SOCIAL CROWDING STRESS ATTENUATES CENTRAL HISTAMINE AND MUSCARINIC CHOLINERGIC SYSTEMS INVOLVED IN THE HYPOTHALAMIC-PITUITARY-ADRENOCORTICAL RESPONSIVENESS

Serum corticosterone responses to intracerebroventricular (icv) admini stration of histaminergic and muscarinic agonists and the hypothalamic histamine content were measured in rats crowded for 3, 7 and 14 days. Social crowding stress drastically reduced the increase in serum cort icosterone levels elicited by dimaprit, a histamine H-2-receptor agoni st, and moderately diminished the corticosterone response to pyridylet hylamine, a histamine H-1-receptor agonist. The corticosterone respons e to icv administration of carbachol, a cholinergic muscarinic recepto r agonist, was also considerably reduced, most significantly after 3 a nd 7 days of crowding. At the same time hypothalamic histamine levels markedly declined in carbachol-treated rats. Corticotropin releasing f actor (CRF) given icv elicited a rise in serum corticosterone levels w hich was identical in rats crowded for 3 days and in control animals. These results show far the first time that significant impairment of t he hypothalamic-pituitary-adrenocortical (HPA) responsiveness to musca rinic receptor stimulation evoked by crowding stress may be connected with a significant diminution in hypothalamic histamine levels. These results also show that not pituitary CRF receptors but rather hypothal amic histaminergic and muscarinic receptors, involved in HPA stimulati on, are desensitized by social stress of crowding.