Developmental changes of glutamate receptors in the rat cerebral cortex and hippocampus

We studied the immunohistochemial localization of the glutamate receptors (GluR-1, -2, and -3,) in the developing rat cerebral cortex and hippocampus using antibodies to GluR1 and to an epitope common to GluR2 and GluR3 (GluR2/3) subunits. In the cerebral cortex, GluR1 immunoreactivity appeared in the neurons from postnatal day (PND) 0, increased with maturation, was highest at PND?10, decreased until PND 30, and thereafter remained at the same level as on PND?0. GluR2/3 immunoreactivity appeared earlier in scattered neurons on embryonal day (ED) 18, increased with maturation and reached a peak between PND?10 and PND?15, after which the immunoreactivity gradually decreased and reached a plateau at PND?30. For both GluR1 and GluR2/3, some of the pyramidal neurons showed intense staining. In the pyramidal layers of the hippocampus, GluR1 and GluR2/3 immunoreactivity was found in all the pyramidal neurons of the CA1–4 area from ED?20. In the dentate gyrus of the hippocampus, GluR1 and GluR2/3 immunoreactivity was found in the neurons of the granule cells after PND?0. Immunoreactivity in the neurons of the subiculum was found after PND?5 and that of the polymorphic cell layers was found after PND?15–20. Our results indicate that the development of glutamate receptor subunits in the rat cerebral cortex and hippocampus is expressed in different spatial patterns and distinct temporal patterns throughout development and is scheduled during the early postnatal period, when synaptic plasticity or synaptic connection occurs in these regions.     

Differential localisation of the metabotropic glutamate receptor mGluR1a and the ionotropic glutamate receptor GluR2/3 in neurons of the human cerebral cortex

Specimens of human cerebral cortex were obtained during neurosurgical operations and studied by immunocytochemistry and electron microscopy, using antibodies to the metabotropic glutamate receptor subunit mGluR1a and the ionotropic glutamate receptor GluR2/3. A small number of non-pyramidal neuronal cell bodies were labelled for mGluR1a. Double immunolabelling with mGluR1a and GluR2/3 showed that most pyramidal cell bodies were labelled for GluR2/3 but not for mGluR1a. Despite the non-colocalisation of these two receptor subtypes in cell bodies, however, many dendrites and dendritic spines were double-labelled for mGluR1a and GluR2/3 at electron microscopy. As there is evidence that most neurons positive for GluR2/3 are pyramidal cells, this suggests that mGluR1a is present in dendrites of pyramidal neurons, despite absent or low levels of immunoreactivity in their cell bodies. Received: 5 May 1997 / Accepted: 24 July 1997     

Expression of glutamate receptor subunits 2/3 and 4 in the hypoglossal nucleus of the rat after neurectomy

The immunoreactivity of glutamate receptor subunits 2/3 (GluR2/3) and 4 (GluR4) was studied following neurectomy of the hypoglossal nucleus (NH). After a short period of survival (at 1, 2, and 7 days postoperation, dpo), GluR2/3 immunoreactivity was barely dectectable in the operated side of HN. During these periods, GluR4 immunoreactivity was present, but was greatly reduced when compared with the GluR4 immunoreactivity in the unoperated side. The data suggest that of the 4 subunits of the AMPA receptor, GluR2/3 is the most susceptible receptor to the early stage of hypoglossal neurectomy, and GluR4 tolerated the lesion more than the others. It is also suggested that both GluR2/3 and 4 may play a very important neuroprotective role in the early stage of neuronal degeneration after axotomy, especially the former. Following a midterm survival period (14, 21, and 35 dpo), GluR2/3 immunoreactivity gradually reappeared in some neurons on the operated side of HN, which may indicate functional recovery. However, the number of GluR4-immunopositive neurons on the operated side of HN was greatly reduced. The reason for such a reduction is not known, but, from the speculative point of view, it is possible that the disappearance of GluR4-positive neurons may be related to their excitotoxic property, especially at 35 dpo, when neuronal cell death had already occurred. Following a long-term period of survival (i.e., 56, 90, and 120 dpo), the numbers of surviving neurons remained fairly constant, suggesting the possible cessation of neuronal death. Received: 15 April 1997 / Accepted: 13 June 1997     

Distribution of glutamate receptor subunit GluR1 and GABA in human cerebral neocortex: a double immunolabelling and electron microscopic study

