Role of GABAB receptors in the control of hypoglossal motoneurons in vivo.

There is little information on GABAB receptor-mediated effects on orofacial motoneurons. We recorded the inspiratory activity from both hypoglossal (XII) nerves in urethane-anesthetized, paralyzed, vagotomized and artificially ventilated rats. A GABAB receptor agonist, baclofen, or antagonist, CGP-35348, was microinjected into one XII nucleus. Baclofen rapidly reduced the XII nerve activity in a dose-dependent manner by over 50%. The antagonist caused a delayed suppression of activity by 40%. We conclude that: (1) GABAB receptors within the XII nucleus may suppress the activity of inspiratory XII motoneurons, but they are not tonically active under the conditions of our experiment; (2) there is a net endogenous excitatory effect in XII motoneurons that is mediated by GABAB receptors located in the reticular formation surrounding the XII nucleus.