Topical application of morphine to the rat somatosensory cortex produces analgesia to tonic pain.

Topical application of 0.01 or 0.1% morphine solution to the somatosensory SI area of the rat cerebral cortex significantly decreased the pain intensity rating in the formalin test without producing motor side effects or sensory deficits. Naloxone partially antagonizes this effect. Topical application of morphine to the striate cortex did not induce analgesia. It is suggested that the primary somatosensory SI area of the cerebral cortex plays a role in opiate pain control.