Blockade of pituitary-adrenal axis activation induced by peripheral immunoneutralization of corticotropin-releasing factor does not affect the behavioral response to social defeat stress in rats.

Specific in vivo immunoneutralization of hypophysiotrophic corticotropin-releasing factor (CRF) and vasopressin (AVP) was used to investigate the respective roles of these circulating peptides in the hormonal response to a social stressor in Wistar rats. The effect of peripheral immunoneutralization on the behavioral response to the same social stressor was also studied. Stress was produced using social defeat in a resident-intruder paradigm, in which an intruder male rat, equipped with an indwelling IV catheter, was confronted with a dominant resident male rat. To minimize the physical component of the agonistic interaction, the intruder was protected immediately after the resident's first attack in a Plexiglas/wire-mesh enclosure. In the following time interval, the resident continued to threaten the protected intruder, which showed a significant increase in plasma immunoreactive adrenocorticotropic hormone (ACTH) and corticosterone levels compared to those measured in rats exposed to a control procedure. The hypothalamo-pituitary-adrenal (HPA) axis response to social defeat stress was totally abolished by pretreatment with an anti-CRF serum (0.3 ml IV) but not an anti-AVP serum (0.75 ml IV). An animal model for anxiety in rodents, the elevated plus-maze, was used to study the emotional response to social defeat stress in rats given anti-CRF serum or normal serum. A significant reduction of open-arm exploration was observed in defeated rats tested 10 min after the end of the agonistic interaction when compared with control animals for all antisera treatment group, indicating an "anxiogenic-like" emotional response to the social defeat stress. Pretreatment with anti-CRF serum did not affect the behavioral performance of defeated and control rats. These results suggest that: 1) social defeat stress produced an activation of the HPA axis in the rat which is mediated by hypophysiotrophic CRF but not AVP; and 2) the peripheral activation of the HPA axis induced by social defeat stress is not involved in the behavioral response to stress as measured in the elevated plus-maze.