Dormancy of inhibitory interneurons in a model of temporal lobe epilepsy.

In humans temporal lobe epilepsy (TLE) is characterized by recurrent seizures, neuronal hyperexcitability, and selective loss of certain neuronal populations in the hippocampus. Animal models of the condition indicate that a diminution of inhibition mediated by gamma-aminobutyric acid (GABA) accounts for the altered function, and it has been hypothesized that the diminution arises because GABAergic basket interneurons are "dormant" as a result of their being disconnected from excitatory inputs. In hippocampal slices, inhibitory postsynaptic potentials (IPSPs) were elicited in CA1 pyramidal cells by activation of basket cells; responses from an animal model of TLE were compared to those from control tissue. IPSPs evoked indirectly by activation of terminals that then excited basket cells were reduced in the epileptic tissue, whereas IPSPs evoked by direct activation of basket cells, when excitatory neurotransmission was blocked, were not different from controls. These results provide support for the "dormant basket cell" hypothesis and have implications for the pathophysiology and treatment of human TLE.

The biochemical basis and pathophysiology of status epilepticus.

Status epilepticus (SE), both convulsive and nonconvulsive, can arise from diverse etiologies in either a normal brain or a previously epileptic brain. SE has a distinct natural history and unattended can lead to profound, life-threatening, systemic metabolic and physiologic disturbances. These factors may account for the poor prognosis associated with this disorder. However, there is mounting experimental evidence that SE itself, independent of metabolic and physiologic disturbances, leads to lasting brain dysfunction.

Self-sustaining limbic status epilepticus. II. Role of hippocampal commissures in metabolic responses.

In prior work, we developed a model of self-sustaining limbic status epilepticus (SSLSE) induced by continuous hippocampal stimulation (CHS). Previous electrographic studies showed that SSLSE was synchronized between the cerebral hemispheres. On the basis of this initial work, we postulated that hippocampal commissures were critical for the initiation and maintenance of SSLSE. In the current experiments, we tested this hypothesis by applying CHS in animals with (CMX) or without (-CMX) hippocampal commissurotomies. In the -CMX group, electrographic SSLSE was synchronized between the stimulated and contralateral sides. In the CMX group, SSLSE developed only on the stimulated sides. Regional cerebral glucose utilization (RCGU) was also studied acutely (1 hour) after CHS using 2-deoxyglucose autoradiography. In the -CMX group, there was symmetrically increased RCGU in the hippocampus, retrohippocampal structures, and associated limbic and subcortical nonlimbic regions. In the CMX group, a similar pattern was found, but confined to the side of stimulation. CMX alone did not change RCGU values from those in control (-CMX, nonstimulated) brain in any of the regions studied. Areas of bilateral neocortical hypometabolism were found in both (CMX and -CMX) SSLSE groups. These results lead to rejection of the hypothesis that hippocampal commissures play an essential role in the initiation and maintenance of SSLSE. Instead, a feedback circuit involving the hippocampus and its adjacent structures seems to be the critical anatomic substrate for SSLSE. The presence of neocortical hypometabolism after CMX indicates that the structures other than the hippocampal commissure (eg, the thalamus or other forebrain commissures) mediate this effect.

Self-sustaining limbic status epilepticus. I. Acute and chronic cerebral metabolic studies: limbic hypermetabolism and neocortical hypometabolism.

Regional cerebral glucose utilization (RCGU) increases during seizures whereas hypometabolism occurs in postictal and interictal states. Recently, we developed a model of nonconvulsive, self-sustaining limbic status epilepticus (SSLSE) in which electrographic seizures persist 12 to 24 hours after 90 minutes of continuous hippocampal stimulation. The present studies define the functional anatomy of SSLSE and the states thereafter. RCGU was studied by 2-deoxyglucose autoradiography in (1) a group of rats acutely (1 hour after induction) during SSLSE, and (2) two groups of rats chronically (1 week or 1 month) after SSLSE. RCGU measurements in these groups were compared with those obtained in naive and electrode-implanted control rats. In the acute group, there were bilateral increases in RCGU in the hippocampus, retrohippocampal structures, and associated limbic and subcortical nonlimbic regions; hypometabolism was found in several neocortical structures. Chronically, RCGU was elevated in certain limbic areas at 7 days but returned to control values at 30 days. On the basis of our findings, we postulate a feedback network involving the hippocampus and neighboring parahippocampal structures (the hippocampal-parahippocampal "loop") as a critical substrate for establishing limbic system status epilepticus. In addition, the results indicate that metabolic responses can persist long after the cessation of status epilepticus and that both increases and decreases in RCGU can be seen in acute limbic status epilepticus.

