Chronic pain induces anxiety with concomitant changes in opioidergic function in the amygdala

Clinically, it has been reported that chronic pain induces depression, anxiety, and reduced quality of life. The endogenous opioid system has been implicated in nociception, anxiety, and stress. The present study was undertaken to investigate whether chronic pain could induce anxiogenic effects and changes in the opioidergic function in the amygdala in mice. We found that either injection of complete Freund's adjuvant (CFA) or neuropathic pain induced by sciatic nerve ligation produced a significant anxiogenic effect at 4 weeks after the injection or surgery. Under these conditions, the selective μ-opioid receptor agonist [D-Ala²,N-MePhe⁴,Gly⁵-ol]- enkephalin (DAMGO)-and the selective δ-opioid receptor agonist (+)-4-[(αR)-α-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl) -3-methoxybenzyl]-N,N-diethyl-benzamide (SNC80)-stimulated [³⁵S] GTPγS binding in membranes of the amygdala was significantly suppressed by CFA injection or nerve ligation. CFA injection was associated with a significant increase in the κ-opioid receptor agonist 2-(3,4- dichlorophenyl)-N-methyl-N-[(IS)-1-phenyl-2-(1-pyrrolidinyl)ethyl]acetamide hydrochloride (ICI 199,441)-stimulated [³⁵S]GTPγS binding in membranes of the amygdala. The intracerebroventricular administration and microinjection of a selective μ-opioid receptor antagonist, a selective δ-opioid receptor antagonist, and the endogenous κ-opioid receptor ligand dynorphin A caused a significant anxiogenic effect in mice. We also found that thermal hyperalgesia induced by sciatic nerve ligation was reversed at 8 weeks after surgery. In the light-dark test, the time spent in the lit compartment was not changed at 8 weeks after surgery. Collectively, the present data constitute the first evidence that chronic pain has an anxiogenic effect in mice. This phenomenon may be associated with changes in opioidergic function in the amygdala.