Methylmercury reduces synaptic transmission and neuronal excitability in rat hippocampal slices.

In a previous study, we pointed out that the neurotoxic action evoked by methylmercury (MeHg), a potent environmental pollutant responsible for fatal food poisoning, is associated with alterations of cellular excitability by irreversible blockade of sodium and calcium currents. Here, we investigated the MeHg effects on synaptic transmission and neuronal plasticity using extracellular field recording in CA1 area of rat hippocampal slices. MeHg caused a fast and drastic depression of evoked field excitatory postsynaptic potentials (fEPSPs) in a concentration-dependent manner with an IC50 of 25.7 µM. This depression was partially caused by the irreversible reduction of axon recruitment deduced from the decrement of the fiber volley (FV) amplitude. Nevertheless, this MeHg-induced synaptic depression represents a true reduction of synaptic efficacy, as judged by input/output curves. In addition, a reduction on presynaptic release of glutamate was detected with the paradigm of paired-pulse facilitation during MeHg application. Moreover, MeHg also reduced population spike (PS) ampxlitude, and this effect was more prominent when the PS was evoked by ortodromic stimulation than by antidromic stimulation. Interestingly, despite these strong effects of MeHg on synaptic transmission and excitability, this compound did not modify the induction of long-term synaptic potentiation (LTP). The effects described here for MeHg were irreversible or very slowly reversible after drug wash-out. In summary, the blockade of sodium and calcium channels by MeHg affects synaptic transmission and cellular excitability but not synaptic plasticity.

Choline induces opposite changes in pyramidal neuron excitability and synaptic transmission through a nicotinic receptor-independent process in hippocampal slices.

Choline is present at cholinergic synapses as a product of acetylcholine degradation. In addition, it is considered a selective agonist for α5 and α7 nicotinic acetylcholine receptors (nAChRs). In this study, we determined how choline affects action potentials and excitatory synaptic transmission using extracellular and intracellular recording techniques in CA1 area of hippocampal slices obtained from both mice and rats. Choline caused a reversible depression of evoked field excitatory postsynaptic potentials (fEPSPs) in a concentration-dependent manner that was not affected by α7 nAChR antagonists. Moreover, this choline-induced effect was not mimicked by either selective agonists or allosteric modulators of α7 nAChRs. Additionally, this choline-mediated effect was not prevented by either selective antagonists of GABA receptors or hemicholinium, a choline uptake inhibitor. The paired pulse facilitation paradigm, which detects whether a substance affects presynaptic release of glutamate, was not modified by choline. On the other hand, choline induced a robust increase of population spike evoked by orthodromic stimulation but did not modify that evoked by antidromic stimulation. We also found that choline impaired recurrent inhibition recorded in the pyramidal cell layer through a mechanism independent of α7 nAChR activation. These choline-mediated effects on fEPSP and population spike observed in rat slices were completely reproduced in slices obtained from α7 nAChR knockout mice, which reinforces our conclusion that choline modulates synaptic transmission and neuronal excitability by a mechanism independent of nicotinic receptor activation.

Dimorphic histopathology of long-standing childhood-onset diabetes.

AIMS/HYPOTHESIS: Childhood diabetes is thought to usually result from autoimmune beta cell destruction (type 1A) with eventual total loss of beta cells. Analysis of C-peptide in children characterised at diabetes onset for autoantibodies shows heterogeneous preservation of insulin secretion in long-standing diabetes. The aim of this study was to characterise the pancreases of childhood-onset diabetes in order to define the pathological basis of this heterogeneity. METHODS: We evaluated 20 cadaveric organ donor pancreases of childhood-onset long-term patients for disease heterogeneity and obtained corresponding C-peptide measurements. RESULTS: Pancreases from the majority of cadaveric donors contained only insulin-deficient islets (14 of 20). The remaining six patients (30%) had numerous insulin-positive cells within at least some islets, with two different histological patterns. Pattern A (which we would associate with type 1A diabetes) had lobular retention of areas with 'abnormal' beta cells producing the apoptosis inhibitor survivin and HLA class I. In pattern B, 100% of all islets contained normal-appearing but quantitatively reduced beta cells without survivin or HLA class I. CONCLUSIONS/INTERPRETATION: Our data demonstrate that C-peptide secretion in long-standing diabetic patients can be explained by two different patterns of beta cell survival,possibly reflecting different subsets of type 1 diabetes.

Modulation of calcium channels by taurine acting via a metabotropic-like glycine receptor.

