Effects of folic and kainic acids on synaptic responses of hippocampal neurones

High dose folic acid supplementation of rats alters synaptic transmission and seizure susceptibility in offspring

Maternal folic acid supplementation is essential to reduce the risk of neural tube defects. We hypothesize that high levels of folic acid throughout gestation may produce neural networks more susceptible to seizure in offspring. We hence administered large doses of folic acid to rats before and during gestation and found their offspring had a 42% decrease in their seizure threshold. In vitro, acute application of folic acid or its metabolite 4Hfolate to neurons induced hyper-excitability and bursting. Cultured neuronal networks which develop in the presence of a low concentration (50 nM) of 4Hfolate had reduced capacity to stabilize their network dynamics after a burst of high-frequency activity, and an increase in the frequency of mEPSCs. Networks reared in the presence of the folic acid metabolite 5M4Hfolate developed a spontaneous, distinctive bursting pattern, and both metabolites produced an increase in synaptic density.

GLUK1 receptor antagonists and hippocampal mossy fiber function

Kainate receptors, one of the three subtypes of ionotropic receptors for the excitatory transmitter l-glutamate, play a variety of functions in the regulation of synaptic activity. Their physiological properties and functional roles have been identified only recently, following the discovery of selective pharmacological tools that allow for isolation of kainate receptor-mediated events. A considerable amount of data indicates that this class of glutamate receptors is located both at the pre- and postsynaptic site, playing a special role in regulating transmission and controlling short- and long-term plasticity. In this review, we summarize some data obtained in our laboratory over the last decade illustrating how various ligands have contributed to our understanding of the physiological role for neuronal kainate receptors. In particular, we show that the GluK1-containing KARs are important for regulating synaptic facilitation and LTP induction at hippocampal mossy fiber synapses.

The history of the pharmacology and cloning of ionotropic glutamate receptors and the development of idiosyncratic nomenclature

In this article, the beginnings of glutamate pharmacology are traced from the early doubts about 'non-specific' excitatory effects, through glutamate- and aspartate-preferring receptors, to NMDA, quisqualate/AMPA and kainate subtypes, and finally to the cloning of genes for these receptor subunits. The development of selective antagonists, crucial to the subtype classification, allowed the fundamental importance of glutamate receptors to synaptic activity throughout the CNS to be realised. The ability to be able to express and manipulate cloned receptor subunits is leading to huge advances in our understanding of these receptors. Similarly the tortuous path of the nomenclature is followed from naming with reference to exogenous agonists, through abortive early attempts at generic schemes, and back to the NC-IUPHAR system based on the natural agonist, the defining exogenous agonist and the gene names.

Characterisation of the effects of ATPA, a GLUK5 kainate receptor agonist, on GABAergic synaptic transmission in the CA1 region of rat hippocampal slices

Kainate receptors are implicated in a variety of physiological and pathological processes in the CNS. Previously we demonstrated that (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid (ATPA), a selective agonist for the GLU(K5) subtype of kainate receptor, depresses monosynaptically evoked inhibitory postsynaptic potentials (IPSPs) in the CA1 region of the rat hippocampus. In the current study, we provide a more detailed characterisation of this effect. Firstly, our data demonstrate a rank order of potency of domoate>kainate>ATPA>alpha-amino-3-(3-hydroxy-5-methyl-4-isoxalolyl)propionic acid Secondly, we confirm that the effects of ATPA are not mediated indirectly via the activation of gamma-aminobutyric acid receptors (i.e. either GABA(A) or GABA(B)). Thirdly, we show that the small increase in conductance induced by ATPA is insufficient to account for the depression of monosynaptic inhibition. Fourthly, we show that the effects of ATPA on IPSPs are antagonised by the GLU(K5)-selective antagonist (3S, 4aR, 6S, 8aR)-6-(4-carboxyphenyl)methyl-1,2,3,4,4a,5,6,7,8,8a-decahydroisoquinoline-3-carboxylic acid (LY382884). However, LY382884 is less potent as an antagonist of the effects of ATPA on IPSPs compared to its depressant effect on EPSPs.

