Possible involvement of the S-adenosyl-L-homocysteine metabolites, adenosine and L-homocysteine, in sleep in rats.

The effects of i.p. injections of adenosine (Ado) and homocysteine at 17.00 h were tested on the rat sleep-waking cycle. Administration of 7 and 50 mg/kg of Ado resulted in a significant increase in sleep in otherwise saline male rats during the environmental-dark period (18.00-06.00 h). Increments of paradoxical sleep were maximal with the smaller dose of Ado. The larger dose increased slow-wave sleep during the first 2 h. In contrast, administration of homocysteine (7 mg/kg) had no effect while a larger dose (50 mg/kg) induced a slight but significant long-lasting decrease of paradoxical sleep. These results were compared with those previously obtained with S-adenosyl-L-homocysteine (SAH), an intracellular precursor of Ado and L-homocysteine. It is proposed that the reported hypnotic action of exogenous SAH in rats could be mediated by Ado receptors.