Hippocampal granule cells express glutamic acid decarboxylase-67 after limbic seizures in the rat

Activity-dependent expression of GAD67 in the granule cells of the rat hippocampus

In the normal granule cells of the dentate gyrus glutamate, GABA and glutamic acid decarboxylase (GAD67) coexist. After kindled seizures, this enzyme is transiently overexpressed and simultaneous glutamatergic and GABAergic transmission in the mossy fiber projection occurs. Since this dual transmission is also seen after acutely-induced seizures, we decided to study the relationship between the expression of GAD67 and the induction of simultaneous glutamatergic and GABAergic transmission by kindled or acutely induced seizures. We also explored whether kindling of the dentate gyrus in vitro, that does not induce epileptiform activity, could induce the expression of GAD67. We confirm that kindling epilepsy induces the expression of GAD67 in the dentate gyrus. Despite the emergence of GABAergic transmission in the mossy fiber projection after a single seizure, GAD67 expression in the dentate gyrus appeared similar to controls, however, in the mossy fibers an enhanced immunostaining was evident. Interestingly, kindling the dentate gyrus in vitro induces a marked GAD67 staining in the granule cells. Our results show that after the activity-dependent emergence of simultaneous glutamatergic and GABAergic transmission from the mossy fibers, GAD67 is expressed in the mossy fibers and, upon long-lasting enduring stimulation periods, also in the dentate gyrus. Thus, this phenomenon does not depend on the presence of epileptic activity, but rather, on increased excitatory input onto the dentate gyrus. This can represent a protective mechanism that can sustain GABA synthesis in an activity-dependent manner.

Vigabatrin protects against hippocampal damage but is not antiepileptogenic in the lithium-pilocarpine model of temporal lobe epilepsy

In temporal lobe epilepsy (TLE), the nature of the structures involved in the development of the epileptogenic circuit is still not clearly identified. In the lithium-pilocarpine model, neuronal damage occurs both in the structures belonging to the circuit of initiation and maintenance of the seizures (forebrain limbic system) as well as in the propagation areas (cortex and thalamus) and in the circuit of remote control of seizures (substantia nigra pars reticulata). In order to determine whether protection of some brain areas could prevent the epileptogenesis induced by status epilepticus (SE) and to identify the cerebral structures involved in the genesis of TLE, we studied the effects of the chronic exposure to Vigabatrin (gamma-vinyl-GABA, GVG) on neuronal damage and epileptogenesis induced by lithium-pilocarpine SE. The animals were subjected to SE and GVG treatment (250 mg/kg) was initiated at 10 min after pilocarpine injection and maintained daily for 45 days. These pilo-GVG rats were compared with rats subjected to SE followed by a daily saline treatment (pilo-saline) and to control rats not subjected to SE (saline-saline). GVG treatment induced a marked, almost total neuroprotection in CA3, an efficient protection in CA1 and a moderate one in the hilus of the dentate gyrus while damage in the entorhinal cortex was slightly worsened by the treatment. All pilo-GVG and pilo-saline rats became epileptic after the same latency. Glutamic acid decarboxylase (GAD67) immunoreactivity was restored in pilo-GVG rats compared with pilo-saline rats in all areas of the hippocampus, while it was increased over control levels in the optical layer of the superior colliculus and the substantia nigra pars reticulata. Thus, the present data indicate that neuroprotection of principal cells in the Ammon's horn of the hippocampus is not sufficient to prevent epileptogenesis, suggesting that the hilus and extra-hippocampal structures, that were not protected in this study, may play a role in the genesis of spontaneous recurrent seizures in this model. Furthermore, the study performed in non-epileptic rats indicates that chronic treatment with a GABAmimetic drug upregulates the expression of the protein GAD67 in specific areas of the brain, independently from the seizures.

Vesicular GABA transporter mRNA expression in the dentate gyrus and in mossy fiber synaptosomes

In the normal granule cells of the dentate gyrus, glutamate and both gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase (GAD) coexist. GAD expression is increased after seizures, and simultaneous glutamatergic and GABAergic neurotransmission from the mossy fibers to CA3 appears, supporting the hypothesis that GABA can be released from the mossy fibers. To sustain GABAergic neurotransmission, the amino acid must be transported into synaptic vesicles. To address this, using RT-PCR we looked for the presence and regulation of expression of the vesicular GABA transporter (VGAT) mRNA in the dentate gyrus and in mossy fiber synaptosomes of control and kindled rats. We found trace amounts of VGAT mRNA in the dentate gyrus and mossy fiber synaptosomes of control rats. In the dentate gyrus of kindled rats with several seizures and of control rats subject to one acute seizure, no changes were apparent either 1 or 24 h after the seizures. However, repetitive synaptic or antidromic activation of the granule cells in slices of control rats in vitro induces an activity-dependent enhancement of VGAT mRNA expression in the dentate. Surprisingly, in the mossy fiber synaptosomes of seizing rats, the levels of VGAT mRNA were significantly higher than in controls. These data show that the granule cells and their mossy fibers, besides containing machinery for the synthesis of GABA, also contain the elements that support its vesiculation. This further supports the notion that local synaptic molecular changes enable mossy fibers to release GABA in response to enhanced excitability.

The spiny rat Proechimys guyannensis as model of resistance to epilepsy: chemical characterization of hippocampal cell populations and pilocarpine-induced changes

At variance with pilocarpine-induced epilepsy in the laboratory rat, pilocarpine administration to the tropical rodent Proechimys guyannensis (casiragua) elicited an acute seizure that did not develop in long-lasting status epilepticus and was not followed by spontaneous seizures up to 30 days, when the hippocampus was investigated in treated and control animals. Nissl staining revealed in Proechimys a highly developed hippocampus, with thick hippocampal commissures and continuity of the rostral dentate gyri at the midline. Immunohistochemistry was used to study calbindin, parvalbumin, calretinin, GABA, glutamic acid decarboxylase, and nitric oxide synthase expression. The latter was also investigated with NADPH-diaphorase histochemistry. Cell counts and densitometric evaluation with image analysis were performed. Differences, such as low calbindin immunoreactivity confined to some pyramidal cells, were found in the normal Proechimys hippocampus compared to the laboratory rat. In pilocarpine-treated casiraguas, stereological cell counts in Nissl-stained sections did not reveal significant neuronal loss in hippocampal subfields, where the examined markers exhibited instead striking changes. Calbindin was induced in pyramidal and granule cells and interneuron subsets. The number of parvalbumin- or nitric oxide synthase-containing interneurons and their staining intensity were significantly increased. Glutamic acid decarboxylase(67)-immunoreactive interneurons increased markedly in the hilus and decreased in the CA1 pyramidal layer. The number and staining intensity of calretinin-immunoreactive pyramidal cells and interneurons were significantly reduced. These findings provide the first description of the Proechimys hippocampus and reveal marked long-term variations in protein expression after an epileptic insult, which could reflect adaptive changes in functional hippocampal circuits implicated in resistance to limbic epilepsy.