Kainate receptors and synaptic transmission

Excitatory glutamatergic transmission involves a variety of different receptor types, each with distinct properties and functions. Physiological studies have identified both post- and presynaptic roles for kainate receptors, which are a subtype of the ionotropic glutamate receptors. Kainate receptors contribute to excitatory postsynaptic currents in many regions of the central nervous system including hippocampus, cortex, spinal cord and retina. In some cases, postsynaptic kainate receptors are co-distributed with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors, but there are also synapses where transmission is mediated exclusively by postsynaptic kainate receptors: for example, in the retina at connections made by cones onto off bipolar cells. Modulation of transmitter release by presynaptic kainate receptors can occur at both excitatory and inhibitory synapses. The depolarization of nerve terminals by current flow through ionotropic kainate receptors appears sufficient to account for most examples of presynaptic regulation; however, a number of studies have provided evidence for metabotropic effects on transmitter release that can be initiated by activation of kainate receptors. Recent analysis of knockout mice lacking one or more of the subunits that contribute to kainate receptors, as well as studies with subunit-selective agonists and antagonists, have revealed the important roles that kainate receptors play in short- and long-term synaptic plasticity. This review briefly addresses the properties of kainate receptors and considers in greater detail the physiological analysis of their contributions to synaptic transmission.

A role for Ca2+ stores in kainate receptor-dependent synaptic facilitation and LTP at mossy fiber synapses in the hippocampus

Compared with NMDA receptor-dependent LTP, much less is known about the mechanism of induction of NMDA receptor-independent LTP; the most extensively studied form of which is mossy fiber LTP in the hippocampus. In the present study we show that Ca2+-induced Ca2+ release from intracellular stores is involved in the induction of mossy fiber LTP. This release also contributes to the kainate receptor-dependent component of the pronounced synaptic facilitation that occurs during high-frequency stimulation. We also present evidence that the trigger for this Ca2+ release is Ca2+ permeation through kainate receptors. However, these novel synaptic mechanisms can be bypassed when the Ca2+ concentration is raised (from 2 to 4 mM), via a compensatory involvement of L-type Ca2+ channels. These findings suggest that presynaptic kainate receptors at mossy fiber synapses can initiate a cascade involving Ca2+ release from intracellular stores that is important in both short-term and long-term plasticity.

Kainate receptors and the induction of mossy fibre long-term potentiation

There is intense interest in understanding the molecular mechanisms involved in long-term potentiation (LTP) in the hippocampus. Significant progress in our understanding of LTP has followed from studies of glutamate receptors, of which there are four main subtypes (alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA), N-methyl-D-aspartate (NMDA), mGlu and kainate). This article summarizes the evidence that the kainate subtype of glutamate receptor is an important trigger for the induction of LTP at mossy fibre synapses in the CA3 region of the hippocampus. The pharmacology of the first selective kainate receptor antagonists, in particular the GLU(K5) subunit selective antagonist LY382884, is described. LY382884 selectively blocks the induction of mossy fibre LTP, in response to a variety of different high-frequency stimulation protocols. This antagonist also inhibits the pronounced synaptic facilitation of mossy fibre transmission that occurs during high-frequency stimulation. These effects are attributed to the presence of presynaptic GLU(K5)-subunit-containing kainate receptors at mossy fibre synapses. Differences in kainate receptor-dependent synaptic facilitation of AMPA and NMDA receptor-mediated synaptic transmission are described. These data are discussed in the context of earlier reports that glutamate receptors are not involved in mossy fibre LTP and more recent experiments using kainate receptor knockout mice, that argue for the involvement of GLU(K6) but not GLU(K5) kainate receptor subunits. We conclude that activation of presynaptic GLU(K5)-containing kainate receptors is an important trigger for the induction of mossy fibre LTP in the hippocampus.

