Effects of electrolytic lesion of dorsolateral periaqueductal gray on analgesic response of morphine microinjected into the nucleus cuneiformis in rat

Abstract

The periaqueductal gray (PAG) and nucleus cuneiformis (CnF), like the rostral ventromedial medulla, have functional roles in descending pain-inhibitory pathway related to morphine antinociception. There is not any evidence concerning the role of different regions of the PAG on antinociceptive effect of morphine administered into the CnF in pain modulatory system. In the present study, we investigate whether electrolytic lesion of dorsolateral periaqueductal gray (dl-PAG) influence the analgesic effect of morphine microinjected into the CnF. 71 adult male Wistar rats weighting 230–280 g cannulated bilaterally into the CnF, concurrently lesion of dl-PAG was done. The tail-flick and formalin tests were performed to measure pain and antinociceptive effect of morphine microinjected into the CnF (2.5 μg/0.3 μl saline per side). The tail-flick latency was measured at 15, 30, 45, 60 and 75 min following morphine microinjection. In formalin test, pain behavior was recorded for 60 min in early (0–5 min) and late (15–60 min) phases after formalin injection. Each rat was given a subcutaneous 50-μl injection of formalin 2.5% into plantar surface of hind paw following morphine administration. The results showed that dl-PAG lesion attenuated the effect of morphine microinjected into the CnF both in tail-flick and formalin tests while dl-PAG lesion solely did not alter basal pain behavior as compared to control group. In conclusion, our results suggest the existence of a direct or indirect projection from CnF to the dl-PAG at least at the level of the morphine antinociception in pain modulation.