected; (2) in the dentate gyrus, the field excitatory postsynaptic potential was decreased while the population spikes were augmented, indicating an increased excitability of the neuronal population. In CA1, both the excitatory postsynaptic potential and spikes were reduced in amplitude. These results indicate that changes in the extracellular concentration of endogenous glutamate influences excitability of the tissue and that inhibition of the uptake processes for putative amino acid neurotransmitters increases the postsynaptic action of synaptically-released endogenous amino acids.

The effect of sensory stimulation, such as stroking of the animals' fur, on activation of the hippocampal CA1 field was studied in paralyzed and locally anaesthetized rats. CA1 population responses evoked either monosynaptically (ipsilateral CA3 stimulation) or trisynaptically (perforant pathway stimulation) decreased markedly when sensory stimulation was applied, and CA1 population spikes were absent during most periods of sensory stimulation. These results demonstrate the strong modulatory role of sensory inputs on hippocampal circuits.

The effects of hypotonic media on extracellular free amino acid levels were studied ‘in vivo’ in the rat dentate gyrus by means of the brain dialysis technique. Extracellular taurine levels increased specifically during perfusions with Krebs-Ringer bicarbonate in which the NaCl concentration was reduced by 25 or 50 mmol/l (hypotonic solutions). These taurine increases were markedly reduced in the presence of furosemide. With further NaCl reductions the enhanced taurine levels remained stable, whereas other amino acids such as glutamate started in increase in a dose-dependent manner. Isoosmolar replacement of NaCl by sucrose did not affect extracellular amino acid levels. These results indicate the possible involvement of taurine in osmoregulatory processes in the brain.

Methods to investigate the in vivo effects and release of neuroactive substances include cortical cups, push-pull cannulae, chemitrodes, and dialytrodes. Critical evaluation of these procedures is necessary in order to interpret related results and to select the most suitable devices for further studies. Recent improvements in the dialytrode include structural modifications and the use of a small, permeable membrane constructed of thin polyester. The dialytrode system and its diffusion rates have been characterized with in vitro studies. In vivo long-term experiments in awake cats have been conducted to test injection rates, diffusion of [14C]urea, temporal variability, pressure factors, and other experimental variables. Using dialytrodes we have measured the normal profile of amino acids present in different cerebral structures and their possible correlations.