Blockade of GABAA receptors in the midbrain periaqueductal gray abolishes nociceptive spinal dorsal horn neuronal activity.

Single spinal dorsal horn neuronal responses to noxious skin heating or innocuous skin brushing were recorded in pentobarbital anesthetized rats. The heat-evoked activity was selectively abolished by blockade of GABAA receptors in the midbrain periaqueductal grey (PAG) by microinjections of 40 or 400 pmol bicuculline. It is concluded that antinociceptive output neurons in the PAG that trigger descending inhibition are maximally active when released from tonic GABAergic inhibition.