The periaqueductal gray: a prerequisite for ACTH-induced excessive grooming

The periaqueductal gray is known to be involved in the expression of a variety of behaviours such as aggression, beta-endorphin-induced immobility and peptide-induced excessive grooming. In order to establish whether the periaqueductal gray (PAG) is indispensible for peptide-induced excessive grooming, lesions were placed in the dorsal part of this structure. Subsequently, the grooming-inducing abilities of adrenocorticotropin (ACTH), beta-endorphin and bombesin were tested. The lesioned animals did not display excessive grooming after intracerebroventricular injection of ACTH. beta-Endorphin administration into the lesioned animals resulted in an extreme display of immobility. Local injection of bombesin into the PAG resulted in reduced scratching behaviour followed by immobility. It was hypothesized that excessive grooming (elicited by ACTH) may be mediated through a non-opioid primary target site-situated in the lesioned region of the PAG-while excessive scratching and immobility (elicited by bombesin or beta-endorphin, respectively) may be mediated through an opioid primary target site (situated in the remaining part of the PAG). Furthermore, the analysis of social behaviour of lesioned animals revealed that these animals reacted towards an unfamiliar partner predominantly with freezing behaviour. The increase of beta-endorphin-induced immobility and socially induced freezing (which is morphologically very similar to beta-endorphin-induced immobility) in lesioned animals supports the hypothesis that the release of opioid peptides such as beta-endorphin in the PAG plays a role in the regulation of social behaviour.