NEUROPHYSIOLOGICAL EFFECTS OF VASOTOCIN AND CORTICOSTERONE ON MEDULLARY NEURONS - IMPLICATIONS FOR HORMONAL-CONTROL OF AMPHIBIAN COURTSHIP BEHAVIOR

Research on a wide variety of vertebrates, from fish to mammals, revea ls that corticosteroid hormones and vasotocin-like neuropeptides can p otently modulate reproductive behaviors. But, it is not clear how the behavioral effects of these chemical messengers relate to functional p roperties of behavior-controlling neurons. This problem was investigat ed in the roughskin newt, Taricha granulosa, an amphibian in which the administration of arginine vasotocin (AVT) facilitates and corticoste rone (CORT) inhibits courtship clasping of females by males. In waking , immobilized male newts, neurophysiological effects of AVT and CORT w ere studied in neurons in the rostral medulla due to the probable role of these neurons in the control of clasping. Topical medullary applic ation of a clasp-facilitating dose of AVT produced a rapid increase in neuronal responsiveness to pressure on the cloaca, a trigger stimulus for clasping responses. Neuronal responses to noncloacal somatic stim uli and to moving visual stimuli were also enhanced. Systemic CORT adm inistration, which has previously been shown to depress newt medullary neuronal sensory responsiveness, reversed the action of AVT such that the peptide depressed sensory responsiveness when applied 30 min afte r CORT. When AVT application prededed CORT injection by 10-17 min, how ever, the usual suppressive CORT effect was reversed and this treatmen t resulted in a rapidly appearing potentiation of neuronal activity an d enhanced somatic sensory responsiveness. If the interval between AVT and CORT was increased to 30 min, the steroid caused a rapid depressi on of firing and a diminished somatic sensory responsiveness in most n eurons, similar to what occurs in newts treated with CORT alone. These results indicate that neurophysiological actions of AVT or CORT depen d on the type of prior hormonal exposure (pretreatment with AVT of COR T) and the time interval between hormone actions.