Anticonvulsant efficacy of gabapentin on kindling in the immature brain.

The anticonvulsant and motor effects of gabapentin (GBP) were evaluated in rat pups aged 16-17 days. Fourteen-day-old rat pups received an implanted stimulating electrode in the amygdala unilaterally. Kindled seizures were produced on day 16 of life by repeatedly applying an electrical current stimulus to the amygdala electrode. Animals received kindling stimulation until they achieved three consecutive generalized convulsions. On day 17, rat pups received one of four doses of GBP 10, 25, 50, or 100 mg/kg. After receiving GBP, rat pups again received electrical stimulation to the amygdala electrode to determine the extent to which GBP prevented the kindled seizure. Anticonvulsant effects were found at doses as low as 10 mg/kg. A separate group of naïve rats received GBP to determine the motor effects of each treatment dose. Impaired motor performance, quantified as time on a balance beam, occurred at doses of >or=50 mg/kg. In summary, our data indicate that in immature rats, GBP exerts an anticonvulsant effect against kindled seizures at doses that do not significantly impair motor performance.

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

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

Laminar and temporal heterogeneity of NMDA/metabotropic glutamate receptor binding in posterior cingulate cortex.

Both N-methyl-D-aspartate (NMDA) and quisqualate/AMPA-insensitive metabotropic glutamate (mGlu) receptors mediate plasticity induction in neocortex, but their interlaminar distribution in cortical microcircuits is largely unknown. We used (+)(3)H-MK801 and (3)H-glutamate binding plus saturating concentrations of NMDA, AMPA, and quisqualate to autoradiographically map NMDA and mGlu receptor sites by lamina in posterior cingulate cortex in adult male rats. Specific binding at NMDA receptor sites in laminae II/III and VI was significantly reduced in comparison to other laminae. Brains prepared from rats killed during dark phase of a 12h/12h light/dark cycle showed a mean 129% increase in overall (+)(3)H-MK801 binding versus light phase brains but retained reduced binding densities in laminae II/III and VI. In contrast to NMDA findings, specific binding at mGlu sites was consistently elevated during light phase in both laminae II/III and VI. Specific (3)H-glutamate binding in dark-phase brains showed an overall 147% increase versus light phase binding but did not retain significant interlaminar heterogeneity. Interpreted in accordance with our physiologically derived models of hippocampo-cortical microcircuitry, these results suggest that spatial and temporal variations in glutamate receptor distribution may play an important role in intracingulate neural processing of afferent input from hippocampus.

Neuroprotective effects of estrogens on hippocampal cells in adult female rats after status epilepticus.

PURPOSE: Estrogens have neuroprotective effects in ischemia, stroke, and other conditions leading to neuronal cell death (e.g., Alzheimer's disease). The present study examined whether estrogens may have neuroprotective effects after seizures. METHODS: The kainic acid model was used to determine if estrogens protect hippocampal cells after status epilepticus in adult female rats. Rats were ovariectomized 1 week before hormone replacement. beta-Estradiol benzoate (EB; 2 microg in 0.1 mL of oil) was injected subcutaneously 48 and 24 hours before seizure testing. We administered kainic acid (16 mg/kg intraperitoneally) and behaviorally monitored the rats for 5 hours. After this time, all rats were injected with pentobarbital (50 mg/kg intraperitoneally) irrespective of seizure severity. Some rats received two additional doses of EB, one immediately and one 24 hours after the seizures. Another group of rats received only these two doses of EB after the seizures, and yet another group of rats received pretreatment with the intracellular EB receptor antagonist tamoxifen before each of four EB injections. Control rats received oil instead of EB. Rats were killed 48 hours after seizures. Neuronal damage was evaluated in silver-impregnated and Nissl-stained sections. RESULTS: Estrogen treatment before kainic acid administration significantly delayed the onset of kainic acid-induced clonic seizures, whereas it did not change the onset of status epilepticus compared with oil-treated controls. Furthermore, estrogen treatment significantly protected against kainic acid-induced seizure-related mortality. In control rats, examination of Nissl-stained and silver-impregnated slides revealed severe neuronal damage in the vulnerable pyramidal neurons of the hippocampal CA3 subfield and in the hilus of the dentate gyrus. Estrogen pretreatment, as well as the combination of pretreatment and posttreatment, significantly reduced the number of argyrophilic neurons in both the CA3 and the dentate gyrus. Posttreatment only had no protective effects. The data indicate that intracellular EB receptors mediate this type of neuroprotective effect, because the tamoxifen pretreatment abolished EB neuroprotection. CONCLUSIONS: Our results suggest that estrogens can be beneficial in protecting against status epilepticus-induced hippocampal damage. Hormonal conditions may have differential effects on underlying epileptic state in some patients. Therefore, more studies are necessary to determine the prospective therapeutic advantage of hormonal treatment in seizure-related damage.