Specimens of human cerebral neocortex were obtained during neurosurgical operations and studied by immunocytochemistry and electron microscopy, using antibodies to the glutamate receptor subunit GluR1 and gamma-aminobutyric acid (GABA). Many GluR1-positive pyramidal neurons and fewer GluR1-positive nonpyramidal neurons were present in the cortex. Non-pyramidal neurons were more heavily labelled for GluR1 than pyramidal neurons. Most GABAergic neurons were labelled for GluR1. The white matter was unstained, except for occasional labelled neurons. This pattern of GluR1 immunostaining is similar to that in rat cerebral cortex, but is different from that in the hippocampus and amygdala, where large numbers of pyramidal or projection neurons, but few non-pyramidal or GABAergic neurons, were labelled for GluR1.     

An immunocytochemical study of glutamate receptors and glutamine synthetase in the hippocampus of rats injected with kainate

Immunocytochemistry was used to study the distribution of the kainate receptors GluR1, GluR2/3 and GluR4 and of the N-methyl-d-aspartate (NMDA) receptor NMDAR1 as well as the astrocyte markers glutamine synthetase (GS) and glial fibrillary acidic protein (GFAP) in the hippocampus of normal and kainate-lesioned rats. Hippocampal pyramidal neurons and dentate granule neurons were labelled heavily for GluR1 and GluR2/3, but only lightly for GluR4. Dense GluR4 immunopositivity was, however, observed in oligodendrocyte-like glial cells. Hippocampal pyramidal neurons and dentate granule neurons were moderately labelled for NMDAR1. Intravenous kainate injections resulted in a decrease in GluR1 and GluR2/3 immunoreactivity on the apical dendrites of pyramidal neurons as early as 7 h postinjection. At 18 h, there was a marked reduction in GluR1 and GluR2/3 receptors in the terminal tuft of dendrites of most hippocampal pyramidal neurons in the affected area, although some cells showed labelling in other portions of the apical dendrites and in basal dendrites. Immunostaining for GluR4 and NMDAR1 was also reduced at this time. At postinjection day 3, only the cell bodies and the basal dendrites of a few scattered pyramidal cells were labelled. Taken together, these results indicate a progressive loss of glutamate receptors, which affects the apical dendritic tree before the basal dendritic tree. The decrease in receptor immunoreactivity could be due to a downregulation of the receptors, since it occurred as early as 7 h postlesion, before cell death was evident in Nissl-stained sections. At long intervals after kainate injection, all pyramidal cells at the centre of the lesion showed a lack of glutamate receptor staining, and no partially labelled pyramidal cells were observed. The periphery of the lesion, however, contained many partially labelled pyramidal neurons among the unlabelled cells and had features of early lesions. The present study also showed an early decrease in GS immunoreactivity in the affected CA fields of the hippocampus (18 h to 3 days postinjection), followed by a medium-term increase (5–68 days) and a late decrease in GS immunoreactivity (81 days). The decrease in GS immunoreactivity at 81 days is not due to an absence of astrocytes, since GFAP staining showed many densely labelled astrocytes in the affected CA field.     

A light- and electron-microscopic study of GluR4-positive cells in cerebral cortex,subcortical white matter and corpus callosum of neonatal,immature and adult rats

The distribution of the [³H]alpha-amino-3-hydroxy-5-methylisoxzalepropionic acid (AMPA) receptor subunit GluR4 was studied in frontal, parietal and temporal cerebral cortex, subcortical white matter and corpus callosum of neonatal, immature and mature rats. In 1-to 2-day-old rats, a few oligodendrocyte progenitors and amoeboid microglia in the supraventricular part of the corpus callosum were immunolabelled for GluR4. At 7 to 10 days, the number of amoeboid microglia and oligodendrocyte progenitors in white matter increased; many neurons in cortex, including pyramidal neurons, were also moderately labelled for GluR4. The pattern of GluR4 immunostaining in 14-day-old rats was different from that in 7-to 10-day-old rats, but similar to the adult, in that there was no immunoreactivity in microglia and oligodendrocyte progenitors in subcortical white matter. A proportion of non-pyramidal neurons in cortex were moderately labelled, while some pyramidal neurons were lightly labelled. A population of small glial cells with features of oligodendrocyte progenitors were densely labelled in cortex.     

