A selective loss of somatostatin in the hippocampus of patients with temporal lobe epilepsy

Somatostatin inhibits excitatory transmission at rat hippocampal synapses via presynaptic receptors

Somatostatin is one of the major peptides in interneurons of the hippocampus. It is believed to play a role in memory formation and to reduce the susceptibility of the hippocampus to seizure-like activity. However, at the cellular level, the actions of somatostatin on hippocampal neurons are still controversial, ranging from inhibition to excitation. In the present study, we measured autaptic currents of hippocampal neurons isolated in single-neuron microcultures. Somatostatin and the analogous peptides seglitide and octreotide reduced glutamatergic, but not GABAergic, autaptic currents via pertussis toxin-sensitive G-proteins. This effect was observed whether autaptic currents were mediated by NMDA or non-NMDA glutamate receptors. Furthermore, somatostatin did not affect currents evoked by the direct application of glutamate, but reduced the frequency of spontaneously occurring excitatory autaptic currents. These results show that presynaptic somatostatin receptors of the SRIF1 family inhibit glutamate release at hippocampal synapses. Somatostatin, seglitide, and octreotide also reduced the frequency of miniature excitatory postsynaptic currents in mass cultures without affecting their amplitudes. In addition, all three agonists inhibited voltage-activated Ca2+ currents at neuronal somata, but failed to alter K+ currents, effects that were also abolished by pertussis toxin. Thus, presynaptic somatostatin receptors in the hippocampus selectively inhibit excitatory transmission via G-proteins of the Gi/Go family and through at least two separate mechanisms, the modulation of Ca2+ channels and an effect downstream of Ca2+ entry. This presynaptic inhibition by somatostatin may provide a basis for its reportedly anticonvulsive action.

Decrease in somatostatin-immunoreactive neurons in the rat amygdaloid complex in a kindling model of temporal lobe epilepsy

In human temporal lobe epilepsy, seizures can begin in the hippocampus, amygdala, or surrounding cortical areas. Histologically, the seizure-induced selective neuronal damage and synaptic reorganization are best documented in the hippocampus. Little information is available about the damage in the other temporal lobe structures or whether the distribution of damage depends on the location of the primary seizure focus. We used an amygdala-kindling model of temporal lobe epilepsy to study whether seizures of amygdaloid origin cause damage to the amygdala and hippocampus. All rats experienced five class 5 generalized seizures. Neuronal damage was assessed by counting the density of GABA-immunoreactive (GABA-ir) and somatostatin-immunoreactive (SOM-ir) neurons in the amygdala and hilus of the dentate gyrus six months after the last seizure. We found that the density of GABA-ir neurons did not differ from that in controls in the contralateral amygdala. The density of SOM-ir neurons was, however, decreased in the lateral (69% of neurons remaining, P < 0.01), basal (67% remaining, P < 0.05), and accessory basal (68% remaining, P < 0.05) nuclei. In the hilus, the densities of GABA-ir and SOM-ir neurons were similar to that in controls. According to our data, a few seizures of amygdaloid origin may cause more severe damage to SOM-ir neurons in the amygdala than in the hilus. Such decrease in SOM-ir neurons which form one subpopulation of GABAergic inhibitory interneurons may increase the local excitability in the amygdala and, therefore, contribute to epileptogenesis.

Status of somatostatin receptor messenger RNAs and binding sites in rat brain during kindling epileptogenesis

In situ hybridization histochemistry with somatostatin sst1-sst5 receptor messenger RNA-selective oligoprobes and quantitative receptor autoradiographic binding studies using [125I]Tyr3-octreotide, [Leu2,D-Trp22,125I-Tyr25]somatostatin-28 and [125I]CGP 23996 ([125I]c[Asn-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Tyr-Thr-Ser]) were performed to determine the level of expression of somatostatin receptor messenger RNA and receptor binding sites in the hippocampal formation, limbic system and cerebral cortex of adult rats electrically kindled in the dorsal hippocampus. In control rats (implanted with electrodes but not electrically stimulated), the somatostatin-1 receptor-selective [125I]Tyr3-octreotide and the non-subtype-selective [Leu3,D-Trp22,125I-Tyr25]somatostatin-28 preferentially labelled the strata oriens and radiatum of the CA1 subfield of the hippocampus, the molecular layer of the dentate gyrus, the subiculum and presubiculum of the hippocampal formation, the inner layer of the frontal cortex, and the lateral and basolateral nuclei of the amygdala. The non-subtype-selective radioligand [125I]CGP 23996 (in 5 mM Mg2+ buffer) preferentially labelled the strata oriens and radiatum of the CA1 subfield of the hippocampus, the subiculum and the basolateral nucleus of the amygdala. Under conditions where primarily somatostatin-2 receptors were labelled, [125I]CGP 23996 (in 120 mM Na+ buffer) showed strong binding in the strata oriens and radiatum of the CA1 subfield of the hippocampus and the frontal cortex, whereas the dentate gyrus, subiculum and amygdala showed only weak signals. During and after kindling, no significant differences were observed between the ipsi- and contralateral sides of the hippocampus. A significant decrease (about 40%) of somatostatin receptor binding sites was observed in the molecular layer of the dentate gyrus with all radioligands (except [125I]CGP 23996 in Na+ buffer, which did not label this area) at stage 2 (pre-convulsive stage) and one week, but not one month, after stage 5 (generalized motor seizures). In contrast to somatostatin receptor binding, no alterations of the messenger RNA levels for sst1-sst5 receptors were found either at stage 2 or at stage 5. Similarly, no changes in receptor binding or messenger RNA levels were observed in the brain of rats which experienced a single afterdischarge. The present study shows a significant and selective decrease of somatostatin-1 receptor binding sites in the dentate gyrus of kindled rats. This is part of the plastic changes induced by kindling and may contribute to the increased sensitivity for the induction of generalized seizures during kindling.

Functional changes in somatostatin and neuropeptide Y containing neurons in the rat hippocampus in chronic models of limbic seizures

Using immunocytochemistry and in situ hybridization analysis of mRNA, we investigated the changes in the expression of somatostatin and neuropeptide Y (NPY) in the rat hippocampal principal neurons in kindling or after electrically induced status epilepticus (SE), two models of limbic epilepsy associated with different chronic sequelae of seizures and seizure-related neuropathology. At the preconvulsive stage 2 of kindling and after three consecutive tonic-clonic seizures (stage 5) but not after a single-discharge (AD), somatostatin and NPY immunoreactivity (IR) were markedly increased in interneurons of the deep hilus and the polymorphic cell layer and their presumed projections to the outer molecular layer of the dentate gyrus. Increased mRNA levels were observed in the same neurons. NPY IR and mRNA were highly expressed in pyramidal-shaped basket cells at both stages of kindling. IR was similar two days after stages 2 or 5 of kindling while less pronounced effects were observed one week after kindling completion. Peptide-containing neurons in the hilus appeared well preserved in spite of an average of 24% reduction of Nissl stained cells (p < 0.01) in the stimulated and contralateral hippocampus at stage 5. No sprouting of mossy fibres in the inner molecular layer was found as assessed by Timm staining. Thirty days after SE, somatostatin IR was slightly reduced or similar to controls in the ventral dentate gyrus and molecular layer in four or six rats (SE-I group) while in the two other post-SE rats (SE-II), somatostatin IR was lost. These changes were associated with a different extent of neurodegeneration as assessed by cell counting of Nissl stained sections. In the granule cells/mossy fibres NPY-IR was transiently expressed at stage 2 and after a single AD. Differently, NPY-IR was persistently enhanced in the mossy fibres of all post-SE rats particularly in the SE-II group. In these rats, NPY immunoreactive fibres were detected in the infrapyramidal region of the stratum oriens CA3 and in the inner molecular layer of the dentate gyrus very likely labeling sprouted mossy fibres. In the hippocampus proper of kindled rats, somatostatin and NPY IR were respectively enhanced in the stratum lacunosum moleculare, the subiculum and in the alveus while no significant changes were observed after SE. Changes in peptide expression were bilateral and involved both the dorsal and the ventral hippocampus. The lasting modifications in peptides IR and mRNA expression in distinct neuronal populations of the hippocampus may reflect functional modifications neurons and play a role in limbic epileptogenesis.

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.

