Abstract
The actions of dextromethorphan (DXM) on the 50 pS conductance state of the N-methyl- d-aspartate (NMDA) receptor-operated channel were studied using outside-out patches obtained from cultured rat hippocampal pyramidal neurons. DXM (5–50 μM) had no effect on the amplitudes of unitary currents but caused concentration-dependent reductions in channel mean open times and the frequency of channel openings. Channel open probability was reduced in a concentration-dependent manner by DXM and was one-half of the control value at a DXM concentration of 6 μM, with the patch potential held at −60 mV. An IC 50 value of 4 μM was obtained for the reduction by DXM of NMDA-evoked rises in [Ca 2+] i in cultured rat hippocampal pyramidal neurons loaded with Fura-2. The results were consistent with drug block of the open NMDA channel with an onward (blocking) rate constant of 7.7 × 10 6 M −1 · s −1 (at −60 mV). The estimated unblocking rate constant was about 10 s −1, a value considerably higher compared to the off-rate constant found for dizocilpine block of the NMDA channel.
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