Malnutrition increases dentate granule cell proliferation in immature rats after status epilepticus.

PURPOSE: Nutritional insults early in life have a profound and often permanent effect on the development of the central nervous system. A direct relationship between malnutrition and epilepsy has not been established; however, it is believed that inadequate nutrition may predispose the brain to seizures. This study was designed to determine whether neonatally malnourished rats are different from nourished rats in terms of flurothyl seizure susceptibility at postnatal day (P)15, in the behavioral manifestations of seizures, and in status epilepticus-induced hippocampal injury. METHODS: Sprague-Dawley rat pups were maintained on a starvation regimen from P2 until P17. Age-matched control rats were not exposed to starvation. At P15, all animals were exposed to flurothyl-induced status epilepticus. At P17, the rats received a single injection of bromodeoxyuridine (50 mg/kg intraperitoneal) to determine the extent of genesis of new cells in the dentate gyrus. At P18, the rats were killed, and the brains were processed for histology and immunohistochemistry. RESULTS: Preliminary analysis indicates that early malnutrition did not modify flurothyl seizure susceptibility or the behavioral manifestations of seizures at P15. Histological assessment did not reveal any evidence of hippocampal cell loss after status epilepticus in either group. Malnutrition per se induced an increase in the genesis of new cells in the anterior dentate granule cell layer. Although exposure to status epilepticus augmented the expression of new cells in the dentate gyrus in both groups, this expression was more pronounced in the malnourished group. CONCLUSIONS: The findings suggest that malnutrition early in life alters dentate plasticity but not the susceptibility to flurothyl seizures. Although status epilepticus can increase the expression of new cells in the dentate gyrus in immature rats, malnutrition followed by status epilepticus further increases dentate granule cell proliferation.

Plasticity of excitatory amino acid transporters in experimental epilepsy.

PURPOSE: To examine the relationship between seizures and excitatory amino acid transporter (EAAT) activity and whether up-regulation of EAAT activity alters epileptogenicity. METHODS: In this study, we exposed rat hippocampal slices to different convulsants before measuring EAAT activity. Rats were exposed to the EAAT inhibitor pyrrolidine-2,4-dicarboxylic acid (PDC) before entorhinal cortex/hippocampal slices were obtained. These slices were exposed to low-Mg2+ buffer while electrophysiological recordings were obtained from the entorhinal cortex. mGluR III acting agents were used to study whether activation of mGluR III could regulate EAAT activity and if this regulation could overcome the effects on EAAT activity induced by the convulsants. RESULTS: Veratridine, kainic acid (KA), and pilocarpine reduced EAAT activity in rat hippocampal slices. L-2-Amino-4-phosphonobutyric acid (an mGluR III agonist) restored EAAT activity and reduced epileptiform activity to near control levels. The saturation curve for glutamate uptake in slices from KA-seized rats killed 2 hours after the first forelimb clonus was displaced to the left, suggesting a compensatory change for the enhanced excitation. On the other hand, rats injected with the EAAT inhibitor PDC (by intracerebroventricular injection) had more severe KA-induced seizures and N-methyl-D-aspartate epileptiform activity than control rats. Furthermore, hippocampal slices from KA- or KA+PDC-treated rats exposed to low Mg2+ reduced their firing rate to nearly zero once they returned to normal solution, whereas their control counterparts continued to fire, although at a lower rate. CONCLUSIONS: These results suggest a significant contribution of EAATs in some experimental epilepsy models and point to their short-term regulation by mGluR III as a possible source of their plasticity.

Age-dependent vulnerability to seizures.