The process of epileptogenesis: a pathophysiological approach

Several recent advances have contributed to our understanding of the processes associated with mesial temporal lobe epilepsy in humans and in experimental animal models. Common pathological features between the human condition and the animal models may indicate a fundamental involvement of the given pathology in the process of epileptogenesis.

Kindling induces transient fast inhibition in the dentate gyrus--CA3 projection

The granule cells of the dentate gyrus (DG) send a strong glutamatergic projection, the mossy fibre tract, toward the hippocampal CA3 field, where it excites pyramidal cells and neighbouring inhibitory interneurons. Despite their excitatory nature, granule cells contain small amounts of GAD (glutamate decarboxylase), the main synthetic enzyme for the inhibitory transmitter GABA. Chronic temporal lobe epilepsy results in transient upregulation of GAD and GABA in granule cells, giving rise to the speculation that following overexcitation, mossy fibres exert an inhibitory effect by release of GABA. We therefore stimulated the DG and recorded synaptic potentials from CA3 pyramidal cells in brain slices from kindled and control rats. In both preparations, DG stimulation caused excitatory postsynaptic potential (EPSP)/inhibitory postsynaptic potential (IPSP) sequences. These potentials could be completely blocked by glutamate receptor antagonists in control rats, while in the kindled rats, a bicuculline-sensitive fast IPSP remained, with an onset latency similar to that of the control EPSP. Interestingly, this IPSP disappeared 1 month after the last seizure. When synaptic responses were evoked by high-frequency stimulation, EPSPs in normal rats readily summate to evoke action potentials. In slices from kindled rats, a summation of IPSPs overrides that of the EPSPs and reduces the probability of evoking action potentials. Our data show for the first time that kindling induces functionally relevant activity-dependent expression of fast inhibition onto pyramidal cells, coming from the DG, that can limit CA3 excitation in a frequency-dependent manner.

Decreased epileptic susceptibility correlates with neuropeptide Y overexpression in a model of tolerance to excitotoxicity

Prior epileptic episodes have been shown to decrease markedly the neuronal damage induced by a second epileptic episode, similar to the tolerance following an episode of mild ischemia. Endogenous neuroprotective effects mediated by various mechanisms have been put forward. This study investigated whether neuroprotection against the excitotoxic damage induced by re-exposure to an epileptic challenge can reflect a change in epileptic susceptibility. Tolerance was elicited in rats by a preconditioning session using intrahippocampal kainic acid (KA) administration followed at 1, 7 and 15-day intervals by a subsequent intraventricular KA injection. The degree of pyramidal cell loss in the vulnerable CA3 subfield contralateral to the KA-injected hippocampus was extensively reduced in animals experiencing KA ventricular administration. This neuroprotection was highly significant 1 and 7 days after injection, but not 15 days after injection. In preconditioned animals, the after-discharge threshold was assessed as an index of epileptic susceptibility. It increased significantly from 1 to 15 days after intrahippocampal KA administration. Finally, an enhancement of neuropeptide Y expression in both non-principal cells and mossy fibers was detected, occurring at the same time as the decrease in epileptic susceptibility. These results provide further evidence of an 'epileptic tolerance' as shown by the substantial neuroprotective effect of a prior episode of epileptic activity upon subsequent epileptic insult and suggest that the prevention of excitotoxic damage after preconditioning results from an endogenous neuroprotective mechanism against hyperexcitability and seizures.

Chronic elevation of brain GABA levels beginning two days after status epilepticus does not prevent epileptogenesis in rats

Vigabatrin (VGB) treatment is neuroprotective in various models of status epilepticus (SE) and delays the development of kindling via mechanisms that are assumed to relate to the elevation of GABA levels in the brain. Here, we tested the hypothesis that a chronic elevation of brain GABA levels obtained by VGB treatment prevents the development of spontaneous seizures (i.e. epilepsy) following SE in rats. Self-sustained SE (SSSE) was induced by stimulating the lateral nucleus of the amygdala. Two days later, chronic VGB (75 mg/kg/day) or saline treatment was started via subcutaneous osmotic minipumps. The development of spontaneous seizures was monitored once a week (24 h at a time) using video-EEG recording. Rats were perfused for histology either at the end of the 10-week drug treatment, or later at the end of an 8-week drug-free follow-up period. Before perfusion for histology, spatial learning and memory perform was tested in the Morris water-maze. Spontaneous seizures were observed in 55% (6/11) of the saline-treated and 73% (8/11) of the VGB-treated rats during the 10-week treatment period. Seizure frequency, severity, and duration were similar in VGB-treated rats and controls during and after the drug-treatment period. VGB treatment did not decrease neuronal damage in various temporal lobe regions or mossy fiber sprouting. VGB treatment also did not attenuate spatial learning or memory impairments. These findings indicate that the augmentation of GABAergic neurotransmission by VGB does not prevent the development of epilepsy when treatment is started 2 days after SE.

Monosynaptic GABAergic signaling from dentate to CA3 with a pharmacological and physiological profile typical of mossy fiber synapses

Mossy fibers are the sole excitatory projection from dentate gyrus granule cells to the hippocampus, where they release glutamate, dynorphin, and zinc. In addition, mossy fiber terminals show intense immunoreactivity for the inhibitory neurotransmitter GABA. Fast inhibitory transmission at mossy fiber synapses, however, has not previously been reported. Here, we show that electrical or chemical stimuli that recruit dentate granule cells elicit monosynaptic GABA(A) receptor-mediated synaptic signals in CA3 pyramidal neurons. These inhibitory signals satisfy the criteria that distinguish mossy fiber-CA3 synapses: high sensitivity to metabotropic glutamate receptor agonists, facilitation during repetitive stimulation, and NMDA receptor-independent long-term potentiation. GABAergic transmission from the dentate gyrus to CA3 has major implications not only for information flow into the hippocampus but also for developmental and pathological processes involving the hippocampus.

