Inositol 1,4,5-trisphosphate (InsP3) mediates calcium release from intracellular stores, thereby regulating cell activity. Neuronal InsP3-levels are strongly depressed by chronic lithium treatment, an effective therapy in manic-depressive psychosis, suggesting a relationship between the intracellular actions of InsP3 and the symptoms of manic-depressive psychosis. InsP3 acts by binding to specific receptors that are coupled to calcium. channels. In the present proposal, the functional domains and regulation of the InsP3-receptor will be studied. The long-term objective will be the elucidation of the molecular mechanisms by which InsP3-receptors release calcium from intracellular stores. The specific aims of the experiments described are: 1. To characterize the structures of the functional and regulatory domains of the InsP3-receptor. In preliminary studies an InsP3-receptor has been purified, molecularly cloned, and functionally expressed in COS cells. Its structure-function relationships will be investigated by expressing and characterizing mutant forms of the receptor and by biochemical assays utilizing site-specific antibodies. 2. To characterize the intrinsic calcium channel of the receptor. Novel expression approaches will be employed to localize the transmembrane regions that form the calcium channel, and mutagenesis experiments will delineate amino acid residues important in channel function. 3. To clone and characterize novel InsP3-receptors by homology with the currently available receptor. The experiments will employ partial clones obtained in preliminary experiments as probes to isolate full-length cDNA's. Novel receptors will be characterized biochemically by sequence-specific antibodies and by expression in heterologous cells. Together, these experiments will provide a molecular description of the functions of the InsP3-receptor and of the pathway(s) by which InsP3 modulates calcium levels in cells.