Forebrain peptides modulate sexually polymorphic vocal circuitry

The peptide arginine-vasopressin (mammals) and its evolutionary precursor a rginine-vasotocin (non-mammals) modulate reproductive physiology and numero us related social behaviours, as do oxytocin (mammals) and its homologues m esotocin and isotocin (fish)(1). The distributions in the brain and/or the behavioural functions of these peptides often differ between the sexes(2-4) , and between species with divergent social structures(3,5,6). Here we pres ent neurophysiological evidence that males with vocal characteristics typic al of females share a pattern of neuropeptide function with females rather than conspecific males. The plainfin midshipman fish (Porichthys notatus) h as two male morphs with different reproductive tactics and vocalizations (a key species-typical behaviour which varies in its physical attributes and contextual usage, depending on the morph's social strategy)(7,8). Forebrain -evoked, rhythmic vocal-motor activity that precisely mimics natural vocali zations was modulated by arginine-vasotocin, isotocin and their antagonists delivered to the preoptic area-anterior hypothalamus, a primary site for b ehavioural integration in all vertebrates. Peptides had different effects i n males that acoustically court females (arginine-vasotocin-sensitive) than in females and sneak-spawning males (isotocin-sensitive), showing that the neuromodulatory mechanisms that establish reproduction-related behaviour c an be dissociated from gonadal sex.