Changes of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors in layer V of epileptogenic, chronically isolated rat neocortex

In vivo chronic partial isolation of neocortical islands results in epileptogenesis that involves pyramidal neurons of layer V. To test whether an alteration in glutamate receptors might contribute to the epileptiform activity, we analysed the time-course of light microscopic changes in expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors using subunit-specific antibodies. The isolation caused a rapid down-regulation of immunoreactivity for GluR1 and GluR2/3 subunits in deep layer V pyramidal neurons within the neocortical island which was evident 24h post-lesion, and within three days was reduced to about 40-60% of the control level. Many pyramidal cells in deep layer V completely lacked GluR2. Between one and four weeks of survival, down-regulation of GluR2/3 and GluR2 involved the majority of pyramidal layer V neurons, except for cells in the upper part of layer V, and those within narrow areas of all sub-laminae of layer V ("micro-islands"). Initial down-regulation was also observed one to three days post-lesion for subunits 1 and 2 of the N-methyl-D-aspartate receptor, but in contrast to GluR2/3 immunoreactivity, NMDAR2A/B immunoreactivity was enhanced three weeks post-lesion. The present data provide evidence for plastic changes in glutamate receptors in neurons of partially isolated neocortical island. A sub-population of layer V neurons remains relatively unaffected, and would presumably be capable of generating fast glutamatergic synaptic potentials necessary for the development of synchronous epileptiform activity.     

Dual (excitatory and inhibitory) calretinin innervation of AMPA receptor-containing neurons in the rat lateral septum

A recent study demonstrated both an extrinsic and an intrinsic calretinin (CR) innervation of the rat septal complex and that a population of the extrinsic calretinin fibers is aspartate/glutamate-containing. The aim of this study was to determine which types (GluR1, GluR2/3, or both) of AMPA receptor-containing lateral septal area neurons are innervated by extrinsic and intrinsic CR neurons and whether the intrinsic CR cells are GABAergic. Light- and electron-microscopic single immunostaining for CR, GluR1, and GluR2/3, as well as light- and electron-microscopic-double immunostaining experiments for CR plus GluR1 and CR plus GluR2/3 were performed in the lateral septal area. Furthermore, the ″mirror″ colocalization technique was employed on consecutive vibratome sections of the septal complex to investigate whether the intrinsic septal CR neurons are GABAergic. The results are summarized as follows: (1) both GluR1- and GluR2/3-immunoreactive neurons are innervated by CR-containing fibers; (2) the majority of these synapses, observed mainly on the soma and, to a lesser extent, on proximal dendrites of AMPA receptor-containing neurons, represent asymmetric synaptic membrane specializations; (3) a minority of CR-containing axon terminals associated with both GluR1- and GluR2/3-immunoreactive neurons form symmetric contacts, predominantly on their soma; and (4) 93% of the lateral septal area CR cells are GABAergic. These observations indicate that both GluR1- and GluR2/3-containing lateral septal area neurons receive a dual intrinsic and extrinsic CR innervation. The former (intrinsic) CR boutons are GABAergic, while the latter form asymmetric synaptic contacts, are excitatory, and probably originate in the supramammillary area, since previous work has demonstrated that a population of supramammillo-septal fibers contain aspartate and/or glutamate. Received: 23 May 1997 / Accepted: 21 August 1997     

Parvalbumin-containing interneurons in rat hippocampus have an AMPA receptor profile suggestive of vulnerability to excitotoxicity

-Amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors mediate excitatory neurotransmission in the central nervous system, and contain combinations of four subunits (GluR1-4). We developed a GluR3-specific monoclonal antibody and quantified the cellular distribution of GluR3 in rat hippocampus. GluR3 immunoreactivity was detected in all pyramidal neurons and most interneurons. In addition, we found a subset of parvalbumin (PV)-containing interneurons in the hippocampus and neocortex that was notable for its intense GluR3 immunoreactivity and lack of GluR2 immunoreactivity. Such an expression pattern of AMPA receptor subunits is likely to make these interneurons selectively vulnerable to excitotoxicity.     

脑缺血再灌注大鼠半暗带皮质谷氨酸受体相互作用蛋白表达的增加

目的 探讨脑缺血再灌注大鼠半暗带皮质中谷氨酸受体相互作用蛋白(GRIP)表达变化的规律.方法 SD雄性大鼠,随机分为脑缺血2h后再灌注1、3、6、12、24、72h组和假手术组(每组6只大鼠),采用线栓-再通法制作大脑中动脉闭塞模型,于再灌注后相应时间点处死大鼠取脑,免疫组织化学方法观察GRIP 在皮质中的表达及分布,...     

