en Pharmacology Pharmacology Original Research: Basic Science in Medicine Original Research: Basic Science in Medicine In the present study the effects of both CCK receptor agonists and antagonists on antinociception induced by morphine in the tail-flick test have been evaluated. M orphine induced dose-dependent antinociception in mice. The response of morphine was potentiated by sulfated cholecystokinin-8 (CCK-8S) but not by unsulfated cholecystokinin-8 (CCK-8U). The CCK receptor antagonists MK-329 and L-365, 260 decreased the potentiation of morphine antinociception induced by CCK-8S.•The antagonists even decreased the response induced by morphine in the pre5ence ofCCK-8U. High doses ofMK-329 and L-365, 260 also potentiated morphine's antinociception. Single administration of the CCK receptor agonists CCK-8 and CCK-8U or CCK receptor antagonists did not elicit any response in the tail-flick test. It is concluded that CCK receptor mechanisms are involved in the modulation of pain response and/or morphine antinociception. CCK agents, morphine, antinociception, mice 261 266 http://mjiri.iums.ac.ir/browse.php?a_code=A-10-298-847&slc_lang=en&sid=1 MR ZARRINDAST 20031947532846006850 20031947532846006850 Yes From the Department of Pharmacology, School of Medicine, Tehran University of Medical Sciences, Tehran, I.R. Iran. M REZAYAT 20031947532846006851 20031947532846006851 No From the Department of Pharmacology, School of Medicine, Tehran University of Medical Sciences, Tehran, I.R. Iran. MR ZOLFAGHARPOOR 20031947532846006852 20031947532846006852 No From the Department of Pharmacology, School of Medicine, Tehran University of Medical Sciences, Tehran, I.R. Iran.