Seizure disorders frequently occur early in life. Seizures are classified as reactive, symptomatic, or idiopathic depending on whether their cause can be identified. Reactive seizures are the result of acute environmental perturbations. Early in life, many stressors can produce seizures and the ultimate outcome may depend on the particular precipitating factor and its intensity. Febrile convulsions are the most common reactive seizures, although they must be differentiated from symptomatic seizures precipitated by fever. Symptomatic seizures are often associated with varying degrees of central nervous system (CNS) insults, including congenital malformations and metabolic storage diseases of the gray matter. These seizures may have age-specific characteristics and may at times be difficult to treat with conventional antiepileptic treatments. To develop a better understanding of the pathophysiology of seizures early in life, we have extensively used animal models of epilepsy. In this chapter, we report our findings with a rat model of developmental cortical dysplasias produced by intrauterine injections of methylazoxymethanol acetate. These rats are more susceptible to kainic acid, flurothyl, and hyperthermic seizures than normal rats. Rats with severe cortical dysplasia are most susceptible to seizures. We have also studied the mechanisms involved in the control of seizures during development because status epilepticus is more prevalent in infants than in adults. Our data suggest that the substantia nigra may play a crucial role in status epilepticus as a function of age. In the adult substantia nigra two regions mediate opposing effects on seizures following infusions of gamma-aminobutyric acid type A (GABAA) agents. One region is located in the anterior substantia nigra, and muscimol infusions in this region mediate anticonvulsant effects. The second region is in the posterior substantia nigra, and here muscimol infusions produce proconvulsant effects. In situ hybridization data demonstrate that, at the cellular level, neurons in the two substantia nigra regions differ in the amount of hybridization grains for GABAA receptor alpha 1 and gamma 2L subunit mRNAs. In developing male rats, only the "proconvulsant" region is present up to the age of 21 days. The transition from the immature to mature substantia nigra mediated seizure control occurs between age 25 and 30 days. The identification of age-dependent functional networks involved in the containment of seizures may lead to possible new pharmacologic strategies to control seizures, thus aiding the development of age-appropriate treatments of seizure disorders.

Flurothyl status epilepticus in developing rats: behavioral, electrographic histological and electrophysiological studies.

Status epilepticus and repeated seizures have age-dependent morphological and neurophysiological alterations in the hippocampus. In the present study, effects of flurothyl-induced status epilepticus were examined in awake and free moving immature (2 weeks old) and adult rats. Without exception, adult rats died of respiratory arrest before the onset of status epilepticus. We were unable to find a concentration of flurothyl that produced status epilepticus and a low mortality in adult rats. In contrast, immature rats survived flurothyl status epilepticus for up to 60 min with a very low mortality. In rat pups, behavioral manifestations correlated with electrographic seizures in both the cortex and hippocampus. Neuropathological damage (cell loss, pyknotic cells or gliosis) was not observed in the immature hippocampus, thalamus, amygdala, substantia nigra or cortex at 24 h, 2 days or 2 weeks after status epilepticus. In addition, no aberrant mossy fiber reorganization or decrease in cells counts were observed in the hippocampus. Young rats did not show alterations in paired-pulse perforant path inhibition following flurothyl status epilepticus. The present findings are consistent with studies in other seizure models, indicating that immature rats are highly resistant to seizure-induced changes.

Enhanced epileptogenicity of area tempestas in the immature rat.

Age-related differences in kindling have been previously observed in terms of behavioral manifestations, seizure generalization and intensity of postictal refractory period. In the present study the development of kindling in the area tempestas (specifically the dorsal endopiriform nucleus) was examined in developing rats (15-16 days of age). Results indicate that kindling can be easily elicited from the immature area tempestas. Rats kindled from area tempestas reached the early kindling stages significantly faster, had longer afterdischarges and a greater number of severe stage 6 seizures than rats kindled from areas above the area tempestas or in the piriform cortex proper. This study indicates that the area tempestas is particularly epileptogenic in the immature rat.

Evidence of enhanced kindling and hippocampal neuronal injury in immature rats with neuronal migration disorders.

