Communication between cells in the nervous system is mediated chemically by neurotransmitters, which are released from presynaptic nerves and interact with specific receptors on postsynaptic cells. The chemical acetylcholine mediates neurotransmission in the peripheral and central nervous systems, interacting with nicotinic acetylcholine receptors, notably at neuromuscular junctions, in autonomic ganglia, and in certain areas of the brain. The function of the nicotinic acetylcholine receptor is to bind acetylcholine and transduce this binding into a depolarization of the postsynaptic membrane via the rapid opening of an ion channel. Receptor activation can be modulated physiologically and pharmacologically by a number of mechanisms. Acetylcholine itself modulates receptor activation homotropically via various desensitization processes, and other transmitters can modulate receptor function heterotropically either by interacting directly with the receptor or through intracellular second messenger systems. This research project will investigate the molecular mechanisms of these modulatory processes and the interactions between them. The neuropeptide substance P appears to heterotropically modulate nicotinic receptor activation by interacting with an allosteric binding site on the receptor. Available evidence suggests that modulation plays a physiological role in the regulation of catecholamine release from the adrenal gland. Although these experiments focus on the mechanism of action of substance P and its effect on agonist-induced desensitization, heterotropic modulation by receptor-coupled second messenger systems will also be investigated. These mechanisms will be studied in various model nicotinic receptor systems including cell lines and purified receptor-enriched membranes, representing both muscle- and neuronal-type nicotinic receptors. Nicotinic receptor responses will be studied over a broad time scale, using both biochemical and biophysical techniques. Elucidation of the mechanisms of these modulatory processes will contribute to the understanding of the physiological and pharmacological regulation of the activity of this important receptor.