Fast and slow depolarizing potentials induced by short pulses of kainic acid in hippocampal neurons

The multifarious hippocampal mossy fiber pathway: a review

The hippocampal mossy fiber pathway between the granule cells of the dentate gyrus and the pyramidal cells of area CA3 has been the target of numerous scientific studies. Initially, attention was focused on the mossy fiber to CA3 pyramidal cell synapse because it was suggested to be a model synapse for studying the basic properties of synaptic transmission in the CNS. However, the accumulated body of research suggests that the mossy fiber synapse is rather unique in that it has many distinct features not usually observed in cortical synapses. In this review, we have attempted to summarize the many unique features of this hippocampal pathway. We also have attempted to reconcile some discrepancies that exist in the literature concerning the pharmacology, physiology and plasticity of this pathway. In addition we also point out some of the experimental challenges that make electrophysiological study of this pathway so difficult.Finally, we suggest that understanding the functional role of the hippocampal mossy fiber pathway may lie in an appreciation of its variety of unique properties that make it a strong yet broadly modulated synaptic input to postsynaptic targets in the hilus of the dentate gyrus and area CA3 of the hippocampal formation.

Distribution and properties of kainate receptors distinct in the CA3 region of the hippocampus of the guinea pig

To characterize the nature of kainate (KA) receptors distinct in the CA3 region of the hippocampus, properties of depolarizations induced by pulses of KA or AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate) applied to dendrites of CA3 neurons with micropipettes were studied in thin transverse slices of the guinea pig hippocampus. KA induced depolarizations at negligible latencies only when administered to the most proximal dendritic areas. The depolarization was unaffected by tetrodotoxin or by a decrease in Ca2+ and an increase in Mg2+ concentrations. The declining slope of the KA-induced depolarization was significantly slower than that of the AMPA-induced depolarization. In comparison with the AMPA-induced depolarization, the KA-induced depolarization was much less susceptible to antagonists such as 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX) and 1-(4-aminophenyl)-4-methyl-7, 8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI52466). 6, 7,8,9-Tetrahydro-5-nitro-1H-benz[g]indole-2,3-dione-3-oxime (NS-102) and (2S,4R)-4-methylglutamate (SYM 2081) were without effects. The threshold concentration of pressure-ejected KA to induce depolarizations was about 200 nM. Excitatory postsynaptic potentials elicited by mossy fiber stimulation were more potently suppressed by CNQX than by GYKI52466. These results indicate that receptors responsible for the slow KA depolarization in the CA3 region of the hippocampus are not AMPA receptors but KA receptors. They are localized in the most proximal part of the apical dendrite and distinct from those observed in primary cultures of hippocampal neurons.

Sensitivity of hippocampal neurones to kainic acid, and antagonism by kynurenate

1. The sensitivity to kainic acid of neurones in the CA1 and CA3 regions of rat hippocampal slices has been examined by microiontophoresis and by superfusion methods. 2. When the iontophoretic currents needed to produce comparable plateaux of firing were compared, neurones in the pyramidal cell layer of the CA3 region were approximately 5 times more sensitive than cells in the CA1 region. No difference was noted in sensitivity to N-methyl-D-aspartate (NMDA) or quisqualate. 3. When kainate was superfused at known concentrations, the threshold for eliciting excitation in CA1 was 2.1 microM. The threshold concentration in CA3 was 0.24 microM. 4. Two weeks after the stereotaxic intrahippocampal injection of colchicine, the granule cells of the dentate gyrus and thus the mossy fibre projections to CA3 were destroyed. In slices prepared from animals thus treated the threshold concentration of kainate for eliciting excitation had risen to 1.64 microM. 5. Kainate was less effective in promoting the development of epileptiform bursts of neuronal firing in colchicine-treated slices than in controls. 6. Kynurenic acid antagonized the excitation of CA1 neurones elicited by kainate, NMDA or quisqualate. In the CA3 region kynurenate antagonized selectively responses to microiontophoretic NMDA, with little effect on responses to kainate or quisqualate. 7. In slices taken from colchicine-treated rats kynurenate was able to block responses to kainate in the CA3 area in parallel with responses to NMDA. 8. Taken together the results suggest that the excitatory responses to kainate in the CA3 region may be partly due to a presynaptic action on mossy fibre terminals to release endogenous amino acids. The differential action of kynurenate in normal and lesioned slices may, therefore, indicate that the postsynaptic kainate receptors are sensitive to antagonism by this compound whereas the presynaptic receptors are resistant to kynurenate.

