Rationale: We previously reported that the central nucleus of the amygdala (CeA) plays a crucial role in the negative affective, rather than somatic, component of morphine withdrawal. However, numerous studies have reported that the central ascending noradrenergic system is implicated in morphine withdrawal syndrome, although the roles of the noradrenergic system within the CeA in the negative affective component remains less clear.
Objectives: The possible role of the noradrenergic system within the CeA in the negative affective component of morphine withdrawal was investigated.
Methods: The extracellular noradrenaline level within the CeA during naloxone-precipitated morphine withdrawal was measured using an in vivo microdialysis experiment on unanesthetized and freely moving rats. The effects of microinjection of beta-adrenoceptor antagonists into the bilateral CeA on the naloxone-precipitated morphine withdrawal-induced conditioned place aversion (CPA) and somatic signs were examined.
Results: The extracellular noradrenaline level within the CeA was transiently elevated during morphine withdrawal. Intra-CeA injections of beta-adrenoceptor antagonists propranolol (30 nmol per side) and timolol (10 nmol per side) significantly attenuated the morphine withdrawal-induced CPA. Similarly, beta(1)-antagonist atenolol (30 nmol per side) or beta(2)-antagonist butoxamine (30 nmol per side) significantly attenuated the CPA. In contrast, they did not affect morphine withdrawal-induced somatic signs, except for propranolol.
Conclusion: These results suggest that the activation of the noradrenergic system within the CeA contributes to naloxone-precipitated morphine withdrawal-induced CPA, rather than somatic signs, through beta(1)- and beta(2)-adrenoceptors.