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) axis to the stimulation of central histamine receptors and on the contents of hypothalamic biogenic amines were investigated. The corticosterone response to intraventricular administration of pyridylethylamine (PEA), a histamine H1-receptor agonist, was significantly higher in isolated than in control rats. The corticosterone response to dimaprit, a histamine H2-receptor agonist, tended to be slightly weaker in the stressed rather than in control rats. PEA significantly diminished the hypothalamic noradrenaline, dopamine and serotonin contents in both control and isolated rats. Dimaprit also decreased the brain NA, but not the DA concentrations in control and isolated rats. The changes in monoamines were not correlated with either the time, direction or magnitude of changes in corticosterone levels. These results suggest that hyperresponsiveness of the HPA system to the stimulation of central H1-histamine receptors during social isolation may depend on changes in the efficacy of H1 receptors but not on changes in the brain monoamine levels.