Ontogeny of Stress-Induced Ultrasonic Vocalization and Pituitary-Adrenal Hormone Secretion in Preweanling Norway Rats

Abstract

Adult organisms rapidly exhibit protective behavioral responses when threatened. In addition, physiological systems are activated reflexively to facilitate survival. How these defensive systems are integrated in order to produce appropriate responding demands a developmental psychobiological analysis. This paper summarizes studies examining in the Norway rat the concurrent ontogeny of 30-50 kHz ultrasonic vocalizations and pituitary-adrenal hormone responses induced by stressors presented during the preweaning period. Postnatal Day 7 male rats exposed to social isolation emit significantly more ultrasounds than Day 14 pups, and by 21 days of age social isolation is no longer effective in eliciting ultrasonic vocalizations. This age-dependent attenuation in occurrence of ultrasounds is accompanied by an increased responsiveness of the pituitary- adrenal system. Studies incorporating the use of psychophysical, that is, electric foot shock, and psychological, that is, conspecific threat, stressors indicate further that exposure to such stimuli significantly reduces in Day 14 rats the emission of ultrasonic vocalization but may potentiate their occurrence in Day 7 pups. Application of foot shock reliably induces ultrasonic emission in normally silent Day 21 pups as in adult rats. The hormonal response to these stressors is similar to that found in pups exposed to social isolation; pituitary adrenocorticotropin and adrenal corticosterone secretion increases significantly in Day 14 pups and a further notable increased secretion occurs in Day 21 males and reflects the intensity of the stressor. The increased secretion of pituitary-adrenal hormones coupled with the modulatory capacity to reduce ultrasonic vocalizations by 14 days of age suggest that systems have developed to provide increased behavioral and physiological flexibility during confrontations with varied threats.