Antinociceptive effects of intrathecal opioids, noradrenaline and serotonin in rats: mechanical and thermal algesic tests.

Antinociceptive effects of intrathecal injections of morphine, methionine-enkephalin, noradrenaline and serotonin were examined in rats, using 3 different methods: the tail-pinch, hot-plate and tail-flick tests. The order of sensitivity of the 3 methods to noradrenaline in the production of antinociception was tail-pinch greater than tail-flick greater than hot-plate, while that of morphine, methionine-enkephalin and serotonin was tail-flick greater than hot-plate greater than tail-pinch. These results indicate that intrathecal noradrenaline, but not methionine-enkephalin and serotonin, produces more potent inhibition of the mechanical nociception than does the thermal one, and suggest a closer relationship of the descending noradrenergic system to the mechanical nociceptive system, in the spinal cord of rats.