Functional activation of somatostatin- and neuropeptide Y-containing neurons in the entorhinal cortex of chronically epileptic rats

The in vitro release of somatostatin and neuropeptide Y, their tissue concentration and immunocytochemical pattern were examined in the entorhinal cortex of chronically epileptic rats. A systemic administration of 12 mg/kg kainic acid causing generalized tonic-clonic seizures for at least 3 h after injection was used to induce, 60 days later, a chronically enhanced susceptibility to seizures in the rats. The release of both peptides under depolarizing conditions was significantly reduced by 15% on average from slices of the entorhinal cortex two days after kainic acid-induced status epilepticus. At 60 days, the spontaneous and 30 mM KCl-induced release of somatostatin was significantly enhanced by 30% on average. The release induced by 100 mM KCl was raised by 70%. The spontaneous, 30 mM and 100 mM KCl-induced release of neuropeptide Y from the same slices was increased, respectively, by 120%, 76% and 36%. The late changes were associated with an increased tissue concentration of neuropeptide Y but not of somatostatin. This was confirmed by immunocytochemical evidence showing that neuropeptide Y-, but not somatostatin-immunoreactive neurons were increased in the entorhinal cortex of kainic acid-treated rats. These results indicate that neurotransmission mediated by somatostatin and neuropeptide Y, two peptides previously shown to play a role in limbic epileptogenesis, is enhanced in the entorhinal cortex of chronically epileptic rats.

Distribution of AMPA receptor subunits in the hippocampal formation of temporal lobe epilepsy patients

The immunocytochemical distribution of the AMPA-selective receptor subunits GluR1 and GluR2/3 were mapped in the human hippocampal formation obtained from surgery for medically intractable temporal lobe epilepsy. GluR2/3 immunoreactivity was detected in all principal cell types of the hippocampal formation, including hilar neurons, granule cells of the dentate gyrus, and pyramidal cells of the cornu ammonis fields and subiculum. GluR2/3 immunostaining typically filled the cell bodies and processes of neurons. A comparison of GluR2/3 immunoreactivity in a sclerotic specimen versus a non-sclerotic specimen demonstrated a profound loss of staining, specifically in the areas where neuronal dropout was occurring, including CA1, CA3 and the hilus. An analysis of GluR1 immunoreactivity in non-sclerotic specimens revealed that it was predominantly localized to cellular processes throughout the cornu ammonis fields, with a sparse staining of the dentate gyrus outer molecular layer and little to no staining of the dentate gyrus inner molecular layer. Similar to the GluR2/3-immunostained patterns, GluR1 immunoreactivity was lost in the cornu ammonis fields of sclerotic hippocampal specimens, corresponding to patterns of neuronal dropout. Our most compelling finding was a unique extensive pattern of GluR1 and Glu2/3 immunoreactivity throughout the molecular layers of the dentate gyrus of severely compromised hippocampi. The altered staining of GluR1 and GluR2/3 complements some of the patterns of axonal sprouting already described for the dentate gyrus, with a conjecture that their anatomy and distribution pattern underlies to some degree the reorganization of the sclerotic hippocampus. A combination of enhanced glutamatergic transmission and changes in neuropeptides that modulate hippocampal circuitry could greatly affect the degree of excitability in the hippocampal formation. The alterations of GluR1 and GluR2/3 immunoreactivity in the dentate gyrus add another component to the concept of reorganization in the epileptic sclerotic hippocampus.

The kindling-activated neuronal network: recruitment of somatostatin-synthesizing neurons

The present study demonstrates the anatomical extent of the kindling-activated neuronal network in general, and specifically the recruitment of extrahippocampal somatostatin (SST)-synthesizing neurons into this network. It has been known that SST neurons of the hippocampal formation are activated during episodes of seizure, however, it was not known if this activation was a local event or extended to other areas in the brain. We were therefore interested in determining if and which SST neurons outside the hippocampal formation might be recruited into this seizure-activated neuronal network. Using the kindling model of seizure elicitation, expression of the Fos protein in activated, depolarized neurons was utilized to identify seizure-activated neurons. Subsequently, the mRNA for SST was identified through in situ hybridization in the same tissue section, allowing the identification of seizure-activated, SST-synthesizing neurons. The results show that: (a) the majority of SST-synthesizing neurons in the forebrain and diencephalon became activated during the kindling development; (b) their recruitment into the kindling-activated neuronal network occurred progressively; and, (c) these SST-synthesizing neurons represented a component of the kindling-activated neuronal network throughout the development of kindling-induced seizures.

Autoradiographic analysis of neuropeptide Y receptor binding sites in the rat hippocampus after kainic acid-induced limbic seizures

Changes in peptide YY receptor binding were investigated at various intervals after limbic seizures induced in rats by an intraperitoneal injection of kainic acid (10-12 mg/kg). Six to 24 h after kainic acid, specific peptide YY binding, representing Y1 and Y2 neuropeptide Y receptor subtypes, was markedly enhanced in the strata radiatum and oriens CA3 (increase by up to 185% and 178% of control values, respectively). Seven and 30 days after kainic acid, a reduction by up to 63% was found. The basal and kainic acid-induced changes in peptide YY binding were mainly represented by Y2 receptor sites. In the hilus of the dentate gyrus, an increase of global peptide YY binding by up to 400% was observed after 24 h which became attenuated to 125% after 30 days. In the molecular layer of the dentate gyrus global peptide YY binding increased by up to 87% between six and 24 h after kainic acid injection and was reduced by 37% after 30 days. Similar changes were observed in the cerebral cortex. Whereas in the hilus of the dentate gyrus peptide YY binding consisted mainly of Y2 sites, it represented predominantly Y1 receptors in the molecular layer and the cortex. The decline in global and Y2 specific peptide YY binding observed at 30 days in the hippocampus proper was prevented in animals protected from seizure-induced brain damage by an anticonvulsant dose of phenobarbital 3 h after injection of kainic acid. In the stratum moleculare of the dentate gyrus, Y2 specific binding was significantly enhanced while global peptide YY binding was slightly decreased compared to controls. These results show lasting changes in neuropeptide Y receptor binding sites after the acute seizures induced by kainic acid. Since neuropeptide Y modulates glutamatergic neurotransmission, these modifications may play an important role in the hippocampal excitability of chronically epileptic rats.

Axonal sprouting and synaptogenesis in temporal lobe epilepsy: possible pathogenetic and therapeutic roles of neurite growth inhibitory factors

Axonal sprouting and synaptic reorganization within the temporal lobe following neuronal injury have been implicated in the pathogenesis of temporal lobe epilepsy (TLE). The molecular species responsible for these structural changes have yet to be fully defined. The recent characterization of molecules whose normal function within the nervous system is to inhibit neurite outgrowth and synaptogenesis prompts the suggestion that a diminution or loss of such molecules might be of relevance to the pathogenesis of TLE. If so, the possibility of developing a novel therapeutic approach to TLE, distinct from currently available symptomatic therapies, to arrest the pathogenetic processes is also raised.

Somatostatin, neuropeptide Y, neurokinin B and cholecystokinin immunoreactivity in two chronic models of temporal lobe epilepsy

Somatostatin-, neuropeptide Y-, neurokinin B- and cholecystokinin-containing neurons were investigated in the rat hippocampus in two chronic models of temporal lobe epilepsy, i.e. 30 days after rapid kindling or electrically induced status epilepticus (post-status epilepticus). After rapid kindling, somatostatin immunoreactivity was strongly increased in interneurons and in the outer and middle molecular layer of the dentate gyrus. In four of six post-status epilepticus rats (status epilepticus I rats), somatostatin immunoreactivity was slightly increased in the dorsal but decreased in the ventral dentate gyrus and molecular layer. Somatostatin immunoreactivity decreased in neurons of the dorsal hilus in the two other post-status epilepticus rats investigated, while a complete loss was found in the respective ventral extension (status epilepticus-II rats). These changes were associated with a different extent of neurodegeneration as assessed by Nissl staining. Similarly, neuropeptide Y immunoreactivity was enhanced in neurons of the hilus and in the middle and outer molecular layer of the dentate gyrus in the dorsal hippocampus of rapidly kindled and status epilepticus-I rats. Neuropeptide Y and neurokinin B immunoreactivity was enhanced in the mossy fibers of all post-status epilepticus rats, but not in the rapidly kindled rats. In status epilepticus-II rats, neuropeptide Y-and neurokinin B-positive fibers were also detected in the infrapyramidal region of the stratum oriens of CA3 and in the inner molecular layer of the dentate gyrus in the dorsal and ventral hippocampus respectively, labeling presumably sprouted mossy fibers. Increased staining of neuropeptide Y and neurokinin B was found in the alveus after rapid kindling. Cholecystokinin immunoreactivity was markedly increased in the cerebral cortex, Ammon's horn and the molecular layer of the dentate gyrus in the ventral hippocampus of rapidly kindled and post-status epilepticus rats. The lasting changes in the immunoreactive pattern of various peptides in the hippocampus may reflect functional modifications in the corresponding peptide-containing neurons. These changes may be involved in chronic epileptogenesis, which evolves in response to limbic seizures.

