In vivo sodium salicylate causes tolerance to acute morphine exposure and alters the ability of high frequency stimulation to induce long-term potentiation in hippocampus area CA1

Effects of morphine on synaptic transmission and plasticity in the hippocampus area CA1 following in vivo sodium salicylate and the potential molecular mechanism were investigated. Population spikes (PS) were recorded from stratum pylamidale of area CA1 following stimulation of Schaffer collaterals in slices taken from control and sodium salicylate injected rats. To induce long term potentiation (LTP), a 100 Hz tetanic stimulation was used. Acute in vitro morphine increased baseline PS amplitude in control slices but not in slices taken from sodium salicylate treated rats. In vivo chronic salicylate did slightly decrease and/or destabilize LTP of CA1 synaptic transmission. We also found that mRNA of NR2A subunit of NMDA receptor was reduced in the hippocampus of sodium salicylate treated rats as compared to control ones. Following LTP induction, the mRNA of NR2A and PP1 (protein phosphatase 1) in slices taken from salicylate-treated rats were more than those of control ones. After long-term exposure to in vitro morphine, high frequency stimulation (HFS) decreased NR2A mRNA level significantly in sodium salicylate treated slices. It is concluded that in vivo sodium salicylate causes tolerance to excitatory effect of morphine and changes the ability of HFS to induce PS LTP in the hippocampus area CA1 in vitro. These changes in synaptic response may be due to alterations in NR2A and PP1 expression.