The behavioral actions of three endogenous brain peptides (TRH, LHRH, and neurotensin) will be utilized as model systems in which to elucidate the neuroanatomical loci and neurochemical systems upon which these peptides act to modulate behavioral, physiological, and pharmacological responses of the organism. Although TRH and LHRH clearly play a prominent role in neuroendocrine regulation, there is now a large literature which indicates that these compounds also exert direct brain effects that are independent of their role in the maintenance of neuroendocrine homeostasis. The use of lesions (electrolyte and chemical), pharmacologic agents, and antisera raised against the various peptides under study will aid in the determination of the brain sites upon which these bioactive peptides act to alter behavioral and physiological responses. In addition each of the peptides will be infused or injected into specific regions to determine "sensitive" loci, i.e., those areas in which peptides elicit their characteristic behavioral responses. The effect of peptide administration on levels of cyclic AMP and cyclic GMP will seek to determine whether these acknowledged second messengers are involved in the peptide effects described. In vitro receptor studies will determine whether these peptides alter the binding of "specific" ligands to CNS neurotransmitter receptors. These studies will better define the CNS effects of endogenous peptides and will contribute to the identification of those neural circuits which mediate the brain-behavioral effects of peptides.