Roles of adenosine receptors in the regulation of kainic acid-induced neurotoxic responses in mice

Kainic acid (KA) is a well-known excitatory and neurotoxic substance. In ICR mice, morphological damage of hippocampus induced by KA administered intracerebroventricularly (i.c.v.) was markedly concentrated on the hippocampal CA3 pyramidal neurons. In the present study, the possible role of adenosine receptors in hippocampal cell death induced by KA (0.1 microg) administered i.c.v. was examined. It has been shown that 3,7-dimethyl-1-propargylxanthine (DMPX; A2 adenosine receptors antagonist, 20 microg) reduced KA-induced CA3 pyramidal cell death. KA dramatically increased the phosphorylated extracellular signal-regulated kinase (p-ERK) immunoreactivities (IR) in dentate gyrus (DG) and mossy fibers. In addition, c-Jun, c-Fos, Fos-related antigen 1 (Fra-1) and Fos-related antigen 2 (Fra-2) protein levels were increased in hippocampal area in KA-injected mice. DMPX attenuated KA-induced p-ERK, c-Jun, Fra-1 and Fra-2 IR. However, 1,3-dipropyl-8-(2-amino-4-chlorophenyl)-xanthine (PACPX; A1 adenosine receptor antagonist, 20 microg) did not affect KA-induced p-ERK, c-Jun, Fra-1 and Fra-2 IR. KA also increased the complement receptor type 3 (OX-42) IR in CA3 region of hippocampus. DMPX, but not PACPX, blocked KA-induced OX-42 IR. Our results suggest that p-ERK and c-Jun may function as important regulators responsible for the hippocampal cell death induced by KA administered i.c.v. in mice. Activated microglia, which was detected by OX-42 IR, may be related to phagocytosis of degenerated neuronal elements by KA excitotoxicity. Furthermore, it is implicated that A2, but not A1, adenosine receptors appear to be involved in hippocampal CA3 pyramidal cell death induced by KA administered i.c.v. in mice.     

Cycloheximide inhibits neurotoxic responses induced by kainic acid in mice

In the present study, we examined the effect of cycloheximide on various pharmacological responses induced by kainic acid (KA) administered intracerebroventricularly (i.c.v.) in mice. In a passive avoidance test, a 20-min cycloheximide (200mg/kg, i.p.) pretreatment prevented the memory impairment induced by KA. The morphological damage induced by KA (0.1microg) in the hippocampus was markedly concentrated in the CA3 pyramidal neurons and cycloheximide effectively prevented the KA-induced pyramidal cell death in CA3 hippocampal region. In immunohistochemical study, KA dramatically increased the phosphorylation of extracellular signal-regulated protein kinase (p-ERK), c-Jun N-terminal kinase 1 (p-JNK1), and calcium/calmodulin-dependent protein kinase II (p-CaMK II). Cycloheximide attenuated the increased p-ERK, p-JNK1, and p-CaMK II levels induced by KA. Furthermore, cycloheximide inhibited the increased c-Fos and c-Jun protein expression levels induced by KA in the hippocampus. The activation of microglia was detected in KA-induced CA3 cell death region by immunostaining with a monoclonal antibody against the OX-42. Cycloheximide inhibited KA-induced increase of OX-42 immunoreactivity. Our results suggest that the increased expression of the c-Fos, c-Jun, and phosphorylation of ERK, JNK1, and CaMK II proteins may play important roles in the memory impairment and the cell death in CA3 region of the hippocampus induced by i.c.v. KA administration in mice. Furthermore, the activated microglia may be related to phagocytosis of degenerated neuronal elements induced by KA.     