Taurine is one of the most abundant free amino acids in the central nervous system, where it displays several functions. However, its molecular targets remain unknown. It is well known that taurine can activate GABA-A and strychnine-sensitive glycine receptors, which increases a chloride conductance. In this study, we describe that acute application of taurine induces a dose-dependent inhibition of voltage-dependent calcium channels in chromaffin cells from bovine adrenal medullae. This taurine effect was not explained by the activation of either GABA-A, GABA-B or strychnine-sensitive glycine receptors. Interestingly, glycine mimicked the modulatory action exerted by taurine on calcium channels, although the acute application of glycine did not elicit any ionic current in these cells. Additionally, the modulation of calcium channels exerted by both taurine and glycine was prevented by the intracellular dialysis of GDP-ß-S. Thus, the modulation of voltage-dependent calcium channels by taurine seems to be mediated by a metabotropic-like glycinergic receptor coupled to G-protein activation in a membrane delimited pathway.

Local and diffuse synaptic actions of GABA in the hippocampus.

In the CNS, gamma-aminobutyric acid (GABA) acts as an inhibitory transmitter via ligand-gated GABAA receptor channels and G protein-coupled GABAB receptors. Both of these receptor types mediate inhibitory postsynaptic transmission in the hippocampus. In addition to these direct postsynaptic actions, GABAB receptor agonists inhibit excitatory transmission through presynaptic receptors on excitatory afferent terminals. However, a physiological role for the GABAB receptors on excitatory nerve endings has not been established. In this study, we have found a brief, heterosynaptic depression of excitatory synaptic transmission in the CA1 region of the hippocampal slice following short-lasting repetitive stimulation and determined that this inhibition is mediated by presynaptic GABAB receptors. The inhibition of GABA uptake greatly enhanced both the presynaptic action of GABA and the slow GABAB-mediated inhibitory postsynaptic current. Transmitter uptake was also found to regulate the "spill-over" of GABA at conventional GABAA synapses. These results suggest that uptake mechanisms restrict the spatial range of both point-to-point synaptic transmission mediated by GABA and its action at a distance.

[Value of autopsy in a neonatal intensive care unit]. / Valor de la autopsia en una unidad de cuidados intensivos neonatológicos.

OBJECTIVES: 1) To identify the profile of the cases requested for autopsy; 2) to analyze the clinocopathological discordance; 3) to investigate predictive factors for unsuspected clinically relevant diagnoses. PATIENTS AND METHOD: All autopsies performed between January 1999 and December 2005 in a tertiary neonatal intensive care unit, were retrospectively reviewed. Clinicopathological concordance was assessed independently by two neonatologists and two pathologists, according to a modification of the Goldman classification. A comparison was made between newborns who had an autopsy performed and those who did not and predictive factors for unsuspected findings were investigated. RESULTS: During the study period, there were 309 deaths, and autopsies were performed in 128 (41.4 %) of these cases. Autopsies were more common in newborns who had gestational age > 36 weeks (p < 0.001), birthweight > 1500 g (p < 0.001) and congenital defects (p < 0.007). However, the probability that the autopsy was granted decreased with increasing death age (p < 0.016). Unsuspected diagnoses were observed in 49.2 % of the autopsies, being a major finding in 21.1 % of the cases. A clinicopathological discordance involving the prognosis was found in four cases (3.1 %). Relevant unsuspected findings could not be predicted from the ante-mortem clinical diagnosis, gestational age, birthweight, sex, and death age. CONCLUSION: The autopsy remains the "gold standard" method to reveal major and unsuspected diagnoses and there is no substitute for it. Postmortem examination should be requested systematically in every neonatal death. However, several factors such as gestational age, birthweight, presence of congenital defects and death age, influence the likelihood of autopsy being granted.

Hippocampal synaptic plasticity and water maze learning in cocaine self-administered rats.

Previously, we have shown that long-term potentiation (LTP) in hippocampus of Lewis rats was significantly modulated by cocaine self-administration. Using a single train of high-frequency stimulation of 100 Hz for 1s (HFS), we found an enhancement of LTP after cocaine self-administration that was maintained even during the extinction of this behavior. However, the effects of cocaine self-administration on a hippocampal-dependent spatial learning task were unknown. Therefore, in the present study our first objective was to analyze if cocaine self-administration might affect the performance in a hippocampus-dependent task, such as the Morris water maze test. Male adult Lewis (LEW) rats self-administered cocaine (1 mg/kg/injection) or saline (0.9% NaCl) for 3 weeks. Three hours after finishing the last self-administration session, animals were submitted to Morris water maze training for 3 consecutives days. A memory test was carried out 24 h after the last training session. No significant differences were found in escape latencies and time spent in the quadrant where the platform was located during training. Given that we did not find any cocaine effect on this spatial learning task, our second objective was to estimate indirectly if brain cocaine levels have failed to modulate LTP in animals that were performing the water maze trials. To this end, we tested if cocaine application to hippocampal slices of naïve subjects was able to evoke LTP. The results indicated that cocaine produced an enhanced LTP in these hippocampal slices. Taking together, the results of the present study suggest that hippocampal LTP-like processes generated after cocaine self-administration are not related to spatial learning hippocampal-dependent tasks, such as the water maze test.

