Physiological Roles of Peptides in the Nervous System

The possibility that peptides play physiological roles in neuronal function has long been suggested but only recently begun to be supported by various lines of evidence. For example, subcutaneous and intrahypothalamic injections of the hypothalamic peptide luteinizing hormone—releasing hormone (LH—RH) in ovariectomized, estrogen-pretreated female rats facilitates the induction of the lordosis reflex within an hour of injection (Moss and McCann, 1973, 1975; Foreman and Moss, 1975; Moss and Foreman, 1976), while intracranial injection of renin, angiotensin I, or angiotensin II leads rapidly to drinking behavior in rats (Epstein et al., 1970; Fitzsimons, 1971; Simpson and Routtenberg, 1973, 1975). That a single substance can organize and integrate the activity of precisely the population of neurons required for such complicated behaviors as mating and drinking strongly indicates an important role in the nervous system for such a substance. The cellular correlates and molecular mechanisms underlying these complicated, peptide-induced behaviors have not been established. The purpose of this chapter is to review recent investigations into the physiological roles of several species of peptide. Most of the material discussed will be from observations made at the cellular level, primarily as these events may relate to the elaboration of endocrine, autonomic, or motor behaviors.