Properties of excitatory amino acid receptors on sustained ganglion cells in the cat retina

Iontophoretic effects of N-methyl-D-aspartate, quisqualate and kainate and a variety of excitatory amino acid receptor antagonists, on retinal ganglion cells, were studied in optically intact eyes of barbiturate anesthetized cats. All three agonists raised the spontaneous firing of both ON- and OFF-sustained retinal ganglion cells, with the potency order of kainate much greater than quisqualate greater than N-methyl-D-aspartate. However, the excitatory amino acid analogues readily saturated the receptors and reduced the visually driven firing of cells with high spontaneous firing, but mimicked an increase in endogenous excitatory amino acid release and raised the visually induced response in cells with low spontaneous firing. The quinoxaline compound, 6-cyano-2,3 dihydroxy-7-nitroquinoxaline and 6-7-dinitroquinoxaline-2,3-dione, blocked the visually driven firing and kainate- and quisqualate-induced excitation, whilst 3[+)-2-carboxypiperazin-4-yl)propyl-1-phosphonate, antagonized the N-methyl-D-aspartate-induced excitation, but failed to block visually driven firing of the retinal ganglion cells. The broadband excitatory amino acid receptor antagonists, such as kynurenate, were also effective in antagonizing the visually driven response and also blocked the N-methyl-D-aspartate- as well as kainate- and quisqualate-induced responses. These results suggest that the receptors at the bipolar/ganglion cell synapse are of the non-N-methyl-D-aspartate type, but that N-methyl-D-aspartate receptors are also present on ganglion cells although their physiological role is unclear.

Protective effects of mossy fiber lesions against kainic acid-induced seizures and neuronal degeneration

The effects of a hippocampal mossy fiber lesion have been determined on neuronal degeneration and limbic seizures provoked by the subsequent intracerebroventricular administration of kainic acid to unanesthetized rats. Mossy fiber lesions were made either by transecting this pathway unilaterally or by destroying the dentate granule cells unilaterally or bilaterally with colchicine. All control rats eventually developed status epilepticus and each temporally discrete seizure that preceded status epilepticus was recorded from the hippocampus ipsilateral to the kainic acid infusion before the contralateral hippocampus. A mossy fiber lesion of the ipsilateral hippocampus prevented the development of status epilepticus in 26% of subjects and in 52% of subjects seizures were recorded from the contralateral hippocampus before the ipsilateral hippocampus. Unlike electrographic records from other treatment groups, those from rats which had received a bilateral colchicine lesion exhibited no consistent pattern indicative of seizure propagation from one limbic region to another. A bilateral, but not a unilateral, mossy fiber lesion also dramatically attenuated the behavioral expression of the seizures. Regardless of its effects on kainic acid-induced electrographic and behavioral seizures, a mossy fiber lesion always substantially reduced or completely prevented the degeneration of ipsilateral hippocampal CA3-CA4 neurons. This protective effect was specific for those hippocampal neurons deprived of mossy fiber innervation. Neurons in other regions of the brain were protected from degeneration only when the mossy fiber lesion also prevented the development of electrographic status epilepticus. These results suggest that the hippocampal mossy fibers constitute an important, though probably not an obligatory, link in the circuit responsible for the spread of kainic acid seizures. Degeneration of CA3-CA4 neurons appears to depend upon (1) the duration of hippocampal seizure activity and (2) an as yet undefined influence of or interaction with the mossy fiber projection which enhances the neurodegenerative effect of the seizures.

Kainic acid induces long-lasting depolarizations in hippocampal neurons only when applied to stratum lucidum

The actions of alpha-kainic acid (KA) were reexamined in thin sections of the hippocampus and the cerebellum of the guinea pig in view of various discrepancies between our previous findings and reports from other laboratories. Brief pulses of KA ejected in st. lucidum in the CA3 region induced short- and long-lasting depolarizations in neurons nearby, whereas those ejected in st. radiatum or st. oriens induced only short-lasting responses. Neurons in CA1 region and Purkinje cells in the cerebellum generated only short-lasting depolarizations in response to KA pulses ejected in their dendritic fields. The short-lasting KA responses in CA1 region were sensitive to gamma-D-glutamylglycine and pentobarbital. The slow KA responses were suppressed by kynurenic acid. They were not accompanied by increases in extracellular potassium concentration. These results suggest that the mossy fiber-innervated portions of the surface membrane of CA3 neurons have a type of KA receptor different from those ubiquitously distributed in central neurons.