Taurine-induced synaptic potentiation and the late phase of long-term potentiation are related mechanistically.

The application of taurine (2-aminoethanesulfonic acid) induces a long-lasting increase of synaptic efficacy and axon excitability (LLP-TAU) in rat hippocampal CA1 area. After taurine withdrawal, LLP-TAU lasted at least 3 h. This fact prompted us to assess whether the mechanisms involved in the maintenance of this particular potentiation were similar to those implicated in the late phase of long-term potentiation (L-LTP). In the presence of KN-62, an inhibitor of calcium/calmodulin-dependent protein kinase, taurine perfusion (10 mM, 30 min) did not affect the induction of LLP-TAU. However, LLP-TAU maintenance was completely suppressed by KT5720, an inhibitor of the cAMP-dependent protein kinase (PKA). Moreover, the late phase of LLP-TAU was blocked by inhibiting protein synthesis with anisomycin. In addition, taurine perfusion increased the phosphorylation of cAMP response element-binding protein (CREB), although did not affect cAMP levels. These features of LLP-TAU do not appear to be caused by the activation of D1/D5 dopamine receptors, as taurine also induced synaptic potentiation in the presence of SCH23390, an antagonist of this type of receptors. Finally, the late phase of both L-LTP and LLP-TAU occluded mutually. These results suggest that taurine triggers the sequence of some of the molecular events involved in the induction of L-LTP.

Taurine-induced synaptic potentiation: role of calcium and interaction with LTP.

Taurine induces a long-lasting potentiation of excitatory synaptic potentials due to the enhancement of both synaptic efficacy and axon excitability in the CA1 area of rat hippocampal slices. In this study, we characterized the role of Ca2+ in the generation of these long-lasting taurine effects. Taurine perfusion in a free-Ca2+ medium did not induce changes in either field excitatory synaptic potentials (fEPSP) slope or fiber volley (FV) amplitude. Intracellular recordings with a micropipette filled with the Ca2+ chelator BAPTA, prevented the EPSP potentiation induced by taurine in the impaled cell, whereas a long-lasting potentiation of the simultaneously recorded fEPSP was obtained. The depletion of intracellular Ca2+ stores by thapsigargin (1 microM), an inhibitor of endosomal Ca2+-ATPase, transformed the taurine-induced potentiation into a transitory process that declined to basal values after taurine withdrawal. Taurine-induced potentiation was not significantly affected by kynurenate (glutamate receptor antagonist), or nifedipine (high-voltage-activated Ca2+ channel antagonist). But, the presence of nickel (50 microM), an antagonist of low-voltage-activated Ca2+ channel, inhibited the taurine-induced potentiation, indicating that Ca2+ influx through this type of Ca2+ channels could account for the Ca2+ requirement of the taurine-induced potentiation. Occlusion experiments between tetanus-induced long-term potentiation (LTP) and taurine-induced potentiation indicate that both processes share some common mechanisms during the maintenance period.

Effects of dihydrokainic acid on extracellular amino acids and neuronal excitability in the in vivo rat hippocampus.

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 CA1 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: 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; 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.

Low chloride-dependent release of taurine by a furosemide-sensitive process in the in vivo rat hippocampus.

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.

Characteristics of CA1 activation through the hippocampal trisynaptic pathway in the unanaesthetized rat.