Kainic acid-induced limbic seizures: electrophysiologic studies.

Surface and depth electroencephalograms (EEGs) were studied after intravenous injections of kainic acid (KA). High frequency oscillations and spikes appeared in the hippocampus at a dose (1 mg per kilogram) that did not affect other structures. Higher doses (greater than or equal to 4 mg per kilogram) led to electrical seizures in limbic structures, similar to those in temporal lobe epilepsy. In hippocampal slices maintained in vitro, 0.1 to 1.0 microM KA produced spontaneous epileptiform spikes, originating in CA3, and increased evoked potentials. Systemic KA is a potent means of inducing limbic seizures with a primary action in the hippocampus. We propose that this selective activation arises when KA augments excitatory glutamatergic synapses in critical epileptogenic areas, such as the CA3 region of the hippocampus.

Cholinergic and adrenergic agents modify the initiation and termination of epileptic discharges in the dentate gyrus.

A unique type of limbic seizures, maximal dentate activation, was used to examine the effects of cholinergic and adrenergic agents on the processes involved in epileptogenesis. The time to onset of maximal dentate activation was used to monitor the initiation of seizures while the duration of maximal dentate activation monitored termination of seizures. The cholinergic agonist pilocarpine shortened maximal dentate activation at 20 mg/kg and lengthened maximal dentate activation at 50 mg/kg, while both doses delayed the onset of maximal dentate activation. Atropine, a cholinergic antagonist, at 50 mg/kg, slowed the rate of lengthening of maximal dentate activation that occurred with repeated stimulation. The beta-adrenergic antagonist propranolol also slowed the rate of lengthening of maximal dentate activation at 3 mg/kg and shortened maximal dentate activation at 10 mg/kg. The alpha 2-agonist clonidine, at 0.5 mg/kg, shortened maximal dentate activation and increased the time to onset; at 0.1 mg/kg, clonidine did not affect maximal dentate activation. Pretreatment with reserpine had no effect on either the time to onset or duration of maximal dentate activation. These results indicate that both cholinergic and adrenergic mechanisms play important roles in the initiation and termination of limbic seizures.

Bilateral maximal dentate activation is critical for the appearance of an afterdischarge in the dentate gyrus.

Recently, a phenomenon has been described in the dentate gyrus termed maximal dentate activation, which is defined by the appearance of bursts of large amplitude population spikes associated with a negative shift of the d.c. potential and a secondary rise of the extracellular potassium level. Previous work has linked maximal dentate activation to kindling of afterdischarges, either when they are elicited in the hippocampus or outside of the hippocampus in the amygdala. Recording bilaterally in the dentate gyrus, it was found that maximal dentate activation occurred on both sides, with the side ipsilateral to the stimulus (either CA3 or angular bundle) being activated first. An afterdischarge did not appear unless bilateral maximal dentate activation had occurred. With repeated stimulation, the time to onset of maximal dentate activation on the two sides of the brain became nearly equal. This was associated with the appearance of afterdischarges. However, complete synchronization of the onset of maximal dentate activation was not necessary for afterdischarge production. Maximal dentate activation and afterdischarges could be readily elicited in rats in which the hippocampal commissures had been cut. It appears that, in the intact brain, the lack of maximal dentate activation on one side of the brain can function as a "brake" for epileptic activity, preventing afterdischarges. Once this brake is removed, by cutting the hippocampal commissures or by initiating maximal dentate activation, the dentate gyrus readily expresses afterdischarges.

Opioid peptide expression in models of chronic temporal lobe epilepsy.