MK-801 is neuroprotective in gerbils when administered during the post-ischaemic period

The neuroprotective effects of the non-competitive N-methyl-D-aspartate receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) have been evaluated in the gerbil hippocampus when the drug was administered i.p. at various times during and after a 5 min period of transient forebrain ischaemia, induced by bilateral common carotid artery occlusion. A single dose of 1, 3 or 10 mg/kg of MK-801 gave significant protection of hippocampal CA1 and CA2 pyramidal neurons when administered during the occlusion and up to 24 h following the period of ischaemia. A dose of 0.3 mg/kg was effective when administered during the occlusion period but gave no protection at 30 min or 2 h post-ischaemia. Experiments in which MK-801 was administered in repeated doses indicated that significant protection was achieved with 1 mg/kg of MK-801 repeated post-ischaemically and with 1 mg/kg MK-801 supplemented with repeated doses of 0.3 mg/kg of MK-801. However 0.3 mg/kg of MK-801 followed by repeated doses of 0.03 mg/kg administered post-ischaemically was not neuroprotective. These results indicate that MK-801 can protect hippocampal neurons from ischaemia-induced neuronal degeneration when it is administered up to 24 h after the insult. These data provide further evidence that therapeutic intervention in the post-ischaemic period can successfully prevent neurodegenerative events, and that the delayed degeneration of hippocampal neurons following an ischaemic insult occurs by an N-methyl-D-aspartate receptor-mediated process.

The neurotoxicity of intrahippocampal kainic acid injection in rats is not accompanied by a reduction of Timm stain

Histopathological changes induced by intrahippocampal injections of low doses of kainic acid (17.5 ng/site) were investigated in rats. Kainic acid produced a selective loss of CA3 pyramidal and hilar neurons. The development of kainic acid-induced neuronal injury was not accompanied by any detectable loss of histologically demonstrable zinc as assessed by means of a modified Timm's sulphide-silver method. It is suggested that the selective injury of hippocampal neurons induced by kainic acid is not contingent on the release of zinc from mossy-fiber terminals.

Quisqualate- and kainate-activated channels in mouse central neurones in culture

1. Quisqualate- and kainate-induced currents were recorded in mouse central neurones in culture, using both 'whole-cell' and 'outside-out' configurations. Experiments were made at room temperature. 2. Both quisqualate- and kainate-induced currents invert at 0 mV when the extracellular and intracellular solutions contain similar concentrations of monovalent cations. The changes of reversal potential produced by changes in the monovalent cation concentrations are consistent with the hypothesis that both agonists activate channels selectively permeable to cations. 3. The spectral analysis of the quisqualate- and kainate-induced currents recorded in the whole-cell mode indicates that the main component of the quisqualate noise has a time constant of 10-15 ms while the main component of the kainate noise has a time constant of 2-3 ms. 4. In outside-out patches most of the quisqualate-induced current was carried by channels with a conductance of about 8 pS. A small fraction of the quisqualate-induced current appears to be carried by two other channels: the 'NMDA channel' (40-50 pS) (Ascher, Bregestovski & Nowak, 1988) and a 'fast' channel, with a conductance of 15-35 pS, which was not activated by low concentrations of L-glutamate or by NMDA (N-methyl-D-aspartate). 5. Most of the kainate-induced current can be attributed to a channel characterized by a conductance of about 4 pS. Here again, outside-out patches revealed the presence of an additional channel, with a conductance of about 20 pS. 6. The results are consistent with the notion that there are at least three distinct receptors for excitatory amino acids, each preferentially activated by either NMDA, quisqualate or kainate, each opening channels with multiple conductance states. Other possibilities are discussed.

Properties of two classes of rat brain acidic amino acid receptors induced by distinct mRNA populations in Xenopus oocytes

The Xenopus laevis oocyte expression system was used to study the molecular composition of mRNAs encoding acidic amino acid (AA) receptors from rat brain. Xenopus oocytes injected with poly(A) mRNA express two general classes of AA receptors. One class consists of AA-gated cation channels. Responses are evoked by N-methyl-D-aspartate (NMDA), by kainate, and to a lesser extent by L-glutamate or quisqualate. The second class of receptor is coupled to an intracellular second messenger pathway activating an oocyte-encoded Ca2+-activated Cl- conductance. This second messenger-coupled AA receptor can be activated by L-glutamate or quisqualate. DL-2-amino-5-phosphonopentanoic acid and D-alpha-aminohexanedioic acid inhibit the AA-gated cation conductances activated by NMDA or kainate with different potencies but do not inhibit the second messenger-coupled AA receptor. Responses to NMDA are enhanced by micromolar level of glycine and are inhibited by Mg2+, Zn2+, or MK-801. Dose-response analysis reveals that the AA-gated cation conductance activated by kainate requires the binding of two agonist molecules. To study the molecular composition, the mRNAs were size-fractionated by denaturing agarose gel electrophoresis. About 20-fold purification in specific activity (nA/ng of mRNA injected) of mRNAs encoding the second messenger-coupled AA receptor was achieved. In contrast, only a slight enrichment of the mRNAs encoding the AA-gated channel was observed. This suggests that the second messenger-coupled AA receptor is encoded by a single size class of mRNA, whereas the AA-gated cation channel(s) is encoded by multiple species of mRNAs or by mRNAs whose size distribution is heterogeneous.

