HIGH INTRACEREBRAL LEVELS OF CRH RESULT IN CRH RECEPTOR DOWN-REGULATION IN THE AMYGDALA AND NEUROIMMUNE DESENSITIZATION

Corticotropin-releasing hormone (CRH) acts at the pituitary level to increase ACTH secretion and, within the central nervous system, to stimulate the sympathoadrenomedullary axis and behavioral activity. In addition, the central administration of CRH has been reported to reduce cellular immunity in the periphery. This study investigated the temporal relationship between CRH receptor regulation and the changes in splenic natural killer (NK) cell and pituitary-adrenocortical hormone responses to a single intracisternal (IC) CRH challenge (acute CRH) 24 h after chronic CRH pretreatment (5 nmol/day IC CRH for 4 days). Chronic CRH significantly decreased by 44.2 +/- 7.8% (P < 0.01) the CRH receptor concentration (beta(max)) in the amygdala. In contrast, the CRH receptor concentration of the anterior pituitary in the chronic CRH group was similar to the pituitary CRH receptor concentration in chronic saline controls. The immunoreactive-CRH concentration of the amygdala measured 24 h after the last IC CRH injection was not different from brain CRH levels in controls receiving chronic saline pretreatment. Consequently, the downregulation of amygdala CRH receptors occurred after transient increases in intracerebral CRH levels and did not result from ex vivo receptor occupancy by residual exogenous CRH sequestrated in brain tissue at the time of the CRH binding assay. Pretreatment with chronic CRH completely abolished the ability of a central CRH injection to suppress splenic NK activity; whereas, pituitary-adrenal responses to a superimposed acute CRH challenge were not significantly altered by chronic CRH pretreatment. These results suggest that the desensitization of the brain processes mediating CRH-induced suppression of splenic NK cytotoxicity is temporally correlated with CRH receptor downregulation in the amygdala but independent of pituitary-adrenal activation. These findings represent the first in vivo demonstration of homologous downregulation of extrahypothalamic CRH receptors and provide further evidence for the role of central CRH in the modulation of immune function.