Expression of the opioid peptides dynorphin and enkephalin is altered within the first 24 h after acutely induced seizures in certain experimental models of epilepsy. Using in situ hybridization, we examined the expression of prodynorphin and preproenkephalin messenger RNA acutely following induction of kindling with recurrent seizures and in two models of chronic temporal lobe epilepsy: (i) rats fully kindled with rapidly recurring hippocampal seizures; and (ii) rats surviving after self-sustaining limbic status epilepticus induced with focal electrical stimulation of the hippocampus. In naive animals, a ventral-dorsal gradient was identified in the expression of both prodynorphin and preproenkephalin messenger RNA in the dentate gyrus and expression of prodynorphin message was demonstrated for the first time in the ventral portion of cornu Ammonis regio superior. After stimulation producing rapidly recurring hippocampal seizures, acute decreases in prodynorphin messenger RNA were seen in the dentate gyrus and cornu Ammonis regio superior at 24 h after the last seizure. In contrast, increases in preproenkephalin messenger RNA expression were seen acutely in the dentate gyrus, with a decrease seen in the entorhinal cortex. The change in prodynorphin message expression in cornu Ammonis regio superior persisted in kindled animals that were studied after one month seizure-free period. There were no changes in preproenkephalin message in kindled animals studied after the one month seizure-free interval. No statistically significant changes were found for either prodynorphin or preproenkephalin message in the post-self-sustaining limbic status epilepticus group at one month following induced seizures. Acute changes in peptide expression may contribute to increased excitation in the dentate gyrus during induction of kindling, while the chronic change identified in cornu Ammonis regio superior may contribute directly to persistently increased excitability in this region.

Kynurenine pathway enzymes in a rat model of chronic epilepsy: immunohistochemical study of activated glial cells.

The kynurenine pathway metabolites quinolinic acid and kynurenic acid have been hypothetically linked to the occurrence of seizure phenomena. The present immunohistochemical study reports the activation of astrocytes containing three enzymes responsible for the metabolism of quinolinic acid and kynurenic acid in a rat model of chronic epilepsy. Rats received 90 min of patterned electrical stimulation through a bipolar electrode stereotaxically positioned in one hippocampus. This treatment induces non-convulsive limbic status epilepticus that leads to chronic, spontaneous, recurrent seizures. One month after the status epilepticus, the rats showed neuronal loss and gliosis in the piriform cortex, thalamus, and hippocampus, particularly on the side contralateral to the stimulation. Astrocytes containing the kynurenic acid biosynthetic enzyme (kynurenine aminotransferase) and the enzymes for the biosynthesis and degradation of quinolinic acid (3-hydroxyanthranilic acid oxygenase and quinolinic acid phosphoribosyltransferase, respectively) became highly hypertrophied in brain areas where neurodegeneration occurred. Detailed qualitative and quantitative analyses were performed in the hippocampus. In CA1 and CA3 regions, the immunostained surface area of reactive astrocytes increased up to five-fold as compared to controls. Enlarged cells containing the three enzymes were mainly observed in the stratum radiatum, whereas the stratum pyramidale, in which neuronal somata degenerated, showed relatively fewer reactive glial cells. Hypertrophied kynurenine aminotransferase- and 3-hydroxyanthranilic acid oxygenase-immunoreactive cells were comparable in their morphology and distribution pattern. In contrast, reactive quinolinic acid phosphoribosyl transferase-positive glial cells displayed diversified sizes and shapes. Some very large quinolinic acid phosphoribosyl transferase-immunoreactive cells were noticed in the molecular layer of the dentate gyrus. In the hippocampus, the number of immunoreactive glial cells increased in parallel to the hypertrophic responses. In addition, pronounced increases in immunoreactivities, associated with hypertrophied astrocytes, occurred around lesioned sites in the thalamus and piriform cortex. These findings indicate that kynurenine metabolites derived from glial cells may play a role in chronic epileptogenesis.

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.

Changes in glutamate receptor and proenkephalin gene expression after kindled seizures.

Changes in gene expression after kindled seizures were examined using microdissection of discrete brain areas and Northern and slot blot analyses. Experimental animals were kindled with either of two protocols: (1) a paradigm in which 50 Hz/10 s stimulus trains were delivered every 30 min through hippocampal electrodes (12 stimulations every other day for 4 days) and (2) a traditional approach in which 50 Hz/10 s stimulus trains were given to the hippocampus three times daily for 16 days. Rats were sacrificed 24 h or 30 days after the last kindled seizure. We first examined the possibility that kindling may affect transcription of mRNA for neurotransmitter receptors. We found significant decreases (22-58%) in AMPA/kainate activated glutamate receptor mRNAs (GluR1, -2, -3 mRNAs) in hippocampus, amygdala/entorhinal cortex and in frontoparietal cortex 24 h but not 30 days after rapidly kindled seizures. However, changes in GABA receptor alpha 1, alpha 2, alpha 4 or beta 1 mRNAs were not observed in any brain region 30 days after traditional kindling or 24 h after rapidly kindled seizures. In addition, we tested whether changes in the expression of proenkephalin could be detected after kindling. We found significant increases (1.7-10 fold) in proenkephalin mRNA in the frontoparietal cortex, hippocampus and in the amygdala/entorhinal cortex 24 h but not 30 days after rapidly kindled seizures. Our findings suggest that changes in glutamate receptor and proenkephalin gene expression are robust, acute sequelae to kindled seizures and may be involved in kindling.