Surgical pathology of epilepsy resections in childhood

In this review, we discuss the important pathological lesions observed in temporal lobectomies and neocortical resections performed for medically refractory seizures in children. A higher percentage of pediatric cases appear to be "lesional" with computed tomography (CT) and magnetic resonance imaging (MRI) and abnormalities and "dual pathology" lesions appear to be more common than pure mesial temporal sclerosis. Almost a third of cases appear to be neuronal migration disorders and low-grade gliomas with some lesions harboring both neoplastic and malformative components. Our experience suggests a role for cytomegalovirus in some cases of Rasmussen's encephalitis.

Vasoactive intestinal polypeptide and its receptor changes in human temporal lobe epilepsy

The distribution of the VIP receptor in the human hippocampus was studied by receptor autoradiography using [3-iodotyrosyl-125I]Vasoactive Intestinal Peptide (VIP) as a ligand, and the relationship of receptor distribution to the distribution of the peptide (visualized by immunocytochemistry) was examined in hippocampi surgically removed from patients with medically intractable temporal lobe epilepsy (TLE) and hippocampi obtained at autopsy from neurologically normal subjects. In the autopsy hippocampi and hippocampi from TLE patients with extrahippocampal temporal lobe lesions [125I]VIP binding was highest in the dentate molecular layer, with lower levels in the fields of Ammon's Horn (CA fields) and the subiculum. In hippocampi from patients with no temporal lobe lesions but considerable hippocampal neuronal loss there were significant elevations in the levels of ligand binding in all CA fields and the subiculum. Ligand binding densities in all CA fields of the patient hippocampi were strongly negatively correlated with neuronal numbers. Immunocytochemical localization of VIP shows no obvious change in the distribution patters of VIP immunoreactivity in the patient groups. This is the first demonstration of VIP and its receptor distribution in the human hippocampus. It is suggested that the elevated levels of receptor binding in the hippocampal seizure focus may indicate a mechanism for greater excitability of neurons and/or for their survivability in the face of the increased excitation and potential for injury in a seizure focus.

Loss of hilar somatostatin neurons following tetanus toxin-induced seizures

A loss of inhibitory interneurons has been reported in the hippocampus following seizure activity in various animal models of epilepsy and in human epileptic tissue. The question of whether particular populations of inhibitory neurons are similarly affected by the chronic block of inhibition that results after tetanus toxin injections directly into the brain has not previously been addressed. In the present study a unilateral intrahippocampal injection of tetanus toxin into the ventral hippocampus was used to produce a chronic epileptic syndrome characterised by brief seizures that recurred intermittently for 6-8 weeks. The results reveal, for the first time, the morphological changes in somatostatin interneurons following tetanus toxin-induced seizures in the rat. A bilateral short-term increase in immunoreactivity of somatostatin neurons is present 1 week after injection. This is accompanied by an increased intensity of somatostatin-immunoreactive axon terminals in the outer molecular layer of the dentate gyrus, which is more marked on the contralateral side. A chronic and significant loss of somatostatin-immunoreactive neurons was noted in the hilus of the dentate gyrus 2 months later. The significance of the chronic loss of the hilar somatostatin neurons in the control of excitatory activity in the dentate gyrus and whether the acute morphological changes are due to a direct action of the toxin on release mechanisms or as a result of seizure activity are discussed.

Functional effects of D-Phe-c[Cys-Tyr-D-Trp-Lys-Val-Cys]-Trp-NH2 and differential changes in somatostatin receptor messenger RNAs, binding sites and somatostatin release in kainic acid-treated rats

In situ hybridization histochemistry for somatostatin receptors-1, -2, -3 and -4 section and receptor autoradiography using [125I]CGP 23996, [125I]somatostatin-28, [125I]seglitide and [125I]Tyr3 octreotide were carried out to determine the expression of somatostatin receptor messenger RNAs and binding sites in the hippocampus and cerebral cortex of rats 21 days following generalized limbic seizures induced by subcutaneous injection of 12mg/kg kainic acid. In control rats, somatostatin-1 to somatostatin-4 receptor messenger RNAs were found in the pyramidal layer and granule cell layer of the dentate gyrus. After kainate treatment, the CA1 subfield displayed a selective decrease in somatostatin-3 and somatostatin-4 receptor hybridization signals of 35 and 41%, respectively, whereas no changes were observed in the remaining hippocampal areas. Somatostatin-1 and somatostatin-2 receptor messenger RNA expression in the hippocampus remained unaffected by kainate treatment. No effect of kainate was observed in the expression of somatostatin receptor messenger RNAs in the cerebral cortex. In control rats, the selective somatostatin-2 receptor ligands, [125I]seglitide and [125I]Tyr3 octreotide and the non-selective somatostatin receptor ligands [125I]CGP 23996 and [125I]somatostatin-28, labelled preferentially the stratum oriens and radiatum CA1, the granule and molecular layers of the dentate gyrus and the deep layers of the cerebral cortex. [125I]somatostatin-28 and [125I]CGP 23996 labelled sites were selectively decreased by 32 and 39%, respectively, in the stratum radiatum CA1 after kainate treatment. [125I]CGP 23996 binding was also decreased by 35% in the stratum oriens CA1 and by 36% on average in the stratum oriens and radiatum CA3. [125I]seglitide and [125I]Tyr3 octreotide binding was not affected by kainate in any hippocampal region. The granule and molecular layers of the hippocampus and the layers IV-VI of the cerebral cortex did not show changes in binding sites for any of the radioligands analysed. A 18 and 35% decrease in the spontaneous and 50 mM KCl-induced somatostatin release from hippocampal slices was found two days after kainate, a likely reflection of neuronal cell loss. No differences in somatostatin release were observed 21 days after kainate treatment. At this latter time, the rats had an enhanced susceptibility to tonic-clonic seizures induced by intraperitoneal injection of 30 mg/kg pentylenetetrazol, a subconvulsant dose in naive rats. Bilateral infusion of 6 micrograms RC 160, a selective somatostatin-2 receptor agonist, in the dentate gyrus 21 days after kainate, significantly reduced (P < 0.05) the number of animals with tonic-clonic seizures induced by pentylenetetrazol.(ABSTRACT TRUNCATED AT 400 WORDS)

Somatostatin neurons are a subpopulation of GABA neurons in the rat dentate gyrus: evidence from colocalization of pre-prosomatostatin and glutamate decarboxylase messenger RNAs

The distribution and extent of glutamate decarboxylase 65 (GAD65) mRNA-labeled neurons that coexpress pre-prosomatostatin mRNA were studied in the rat dentate gyrus of the dorsal and ventral hippocampal formation. The distribution of each group of neurons was determined initially by nonradioactive in situ hybridization experiments with digoxigenin-labeled riboprobes for GAD65 mRNA and pre-prosomatostatin mRNA. Double labeling experiments were then conducted with digoxigenin-labeled riboprobes for GAD65 mRNA and 35S-labeled riboprobes for pre-prosomatostatin mRNA. In the dorsal and ventral dentate gyrus, GAD65 mRNA-containing neurons were highly concentrated in the hilus and in the innermost part of the granule cell layer whereas only a few labeled neurons were scattered in the molecular layer. Pre-prosomatostatin mRNA-containing neurons were primarily located in the hilus and were virtually absent from the molecular and granule cell layers. The simultaneous detection of GAD65 and pre-prosomatostatin mRNAs in the same sections showed that the vast majority of pre-prosomatostatin mRNA-containing neurons in the hilus of the dentate gyrus were also labeled for GAD65 mRNA. In contrast many GAD65 mRNA-labeled neurons did not contain pre-prosomatostatin mRNA. These included all neurons in the molecular layer, neurons within the inner granule cell layer and neurons interspersed amongst double labeled neurons in the hilus. Quantitative analyses indicated that a very high percentage of hilar pre-prosomatostatin mRNA-containing neurons coexpressed GAD65 mRNA in the dorsal (96%) and ventral (92%) dentate gyrus. In contrast only a part of the total population of hilar GAD65 mRNA-containing neurons were also labeled for pre-prosomatostatin mRNA in the dorsal (43%) and ventral (53%) dentate gyrus. In the CA3c region, the percentages of neurons containing both mRNAs were similar to those observed in the hilus. The findings demonstrate that the vast majority of hilar somatostatin neurons, which have previously been shown to be extremely vulnerable to ischemia and seizure-induced damage, are GABA neurons. However, the total population of GAD65 mRNA-containing neurons in the hilus is substantially larger than the somatostatin-containing subgroup, and these findings reinforce the suggestion that GABA neurons are a major component of the diverse group of neurons in the hilus of the dentate gyrus.

