Effects of substantia nigra gamma-vinyl-GABA infusions on flurothyl seizures in adult rats

Seizure onset and offset pattern determine the entrainment of the cortex and substantia nigra in the nonhuman primate model of focal temporal lobe seizures

Temporal lobe (TL) epilepsy is the most common form of drug-resistant epilepsy. A major focus of human and animal studies on TLE network has been the limbic circuit and the structures composing the temporal lobe. However, there is also evidence suggesting an active role of the basal ganglia in the propagation and control of temporal lobe seizures. Evidence suggests that the network involved in temporal lobe seizure may depend on their onset and offset pattern but studies on the relationship between the patterns and extralimbic activity are limited. Here, we characterize the involvement of the substantia nigra (SN) and somatosensory cortex (SI) during temporal lobe seizures induced in two nonhuman primates (NHP). The seizure onset and offset patterns were manually classified and spectral power and coherence were calculated. We then analyzed the three first and last seconds of the seizure as well as 3-second segments of recorded in pre-ictal and post-ictal periods and compared the changes based on the seizure onset and offset patterns. Our results demonstrated an involvement of the SN and SI dependent on the seizure onset and offset pattern. We found that seizures with both low amplitude fast activity (LAF) and high amplitude slow activity (HAS) onset patterns were associated with an increase in activity of the SN while the change in activity was limited to LAF seizures in the SI. However, the increase of HPC/SI coherence was similar for both type of onset, while the increase in HPC/SN coherence was specific to the farther-spreading LAF onset pattern. As for the role of the SN in seizure cessation, we observed that the coherence between the HPC/SN was reduced during burst suppression (BS) compared to other termination phases. Additionally, we found that this coherence returned to normal levels after the seizure ended, with no significant difference in post-ictal periods among the three types of seizure offsets. This result suggests that the SN might be involved differently in the termination of the BS seizure pattern. This study constitutes the first demonstration of temporal lobe seizures entraining the SN in the primate brain. Moreover, these findings provide evidence that this entrainment is dependent on the seizure onset pattern and support the hypothesis that the SN might play a role in the maintenance and termination of some specific temporal lobe seizure.

Not Part of the Temporal Lobe, but Still of Importance? Substantia Nigra and Subthalamic Nucleus in Epilepsy

The most researched brain region in epilepsy research is the temporal lobe, and more specifically, the hippocampus. However, numerous other brain regions play a pivotal role in seizure circuitry and secondary generalization of epileptic activity: The substantia nigra pars reticulata (SNr) and its direct input structure, the subthalamic nucleus (STN), are considered seizure gating nuclei. There is ample evidence that direct inhibition of the SNr is capable of suppressing various seizure types in experimental models. Similarly, inhibition via its monosynaptic glutamatergic input, the STN, can decrease seizure susceptibility as well. This review will focus on therapeutic interventions such as electrical stimulation and targeted drug delivery to SNr and STN in human patients and experimental animal models of epilepsy, highlighting the opportunities for overcoming pharmacoresistance in epilepsy by investigating these promising target structures.

Bypassing the Blood-Brain Barrier: Direct Intracranial Drug Delivery in Epilepsies

Epilepsies are common chronic neurological diseases characterized by recurrent unprovoked seizures of central origin. The mainstay of treatment involves symptomatic suppression of seizures with systemically applied antiseizure drugs (ASDs). Systemic pharmacotherapies for epilepsies are facing two main challenges. First, adverse effects from (often life-long) systemic drug treatment are common, and second, about one-third of patients with epilepsy have seizures refractory to systemic pharmacotherapy. Especially the drug resistance in epilepsies remains an unmet clinical need despite the recent introduction of new ASDs. Apart from other hypotheses, epilepsy-induced alterations of the blood-brain barrier (BBB) are thought to prevent ASDs from entering the brain parenchyma in necessary amounts, thereby being involved in causing drug-resistant epilepsy. Although an invasive procedure, bypassing the BBB by targeted intracranial drug delivery is an attractive approach to circumvent BBB-associated drug resistance mechanisms and to lower the risk of systemic and neurologic adverse effects. Additionally, it offers the possibility of reaching higher local drug concentrations in appropriate target regions while minimizing them in other brain or peripheral areas, as well as using otherwise toxic drugs not suitable for systemic administration. In our review, we give an overview of experimental and clinical studies conducted on direct intracranial drug delivery in epilepsies. We also discuss challenges associated with intracranial pharmacotherapy for epilepsies.

