Effect of subcutaneous injection of morphine on activity of neurons in the nucleus cuneiformis of rat

The similarities between periaqueductal gray matter and the nucleus cuneiformis in both ultrastructural and functional levels suggest that this nucleus may play an important role in the morphine-induced analgesia. This study was designed to determine neuronal activity and responsiveness to peripheral morphine administration in the nucleus coneiformis of rat. In this study, neural activity of cuneiform neurons in response to peripheral administration of morphine was recorded by extracellular single unit recording technique. Firing rate of neurons was recorded in four groups: intact group (n=19) to determine the spontaneous (baseline) activity, saline group (n=20), morphine group (n=39) and morphine + naloxone group (n=12), before and after drug administration. Our findings showed that the firing rate in majority of cuneiform neurons decreased after morphine (3.8 mg/kg; SC) administration. Activity of neurons (n=39) in the cuneiform neurons was reduced significantly (P<0.01) after morphine injection (6.66±0.67 spike/sec) in comparison with pre-injection time (12.47±1.84 spike/sec) and the saline group (11.6±1.58 spike/sec). The firing rate and response pattern of many of neurons in response to peripheral application of morphine were reversed after naloxone injection (2 mg/kg; SC) in this nucleus. Based on the above findings, we suggest that the changes of activity pattern in spontaneous activity of cuneiform neurons in response to peripheral administration of morphine maybe resulted from direct action of morphine on opioid receptors in the nucleus cuneiformis. Nevertheless, the role of pain transmission and modulation pathways are still important in the antinociceptive effect of morphine as well.