Activation of somatostatin-synthesizing neurons in the hippocampal formation through kindling-induced seizures

The present study was designed to determine if and to what extent somatostatin (SST) synthesizing neurons of the hippocampal formation are activated during seizures, elicited through kindling of the perforant pathway. Tissue was used and analyzed from animals which had experienced a single after discharge, or a stage 3 or stage 5 seizure. The protein expression of the oncogene c-fos in activated, depolarizing neurons was utilized to identify seizure-activated SST-synthesizing neurons. Combined immunocytochemical and in situ hybridization methods were used to identify these double-labeled, Fos protein, and SST mRNA-containing neurons. The results were quantified and compared across seizure stages. The resulting data demonstrate that at every stage of seizure development, a majority of SST-synthesizing neurons is activated, but that these activated SST mRNA-containing neurons represent only a minority of all seizure-activated, Fos-expressing neurons in the hippocampal formation. The data further reveal a numerical hierarchy in which the majority of double-labeled neurons is present in the hilus of the dentate, followed by the stratum oriens of CA1. It is concluded that SST-synthesizing neurons represent an integral component of the kindling activated neuronal network and, since the SST synthesizing neurons represent the minority of all seizure-activated neurons in the hippocampal formation, that this neuronal network is likely to be of considerable neurochemical complexity.

Alterations of inhibitory synaptic responses in the dentate gyrus of temporal lobe epileptic patients

The number of orthodromically evoked population spikes was used to classify brain slice tissue from the dentate gyrus of temporal lobe epileptic patients as "more excitable" (multiple population spikes) or "less excitable" (a single population spike). During orthodromic stimulation, "more excitable" tissue exhibited less paired-pulse depression in comparison to "less excitable" tissue. During antidromic stimulation, both multiple population spikes and paired-pulse depression were observed in "more excitable" tissue. "Less excitable" tissue exhibited a single antidromic spike and often no antidromically evoked paired-pulse depression. The strength of antidromic paired-pulse depression was correlated positively with the number of antidromic spikes and was correlated negatively with orthodromic paired-pulse depression. Although orthodromic and antidromic paired-pulse depression were correlated to the number of orthodromically evoked population spikes, this correlation was not as strong as that between orthodromic paired-pulse depression, antidromic paired-pulse depression, and number of antidromically evoked population spikes. The antidromic paired-pulse depression observed in tissue exhibiting antidromically evoked multiple population spikes was enhanced rather than blocked by bicuculline. In addition, the blockade of the antidromic paired-pulse depression by CNQX indicated that this inhibition is mediated by an AMPA-type glutamatergic synapse. We suggest that alterations in circuitry occur in the dentate gyrus of some temporal lobe epileptic patients and were manifested by both a loss of inhibitory input as well as an increase of inhibition, which was dependent on the pathway of stimulation. The results of pairing antidromic and orthodromic stimuli were consistent with these conclusions.

Quantitative autoradiographic study of somatostatin receptors in the adult human cerebellum

The evolution of the distribution and density of somatostatin receptors was studied in the human cerebellum during ageing. The brain tissues were collected 3-30 h after death from 20 individuals aged from 28 to 86 years. In vitro autoradiographic experiments were performed on blocks of vermis and of right and left cerebellar hemispheres, using [125I-Tyr0,DTrp8]S14 as a radioligand. In the vermis, the mean concentrations of somatostatin receptors in the molecular layer, the granular layer and the medulla were 140 +/- 9, 150 +/- 22 and 61 +/- 13 fmol/mg proteins, respectively. For each individual, the density of sites in the two lateral lobes was similar. The mean concentrations of somatostatin receptors in the molecular layer, the granular layer and the medulla were 152 +/- 17, 190 +/- 20 and 56 +/- 11 fmol/mg proteins, respectively. The mean level of somatostatin receptors and the type of distribution of the receptors were not correlated to the age of the patients. Different distribution patterns of somatostatin receptors were noted among the patients studied. In the majority of patients (11/20), the density of somatostatin receptors was higher in the granular layer than in the molecular layer. Conversely, in four patients, the density of somatostatin receptors was higher in the molecular layer. The other individuals exhibited similar concentrations of somatostatin receptors in the granular and molecular layers. The present study indicates that the adult human cerebellum contains a high concentration of somatostatin receptors (> 100 fmol/mg proteins) and that the receptor level does not decline during ageing.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of mRNAs encoding two forms of glutamic acid decarboxylase in the rat hippocampal formation

The mRNAs for two forms of glutamic acid decarboxylase (GAD65 and GAD67) were localized in the rat hippocampal formation by nonradioactive in situ hybridization methods with digoxigenin-labeled cRNA probes. Some neurons in all layers of the hippocampus and dentate gyrus were readily labeled for each GAD mRNA, and the patterns of labeling for GAD65 and GAD67 mRNAs were very similar. All major groups of previously described GAD- and GABA-containing neurons appeared to be labeled for each GAD mRNA. Such findings suggest that most GABA neurons in the hippocampal formation contain both GAD mRNAs. When the labeling of neurons in the hippocampal formation and cerebral cortex was compared in the same sections, the intensity of neuronal labeling for GAD67 mRNA was generally similar in the two regions. However, the intensity of labeling for GAD65 mRNA was generally stronger for many neurons in the hippocampal formation than for most neurons in the cerebral cortex. Neurons in the hilus of the dentate gyrus were particularly well labeled for GAD65. The nonradioactive labeling for the GAD mRNAs was confined to the cytoplasm of neuronal cell bodies, and this allowed a clear visualization of the relative number and location of labeled neurons. Several distinct patterns of GAD mRNA-containing neurons were observed among different regions of the hippocampal formation. In the hilus of the dentate gyrus, GAD mRNA-containing neurons were numerous in the regions deep to the granule cell layer as well as in more central parts of the hilus. Within CA3, the densities (quantities) of labeled neurons varied among the regions. In the inner or hilar segment of CA3, the density of labeled neurons was often lower than that in the outer part of CA3 where numerous labeled neurons were distributed throughout all layers. In CA1, GAD mRNA-labeled neurons were distributed in a relatively laminar pattern with the highest density in stratum pyramidale and moderate densities in stratum oriens and at the interface between strata radiatum and lacunosum-moleculare. Lower densities were found within the latter two layers. The prominent localization of the two GAD mRNAs in the hippocampal formation suggests that a dual system for GABA synthesis is necessary for normal GABAergic function in this brain region. Most putative GABA neurons contain relatively high levels of GAD67 mRNA as might be expected if this GAD form is responsible for the synthesis of GABA for metabolic and baseline synaptic function.(ABSTRACT TRUNCATED AT 400 WORDS)

Somatostatin messenger RNA-containing neurons in Alzheimer's disease: an in situ hybridization study in hippocampus, parahippocampal cortex and frontal cortex

The level of expression of somatostatin messenger RNA-containing neurons in human brain was visualized and quantified by in situ hybridization with a 35S-labelled oligonucleotide complementary to amino acids 96-111 of the preprosomatostatin complementary DNA sequence. The analysis was carried out in the frontal and parahippocampal cortices and hippocampus of six age- and post mortem delay-matched Alzheimer's disease and control brains. By northern blot analysis, in frontal cortex samples, 18S rRNA degradation was identical in control and Alzheimer brains and somatostatin messenger RNAs migrated as a single band of 1 kb. By in situ hybridization, specificity was demonstrated by abolition of the signal using either an excess of unlabelled antisense probe or using a labelled sense probe. Somatostatin messenger RNA-containing neurons displayed a similar regional and subregional distribution in control subjects and patients with Alzheimer's disease, being more abundant in the frontal cortex, followed by the hippocampus and the parahippocampal cortex. An overall reduction of labelled cell density was observed in patients with Alzheimer's disease (frontal cortex gray matter:--41%; white matter:--66%; hippocampus:--44%; parahippocampal cortex white matter:--40%). Due to a great variation between brains, this decrease only reached significance in the parahippocampal cortex (-59%, P < 0.05). A significantly lower level of expression of somatostatin messenger RNA per somatostatinergic cell was observed in the hippocampus of Alzheimer's disease patients (-47%, P < 0.05), but not in frontal cortex gray (-17%) and white (-36%) matter and parahippocampal cortex gray (-42%) and white (-29%) matter. These data are in accordance with the distribution of somatostatin cells as visualized by immunohistochemistry in human brain. They indicate that the ability of cortical cells to express somatostatin messenger RNA is partially preserved in Alzheimer disease brains and that the decrease in the amount of somatostatin messenger RNA per cell is restricted to the hippocampal formation.

