Asian Journal of Animal and Veterinary Advances1683-99191996-3289Academic Journals Inc.10.3923/ajava.2007.15.20MokhtariM. ShariatiM. RezaeianL. 1200721The present study intends to assess the effects of morphine, L-NAME and also the interaction of morphine and L-NAME together on the antinociceptive activity by using the formalin test as a test of nociception. The experiments were carried out on seventy adult male rats, weighting approximately 200-210 g. The animals were divided into seven groups, considered as control, sham and experimental groups, respectively. The experimental groups were divided into five groups which the first group that received 2 mg kg-1 of morphine, the other four groups received morphine in a dose of 2 mg kg-1 accompanied by L-NAME in the dose of 15, 20 and 40 mg kg-1 and the last group received only 40 mg kg-1 of L-NAME. The drugs were intraperitoneally injected 15 min before the formalin test. Minutes 0-5 and 15-60 were designated, respectively as the acute and chronic phases of pain. Statistical analysis showed that the groups which received morphine accompanied by different doses of L-NAME and those which received only morphine and L-NAME, showed more reduction of pain in comparison with the control group, specially in the second phase of the formalin test. The finding overall showing that two drugs act synergically. On the other hand, L-NAME decreases the intracellular signal activity of both cGMP and cAMP, which inhibiting of NO synthase. Consequently, the antinociceptive effect of morphine is reinforced.]]>Alvarez, M., J. William and E. Bockstacle,20002040914098Andrew, M.,200430722766Brundege, J., L. Dlao, W. Proctor and T. Dunwiddie,19963612011210Chizuko, W., S. Tsukasa, O. Kazuhiro and S. Chikai,2003106269283Dubuisson, D. and S.G. Dennis,19774161174Garthwaite, J.,1991146067Garthwaite, J. and C. Boulton,199557683706Herdegen, T., S. Rudiger, B. Mayer, R. Bravo and M. Zimmermann,199422245258Hibbs, Jr. J.B., R.R. Taintor, Z. Vavrin and E.M. Rachlin,19881578794Jungi, T., H. Adler, M. Thony, M. Krampe and E. Peterhans,199654323330Khotib, J., M. Narita, M. Suzuki, Y. Yajima and T. Suzuki,200446531540Machelska, H., D. Labuz, R. Przewlocki and B. Prze-wlocka,1997282977984Meller, S.T. and G.F. Gebhort,199353127136Moncada, S., R.M.J. Palmer and E.A. Higgs,19893817091715Furst, S.,199948129141Stoclet, J., B. Muller, K. Gyorgy, R. Andriantsio-thaina and A. Kleschyov,1999375139155Tjolsen, A., O. Berge, S. Hunskaar, J. Rosland and K. Hole,199251517Terayama, R., Y. Guan, R. Dubner and K. Ren,20001119151919Vaupel, O., A. Kimes and E. London,199513315322Willis, W.,2001933142156