Gonadal steroids play a critical role in the regulation of male copulatory behavior. Evidence suggests that steroids regulate mating behavior by acting on three target areas in the limbic system, the medial nucleus of the amygdala (M), the bed nucleus of the stria terminalis (BNST), and the medial preoptic area (MPOA). Neurons in these areas accumulate gonadal steroids and play a critical role in the regulation of mating behavior. However, the mechanisms by which steroids influence the function of neurons involved in the regulation of copulation are not fully understood. Recent studies in my lab and others have shown that neurons and terminals in these areas contain a variety of neuroactive peptides, and that the levels of these peptides are regulated by gonadal steroids. Since these neuropeptides may play a role in neural transmission, the levels of neuropeptides present in these neurons may influence their ability to mediate mating behavior. Thus, one way by which steroids may influence mating behavior is by regulating the levels of these neuropeptides. The goal of the studies in this proposal is to determine whether steroids regulate male .hamster mating behavior by regulating the production of peptide neurotransmitters within the medial nucleus of the amygdala, the bed nucleus of the stria terminalis and the medial preoptic area. Currently, there is little information on the specific types of neuropeptides contained in neurons in M, MPOA, or BNST that are involved in the regulation of mating behavior. Moreover, to clarify the functional organization of peptidergic neurons involved in mating behavior, it is important to determine whether specific neuropeptides are present in separate neurons, or whether they co-occur in individual neurons. tract tracing. Finally, to help clarify the cellular mechanisms by which steroids regulate peptide levels, it is important to determine whether testosterone itself, or one of its androgenic or estrogenic metabolites, regulates peptide levels in neurons in M, MPOA and BNST. To address these issues, I propose to carry out several anatomical and behavioral studies. First, I will use single-and double-label immunohistochemical techniques to identify the neuropeptides contained in neurons within M, MPOA and BNST. Second, I will combine retrograde pathway tracing and immunohistochemical techniques to determine which peptides are contained in projection neurons of M, MPOA and BNST (i.e., neurons in the neural circuits mediating mating behavior). Third, for neuropeptides implicated by the above studies to play a role in neurotransmission between areas involved in mating behavior, I will inject these peptides into M, MPOA and/or BNST and determine their effects on mating behavior. Finally, I will determine the effects of treatment with testosterone or its metabolites (in castrated animals) on peptide levels in neurons in M, MPOA and BNST. These studies will clarify the role of neuropeptides in the regulation of mating behavior, and will clarify the mechanisms by which steroids influence the function of neurons involved in the regulation of mating behavior.