Prenatal exposure to morphine alters kinetic properties of NMDA receptor-mediated synaptic currents in the hippocampus of rat offspring

Whole-cell patch-clamp recordings of pharmacologically isolated N-methyl-D-asparate (NMDA) receptor-mediated evoked excitatory postsynaptic currents (EPSCs) were made, to study whether prenatal exposure to morphine affected functional properties of synaptic NMDA receptors in hippocampal slices of 2-week-old rat offspring from morphine-addicted mothers. The saturated amplitude of synaptic NMDA receptor-mediated EPSCs from morphine-treated offspring was about twofold larger than that from vehicle-control offspring. The apparent dissociation constant (K _d) values of NMDA receptors for Mg ²⁺ at 0 mV were 7.5 ± 1.4 and 7.9 ± 1.3 mM in slices from vehicle-control and morphine-treated offspring, respectively. In addition, no distinguishable changes in the voltage-dependent nature and the reversal potential of NMDA receptors occurred in morphine-treated offspring, suggesting no alterations of Mg ²⁺ blockade and ion selectivity to NMDA receptors. The 10-90% rise times of NMDA receptor-mediated EPSCs in morphine-treated offspring became longer than those in vehicle-control offspring. The decay of NMDA receptor-mediated EPSCs in both morphine-treated and vehicle-control offspring could be described by the sum of a fast and a slow exponential function. The slow, but not fast, decay times of synaptic NMDA receptor-mediated currents in morphine-treated offspring became slower than those in vehicle-control offspring. Collectively, these results suggest that prenatal exposure to morphine altered kinetic properties of synaptic NMDA receptors in hippocampal CA1 pyramidal neurons of rat offspring during early life. The extended duration of synaptic NMDA receptor-mediated currents presumably provided more Ca ²⁺ entry through NMDA receptors in morphine-treated offspring, and its further prolongation by depolarization in such young offspring strengthened NMDA receptor-dependent functions. Thus, in light of pathophysiological implications within the central nervous system of morphine-treated offspring during early life, the present study may provide important insights and serve as a basis for therapeutic intervention in conditions under which NMDA receptors become abnormal.