Seizures, brain damage and brain development

Recent evidence suggests that hippocampal damage can be both the result of seizure activity and the cause of further chronic epilepsy. A review of current models of status epilepticus-induced brain damage reveals that excitotoxic mechanisms probably mediate the lesions in most brain regions. NMDA receptors appear to play a dominant role, although non-NMDA glutamate receptors are important in several specific neuronal populations. In the immature brain, a number of unique metabolic features determine a different set of vulnerabilities, resulting in a brain which is more resistant than the adult's to certain mechanisms of brain damage, but quite vulnerable to others. The inhibition of growth by severe seizure activity has implications for the developing brain that have not yet been fully explored. The mechanisms by which seizure-induced hippocampal lesions cause chronic epilepsy have been explored in several recent animal models. A rearrangement of hippocampal circuits may result from death of selected populations of inhibitory neurons, or from misdirected regeneration by excitatory neurons. It could lead to chronic epilepsy through loss of normal inhibition, through sprouting of new excitatory connections, through conservation of excitatory connections which in a healthy brain would be pruned during development, or through facilitation of kindling by one of these mechanisms. These recent results are beginning to reconcile the pathology seen in human hippocampi ablated for intractable epilepsy with that observed in experimental animals, and offer the promise of even greater advances in the future. They suggest a mechanism for Gower's dictum that "seizures beget seizures" and highlight the importance of the interneurons of the dentate gyrus in epileptogenesis.

Surgical pathology of epilepsy: a review

This review discusses the neuropathological issues pertaining to temporal lobectomies and neocortical resections for medically refractory seizures of childhood. Most cases in our pediatric series are "lesional" and have CT/MRI abnormalities that contribute to a higher incidence of "dual pathology" lesions rather than pure mesial temporal sclerosis. Almost up to two thirds of our cases are represented by neuronal migration disorders and low grade gliomas. We have also encountered lesions that have both neoplastic and malformative foci, some of which behave more like tumors, while others defy precise classification as tumor or malformation. Our preliminary data indicate a role for cytomegalovirus in some cases of Rasmussen's encephalitis.

Characterization of two alternatively spliced forms of a metabotropic glutamate receptor in the central nervous system of the rat

Amplification of complementary DNA by the polymerase chain reaction and anti-peptide antibodies were used to characterize the expression of two alternatively spliced forms of a metabotropic glutamate receptor (mGluR1 alpha and mGluR1 beta) in the central nervous system of the rat. Polymerase chain reaction analysis showed that mGluR1 alpha was the predominate of the two forms in the cerebellum, diencephalon, mesencephalon, olfactory bulb and brainstem, while mGluR1 beta was the major form present in the hippocampus. Approximately equal amounts of the two receptors were expressed in the cerebral cortex, septum and striatum. Immunochemical analyses of the two receptors were conducted in the rat cerebellum and hippocampus. An mGluR1 alpha-specific antibody labelled a protein with a relative molecular weight of 146,000 on immunoblots of the hippocampus and cerebellum. Immunoblot analysis of the developmental expression of mGluR1 alpha in the hippocampus and cerebellum demonstrated that in both structures, the levels of mGluR1 alpha were at or near their maximum levels in the adult brain. In contrast, two mGluR1 beta-specific antibodies failed to detect mGluR1 beta on immunoblots of brain tissue, thus precluding an immunocytochemical analysis of this receptor. Although low levels of a higher-molecular weight protein, possibly a dimeric form of mGluR1 beta were seen with one of the mGluR1 beta-specific antibodies, we hypothesize that some of the mGluR1 beta present in brain tissue may undergo proteolytic cleavage of the carboxy terminus. Immunocytochemical analysis of mGluR1 alpha showed that very high levels of this receptor were expressed in Purkinje cell bodies and dendrites. In the granule cell layer, some Golgi neurons were immunostained. The granule cells were not labelled. In the hippocampus, mGluR1 alpha immunoreactivity was present in interneurons of the stratum oriens and the dentate hilar region. Double-labelling studies demonstrated that these interneurons were also immunopositive for the neuropeptide somatostatin. The presence of mGluR1 alpha in cells of the hippocampus that are associated with the release of somatostatin, suggest that this receptor could play a role in regulating hippocampal excitability in both normal and epileptic tissues.

Selective protection of neuropeptide containing dentate hilar interneurons by non-NMDA receptor blockade in an animal model of status epilepticus

We used a 24 h perforant path stimulation model of status epilepticus to study the role of non-NMDA receptors in the loss of hilar interneurons and paired pulse inhibition associated with the model. In one experiment, NBQX administered i.v. at 1.0 mg/kg/h significantly reduced the loss of hematoxylin and eosin-stained hilar neurons from 360.2 to 125.3 but failed to protect against the loss of paired pulse inhibition. In a second experiment, i.v. NBQX at 1.5 mg/kg/h significantly protected against loss of SS- and NPY-positive hilar interneurons but also failed to protect against loss of paired pulse inhibition. These results demonstrate that the neuronal loss associated with sustained stimulation of this excitatory pathway is mediated in part through non-NMDA receptors. The lack of protection against loss of paired pulse inhibition suggests that SS- and NPY-immunoreactive interneurons may not be responsible for frequency-dependent paired-pulse inhibition of dentate granule cells.

Somatostatin receptor-GTP binding regulatory protein-adenylyl cyclase system in hippocampal membranes of strychnine-treated rats

Wistar rats were injected with either a non-convulsive dose (37.5 micrograms/100 g body weight (b.wt.), intravenously (i.v.)) or a convulsive dose (50 or 80 micrograms/100 g b.w.t, i.v.) of strychnine. Binding of 125I-Tyr11-somatostatin (125I-Tyr11-SS) to its specific receptors was measured in hippocampal membranes 15 min after strychnine injection at these three doses. The non-convulsive dose of strychnine did not affect binding of SS in the hippocampus whereas both convulsive doses decreased the number of specific SS receptors without influencing their apparent affinity. Somatostatin-like immunoreactivity (SSLI), SS-modulated adenylyl cyclase (AC) activity and the inhibitory guanine-nucleotide binding regulatory protein were also measured in rats treated with 80 micrograms/100 g b.wt. of strychnine. SSLI content remained stable. No significant differences were seen for the basal and forskolin (FK)-stimulated AC enzyme activities in the hippocampus of strychnine-treated rats when compared to the control group. The capacity of SS to inhibit basal and FK-stimulated AC activity in the hippocampus was significantly lower in the strychnine group than in the control group. The ability of the stable GTP analogue 5'-guanylylimidodiphosphate [Gpp(NH)p] to inhibit FK-stimulated AC activity was also decreased in hippocampal membranes from strychnine-treated rats. These results suggest that the attenuated inhibition of AC by SS in hippocampal membranes from strychnine-treated rats may be caused by decreases in both Gi activity and in the number of SS receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Colocalization of nitric oxide synthase and somatostatin immunoreactivity in rat dentate hilar neurons

Distribution of nitric oxide synthase (NOS), somatostatin (SSN), and parvalbumin (PV) was studied in the rat hippocampus by immunohistochemical methods. The aim was to explore the interrelationship between SSN-immunoreactive (SSN-IR) neurons in the dentate hilus, which have been shown to be vulnerable to a number of pathophysiological insults, and the presence or absence of NOS and/or PV in the same subset of dentate hilar neurons. Small NOS-IR neurons were scattered in the pyramidal, oriens, and radiatum layers of the CA1-CA3 areas and in the subiculum, where larger NOS-IR neurons were occasionally noted. In the area dentata, NOS-IR neurons, which were composed of small and large polymorphic cells, appeared as a single file at the hilar border with the granule cell layer and clustered in the hilus in fairly high density. Double-labeling techniques showed that most NOS-IR neurons in the hilus were SSN-IR, whereas coexistence of NOS and PV immunoreactivity or SSN and PV immunoreactivity was low in dentate hilar neurons. In other areas of the hippocampus, colocalization of NOS and SSN in the same neurons was much less frequent. Thus, SSN-IR neurons in the dentate hilus constitute a population of neurons that contain the enzyme NOS as well. The presence of NOS coupled to the lack or low level of PV in this group of neurons may provide a neurochemical basis for their high susceptibility to certain pathophysiological insults.

Somatostatin-containing neurons in rat organotypic hippocampal slice cultures: light and electron microscopic immunocytochemistry

Light and electron microscopic immunocytochemical techniques were used to study the interneuron population staining for somatostatin (SRIF) in cultured slices of rat hippocampus. The SRIF immunoreactive somata were most dense in stratum oriens of areas CA1 and CA3, and in the dentate hilus. Somatostatin immunoreactive cells in areas CA1 and CA3 were characteristically fusiform in shape, with dendrites that extended both parallel to and into the alveus. The axonal plexus in areas CA1 and CA3 was most dense in stratum lacunosum-moleculare and in stratum pyramidale. Electron microscopic analysis of this area revealed that the largest number of symmetric synaptic contacts from SRIF immunoreactive axons were onto pyramidal cell somata and onto dendrites in stratum lacunosum-molecular. In the dentate gyrus, SRIF somata and dendrites were localized in the hilus. Hilar SRIF immunoreactive neurons were fusiform in shape and similar in size to those seen in CA1 and CA3. Axon collaterals coursed throughout the hilus, projected between the granule cells and into the outer molecular layer. The highest number of SRIF synaptic contacts in the dentate gyrus were seen on granule cell dendrites in the outer molecular layer. Synaptic contacts were also observed on hilar neurons and granular cell somata. SRIF synaptic profiles were seen on somata and dendrites of interneurons in all regions. The morphology and synaptic connectivity of SRIF neurons in hippocampal slice cultures appeared generally similar to intact hippocampus.

