Effects of opioid microinjections in the nucleus of the solitary tract on the sleep-wakefulness cycle states in cats

Abstract

Background: Previous studies have shown that the region of the nucleus of the solitary tract (NST) is involved in the control of electrocortical activity and in sleep mechanisms. It also is well known that this region contains the highest concentration of opioid receptors within the medullary brainstem. The involvement of the NST opioid system in sleep-wakefulness states was evaluated. Methods: Ten cats were implanted with electrodes for chronic polygraphic recordings of their sleep-wakefulness states and provided with an implanted guide cannula stereotaxically aimed at the NST region. Microinjections of saline, morphine sulfate, morphiceptin (specific μ agonist), D-pen-2-D-pen-5-enkephalin (δ agonist), and U-50488H (κ agonist) were given to the freely moving animals (doses 0.8-2.4 x 109 M, in a volume of 0.05 μl of saline). After microinjections, sleep-wakefulness recordings were obtained for 8 h. Results: Morphine microinjections in NST provoked a dose-dependent enhancement of all the polygraphic and behavioral manifestations of slow wave sleep. This effect was Mocked by the prior intraperitoneal administration of naloxone. The μ and δ agonists also produced a hypnotic effect by enhancing slow wave sleep. By contrast, the κ agonist caused no changes in sleep-wakefulness states. Conclusions: These results indicate that endogenous opioids could be involved in controlling electrocortical activity generated by NST and that activation of μ and δ NST opioid receptors enhanced the electroencephalographic synchronization associated with behavioral slow wave sleep in cats.