Differential regulation of adult and embryonic glutamate decarboxylases in rat dentate granule cells after kainate-induced limbic seizures

In adult brain, the inhibitory GABAergic neurons utilize two distinct molecular forms of the GABA-synthesizing enzyme glutamate decarboxylase (GAD), GAD65 and GAD67. During embryonic development, two truncated forms of GAD67 are also expressed (GAD25 and GAD44), which are translated from two embryonic-specific splice variants of GAD67 messenger RNA. It has recently been established that the excitatory dentate granule cells, in addition to the neurotransmitter glutamate, also contain low levels of GABA and GAD67, which are increased after limbic seizures. To study the seizure-induced activation of glutamate decarboxylase, we investigated the expression of both embryonic and adult glutamate decarboxylase messenger RNAs in the adult rat hippocampus after kainic acid administration by semi-quantitative reverse transcription-coupled polymerase chain reaction, in situ hybridization and immunoblotting. We observed a rapid induction of the embryonic glutamate decarboxylase messenger RNA in the granule cells of dentate gyrus. The expression of embryonic glutamate decarboxylase transcripts, identified here as the splice variant that contains exon 7/B, peaked at about 2h after kainic acid injection and gradually returned to nearly basal levels by 24h. Strikingly, this transient induction of embryonic glutamate decarboxylase messenger RNA was not accompanied by concomitant synthesis of its corresponding protein product GAD25. In contrast, the adult GAD67 messenger RNA and protein were both clearly up-regulated in granule cells, albeit with a certain delay, reaching a maximum around 4-6h after kainic acid injection and gradually returned to control levels by 24h. GAD65 remained unchanged at both messenger RNA and protein levels during the studied period. These characteristic and highly reproducible changes in the synthesis of glutamate decarboxylases indicate that GAD67 is the predominant form of glutamate decarboxylases involved in the elevated synthesis of GABA during seizures and suggest that the transient induction of the embryonic GAD67 messenger RNA that contains exon 7/B, but not GAD25 protein, may exert a role solely in the subsequent up-regulation of adult GAD67 transcription. Expression of the messenger RNA encoding for an alternatively spliced, truncated form of the GABA-synthesizing enzyme glutamate decarboxylase was detected in dentate granule cells briefly after kainic acid-induced seizures. Just as during embryonic development, expression of the alternatively spliced messenger RNA was transient and followed by transcription of its adult form, indicating a possible recapitulation of an embryonic program of gene expression in adult granule cells after epileptic seizures.

Seizures induce simultaneous GABAergic and glutamatergic transmission in the dentate gyrus-CA3 system

Monosynaptic and polysynaptic responses of CA3 pyramidal cells (PC) to stimulation of the dentate gyrus (DG) are normally blocked by glutamate receptor antagonists (GluRAs). However, after kindled seizures, GluRAs block the monosynaptic excitatory postsynaptic potential (EPSP) and isolate a monosynaptic inhibitory postsynaptic potential (IPSP), suggesting that mossy fibers release GABA. However, kindling epilepsy induces neuronal sprouting, which can underlie this fast inhibitory response. To explore this possibility, the synaptic responses of PC to DG stimulation were analyzed in kindled epileptic rats, with and without seizures, and in nonepileptic rats, immediately after a single pentylenetetrazol (PTZ)-induced seizure, in which sprouting is unlikely to have occurred. Excitatory and inhibitory synaptic responses of PC to DG stimulation were blocked by GluRAs in control cells and in cells from kindled nonseizing rats, confirming that inhibitory potentials are disynaptically mediated. However, a fast IPSP could be evoked in kindled epileptic rats and in nonepileptic rats after a single PTZ-induced seizure. The same response was induced after rekindling the epileptic nonseizing rats. This IPSP has an onset latency that parallels that of the control EPSP and is not altered under low Ca(2+) medium or halothane perfusion. In addition, it was reversibly depressed by L(+)-2-amino-4-phosphonobutyric acid (L-AP4), which is known to inhibit transmitter release from mossy fibers. These results demonstrate that seizures, and not the synaptic rearrangement due to an underlying epileptic state, induce the emergence of fast inhibition in the DG-CA3 system, and suggest that the mossy fibers underlie this plastic change.

Specific frequencies of spontaneous Ca2+ transients upregulate GAD 67 transcripts in embryonic spinal neurons

Spontaneous Ca2+ transients expressed prior to synaptogenesis regulate the developmental appearance of GABA in cultured Xenopus spinal neurons. We find that glutamic acid decarboxylase (GAD) immunoreactivity is also Ca(2+)-dependent and parallels the appearance of GABA. We show that xGAD 67 transcripts first appear in the embryonic spinal cord during the period in which these Ca2+ spikes are generated, in a pattern that is temporally and spatially appropriate to account for differentiation of GABAergic interneurons. RNase protection and competitive quantitative RT-PCR demonstrate that transcript levels are approximately threefold greater when neurons are cultured in the presence of extracellular Ca2+ that permits generation of transients than when cultured in its absence. The frequency of spontaneous Ca2+ spikes plays a crucial role in the regulation of transcripts, since reimposition of Ca2+ transients at the frequency generated in cultured neurons rescues normal expression. We conclude that naturally occurring low frequencies of these Ca2+ transients regulate levels of xGAD 67 mRNA in differentiating neurons.

Testing the disinhibition hypothesis of epileptogenesis in vivo and during spontaneous seizures

The "disinhibition" hypothesis contends that (1) seizures begin when granule cells in the dentate gyrus of the dorsal hippocampus are disinhibited and (2) disinhibition occurs because GABAergic interneurons are excessively inhibited by other GABAergic interneurons. We tested the disinhibition hypothesis using the experimental model that inspired it-naturally epileptic Mongolian gerbils. To determine whether there is an excess of GABAergic interneurons in the dentate gyrus of epileptic gerbils, as had been reported previously, GABA immunocytochemistry, in situ hybridization of GAD67 mRNA, and the optical fractionator method were used. There were no significant differences in the numbers of GABAergic interneurons. To determine whether granule cells in epileptic gerbils were disinhibited during the interictal period, IPSPs were recorded in vivo with hippocampal circuits intact in urethane-anesthetized gerbils. The reversal potentials and conductances of IPSPs in granule cells in epileptic versus control gerbils were similar. To determine whether the level of inhibitory control in the dentate gyrus transiently decreases before seizure onset, field potential responses to paired-pulse perforant path stimulation were obtained from the dorsal hippocampus while epileptic gerbils experienced spontaneous seizures. Evidence of reduced inhibition was found after, but not before, seizure onset, indicating that seizures are not triggered by disinhibition in this region. However, seizure-induced depression of inhibition may amplify and promote the spread of seizure activity to other brain regions. These findings do not support the disinhibition hypothesis and suggest that in this model of epilepsy seizures initiate by another mechanism or at a different site.