Somatostatin-immunoreactivity in the hippocampus of mouse, rat, guinea pig, and rabbit

The hippocampi of species commonly used for in vitro physiologic studies were examined to determine if there were species-specific and regional differences in somatostatin immunoreactivity. The distributions of somatostatin-immunoreactive somata and fiber plexuses were determined, and the concentration of somatostatin along the septotemporal axis of the hippocampus was measured using a radioimmunoassay. There are many similarities in the patterns of somatostatin immunoreactivity in the hippocampi of mice, rats, guinea pigs, and rabbits. All species had a relatively even distribution of somatostatin-positive perikarya across three fields of the hippocampus (dentate gyrus, CA3, and CA1-2), a similar distribution of somatostatin-immunoreactive perikarya across the strata of the CA1-2 field and the dentate gyrus; and more somatostatin-positive cells in temporal than in septal hippocampus. However, there are species-specific differences in the distribution of somatostatin-immunoreactive perikarya across the strata of CA3. In addition, unlike the other species examined, mice appeared not to have a somatostatin-immunoreactive fiber plexus in the molecular layer of the dentate gyrus. The functional significance of these differences remains to be determined.

Hippocampal synaptic pathology in patients with temporal lobe epilepsy

Immunostaining of synaptic terminals was studied in the hippocampus of 26 patients who had surgical resections for intractable temporal lobe epilepsy. Two monoclonal antibodies (EP10 and SP12) reactive with distinct synaptic antigens were used on paraffin-embedded tissues. The results indicated qualitative reductions on synaptic terminals in CA4 and other regions where cell loss is reported. The inner molecular layer of the dentate gyrus was observed to have increased synaptic immunostaining. Synaptic terminal loss in CA4 and redistribution in the molecular layer were most frequent in cases with hippocampal sclerosis. However, both forms of synaptic pathology were also noted in most cases where the pathological findings were classified as indefinite, and in some cases associated with mass lesions of the temporal lobe. These results support the importance of neuronal loss and synaptic reorganization as possible mechanisms of illness in epilepsy. They also indicate that synaptic immunostaining may be a useful adjunct to routine neuropathological diagnostic techniques.

Immediate early gene expression in experimental epilepsy

Neuronal excitation by experimentally induced seizures elicits the rapid induction of a set of genes called immediate early genes (IEGs). The gene products of fos, jun and Krox, multimember gene families that belong to the class of IEGs, participate in a fundamental biological control mechanism, the regulation of gene transcription. IEG encoded proteins act as third messengers in an intracellular signal transduction cascade between neural cell surface receptors, cytoplasmic second messenger systems and specific target genes in the nucleus, a process for which the term 'stimulus transcription coupling' has been given. Almost all types of seizures cause dynamic alterations of IEG expression in neurons of the limbic system, but also in non-limbic areas, such as the cortex, striatum and thalamus. IEG encoded transcription factors are thought to up- or down-regulate effector genes with preferential expression in the central nervous system, including genes for neurotransmitters, growth factors, receptors, synaptic and axonal proteins. If the concept holds true that IEGs act as molecular switches converting epileptic short-term excitation of neurons into alterations of the molecular phenotype, future research may help to explain hitherto unexplained phenomena in epileptogenesis including changes of synaptic efficacy, kindling and sprouting.

Pathology of temporal lobectomy for refractory seizures in children. Review of 20 cases including some unique malformative lesions

Significant pathological abnormalities were encountered in a series of 20 temporal lobectomies in children with intractable complex partial seizures. In particular, "dual pathology" (mesial temporal sclerosis with other lesions) was found rather than mesial temporal sclerosis as the only lesion. Unusual pathological findings included capillary penetration of neurons in a neuronal heterotopia in one patient, and foci of extensive cortical disorganization in some cases of mixed tumors and gangliogliomas. A high proportion of neuronal migration disorders was also seen with overlapping pathological features between cortical dysplasia and tuberous sclerosis. In this correlative clinical, radiological, electroencephalographic, and pathological study, some of the pathological lesions in children did not fit the classical categories of neoplasia and malformation and transitional forms were rarely encountered.

Spatiotemporal induction of immediate early genes in the rat brain after limbic seizures: effects of NMDA receptor antagonist MK-801

Fos, jun and krox belong to multigene families coding for transcription factors. These cellular immediate early genes (IEGs) are thought to be involved in coupling neuronal excitation to changes of target gene expression. Immunocytochemistry with specific antisera was used to assess regional levels of six IEG-encoded proteins (c-Fos, Fos B, Krox-24, c-Jun, Jun B, Jun D) in the rat forebrain after kainic acid-induced limbic seizures. The results demonstrate a complex spatial pattern of IEG induction and/or suppression in limbic and non-limbic structures. The sequence of induction within hippocampal subpopulations was identical for all IEGs investigated, following the order dentate gyrus, CA1 and CA3, and irrespective of different temporal profiles for individual transcription factors. Since Fos and Jun proteins act via homo- and heterodimer complexes at specific DNA sites, our data imply that the postictal combinatorial changes of these dimers allow a sequential and differential regulation of target gene expression in specific forebrain regions. Pretreatment with the non-competitive NMDA receptor antagonist MK-801 did not affect kainate-induced expression of IEGs in the limbic system, indicating that IEG induction in these regions is mediated by high-affinity kainate and AMPA receptors rather than NMDA receptors. In contrast, MK-801 abolished IEG induction in the somatosensory cortex and striatum, suggesting that IEG expression in non-limbic neurons occurs transsynaptically and is mediated by NMDA receptors.

Quantitative autoradiography of somatostatin receptors in the rat limbic system

The distribution of somatostatin receptors (SRIF-R) was analyzed in the limbic system of the adult rat by in vitro autoradiography with [125I-Tyr0,DTrp 8]S14 as a radioligand. Precise quantification of the density of binding sites, at 0.2 mm intervals throughout the different areas revealed a marked heterogeneity of labeling in most structures. In particular, SRIF-R were concentrated in the basal (104.4 +/- 3.3 fmol/mg proteins) and basolateral amygdaloid nuclei (94.8 +/- 4.3 fmol/mg proteins), and in the nucleus of the lateral olfactory tract (121.6 +/- 2.4 fmol/mg proteins), whereas moderate densities were detected in the amygdalo-hippocampal nucleus (76.4 +/- 2.8 fmol/mg proteins). The medial (41.3 +/- 1.9 fmol/mg proteins) and the central (24.0 +/- 1.4 fmol/mg proteins) amygdaloid nuclei contained lower SRIF-R concentrations. It appears from these observations, in the light of the anatomical pathways of the amygdala, that intra-amygdalian SRIF-containing neurons project to the amygdalo-hippocampal nucleus, and that SRIF-R in the basolateral complex are the target of afferents from limbic cortical areas. SRIF-R were detected at different levels of the hippocampal formation but their distribution was more restricted than that of SRIF-containing fibers. The maximal density of sites was detected in the ventral and dorsal parts of the subiculum (115.0 +/- 3.4 and 87.0 +/- 2.8 fmol/mg proteins, respectively) and in the parasubiculum (100.1 +/- 5.4 fmol/mg proteins). In Ammon's horn, the stratum oriens and stratum radiatum of the CA1 field were the only sites enriched in SRIF-R (74.1 +/- 2.0 and 74.6 +/- 1.9 fmol/mg proteins, respectively). The apparent lack of receptors in the pyramidal cell layer indicated that, in Ammon's horn, SRIF is involved in intra-hippocampal communication. Low levels of receptors were found in the hippocampal CA2 and CA3 fields. SRIF-R in the dentate gyrus were mainly concentrated in the molecular layer (57.3 +/- 1.2 fmol/mg proteins). A very high density of sites was also observed in the entorhinal cortex (up to 123.1 +/- 1.5 fmol/mg proteins). A clear mismatch between SRIF and SRIF-R was detected in the septum and the habenula. In the profound layers of the cingulum and retrosplenial cortex, a heterogeneous distribution of SRIF-R was observed. High concentrations of sites were detected in the rostral zone of the cingulate cortex (93.4 +/- 2.0 fmol/mg proteins) while the posterior cingulate only exhibited moderate concentrations of sites (66.5 +/- 0.7 fmol/mg proteins).(ABSTRACT TRUNCATED AT 400 WORDS)

Unilateral hemispheric memory and hippocampal neuronal density in temporal lobe epilepsy

We examined the relationship of preoperative unilateral memory function and quantitative hippocampal histology in patients undergoing anterior temporal lobectomy for the treatment of complex partial seizures. Recognition memory (objects, words, figures) was assessed preoperatively for each hemisphere by the intracarotid amobarbital procedure in 23 patients (mean age at the time of operation, 30.2 yr; standard deviation, 9.2; mean age at the time of seizure onset, 12.3 yr; standard deviation, 8.6) without tumor. Memory scores were the total number of items recognized, adjusted for guessing. Histological examination of the anterior 20 to 30 mm of hippocampal tissue was accomplished in all patients. The degree of unilateral memory impairment ipsilateral to the seizure focus was significantly correlated with decreased neuronal density in the hilar (r = 0.66, P < 0.001) and dentate granule (r = 0.61, P < 0.002) regions, but not in the CA1 (r = 0.10, P = not significant) or CA2-3 (r = 0.35, P = not significant) regions. Memory performance with the contralateral hemisphere was not significantly correlated with ipsilateral hippocampal densities. These data support the role of the hippocampus in human memory and show further evidence of hippocampal subfield specificity in the relationship between memory performance and neuronal cell loss. Further studies of the dentate granule and hilar regions in relation to human memory are warranted.

