Alpha-melanocyte-stimulating hormone attenuates behavioral effects of corticotropin-releasing factor in isolated guinea pig pups

Abstract

During a 3-hr period of social isolation in a novel environment, guinea pig pups exhibit an initial active phase of behavioral responsiveness, characterized primarily by vocalizing, which is then followed by a stage of passive responsiveness in which pups display a distinctive crouch, eye-closing, and extensive piloerection. Prior treatment of pups with alpha-melanocyte-stimulating hormone (alpha-MSH) reduces each of the passive behaviors. The onset of passive responding during separation can be accelerated with peripheral injection of corticotropin-releasing factor (CRF). To examine whether CRF produces its effects through a mechanism similar to that of prolonged separation, we examined the effect of administering alpha-MSH to pups injected with CRF. As expected, CRF markedly enhanced passive responding during a 60-min period of separation. alpha-MSH delivered by either intracerebroventricular infusion or intraperitoneal injection significantly reduced each of the passive behavioral responses without significantly affecting active behavior. These findings, together with previous results indicating that it is the anti-inflammatory property of alpha-MSH that is responsible for its behavioral effects during prolonged separation, suggest that peripheral CRF speeds the induction of passive responding through a mechanism involving enhanced proinflammatory activity.

FIGURE 1

Effects of central (ICV) administration of α-MSH on CRF-induced active and passive behaviors. Medians and semi-interquartile ranges (numbers in parentheses) are depicted. Levels of significance were calculated with Wilcoxon matched-pairs tests for within-subject comparisons and Mann–Whitney tests for between-subject comparisons (^*p <.05; ^**p <.01; ^***p <.001).

FIGURE 2

Effects of peripheral (SC) administration of α-MSH on CRF-induced changes in active and passive behaviors. Medians and semi-interquartile ranges (number in parentheses) are depicted. Levels of significance were calculated with Wilcoxon matched-pairs tests for within-subject comparisons and Mann–Whitney tests for between-subject comparisons (^*p <.05; ^**p <.01; ^***p <.001).