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 collatera l-CA1 pyramidal cell synapses of the rat hippocampal slices. Orthodromic po pulation spike (OPS) amplitude and delay (peak latency) were measured as in dices 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 morphin e could not mimic the LTP enhancing effects of chronic morphine administrat ion, however, attenuated the amount of LTP of OPS amplitude in slices of de pendent rats. This study supports the hypothesis that the susceptibility of CAI synapses to plastic changes increases by chronic, not acute exposure t o morphine and suggests that a withdrawal phenomenon might be an underlying mechanism for the observed augmented LTP of OPS amplitude in slices of dep endent rats. (C) 1999 Elsevier Science B.V. All rights reserved.