Background: Glycine is a major inhibitory neurotransmitter in the adult central nervous system (CNS), and its receptors (GlyRs) are well known for their effects in the spinal cord and the lower brainstem. Accumulating evidence indicates that GlyRs are more widely distributed in the CNS, including many supraspinal regions. Previous in vitro studies have demonstrated that ethanol potentiates the function of these brain GlyRs, yet the behavioral role of the brain GlyRs has not been well explored.
Methods: Experiments were conducted in rats. The loss of righting reflex (LORR) was used as a marker of the hypnotic state. We compared the LORR induced by systematic administration of ethanol and of ketamine in the absence and presence of the selective glycine receptor antagonist strychnine. Ketamine is a general anesthetic that does not affect GlyRs.
Results: Systemically administered (by intraperitoneal injection) ethanol and ketamine dose-dependently induced LORR in rats. Furthermore, systemically administered (by subcutaneous injection) strychnine dose-dependently reduced the percentage of rats exhibiting LORR induced by ethanol, increased the onset time, and decreased the duration of LORR. Strychnine had no effect, however, on the LORR induced by ketamine.
Conclusions: Given that hypnosis is caused by neuronal depression in upper brain areas, we therefore conclude that brain GlyRs contribute at least in part to the hypnosis induced by ethanol.