Early induction of mRNA for calbindin-D28k and BDNF but not NT-3 in rat hippocampus after kainic acid treatment.

The influence of kainic acid (KA), which induces acute seizures, on expression of mRNA for the calcium-binding protein, calbindin-D28k, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and early-response genes [c-fos, zif268 (NGFI-A), nur77 (NGFI-B)] was examined in rat hippocampus by Northern blot analysis. A significant increase (3.2-fold) in BDNF mRNA was observed 1 h after KA injection (12 mg/kg i.p.) and peak expression (9.4-fold) occurred 3 h after KA. The induction of BDNF mRNA was preceded by the induction of c-fos, mRNA (30 min after KA) and was followed by the induction of calbindin-D28k mRNA (3.5-fold 3 h after KA; a maximal response was at 3-6 h after KA). Region-specific changes, analyzed by immunocytochemistry and in situ hybridization, indicated that the most dramatic increases in calbindin protein and mRNA after KA treatment were in the dentate gyrus. Although calbindin-D28k and BDNF mRNAs were induced, a 3.4-3.8-fold decrease in NT-3 mRNA was observed by Northern analysis 3-24 h after KA treatment. Calbindin-D28k gene expression was also examined in rats with a chronic epileptic state characterized by recurrent seizures established with an episode of electrical stimulation-induced status epilepticus (SE). When these animals were examined 30 days post-SE, no changes in hippocampal calbindin-D28k mRNA were observed. Our findings suggest that the induction of calbindin-D28k mRNA (which may be interrelated to the induction of BDNF mRNA) is an early response which may not be related to enhanced neuronal activity or seizures per se, but rather to maintaining neuronal viability.

In vitro and in vivo transplantation of fetal rat brain cells following incubation with various anatomic tracing substances.

Implantation of fetal brain regional anlage into host brains ('brain transplantation') holds promise as a plausible treatment for certain human neurodegenerative disorders. Improvements in experimental brain transplantation techniques include: (1) utilization of brain cells in tissue culture as opposed to freshly prepared cell suspensions as a transplantation source, (2) prelabeling of fetal brain cells with inert, non-toxic tracer substances to allow subsequent (a) unequivocal identification of those cells as being fetally derived, and (b) anatomical and immunohistochemical identification of transplanted neurons, and (3) development of in vitro models for transplantation to allow physiological studies of connections formed between fetal neurons and host brain tissue. We examined the ability of brain cell suspensions derived from rat fetuses 15-17 gestational days old to accumulate and retain anatomic tracing substances, including Phaseolus vulgaris leucoagglutinin (PHA-L), rhodamine-labeled latex microspheres (RLM) and fluorogold (FG). All tracers were rapidly accumulated by fetal brain cells, but only PHA-L and RLM were retained following implantation into adult hosts or in tissue culture in vitro. PHA-L-labeled fetal brain cells transplanted in vivo showed morphological characteristics similar to fetal neurons kept in tissue culture in vitro. RLM- or PHA-L-labeled fetal brain cells can be co-cultured with rat brain slices maintained in long-term roller culture. This in vitro system will allow identification and physiological or immunohistochemical study of interactions between fetally derived and host brain neurons.

Closely spaced recurrent hippocampal seizures elicit two types of heightened epileptogenesis: a rapidly developing, transient kindling and a slowly developing, enduring kindling.

Kindling is widely accepted as a model of chronic epilepsy as well as a model of plasticity in the nervous system. Conventional kindling studies have used infrequent stimuli (separated by many hours) to establish a fully kindled state in which enhanced responses (kindled motor seizures and protracted afterdischarges) are consistently triggered by stimuli that initially did not elicit such responses. The enhanced responses occur even after a prolonged stimulus-free interval. Whereas the establishment of a kindled state with traditional stimulus protocols takes several weeks, our previous work showed that kindling could take place much more quickly when the interstimulus interval was set at 30 min (rapid kindling). In this report we tested whether rapid kindling protocols share with traditional kindling protocols the ability to establish a fully kindled state. Using different stimulus protocols involving recurrent hippocampal seizures, we characterized two types of kindling. 'Rapid kindling' developed over hours, but was transient, with a decay rate of a few days so that a fully kindled state did not persist. In contrast, 'slow kindling' developed over several weeks and was enduring, apparently permanent, being associated with a fully kindled state. These findings suggest that, while having certain similarities, the two types of kindling arise from dissimilar mechanisms. The existence of these two types of kindling has implications for epileptogenesis in humans. Moreover, the protocols developed in this work provide a useful means to control for the effects of seizures that are not related to mechanisms underlying a fully kindled state.

