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 f eeding. We have investigated the effects of a bolus subcutaneous injection of 10 nmol/kg body weight of orexins on pituitary-adrenocortical function i n both normal and ether- or cold-stressed rats. The blood concentrations of adrenocorticotropic hormone (ACTH), aldosterone and corticosterone were me asured 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 corticoster one concentration significantly at 60 min without affecting that of ACTH. O rexin-A did not affect the pituitary-adrenocortical response to ether stres s, while orexin-B increased ACTH and corticosterone responses significantly at 120 and 60 min respectively. Both orexins magnified the ACTH response t o cold stress significantly, without altering aldosterone or corticosterone blood concentrations. We conclude that orexins (i) evoke a transient activ ation 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 th e type of stressor.