The nervous system operates by conducting impulses from one neuron to another by synaptic transmission. The essential events in synaptic transmission are the release of neurotransmitter from a presynaptic neuron and the binding of that neurotransmitter to receptor proteins on the membrane of a postsynaptic cell. This process of synaptic transmission is fundamental to the functioning of the nervous system. In addition to endogenous neurotransmitters, many exogenous compounds, including therapeutic and non-therapeutic agents, bind to neurotransmitter receptors and, thereby, alter neuronal function. In many situations, repeated or continuous exposure to drugs or neurotransmitters results in changes in the responsiveness of neurons to that drug or neurotransmitter. Frequently this change involves a decreased sensitivity to that drug. With therapeutic agents, this may be observed as the development of tolerance or tachyphylaxis. At the cellular level, a variety of changes are observed following repeated or continuous exposure of neurons to certain neurotransmitters. These changes include desensitization, internalization and resensitization. These cellular processes may also be involved in the development of dependence to drugs and in the development of side effects to certain drugs. The overall goal of this proposal is to examine the molecular mechanisms involved in desensitization internalization and resensitization of receptors for substance P and gonadotrophin-releasing hormone in single neurons isolated from sympathetic ganglia. Sympathetic neurons have receptors for both of these peptides an respond to the peptides with changes in cell membrane conductance. By monitoring this conductance, the cellular state with regard to desensitization, internalization and resensitization can be assessed. The techniques used to study single cells permit the precise control of intracellular and extracellular solutions. These techniques will permit the cellular changes in response to neurotransmitters to be studied in single cells with precision that has not been previously achieved. The major questions to be addressed in these studies are: 1) What are the molecular mechanisms involved in the internalization of the substance P receptor in single neurons? 2) What are the requirements for resensitization of the substance P receptor? 3) What is the mechanism for desensitization of responses to gonadotrophin-releasing hormone?