Protective effect of topiramate on kainic acid–induced cell death in mice hippocampus

The protective effect of topiramate (TPM) on seizure-induced neuronal injury is well known; however, its molecular basis has yet to be elucidated. We investigated the effect and signaling mediators of TPM on seizure-induced hippocampal cell death in kainic acid (KA)-treated ICR mice. KA-induced hippocampal cell death was identified by terminal deoxynucleotidyl transferase–mediated dUTP nick end labeling. Immunoreactivity (IR) of p-Erk, p-Jnk, p-P38, and caspase-3, and caspase-3 activity were observed in the hippocampal region 3 h after KA (0.1 μg/5 μL, i.c.v.) administration, and/or TPM (100 mg/kg, i.p.) pretreatment. TPM attenuated seizure-induced neuronal cell death and reduced KA-induced p-Erk IR in the CA3 region of the hippocampus, but did not affect p-Jnk and p-P38. In addition, TPM reduced caspase-3 IR and activation by KA. KA-induced seizures were also suppressed by TPM pretreatment. TPM inhibits seizures, and decreases Erk phosphorylation and caspase-3 activation by KA, thereby contributing to protection from neuronal injury.     

Protective effect of topiramate on kainic acid-induced cell death in mice hippocampus.

The protective effect of topiramate (TPM) on seizure-induced neuronal injury is well known; however, its molecular basis has yet to be elucidated. We investigated the effect and signaling mediators of TPM on seizure-induced hippocampal cell death in kainic acid (KA)-treated ICR mice. KA-induced hippocampal cell death was identified by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling. Immunoreactivity (IR) of p-Erk, p-Jnk, p-P38, and caspase-3, and caspase-3 activity were observed in the hippocampal region 3 h after KA (0.1 microg/5 microL, i.c.v.) administration, and/or TPM (100 mg/kg, i.p.) pretreatment. TPM attenuated seizure-induced neuronal cell death and reduced KA-induced p-Erk IR in the CA3 region of the hippocampus, but did not affect p-Jnk and p-P38. In addition, TPM reduced caspase-3 IR and activation by KA. KA-induced seizures were also suppressed by TPM pretreatment. TPM inhibits seizures, and decreases Erk phosphorylation and caspase-3 activation by KA, thereby contributing to protection from neuronal injury.     

Enhanced hippocampal neurodegeneration after traumatic or kainate excitotoxicity in GFAP-null mice.

Astrocytes perform a variety of functions in the adult central nervous system. Recent evidence suggests that the upregulation of glial fibrillary acidic protein (GFAP), an astrocyte-specific intermediate filament component, is a biological marker of neurotoxicity after cerebral injury. We herein compared the response to traumatic brain injury or kainic acid (KA)-induced neurotoxicity in GFAP knockout (GFAP-KO) and wild-type (WT) mice. Seventy-two hours after injury, all GFAP-KO mice showed hippocampal CA3 neurodegeneration, whereas WT mice did not show neurodegeneration. Seventy-two hours after KA administration, GFAP-KO mice were more susceptible to KA-induced seizures and had an increased number of pyknotic damaged CA3 neurons than did WT mice. These results indicate that GFAP plays a crucial role in pyramidal neuronal survival after injury or KA-induced neurotoxicity.     

Resistance of immature hippocampus to morphologic and physiologic alterations following status epilepticus or kindling.

Seizures in adult rats result in long-term deficits in learning and memory, as well as an enhanced susceptibility to further seizures. In contrast, fewer lasting changes have been found following seizures in rats younger than 20 days old. This age-dependency could be due to differing amounts of hippocampal neuronal damage produced by seizures at different ages. To determine if there is an early developmental resistance to seizure-induced hippocampal damage, we compared the effects of kainic acid (KA)-induced status epilepticus and amygdala kindling on hippocampal dentate gyrus anatomy and electrophysiology, in immature (16 day old) and adult rats. In adult rats, KA status epilepticus resulted in numerous silver-stained degenerating dentate hilar neurons, pyramidal cells in fields CA1 and CA3, and marked numerical reductions in CA3c pyramidal neuron counts (-57%) in separate rats. Two weeks following the last kindled seizure, some, but significantly less, CA3c pyramidal cell loss was observed (-26%). Both KA status epilepticus and kindling in duced mossy-fiber sprouting, as evidenced by ectopic Timm staining in supragranular layers of the dentate gyrus. In hippocampal slices from adult rats, paired-pulse stimulation of perforant path axons revealed a persistent enhancement of dentate granule-cell inhibition following KA status epilepticus or kindling. While seizures induced by KA or kindling in 16-day-old rats were typically more severe than in adults, the immature hippocampus exhibited markedly less KA-induced cell loss (-22%), no kindling-induced loss, no detectable synaptic rearrangement, and no change in dentate inhibition. These results demonstrate that, in immature rats, neither severe KA-induced seizures nor repeated kindled seizures produce the kind of hippocampal damage and changes associated with even less severe seizures in adults. The lesser magnitude of seizure-induced hippocampal alterations in immature rats may explain their greater resistance to long-term effects of seizures on neuronal function, as well as future seizure susceptibility. Conversely, hippocampal neuron loss and altered synaptic physiology in adults may contribute to increased sensitivity to epileptogenic stimuli, spontaneous seizures, and behavioral deficits.     