Granule-like neurons at the hilar/CA3 border after status epilepticus and their synchrony with area CA3 pyramidal cells: functional implications of seizure-induced neurogenesis

A group of neurons with the characteristics of dentate gyrus granule cells was found at the hilar/CA3 border several weeks after pilocarpine- or kainic acid-induced status epilepticus. Intracellular recordings from pilocarpine-treated rats showed that these "granule-like" neurons were similar to normal granule cells (i. e., those in the granule cell layer) in membrane properties, firing behavior, morphology, and their mossy fiber axon. However, in contrast to normal granule cells, they were synchronized with spontaneous, rhythmic bursts of area CA3 pyramidal cells that survived status epilepticus. Saline-treated controls lacked the population of granule-like cells at the hilar/CA3 border and CA3 bursts. In rats that were injected after status epilepticus with bromodeoxyuridine (BrdU) to label newly born cells, and also labeled for calbindin D(28K) (because it normally stains granule cells), many double-labeled neurons were located at the hilar/CA3 border. Many BrdU-labeled cells at the hilar/CA3 border also were double-labeled with a neuronal marker (NeuN). Taken together with the recent evidence that granule cells that are born after seizures can migrate into the hilus, the results suggest that some newly born granule cells migrate as far as the CA3 cell layer, where they become integrated abnormally into the CA3 network, yet they retain granule cell intrinsic properties. The results provide insight into the physiological properties of newly born granule cells in the adult brain and suggest that relatively rigid developmental programs set the membrane properties of newly born cells, but substantial plasticity is present to influence their place in pre-existing circuitry.

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.

Presence of gamma-aminobutyric acid transporter mRNA in interneurons and principal cells of rat hippocampus

After release, neurotransmitters are removed from the extracellular space by high-affinity uptake. Specific sodium-dependent transporters serve this function for the inhibitory transmitter gamma-aminobutyric acid (GABA). However, it is largely unknown to which proportion GABA is taken up by GABAergic interneurons, glia cells or principal neurons. We analyzed the distribution of mRNA for the main GABA-transporter subtype in the hippocampus, GAT-1, in adult rats. Most interneurons were strongly stained for GAT-1 mRNA, indicating re-uptake by the GABA-releasing cells. Surprisingly, prominent signals for GAT-1 were also found throughout the principal cell layers (granule and pyramidal cells). These data indicate that GABA transporters may be present in non-GABAergic projection cells of the rat hippocampus which contribute to the clearance of GABA from the extracellular space.

Neurodegenerative and morphogenic changes in a mouse model of temporal lobe epilepsy do not depend on the expression of the calcium-binding proteins parvalbumin, calbindin, or calretinin

The functional role of the calcium-binding proteins parvalbumin, calretinin, and calbindin D-28k for epileptogenesis and long-term seizure-related alterations of the hippocampal formation was assessed in single- and double-knockout mice, using a kainate model of mesial temporal lobe epilepsy. The effects of a unilateral intrahippocampal injection of kainic acid were assessed at one day, 30 days, and four months post-injection, using various markers of GABAergic interneurons (GABA-transporter type 1, GABA(A)-receptor alpha1 subunit, calretinin, calbindin D-28k, somatostatin, and neuropeptide Y). Parvalbumin-deficient, parvalbumin/calbindin-deficient, and parvalbumin/calretinin-deficient mice exhibited no difference in cytoarchitecture of the hippocampal formation and in the number, distribution, or morphology of interneurons compared to wild-type mice. Likewise, mutant mice were not more vulnerable to acute kainate-induced excitotoxicity or to long-term effects of recurrent focal seizures, and exhibited the same pattern of neurochemical alterations (e.g., bilateral induction of neuropeptide Y in granule cells) and morphogenic changes (enlargement and dispersion of dentate gyrus granule cells) as wild-type animals. Quantification of interneurons revealed no significant difference in neuronal vulnerability among the genotypes.These results indicate that the calcium-binding proteins investigated here are not essential for determining the neurochemical phenotype of interneurons. Furthermore, they are not protective against kainate-induced excitotoxicity in this model, and do not appear to modulate the overall level of excitability of the hippocampus. Finally, seizure-induced changes in gene expression in granule cells, which normally express high levels of calcium-binding proteins, apparently were not affected by the gene deletions analysed.

Altered hippocampal expression of neuropeptide Y, somatostatin, and glutamate decarboxylase in Ihara's epileptic rats and spontaneously epileptic rats

By in situ hybridization and immunocytochemistry, expression of neuropeptide Y (NPY), somatostatin and glutamate decarboxylase 65 (GAD65) was studied in the hippocampus of two different epileptic mutant rats, Ihara's epileptic rat (IER) and the spontaneously epileptic rat (SER). GAD65 mRNA expression was enhanced in interneurons of the hippocampus in young IER, that had not yet developed generalized seizures. In older IER and older SER that both showed spontaneous seizures, marked increases of NPY mRNA in hippocampal granule cells and interneurons were found, as well as elevated GAD65 mRNA levels in interneurons. NPY immunoreactivity was enhanced in hilar interneurons and the dentate gyrus of older IER. In addition, some older IER stained heavily for NPY in mossy fibers. These findings suggest that up-regulation of NPY and GAD65 synthesis may be important in epileptogenesis.

Amino acid neurotransmitter metabolism in neurones and glia following kainate injection in rats

Limbic seizure was induced in rats by intraperitoneal injection of the glutamate receptor agonist kainic acid. After 14 days [1-13C]glucose and [1,2-13C]acetate were injected subcutaneously and the rats killed 15 min later. Analysis of brain extracts was performed using 13C-magnetic resonance spectroscopy and high performance liquid chromatography. No significant differences between the two groups of rats were found for label concentration in blood or total metabolite tissue levels. Only astrocytes are able to utilize acetate as a substrate, whereas glucose is thought to be metabolized predominantly in the neuronal tricarboxylic acid cycle. Thus information about neuronal and astrocytic metabolism could be obtained in the same animal. A significant increase in label derived from [1-13C]glucose was observed in metabolites such as glutamate, gamma-aminobutyric acid, aspartate, and succinate (all of which are mainly labelled in neurones). The increased labelling of glutamine in epileptic rats might be due to transfer of labelled glutamate from neurones to astrocytes. Astrocytic metabolism of acetate and transfer of glutamine to neurones were not affected. The results suggest that increased neuronal activity 2 weeks following epileptic seizures produces increased amino acid turnover in neurones. Changes in astrocytic metabolism were not detected.