PURPOSE: Neuronal migration disorders (NMD) are often found in patients with epilepsy. However, the mechanisms linking these two pathologies are not yet fully understood. In this study, we evaluated whether NMD increased kindling seizure susceptibility and seizure-induced acute neuronal damage in the immature brain. METHODS: Experimental NMD were produced by exposing pregnant rats (gestation day 15) to methylazoxymethanol acetate (MAM, 25 mg/kg, ip). Seizures were induced in rat pups (postnatal day 15) transplacentally exposed to MAM and controls by hippocampal kindling. Afterdischarge (AD) threshold and duration, seizure stage, and number of stimulations required to reach each seizure stage were recorded. Acute seizure-induced damage was histologically assessed in Nissl-stained and silver-impregnated hippocampal tissue 24 h after kindling. RESULTS: Rat pups with NMD had a significantly lower AD threshold than controls (91+/-18 vs. 163+/-23 microA; p < 0.05). Furthermore, rats with NMD required fewer stimulations to reach seizure stage 3.5 and 4 than did controls. Additionally, rats with NMD had longer AD the second day of stimulation (2,094+/-416 s vs. 1,755+/-353 s; p < 0.05). Histologic examination revealed that in rats with NMD, acute seizure-induced neuronal hippocampal damage occurred bilaterally in CA3 hippocampal neurons. CONCLUSIONS: The lowered AD threshold and more rapid kindling to stages 3.5 and 4 indicate that in the presence of severe NMD, hippocampal kindling is facilitated. Furthermore, this study suggests that in the immature brain, seizure-induced hippocampal neuronal damage occurs if there is an underlying pre-existing pathology.

Hippocampal sclerosis revisited.

Studies dating back more than 150 years reported a relationship between hippocampal sclerosis and epilepsy. Retrospective studies of patients who underwent temporal lobectomy for intractable partial epilepsy found a relationship between a history of early childhood convulsions, hippocampal sclerosis, and the development of temporal lobe epilepsy. Many believe that febrile seizures lead to hippocampal damage and this in turn predisposes the patient to the development of temporal lobe epilepsy. Studies in adult rats have shown that seizures can lead to hippocampal damage and unprovoked recurrent seizures. However, many questions remain as to the relevance of early childhood seizures to hippocampal sclerosis and temporal lobe epilepsy. Human prospective epidemiologic studies have not shown a relationship between early childhood seizures and temporal lobe epilepsy. Recent MRI studies in humans suggest that a preexisting hippocampal lesion may predispose infants to experience febrile seizures, later on hippocampal sclerosis, and possibly temporal lobe epilepsy may occur. Unlike the studies in adult rats, normal immature rats with seizures have not been shown to develop hippocampal damage or unprovoked seizures in adulthood. Furthermore, animal studies reveal that preexisting brain abnormalities can predispose to hippocampal damage following seizures early in life. This paper reviews evidence for and against the view that early childhood convulsions, hippocampal sclerosis, and temporal lobe epilepsy are related, while also exploring clinical and animal studies on how seizures can lead to hippocampal damage, and how this can result in temporal lobe epilepsy. By better understanding the cause and effect relationship between early childhood seizures and hippocampal injury in normal and abnormal brains specific treatments can be developed that target the pathogenesis of epilepsy.

Transplacentally induced neuronal migration disorders: an animal model for the study of the epilepsies.

Recent clinical and laboratory data suggest that there is a link between neuronal migration disorders (NMD) and increased seizure threshold. To characterize an animal model with features similar to human NMD and to assess seizure susceptibility, NMD were induced in the rat at the time of neuroblastic division (PG15) and three other gestational ages (PG 13, PG14, PG16) by transplacental exposure to methylaxozymethanol (MAM, 25 mg/kg). Offspring pups were monitored for spontaneous and electrographic seizures. At postnatal day 14, randomly selected rat pups were sacrificed for histological examination. In other MAM-exposed pups and controls, status epilepticus was induced by intraperitoneal administration of kainic acid. On histology, NMD were found in all PG 15 MAM-exposed rats, in comparison to 63% of PG 13, 70% of PG 14, 80% of PG16. Histological features included cortical laminar disorganization, ectopic neurons in the subcortical white matter and in cortical layer I, persistent granular layer, marginal glioneuronal heterotopia, and discrete areas of neuronal ectopia in the CA1 subfield of the hippocampus. Based on the severity of the neuronal migration abnormalities, rats were divided into three categories: severe, moderate, and mild. Severe and moderate NMD were only found in the PG 15 MAM-exposed rats. EEG recording in rats with NMD did not disclose spontaneous seizures; however, rats with severe NMD had higher slow wave activity compared to controls (P < .05). MAM-exposed rats with severe NMD were more susceptible to kainic-induced seizures compared to controls (P < .05). In rats with severe NMD, kainic acid-induced status epilepticus produced hippocampal damage in the CA3/4 region. These results demonstrate that MAM-induced NMD have histological and electrographic characteristics similar to human NMD. The severity of neuronal abnormality depends on the time of transplacental exposure as the most severe NMD were found after exposure to MAM at the time of neuroblastic division. The degree of NMD positively correlates with seizure susceptibility, since only rats with severe NMD have decreased seizure threshold. The occurrence of status epilepticus-induced hippocampal damage in pups with severe NMD suggests that the severely compromised hippocampus is less resistant to seizure-induced injury than the normal developing brain.

