Changes in rat sleep after single and repeated injections of the long-acting somatostatin analog octreotide

Somnogenic activity is attributed to both growth hormone (GH) and GH-releasing hormone (GHRH). The aim of our experiments was to study sleep after suppression of the somatotropic axis by means of administration of a long-lasting somatostatin analog, octreotide. Rats received subcutaneous injections of physiological saline (baseline), octreotide (1, 10, and 200 microg/kg), or a control solution just before light onset, and sleep-wake activity and cortical brain temperature were recorded for 23 h. Each dose of octreotide slightly promoted rapid eye movement sleep (REMS) during the 12-h light period. Non-REM sleep (NREMS) was strongly suppressed for 1 h in response to 10 and 200 microg/kg octreotide. This was followed by slight increases in NREMS time and significant enhancements in electroencephalogram slow-wave activity during NREMS after 200 microg/kg octreotide. The octreotide-induced inhibition of the somatotropic axis, as determined by plasma GH levels, vanished by the time sleep increased. Another group of rats received 10 microg/kg octreotide twice a day for 5 days. This treatment elicited persistent decreases in both NREMS time and NREMS intensity. The results support the previously reported REMS-promoting activity of somatostatin in rats. The decreases in sleep after repeated injections of octreotide are attributed to a withdrawal of the normal sleep-promoting activity of GH. The role of GHRH-GH in octreotide-induced acute suppression of NREMS is currently not clear. Other mechanisms, such as mimicking central transmitter functions of somatostatin by octreotide, should also be considered.