Effects of neonatal administration of vasopressin on cardiac and behavioral responses to emotional stress in adult male rats.

Arginine-8-vasopressin (AVP) was administered subcutaneously on postnatal days 3-7 in a high (10 micrograms/100 g b.wt.) or a low dose (1 microgram/100 g b.wt.) to male Wistar rats. Control pups were untreated or saline injected. Behavioral observations in a complex maze after maturation indicated that neonatal administration of AVP increases exploratory behavior in this novel environment in a dose-dependent way. Cardiac monitoring during the conditioned emotional stress of fear of inescapable electric footshock showed that only the high dose of AVP attenuates the bradycardiac stress response. The analysis of cardiac responses also suggested an adult hyposensitivity to AVP in rats treated neonatally with AVP. In addition, the low dose of neonatal AVP was impairing the retention of a passive avoidance behavior. The data indicate that the neonatal administration of AVP exerts long-term effects upon the behavioral adaptation to novelty and memory processes related to emotional stress. That neonatal AVP is less effective in influencing adult vagally mediated cardiac stress responses suggests differences in the developmental sensitivity ("critical periods") of the central vasopressinergic systems involved in the regulation of behavior and autonomic functioning.