Non-opioid antitussives and methadone differentially influence hippocampal long-term potentiation in freely moving rats

Long-term potentiation (LTP) of monosynaptically evoked field potentials (MEFP) in the dentate gyrus of freely moving rats following tetanization of the perforant pathway was investigated after peripheral application of substances which have been shown to influence NMDA receptor-mediated effects (dextromethorphan, methadone) as well as structurally related substances with similar antitussive effects (codeine, normethadone). The noncompetitive NMDA receptor antagonist MK 801 was also tested for comparison. Whereas under control conditions the field e.p.s.p. (excitatory postsynaptic potential) and the population spike of the MEFP were largely uninfluenced by these substances, different effects were seen after the induction of LTP. MK 801 (0.2 mg/kg i.p.) suppressed the induction of LTP of both the field e.p.s.p. and the population spike. Dextromethorpan (40 mg/kg i.p.) also prevented the potentiation of the field e.p.s.p. and the population spike, thus resembling MK 801 in its effect. Codeine (20 mg/kg i.p.), the levorotatory structural analogue of dextromethorphan had no effect. Methadone and normethadone did not influence the potentiation of the field e.p.s.p. or interfere with the induction of potentiation of the population spike but depressed its maintenance. The results obtained with MK 801 confirm those reported by others. Comparison of the effects of dextromethorphan with those of MK 801, suggests that there is a direct interaction with the NMDA receptor-ionophore complex. The effects of methadone and normethadone appear not to be linked to an interaction with opioid receptors, since naloxone did not influence the suppression of LTP caused by methadone. The possibility of interference with the NMDA receptor-ionophore complex is discussed.