Ethyl pyruvate attenuates kainic acid-induced neuronal cell death in the mouse hippocampus

Recent studies have shown that ethyl pyruvate (EP) acts as an anti-inflammatory molecule in several cell lines including RAW264.7 macrophages. However, the potential therapeutic value of EP for the treatment of the pathologic brain has not been investigated fully. In the present study, we examined whether EP has a beneficial effect on KA-induced neuronal cell death. Intracerebroventricular (i.c.v.) injection of 0.94 nmol (0.2 mug) of KA produced typical neuronal cell death in the CA1 and CA3 pyramidal layers of the hippocampus, and the systemic administration of EP significantly attenuated KA-induced neuronal cell death in these regions. Ethyl pyruvate was found to exert a protective effect when it was injected as late as 12 hr after KA-injection. Moreover, this EP-induced neuroprotection was accompanied by reduced levels of reactive gliosis and COX-2, IL-1beta, and TNF-alpha in the hippocampus. In addition, in passive avoidance tests, KA-induced memory impairment was improved markedly by EP. These results suggest that EP has a therapeutic potential for suppressing KA-induced pathogenesis in the brain.     

Changes in the activity of gamma-glutamyl transpeptidase induced by kainic acid and surgical lesions of the hippocampal formation in young rats

To study possible functional involvement of gamma-glutamyl transpeptidase (GGT) in glutamate transmitter metabolism we lesioned putative glutamatergic structures of the rat hippocampal formation by intracerebroventricular (i.c.v.) injection of kainic acid (KA) or by surgical CA3 axotomy. Unilateral injection of KA into the left lateral cerebral ventricle of 30-day-old rats resulted in decreased GGT activity in hippocampal areas CA3, Ca1 ipsilaterally, and in the contralateral area CA1, four hours after the induction of the chemical lesion. Four days after the injection, the enzyme activity was decreased in all hippocampal areas with the exception of the contralateral dentate gyrus. Four days after bilateral i.c.v. injection of KA, lower GGT levels were found than was seen after bilateral surgical lesion of the CA3 pyramidal cell axons (Schaffer's collaterals). The surgical lesion was followed by a decrease of GGT only in the stratum pyramidale and stratum radiatum of area CA1. In contrast to the effects in 30-day-old rats, unilateral i.c.v. injection of KA on postnatal day 12 did not alter the GGT activity in any studied hippocampal area presumably because of incomplete maturation of structures required for KA vulnerability.     

Protective effects of extracellular glutathione against Zn2+-induced cell death in vitro and in vivo

The central nervous system reserves high concentrations of free Zn(2+) in certain excitatory synaptic vesicles. In pathological conditions such as transient cerebral ischemia, traumatic brain injury, and kainic acid (KA)-induced seizure, free Zn(2+) is released in excess at synapses, which causes neuronal and glial death. We report here that glutathione (GSH) can be used as an effective means for protection of neural cells from Zn(2+)-induced cell death in vitro and in vivo. Chronic treatment with 35 microM Zn(2+) led to death of primary cortical neurons and primary astrocytes. The Zn(2+) toxicity of cortical neurons was partially protected by 1 mM of GSH, whereas the Zn(2+) toxicity of primary astrocyte cultures was blocked completely by 100 microM of GSH. To evaluate the beneficial effects of GSH in vivo, an excitotoxin-induced neural cell death model was established by intracerebroventricular (i.c.v.) injection of 0.94 nmol (0.2 microg) KA, which produced selective neuronal death, especially in CA1 and CA3 hippocampal regions. The i.c.v. co-injection of 200 pmol of GSH significantly attenuated KA-induced neuronal cell death and reactive gliosis in hippocampus. The results of this study suggest the contribution of Zn(2+) in the excitotoxin-induced neural cell death model and a potential value of GSH as a therapeutic means against Zn(2+)-induced pathogenesis in brain.     

