Peripheral electric stimulation inhibits morphine-induced place preference in rats.

Conditioned place preference (CPP) is a commonly used model to detect rewarding effect of drugs. To observe the effect of peripheral electric stimulation (PES) on morphine-induced CPP, we trained the rats with morphine in a CPP paradigm. Twelve hours before the testing phase, rats were given PES via stainless-steel needles with frequencies of 2, 100, or 2/100 Hz, respectively. PES of 2 and 2/100 Hz significantly decreased CPP in morphine-trained animals in a naloxone reversible manner, while PES of 100 Hz, foot shock, needle insertion, or plain restraining, showed no effect. Thus, PES with a low-frequency component (2 Hz) could specifically inhibit the expression of morphine-induced CPP, presumably via activation of opioid receptors.