We investigated the site of action of two sleep-inducing substances, viz., muramyl dipeptide (MDP) and uridine. Localized electrolytic lesions were made bilaterally in the lateral preoptic hypothalamus (LPO) in rats and nocturnal 10-h i.c.v. infusions of MDP and uridine were performed before and after the LPO lesions. MDP increased only slow-wave sleep (SWS) in both intact and LPO-lesioned rats. Uridine promoted both SWS and paradoxical sleep (PS) before the LPO lesions whereas it increased only PS after the lesions. These results suggest that the LPO is crucial for SWS-promoting action of uridine but not MDP. © 1993.
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