Lactation Reduces Glial Activation Induced by Excitotoxicity in the Rat Hippocampus

Motherhood induces a series of adaptations in the physiology of the female, including an increase of maternal brain plasticity and a reduction of cell damage in the hippocampus caused by kainic acid (KA) excitotoxicity. We analysed the role of lactation in glial activation in the hippocampal fields of virgin and lactating rats after i.c.v. application of 100 ng of KA. Immunohistochemical analysis for glial fibrillary acidic protein (GFAP) and ionised calcium binding adaptor molecule 1 (Iba-1), which are markers for astrocytes and microglial cell-surface proteins, respectively, revealed differential cellular responses to KA in lactating and virgin rats. A significant astrocyte and microglial response in hippocampal areas of virgin rats was observed 24 h and 72 h after KA. By contrast, no increase in either GFAP- or Iba-1-positive cells was observed in response to KA in the hippocampus of lactating rats. Western blot analysis of GFAP showed an initial decrease at 24 h after KA treatment, with an increase at 72 h in the whole hippocampus of virgin but not of lactating rats. The number of GFAP-positive cells was increased by lactation in the dentate gyrus of the hippocampus but not in CA1 and CA3 areas. The present results indicate that lactating rats exhibit diminished responses of astrocyte and microglial cells in the hippocampus to damage induced by KA, supporting the notion that the maternal hippocampus is resistant to excitotoxic insults.     

Immediate and delayed hippocampal neuronal loss induced by kainic acid during early postnatal development in the rat

The degree to which the neonatal hippocampus is resistant to the effects of excitotoxins, such as kainic acid (KA) remains uncertain. Previously, we showed delayed loss of hippocampal neurons during pubescence in neonatal rats subjected to intracerebroventricular (i.c.v.) KA administration (10 nmol) at postnatal day 7 (P7). To further characterize the time course as well as the underlying mechanisms of this neuronal loss, we administered i.c.v. KA (10 or 50 nmol) to P7 preweanling rats. Brain sections were then examined at several neurodevelopmental time points (i.e., P8, P14, P25, P40, P60 and P75) using thionin staining and three-dimensional, non-biased cell counting to assess neuronal loss, and immunohistochemistry and electron microscopy to search for evidence of necrosis and apoptosis. Dose-dependent acute neuronal loss was observed at P8-P14 in hippocampal subfields CA3a and CA3c. Transient heat shock protein (HSP-70) immunostaining accompanied this acute neuronal loss. Progressive neuronal loss then continued in CA3 until P75, but without concomitant HSP-70 immunostaining. Progressive neuronal cell loss was also observed in the CA1 subfield of the hippocampus beginning at pubescence (i.e., P40) and continuing until P75. The appearance of TUNEL-positive hippocampal neurons accompanied the delayed neuronal loss in both CA3 and CA1 and electron micrographs confirmed that neurons in these subfields were undergoing apoptosis. KA administration (i.c.v.) to preweanling rats caused both immediate and delayed damage to hippocampal neurons. The effect of KA was dose-dependent, and the delayed neuronal damage occurred through an apoptosis-mediated mechanism. These findings may be relevant to the pathogenesis of some neuropsychiatric disorders, where early CNS injury is not apparent until the onset of clinical symptoms in young adulthood.     

