MODULATION OF THE HYPOTHALAMO-PITUITARY-ADRENOCORTICAL RESPONSES TO CYTOKINES IN THE RAT BY LIPOCORTIN-1 AND GLUCOCORTICOIDS - A ROLE FOR LIPOCORTIN-1 IN THE FEEDBACK INHIBITION OF CRF-41 RELEASE

Our recent studies indicate that lipocortin 1 (LC1), a putative second messenger protein for the anti-inflammatory steroids in peripheral tissues, may also contribute to the regulatory actions of the glucocorticoids on the hypothalamo-pituitary-adrenal axis. In the present study we have used in vitro and in vivo models to compare the effects of adrenalectomy, LC1 and dexamethasone on the cytokine-induced secretion of the 41-amino acid corticotrophin releasing factor (CRF-41) and arginine vasopressin (AVP) by the rat hypothalamus. In addition, western blot analysis was used to examine the influence of dexamethasone on the expression of LC1 in the hypothalamus. In vitro, interleukins- (IL-) 1alpha (100 and 200 pg/ml), 1beta (0.5 and 1.0 ng/ml) and 8 (0.25-1.0 ng/ml) readily initiated the release of CRF-41 and AVP from hypothalami removed from intact rats. IL-6 (10 and 20 ng/ml) was also an effective CRF-41 secretagogue but, unlike the other interleukins tested, it was ineffective with regard to AVP. Adrenalectomy 7-14 days prior to autopsy increased significantly (p < 0.01) the magnitude of the CRF-41 responses to IL-1alpha, IL-1beta and IL-6 but not to IL-8. In contrast however, while hypothalamic tissue from adrenalectomized rats, unlike that from intact animals, responded to IL-6 (5-20 ng/ml) with a pronounced hypersecretion of AVP, the AVP responses to IL-1alpha and IL-1beta were largely unaffected by adrenalectomy as too were those to IL-8. The marked increases in CRF-41 and AVP release from hypothalami from adrenalectomized rats initiated in vitro by IL1alpha, IL-1beta, IL-6 and IL-8 were readily overcome by preincubation of the tissue with dexamethasone (10(-7) M). In addition, the steroid caused 'externalization' of two species of immunoreactive (ir-) LC1 (37 and 58 kDa) by the hypothalamic cells but failed to influence the total LC1 content of the tissue. The inhibitory effects of dexamethasone on the cytolcine-induced release of CRF-41 in vitro were mimicked by LC1 (10 ng/ml) which alone had no effect on the basal release of the peptide. However, unlike dexamethasone, LC1 failed to influence the concomitant release of AVP from the hypothalamic tissue elicited by IL1alpha, IL-1beta or IL-8 and potentiated that evoked by IL-6. In vivo central administration of IL-1beta (5-10 ng in 3 mul) or IL-6 (7.5-30 ng) in 3 mul) via an indwelling intracerebroventricular (i.c.v.) cannula initiated significant (p < 0.01) dose-dependent increases in the serum corticosterone concentration; these responses were inhibited by prior injection into the third ventricle of LC1 (0.3-1.2 mug/3 mul, i.c.v.) which alone, in the highest dose tested, precipitated a small increase in corticosterone release. The results support the concept that cytokines play a key role in initiating the release 6f CRF-41 from the hypothalamus and provide novel evidence to suggest that the acute inhibitory effects of the corticosteroids on these responses may be mediated in part by LC1. In addition they demonstrate the ability of certain cytokines to stimulate the release of AVP from isolated hypothalamic tissue and indicate that, although these effects are dexamethasone sensitive, they are not blocked by LC 1.