Effects of morphine and lidocaine administration into the cuneiformis nucleus of rats on acute and chronic pain modulation by formalin test

The cuneiformis nucleus (CnF), which rest in the ventrolateral portion of the periaqueductal gray matter, has opioid receptors that may modulate acute pain. The present study was conducted to evaluate the role of morphine injection into the area of rat’s brain CnF in the modulation of chronic pain and antinociceptive effect of morphine on this area. In this experimental study, 40 NMRI male rats were used for morphine and lidocaine microinjection into the CnF through a guide cannula by stereotaxic instrument in accordance with Paxinos and Watson characteristics. Chronic pain was induced by injection of 50 ml of formalin 2.5% into the hind paw in rats and the number of grasping (biting and/or licking) of the area in phase I or acute phase (0-5 min) and phase II or chronic phase (15-60 min) were regarded as pain indices. The obtained data was analyzed by means of statistical tests including Analysis of Variance and Tukey post-hoc test at the significant level of P≤0.05. Biting frequency was almost the same in control (saline) and intact groups but it was higher in CnF morphine- and lidocaine-microinjected groups in phase I (P<0.01) and phase II only for morphine-microinjected group (P<0.05) as compared with control (saline-treated) rats. The co-administration of morphine and lidocaine into the CnF caused a marked analgesic response (even higher than solely morphine microinjection) in phase I (P<0.0001) and phase II (P<0.05) as compared with lidocaine- and morphine-treated rats in formalin test. CnF has µ receptors and modulates chronic pain system as well. On the other hand, the role of morphine in acute and chronic pain descending systems is more important than lidocaine. So, it seems that CnF causes a significant analgesic response directly or through other brainstem structures in acute and/or chronic peripheral pain processing.