Evidence for opioid and nonopioid processes mediating adaptive responses of infant rats that are repeatedly isolated.

Abstract

Previous research examining the ability of neonatal rats to adapt to repeated isolation demonstrated that an opioid-dependent decline in ultrasonic vocalizations occurred across a series of isolations (Goodwin, Molina, & Spear, 1994). These findings were expanded in the present study. In the first experiment, the decline in vocalization rates was found to result from the release of endogenous opioids throughout the series of isolations. Although naltrexone attenuated the decline in calling rates relative to vehicle-treated subjects, there was still a significant decline in calling rates following opioid receptor blockade. In the second experiment, two injections of naltrexone did not attenuate the decline in calling rates any more than a single injection did, suggesting that there must also be some nonopioid process that modulates this decline. In both experiments, activity levels and, in the first experiment, the amount of body heat lost in the repeatedly isolated subjects declined in a nonnaltrexone reversible manner. In a final study, after calling rates had been suppressed by a series of isolations, a brief exposure to the mother was found to restore baseline calling rates, suggesting the decline is not the consequence of fatigue. The attenuation of vocalization rates, activity, and loss of body heat are adaptive responses of infant rats to isolation; however, of these three, only the attenuation of vocalization rates is consistently modulated by the release of endogenous opioids.