Repeated 2 Hz peripheral electrical stimulations suppress morphine-induced CPP and improve spatial memory ability in rats

Our previous studies have shown that 2 Hz peripheral electrical stimulation (PES) can suppress morphine-induced conditioned place preference (CPP) in the rat, although the mechanisms remain unclear. Since CPP involves the mechanism of learning and memory, it is rational to ask whether the suppressive effect of repeated 2 Hz PES on morphine-induced CPP is due to an impairment of the function of spatial learning and memory. Rats were trained with 4 mg/kg morphine, i.p. for 4 days to establish the CPP. Twenty-four hours after the CPP testing, they were given PES at 2 Hz once a day for 1, 3 or 5 days, followed by another CPP testing. The results showed that (1) the morphine-induced CPP was significantly inhibited by 3 or 5 consecutive sessions, but not by single session of 2 Hz PES. (2) A test of spatial leaning and memory ability using the Morris water maze task revealed that 2 Hz PES per se exhibited a promoting, rather than a deteriorating effect on the ability of spatial memory. (3) 2 Hz PES by itself produced a moderate yet significant CPP. The results imply that (a) a low frequency PES can produce a rewarding effect as revealed by the CPP testing, which may account, at least in part, for its suppressive effect on morphine induced CPP, (b) the suppressive effect of PES on morphine induced CPP is not due to a deteriorating effect on the ability of spatial memory.