Consequences of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor blockade during status epilepticus in the developing brain

To investigate if AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor activation contributes to acute manifestations and long term consequences of status epilepticus (SE), we administered the AMPA receptor antagonist NBQX to P35 rats undergoing kainic acid (KA)-induced SE. NBQX (30 mg/kg/dose) given intraperitoneally (i.p.) at 30, 60 and 90 min after i.p. KA injection (12 mg/kg) reduced severity of SE. When tested as adults, rats that had received KA and NBQX were similar to controls with no long term impairment in visuospatial memory (assessed by the water maze test), or histologic damage in the CA1 or CA3 hippocampal subfields. However, both P35 groups, those receiving KA alone and those receiving KA and NBQX, had similar rates of spontaneous recurrent seizures (SRS). In P15 rats, NBQX resulted in increased acute mortality from KA associated SE. These results indicate that the effects of NBQX on KA-induced SE are age dependent, and that non-NMDA receptor activation contributes to the acute manifestations and to the long term sequelae seen after KA-induced SE in the prepubescent rat brain.     

The β2-adrenoceptor agonist clenbuterol elicits neuroprotective,anti-inflammatory and neurotrophic actions in the kainic acid model of excitotoxicity

Excitotoxicity is a mechanism of neuronal cell death implicated in a range of neurodegenerative conditions. Systemic administration of the excitotoxin kainic acid (KA) induces inflammation and apoptosis in the hippocampus, resulting in neuronal loss. Evidence indicates that stimulation of glial β₂-adrenoceptors has anti-inflammatory and neurotrophic properties that could result in neuroprotection. Consequently, in this study we examined the effect of the β₂-adrenoceptor agonist clenbuterol on KA-induced inflammation, neurotrophic factor expression and apoptosis in the hippocampus. Clenbuterol (0.5 mg/kg) was administered to rats one hour prior to KA (10 mg/kg). Epileptic behaviour induced by KA was assessed for three hours following administration using the Racine scale. Twenty-four hours later TUNEL staining in the CA3 hippocampal subfield and hippocampal caspase-3 activity was assessed to measure KA-induced apoptosis. In addition, expression of inflammatory cytokines (IL-1β and IFN-γ), inducible nitric oxide synthase (iNOS), kynurenine pathway enzymes indolamine 2,3-dioxygenase (IDO) and kynurenine monooxygenase (KMO), the microglial activation marker CD11b, and the neurotrophins BDNF and NGF were quantified in the hippocampus using real-time PCR. Whilst clenbuterol treatment did not significantly alter KA-induced epileptic behavior it ameliorated KA-induced apoptosis, and this neuroprotective effect was accompanied by reduced inflammatory cytokine expression, reduced expression of iNOS, IDO, KMO and CD11b, coupled with increased BDNF and NGF expression in KA-treated rats. In conclusion, the β₂-adrenoceptor agonist clenbuterol has anti-inflammatory and neurotrophic actions and elicits a neuroprotective effect in the KA model of neurodegeneration.     

Repeated intracerebroventricular infusion of nicotine prevents kainate-induced neurotoxicity by activating the alpha7 nicotinic acetylcholine receptor

We examined whether (-)-nicotine infusion can affect kainic acid (KA)-induced neurotoxicity in rats. Although treatment with a single nicotine infusion (0.5 or 1.0 microg/side, i.c.v.) failed to attenuate KA-induced neurotoxicity, repeated nicotine infusions (1.0 microg/side/day for 10 days) attenuated the seizures, the severe loss of cells in hippocampal regions CA1 and CA3, the increase in activator protein (AP)-1 DNA binding activity, and mortality after KA administration. alpha-Bungarotoxin and mecamylamine blocked the neuroprotective effects of nicotine. These results suggest that repeated nicotine treatment provides alpha7 nicotinic acetylcholine receptor-mediated neuroprotection against KA toxicity.     

