Effects of morphine on neuronal and behavioural responses to visceral and somatic nociception at the level of spinal cord.

This study was undertaken to examine the role of morphine in modulation of nociception in visceral and somatic pain tests at the level of the spinal cord, using neurophysiological and behavioural reflex assays. In the neurophysiological study, we recorded extracellularly the activity of the single viscero-somatic convergent neurons of the spinal dorsal horn, which was evoked by the colorectal distention (80 mmHg) of noxious visceral stimulation and the radiant heat (51 degrees C) of noxious somatic stimulation, in decerebrated, spinally transected cats. Spinally administered morphine (200 micrograms) produced significant suppression of noxiously evoked activity by both stimuli in a time-dependent manner. In addition, intravenously administered naloxone reversed the suppressive effects of morphine. In the behavioral reflex study, colorectal distension threshold and tail-flick latency were measured in rats chronically implanted with lumbar intrathecal catheter. Intrathecally administered morphine significantly elevated the colorectal distension threshold and prolonged the tail-flick latency in a time- and dose-dependent manner. The results of the present study demonstrated that spinal morphine was capable of suppressing the evoked activity of the viscero-somatic convergent neurons, resulting in suppression visceral and somatic pain behavioral reflexes.