Objective:The aim of this study was to evaluate if nitric oxide (NO) in the central amygdala (CeA) is involved in the expression of withdrawal aspects induced by morphine.Materials and Methods:Male Wistar rats (weighing 200-250 g) were bilaterally cannulated in the CeA and conditioned to morphine using an unbiased paradigm. Morphine (2.5-10 mg/kg) was subcutaneously injected once a day throughout the conditioning phase of the procedure. This phase also included 3-saline paired sessions. Naloxone (0.1-0.4 mg/kg, intraperitoneally [i.p.]), an antagonist of opioid receptors, was administered i.p. 10 min prior to testing of morphine-induced withdrawal features. The NO precursor, L-arginine (0.3-3 μg/rat) was intra-CeA injected prior to testing of naloxone response. To evaluate the involvement of NO system an inhibitor of NO synthase (NOS), NG-nitro-L-arginine methyl ester (L-NAME) (0.3-3 μg/rat), was injected ahead of L-arginine. Control group received saline solely instead of drug. As a complementary study, the activation of NOS was studied by nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d).Results:Morphine induced a significant increase in wet dog shaking and grooming behaviors compared with controls. Injection of naloxone pre-testing of morphine response significantly reversed the response to morphine. However, pre-microinjection of L-arginine intra-CeA recovered the response to morphine. Injection of L-NAME intra-CeA ahead of L-arginine though had no effect behaviorally, but, inhibited the NOS which has been evidenced by NADPH-d.Conclusion:The present study shows that NO in the CeA potentiates the expression of conditioned withdrawal induced by morphine paired with naloxone.