The development of excitatory transmitter amino acid-containing neurons in the rat visual cortex

The excitatory amino acids l-glutamate and l-aspartate are believed to be utilized as neurotransmitters by the pyramidal neurons in the mammalian cerebral cortex. These cells can be recognized early in development, while glutamate might play an important part in the maturation and plasticity of the cortex. Here, we used light and electron microscopic immunocytochemistry to study the time of appearance and maturation of glutamate and aspartate in neurons of the rat visual cortex. Glutamate- and aspartate-immunoreactive cells were first detected in deep cortical layers at postnatal day 3. During the next 3 weeks, labelled neurons were observed progressively in more superficial layers, but did not demonstrate their adult pattern of distribution until postnatal week 4. Electron microscopic analysis showed that glutamate- and aspartate-labelled neurons gradually develop their cytological and synaptic features during the first 4 postnatal weeks, with this process of differentiation originating in the deep cortical layers and progressively extending to the superficial layers. These findings suggest that cortical pyramidal neurons begin to express detectable levels of transmitter glutamate and/or aspartate after they have completed their migration. Their neurochemical differentiation follows an inside-out pattern similar to the pattern described for the genesis and morphological differentiation of this population of cortical neurons.     

Brainstem and spinal projections of augmenting expiratory neurons in the rat

To examine the role of Ca(2+) entry through AMPA receptors in the pathogenesis of the ischemia-induced cell death of hippocampal neurons, we delivered cDNA of Q/R site-unedited form (GluR2Q) of AMPA receptor subunit GluR2 in the hippocampus by using an HVJ-liposome-mediated gene transfer technique. Two days prior to transient forebrain ischemia, we injected an HVJ-liposome containing cDNA of the GluR2Q-myc fusion gene into a rat unilateral hippocampus. In the absence of ischemic insult, overexpression of Ca(2+)-permeable GluR2Q did not cause any neurodegeneration in the cDNA-injected hippocampus. In ischemic rats, overexpression of Ca(2+)-permeable GluR2Q markedly promoted ischemic cell death of CA1 pyramidal neurons, while complete rescue of CA1 pyramidal neurons from ischemic damage occurred in the hippocampal hemisphere opposite the GluR2Q expression. Overexpression of the Q/R-site edited form (GluR2R) of subunit GluR2 did not affect the ischemia-induced damage of CA1 pyramidal neurons. From these results, we suggest that the Ca(2+)-permeability of AMPA receptors does not have a direct contribution to glutamate receptor-mediated neurotoxicity but has a promotive action in the evolution of ischemia-induced neurodegeneration of vulnerable neurons.     

Activation of metabotropic glutamate receptors does not alter the phosphorylation state of GluR1 AMPA receptor subunit at serine 845 in perirhinal cortical neurons

Activation of group I and group II metabotropic glutamate receptors (mGluRs) is thought to be required for long-term depression (LTD) of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor-mediated synaptic transmission in the perirhinal cortex. However, little is known about how activation of mGluRs leads to this form of synaptic plasticity. AMPA receptor phosphorylation has been implicated in several forms of modulation of synaptic transmission. In the CA1 area of the hippocampus, N-methyl-d-aspartate (NMDA) receptor-dependent LTD is associated with the reduced phosphorylation of the GluR1 AMPA receptor subunit at serine 845 (GluR1-S845). Immunoblot analysis of perirhinal cortical neurons using GluR1 and GluR1-S845 phosphorylation state specific antibodies showed that stimulation of adenylyl cyclase (AC) with forskolin (FSK) dramatically increased PKA-mediated phosphorylation of GluR1-S845. However, selective or simultaneous application of mGluR5 agonist (S)-3,5-dihydroxyphenylglycine (CHPG) and mGluR2/3 agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) did not produce detectable changes in GluR1-S845 phosphorylation. These results indicate that in the perirhinal cortex mGluR activation does not alter the phosphorylation state of GluR1-S845. Therefore, it is likely that the process involved in the modification of AMPA receptors in mGluR activation dependent LTD in the perirhinal cortex is mechanistically distinct from NMDA receptor-mediated LTD described in hippocampal neurons.     