The hippocampal CA1 field is activated by the entorhinal cortex mainly through the hippocampal excitatory trisynaptic circuit. Field responses of the CA1 region were evoked by ipsilateral CA3 or perforant path volley (mono- or trisynaptic activation, respectively) in paralyzed, locally anaesthetized rats and studied as a function of the stimulus patterns presented. The relationship of these responses with the concomitant EEG was also explored. Results showed that mono- and especially trisynaptically evoked responses were progressively enhanced by increasing the stimulus frequency from 0.1 to 1.0 Hz. At specific intensities the trisynaptically evoked population spike (PS) was present only with a rather fixed frequency of stimulation (approximately 0.5 Hz). PS was produced in 100% of the responses using 0.7 Hz, indicating the existence of a threshold-like level for this stimulus parameter. The frequency of presented paired pulses differentially affected pair-pulse facilitation of mono- and trisynaptically evoked excitatory postsynaptic potentials (EPSP): higher frequency decreased the former and increased the latter. All evoked responses studied (i.e. EPSP and PS) showed steep increments and decrements in amplitude, clearly developing several clusters. Moreover, the amplitude distribution of trisynaptic PS often varied spontaneously from maximal to negligible values, showing an all-or-none distribution. Clustering was interpreted as evidence of the existence in the hippocampus of functional neuronal aggregates. All-or-none distribution of trisynaptic PS was found to be associated with the EEG pattern, PS amplitude being maximal during irregular EEG activity and minimal during theta rhythm. Present results suggest that (1) the entorhinal cortex may exert modulatory actions on CA1 by a mechanism widely based on the frequency of the input; (2) information transfer from the entorhinal cortex to other brain areas throughout the hippocampus is biased by hippocampal EEG; and (3) electronic coupling may be functionally predominant in the hippocampus.

Sensory modulation of hippocampal transmission. II. Evidence for a cholinergic locus of inhibition in the Schaffer-CA1 synapse.

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.

Sensory modulation of hippocampal transmission. I. Opposite effects on CA1 and dentate gyrus synapsis.

Neuronal transmission through hippocampal subfields exhibits a high degree of modulation and appears dependent on the behavioral state and hippocampal EEG. Sensory inputs, which profoundly modify the hippocampal EEG, may be involved in modulating hippocampal excitability. Field responses of the CA1 region, evoked by ipsilateral CA3 or perforant path stimulation, as well as dentate gyrus potentials evoked by perforant path stimulation were recorded in paralyzed and locally anesthetized rats and studied before, during and after sensory stimulation, consisting of gentle stroking of the animal's fur. On some occasions the CA1 was also antidromically driven from the posterior alveus in order to study the recurrent inhibitory loop and paired pulses were applied to the perforant pathway to study recurrent inhibition in the dentate gyrus. Evoked responses were averaged and field excitatory postsynaptic potential (EPSP) slope and population spike (PS) amplitude measured. In addition the positive wave which follows the population spike, which corresponds in part to the recurrent IPSP, was also evaluated. Sensory stimulation, which evoked a high-amplitude 5-6 Hz theta (theta)-rhythm in the hippocampal EEG, drastically depressed the efficacy of Schaffer collateral volleys in discharging the CA1 cells. The EPSP-PS curves, however, were not altered revealing that cellular excitability was unaffected. The inhibitory CA1 loop appeared to be unaltered. In contrast, the dentate gyrus responses to perforant pathway stimulation were enhanced during periods of sensory stimulation and the cellular excitability increased, as judged by the shift to the left of EPSP-PS relation. In addition, the recurrent inhibition appeared to be reduced during sensory stimulation. Present results demonstrate that sensory stimulation causes modulation of information transfer through the hippocampus. This modification of hippocampal transmission may serve to properly gate the information reaching the CA1 and dentate gyrus.

Taurine activates GABA(A) but not GABA(B) receptors in rat hippocampal CA1 area.

We investigated if taurine, an endogenous GABA analog, could mimic both hyperpolarizing and depolarizing GABA(A)-mediated responses as well as pre- and postsynaptic GABA(B)-mediated actions in the CA1 region of rat hippocampal slices. Taurine (10 mM) perfusion induced changes in membrane potential and input resistance that are compatible with GABA(A) receptor activation. Local pressure application of taurine and GABA from a double barrel pipette positioned along the dendritic shaft of pyramidal cells revealed that taurine evoked a very small change of membrane potential and resistance compared with the large changes induced by GABA in these parameters. Moreover, in the presence of GABA(A) antagonists, local application of GABA on the dendrites evoked a GABA(B)-mediated hyperpolarization while taurine did not induce any change. Taurine neither mimicked baclofen inhibitory actions on presynaptic release of glutamate and GABA as judging by the lack of taurine effect on paired-pulse facilitation ratio and slow inhibitory postsynaptic potentials, respectively. These results show that taurine mainly activates GABA(A) receptors located on the cell body, indicating therefore that if taurine has any action on the dendrites it will not be mediated by either GABA(A) or GABA(B) receptors activation.

Postsynaptic action of endogenous GABA released by nipecotic acid in the hippocampus.

