Effects of Electrolytic Lesions of the Ventrolateral Periaqueductal Gray and Nucleus Raphe Magnus on Morphine - Induced Antinociception in the Nucleus Cuneiformi

The nucleus cuneiformis (NCF) and ventrolateral periaqueductal gray (vlPAG), two adjacent areas, mediate the central pain modulation and project to the nucleus raphe magnus (NRM). This study examined whether the antinociceptive effect of morphine microinjected into the NCF is influenced by inactivation of vlPAG and NRM in rats. Animals were bilaterally microinjected with morphine (2.5 µg/0.3 µl saline) into the NCF. Electrolytic lesions were made in vlPAG (0.1 mA, 45 sec) and/or NRM (1 mA, 30 sec). Tail-flick latency (TFL) was measured at 30, 60, 90 and 120 min after microinjection. The results showed that TFLs are significantly decreased in vlPAG+NRM lesions group at 30 (P<0.001) and 60 (P<0.01) min after intra- NCF administration of morphine whereas TFLs did not affect in solely vlPAG lesion animals. Our findings show that concurrent lesions of NRM and vlPAG completely reversed the analgesic effect of morphine in NCF. However, vlPAG do not play a critical role directly in pain modulatory system elicited from NCF, at least at the level of morphine-induced analgesia. It can be concluded that its interactive effect in descending pain modulation from NCF to NRM should not be neglected.