Neurochemical characterization of AMPA receptor-containing neurons in the mediolateral septal area of the rat

 The lateral septum receives a massive innervation by excitatory amino acid-containing limbic cortical and hypothalamic afferents, and previous studies have described a wide distribution of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor-containing neurons in this area. The aim of this study was to determine whether different subtypes of AMPA receptors are expressed in the same neurons. Furthermore, considering the fact that a population of lateral septal cells, the ”somatospiny neurons,” are GABAergic calbindin-containing cells, the coexistence of each subtype of AMPA receptor with calbindin was also investigated. Colocalization experiments were performed on adjacent vibratome sections of the lateral septal area for GluR1 and GluR2/3 AMPA-receptor subunits, GluR1 and calbindin, GluR2/3 and calbindin, as well as GluR1 plus calbindin and GluR2/3 plus calbindin, using the ”mirror” colocalization technique. The results are summarized as follows: (1) GluR1 is present in the soma and most intensively expressed in dendrites and somatic and dendritic spines; while GluR2/3 is associated with the soma and proximal dendrites of the neurons. (2) Forty-one percent of the AMPA receptor-containing neurons cocontain GluR1 and GluR2/3. (3) Thirty-eight percent of GluR1- and 28% of GluR2/3-labeled cells express calbindin. (4) Sixty-two percent of the calbindin-immunoreactive neurons contain GluR1 and 51% of them express GluR2/3. (5) Half of the neurons expressing both GluR1 and GluR2/3 also contain calbindin. (6) The distribution of GluR1 plus GluR2/3-containing, GluR1 plus calbindin-containing, and GluR2/3 plus calbindin-containing neurons in the lateral septum are homogeneous. This study indicates the existence of multiple populations of AMPA receptor- and calbindin-containing neurons in the lateral septal area. Received: 9 August 1996 / Accepted: 7 November 1996     

Activation of muscarinic receptors modulates NMDA receptor-mediated responses in auditory cortex

 The present study examines the ability of muscarinic receptor activation to modulate glutamatergic responses in the in vitro rat auditory cortex. Whole-cell patch-clamp recordings were obtained from layer II-III pyramidal neurons and responses elicited by either stimulation of deep gray matter or iontophoretic application of glutamate receptor agonists. Iontophoresis of the muscarinic agonist acetyl-β-methylcholine (MCh) produced an atropine-sensitive reduction in the amplitude of glutamate-induced membrane depolarizations that was followed by a long-lasting (at least 20 min) response enhancement. Glutamate depolarizations were enhanced by MCh when elicited in the presence of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/ kainate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or 2,3-diyhdroxy-6-nitro-7-sulfamoyl, benzo(F)quinoxaline (NBQX) but not the NMDA antagonists d-2-amino-5-phosphonovaleric acid (APV) or MK-801 hydrogen maleate. The magnitude of enhancement was voltage-dependent with the percentage increase greater at more depolarized membrane potentials. An involvement of NMDA receptors in these MCh-mediated effects was tested by using AMPA/kainate receptor antagonists to isolate the NMDA-mediated slow excitatory postsynaptic potential (EPSP) from other synaptic potentials. The slow EPSP and iontophoretic responses to NMDA were similarly modified by MCh, i.e., both being reduced during and enhanced (15–55 min) following MCh application. Cholinergic modulation of NMDA responses involves the engagement of G proteins, as enhancement was prevented by intracellular infusion with the nonhydrolyzable GDP analog guanosine-5′-O-(2-thiodiphosphate) trilithium salt (GDPβS). GDPβS was without effect on the early MCh-induced response suppression. Our results suggest that acetylcholine, acting at muscarinic receptors, produces a long-lasting enhancement of NMDA-mediated neurotansmisson in auditory cortex, and that this modulatory effect is dependent upon a G protein-mediated event. Received: 7 March 1996 / Accepted: 14 August 1996     

Expression of Fos-like immunoreactivity in the brain and spinal cord of rats following middle cerebral artery occlusion

 This study examined c-fos protein expression in the brain and spinal cord of rats following permanent occlusion of the middle cerebral artery (MCA) above the rhinal fissure. At 1 h after right-sided MCA occlusion, Fos-like immunoreactivity (Fos-LI) was detected in neurons not only in the ipsilateral cerebral cortex but also in the spinal cord. In the latter, Fos-LI was localized in the nucleus and perikarya of neurons in the grey matter, notably the large motor neurons in the ventral horn. Fos-LI was most intense at 2–4 h, but became undetectable after 48 h in the cerebral cortex and 72 h in the spinal cord. In sham-operated animals, Fos-LI was almost undetectable or virtually absent. It was also not detected in the core territory supplied by the MCA at any time points after arterial occlusion. When the ischaemia-induced neuronal damage in both the cerebral cortex and spinal cord was evaluated by Nissl staining, some neurons appeared atrophic. We conclude that the induction of Fos-LI in neurons of the cerebral cortex and spinal cord is linked respectively to early onset–short stimulation and persistent excitatory or disinhibition phenomenon as a result of focal ischaemic brain injury. Received: 16 September 1996 / Accepted: 29 November 1996     

Localization of AMPA receptors in the hippocampus and cerebellum of the rat using an anti-receptor monoclonal antibody.

