Chronic in vivo morphine administration facilitates primed-bursts-induced long-term potentiation of Schaffer collateral-CA1 synapses in hippocampal slices in vitro

In this study, the effects of chronic morphine administration (20-30 days) on long-term potentiation (LTP) were investigated at the Schaffer collateral-CA1 pyramidal cell synapses of the rat hippocampal slices. Orthodromic population spike (OPS) amplitude and delay (peak latency) were measured as indices of increase in synaptic efficacy. The amounts of LTP of OPS delay and LTP of OPS amplitude were higher in slices from dependent rats. Perfusion of slices from control and dependent rats with morphine containing ACSF and delivering tetanic stimulation, showed that short-term presence of morphine could not mimic the LTP enhancing effects of chronic morphine administration, however, attenuated the amount of LTP of OPS amplitude in slices of dependent rats. This study supports the hypothesis that the susceptibility of CA1 synapses to plastic changes increases by chronic, not acute exposure to morphine and suggests that a withdrawal phenomenon might be an underlying mechanism for the observed augmented LTP of OPS amplitude in slices of dependent rats.