METAMIZOL POTENTIATES MORPHINE EFFECTS ON VISCERAL PAIN AND EVOKED C-FOS IMMUNOREACTIVITY IN SPINAL-CORD

In a model of visceral pain consisting of intraperitoneal injection of acetic acid (writhing test), simultaneous administration of subanalge sic doses of metamizol (150 mg/kg) and morphine (0.2 mg/kg) resulted i n a potent analgesia (19 +/- 1 vs. 2.3 +/- 0.8 writhes; P < 0.05). Whi le the analgesic effect of morphine (2 mg/kg) was antagonized by nalox one (1 mg/kg), the opioid antagonist did not reverse the analgesia ind uced by the combination of metamizol and morphine. Potentiation by met amizol was also observed as a bilateral decrease in stimulus-evoked c- Fos induction in superficial laminas (I-II) of the dorsal spinal cord after drug combination compared to single administration (66.5 +/- 2.2 vs. 80.7 +/- 4.2; P < 0.05). Conversely, the number of nuclei immunos tained with an antibody that recognizes all proteins of the Fos family was not modified by the same dose combination compared to single trea tment (21.1 +/- 1.3 vs. 20.2 +/- 1.2). Furthermore, in a model of soma tic pain consisting of peripheral thermal stimulation of the paws, sim ultaneous administration of metamizol (100-250 mg/kg) and morphine (0. 5 mg/kg) failed to modify flexor reflex latency. (C) 1998 Elsevier Sci ence B.V. All rights reserved.