Neuroactive amino acids in focally epileptic human brain: a review

Studies of neuroactive amino acids and their regulatory enzymes in surgically excised focally epileptic human brain are reviewed. Concentrations of glutamate, aspartate and glycine are significantly increased in epileptogenic cerebral cortex. The activities of the enzymes, glutamate dehydrogenase and aspartate aminotransferase, involved in glutamate and aspartate metabolism are also increased. Polyamine synthesis is enhanced in epileptogenic cortex and may contribute to the activation of N-methyl-D-aspartate (NMDA) receptors. Nuclear magnetic resonance spectroscopy (NMRS) reveals that patients with poorly controlled complex partial seizures have a significant diminution in occipital lobe gamma aminobutyric acid (GABA) concentration. The activity of the enzyme GABA-aminotransaminase (GABA-T) which catalyzes GABA degradation is not altered in epileptogenic cortex. NMRS studies show that vigabatrin, a GABA-T inhibitor and effective antiepileptic, significantly increases brain GABA. Glutamate decarboxylase (GAD), responsible for GABA synthesis, is diminished in interneurons in discrete regions of epileptogenic cortex and hippocampus. In vivo microdialysis performed in epilepsy surgery patients provides measurements of extracellular amino acid levels during spontaneous seizures. Glutamate concentrations are higher in epileptic hippocampi and increase before seizure onset reaching potentially excitotoxic levels. Frontal or temporal cortical epileptogenic foci also release aspartate, glutamate and serine particularly during intense seizures or status epilepticus. GABA in contrast, exhibits a delayed and feeble rise in the epileptic hippocampus possibly due to a reduction in the number and/or efficiency of GABA transporters.

The GABA paradox: multiple roles as metabolite, neurotransmitter, and neurodifferentiative agent

GABA, which is present in the brain in large amounts, is distributed among distinctly different cellular pools, possibly reflecting its multiple functions as metabolite, neurotransmitter, and neurotrophin. Its metabolic enzymes also exhibit heterogeneity, because glutamate decarboxylase exists in two isoforms with different subcellular distribution and regulatory properties. Moreover, recent evidence points to a more pronounced regulatory role of the tricarboxylic acid cycle than hitherto anticipated in the biosynthetic machinery responsible for formation of GABA from glutamine. Additionally, GABAergic neurons may contain distinct populations of mitochondria having different turnover rates of the tricarboxylic acid cycle with different levels of association with GABA synthesis from 2-oxoglutarate via glutamate. These aspects are discussed in relation to the different functional roles of GABA and its prominent involvement in epileptogenic activity.

Excitatory granule cells of the dentate gyrus exhibit a double inhibitory neurochemical content after intrahippocampal administration of kainate in adult mice

We have previously demonstrated that, in the adult mouse, injection of kainate/AMPA receptors agonists into the dorsal hippocampus induces major structural modifications of the dentate gyrus granule cells. Such changes are mediated by the brain-derived neurotrophic factor (BDNF). Considering previous involvements of BDNF in activity-linked regulations of hippocampal neuronal phenotype, changes of neurochemical contents were further investigated. It is shown that excitatory granule cells rapidly acquire a strong immunoreactivity for the inhibitory neurotransmitters GABA and neuropeptide-Y, with different patterns for both molecules. GABA immunoreactivity appeared first in mossy fibers, before extending to cell bodies and dendrites. Analysis of glutamic acid decarboxylase revealed slight increases in mossy fibers and no somatic labeling. In contrast to GABA, neuropeptide-Y labeling was observed first in granule cell soma and then in mossy fibers, with a centrifugal gradient. All labelings were transient, but slight amounts of GABA and NPY were kept in some cell bodies for at least 6 months. Confocal microscope analysis of double GABA/NPY labelings revealed colocalization of both mediators in the same neurons. The specificity of kainate-linked changes was suggested by lack of immunoreactivity for somatostatin. These results show that the capacities of mature granule cells to adapt environmental modifications can concern neurochemical contents, by synthesis and/or uptake of specific molecules. The fact that adaptive changes are rapid and transient suggests a direct response to kainate, in order to limit its potentially deleterious effects. Colocalization of GABA and neuropeptide-Y indicates that the dentate gyrus granule cells can use several pathways to this aim.

5'-Nucleotidase activity of mossy fibers in the dentate gyrus of normal and epileptic rats

Sprouting of mossy fibers in the hippocampus of rats that underwent limbic epileptogenesis by amygdala kindling or kainate injection was studied at the light microscopic and ultrastructural levels by cytochemical demonstration of the enzyme 5'-nucleotidase. This adenosine-producing ectoenzyme has previously been shown to characterize malleable terminals during brain development and lesion-induced synaptogenesis, but to be otherwise associated with glial membranes. At the light microscopic level, kainate-treated but not control or kindled rats showed 5'-nucleotidase activity in the CA3 region and in the inner molecular layer of the dentate gyrus. At the ultrastructural level, in control animals, the synapses of the molecular and granular layers were enzyme negative. Only some mossy fiber boutons of the dentate hilus exhibited 5'-nucleotidase activity. In epileptic rats, synaptic labeling within the hilus appeared more intense. Moreover, 5'-nucleotidase-containing terminals within the inner molecular layer, presumably ectopic mossy fiber boutons, were found in both kindled and kainate-treated rats. It is concluded that, in both the normal and epileptic hippocampus, 5'-nucleotidase is associated with axons capable of a plastic sprouting response. The synaptic enzyme may attenuate the glutamatergic transmission of mossy fibers, in particular of the aberrant mossy fibers in epileptic rats, by producing the inhibitory neuromodulator adenosine. Alternatively, 5'-nucleotidase may influence synapse formation by its putative non-enzymatic, adhesive functions.

Localization of cells preferentially expressing GAD(67) with negligible GAD(65) transcripts in the rat hippocampus. A double in situ hybridization study

Two major forms of glutamic acid decarboxylase (GAD) are present in the mammalian brain, a 65-kDa isoform (GAD(65)) and a 67-kDa isoform (GAD(67)), and it is usually assumed that all GABAergic neurons contain both. The two forms have not yet been colocalized to the same neurons, because the GAD(65) protein is found almost exclusively in axon terminals, while GAD(67) is found predominantly in the cell body. Using double in situ hybridization (DISH) with both radioactive [35S] and non-radioactive (digoxigenin, DIG) probes, the distributions of GAD(65) and GAD(67) mRNA have been simultaneously examined in the rat hippocampus. The results suggest that [35S] radioprobes are slightly more sensitive than DIG probes, and that the reversal of labels is necessary in DISH studies to determine whether a neuronal subtype which expresses only one isoform of GAD may be present. The data indicate that the majority of cells (90%) showing labeling were labeled for both GAD(65) and GAD(67) mRNA. In sectors CA1 and CA3 approximately 5-10% of the cells positive for GAD(67) showed little or no detectable GAD(65) mRNA. In the hilus, however, GAD(65) levels were higher, and all cells seem to express both GAD(65) and GAD(67) mRNA. Taken together, these results support the view that most GABAergic neurons in the hippocampus express both GAD(65) and GAD(67). However, it appears that some interneurons in the CA subfields differ from "classic" GABAergic interneurons by preferentially expressing the 67-kDa isoform of GAD under baseline conditions, with GAD(65) mRNA levels very low or absent.