The induction of N-methyl-D-aspartate receptor-dependent long-term potentiation

The role of N-methyl-D-aspartate (NMDA) receptors in the induction of long-term potentiation (LTP) was established during the 1980s. In this article I present a personal reflection upon the role that my colleagues and I played in the discovery of the mechanism of induction of NMDA receptor-dependent LTP.

Antagonists of GLU(K5)-containing kainate receptors prevent pilocarpine-induced limbic seizures

Developments in the molecular biology and pharmacology of GLU(K5), a subtype of the kainate class of ionotropic glutamate receptors, have enabled insights into the roles of this subunit in synaptic transmission and plasticity. However, little is known about the possible functions of GLU(K5)-containing kainate receptors in pathological conditions. We report here that, in hippocampal slices, selective antagonists of GLU(K5)-containing kainate receptors prevented development of epileptiform activity--evoked by the muscarinic agonist, pilocarpine--and inhibited the activity when it was pre-established. In conscious rats, these GLU(K5) antagonists prevented and interrupted limbic seizures induced by intra-hippocampal pilocarpine perfusion, and attenuated accompanying rises in extracellular L-glutamate and GABA. This anticonvulsant activity occurred without overt side effects. GLU(K5) antagonism also prevented epileptiform activity induced by electrical stimulation, both in vitro and in vivo. Therefore, we propose that subtype-selective GLU(K5) kainate receptor antagonists offer a potential new therapy for epilepsy.

Characterisation of the effects of ATPA, a GLU(K5) receptor selective agonist, on excitatory synaptic transmission in area CA1 of rat hippocampal slices

Kainate receptors are involved in a variety of synaptic functions in the CNS including the regulation of excitatory synaptic transmission. Previously we described the depressant action of the GLU(K5) selective agonist (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid (ATPA) on synaptic transmission in the Schaffer collateral-commissural pathway of rat hippocampal slices. In the present study we report several new features of the actions of ATPA at this synapse. Firstly, the effectiveness of ATPA is developmentally regulated. Secondly, the effects of ATPA decline during prolonged or repeated applications. Thirdly, the effects of ATPA are not mediated indirectly via activation of GABA(A), GABA(B), muscarinic or adenosine A(1) receptors. Fourthly, elevating extracellular Ca(2+) from 2 to 4 mM antagonises the effects of ATPA. Some differences between the actions of ATPA and kainate on synaptic transmission in the Schaffer collateral-commissural pathway are also noted.

Synaptic activation of a presynaptic kainate receptor facilitates AMPA receptor-mediated synaptic transmission at hippocampal mossy fibre synapses

The development of GluR5-selective kainate receptor ligands is helping to elucidate the functions of kainate receptors in the CNS. Here we have further characterised the actions of a GluR5 selective agonist, ATPA, and a GluR5 selective antagonist, LY382884, in the CA3 region of rat hippocampal slices. In addition, we have used LY382884 to study a novel synaptic mechanism. This antagonist substantially reduces frequency facilitation of mossy fibre synaptic transmission, monitored as either AMPA or NMDA receptor-mediated EPSCs. This suggests that GluR5-containing kainate receptors on mossy fibres function as autoreceptors to facilitate the synaptic release of L-glutamate, in a frequency-dependent manner.

A critical role of a facilitatory presynaptic kainate receptor in mossy fiber LTP

The mechanisms involved in mossy fiber LTP in the hippocampus are not well established. In the present study, we show that the kainate receptor antagonist LY382884 (10 microM) is selective for presynaptic kainate receptors in the CA3 region of the hippocampus. At a concentration at which it blocks mossy fiber LTP, LY382884 selectively blocks the synaptic activation of a presynaptic kainate receptor that facilitates AMPA receptor-mediated synaptic transmission. Following the induction of mossy fiber LTP, there is a complete loss of the presynaptic kainate receptor-mediated facilitation of synaptic transmission. These results identify a central role for the presynaptic kainate receptor in the induction of mossy fiber LTP. In addition, these results suggest that the pathway by which kainate receptors facilitate glutamate release is utilized for the expression of mossy fiber LTP.

