The overall goal of this project is to investigate strategies for the drug treatment of epilepsy through pharmacological studies in animal models. Research was continued evaluating the role of neuroactive steroids in epilepsy and their possible uses in epilepsy therapy. Neuroactive steroids are endogenous steroid hormones (and their synthetic analogs) that rapidly alter the excitability of neurons by direct actions on membrane ion channels, including the GABA-A and NMDA receptors. In prior reporting periods, several structurally-related neuroactive steroids were demonstrated to be potent anticonvulsants in the mouse pentylenetetrazol (PTZ) seizure test, an effect that was associate with their ability to potentiate GABA-evoked chloride currents. In addition, it was shown that GABA-potentiating neuroactive steroids are highly protective in models of status epilepticus. Chronic treatment studies indicated that unlike other GABA potentiating drugs tolerance does not develop to the anticonvulsant activity of neuroactive steroids, supporting their potential utility in seizure therapy. Additional studies have examined the anticonvulsant activity of progesterone with the aim of exploring why fluctuations in seizure frequency often occur in women during the menses and following pregnancy. Progesterone was demonstrated to be an effective anticonvulsant in the PTZ test, and this activity was further shown to be due to conversion of progesterone (via 5alpha-hydroxylase isoenzymes) to the GABA-potentiating neuroactive steroid allopregnanolone. The results suggest that neuroactive steroids could be of utility in the treatment of seizure exacerbations that occur with changing progesterone levels during the menstrual cycle (catamenial epilepsy) and following pregnancy. In the present reporting period, studies were conducted on pregnenolone sulfate (PS), an endogenous neuroactive steroid that has previously been shown to antagonize GABA-A-receptor-mediated inhibitory responses and potentiate NMDA receptor-mediated excitatory responses in vitro. To assess the actions of the steroid as a modulator of seizure susceptibility in vivo, PS was administered intracerebroventricularly in mice. PS elicited limbic, clonic and tonic seizures that progressed to status epilepticus. Protection against PS-induced seizures and lethality was conferred by the GABA-A receptor positive allosteric modulators clonazepam and allopregnanolone, and by the NMDA receptor antagonists dizocilpine and (R)-CPP. The pharmacological profile suggested that the convulsant actions of PS are mediated predominantly via its effects on GABA-A receptors, and also possibly by effects on NMDA receptors. These observations raise the possibility that PS may serve as an endogenous regulator of seizure susceptibility.
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