Spiny calretinin-immunoreactive neurons in the hilus and CA3 region of the rat hippocampus: local axon circuits, synaptic connections, and glutamic acid decarboxylase 65/67 mRNA expression

We have used the Golgi method and Golgi electron microscopic techniques to analyze the axonal arborization and efferent connections of spiny calretinin-immunoreactive neurons in the CA3 region and hilus of the rat hippocampal formation. In the hilus, the axons of spiny calretinin-immunoreactive neurons sent out numerous collaterals that arborized in the hilar region and the molecular layer. In the CA3 region, these axons extended mainly to the stratum radiatum and pyramidal layer but also to the stratum oriens and stratum lacunosum-moleculare. Axonal varicosities were distributed widely throughout the axonal collaterals. Electron microscopic studies revealed that the axon terminals of spiny calretinin-immunoreactive neurons established synaptic contacts mainly with dendritic shafts. We next analyzed the expression of glutamic acid decarboxylase (GAD65/67) mRNAs in spiny nonpyramidal neurons that were identified by calretinin immunoreactivity. We found that spiny calretinin-positive neurons in the CA3 region and hilus of the rat hippocampal formation expressed the two isoforms of GAD: GAD65 and GAD67 mRNAs. These findings show that the spiny calretinin-immunoreactive neurons of hippocampus give rise to local axonal arborizations, suggesting that they are inhibitory.

Galanin modulation of seizures and seizure modulation of hippocampal galanin in animal models of status epilepticus

We examined the role of hippocampal galanin in an animal model of status epilepticus (SE). Control rats showed abundant galanin-immunoreactive (Gal-IR) fibers in the dentate hilus, whereas no Gal-IR neurons were observed. Three hours after the onset of self-sustaining SE (SSSE), induced either by intermittent stimulation of the perforant path for 30 min (PPS) or by injection of lithium and pilocarpine, Gal-IR fibers disappeared in the hilus and remained absent for up to 1 week afterward. Twelve hours after the induction of SE by PPS or 3 hr after pilocarpine administration, Gal-IR neurons appeared in the hilus; these neurons increased in number after 1 d and gradually declined 3 and 7 d later. Galanin concentration in the hippocampus, measured by ELISA, significantly decreased on the plateau of SSSE and increased 24 hr after PPS. Galanin (0.05 nmol) injected into the hilus prevented the induction of SSSE, and 0.5 nmol of galanin stopped established SSSE. These effects were attenuated by galanin receptor antagonists (M35 > M40 >/= M15). 2-Ala-galanin (5 nmol), a putative agonist of galanin type 2 receptors, prevented but was unable to stop SSSE. M35 facilitated the development of SSSE when given before PPS. We suggest that hippocampal galanin acts as an endogenous anticonvulsant via galanin receptors. SE-induced galanin depletion in the hippocampus may contribute to the maintenance of seizure activity, whereas the increase of galanin concentration and the appearance of galanin-immunoreactive neurons may favor the cessation of SSSE. The seizure-protecting action of galanin SSSE opens new perspectives in the treatment of SE.

Selective changes in single cell GABA(A) receptor subunit expression and function in temporal lobe epilepsy

Temporal lobe epilepsy is the most prevalent seizure disorder in adults. Compromised inhibitory neurotransmitter function in the hippocampus contributes to the hyperexcitability generating this condition, but the underlying molecular mechanisms are unknown. Combining patch-clamp recording and single-cell mRNA amplification (aRNA) techniques in single dentate granule cells, we demonstrate that expression of GABA(A) receptor subunit mRNAs is substantially altered in neurons from epileptic rats. These changes in gene expression precede epilepsy onset by weeks and correlate with profound alterations in receptor function, indicating that aberrant GABA(A) receptor expression and function has an essential role in the process of epileptogenesis.

Changes in extracellular glutamate and GABA levels in the hippocampal CA3 and CA1 areas and the induction of glutamic acid decarboxylase-67 in dentate granule cells of rats treated with kainic acid

For the evaluation of glutamatergic and GABAergic transmission during seizures, rat hippocampal CA1 and CA3 areas were separately assessed by brain microdialysis, and extracelluar glutamate and GABA were measured through the course of the seizures after a systemic administration of kainic acid (KA). The generalized convulsion started at about 1.5 h and was suppressed by diazepam at 2 h after the KA treatment. In the CA3 area, extracellular glutamate started to increase soon after the KA injection and returned to the control level at about 1.5 h. A decrease and then slight increase of the extracellular glutamate level in CA3 followed the diazepam injection. In the CA1 area, in contrast, a long-lasting decrease of extracellular glutamate was observed. The extracellular GABA concentration in the CA3 area increased immediately after the systemic administration of KA and returned to the normal level at about 3.5 h. A second increase in the extracellular GABA in this area began at about 4.5 h after the KA treatment. In the CA1 area, an increase of extracellular GABA began at about 3.5 h after KA administration (much later than that observed in the CA3 area) and was maintained throughout the observation. In situ hybridization showed a transient expression of glutamic acid decarboxylase (GAD)-67 mRNA in the granule cell layer of the dentate gyrus at 4 and 6 h, whereas GAD65 mRNA was unaffected. GABA immunoreactivity in the same area and mossy fibers in the CA3 were increased most significantly at 8 h after administration of KA. The possible relation of GABA induction in mossy fibers with the delayed increase in extracellular GABA in CA3 was discussed.

Regulation of cell-type specific expression of lacZ by the 5'-flanking region of mouse GAD67 gene in the central nervous system of transgenic mice

The transcriptional regulation of the murine gene encoding the 67-kDa form of glutamic acid decarboxylase (GAD67) was studied by beta-galactosidase histochemistry in transgenic mice carrying fusion genes between progressively longer portions of the 5'-upstream regulatory region of GAD67 and E. coli lacZ. No expression was detected in brains of mice carrying 1.3 kb of upstream sequences including a housekeeping and two conventional promoters, and two negative regulatory elements with homology to known silencers. In mice carrying the same portion of the promoter region plus the first intron, lacZ expression in the adult central nervous system was found in few, exclusively neuronal sites. The number of correctly stained GABAergic centres increased dramatically with increasing the length of the 5'-upstream region included in the construct which suggests that multiple putative spatial enhancers are located in this region. Their action is influenced by epigenetic mechanisms that may be due to site-of-integration and transgene copy-number effects. Additional cis-acting elements are needed to obtain fully correct expression in all GABAergic neurons of the adult central nervous system.

