Analgesic effect of morphine injected into habenula, nucleus accumbens or amygdala of rabbits

Previously we reported that the analgesic effect elicited by iv morphine was partially antagonized by microinjection of naloxone into nucleus accumbens, habenula or amygdale of rabbits. In the present study morphine was injected into these nuclei to evaluate its effect on pain threshold as measured by radiant heat-head jerk test. Unilateral injection of 10 μg morphine into habenula, 20 μg into amygdale or bilateral injections (5μg ×2) into nuclei accumbens resulted in an increase of pain threshold the maximal level 20-30 min (by 70-90%), and did not return to normal level within 50 min. No significant changes in pain threshold were noticed when morphine was injected to the vicinity of the above-mentioned nuclei, or when saline (1-2μl) was injected into the nucleus. The results suggest that nucleus accumbens, habenula and amygdale each may play an important role in mediating morphine analgesia.