Sex dimorphism in seizure-controlling networks

Males and females show a different predisposition to certain types of seizures in clinical studies. Animal studies have provided growing evidence for sexual dimorphism of certain brain regions, including those that control seizures. Seizures are modulated by networks involving subcortical structures, including thalamus, reticular formation nuclei, and structures belonging to the basal ganglia. In animal models, the substantia nigra pars reticulata (SNR) is the best studied of these areas, given its relevant role in the expression and control of seizures throughout development in the rat. Studies with bilateral infusions of the GABA(A) receptor agonist muscimol have identified distinct roles of the anterior or posterior rat SNR in flurothyl seizure control, that follow sex-specific maturational patterns during development. These studies indicate that (a) the regional functional compartmentalization of the SNR appears only after the third week of life, (b) only the male SNR exhibits muscimol-sensitive proconvulsant effects which, in older animals, is confined to the posterior SNR, and (c) the expression of the muscimol-sensitive anticonvulsant effects become apparent earlier in females than in males. The first three postnatal days are crucial in determining the expression of the muscimol-sensitive proconvulsant effects of the immature male SNR, depending on the gonadal hormone setting. Activation of the androgen receptors during this early period seems to be important for the formation of this proconvulsant SNR region. We describe molecular/anatomical candidates underlying these age- and sex-related differences, as derived from in vitro and in vivo experiments, as well as by [(14)C]2-deoxyglucose autoradiography. These involve sex-specific patterns in the developmental changes in the structure or physiology or GABA(A) receptors or of other subcortical structures (e.g., locus coeruleus, hippocampus) that may affect the function of seizure-controlling networks.

GABAergic interneuron development and function is modulated by the Tsc1 gene

Tuberous sclerosis complex (TSC) is a genetic disease with severe neurologic and psychiatric manifestations including epilepsy, developmental delay, and autism. Despite much progress in defining abnormal signaling pathways including the contribution of increased mTORC1 signaling, specific abnormalities that underlie the severe neurologic features in TSC remain poorly understood. We hypothesized that epilepsy and autism in TSC result from abnormalities of γ-aminobutyric acidergic (GABAergic) interneurons. To test this hypothesis, we generated conditional knockout mice with selective deletion of the Tsc1 gene in GABAergic interneuron progenitor cells. These interneuron-specific Tsc1 conditional knockout (CKO) mice have impaired growth and decreased survival. Cortical and hippocampal GABAergic interneurons of CKO mice are enlarged and show increased mTORC1 signaling. Total numbers of GABAergic cells are reduced in the cortex with differential reduction of specific GABAergic subtypes. Ectopic clusters of cells with increased mTORC1 signaling are also seen suggesting impaired interneuron migration. The functional consequences of these cellular changes are evident in the decreased seizure threshold on exposure to the proconvulsant flurothyl. These findings support an important role for the Tsc1 gene during GABAergic interneuron development, function, and possibly migration.

The role of substantia nigra pars reticulata in modulating clonic seizures is determined by testosterone levels during the immediate postnatal period

GABAergic activation of substantia nigra pars reticulata (SNR) at postnatal day (PN) 15 has sex-specific features on seizure control in vivo and electrophysiological responses in vitro. In males, the GABA(A)-receptor agonist muscimol has proconvulsant effects and induces depolarizing responses. In females, muscimol has no effect on seizures and evokes hyperpolarizing responses. We determined the time period during which sex hormones must be present to produce the sex-specific muscimol effects on seizures and their influence on SNR GABA(A) receptor-mediated postsynaptic currents. Exposure to testosterone or its metabolites (estrogen or dihydrotestosterone) during PN0-2 in females or males castrated at PN0 was sufficient to produce proconvulsant muscimol effects but did not affect the in vitro GABA responses, which remained hyperpolarizing. The data suggest that the PN0-2 period is critical for the development of the seizure-controlling SNR system; the hormonal effect on seizure control is independent from their effect on GABA conductance.