Kainate treatment alters TGF-beta3 gene expression in the rat hippocampus

In order to evaluate the role of transforming growth factor (TGF)-beta3 in the neurodegenerative process, we examined the levels of mRNA and immunocytochemical distribution for TGF-beta3 in the rat hippocampus after systemic kainic acid (KA) administration. Hippocampal TGF-beta3 mRNA level was reduced 3 h after KA injection. However, the levels of TGF-beta3 mRNA were elevated 1 day post-KA and lasted for at least 30 days. A mild TGF-beta3 immunoreactivity (TGF-beta3-IR) in the Ammon's horn and a moderate TGF-beta3-IR in the dentate granule cells were observed in the normal hippocampus. The CA1 and CA3 neurons lost their TGF-beta3-IR, while TGF-beta3-positive glia-like cells proliferated mainly throughout the CA1 sector and had an intense immunoreactivity at 7, 15 and 30 days after KA. This immunocytochemical distribution of TGF-beta3-positive non-neuronal populations was similar to that of glial fibrillary acidic protein (GFAP)-positive cells. Double labeling immunocytochemical analysis demonstrated colocalization of TGF-beta3- and GFAP-immunoreactivity in the same cells. These findings suggest a compensatory mechanism of astrocytes for the synthesis of TGF-beta3 protein in response to KA-induced neurodegeneration. In addition, exogenous TGF-beta3 (5 or 10 ng/i.c.v.) significantly attenuated KA-induced seizures and neuronal damages in a dose-related manner. Therefore, our results suggest that TGF-beta3 plays an important role in protective mechanisms against KA-induced neurodegeneration.     

Selective mGluR1 Antagonist EMQMCM Inhibits the Kainate-Induced Excitotoxicity in Primary Neuronal Cultures and in the Rat Hippocampus

Abundant evidence suggests that indirect inhibitory modulation of glutamatergic transmission, via metabotropic glutamatergic receptors (mGluR), may induce neuroprotection. The present study was designed to determine whether the selective antagonist of mGluR1 (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate (EMQMCM), showed neuroprotection against the kainate (KA)-induced excitotoxicity in vitro and in vivo. In in vitro studies on mouse primary cortical and hippocampal neuronal cultures, incubation with KA (150 μM) induced strong degeneration [measured as lactate dehydrogenase (LDH) efflux] and apoptosis (measured as caspase-3 activity). EMQMCM (0.1-100 μM) added 30 min to 6 h after KA, significantly attenuated the KA-induced LDH release and prevented the increase in caspase-3 activity in the cultures. Those effects were dose- and time-dependent. In in vivo studies KA (2.5 nmol/1 μl) was unilaterally injected into the rat dorsal CA1 hippocampal region. Degeneration was calculated by counting surviving neurons in the CA pyramidal layer using stereological methods. It was found that EMQMCM (5-10 nmol/1 μl) injected into the dorsal hippocampus 30 min, 1 h, or 3 h (the higher dose only) after KA significantly prevented the KA-induced neuronal degeneration. In vivo microdialysis studies in rat hippocampus showed that EMQMCM (100 μM) significantly increased γ-aminobutyric acid (GABA) and decreased glutamate release. When perfused simultaneously with KA, EMQMCM substantially increased GABA release and prevented the KA-induced glutamate release. The obtained results indicate that the mGluR1 antagonist, EMQMCM, may exert neuroprotection against excitotoxicity after delayed treatment (30 min to 6 h). The role of enhanced GABAergic transmission in the neuroprotection is postulated.     

Influence of the plasma glucose level on brain damage after systemic kainic acid injection in the rat

Summary Systemic administration of kainic acid (KA), 11 mg/kg body weight, to hyperglycemic rats induced lethal seizures in all animals, while 40% of normoglycemic rats survived the KA treatment and all hypoglycemic rats survived. An inverse correlation (P<0.01) between the plasma glucose level and survival during KA-induced seizures was demonstrated (Chi-square-test). Histopathological observations on the surviving rats clearly divided them into a group with severe hippocampal CA-1 damage and a group with mild hippocampal CA-1 damage. Hippocampal pyramidal cells and CA-1 interneurons were counted 3 weeks after the insult. The pyramidal cell loss in the CA-1 region was significant within mildly, as well as severely, affected rats with normo- and with hypoglycemia. CA-1 interneurons and CA-4 interneurons were only lost in the severely affected group. Hypoglycemia seemed to protect those CA-1 interneurons situated close to the alveus and within the stratum radiatum in these animals. The increased mortality in the hyperglycemic rats could be due to increased brain lactate accumulation, but extracerebral damage of hyperglycemia in association with KA is also a possibility. The study indicated a correlation between loss of interneurons and pronounced CA-1 pyramidal cell death and furthermore that hypoglycemia possibly protected some interneurons against KA.     

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.