Alpha 2-adrenoceptor modulation of nociception in rat spinal cord: location, effects and interactions with morphine

The effects of intrathecal clonidine alone and prior to intrathecal morphine were studied on electrically evoked A beta and C fibre activity in the dorsal horn of the halothane-anaesthetised rat. Clonidine reduced C fibre-evoked activity in a dose-dependent manner, to a maximum 52% inhibition which was reversed by rauwolscine and yohimbine but not naloxone. High doses of clonidine also produced small inhibitions of A fibre-evoked activity. Clonidine potentiated the inhibitory action of intrathecal morphine on electrically evoked C fibre activity but not A fibre activity. In addition, the location of alpha 2-adrenoceptor and opiate binding sites in consecutive sections of rat lumbar cord was investigated using in vitro autoradiography with selective ligands, and it was demonstrated that both opiate and alpha 2-receptor types are present within the same superficial layers of the dorsal horn of the same animal. The results indicate that alpha 2-adrenoceptors and opiate receptors can interact in the modulation of nociceptive transmission in rat spinal cord.