Sex-specific control of flurothyl-induced tonic-clonic seizures by the substantia nigra pars reticulata during development

The substantia nigra pars reticulata (SNR) plays an important age- and sex-specific role in control of clonic seizures. Its involvement in control of tonic-clonic seizures is contradictory. We investigated the role of the SNR in the tonic-clonic seizures induced in male, female and neonatally castrated male rats using flurothyl. In adult female rats, vaginal impedance determined the changes in progesterone/estrogen ratio. Rats at various postnatal ages received infusions of muscimol or vehicle in the SNRanterior or SNRposterior. Furthermore, in 15-day-old (P15) and adult male rats, ZAPA (a GABA(A) receptor agonist) or AP7 (an NMDA receptor antagonist) was infused. The developmental profile of tonic-clonic seizure threshold differed between male and female rats possibly due to early postnatal testosterone surge in male rats. On the other hand, changing estrogen/progesterone ratio in cycling adult female rats had no effect on seizure threshold. Intranigral muscimol had proconvulsant effects on tonic-clonic seizures only in immature rats, and this effect was dependent on the perinatal testosterone surge. ZAPA had anticonvulsant effects in P15 rats but was not effective in adult rats. Only AP7 had anticonvulsant effects in both adult and P15 rats. Results indicate that thresholds for flurothyl-induced tonic-clonic seizures develop under the control of postnatal testosterone. Although GABAergic inhibition in the SNR affects tonic-clonic seizures in developing rats, only the NMDA antagonist had consistent anticonvulsant effects throughout development.

Deep brain stimulation in epilepsy

Since the pioneering studies of Cooper et al. to influence epilepsy by cerebellar stimulation, numerous attempts have been made to reduce seizure frequency by stimulation of deep brain structures. Evidence from experimental animal studies suggests the existence of a nigral control of the epilepsy system. It is hypothesized that the dorsal midbrain anticonvulsant zone in the superior colliculi is under inhibitory control of efferents from the substantia nigra pars reticulata. Inhibition of the subthalamic nucleus (STN) could release the inhibitory effect of the substantia nigra pars reticulata on the dorsal midbrain anticonvulsant zone and thus activate the latter, raising the seizure threshold. Modulation of the seizure threshold by stimulation of deep brain structures-in particular, of the STN-is a promising future treatment option for patients with pharmacologically intractable epilepsy. Experimental studies supporting the existence of the nigral control of epilepsy system and preliminary results of STN stimulation in animals and humans are reviewed, and alternative mechanisms of seizure suppression by STN stimulation are discussed.

Fos expression and 2-deoxyglucose uptake following seizures in developing genetically epilepsy-prone rats

Juvenile genetically epilepsy-prone rats (GEPR)-3s display one of three types of seizures in response to sound: a typical class 3 seizure consisting of an explosive running/bouncing episode followed by a clonic seizure (audiogenic response score, ARS-3); an ARS-3 seizure followed by a forebrain seizure that includes facial and forelimb (F&F) clonus with rearing (ARS-3f); or, a running/bouncing episode followed by a severe tonic seizure with complete hindlimb extension (ARS-9) not accompanied with subsequent F&F clonus. The adult seizure phenotype, manifest in all GEPR-3s by age 45 days of age, consists of an ARS-3 not followed by F&F clonus or tonic extension. The present studies sought to determine the neuronal networks activated during these various developmental convulsive patterns by examining anatomical patterns of [(14)C]2-deoxyglucose (2-DG) uptake or immediate-early-gene (Fos) expression subsequent to seizures. Many, but not all, brain areas of control rats showed age-related increases in Fos expression in response to the acoustic stimulation. An age effect was not observed in 2-DG uptake. In GEPRs, the profiles of Fos expression and 2-DG uptake following seizures were often parallel; however, there were notable exceptions. For example, increased 2-DG uptake in the cochlear nuclei, central region of the inferior colliculi, and the substantia nigra were not accompanied by increased Fos expression in these areas regardless of the seizure phenotypes. Reciprocally, other regions, particularly in the amygdala, ventromedial hypothalamus and parabrachial areas, displayed intense seizure related Fos labeling without detectable increases in 2-DG uptake. Fos and 2-DG uptake patterns in response to acoustic stimulation varied according to brain region, seizure phenotype and severity. In general, the degree of 2-DG uptake correlated with seizure severity. For example, the ARS-9 seizures, being the most intense, resulted in significant increases in 2-DG uptake in almost all brain regions examined. 2-DG uptake following the ARS-3f and ARS-3 seizures, although increased, did not reach statistical significance in most brain areas. In contrast to the 2-DG findings, a seizure-severity dependent effect was not seen with Fos. Rather, the induction of Fos associated with acoustic stimulation and seizure was more associated with age and seizure-phenotype. Thus, the developmental profiles of Fos expression and 2-DG uptake in response to seizures are distinctly different and concurrent examination of both markers is useful in the identification of brain circuitry involved in seizure development.