Cellular and synaptic basis of kainic acid-induced hippocampal epileptiform activity.

The effects of kainic acid (KA) were studied using extracellular and intracellular recordings in the hippocampal slice preparation. In sufficient concentrations, KA led to a loss of all evoked responses. However, the amount of drug needed for this varied according to anatomic region. CA3 was more sensitive (1 microM) than CA1 or the dentate gyrus (10 microM). These results can be understood in terms of a profound and long-lasting depolarization of neurons. Lower concentrations of KA (0.05-0.1 microM) did not change the resting membrane potential or input resistance of hippocampal pyramidal cells but produced spontaneous epileptiform activity which originated in CA3 and propagated to CA1. Epileptiform discharges were not present in the dentate gyrus. Coincident with the induction of paroxysms, the following changes were observed: (1) an increase in the excitability of CA3 and CA1 pyramidal cells as measured by a left shift in the input-output curves of evoked responses and a lowered threshold stimulus intensity necessary for activation of action potentials in single neurons; (2) augmentation and synchronization of bursting in pyramidal cells; and (3) prolonged EPSPs without an increase in their amplitude. These findings indicate that multiple changes, involving both the properties of single neurons and synaptic connections, are involved in the development of hippocampal paroxysms and that CA3 and CA1 have different roles in the generation of these discharges.

Decreased heterosynaptic and homosynaptic paired pulse inhibition in the rat hippocampus as a chronic sequela to limbic status epilepticus.

We studied a rat model of chronic epilepsy that shares key features with certain patients with temporal lobe epilepsy. This model relies on a previous period of limbic system status epilepticus established by focal stimulation to one hippocampus. Animals were examined 1 month after recovery from such status epilepticus and compared to unstimulated controls and to animals that received stimulation but did not develop status epilepticus. Two experimental procedures were employed to study changes in paired pulse inhibition of population spike (PS) discharges elicited in CA1 pyramidal cells. One procedure (homosynaptic) delivered two identical stimuli to the CA3 region contralateral to the recording site; the other procedure (heterosynaptic) delivered a conditioning stimulus to the ipsilateral angular bundle and a separate test stimulus to the contralateral CA3. For both procedures, influences of stimulus intensities and of interpulse intervals on the potency of paired pulse inhibition were determined. Based on the results, standardized protocols that assayed the maximal amount of paired pulse inhibition were developed. With the homosynaptic protocol, there was one period of inhibition (interpulse intervals up to 300 ms). Animals that previously experienced limbic status epilepticus had markedly less paired pulse inhibition under these conditions than did controls. The stimulated, non-status epilepticus animals were not different from controls. For the heterosynaptic protocol, there were 2 phases of paired pulse inhibition, early (< 50 ms) and late (> 300 ms), separated by a period of paired pulse facilitation. After status epilepticus there were, compared to controls, decreases in both early and late phases of inhibition. The stimulated, non-status epilepticus animals were not different from controls. For the paired pulse facilitation, there was no difference between the animals that experienced status epilepticus and controls. These findings indicate a profound and enduring disturbance of GABA-mediated inhibition in this model. The heterosynaptic paired pulse protocol deals with a number of confounding issues associated with the homosynaptic protocol in this regard. Furthermore, the results suggest the inhibitory disturbance is diffuse, affecting various inhibitory circuits in the hippocampus.

Morphometric effects of intermittent kindled seizures and limbic status epilepticus in the dentate gyrus of the rat.

The effect of recurrent seizures on the hippocampus has been controversial for many years. To determine the effect different seizure paradigms had on the structure of the dentate gyrus, we conducted histological studies on the dentate gyrus (DG) from three groups of rats: (1) those that had experienced 1500 intermittent kindled seizures; (2) those that had experienced a single episode of limbic status epilepticus (SE); and (3) control rats that had been implanted with electrodes. When compared to controls the DG of SE rats was overall slightly, but non-significantly, smaller, but the DG of rats with 1500 kindled seizures was significantly larger. The decrease of size following SE was attributable to a significant atrophy of the molecular layer. The increase in area associated with kindling was the result of an enlargement of the molecular layer and the hilus. Absolute neuronal counts showed a decrease in the hilus after SE but no change following kindling, but both groups had decreased neuronal densities in the hilus when compared to controls. The decreased density after SE was secondary to neuronal loss, but the decrease in neuronal density following kindling was the result of the expansion of the hilar neuropil without change in the number of neurons. This study extends our previous findings in Ammon's horn and indicates that SE induces significant neuronal loss, but numerous intermittent kindled seizures have no effect on neuronal numbers in the DG.

