Evidence against an exclusive role of glutamate in kainic acid neurotoxicity

Increased expression of mRNA encoding interleukin-1beta and caspase-1, and the secreted isoform of interleukin-1 receptor antagonist in the rat brain following systemic kainic acid administration

Kainic acid, an analogue of glutamate, injected systemically to rats evokes seizures that are accompanied by nerve cell damage primarily in the limbic system. In the present study, we have analyzed the temporal profile of the expression of the cytokines interleukin-1beta (IL-1beta) and IL-1 receptor antagonist (IL-1ra), and the related IL-1beta-converting enzyme (ICE/caspase-1), in different regions of the rat brain in response to peripheral kainic acid administration (10 mg/kg, i.p.). In situ hybridization histochemistry experiments revealed that IL-1beta mRNA-expressing cells, morphologically identified as microglial cells, were mainly localized to regions showing pronounced neuronal degeneration; hippocampus, thalamus, amygdala, and certain cortical regions. The strongest expression of IL-1beta mRNA was observed after 12 hr in these regions. A weak induction of the IL-1beta mRNA expression was observed already at 2 hr. Similar results were obtained by RT-PCR analysis, showing a significantly increased expression of IL-1beta mRNA in the hippocampus and amygdala after 12 hr. In addition, RT-PCR analysis revealed that IL-1ra mRNA, and specifically mRNA encoding the secreted isoform of IL-1ra (sIL-1ra), was strongly induced in the hippocampus and amygdala at 12 and 24 hr post-injection. RT-PCR analysis of mRNA encoding caspase-1 showed a significantly increased expression in the amygdala after 12 hr. In conclusion, in response to systemic kainic acid injection IL-1beta mRNA is rapidly induced and followed by induction of IL-1ra mRNA and caspase-1 mRNA, supporting a role of the IL-1 system in the inflammatory response during excitotoxic damage.

The effect of kainic, quinolinic and beta-kainic acids on the release of endogenous amino acids from rat brain slices

It has been suggested that the neurotoxic properties of quinolinic acid and kainic acid may, at least in part, involve an indirect action on nerve terminals containing glutamate. In the present study it is confirmed that kainate causes the release of endogenous glutamate from rat hippocampal slices, but that quinolinic acid does not share this activity. In addition beta-kainic acid was found to depress the potassium evoked release of endogenous glutamate at relatively high concentrations and this effect may underlie the anticonvulsant properties of this substance.

Intrahippocampal injection of kainic acid produces significant pyramidal cell loss in neonatal rats

Previous reports have indicated that pyramidal cells in the developing rat hippocampal formation are not destroyed by intraventricular or intraperitoneal administration of kainic acid. We examined the neurotoxic properties of kainic acid and ibotenic acid following intrahippocampal injection in neonatal rats and found significant pyramidal cell death following injection of 1.0 microgram kainic acid in 6, 7 and 9-day-old pups. At doses 2.5 or five times this amount, significant pyramidal cell loss was obtained in 5-day-old rats as well. The susceptibility of pyramidal neurons to kainic acid increased as a function of age. The developing hippocampus was considerably more vulnerable to ibotenic acid compared with kainic acid, in contrast to the order of potency reported in adult rats. The increased sensitivity of CA3 pyramidal cells parallels the development of the mossy fiber innervation to the dendrites of these cells supporting the twofold mechanism suggested by Coyle for kainic acid neurotoxicity; that is, a direct cytotoxic action via postsynaptic receptors as well as increased sensitivity due to the presence of excitatory inputs.

In vitro neurotoxicity of excitatory acid analogues during cerebellar development