Toxicity induced by a polyglutamated folate analog is attenuated by NAALADase inhibition

Folates have been shown to be neurotoxic and convulsive. Endogenously, folates exist in the brain in a polyglutamated form with 1-7 terminal glutamates (approx. 1 microM). The brain enzyme N-acetylated alpha-linked acidic dipeptidase (NAALADase) has been shown to remove sequentially the gamma-linked glutamates from folic acid polyglutamates. We report that, at high concentrations (300 microM-30 mM), a folic acid hexaglutamate analog is dose-dependently toxic to dissociated rat cortical cultures and that this toxicity is reversed by 2-PMPA, a potent and selective NAALADase inhibitor. These data suggest a new mechanism for folic acid toxicity.

The GluR5 subtype of kainate receptor regulates excitatory synaptic transmission in areas CA1 and CA3 of the rat hippocampus

Activation of kainate receptors depresses excitatory synaptic transmission in the hippocampus. In the present study, we have utilised a GluR5 selective agonist, ATPA [(RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid], and a GluR5 selective antagonist, LY294486 [(3SR,4aRS,6SR,8aRS)-6-([[(1H-tetrazol-5-y l)methyl]oxy]methyl)-1,2,3,4,4a,5,6,7,8,8a-decahydroisoquinoline-3 -carboxylic acid], to determine whether GluR5 subunits are involved in this effect. ATPA mimicked the presynaptic depressant effects of kainate in the CA1 region of the hippocampus. It depressed reversibly AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) receptor-mediated field excitatory postsynaptic potentials (field EPSPs) with an IC50 value of approximately 0.60 microM. The dual-component excitatory postsynaptic current (EPSC) and the pharmacologically isolated NMDA (N-methyl-D-aspartate) receptor-mediated EPSC were depressed to a similar extent by 2 microM ATPA (61 +/- 7% and 58 +/- 6%, respectively). Depressions were associated with an increase in the paired-pulse facilitation ratio suggesting a presynaptic locus of action. LY294486 (20 microM) blocked the effects of 2 microM ATPA on NMDA receptor-mediated EPSCs in a reversible manner. In area CA3, 1 microM ATPA depressed reversibly mossy fibre-evoked synaptic transmission (by 82 +/- 10%). The effects of ATPA were not accompanied by any changes in the passive properties of CA1 or CA3 neurones. However, in experiments where K+, rather than Cs+, containing electrodes were used, a small outward current was observed. These results show that GluR5 subunits comprise or contribute to a kainate receptor that regulates excitatory synaptic transmission in both the CA1 and CA3 regions of the hippocampus.

Mathematical model of the CA1 region of the rat hippocampus

A mathematical transcription of the intrinsic circuit of the CA1 region of the rat dorsal hippocampus was made and the model parameters adjusted according to experimental data from intracellular recordings and single channel kinetics. This model was able to simulate well the profile of the field potentials recorded extracellularly and the well known phenomenon of the paired-pulse depression. The results suggest that the depression of the second pulse, often interpreted in the literature as resulting from GABA(A) inhibition, can also be due to 'shunting' effects on the CA1 pyramids' membrane. The rhythmic oscillations of the field potential (EEG) was obtained as an emergent property of the network dynamics. The frequency of the field oscillation followed the main synaptic input in the region (Schaffer collaterals).