The primary amino acid sequences of the kainate binding proteins from the amphibian and avian central nervous systems are homologous with the functional alpha-amino-3-hydroxyl-5-methyl-isoxazole-4-propionate receptors that have been cloned from rat brain. In this study, we have analysed the anatomical and subcellular distribution of the alpha-amino-3-hydroxyl-5-methyl-isoxazole-4-propionate receptors in the rat hippocampus and cerebellum, using a monoclonal antibody that was raised against a kainate binding protein purified from frog brain. Immunoblots of rat hippocampus and cerebellum, and membranes from COS cells transfected with rat brain alpha-amino-3-hydroxyl-5-methyl-isoxazole-4-propionate receptor cDNAs (GluR1, GluR2, or GluR3) showed a major immunoreactive band migrating at a relative molecular weight of 107,000. In the cerebellum, an additional immunoreactive protein of approximately 128,000 mol. wt was also seen on immunoblots probed with the antibody. The distribution of this protein is apparently restricted to the cerebellum since the 128,000 mol. wt band was not present in other brain areas examined. The identity of the 128,000 mol. wt cerebellar protein is not known. Immunocytochemical analyses of the hippocampus demonstrated that alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate receptor subunits are present in the cell bodies and dendrites of pyramidal cells. The granule cells were also immunostained. All of the pyramidal cell subfields were heavily labeled. In the pyramidal cell bodies, a high level of immunoreactivity was observed throughout the cytoplasm. In the cerebellum, the Purkinje cell bodies and dendrites also displayed very high levels of immunoreactivity. In addition to the Purkinje neurons, the Bergmann glia and some Golgi neurons were clearly immunostained. Subcellular fractionation and lesioning experiments using the excitotoxin domoic acid indicated that the alpha-amino-3-hydroxyl-5-methyl-isoxazole-4-propionate receptor subunits were associated with postsynaptic membranes. Direct visualization of the immunoreactivity using electron microscopy confirmed the postsynaptic localization of the staining in the dendritic areas in both the hippocampus and the cerebellum. Thus, unlike the kainate binding proteins, which are found primarily extrasynaptically in the frog and on glial cells in the chicken cerebellum, the GluR1, GluR2, and GluR3 receptor subunits are localized to the postsynaptic membrane in the dendrites of neurons in the rat central nervous system.     

Ultrastructure and immunocytochemical distribution of GABA in layer III of the rat medial entorhinal cortex following aminooxyacetic acid-induced seizures

 Layer III of the entorhinal cortex (EC) is lesioned in patients with temporal lobe epilepsy (TLE). A similar neuropathology is also present in different animal models of TLE. For example, injection of the ”indirect” excitotoxin aminooxyacetic acid (AOAA) into the EC of rats causes behavioral seizures and preferential loss of neurons in layer III of the medial EC. The animals also develop hyperexcitability of the EC and the hippocampal region CA1. To further explore the neuropathological changes within the EC, the ultrastructure and distribution of GABA-like immunoreactivity were assessed in layer III, 28 days after an intraentorhinal AOAA injection. At this time point, light microscopic preparations revealed that a large proportion of pyramidal (putative excitatory) neurons in layer III of the medial EC had degenerated, whereas GABA-immunoreactive neurons had survived. In immunogold-labeled ultrathin sections, the lesioned neuropil was found to contain morphologically intact GABA-containing neurons and nerve terminals. Pathologically swollen dendrites and electron-dense neuronal profiles were present in the lesioned sector as well. The majority of the electron-dense profiles was identified as degenerating dendritic spines that were closely apposed to strongly glutamate-immunopositive axon terminals. Thus, the entorhinal chemoarchitecture is dramatically altered following an episode of AOAA-induced epileptic seizures. One possible consequence of this pathology is a reduced ”drive” of the surviving layer III GABA neurons, which in turn may cause hyperexcitability of the EC and the hippocampus. These findings may be of relevance for the genesis and spread of temporal lobe seizures. Received: 8 September 1998 / Accepted: 1 December 1998