Voltage-dependent interaction of open-channel blocking molecules with gating of NMDA receptors in rat cortical neurons

J Physiol. 1996 Jun 1;493 ( Pt 2)(Pt 2):425-45. doi: 10.1113/jphysiol.1996.sp021394.


1. The mechanisms by which four adamantane derivatives (IEM-1857, -1592, -1460 and -1754) block the open NMDA-activated channel were studied at membrane voltages (Vm) from -170 to +30 mV. The rate constants of channel block (k+) and of channel unblock (k-) were measured from the fully resolvable flicker of single-channel currents induced by each compound. 2. The k+ of each compound exhibited a similar exponential dependence on voltage over the Vm range studied. 3. The k- of IEM-1857 and IEM-1592 over the Vm range studied, and of IEM-1754 and IEM-1460 from -30 to -90 mV, exhibited similar exponential dependencies on voltage. However, the k- of IEM-1754 and IEM-1460 at Vm values more hyperpolarized than -90 mV were much more steeply voltage dependent, suggesting that at these Vm values the two drugs can occupy a deeper binding site. 4. Each of the drugs induced a concentration-dependent prolongation of the mean burst length at -90 mV, suggesting that while blocking they can interfere with channel closure. 5. The prolongation of mean burst length induced by the largest drug (IEM-1857) increased with hyperpolarization. The increase was consistent at each Vm with the predictions of the sequential scheme of block, suggesting that channel closure is prevented when IEM-1857 is bound. The prolongation of burst length induced by the smallest drug (IEM-1754) was less than predicted by the sequential scheme and the deviation increased with hyperpolarization. 6. The IEM-1857 concentration-dependence of number of blockages per unit open time had a slope equal to k+ at -150 mV. The IEM-1754 concentration-dependence of number of blockages per unit open time revealed a slope about two times less than k+ for this compound at -150 mV. 7. The mean patch current was not significantly altered by 3 microM IEM-1857 at Vm values from -90 to -150 mV, as expected of a drug that prevents channel closure when blocking. Mean patch current significantly decreased with hyperpolarization beyond -90 mV in the presence of 1 microM IEM-1754. 8. The data suggest that there are two blocking sites at different depths within the NMDA-activated channel. Channel closure is prevented when any of the IEM drugs occupy the shallow blocking site. Channel closure is permitted during occupation of a deeper blocking site that can be reached only by the smaller IEM drugs at hyperpolarized voltages.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adamantane / analogs & derivatives*
  • Adamantane / pharmacology
  • Animals
  • Axons / drug effects
  • Axons / metabolism
  • Axons / physiology
  • Cells, Cultured
  • Cerebral Cortex / cytology
  • Cerebral Cortex / drug effects
  • Cerebral Cortex / metabolism*
  • Electrophysiology
  • Ion Channel Gating / physiology*
  • Kinetics
  • Magnesium / metabolism
  • Membrane Potentials / drug effects
  • Neurons / drug effects
  • Neurons / metabolism
  • Neurons / physiology*
  • Patch-Clamp Techniques
  • Quaternary Ammonium Compounds / pharmacology*
  • Rats
  • Rats, Sprague-Dawley
  • Receptors, N-Methyl-D-Aspartate / antagonists & inhibitors*


  • IEM 1754
  • IEM 1857
  • Quaternary Ammonium Compounds
  • Receptors, N-Methyl-D-Aspartate
  • Magnesium
  • Adamantane