A hippocampal GluR5 kainate receptor regulating inhibitory synaptic transmission

The principal excitatory neurotransmitter in the vertebrate central nervous system, L-glutamate, acts on three classes of ionotripic glutamate receptors, named after the agonists AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxalole-4-propionic acid), NMDA (N-methyl-D-aspartate) and kainate. The development of selective pharmacological agents has led to a detailed understanding of the physiological and pathological roles of AMPA and NMDA receptors. In contrast, the lack of selective kainate receptor ligands has greatly hindered progress in understanding the roles of kainate receptors. Here we describe the effects of a potent and selective agonist, ATPA ((RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl)propanoic acid) and a selective antagonist, LY294486 ((3SR, 4aRS, 6SR, 8aRS)-6-((((1H-tetrazol-5-yl) methyl)oxy)methyl)-1, 2, 3, 4, 4a, 5, 6, 7, 8, 8a-decahydroisoquinoline-3-carboxylic acid), of the GluR5 subtype of kainate receptor. We have used these agents to show that kainate receptors, comprised of or containing GluR5 subunits, regulate synaptic inhibition in the hippocampus, an action that could contribute to the epileptogenic effects of kainate.

The basomedial and basolateral amygdaloid nuclei contribute to the induction of long-term potentiation in the dentate gyrus in vivo

Recent behavioural studies have provided evidence that the amygdala modulates hippocampal-dependent memory. To test the possibility that the amygdala modulates hippocampal synaptic plasticity, we investigated the effects of surgical lesions of the amygdaloid nuclei on the induction of long-term potentiation (LTP) in the dentate gyrus of anaesthetized rats. Previously we reported that LTP in the dentate gyrus was attenuated by lesion of the basolateral amygdala, but was not affected by lesion of the central amygdala. In the present study, dentate gyrus LTP was significantly attenuated by basomedial amygdala lesion but not by medial amygdala lesion. These results suggest that, among the amygdaloid nuclei, the basomedial and basolateral nuclei are involved in the modulation of hippocampal plasticity. The roles of the basomedial and basolateral amygdala were further supported by experiments examining the effects of electrical stimulation of these nuclei. High-frequency stimulation of the basomedial amygdala alone did not induce dentate gyrus LTP, but when applied at the same time as tetanic stimulation of the perforant path increased the magnitude of the dentate gyrus LTP. Similarly, high-frequency stimulation of the basolateral amygdala enhanced LTP induced by tetanic stimulation of the perforant path. Furthermore, facilitation of dentate gyrus LTP by basomedial or basolateral amygdala stimulation was observed even in rats lesioned in either amygdala, suggesting that neurons in the basomedial and basolateral amygdala can modulate dentate gyrus LTP independently. Activity-dependent facilitation of hippocampal plasticity by the basomedial and basolateral amygdala may underlie memory processing associated with emotion.

Attenuated hippocampal long-term potentiation in basolateral amygdala-lesioned rats

Possible involvement of the amygdaloid input in long-term potentiation (LTP) in the medial perforant path-dentate gyrus granule cell synapses in vivo was investigated by evaluating the effects of lesions of the amygdaloid nucleus. The dentate gyrus synaptic potential evoked by low-frequency test stimulation did not change following lesions of the basolateral and central amygdala. However, when tetanic stimulation (30 pulses at 60 Hz) was applied 60 min after lesioning of the ipsilateral basolateral amygdala, the magnitude of LTP was significantly attenuated. Since lesions of the ipsilateral central amygdala and the contralateral basolateral amygdala did not affect the dentate gyrus LTP, the attenuation of the dentate gyrus LTP is a specific effect of acute lesions of the ipsilateral basolateral amygdala. The basolateral amygdaloid lesions significantly attenuated both LTP induced by weak (20 pulses at 60 Hz) and strong (100 pulses at 100 Hz) tetanus, indicating that the effect of the lesions does not depend on the strength of tetanus applied to induce LTP. When the ipsilateral basolateral amygdala was destroyed after application of tetanus, it did not affect the established LTP. The attenuation of LTP was also observed after the basolateral amygdala-lesioned rats were allowed a recovery period of 2 weeks. This is the first report providing evidence that the ipsilateral basolateral amygdala modulates hippocampal synaptic plasticity in vivo.

