SOCIALLY DEFEATED MALE-RATS DISPLAY A BLUNTED ADRENOCORTICAL-RESPONSE TO A LOW-DOSE OF 8-OH-DPAT

The study examined in male Wistar rats the influence of social defeat on the neuroendocrine stress response system using injection of the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), as the pharmacological challenge. Social defeat was defined by the submissive postures displayed by the Wistar rats which were threatened and attacked by Tryon Maze Dull S3 rats for 10 min. 18-20 h after social defeat, the defeated rats were injected intravenously (i.v.) with a low and high dose of 8-OH-DPAT in their home cages. Blood samples were withdrawn from the freely moving cannulated rats for determination of plasma corticosterone and catecholamines. The corticosterone response to the low dose of 8-OH-DPAT (0.05 mg/kg, i.v.) was significantly diminished in the defeated rats as compared to the controls, but this dose failed to affect catecholamine concentrations. The high dose of 8-OH-DPAT (0.15 mg/kg, i.v.) significantly elevated corticosterone and adrenaline levels in defeated and control rats to the same extent, whereas no effect on noradrenaline was found. The present data thus indicate that social defeat blunts 5-HT1A receptor-mediated adrenocortical activation probably via a decrease in the sensitivity of a population of postsynaptic 5-HT receptors.