EFFECTS OF SYSTEMIC GHRH ON SLEEP IN INTACT AND HYPOPHYSECTOMIZED RATS

The role of pituitary growth hormone (GH) in the mediation of enhanced sleep elicited by GH-releasing hormone (GHRH) was studied in the rat. Intact and hypophysectomized (HYPOX) rats received systemic injection s of GHRH or physiological saline. GHRH (0.5, 5.0, or 50 mu g/kg in th e intact rats and 0.5 or 50 mu g/kg in HYPOX rats) was injected 6 h af ter light onset (P.M. injection) or just before light onset (A.M. inje ction, 0.5 mu g/kg in both A.M. groups). Sleep-wake activity and brain cortical temperature were recorded for 23 h (12 h light + 11 h dark). A.M. injection of GHRH did not alter sleep in normal or HYPOX rats. E ach dose of P.M. GHRH increased rapid-eye-movement sleep (REMS) during 6 h postinjection in the intact rats. Hypophysectomy abolished the RE MS-promoting activity of GHRH. P.M. injection of 0.5 mu g/kg GHRH incr eased non-REM sleep (NREMS) and enhanced electroencephalogram slow-wav e activity during NREMS in both the intact and the HYPOX rats. The NRE MS-promoting activity disappeared when the dose of GHRH was increased in the intact rats, whereas a tendency to enhanced NREMS was still obs erved after 50 mu g/kg GHRH in the HYPOX rats. GHRH stimulated GH secr etion dose dependently in the intact rats. A.M. injection of 0.5 mu g/ kg GHRH tended to be less effective in stimulating GH release than the same dose administered P.M. The results confirm the time-of-day varia tions in the GHRH effects on sleep previously reported in human subjec ts. It is likely that pituitary GH is involved in the mediation of the REMS-promoting activity of GHRH but not in the NREMS-promoting activi ty of GHRH. Nevertheless, the results do not exclude the possibility t hat GH may modulate NREMS.