The expression of GABA(A) receptor subunits in the substantia nigra is developmentally regulated and region-specific.

The substantia nigra pars reticulata (SNR) controls the spread of seizures. GABA(A)ergic drug (agonist or antagonist) infusions into the SNR have age-specific and site-specific effects on flurothyl-induced seizures. Developmental and cell-specific regulation of GABA(A) receptor subunit expression may be responsible for these specific effects. To test this hypothesis, in situ hybridization was used to examine regional expression of alpha1 and gamma2L GABA(A) receptor subunit mRNAs in the SNR during development. Distinct temporal and spatial patterns of expression were observed. In rats at postnatal days (PN) 21-60, fewer neurons were labeled with probes directed to alpha1 and gamma2L subunits in SNRanterior compared with SNRposterior. In addition, neurons in SNRanterior contained higher amounts of hybridization grains than in SNRposterior. In PN 15 rats, the labeling of neurons was relatively diffuse throughout the anterior and posterior SNR regions with moderate amounts of hybridization grains for both subunits. The finding of age-related differential distribution of alpha1 and gamma2L subunit mRNAs in the SNR suggests that GABA(A) receptor heterogeneity may play a role in the age-specific and site-specific effects of GABA(A)ergic agents on seizures in the SNR.

Developmental regulation of glutamate and GABA(A) receptor gene expression in rat hippocampus following kainate-induced status epilepticus.

In adult rats, kainic acid-induced status epilepticus markedly reduces GluR2 (the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid, AMPA subunit that limits Ca2+ permeability), receptor mRNA in the vulnerable CA3 and may contribute to delayed neurodegeneration. In rat pups resistant to kainate seizure-induced hippocampal neurodegeneration by silver impregnation, glutamate or GABA(A) alpha1-receptor mRNAs were unaltered in CA3 neurons 24 h after status epilepticus. In the dentate gyrus, GluR1 and GluR2 mRNAs were transiently increased in P14 but not P5 pups. Immunocytochemistry revealed no apparent differences in the distribution patterns of GluR1, GluR2, or GluR2/3 receptor proteins in the CA3 or dentate gyrus of P14 pups. Status epilepticus-induced alterations in receptor GluR2 and GABA(A) alphal mRNAs and AMPA protein expression vary with developmental age. Sustained expression at young ages may contribute to the resistance of developing hippocampal neurons to seizure-induced damage.

Increased seizure susceptibility in adult rats with neuronal migration disorders.

Recent data show that neuronal migration disorders (NMD) lower the seizure threshold in the immature brain. To assess if this is an age-related phenomenon, kainic acid (KA) was administered to induce status epilepticus in adult rats with NMD. Results of the present study demonstrate that adult rats with NMD had a shorter latency to seizures and longer duration of status epilepticus compared to age-related controls. Furthermore, in rats with NMD seizures were more severe and status epilepticus-induced mortality was worse than in age-matched controls. These data confirm that NMD lower the seizure threshold in the adult rat. The results of the present study combined with our previous studies in the immature rat, suggest that the facilitating effects of NMD on seizures are not age dependent.

Neuronal migration disorders increase susceptibility to hyperthermia-induced seizures in developing rats.