The neurotoxic effects of the selective excitatory amino acid receptor agonists, quisqualate, kainate and N-methyl-D-aspartate, were studied in slice preparations of cerebellum from rats at different stages of postnatal development. With increasing age, (i) Purkinje cells became more vulnerable to kainate and quisqualate but remained insensitive to N-methyl-D-aspartate (up to 300 microM); (ii) Golgi cells became more sensitive to kainate, quisqualate and N-methyl-D-aspartate; (iii) granule cells became more sensitive to kainate, less sensitive to N-methyl-D-aspartate and remained unaffected by quisqualate (up to 100 microM), and (iv) basket and stellate cells and, up to 14 days of age, neurones of the deep cerebellar nuclei, became more vulnerable to kainate and quisqualate, but their sensitivity to N-methyl-D-aspartate stayed the same. The neurotoxicity of N-methyl-D-aspartate, but not that of kainate in 8-day-old cerebellar slices was prevented by 2-amino-5-phosphonovaleric acid; tetrodotoxin did not affect the toxicity of the agonists in 8-day-old or adult slices. The results with kainate are consistent with other studies indicating an insensitivity of the immature brain to its neurotoxic effects, but suggest that this property is not a peculiarity of kainate. Alterations in excitatory potency can explain some of the observed developmental changes. However, other observations cannot readily be accounted for on the basis of either changes in excitatory potency, the functional maturation of cerebellar circuits, changes in synaptic density, or the developmental appearance of Ca2+ channels in susceptible cells, suggesting that additional factors play an important role in the neurotoxic effects of the excitants.

Quinolinic acid: a pathogen in seizure disorders?

The evidence for an involvement of QUIN in human seizure disorders is clearly circumstantial. Importantly, QUIN is not a classical neurotransmitter and may thus play only a negligible or no role at all in normal brain function (Foster et al., 1984). We have yet to understand if and how such a possibly inert metabolite may turn into a pathogen. Several crucial questions remain to be addressed before a case can be made for a 'quinolinic acid hypothesis' of temporal lobe epilepsy. Among the most prominent ones figure the extracellular concentration of QUIN in the human brain under normal and pathological ('epileptic') conditions, the relationship between QUIN metabolism in the brain and its extracellular concentration and, a related issue, the regulation of cerebral QUIN metabolism (i.e., turnover). It is of equal importance to assess if NMDA-receptors, particularly those in the hippocampus and other parts of the limbic system, can exert a modulatory function upon brain QUIN. Unquestionably, future experiments with selective NMDA-antagonists will prove useful for the elucidation of such possible (feedback) interactions.

Excitotoxic models for neurodegenerative disorders

In recent years, considerable interest has been shown in the neurotoxin properties of excitatory amino acids and their possible relevance for the study of human neurodegenerative disorders. The term "excitotoxin" has been coined for a family of acidic amino acids which are neuroexcitants and produce a characteristic type of "axon-sparing" neuronal lesion. Intracerebral infusions of kainic and ibotenic acids, the two most commonly used excitotoxins, result in a morphological and biochemical picture in experimental animals which resembles that observed in the brains of Huntington's disease and epilepsy victims. The emergence of such animal models for neurodegenerative disorders has led to the hypothesis that endogenous excitotoxins may exist which are linked to the pathogenesis of human diseases. The most promising candidate discovered so far is quinolinic acid, a hepatic tryptophan metabolite which has recently also been found to occur in brain tissue. The particular excitotoxic properties of quinolinic acid warrant a thorough investigation of its metabolic and synaptic disposition in normal and abnormal brain function. While little is known about the mechanisms by which excitotoxins cause selective neuronal death, most current speculations propose the participation of specific synaptic receptors for acidic amino acids. The recent development of selective antagonists of such receptors has aided in the elucidation of excitotoxic mechanisms. Although a biochemical link between endogenous excitotoxins and human neurodegenerative disorders remains elusive at present, pharmacological blockade of excitotoxicity may constitute a novel therapeutic strategy for the treatment of these disease states.

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

The actions of the neurotoxic amino acids folate and kainate have been compared on ortho-and antidromic responses evoked in CA1, CA3 and the dentate gyrus of slices of rat hippocampus maintained in vitro. Both in CA1 and the dentate gyrus superfusion of these acids caused an increase in amplitude of the population spike discharging from an excitatory postsynaptic potential which either remained unaffected or was reduced. In the CA3 region kainate and folate had broadly similar actions to enhance the probability of cell firing to synaptic excitation, and also caused epileptiform discharges to occur spontaneously or in response to electrical stimulation. Spontaneous and evoked population bursts in CA3 did not persist in low calcium/high magnesium medium indicating their dependence on intact synaptic transmission; spontaneously occurring bursts in CA1 were eliminated with the latter treatment or when the axonal connections between it and CA3 were cut. Following folate superfusion the commissural-evoked response in CA3 showed large and variable shifts of the latency which were dependent on the stimulus intensity and its timing after a spontaneous population discharge. Although all of the effects of folate were reproduced by bicuculline, no evidence for a decreased recurrent inhibition in CA1 was obtained although this was observed with kainate. The finding that folate and kainate produced their effects in the absence of a detectable effect on the antidromic population spike suggests a mechanism of action other than neuronal depolarization. The implications of these data for the neurotoxic mechanism(s) and the receptor homologies of folate and kainate are discussed.