Intracellular and whole-cell recording from CA1 pyramidal cells and dentate granule cells was used to study the release of endogenous GABA by nipecotic acid. Local application of nipecotic acid produced responses that could be entirely blocked by a combination of the GABAA receptor antagonist picrotoxin and the GABAB receptor antagonist CGP 35348. These responses were due to the heteroexchange release of endogenous GABA because they were blocked by low Na+ which blocks the GABA transporter and by SKF 89976 which is a competitive antagonist of the GABA transporter. Local application of nipecotic acid could, depending on the location, evoke pure GABAA or pure GABAB responses supporting proposals that GABAA and GABAB receptors can be segregated at separate inhibitory synapses.

Does taurine act as an osmoregulatory substance in the rat brain?

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.

Extracellular taurine increase in rat hippocampus evoked by specific glutamate receptor activation is related to the excitatory potency of glutamate agonists.

Taurine increases in brain extracellular space due to glutamate agonists were studied in vivo in the rat hippocampus using a dialysis technique, both in the absence and in the presence of glutamate receptor antagonists. Extracellular taurine levels increased during perfusions of agonists, listed in descending order of potency: kainate (KA), N-methyl-D-aspartate (NMDA), and quisqualate (QA). While taurine increases due to KA or QA perfusions were inhibited by 6,7-dinitro-quinoxaline-2,3-dione (DNQX), those induced by NMDA were abolished in the presence of 3-(carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP). These results indicate that increases in extracellular taurine levels evoked by NMDA, KA or QA in the rat hippocampus are caused by activation of their specific receptors. Field potentials, concomitantly recorded, were quickly abolished during NMDA or KA perfusions (0.1 mM), while QA (0.25 mM) induced the appearance of bicuculline-like evoked responses. Since taurine has been proposed as an osmoregulatory substance in the rat brain, and cell swelling is known to be an early component of glutamate agonists neurotoxicity, the increases in extracellular taurine reported here could be due to taurine released through an osmoregulatory process, counteracting the neurotoxic cellular oedema induced by glutamate agonists.

Synaptic transmission at the Schaffer-CA1 synapse is blocked by 6,7-dinitro-quinoxaline-2,3-dione. An in vivo brain dialysis study in the rat.

6,7-Dinitro-quinoxaline-2,3-dione (DNQX, FG 9041), a new non-N-methyl-D-aspartate (NMDA) glutamate receptor antagonist, has been reported to block non-NMDA receptor-mediated excitatory amino acidic responses in cultured neurons. We have perfused this compound in vivo through a dialysis fiber placed in the CA1 regions of anesthetized rats to test its effects on CA1 field-evoked potentials. Perfusions of 25-100 microM DNQX completely abolished field excitatory postsynaptic potentials (EPSP) and orthodromic population spikes (PS). This effect was dose-dependent and was reversed after washing with fresh Krebs-Ringer-bicarbonate. Antidromic population spikes and fiber volley potentials were unaffected by perfusions of DNQX up to 100 microM. On the contrary, perfusion of 50 microM D-2-amino-5-phosphonovalerate, a specific NMDA receptor antagonist, left unchanged both field EPSP and orthodromic PS. Results demonstrate that low-frequency transmission at the Schaffer collaterals-CA1 synapse is mediated by non-NMDA glutamate receptors.

Nicotinamide adenine dinucleotides mimic adenosine inhibition on synaptic transmission by decreasing glutamate release in rat hippocampal slices.

To assess the possible inhibitory action of nicotinamide adenine dinucleotides on the synaptic release of glutamate, electrophysiological and biochemical experiments were performed on rat hippocampal slices. Perfusion of adenosine, beta-nicotinamide adenine dinucleotide (NAD) or beta-nicotinamide adenine dinucleotide phosphate (NADP), reversibly inhibited the field excitatory postsynaptic potentials (fEPSP). Dose-response curves for their inhibitory action showed that these three substances had a similar potency in the range of concentrations from 0.1 microM to 100 microM. NADP and adenosine (100 microM) halved the K(+)-induced release of endogenous glutamate and aspartate, leaving gamma-amino-butyric acid (GABA) levels unchanged. 3-Isobutyl-1-methylxanthine (IBMX) 200 microM, an antagonist of the P1-purinoreceptors, antagonized the depressant effects of these coenzymes on both fEPSP and also on amino acid release. Based on these results we propose that nicotinamide adenine dinucleotides, similar to adenosine, inhibit excitatory synaptic transmission in the rat hippocampus by decreasing glutamate release from synaptic terminals.