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Summary:Purpose: Development and sex hormones are important determinants of seizure susceptibility. Seizures develop in the immature brain more readily than in the mature brain. Male children experience a higher incidence of epilepsy or unprovoked seizures than do female children. Sex-specific differences in the development of seizure-suppressing neuronal networks may account, at least in part, for this increased age-and sex-related susceptibility to seizures. The control of seizures can be influenced by the substantia nigra pars reticulata (SNR) in an age-and sex-specific manner. In the adult male rat SNR, two topographically discrete regions (SNRanterior and SNRposterior) mediate distinct effects on seizures, by using divergent output networks in response to localized infusions of γ -aminobutyric acid (GABA)A agents, such as muscimol. The GABAA-sensitive "anticonvulsant" region is located in the SNRanterior, whereas the GABAA-sensitive "proconvulsant region is in the SNRposterior. In immature postnatal day (PN)15-21 male rats, the SNR is not topographically segregated, and GABAAergic drug infusions produce similar effects when applied in the SNRanterior or SNRposterior. Only a GABAA-sensitive proconvulsant network is evident. By contrast, female SNR does not contain any region that mediates muscimol-related proconvulsant effects. As with the adult, immature female rats do not develop a proconvulsant SNR region at any age.Methods: We measured the effects of SNR muscimol infusions on seizures in male rats castrated at birth to better understand the effects of testosterone on the formation of age-and sex-specific features of the SNR.Results: Neonatal castration permanently alters the maturation of the muscimol-sensitive SNR effect on seizures. The SNR of neonatally castrated rats develops functionally like the "female" SNR. The "proconvulsant" SNR region does not develop in the absence of testosterone in the immediate postnatal period. The "male" type of SNR effects can be induced in neonatally castrated rats by restoration of testosterone levels or in female rats by artificially increasing testosterone levels. Dihydrotestosterone and estrogen, produced by the reduction and aromatization of testosterone, respectively, are the direct mediators of testosterone actions. At PN0, only β estrogen receptors are equally expressed in the SNRs of males and females and may be responsible for testosterone-mediated effects in both sexes.Conclusions: The phenotype of SNR GABAergic neurons, as characterized by GABAA-receptor subunit composition, by muscimol-induced electrophysiologic responses, and by connectivity of output networks each may be altered by the presence of testosterone. Higher KCC2 messenger RNA (mRNA) expression in female PN15 SNR neurons compared with males may be responsible for sex-related differences in muscimolinduced electrophysiologic responses. In summary, a growing body of compelling evidence identifying sex-related differences in the SNR implicates postnatal testosterone as a critical factor in the dev...

Summary: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 I11 acting agents were used to study whether activation of mGluR I11 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. ~-2-Amino-4-phosphonobutyric acid (an mGluR I11 agonist) restored EAAT activity and reduced epileptiform activity to near control levels.The saturation curve for glutamate uptake in slices from KAseized 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 111 as a possible source of their plasticity.

Summary:Purpose: Excitatory amino acid transporter (EAAT) activity prevents Glu from reaching toxic levels, but their contribution to epileptogenesis remains controversial. We examined how the convulsant veratridine causes inhibition of EAAT activity and how it differs from the effects of another convulsant, high (50 mM) K + , that also increases Na + conductance.Methods: Transverse rat hippocampal slices were incubated for 1 h with 100 M veratridine in oxygenated artificial cerebrospinal fluid (aCSF) with or without extracellular Ca 2+ . The medium was replaced by 50 M [ 3 H]glutamate in aCSF, and the slices incubated for 10 min at 37°C. The slices were washed 3 times with cold aCSF after removal of the extracellular medium, and the radioactivity was quantified after solubilization of the slices.Results: Veratridine caused a time-and dose-dependent decrease, whereas high K + had no effect on EAAT activity. The effects of veratridine on EAAT activity were not prevented by tetrodotoxin (TTX; 10 M). Coincubation of ouabain with veratridine resulted in further decreases of EAAT activity. Removal of extracellular Ca 2+ potentiated the inhibitory effects of veratridine (and other convulsants) on EAAT activity. Chelation of intracellular Ca 2+ with BAPTA also increased the inhibitory effects of veratridine on EAAT activity.Conclusions: Veratridine caused changes Ca 2+ dynamics that led to inhibition of EAAT activity. Such changes in EAAT activity can contribute to the sustained epileptiform activity caused by veratridine.

Glutamate (Glu) uptake is the primary mechanism for its removal from the synapse. In genetic audiogenic seizures (AGS), Glu uptake is elevated prior to the appearance of seizures. Increased Glu uptake is also observed in synaptosomes from normal mice preincubated with lithium or nitroarginine, an NO synthase inhibitor. Pertussis and cholera toxins cause a marked reduction in Glu uptake. In contrast, neither lithium nor nitroarginine affected Glu uptake by synaptosomes from genetic epileptic mice. Arachidonic acid inhibits Glu uptake, whereas synaptosomes from epileptic mouse brain appear to be more sensitive to arachidonic acid as indicated by a shift of the inhibition curve to the left. These observations are indicative of the possible regulation of Glu uptake by second messengers and its alteration in genetic epilepsy.