GABAergic neurons are spared after intrahippocampal kainate in the rat

The present study used Nissl stains and glutamate decarboxylase immunoreactivity (GAD-IR) to quantify the acute and chronic toxicity of kainic acid (KA) on focal and remote hippocampal principal neurons (i.e., pyramidal and granule cells) and on putative inhibitory neurons (GAD-IR or GABAergic) following intrahippocampal KA administration. Concentrations of 0.5, 1.0, 1.25 or 1.5 micrograms KA/0.2 microliters were injected unilaterally into the posterior hippocampus of rats (n = 32), with survival periods of 1, 3, 5, 14, 21, 30 and 60 days. The age-matched control animals (n = 10) received an intrahippocampal injection of 0.2 microliter saline (sham control, n = 4) or no injection (normal, n = 6). The ipsilateral (KA+) cell counts demonstrated a selective vulnerability of CA3 and CA4 pyramidal neurons which was maximal at 14 days and unchanged to 60 days. However, in the same region, putative inhibitory (GAD-IR) neurons were resistant to the neurotoxic effects of KA. Contralateral (KA-) pyramidal cell and GAD-IR neuron densities were equivalent to controls. The present data demonstrate a selective resistance to KA by GABA neurons compared to the vulnerability of pyramidal neurons. Because GABA neurons are relatively spared in the KA focus, loss of GABAergic inhibitory neurons is probably not a mechanism for the seizure sensitivity in the KA model.

Alterations in hippocampal extracellular amino acids and purine catabolites during limbic seizures induced by folate injections into the rabbit amygdala

The effects on hippocampal extra- and intracellular amino acids of focal injection of folic acid into the amygdala in the rabbit were studied with brain dialysis. Folate seizures were accompanied by pronounced elevations of extracellular alanine and phosphoethanolamine. The increase of extracellular alanine was related to an enhanced level of this amino acid in total hippocampal tissue, whereas phosphoethanolamine was unaltered in tissue biopsies. Folate seizures did not significantly affect extracellular aspartate and extracellular glutamate was only slightly elevated (50-75% over baseline values). The tissue concentration of glutamate remained at control levels during the seizures and tissue aspartate was decreased by 28%. Extracellular glutamine decreased rapidly after folate injection with a concomitant increase of total hippocampal glutamine. Neither extra- nor intracellular taurine was affected by folate epilepsy. The experiments also encompassed measurements of the extracellular purine catabolites inosine, hypoxanthine and xanthine. Folic acid-induced epilepsy produced changes in these compounds indicative of moderately accelerated degradation of adenosine 5'-triphosphate. The findings support the view of glutamate as a mediator of epilepsy-related brain damage. It is, however, questionable if the small enhancement of extracellular glutamate is enough to provoke neuronal necrosis associated with folate epilepsy.

Characterization of an N-methyl-D-aspartate receptor component of synaptic transmission in rat hippocampal slices

The involvement of N-methyl-D-aspartate receptors in synaptic transmission from Schaffer collateral-commissural fibres to CA1 neurons has been investigated in rat hippocampal slices. When the perfusion medium was changed from one containing 1 mM Mg2+ to one with no added Mg2+ there was a pronounced increase in the amplitude of the population spike, the appearance of secondary population spikes and in some slices spontaneous epileptiform discharges developed. The secondary and spontaneous population spikes were abolished by the selective N-methyl-D-aspartate antagonist, D-2-amino-5-phosphonovalerate. The effects on the primary population spike depended on the strength of synaptic activation. At low intensities, the N-methyl-D-aspartate antagonist reduced or abolished this response whereas at high intensities the primary population spike was slightly increased in amplitude by this compound. Mg2+ had dose-dependent (20-500 microM) effects on synaptic responses which were identical to those of D-2-amino-5-phosphonovalerate. Increasing the Ca2+ concentration over a range of 1-3 mM also reduced or abolished secondary population spikes and, at low stimulus intensities, the primary population spike. At higher stimulus intensities, however, the primary population spike was insensitive to the Ca2+ concentration over this range. These results demonstrate the major extent to which N-methyl-D-aspartate receptors can contribute to synaptic transmission and epileptiform activity in the CA1 region of the hippocampus. They also show that an important role of Mg2+ in this region is to prevent significant activation of this receptor system during low-frequency synaptic transmission.

