Morphine suppresses plasma catecholamine responses to laparotomy but not to 2-deoxyglucose

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Abstract
The increase of plasma catecholamines that occurs during surgery can be reduced by administration of morphine. To test the hypothesis that morphine specifically blocks nociceptive stimulation during surgery, the effects of morphine administration on the plasma catecholamine response to a laparotomy in pentobarbital-anesthetized dogs were compared with the effects of morphine on the plasma catecholamine response to the neuroglucopenic agent, 2-deoxy-D-glucose (2DG, 300 mg/kg i.v.). In control dogs, plasma epinephrine (Epi) and plasma norepinephrine (NE) both increased progressively with time following a midline laparotomy (.DELTA. Epi by 50 min, + 133 .+-. 42 pg/ml, P < 0.01 and .DELTA. NE by 50 min, + 108 .+-. 38 pg/ml, P < 0.01, mean n = 12). 2-DG produced a similar increase of both plasma NE and Epi. In dogs that received the anesthesia alone, plasma catecholamines did not increase from baseline during the experiment. The analgesic morphine (15 mg i.v.), given 15 min after the completion of laparotomy, not only prevented the progressive rise of plasma catecholamines after laparotomy, but also caused a small but significant decline (P < 0.05). Naloxone (0.04 mg i.v.) totally reversed the suppressive effects of morphine, restoring both catecholamines to the levels of their time-related control. Neither morphine nor naloxone affected the plasma NE and Epi increases following the administration of 2DG. Evidently, morphine suppression of plasma catecholamines during surgery is not due to a generalized attenuation of sympathetic outflow, but rather to a specific interaction with an opiate receptor that either mediates analgesia or lies within the neural pathway stimulated by laparotomy but not by 2DG.