The ability of steroid hormones to influence profoundly the excitability of the CNS is well documented. The broad objective of this proposal is to explore pharmacologically the molecular mechanisms whereby steroids modulate CNS excitability. Abnormal activation of amino acid receptors has been proposed to play a role in the etiology of psychiatric disorders such as anxiety, depression and schizophrenia. Understanding the mechanisms of steroid actions on the CNS may lead to new strategies for the treatment of psychiatric disorders. During the course of our studies we came upon the unexpected finding that pregnenolone sulfate (PS), an abundant neurosteroid, acts as a positive allosteric modulator at the NMDA receptor while inhibiting the kainate, AMPA, glycine, and GABA responses. A major focus of the research plan will be to test our working hypothesis that steroids such as PS regulate the balance between excitation and inhibition on neurons derived from the vertebrate CNS by acting on excitatory and inhibitory amino acid receptors. Toward this end, whole-cell voltage-clamp and current-clamp techniques will be utilized to character electrophysiologically the effects of PS and related steroids on excitatory and inhibitory amino acid receptor-mediated responses of embryonic chick spinal cord neurons maintained in primary monolayer cell culture. The proposed study has three major parts: First, we will screen a series of steroids for activity on amino acid receptor-mediated currents. Secondly, we will determine the potency, efficacy, and mode of action for PS and each steroid identified, with the goal of elucidating the mechanism(s) of steroid action(s) through a study of their structure-activity relationships. In particular, we will focus our studies on PS and on other steroids that are active at NMDA and non-NMDA glutamate receptors. Finally, the effects of steroids on excitatory and inhibitory synaptic transmission at single synapses will be investigated. In the long run, we will determine whether chronic treatment with active steroids such as PS, progesterone, and reduced metabolites of progesterone induces functional changes of excitatory or inhibitory amino acid receptors and of synaptic transmission at identified synapses. These studies will provide a strong basis for evaluating the role of steroids as modulators of neuronal function, and a foundation for development of novel steroid-based drugs for the treatment of psychiatric disorders.
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