Folic acid protects chick retinal neurons against the neurotoxic action of excitatory amino acids

In this study we have examined the neurotoxic effects of folic acid (FA), alone or in combination with selected excitotoxins using in vitro preparations of chick retina. Folic acid alone at concentrations of up to 10 mM had no effect. Co-incubation of 10 mM FA with 2 microM kainic acid (KA) protected all cell types susceptible to KA toxicity, namely amacrine and bipolar cells. At lower concentrations the protective effect of FA to susceptible cell types was found to be dose-dependent. The rank order of cells which are protected by FA, in order of the lowest concentration of FA required, was amacrine, inner bipolar and outer bipolar cells. The effect of FA against KA is a very weak one, as a 130-400-fold concentration of FA is required to protect amacrine cells from KA and a 1000-5000-fold dose of FA required to protect bipolar cells. However, FA (2 mM) also protects susceptible retinal neurons from the neurotoxic effects of 40 microM N-methyl-DL-aspartic acid (NMDLA) and 60 microM quinolinic acid (QUIN); only requiring respectively 50 and 33 times the concentration of FA. Interestingly 10 mM FA had little effect against 40 microM quisqualic acid (QUIS). Thus FA antagonizes the effects of KA, NMDLA, QUIN and to a small extent QUIS. Although its action may be mediated through several receptor types, FA appears to be a more potent antagonist of the N-methyl-D-aspartic acid (NMDA)-preferring than the KA-preferring or QUIS-preferring receptor.

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.

Loss of inhibition in the CA1 region of the kainic acid lesioned hippocampus is not associated with changes in postsynaptic responses to GABA

Somatic and dendritic responses to gamma-aminobutyric acid (GABA) were recorded intracellularly from CA1 pyramidal cells in slices of the hippocampus ipsilateral and contralateral to a unilateral kainic acid lesion of the CA3 region. Ipsilateral CA1 cells show a loss of GABA-mediated synaptic inhibition. However, somatic GABA responses and the sensitivity of cells to GABA were very similar in ipsilateral and contralateral cells. This was also true for dendritic applications of GABA.

Pre-and postsynaptic actions of baclofen: blockade of the late synaptically-evoked hyperpolarization of CA1 hippocampal neurones

Using intracellular recording techniques, the effects of beta-p-chlorophenyl-GABA (baclofen) on passive membrane properties and postsynaptic potentials of CA1 pyramidal neurones were investigated. In experiments where only the hyperpolarizing action of baclofen was precluded by conventional current clamp techniques, 20 microM ( +/- ) baclofen blocked the early GABA-mediated IPSP and also a late hyperpolarization which, since it could be evoked by orthodromic stimulation subthreshold for spike firing, would not be expected to be produced by a Ca2+-activated increase in potassium conductance (AHP), but to be a transmitter-mediated event. In addition the conductance increase associated with this late IPSP evoked by subthreshold stimulation and also that associated with the AHP produced by spike activation were abolished. Baclofen also appeared to increase the duration of EPSPs, an event possibly related to loss of IPSPs. The hyperpolarization produced by baclofen was associated with an increased conductance of the resting membrane, an event possibly associated with an elevated potassium flux. To preclude this postsynaptic effect as a cause of reduced synaptic responses, tetraethylammonium chloride (TEA), a compound which decreases conductance and depolarizes the membrane of CA1 pyramidal neurones by a reduction of a 'leak' or resting potassium conductance (gK), was added to the bathing medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Folic acid has a disinhibitory action in the rat hippocampal slice preparation