Sex and the substantia nigra: administration, teaching, patient care, and research

The immature brain is most susceptible to the development of seizures. The substantia nigra may play a crucial role in the control of seizures as a function of age. In the adult substantia nigra pars reticulata (SNR), there are two regions that mediate opposing effects on seizures after infusions of GABA(A) agents. One region is located in the anterior SNR, and localized muscimol infusions mediate anticonvulsant effects. These anticonvulsant effects use a circuitry that may involve the ventromedial thalamic nucleus, the deep layer of the superior colliculus, or both. The second region is in the posterior SNR, and muscimol infusions produce proconvulsant effects, perhaps mediated by the striatum, the globus pallidus, the deep layer of the superior colliculus, or all three. In developing male rats, only the proconvulsant region is present up to the age of 21 days. In ongoing studies, it has been shown that, in the male rat, the transition from the immature to mature SNR-mediated seizure control occurs between the ages of 25 and 30 days, just before adolescence. In male rats castrated on the day of birth, the ensuing depletion of testosterone accelerates the development of the anterior SNR with its anticonvulsant features. Castration does not alter the development of the proconvulsant region. In the developing female SNR, muscimol infusions produce only anticonvulsant effects. The data indicate that gonadal hormones may have an important role in the maturation of systems involved in the containment of seizures.

Developmental regulation of regional functionality of substantial nigra GABAA receptors involved in seizures

GABAergic (gamma-aminobutyric acid) transmission in the substantia nigra pars reticulata is critical for seizure control. We tested the hypothesis that there is a differential regional distribution and functionality of nigral GABAA receptor sites that is developmentally regulated. In adult rats, we determined the effects on flurothyl seizures of (Z)-3-[(aminoiminomethyl)thio]prop-2-enoic acid (ZAPA, a presumed agonist of the low-affinity GABAA receptor site), bicuculline (an antagonist of the low-affinity GABAA receptor site) and gamma-vinyl-GABA (a GABA-transaminase inhibitor), infused bilaterally in anterior or posterior substantia nigra pars reticulata. ZAPA infusions (8 micrograms) were anticonvulsant in anterior substantia nigra but proconvulsant in posterior substantia nigra. Bicuculline infusions (100 ng) were proconvulsant in anterior substantia nigra but ineffective in posterior substantia nigra. An anticonvulsant dose of gamma-vinyl-GABA, when infused in anterior substantia nigra, was proconvulsant when infused in posterior substantia nigra. In 15 day old rats, the effects of ZAPA, were biphasic: 2 micrograms was anticonvulsant while 8 micrograms was proconvulsant. There was no regional specificity. The data suggest that with maturation there is functional segregation of specific GABAA receptor subtypes involved in substantia nigra-mediated seizure control.

Neither intranigral fluoxetine nor 5,7-dihydroxytryptamine alter audiogenic seizures in genetically epilepsy-prone rats

Previous studies have shown that widespread depletion of brain 5-hydroxytryptamine (5-HT, serotonin) exacerbates audiogenic seizures in genetically epilepsy-prone rats (GEPRs), while elevations in brain 5-HT attenuate these seizures. However, the location of the central nervous system site(s) at which 5-HT exerts its anticonvulsant action on audiogenic seizures, remains unknown. The substantia nigra has been shown to exert modulatory actions over both brainstem and forebrain driven seizures in normal rats, and receives a rich serotonergic innervation. The present study was designed to determine if 5-HT exerts its modulatory effect on audiogenic seizures by an action in the substantia nigra. Microinfusion of 5,7-dihydroxytryptamine (4 micrograms/0.25 microliter bilateral) into the substantia nigra of GEPRs which display a moderate seizure (GEPR-3s) failed to alter the audiogenic seizure. Consistent with these findings, microinfusions of fluoxetine-HCl into the substantia nigra of severe seizure GEPRs (GEPR-9s) failed to alter any aspect of the audiogenic seizure. This effect was observed when fluoxetine was infused alone, or in combination with systemic administration of 5-hydroxytryptophan (75 mg/kg, i.p.). The present findings argue against a modulatory role of nigral 5-HT on audiogenic seizures in GEPRs.

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.