Effects of a subconvulsant dose of kainic acid on afterdischarges elicited by cortical stimulation in rats

The aim of this study was to test the hypothesis that nonconvulsive seizures elicited by a low dose of kainic acid may induce acute as well as chronic changes in brain function. Cortical epileptic afterdischarges (ADs) characterized by spike-and-wave rhythm and clonic seizures of facial and forelimb muscles were elicited in adult male rats with chronically implanted electrodes. Four stimulations were given in each of four weekly sessions. In the second session, 26 animals were injected with kainic acid (6 mg/kg i.p.) and 19 rats received no injection. The acute effects of kainic acid were to increase the intensity of movements accompanying stimulation and abruptly prolong ADs. Epileptic ADs were followed by a depression of electrocorticographic activity in both noninjected and kainic acid groups. In addition, when kainate was administered, interictal spike activity was registered mostly in the occipital region. One and two weeks after kainate administration, i.e. in the third and fourth stimulation sessions, there was an increased incidence of transitions from spike-and-wave ADs to another, limbic type of afterdischarge. This functional change persisted although no obvious neuronal death was found in the hippocampi of 12 other rats that received the same dose of kainic acid.

GABA(A) receptor subunits in the rat hippocampus III: altered messenger RNA expression in kainic acid-induced epilepsy

Kainic acid-induced seizures in rats represent an established animal model for human temporal lobe epilepsy. The neuropathological sequelae include acute status epilepticus followed by neurodegeneration in the CA1 and CA3 sector of the Ammon's horn and of interneurons in the hilus of the dentate gyrus. After about three weeks spontaneous recurrent seizures become manifest. We investigated changes in messenger RNA expression of 13 GABA(A) receptor subunits in the hippocampus of rats in the initial phase (6 h, 12 h and 24 h) after acute kainic acid-induced status epilepticus and seizure-related neuronal cell damage during and after acquisition of spontaneous recurrent seizures (seven and 30 days after kainic acid injection). In the granule cell layer, initial (after 6 to 12 h) decreases in (alpha2, alpha3, alpha5, beta1, beta3, gamma2 and delta messenger RNAs (by about 25 to 50%) were accompanied by increases (by about 50%) in alpha1, alpha4, and beta2 messages. At later intervals (after seven to 30 days), expression of alpha2, alpha4, beta3 and gamma2 messenger RNAs recovered to control values, with alpha5 and delta messenger RNA still being reduced (by 15 and 40% below control levels, respectively). Concentrations of the transcripts encoding for alpha1, alpha3, beta1, beta2, became markedly enhanced (between 20 and 50% of controls). Within the pyramidal cell layers CA1 and CA3, decreases in alpha2, alpha4, alpha5, beta(1-3) and gamma2 messenger RNAs were detected after seven to 30 days, reflecting pronounced neurodegeneration in these areas. The alpha1 transcript was decreased in CA3 after 24 h and increased to control levels indicating compensatory up-regulation of this message after seven days. Messenger RNAs encoding for alpha3-, gamma1-, and gamma3-subunits were detected at rather low levels, alpha6 was not present in the hippocampus. Our data suggest a fast but transient change in the expression of messenger RNAs encoding for different subunits of the GABA(A) receptor in the granule cell layer of the dentate gyrus. This is followed by a lasting augmentation of messenger RNAs encoding different GABA(A) receptor subunits in the same cell layer indicating long-lasting GABAergic inhibition. Changes within the pyramidal cell layer are mostly determined by concomitant neurodegenerative processes.

GABA(A) receptor subunits in the rat hippocampus II: altered distribution in kainic acid-induced temporal lobe epilepsy

Intraperitoneal injection of kainic acid in the rat represents a widely used animal model of human temporal lobe epilepsy. Injection of kainic acid induces acute limbic seizures which are accompanied by seizure-induced brain damage and late spontaneous recurrent seizures. There is considerable evidence for an altered transmission of GABA in human temporal lobe epilepsy and in the kainic acid model. We therefore investigated by immunocytochemistry the distribution of 13 GABA receptor subunits in the hippocampus of rats 12 h, 24 h, and two, seven and 30 days after injection of kainic acid. Within the molecular layer of the dentate gyrus, decreases in alpha2- and delta- and slight increases in alpha1, beta2- and beta3-immunoreactivities were observed at early intervals (12 to 24 h) after kainic acid injection. These changes were succeeded by marked increases in alpha1-, alpha2-, alpha4-, alpha5-, beta1-, beta3-, gamma2- and delta-immunoreactivities in the same area after seven to 30 days. Within the hippocampus proper, changes in expression of GABA(A) receptor subunits were demarcated by considerable neurodegeneration of CA1 and CA3 pyramidal neurons. All subunits present within dendritic areas of CA1 and CA3 were affected. These were alpha1, alpha2, alpha5, beta1-beta3, gamma2 and alpha4 (present only in CA1). Decreases in these subunits were followed by increased expression of alpha2-, alpha5-, beta3-, gamma2- and delta-subunits in the hippocampus proper notably in CA3 at later intervals (up to 30 days). Alpha1-, beta2-, gamma2- and delta-subunits were found in presumed GABA containing interneurons throughout the hippocampus. Their immunoreactivity was augmented after two to seven days. Some alpha4-, gamma3- and delta-immunoreactivity was also found in astrocytes 48 h after kainic acid injection. Our data indicate an impairment of GABA-mediated neurotransmission due to a lasting loss of GABA(A) receptor containing cells after kainic acid-induced seizures. The seizure-induced loss in GABA(A) receptors within the hippocampus may in part be compensated by increased expression of GABA(A) receptor subunits within the molecular layer of the dentate gyrus and in pyramidal cells.

Structure and alternative promoters of the mouse glutamic acid decarboxylase 67 gene

gamma-Aminobutyric acid is synthesized by glutamic acid decarboxylase (GAD), which has two forms, GAD65 and GAD67. Genomic clones coding mouse GAD67 (mGAD67) have been isolated. The restriction map of the overlapping clones covers a region of more than 45 kb of genomic DNA. The mGAD67 gene contains 16 translated exons in addition to an exon which is preferentially expressed in foetal brain. The rapid amplification of 5'-cDNA ends showed that mGAD67 gene transcripts have two different 5'-untranslated regions. Analysis of the genomic clones encompassing the 5'-exons revealed that the two transcripts arose from a single gene by alternative splicing using two different donor sites and a common acceptor. The exons were found 1.5 and 0.6 kb upstream of exon 1. The corresponding promoter regions of these exons have a number of putative regulatory elements, including Sp1- and Krox-24-binding sites. Analysis of mGAD67 transcripts demonstrated that each of the 5'-untranslated exons was expressed in mouse brain. In contrast, exon 0A, but not exon 0B, was expressed in mouse testis and pancreas. These results suggest that these transcripts may be regulated under the control of independent promoters.

Induction of tolerance and mossy fibre neuropeptide-Y expression in the contralateral hippocampus following a unilateral intrahippocampal kainic acid injection in the rat

We have previously reported an ectopic expression of neuropeptide-Y (NPY) immunoreactivity in mossy fibres (MFs) in the contralateral hippocampus following a unilateral intrahippocampal (IH) injection of kainic acid (KA). In the present study we report that, in addition to MF NPY expression, unilateral IH KA injections also induce tolerance towards a subsequent intracerebroventricular (ICV) contralateral KA injection, resulting in a reduction in the number of overt seizures and degree of cell loss.

