Morphine-induced conditioned place preference in rats is inhibited by electroacupuncture at 2 Hz: Role of enkephalin in the nucleus accumbens

Our previous studies have demonstrated that morphine-induced conditioned place preference (CPP) can be inhibited by 2 Hz electroacupuncture (EA). This inhibition can be blocked by either the opioid receptor antagonist naloxone (i.p.) or lesion in the nucleus accumbens (NAc), providing evidence that endogenous opioid system in the NAc mediates the effects of EA. Here we report that 1) A single session of 2 Hz EA produced a significant increase of the content of enkephalin in the NAc of morphine-induced CPP rats, and this effect was stronger in three consecutive sessions of EA; 2) Intracerebroventricular injection of the μ-opioid receptor antagonist CTAP or δ-opioid receptor antagonist NTI, but not κ-opioid receptor antagonist nor-BNI, dose-dependently reversed the inhibitory effects of 2 Hz EA on the expression of morphine-induced CPP; 3) Three consecutive sessions of 2 Hz EA up-regulated the mRNA level of preproenkephalin in the NAc of morphine-induced CPP rats. The results suggest that the inhibitory effects of 2 Hz EA on the expression of the morphine CPP is mediated by μ- and δ-, but not κ-opioid receptor, possibly via accelerating both the release and synthesis of enkephalin in the NAc. These findings support the possibility of using 2 Hz EA for the treatment of opiate addiction.