The substantia nigra pars reticulata, seizures and Fos expression

The induction of the proto-oncogene c-fos has been used extensively to identify spatially distributed neural systems activated by seizures. The substantia nigra pars reticulata (SNpr) has been implicated as a critical structure in neural networks involved in the modulation of seizure expression, yet the SNpr has not been reported to express Fos following seizures induced in a variety of seizure paradigms. In this study we determined whether (1) the temporal characteristics of Fos induction in the SNpr were different than those of other brain areas following kindled seizures, (2) neurons in the SNpr possess the cellular machinery to express Fos, (3) Fos can be induced in SNpr by direct electrical stimulation, and (4) Fos expression is induced in the SNpr following kainate or pilocarpine-induced status epilepticus. Results indicate that Fos is not induced in SNpr at any time point (1-12 h) after kindled seizures, and that serum response factor, a constitutively expressed nuclear protein necessary for Fos expression, is present in SNpr neurons. Results further indicate that Fos expression in the SNpr is induced following either direct electrical stimulation or pilocarpine status, but not status elicited by kainate. We conclude that, in so far as the SNpr represents a critical structure for modulating seizure expression, seizure activity does not represent a sufficient stimulus to induce Fos in SNpr neurons. Further, the neural networks defined by Fos expression following seizure may be incomplete, and should be interpreted conservatively.

The development of epilepsy in the paediatric brain

The immature central nervous system (CNS) is more susceptible to the development of seizures than its adult counterpart. Developmental studies of experimental seizures have suggested that young animals have unique behavioural seizure patterns, including the presence of bilateral, though asymmetric, convulsions. There are differences in the mechanisms responsible for the generation of seizures, propagation patterns and seizure arrest and recurrences. These differences are due to local factors as well as factors that affect neural systems consisting of long neuronal circuits. The substantia nigra, a site involved in the control of seizures, will be used as an example to demonstrate how evolving neurobiological processes modulates the suppression or exacerbation of seizures with age. Evidence will also be presented indicating that early in life, seizures may not produce hippocampal damage. An understanding of the age-related differences is important for the development of rational approaches to treating seizures and their consequences.

Seizure suppression in kindling epilepsy by intracerebral implants of GABA- but not by noradrenaline-releasing polymer matrices

Gamma-aminobutyric acid (GABA)-releasing polymer matrices were implanted bilaterally, immediately dorsal to the substantia nigra, in rats previously kindled in the amygdala. Two days after implantation, rats with GABA-releasing matrices exhibited only focal limbic seizures in response to electrical stimulation, whereas animals with control matrices devoid of GABA had generalized convulsions. GABA release from the polymer matrices was high during the first days after implantation, as demonstrated both in vitro and, using microdialysis, in vivo. The anticonvulsant effect was no longer observed at 7 and 14 days at which time GABA release was found to be low. In a parallel experiment, polymer matrices containing noradrenaline (NA) were implanted bilaterally into the hippocampus of rats with extensive forebrain NA depletion induced by an intraventricular 6-hydroxydopamine injection. No effect on the development of hippocampal kindling was observed, despite extracellular NA levels exceeding those of rats with intrahippocampal locus coeruleus grafts that have previously been shown to retard kindling rate. The results indicate that GABA-releasing implants located in the substantia nigra region can suppress seizure generalization in epilepsy, even in the absence of synapse formation and integration with the host brain. In contrast, the failure of NA-releasing polymer matrices to retard the development of seizures in NA-depleted rats suggests that such an effect can only be exerted by grafts acting through a well-regulated, synaptic release of NA.

Further evidence of involvement of substantia nigra GABAB receptors in seizure suppression in developing rats

To determine whether the substantia nigra GABAergic anticonvulsant effects depend on GABAB receptor activation, we tested the effects of intranigral CGP 35,348 (a GABAB receptor antagonist) alone or in combination with gamma-vinyl-GABA (GVG; a GABA-transaminase inhibitor) on flurothyl seizures in rat pups and adult rats. In pups, nigral infusions of CGP 35,348 decreased the thresholds for clonic and tonic seizures and attenuated the anticonvulsant effects of GVG. In adults, nigral infusions of CGP 35,348 did not alter seizure thresholds. The data suggest that, in rat pups, nigral GABAB receptors regulate, in part, flurothyl-induced seizures.

