Modulatory effects of orexins on the function of rat pituitary-adrenocortical axis under basal and stressful conditions

Orexins A and B are two hypothalamic peptides, involved in the control of feeding. We have investigated the effects of a bolus subcutaneous injection of 10 nmol/kg body weight of orexins on pituitary-adrenocortical function in both normal and ether- or cold-stressed rats. The blood concentrations of adrenocorticotropic hormone (ACTH), aldosterone and corticosterone were measured by specific radioimmune assay 60 and 120 min after the injection. In non-stressed rats, orexin-A raised the blood levels of the three hormones significantly at 60 min, and orexin-B increased aldosterone and corticosterone concentration significantly at 60 min without affecting that of ACTH. Orexin-A did not affect the pituitary-adrenocortical response to ether stress, while orexin-B increased ACTH and corticosterone responses significantly at 120 and 60 min respectively. Both orexins magnified the ACTH response to cold stress significantly, without altering aldosterone or corticosterone blood concentrations. We conclude that orexins (i) evoke a transient activation of the rat pituitary-adrenocortical axis, acting on both its central and peripheral branch; and (ii) exert a minor modulatory role on pituitary-adrenocortical responses to stresses, their effects being dependent upon the type of stressor.