The effects of folic acid on synaptic transmission in the hippocampal slice have been studied. Application of folic acid (0.1-1 mM) increased the size of population spikes recorded extracellularly in the CAl pyramidal cell layer and caused the appearance of multiple population spikes. Intracellular recording revealed that folic acid had no consistent effect on the membrane potential, but greatly reduced the rapid chloride-mediated phase of the inhibitory postsynaptic potential (IPSP) evoked by ortho- and antidromic stimulation. The slower, potassium-mediated phase of the IPSP was usually enhanced. Furthermore, folic acid abolished spontaneous IPSPs recorded with potassium chloride-filled microelectrodes. All of these effects were quickly reversible when the drug was washed from the chamber. Finally bath-applied folic acid reduced the hyperpolarization produced by iontophoretically applied GABA. Based on these results, we conclude that folic acid produces its excitatory effects on hippocampal pyramidal cells by a disinhibitory action which involves a postsynaptic blockade of GABA responses.

Electrophysiological evidence that nipecotic acid can be used in vivo as a false transmitter

Dentate gyrus of the rat hippocampal formation was perfused with the potent inhibitor of GABA uptake, nipecotic acid, by means of an implanted dialytrode. Evoked population spikes in dentate gyrus were decreased in amplitude and often abolished during perfusion. However, multiple (2-4) population spikes developed shortly after nipecotic acid withdrawal. This excitability increase, which presented a pattern of repetitive discharge resembling that following blocking of GABAergic transmission was interpreted as electrophysiological evidence that nipecotic acid can act as a false transmitter 'in vivo', as previously postulated from uptake and release 'in vitro' studies.

Injections of picrotoxin and bicuculline into the amygdaloid complex of the rat: an electroencephalographic, behavioural and morphological analysis

Bicuculline methiodide (0.5-3 nmol) and picrotoxin (0.5-4 nmol) were injected uni- or bilaterally into the rat amygdala and the resulting behavioural, electroencephalographic and morphological alterations were studied. In rats treated unilaterally with lowest doses of either bicuculline or picrotoxin (0.5 and 1 nmol) increase in the locomotor activity, occasional myoclonus of the hindlimbs and wet dog shakes were observed. At doses of 2-3 nmol, both gamma-aminobutyrate antagonists produced a sequence of repetitively occurring behavioural alterations including limbic gustatory automatisms, tremor and myoclonus of the forelimbs, head nodding and rearing, that developed over 15-30 min and built up progressively into the recurrent motor limbic seizures lasting for 1-6 h. In animals injected bilaterally with either bicuculline (0.5-3 nmol) or picrotoxin (0.5-3 nmol) motor limbic seizures rapidly developed into the status epilepticus lasting for several hours. Bicuculline and picrotoxin produced both ictal and interictal epileptiform activity in the electroencephalogram. A spectrum of electroencephalographic changes consisted of high voltage fast activity, slow and fast voltage spiking, paraoxysmal bursts and periods of postictal depression. The earliest electrographic alterations appeared in the amygdala and then rapidly spread to cortical areas. Electrographic seizures started 1-10 min after unilateral injections of large doses of bicuculline and pictrotoxin (2-4 nmol). Ictal periods lasted for 1-2 min, recurred every 5-10 min and were followed by periods of depression of the electrographic activity. Bilateral injections of large doses of both gamma-aminobutyrate antagonists (2-3 nmol) resulted in the status epilepticus. Morphological examination of frontal forebrain sections with light microscopy revealed a widespread damage to the amygdala, olfactory cortex, substantia nigra, thalamus, hippocampus and neocortex. Pretreatment of animals with diazepam prevented the build-up of convulsive activity and brain damage produced by bicuculline or picrotoxin. Muscimol retarded the appearance and shortened the duration of convulsive activity, but did not alter the sequence and intensity of seizures. The results indicate that gamma-aminobutyrate antagonists, bicuculline and picrotoxin when directly applied to the amygdala can elicit in rats motor limbic seizures, epileptic changes in the electroencephalogram indicative of repetitive limbic seizures, and status epilepticus accompanied by seizure-related brain damage.(ABSTRACT TRUNCATED AT 400 WORDS)