Pharmacological interaction between neuropeptides and morphine or pentazocine in rat spinal cord

Male Wistar rats were treated with morphine or pentazocine subcutaneously (sc) and then intrathecally (it) by methionine- or leucine-enkephalin, neurotensin, substance P or cholecystokinin octapeptide 26-33. Then antinociceptive effect was measured during 1 h using tail-immersion test. Leucine-enkephalin potentiated and methionine-enkephalin antagonized morphine or pentazocine analgesia. Neurotensin, substance P and cholecystokinin acted biphasically on morphine induced antinociception. After short elevation, the diminution of antinociceptive effect was seen. Neurotensin diminished but substance P and cholecystokinin elevated analgesic effect elicited by pentazocine. Experimental model employed by us may be useful for preliminary screening of pharmacological interactions between neuropeptides and opioid analgesics on the spinal level. Our data confirm the results of other authors that individual enkephalins have different pharmacological features.