Sensorimotor Processing Model: How Vasotocin and Corticosterone Interact and Control Reproductive Behaviors in an Amphibian

This chapter reviews research on amphibians, focusing on questions about the hormonal mechanisms that regulate reproductive behaviors. In the roughskin newt (Taricha granulosa) and other amphibians, reproductive behaviors are activated and maintained by the usual set of sex steroid hormones—testosterone, 5αdihydrotestosterone, and 17β-estradiol. Sex steroids appear to act on the brain and activate reproductive behaviors, at least in part by stimulating the synthesis of arginine vasotocin (AVT) in specific neurons. AVT is a behaviorally active peptide that can enhance specific behaviors in amphibians (female sexual receptivity and egg-laying behaviors; male frog calling and courtship behaviors). Behavioral and neurophysiological studies reveal that AVT modulates behaviors in Taricha by acting on neuronal pathways associated with sensorimotor processing, not by acting on the animal’s general state of arousal. When animals perceive harsh or life-threatening conditions, hormones in the stress axis typically suppress reproductive behaviors. In Taricha, corticosterone (CORT) rapidly inhibits male courtship by nongenomic mechanisms that use a membraneassociated corticosteroid receptor (mCR). This mCR appears to be a member of the G-protein-coupled receptor superfamily and has similarities to kappa opioid-like receptors. Neurophysiological recordings in medullary neurons show that CORT rapidly depresses spontaneous and stimulus-coupled neuronal activity and provide strong evidence that the behavioral effects of CORT involve the modulation of sensorimotor processing. Other studies in Taricha reveal that the propensity to exhibit courtship is affected by context-dependent interactions between AVT and CORT. We conclude that the hormonal control of amphibian reproductive behavior operates in diverse ways that depend on the specific neural subsystem affected (e.g., tectal, reticulospinal, or intraspinal), as well as concurrent interactions between steroids and neuropeptides in a subsystem.