Delayed decrease of calbindin immunoreactivity in the granule cell-mossy fibers after kainic acid-induced seizures

Kainic acid (KA) administration induces an abnormal excitation and spontaneous recurrent seizures. Alterations of granule cell properties may be potential mechanisms. In this study, dynamic alterations of calbindin, a calcium binding protein particularly abundant in the granule cells, have been investigated immunocytochemically in the rat hippocampus after the KA-induced seizures. The calbindin immunoreactivity decreased slightly in the CA1/CA2 fields already after 1 and 3 days, and was lost partly or completely in the pyramidal layer after 10 days. From day 21, the calbindin immunoreactivity decreased in dendrites and soma of the granule cells and mossy fibers. The alterations remained at least to day 90, while no evident neuronal loss occurred in the granule cells. This may reflect a disturbance of calcium homostasis in the granule cells after seizures. The delayed decrease of calbindin has a time course similar to the occurrence of spontaneous recurrent seizures, suggesting a possible correlation between the two events.

Adrenocorticotropic hormone immunoreactivity in the hippocampal formation of temporal lobe epilepsy patients

PURPOSE

We wished to identify immunocytochemically the distribution of proopiomelanocortin-related peptides in the hippocampal formation of patients with epilepsy.

METHODS

Surgical hippocampal specimens from temporal lobe epilepsy (TLE) patients and autopsy control tissue were examined immunocytochemically for ACTH, alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin.

RESULTS

There was a dense distribution of ACTH-immunoreactive neurons in the hippocampal formation of patients with mesial TLE syndrome (MTLE). These hippocampal specimens showed significant cell loss. ACTH-positive neurons were most prominent in the subiculum, with scattered ACTH-immunoreactive neuronal elements distributed in the cornu ammonis fields and hilus. Light ACTH immunoreactivity was detected in the tumor-related epileptic hippocampal specimens, which showed minimal cell loss. Although autopsy control tissue from the hypothalamus showed intense ACTH staining patterns in cells and fibers, there was little or no ACTH immunoreactivity in the autopsy hippocampal tissue. The expression of ACTH immunoreactive elements was correlated with patterns of cell loss. No alpha-MSH- or beta-endorphin-immunoreactive neurons were detected in any of the hippocampal specimens.

CONCLUSIONS

ACTH has anticonvulsant properties, and its novel expression in the glutamatergic subicular neurons, which provide the main outflow of the hippocampal formation, may represent an attempt by the damaged hippocampal circuit to restore the balance of excitatory/inhibitory neurotransmission in TLE.

Basal expression and induction of glutamate decarboxylase and GABA in excitatory granule cells of the rat and monkey hippocampal dentate gyrus

The excitatory, glutamatergic granule cells of the hippocampal dentate gyrus are presumed to play central roles in normal learning and memory, and in the genesis of spontaneous seizure discharges that originate within the temporal lobe. In localizing the two GABA-producing forms of glutamate decarboxylase (GAD65 and GAD67) in the normal hippocampus as a prelude to experimental epilepsy studies, we unexpectedly discovered that, in addition to its presence in hippocampal nonprincipal cells, GAD67-like immunoreactivity (LI) was present in the excitatory axons (the mossy fibers) of normal dentate granule cells of rats, mice, and the monkey Macaca nemestrina. Using improved immunocytochemical methods, we were also able to detect GABA-LI in normal granule cell somata and processes. Conversely, GAD65-LI was undetectable in normal granule cells. Perforant pathway stimulation for 24 hours, which evoked population spikes and epileptiform discharges in both dentate granule cells and hippocampal pyramidal neurons, induced GAD65-, GAD67-, and GABA-LI only in granule cells. Despite prolonged excitation, normally GAD- and GABA-negative dentate hilar neurons and hippocampal pyramidal cells remained immunonegative. Induced granule cell GAD65-, GAD67-, and GABA-LI remained elevated above control immunoreactivity for at least 4 days after the end of stimulation. Pre-embedding immunocytochemical electron microscopy confirmed that GAD67- and GABA-LI were induced selectively within granule cells; granule cell layer glia and endothelial cells were GAD- and GABA-immunonegative. In situ hybridization after stimulation revealed a similarly selective induction of GAD65 and GAD67 mRNA in dentate granule cells. Neurochemical analysis of the microdissected dentate gyrus and area CA1 determined whether changes in GAD- and GABA-LI reflect changes in the concentrations of chemically identified GAD and GABA. Stimulation for 24 hours increased GAD67 and GABA concentrations sixfold in the dentate gyrus, and decreased the concentrations of the GABA precursors glutamate and glutamine. No significant change in GAD65 concentration was detected in the microdissected dentate gyrus despite the induction of GAD65-LI. The concentrations of GAD65, GAD67, GABA, glutamate and glutamine in area CA1 were not significantly different from control concentrations. These results indicate that dentate granule cells normally contain two "fast-acting" amino acid neurotransmitters, one excitatory and one inhibitory, and may therefore produce both excitatory and inhibitory effects. Although the physiological role of granule cell GABA is unknown, the discovery of both basal and activity-dependent GAD and GABA expression in glutamatergic dentate granule cells may have fundamental implications for physiological plasticity presumed to underlie normal learning and memory. Furthermore, the induction of granule cell GAD and GABA by afferent excitation may constitute a mechanism by which epileptic seizures trigger compensatory interictal network inhibition or GABA-mediated neurotrophic effects.

Immunocytochemical localization of GABA immunoreactivity in dentate granule cells of normal and kindled rats

In order to identify lasting alterations in gamma-aminobutyric acid (GABA) neurons in the kindling model of epilepsy, immunocytochemical techniques were used to quantify the number of GABA-immunoreactive (IR) neurons in different regions of the hippocampal formation (HCF) of amygdala-kindled rats, 40 days after the last fully kindled seizure. A new, highly specific monoclonal GABA antibody was used for these experiments. Unexpectedly, the antibody not only stained neurons in CA1, CA3, and hilus, but also intensively stained granule cells (GCs) in the dentate gyrus (DG) of both kindled rats and non-kindled controls, indicating that GCs may be capable of synthesizing GABA. Comparison with a polyclonal GABA and a glutamate decarboxylase antibody showed that staining of GCs with the monoclonal GABA antibody was much more intense. The number of GABA-IR cells that were counted in different regions of the HCF, including the DG, did not differ significantly between kindled rats and controls, which does not support the hypothesis of loss of hippocampal GABAergic neurons to explain the permanency of kindled epileptogenesis.