Cyclic GMP is a vital second messenger in the CNS. It has been shown to have specific actions at presynaptic and postsynaptic sites that induce dramatic short-term alterations in synaptic efficacy. The central hypotheses of this proposal, that these actions act synergistically to affect information flow through visual cortex and that similar actions play a role in longer lasting forms of plasticity, will be tested with four specific aims. In the first aim, the mechanism by which cGMP enhances NMDA receptor responses will be studied. Electrophysiological measurements will be used to confirm that elevation of cGMP decreases receptor desensitization. Recombinant NMDA receptor subunits, including those that change in postnatal development, will be used to determine whether cGMP causes direct phosphorylation of one or more subunits. Finally, the small depolarization induced in some cortical neurons by cGMP will be studied to determine how these actions combine to increase selectively the proportion of the glutamate response passing through the NMDA receptor.
The second aim will study the mechanism by which cGMP can dramatically inhibit cortical GABAergic responses at the presynaptic terminal. A combination of electrophysiological measurements and calcium imaging studies will be carried out to test the hypothesis that elevation of cGMP levels leads to inhibition of both plasma membrane voltage-dependent calcium channels and the endoplasmic reticulum InsP3 receptor calcium channel. In addition, cGMP-mediated effects on the downstream mobilization of synaptic vesicles will be tested.
The third aim will study the mechanism of cGMP-induced membrane depolarization in cortical neurons. Past work has shown that this can occur through cyclic-nucleotide gated cation channels. Bcng channels, responsible for the Ih current in many cell types, are also regulated by cyclic nucleotides. The development and distribution of these channels in different cortical cells will be studied. The effects of cGMP and cAMP on these cannels will be studied to determine how activation can alter synaptic responses of cortical cells. Finally, actions of cGMP will be studied using stimulus paradigms that cause longer lasting forms of synaptic plasticity. Such experiments will test the hypothesis that the short-term modulation studied mechanistically in this proposal are also important for longer lasting potentiation or depression of cortical synaptic responses. This proposal is a coherent program that will explain actions of a vital second messenger at the molecular level and show how it affects the physiology of individual cells and cell assemblies within visual cortex.
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