Amino acid levels in the cerebrospinal fluid of newly diagnosed epileptic patients: effect of vigabatrin and carbamazepine monotherapies

We studied the CSF amino acid levels of 42 patients with newly diagnosed epilepsy before treatment with antiepileptic medication and during monotherapy with either vigabatrin or carbamazepine. The present study shows that patients with newly diagnosed epilepsy have elevated levels of the excitatory amino acid glutamate in CSF. Vigabatrin monotherapy effectively prevents the appearance of seizures in patients with high baseline CSF glutamate levels. In these patients, vigabatrin not only elevates the levels of gamma-aminobutyric acid, but also decreases the elevated levels of glutamate in CSF, which may also be important to the antiepileptic efficacy of vigabatrin. Patients with low CSF glutamate levels did not benefit from vigabatrin-induced changes in amino acid levels and successful monotherapy with carbamazepine did not affect CSF amino acid levels. The elevation of gamma-aminobutyric acid is thus not the only way to achieve seizure control and there are several factors underlying the generation and control of seizures. Follow-up of the patients with high baseline glutamate CSF levels will show if the observed abnormalities are related to the severity of epilepsy in individual patients and if early treatment with vigabatrin of these patients could prevent the development of intractable epilepsy.

Cerebrospinal fluid levels of neuropeptides, cortisol, and amino acids in patients with epilepsy

We measured lumbar cerebrospinal fluid (CSF) levels of somatostatin, cholecystokinin, neurotensin, atrial natriuretic factor, vasoactive inhibitory peptide, neuropeptide Y, adrenocorticotrophic hormone, corticotropin releasing hormone, beta-endorphin, metenkephalin, cortisol, alanine, glycine, aspartate, glutamate, taurine, and gamma-aminobutyric acid in 25 inpatients with epilepsy at known interictal and postictal times and in 11 neurologically normal volunteers. There were no significant differences between interictal or postictal complex partial seizures (CPS), postictal generalized tonic-clonic seizures (GTC), and control CSF neuropeptide, cortisol, and amino acid (AA) levels. However, there were nonsignificant trends for CSF levels of several neuropeptides to be increased after CPS and GTC as compared with interictal baseline levels. There were significant correlations between levels of certain CSF neuropeptides or (AAs) and serum antiepileptic drug (AED) levels. Several correlations were noted between CSF levels of AAs, including a correlation between the excitatory neurotransmitters aspartate and glutamate identified only after CPS.

Hilar somatostatin neurons are more vulnerable to an ischemic insult than CA1 pyramidal neurons

The effects of a very short ischemic insult on hilar somatostatin (SS) neurons were investigated in the gerbil hippocampus by means of immunocytochemistry and in situ hybridization histochemistry. A selective and significant loss of 40% of hilar SS neurons took place after 1 day, and a 60% loss after 7 days following 2 min of ischemia, while no SS neurons were lost during recirculation after 1 min of ischemia. Repeated 2-min periods of ischemia, which induced ischemic tolerance by vulnerable CA1 neurons, caused almost complete loss of hilar SS neurons. This study clearly demonstrates that hilar SS neurons are more vulnerable to ischemic insult than CA1 pyramidal neurons. Ischemic tolerance may be induced during the progressive loss of SS neurons in the hilus by changes in their synaptic connections to CA1 pyramidal neurons.

Anti-somatostatin antibody enhances the rate of hippocampal kindling in rats

A somatostatin-specific antibody (Ab) (1:250) was continuously infused into the stimulated dorsal hippocampus of rats from 4 days before to 26 days after the beginning of kindling or until the first stage 5. Controls received boiled Ab. The number of stimulations to the first stage 5 were reduced by 41 +/- 4% (P < 0.01, Student's t-test) in animals infused with the Ab compared to controls. The cumulative after-discharge in the stimulated hippocampus was slightly, although not significantly, reduced. Kindling was not affected when the Ab was infused only during the first 10 stimulations (stage 2). Histological analysis showed no neurotoxic effects in the hippocampus as a consequence of Ab infusion.

Somatostatin release is enhanced in the hippocampus of partially and fully kindled rats

The release of somatostatin (somatostatin-like immunoreactivity) from hippocampal slices during the development of hippocampal kindling in rats was measured under resting and depolarizing conditions. Preliminary experiments in naive rats showed that the spontaneous efflux of somatostatin (4.0 +/- 0.3 fmol/ml every 10 min) was independent of external Ca2+ but was reduced to 71.5 +/- 6% of baseline (P < 0.05) during 20 min incubation with 5 microM tetrodotoxin. Neuronal depolarization with 25, 50 and 100 mM KCl induced a Ca(2+)-dependent somatostatin release, respectively 4.3 +/- 0.4, 16.7 +/- 1.6 and 22.0 +/- 1.3 times baseline (P < 0.01). Veratridine caused a dose-dependent Ca2+ and tetrodotoxin (5 microM) sensitive release ranging from 6.5 +/- 0.1 to 13.0 +/- 1.4 times baseline at 1.4 microM and 50 microM respectively (P < 0.01). One week after the last of three consecutive stage 5 seizures (full seizure expression) or 48 h after the last stage 2 stimulation (preconvulsive stage), 50 mM KCl-induced somatostatin release was significantly higher (1.8 +/- 0.1, P < 0.01) than in shams (animals implanted with electrodes but not stimulated) in the stimulated and contralateral hippocampus. Somatostatin release measured under resting conditions was increased by 1.5 times in the stimulated hippocampus at stage 2 (P < 0.05) and by 2.2 and 1.7 times in both hippocampi at stage 5 (P < 0.01). Forty-eight hours after the induction of a single afterdischarge no significant changes were found in either spontaneous or 50 mM KCl-induced release of somatostatin.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurons in the ventral subiculum, amygdala and entorhinal cortex which project to the nucleus accumbens: their input from somatostatin-immunoreactive boutons

Neurons in the hippocampus, amygdala and entorhinal cortex which project to the nucleus accumbens were labelled retrogradely following injection of horseradish peroxidase. The injections were targetted on the medial part of the nucleus accumbens, but some injection sites included the whole nucleus. Projection neurons in all three areas were found to be spiny, and from the entorhinal cortex and ventral subiculum of the hippocampus they were pyramidal neurons. Somatostatin (S28(1-12)-immunoreactive neurons were found in all parts of the three limbic areas examined. They were found to have various morphologies, but in the electron microscope all had the ultrastructural characteristics of interneurons. In the hippocampus the stratum lacunosum was found to contain the most immunoreactive fibres while most cells lay in the stratum oriens. In the amygdala the densest staining for both cells and fibres was in the central nucleus. In the entorhinal cortex somatostatin-immunoreactive fibres and cells seemed to have no preferential distribution. Examination of somatostatin-immunoreactive profiles in the electron microscope revealed that the majority of synaptic contacts were made with dendrites, many of which were spine-bearing. In the light microscope somatostatin-immunoreactive fibres could be seen to lie near the somata and proximal dendrites of neurons that projected to the nucleus accumbens. In the electron microscope it was found that somatostatin-immunoreactive boutons were in symmetrical synaptic contact with the somata and proximal dendrites of neurons in the ventral subiculum, entorhinal cortex and amygdala which project to the nucleus accumbens.