Baclofen prevents rapid amygdala kindling in adult rats

This study investigates the effect of the gamma-aminobutyric acid (GABAB) agonist, baclofen, on amygdala kindling in adult rats. Baclofen has been reported to be anticonvulsant in a variety of seizure models and prevents kindling in immature rats. These experiments describe the effects of baclofen (2, 5 and 10 mg/kg, i.p.) on the afterdischarge threshold and kindling rate. Baclofen, 10 mg/kg, significantly increased the afterdischarge threshold in the amygdala. Baclofen at 5 and 10 mg/kg, retarded the rate of kindling as measured by the number of stimuli required to advance to subsequent seizure stages. These results suggest that baclofen may decrease the local excitability of the amygdala and retard the rate of seizure spread (or generalization) throughout the brain. Baclofen, acting at GABAB receptors exerts an anticonvulsant effect on amygdala kindling in these experiments.

The influence of thalamic GABA transmission on the susceptibility of adult rats to flurothyl induced seizures

There is considerable evidence that thalamic nuclei are involved in the propagation and regulation of seizures. In the present study, we investigated the possible role in seizure mechanisms of GABAergic transmission in two thalamic nuclei, the posterior nucleus (PO) and the ventromedial nucleus (VM). Several GABAergic drugs were bilaterally microinfused into PO or VM of adult rats via chronically implanted cannulae, before testing the rats' susceptibility to seizures induced by flurothyl. In PO, infusions of the GABA elevating agent gamma-vinyl-GABA (20 micrograms) or of the GABAA receptor agonist muscimol (100 ng) suppressed both clonic and tonic seizures. Infusions into PO of the GABAA receptor antagonist bicuculline (100 ng) facilitated both these seizure types. Administration of the GABAB receptor agonist baclofen (200 ng) also suppressed clonic seizure susceptibility. Drug infusions into VM, however, did not significantly modify the susceptibility to seizures. These findings lead us to conclude that GABAergic transmission in the vicinity of the PO, but perhaps not in VM, affects flurothyl seizure susceptibility. We hypothesize that GABA synapses in PO may be part of a seizure propagation or control circuit including striatum, substantia nigra, and superior colliculus.

Alterations in regional brain GABA concentration and turnover during pregnancy

During pregnancy, mice are more susceptible to flurothyl-induced seizures than are nonpregnant control mice. The potential role of brain GABA in mediating this behavior was examined in the present study. GABA concentrations in the cerebellum, hippocampus, striatum, midbrain, and cortex from individual control, pregnant (days 17-18) and delivery-day Heterogeneous Stock mice were assayed using a fluorometric method. Turnover of GABA was assessed by inhibiting metabolism with aminooxyacetic acid and measuring GABA accumulation over the next 2 h. Steady-state GABA concentrations decreased significantly from control in all brain regions during pregnancy. Reductions in GABA concentrations were approximately 25-30% in the affected regions. At parturition, GABA concentrations in the cerebellum and cortex returned to control levels, but hippocampal, striatal, and midbrain GABA levels remained significantly depressed. All the indices of GABA turnover--first-order rate constant, half-life, initial rate of synthesis, and turnover rate (product of first-order rate constant and initial concentration)--showed a significant reduction in pregnancy, which was continued through the time of delivery in all brain regions except the hippocampus. Half-life values for GABA increased nearly fourfold in the cerebellum and cortex. These results show that there is a significant alteration in GABAergic systems during pregnancy and parturition. We suggest that the reduction in GABA turnover is a compensatory anticonvulsant mechanism to offset the inherent seizure susceptibility brought about by the reduced level of the major inhibitory neurotransmitter in the brain.

The proconvulsant effect of nigral infusions of THIP on flurothyl-induced seizures in rat pups

The substantia nigra gamma-aminobutyric acid (GABA) system is crucial for seizure control. Our previous work indicates that in 16-day-old rat pups, nigral administration of the GABAA receptor agonist muscimol facilitates flurothyl-induced seizures, whereas it suppresses seizures in adult rats. To determine whether the proconvulsant effect of muscimol in rat pups may be mediated by nigral GABAA receptors, in the present study we applied a selective GABAA receptor agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP). Bilateral nigral infusions of THIP (500 or 700 ng) significantly decreased the thresholds for flurothyl seizures in a dose-dependent fashion. Doses of 350 ng or less did not significantly modify the susceptibility to seizures. An anticonvulsant action of THIP could not be detected at any dose. Administration of an effective THIP dose (500 ng) 2 mm dorsal to the SNR had no influence on seizures. These findings suggest that in rat pups the proconvulsant effect of nigral GABAA receptor agonists may be attributed to unique pharmacologic characteristics of GABAA receptors during development.