Postnatal development of the chemosensitivity of rat cerebellar Purkinje cells to excitatory amino acids. An in vitro study

In vitro sagittal slices of immature rat cerebellum were used to study the development of the sensitivity of Purkinje cells (PCs) to L-aspartate (L-Asp), L-glutamate (L-Glu) and related derivatives. As early as postnatal day 0 all PCs already displayed clear excitatory responses to short iontophoretic applications of L-Asp, L-Glu and quisqualate while in the same conditions no effect of N-methyl-D,L-aspartate (NMDLA) was detected. By postnatal day 5, i.e. after the onset of the synaptogenesis, the sensitivity of PCs to L-Asp, L-Glu and quisqualate significantly increased up to values similar to those recorded in adult rat cerebellum and surprisingly nearly all (87%) the recorded cells now also displayed excitatory responses to NMDLA. Although this sensitivity of PCs to NMDLA was significantly lower than that observed with the other drugs, it persisted until the end of the first postnatal month when the adult type of connectivity is already well established but at this stage only 30 per cent of the tested cells were still sensitive to the agonist. During this period, excitatory responses elicited by NMDLA were selectively antagonized by 2-amino-5-phosphonovalerate (2-APV), suggesting that during postnatal development, NMDA receptor types are transiently expressed on PCs membranes since in the adult, NMDLA no longer had an excitatory effect. Instead, this drug now exerted a preferential antagonistic action on the excitatory response elicited by L-Asp. Also in the adult, no major changes occurred in the sensitivity of PCs to L-Asp, L-Glu and quisqualate when these drugs were ejected at a dendritic site whereas, when ejected at the somatic level, the sensitivity of the cell appeared 2-3 times lower.

Chemoresponsiveness of intracellular nuclei neurones to L-aspartate, L-glutamate and related derivatives in rat cerebellar slices maintained in vitro

The sensitivity of intracerebellar nuclei neurones to pulse applications of L-aspartate, L-glutamate, N-methyl-D,L-aspartate and quisqualate was tested in rat cerebellar slices maintained in vitro. The responses of the nuclear neurones to the four agonists consisted of a transient and dose-dependent increase in their firing of simple spikes. When suprathreshold currents were used, quisqualate induced the highest increase in the spike discharge frequency of the cells. Quisqualate mediated responses were unaffected by steady applications of 2-amino-5-phosphonovalerate, whereas the sensitivity of the responses induced by the three other agonists was in the order N-methyl-D,L-aspartate, L-aspartate, L-glutamate. When the superfusing solution was devoid of Mg2+ ions, N-methyl-D,L-aspartate and L-aspartate mediated responses were much potentiated, while quisqualate induced responses were not enhanced. In such a medium, L-glutamate elicited responses were more or less potentiated depending on cells. These results suggest that rat intracerebellar nuclei neurones bear both N-methyl-D-aspartate and non-N-methyl-D-aspartate, probably quisqualate, receptors, and that L-aspartate and L-glutamate have a mixed action upon both types. L-Aspartate preferentially activates N-methyl-D-aspartate receptors, whereas L-glutamate predominantly acts via non-N-methyl-D-aspartate receptors. Furthermore, the potency of L-glutamate in activating N-methyl-D-aspartate receptors appears to vary as a function of the cells.

Blocking action of pentobarbital on receptors for excitatory amino acids in the guinea pig hippocampus

The actions of pentobarbital sodium (Pent) on receptors for glutamate (Glu) and related compounds were studied in thin sections of the guinea pig hippocampus. Depolarizations induced by Glu and quisqualate (Quis) in CA3 neurons were reduced in amplitude during iontophoretic administration of Pent. This action of Pent was not accompanied by any noticeable changes in membrane potential or neuron input resistance. Depolarizations induced by N-methyl-D-aspartate were less sensitive to Pent. The fast kainate (KA) response was as susceptible as the Glu response, whereas the slow KA response was unaffected by Pent in three quarters of the neurons examined. Pent suppressed the Glu response at lower concentrations than required to potentiate responses to gamma-amino butyric acid. Excitatory postsynaptic potentials (EPSPs) elicited by stimulation of mossy fibers were suppressed by Pent. The EPSPs were a little more resistant to Pent than were the Glu responses. These results indicate that Pent blocks receptors for excitatory amino acids in the hippocampus. Of the three different populations of the receptors, Quis receptors are the most sensitive to Pent and KA receptors are the least sensitive. The suppression of the EPSPs is in accordance with the notion that Glu is the transmitter released from mossy fibers.