Kainate resistant neurons in peripheral ganglia of the rat

Neurons of dorsal root ganglia and of superior cervical ganglia did not display any morphological signs of degeneration when kainic acid (KA) was administered either systemically (20 and 40 mg/kg) or when it was directly injected (1 microgram). The KA doses used were sufficient to result in heavy destruction of hippocampal CA3/CA4 neurons and neurodegeneration of various brain regions after intracerebroventricular or local application, respectively. The resistance of the peripheral neurons to KA is discussed as a consequence of lacking glutamate inputs.

Effects of kainic and other amino acids on synaptic excitation in rat hippocampal slices: 1. Extracellular analysis

The actions of kainic acid on excitatory synaptic responses in rat hippocampal slices have been investigated and compared with the effects of other excitatory amino acids. Kainate administered iontophoretically or via the superfusate produced a large and long lasting potentiation of the population spike evoked in the CA1 region by Schaffer collateral-commissural stimulation. This potentiation was associated with a reduction in the field EPSP recorded in the dendritic region (stratum radiatum) but with no change in the presynaptic fibre volley or with any long lasting alteration in the antidromic population spike. The results suggest that one effect of kainate may be to produce dendritic depolarisation in CA1 pyramidal neurones. Kainate in equivalent amounts elicited similar potentiations of the population spike recorded in the dentate gyrus in response to either lateral or medial perforant path stimulation. Smaller amounts of kainate than those required to affect either CA1 or dentate pathways were able to potentiate the mossy fibre-evoked population spike in the CA3 region. Folic acid, which shares kainate's ability to produce seizures and distant brain damage when injected into the brain, elicited similar potentiations of synaptic excitation. Higher doses of folate than of kainate were required which is consistent with its weaker epileptogenic actions in vivo. In contrast, N-methyl-aspartate, ibotenate, L-glutamate and L-aspartate were unable to mimic kainate's potentiating action. In higher doses the substances depressed the population spike for long periods. These data suggest that potentiation of synaptic events may underlie the ability of kainate (and folate) to elicit seizures and distant brain damage.

Effects of in vivo administration of kainic acid on the extracellular amino acid pool in the rabbit hippocampus

The effect of local administration of kainic acid in the rabbit hippocampus was studied; the hippocampus was perfused continuously in the freely moving animal with an implanted 0.3-mm dialysis fiber. The pattern of endogenous amino acids in the perfusate, reflecting extracellular amino acids, was monitored with liquid chromatography separation and fluorimetric detection of amino acid derivatives. Kainic acid was included in the perfusion medium for up to 70 min at 0.1-1.0 mM and, with time, induced epileptiform activity. Endogenous glutamic acid, taurine, and phosphoethanolamine levels were increased selectively at the lower perfusion concentrations of kainic acid. Long perfusion periods with higher concentrations increased the levels of virtually all amino acids. Perfusion of the hippocampus with depolarizing concentrations of potassium gave an amino acid response partly similar to that seen with kainic acid treatment. However, one notable difference between the two responses was that the extracellular concentration of glutamine, although not influenced by kainic acid, was significantly decreased after high potassium concentrations. These results confirm previous notions that kainic acid has a primarily excitatory effect, one manifestation of this effect being the release of glutamic acid.

Comparison of ibotenate and kainate neurotoxicity in rat brain: a histological study

The neurotoxic properties of ibotenate and kainate after intracerebral application were compared in several regions of the rat brain. Ibotenate, being 5-10 times less toxic than kainate, caused lesions which were generally found to extend spherically from the tip of the injection cannula. In contrast, kainate injections often resulted in neuronal degeneration distant from the site of infusion, thus severely limiting its use as a tool for causing lesions in neurobiological studies. In some of the brain regions examined (hippocampus, septum), neurons appeared differentially susceptible to kainate but uniformly vulnerable to ibotenate. Some cell groups, such as those in the medial septum and the locus coeruleus, proved highly resistant to kainate but could be selectively ablated by ibotenate. These findings, together with differences between the two toxins in the evolution of neuronal degeneration (exemplified here in the hippocampal formation), appear to support previous suggestions that ibotenate and kainate exert their excitotoxic actions via different mechanisms. On the other hand, neuropathological changes caused in the cerebellum did not differ, since both ibotenate and kainate preferentially destroyed granule cells. Two nuclei, the arcuate nucleus of the hypothalamus and the nucleus of the fifth nerve, were found to be extremely resistant to either neurotoxin.

