Effect of systemic morphine on neurons in the lateral reticular nucleus area of the rat.

The present study is undertaken to investigate the effect of systemic morphine on neurons in the lateral reticular nucleus (LRN) using extracellular recording techniques. The spontaneous activities of 64 neurons in the LRN area were tested with morphine (3-5 mg/kg, IV). Morphine excited 23 and inhibited 28 neurons tested, and 13 neurons were not affected. Of the 28 neurons inhibited, 20 were identified as nociceptive and the remaining 6 were nonnociceptive. Of the 23 neurons excited by morphine, 18 were nociceptive and 5 were nonociceptive. Systemic naloxone (0.3-0.5 mg/kg) significantly reversed the morphine effect in 15 out of 19 neurons excited and 19 out of 20 neurons inhibited by morphine. Thirteen out of 64 neurons were further identified as reticulospinal neurons, of which four were excited and four were inhibited by morphine. The remaining five were not affected. The results demonstrate that a similar proportion of neurons in the LRN area were either excited or inhibited by systemic morphine, and the majority of them are nociceptive neurons. It is suggested that different types of neurons in the LRN area may have different functions in morphine analgesia.