Investigating the role of anaesthetics on the rostral ventromedial medulla: implications for a GABAergic link between ON and OFF cells.

Two classes of cells in the rostral ventromedial medulla (RVM), ON and OFF, are important in the descending modulation of noxious input. It is thought that these two cell classes communicate, at a local level, with each other to control the RVM's descending modulation. Systemic morphine was given to rats anaesthetized with either pentobarbital (a GABA agonist) or with ketamine (a non-competitive NMDA receptor antagonist) and xylazine. The spiking activity of the ON and OFF cells was recorded. The typical ON cell inhibition by morphine was observed in rats under both anaesthetics. However, the OFF cell excitatory response to morphine was seen in the ketamine but not in the pentobarbital-anaesthetized animals. The implications for a GABAergic connection between these two classes of cells are discussed.