An anatomical and neurophysiological investigation of neural mechanisms in the hindbrain underlying vasotocin and corticosterone effects on a reproductive behavior of roughskin newts (Taricha granulosa)

Arginine vasotocin (AVT), a neurohypophyseal peptide in non-mammalian vertebrates, regulates a variety of social and reproductive behaviors. In male roughskin newts (Taricha granulosa), AVT enhances clasping of females, a component of courtship behavior. In contrast, corticosterone (CORT) has the opposite effect, rapidly depressing male courtship clasping. Additional evidence shows that AVT and CORT act synergistically rather than antagonistically in the brain. Neurophysiological and behavioral studies have shown the medulla of the hindbrain to be critical for control of clasping and for AVT and CORT regulation of clasping. The present studies were aimed at two primary objectives: anatomical identification of medullary neurons that are the target cells for AVT and identification of the effects of AVT and CORT on medullary neurons that underlie effects of these hormones on clasping. A new, fluorescent conjugate of AVT and the dye Oregon Green (AVT-OG) was functionally validated for in vivo identification of target AVT cells in the medulla. AVT-OG application to the medulla of male newts resulted in conjugate internalization by a widespread and functionally diverse neuronal population, including reticulospinal neurons of known importance for clasping. To examine clasping-related hormonal effects on single medullary neurons, a behaving, decerebrate preparation was utilized. Medullary AVT application enhanced neuronal activity, particularly neuronal responsiveness to clasptriggering cloacal stimuli and the peptide concurrently enhanced strength and speed of clasping. Systemic CORT injection caused opposite effects, rapidly depressing neuronal responsiveness and concurrent clasping. In contrast, CORT, when administered after AVT, potentiated neuronal responsiveness to cloacal pressure and enhanced clasping beyond the effects seen with AVT acting alone. These results indicate that AVT is probably acting on a very diverse population of medullary neurons to produce effects on sensorimotor processing that regulate clasping. The neurophysiological results show a strong dependency between the effects of AVT or CORT on medullary neuronal responsiveness to clasp-triggering stimuli and the associated hormone effects on clasping. In addition, the results confirm that synergistic neurophysiological effects of AVT and CORT are associated with correspondingly synergistic effects on clasping. These findings illustrate that the neurobehavioral actions of the stress hormones AVT and CORT are highly interdependent.