EFFECT OF SOCIAL-ISOLATION ON CORTICOSTERONE SECRETION ELICITED BY HISTAMINERGIC STIMULATION

The effects of social stress of isolation for 3, 7, and 14 days on the responsiveness of the hypothalamic-pituitary-adrenocortical (HPA) axi s to the stimulation of central histamine receptors and on the content s of hypothalamic biogenic amines were investigated. The corticosteron e response to intraventricular administration of pyridylethylamine (PE A), a histamine H-1-receptor agonist, was significantly higher in isol ated than in control rats. The corticosterone response to dimaprit, a histamine H-2-receptor agonist, tended to be slightly weaker in the st ressed rather than in control rats. PEA significantly diminished the h ypothalamic noradrenaline, dopamine and serotonin contents in both con trol and isolated rats. Dimaprit also decreased the brain NA, but not the DA concentrations in control and isolated rats. The changes in mon oamines were not correlated with either the time, direction or magnitu de of changes in corticosterone levels. These results suggest that hyp erresponsiveness of the HPA system to the stimulation of central H-1-h istamine receptors during social isolation may depend on changes in th e efficacy of H-1 receptors but not on changes in the brain monoamine levels.