Effects of ibotenic acid-induced neuronal degeneration in the medial preoptic area and the lateral hypothalamic area on sexual behavior in the male rat

It is well known that electrolytic lesions in the medial preoptic area (MPOA) and the lateral hypothalamic area (LHA) seriously impair masculine sexual behavior in the rat. We here report that bilateral infusions of the neurotoxin, ibotenic acid (IBO), in the MPOA were as effective as electrolytic lesions in eliminating copulation whereas no behavioral effects were detected following similar infusions in the LHA. Histological examination of MPOA and LHA following IBO exposure revealed extensive degeneration of neuronal cell bodies with little evidence of non-specific damage. Also, immunohistochemical studies suggested that the serotonergic innervation of the MPOA remained largely intact in spite of IBO treatment; similarly, the damage inflicted by IBO in LHA on tyrosine hydroxylase-immunoreactive fibers in the medial forebrain bundle was insignificant. These data suggest that: (i) the functional integrity of MPOA nerve cell bodies is necessary for the expression of sexual behavior, and (ii) disruption of mating produced by electrolytic LHA lesions is due to disruption of medial forebrain bundle fiber systems. Behavioral observations of non-copulating males suggested that the MPOA injury did not interfere with all aspects of their sexual interaction with the estrous female; rather, they appeared specifically unable to perform the reflexive pelvic thrust pattern normally associated with mounting. We here report, however, that the ability to perform mounts with pelvic thrusts was temporarily restored in the vast majority of MPOA-injured males by the i.p. administration of the ergot derivative, lisuride. About 50% of these MPOA-damaged males even ejaculated, often after a low number of intromissions and short ejaculation latencies. On the other hand, injections of naloxone (an opiate receptor antagonist) failed to activate mounting in MPOA-lesioned or castrated rats. On the basis of these findings the possible ways in which steroid hormone-sensitive brain areas might interact with monoamine-containing pathways are discussed.U

Neurotransmitter receptor ligand binding and enzyme regional distribution in the pigeon visual system

The relative importance of acetylcholine, dopamine, endogenous opiates, gamma-aminobutyric acid (GABA), glutamate, glycine, noradrenaline, and serotonin as transmitters in the pigeon visual system was estimated by measuring the activity of choline acetyltransferase (ChAT), glutamic acid decarboxylase (GAD), and aromatic amino acid decarboxylase (AAD) as well as the binding of dihydroalprenolol, etorphine, kainic acid, muscimol, serotonin, spiroperidol, strychnine, and quinuclidinyl benzilate (QNB) in the tectum opticum, nucleus rotundus, ectostriatum, dorsolateral thalamus, and hyperstriatum (Wulst). As a nonvisual reference structure, the paleostriatal complex was included in the examination. The regional distribution of most of these parameters was very similar to data reported in the mammalian CNS supporting the hypothesis that the avian tectofugal and thalamofugal visual systems are homologous to the mammalian tecto-thalamo-cortical and retino-geniculo-striate pathways, respectively. On the basis of the low values of their parameters, some transmitters can be excluded as significant contributors in a number of structures. As a hypothesis for further investigations, the presence of cholinergic and serotoninergic systems in the Wulst, possibly originating in the dorsolateral thalamus and nucleus raphe, respectively, and of glycinergic and dopaminergic terminals in the paleostriatal complex is proposed.

Monosodium glutamate: increased neurotoxicity after removal of neuronal re-uptake sites

Microinjections of monosodium glutamate (MSG; 300 microgram/0.5 microliter) into the hippocampus of the adult rat result in only marginal damage to local neurons. Perforant path transections, removing glutamatergic afferents to hippocampal granule cells, make the latter markedly more vulnerable to a subsequent MSG injection. The principle of modulating toxic effects of MSG by interfering with its neurotransmitter role may have significant impact on our understanding of human neurodegenerative disorders.