Abstracts

Previous developmental studies in vitro suggested that the inhibitory neurotransmitter GABA exerts depolarizing and excitatory actions on the immature neurons and that depolarizing GABA is causally linked to the ictal activity during the critical developmental period of enhanced excitability. However, little is known on the role of GABA in the generation of neonatal seizures in vivo. This is a critical issue since neonatal seizures in humans are frequently treated with GABA-ergic drugs. Here we studied the effects of GABA(A) acting drugs on electrographic seizures induced by local hippocampal infusion of epileptogenic agents in non-anaesthetized rats in vivo during the second postnatal week. Thirty-six Sprague-Dawley rat pups of postnatal days [P] 8-12 were used in the study. Extracellular recordings were performed from the CA3 pyramidal cell layer of the hippocampus. A burr hole of 0.5 mm in diameter was drilled in the skull above the hippocampus and the dura cut and removed. A wire electrode (50 [mu]m in diameter) for extracellular field potential recordings was inserted into the application cannula. The application cannula with recording electrode was positioned into the CA3 pyramidal cell layer of the hippocampus under stereotaxic and electrophysiological guidance. Reference and ground electrodes were implanted into the cerebellum. After surgery the isoflurane anesthesia was stopped, and the pups were left to recover from anesthesia for 10-15 minutes, and then electrophysiological data were recorded uninterrupted for 60-120 min. Drugs were applied locally through the stainless-steel cannula (inner diameter 100 [micro]m) using microsyringes. Hippocampal seizures were induced by brief repetitive microinjections of high-K⁺ (10 mM) / Mg²⁺ - free ACSF into the stratum radiatum of CA3 hippocampus in the vicinity of the recording electrode. We found that the induction of ictal-like events was facilitated by co-infusion of 10 mM K⁺ / Mg²⁺ - free ACSF together with the GABA(A) antagonist bicuculline. Tonic-clonic discharges appear on 6-9^th application in 10 mM K⁺ / Mg²⁺ - free ACSF and on 1-3 application in the presence of 15 ml bicuculine. Also the amplitude of seizures obtained in the presence of bicuculine was significantly increased comparatively to the amplitude of seizures obtained with10 mM K⁺ / Mg²⁺ alone. Moreover, the infusion of bicuculline in isolation often caused ictal-like activity. Co-infusion of the GABA(A) receptor agonist isoguvacine or the GABA(A) positive allosteric modulator diazepam completely prevented 10 mM K⁺ / 0 mM Mg²⁺ - induced seizures. Our results indicate that in the in vivo conditions, GABA, acting via GABA(A) receptors, exerts an anticonvulsive actions during the critical developmental period of enhanced excitability These results demonstrate that even in very young rats GABAergic agents are anticonvulsant rather than proconvulsant. (Supported by the NINDS (R01NS041595).)