PURPOSE: Retrospective studies suggest that adult patients with intractable epilepsy may have a history of febrile seizures in childhood. Risk factors for a febrile seizure may include the rate of increase in the core temperature (T-core), its peak (Tmax), the duration of the temperature increase, or an underlying brain pathology. Recently, neuronal migration disorders (NMD) have been diagnosed with increasing frequency in patients with epilepsy, but the link between NMD, febrile seizures, and epilepsy is unclear. We studied rat pups rendered hyperthermic to ascertain the incidence of seizures, mortality, and extent of hippocampal cell loss in each group. METHODS: We exposed 14-day-old rat pups with experimentally induced NMD (n = 39) and age-matched controls (n = 30) to hyperthermia (core body temperature > 42 degrees C). RESULTS: The incidence of hyperthermia-induced behavioral seizures and mortality rate were significantly higher in rats with NMD than in controls (p < 0.05). The longer duration of hyperthermia resulted in a higher incidence of behavioral seizures and higher mortality rate (p < 0.05). In rats with NMD, hyperthermia resulted in hippocampal pyramidal cell loss independent of seizure activity; the extent of neuronal damage correlated positively with the duration of hyperthermia. In control rats, occasional neuronal loss and astrocytosis occurred only after prolonged hyperthermia. CONCLUSIONS: In immature rats, NMD lower the threshold to hyperthermia-induced behavioral seizures and hyperthermia in the presence of NMD may cause irreversible hippocampal neuronal damage.

Kainic acid-induced seizures enhance dentate gyrus inhibition by downregulation of GABA(B) receptors.

Seizures cause a persistent enhancement in dentate synaptic inhibition concurrent with, and possibly compensatory for, seizure-induced hippocampal hyperexcitability. To study this phenomenon, we evoked status epilepticus in rats with systemic kainic acid (KA), and 2 weeks later assessed granule cell inhibition with paired-pulse stimulation of the perforant path (PP) in vitro. Controls demonstrated three components of paired-pulse inhibition: early inhibition (10-30 msec), intermediate facilitation (30-120 msec), and late inhibition (120 msec to 120 sec). After seizures, inhibition in all components was enhanced significantly. The GABA(A) antagonist bicuculline blocked only early enhanced inhibition, demonstrating that both GABA(A) and GABA(B) postsynaptic receptors contribute to seizure-induced enhanced inhibition. In controls, the GABA(B) antagonist CGP 35348 increased both GABA(A) and GABA(B) responses in granule cells, suggesting that CGP 35348 acts presynaptically, blocking receptors that suppress GABA release. In contrast, slices from KA-treated rats were markedly less sensitive to CGP 35348. To test the hypothesis that GABA(B) receptors regulating GABA release are downregulated after seizures, we measured paired-pulse suppression of recurrent IPSPs, or disinhibition, using mossy fiber stimuli. Early disinhibition (< 200 msec) was reduced after seizures, whereas late disinhibition remained intact. CGP 35348 blocked the early component of disinhibition in controls and, to a lesser extent, reduced disinhibition in KA slices. However, paired monosynaptic IPSPs recorded intracellularly showed no difference in disinhibition between groups. Our findings indicate that seizure-induced enhancement in dentate inhibition is caused, at least in part, by reduced GABA(B) function in the polysynaptic recurrent inhibitory circuit, resulting in reduced disinhibition and heightened GABA release.

Age-related differences in the effects of GABAA agonists microinjected into rat substantia nigra: pro- and anticonvulsant actions.

GABAergic transmission in the substantia nigra pars reticulata (SNR) has an important role in the control of experimental seizures. In the flurothyl seizure model, SNR microinjection of the selective GABAA receptor agonist muscimol results in a biphasic dose-response curve in adults: Intermediate doses are anticonvulsant, but high doses have proconvulsant effects. Another GABAA agonist, THIP (4,5,6,7-tetrahydroisoxazolo-[5,4-c]pyridin-3-ol), also produces anticonvulsant effects at lower doses, whereas higher doses tend to produce a proconvulsant effect. In 16-day-old rat pups, no anticonvulsant but only proconvulsant effects of muscimol occur, and at lower doses than in adults. These data suggest that the immature SNR is significantly more sensitive to the proconvulsant effects of GABAA receptor agonists than is the SNR of adults. We hypothesize that the age-related differences in nigral GABAergic response may be due to ontogenic changes in GABAA-sensitive neuronal circuits in the SNR.