Glial response to neuronal activity: GFAP-mRNA and protein levels are transiently increased in the hippocampus after seizures.

We have recently demonstrated that electrically induced seizures lead to dramatic increases in mRNA for GFAP in areas in which seizures occur. The present study evaluates the time course of the changes in the GFAP-mRNA levels after seizures and the relationship between these changes and GFAP protein levels to understand the role of neuronal activity in regulating glial gene expression. GFA protein and mRNA levels were measured in hippocampi from rats in which seizures were induced by: (1) 50-Hz stimulus trains delivered 12 times over the course of 1 day via indwelling electrodes implanted chronically in the CA3 region of the hippocampus; and (2) intraperitoneal injections of pentylenetetrazol. In the case of the electrically induced seizures, we also compared the glial response in animals that had never experienced a seizure with the response in animals that previously had been kindled but had not experienced a seizure for 30 days. Electrically induced seizures led to rapid transient increases in GFAP-mRNA levels in the hippocampus ipsi- and contralateral to the stimulation. GFAP-mRNA increased about five-fold 1 day after the end of seizure activity and returned to near-control levels by 4 days. There were no detectable increases in GFA protein at 1 day but by 2 days GFA protein levels had increased about two-fold. GFA protein levels remained elevated until 4 days poststimulation and then began to decrease. The responses were similar when seizures were induced in kindled animals, except that the GFAP protein levels remained elevated for somewhat longer.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of extracellular ionic changes in upregulating the mRNA for glial fibrillary acidic protein following spreading depression.

While spreading depression has been shown to be a powerful stimulus in upregulating glial fibrillary acidic protein (GFAP) mRNA expression, the specific physiological signal underlying the upregulation is unknown. During spreading depression, extracellular ionic concentrations are altered markedly. The present study evaluates the role of these changes in extracellular ionic concentrations as potential signals influencing GFAP mRNA expression. Gel foam pledgets saturated with artificial cerebrospinal fluid (CSF) solutions in which [Na+], [Ca2+], [K+] and [H+] were altered one at a time to match concentrations seen in spreading depression were applied to exposed parietal cortex for one hour. Dot and in situ hybridization techniques were used to evaluate GFAP mRNA levels. We found that CSF containing 60 mM KCl produced a dramatic upregulation of GFAP mRNA levels throughout the cerebral cortex of the ipsilateral hemisphere without causing detectable tissue damage. The pattern and time course of the change were similar to those following application of 3 M KCl. Alteration of other ionic species did not affect GFAP mRNA levels. However, the upregulation of GFAP mRNA was not likely due directly to the increased [K+], but rather to the spreading depression that the elevated [K+] induced. This was demonstrated by the finding that the upregulation in GFAP mRNA induced by the potassium exposure was totally blocked by prior administration of MK-801, an NMDA antagonist that blocks spreading depression. These results demonstrate that an upregulation in GFAP mRNA can occur in the absence of degeneration debris and that the initiating events can be related to physiological changes, but that changes in extracellular ionic concentrations are not the likely molecular signals underlying the upregulation.

Functional mapping of limbic seizures originating in the hippocampus: a combined 2-deoxyglucose and electrophysiologic study.

The pathways by which seizures spread from the hippocampus were studied both with multiple electroencephalographic recordings and 2-deoxyglucose autoradiography. The rapid kindling model described in the previous report was employed to compare mild versus severe limbic seizures. Seizures were accompanied by an increased glucose utilization in localized brain areas. The transition from mild to severe limbic seizures involved a greater spatial extent of paroxysmal electroencephalographic activity and metabolic signals. However, electrical recordings proved more sensitive in mapping seizures, as regions shown to be involved in mild or severe limbic seizures with electrical recordings did not necessarily show an increased glucose metabolism. Three types of circuits are important in dissemination of these seizures: interhippocampal connections, pathways leading out of the hippocampus to other limbic regions, and connections to certain extralimbic areas. The nucleus accumbens, amygdala, and substantia nigra emerge as important relay points in the spread of hippocampal-based seizures.