Biochemical and morphological changes in the rat hippocampus following transection of the fimbria-fornix

According to electrophysiological studies, the subcortically denervated hippocampus has been suggested as a model for limbic epilepsy. We investigated a) whether fimbrial lesioning leads to any biochemical or morphological changes in the rat hippocampus, b) if these changes give any explanation to the previously indicated hyperexcitability, and c) if the changes are in line with the findings in other experimental models and human epilepsy. The fimbria-fornix transection was done by aspiration. Four months later, spontaneous EEG activities were recorded, and the hippocampal formation was processed for histology. In addition, a separate group of lesioned rats was used for hippocampal amino acid analysis. Hyperexcitable functioning of the hippocampus was seen as frequent and rhythmic spiking activity in 25% of the fimbria-fornix-lesioned rats, although the rest of them had spikes occasionally. The amino acids analysis revealed a notable decrease in the concentration of GABA but no significant changes in the amount of excitatory amino acids. This suggests impaired GABAergic functioning but does not exclude possible abnormalities in the release of both excitatory and inhibitory amino acids. The number of somatostatin-immunoreactive (SOM-IR) neurons, a subpopulation of GABAergic neurons, was decreased in all the areas of the hippocampus (CA3 > CA1 > hilus), but this was statistically significant only in the CA3 area. Interestingly, it is the region from which interictal spiking activity in the subcortically denervated rat presumable originates. Immunostaining for synaptophysin showed a dense band of granules in the inner molecular layer of the dentate gyrus, indicating probable synaptic reorganization of associational afferents.

Somatostatin- and neuropeptide Y-like immunoreactivity in the dentate area, hippocampus, and subiculum of the domestic pig

With the principal aim of providing baseline observations for future experimental studies, the distribution of somatostatin-like and neuropeptide Y-like immunoreactivities is described in the dentate area, hippocampus, and subiculum of the domestic pig (Sus scrofa domesticus) and compared with the distribution described in other mammals. Intensely stained somatostatin-like immunoreactive nerve cell bodies were present throughout the region, with highest densities in the dentate hilus, stratum radiatum and stratum oriens of the hippocampal regio inferior, stratum oriens of the hippocampal regio superior, and in the subicular cell layer. Somatostatin-like immunoreactive terminals were represented by both stained fibers and stained puncta. Scattered somatostatin-like immunoreactive nerve fibers were seen in most areas, but regular fiber plexuses were present in the dentate molecular layer and dentate hilus, stratum moleculare of the hippocampus, and in the subicular plexiform layer. Somatostatin-like immunoreactive puncta were seen in the dentate molecular layer, stratum moleculare of the hippocampus, and in the subicular plexiform layer. Neuropeptide Y-like immunoreactive nerve cell bodies were less numerous than somatostatin-like immunoreactive ones. They were mainly seen in the dentate granule cell layer and dentate hilus, stratum radiatum and stratum oriens of the hippocampus, and in the subicular cell layer. Intensely stained neuropeptide Y-like immunoreactive fibers were numerous, and present in all areas examined. They formed fiber plexuses in the dentate molecular layer and dentate hilus, stratum moleculare of the hippocampal regio superior, and in the subicular plexiform layer. Neuropeptide Y-like immunoreactive puncta were present in the dentate molecular layer, stratum moleculare of the hippocampus, and in the subicular plexiform layer. Consistent and very characteristic variation in the distribution of somatostatin-like and neuropeptide Y-like immunoreactivity was found along the septotemporal axis of the hippocampus. The distribution of somatostatin-like and neuropeptide Y-like neurons and terminals in the domestic pig displayed striking similarities with the basic pattern of organization of these neuropeptides in other species, although more subtle species-specific characteristics were also observed in the pig.

Functional changes in neuropeptide Y- and somatostatin-containing neurons induced by limbic seizures in the rat

The influence of sustained epileptic seizures evoked by intraperitoneal injection of kainic acid on the gene expression of the neuropeptides somatostatin and neuropeptide Y and on the damage of neurons containing these peptides was studied in the rat brain. Injection of kainic acid induced an extensive loss of somatostatin and, though less pronounced, of neuropeptide Y neurons in the inner part of the hilus of the dentate gyrus. Neuropeptide Y-immunoreactive neurons located in the subgranular layer of the hilus, presumably pyramidal-shaped basket cells, were spared by the treatment. Although neuropeptide Y messenger RNA was not detected in granule cells of control rats, it was found there after kainic acid seizures at all time intervals investigated (12 h to 90 days after injection of kainic acid). High concentrations of neuropeptide Y messenger RNA were especially observed 24 h after injection of kainic acid. At this time neuropeptide Y messenger RNA was also transiently observed in CA1 pyramidal cells. Neuropeptide Y synthesis in granule cells in turn gave rise to an intense immunoreactivity of the peptide in the terminal field of mossy fibers which persisted for the entire time period (90 days) investigated. In addition, neuropeptide Y messenger RNA concentrations were also drastically elevated in presumptive basket cells located at the inner surface of the granule cell layer, especially at the "late" time intervals investigated (30-90 days after kainic acid). These data support the concept that extensive activation of granule cells by limbic seizures contributes to the observed neuronal cell death in CA3 pyramidal neurons and interneurons of the hilus. Consecutively, basket cells containing neuropeptide Y and presumably GABA might be activated and participate in recurrent inhibition of granule cells. Neuropeptide Y-immunoreactive fibers observed in the inner molecular layer at "late" time intervals after kainic acid may result either from collateral sprouting of mossy fibers or from basket cells extensively expressing the peptide. It is speculated that neuropeptide Y synthesized and released at a high rate from granule cells and basket cells may exert a protective action against seizures.

Increased expression of GAD mRNA during the chronic epileptic syndrome due to intrahippocampal tetanus toxin

A few mouse minimum lethal doses (MLD) of tetanus toxin injected into rat hippocampus triggers prolonged changes in neuronal function. Spontaneously recurring epileptic discharges arise in both the injected and the contralateral, uninjected hippocampus. The seizures remit after about 6 weeks, to be succeeded by a permanent depression of hippocampal neuronal responses. There is no evidence of any loss of pyramidal cells at this low dose of toxin. Here we studied presumptive inhibitory, GABAergic neurons, using in situ hybridization (ISH) with a probe directed against the mRNA encoding glutamic acid decarboxylase (GAD), at each of 1, 2, 4 and 8 weeks after injection of tetanus toxin. Epileptic activity was recorded from hippocampal slices prepared from both injected and contralateral hippocampi of rats at each time point, unexpectedly persisting until 8 weeks. There were no significant differences in the numbers of neurons containing GAD mRNA between toxin- and vehicle-injected and control rats in any hippocampal subfield, at any survival time, except for an apparently transient loss of hilar signal in vehicle-injected rats at 1 and 2 weeks which we attribute to a significant, transient loss of neuronal GAD mRNA to below the threshold for detection by ISH using this probe. In contrast there was a marked increase in GAD mRNA in the toxin-injected group, which reached a peak at 4 weeks, and returned to control levels by 8 weeks. The changes were bilateral and were most marked in the hilus of the dentate area, but were also significant in CA3 and CA1. Upregulation of GAD mRNA was preceded by an increase in the levels of the mRNA for the alpha subunit of the GTP binding protein, Gs (Gs alpha), at 2 weeks which affected the GABAergic neurons selectively, and not the pyramidal or granule cells. These marked changes in GAD mRNA may contribute to putative adaptive responses within GABAergic neurons, which would help contain epileptic activity in these chronic foci. The changes in GAD expression may be due to mechanisms acting through an increase in mRNA encoding Gs alpha.

Enhanced GABAergic inhibition preserves hippocampal structure and function in a model of epilepsy

Extensive electrical stimulation of the perforant pathway input to the hippocampus results in a characteristic pattern of neuronal death, which is accompanied by an impairment of cognitive functions similar to that seen in human temporal lobe epilepsy. The excitotoxic hypothesis of epileptic cell death [Olney, J. W. (1978) in Kainic Acid as a Tool in Neurobiology, eds. McGeer, E., Olney, J. W. & McGeer, P. (Raven, New York), pp. 95-121; Olney, J. W. (1983) in Excitotoxins, eds. Fuxe, K., Roberts, P. J. & Schwartch, R. (Wenner-Gren International Symposium Series, Macmillan, London), Vol. 39, pp. 82-96; and Rothman, S. M. & Olney, J. W. (1986) Ann. Neurol. 19, 105-111] predicts an imbalance between excitation and inhibition, which occurs probably as a result of hyperactivity in afferent pathways or impaired inhibition. In the present study, we investigated whether the enhancement of gamma-aminobutyric acid (GABA)-mediated (GABAergic) inhibition of neurotransmission by blocking the GABA-metabolizing enzyme, GABA transaminase, could influence the histopathological and/or the behavioral outcome in this epilepsy model. We demonstrate that the loss of pyramidal cells and hilar somatostatin-containing neurons can be abolished by enhancing the level of synaptically released GABA, and that the preservation of hippocampal structure is accompanied by a significant sparing of spatial memory as compared with placebo-treated controls. These results suggest that enhanced GABAergic inhibition can effectively block the pathophysiological processes that lead to excitotoxic cell death and, as a result, protect the brain from seizure-induced cognitive impairment.