Subunit-specific mechanisms and proton sensitivity of NMDA receptor channel block.
We have compared the potencies of structurally distinct channel blockers at recombinant NR1/NR2A, NR1/NR2B, NR1/NR2C and NR1/NR2D receptors. The IC50 values varied with stereochemistry and subunit composition, suggesting that it may be possible to design subunit-selective channel blockers. For dizocilpine (MK-801), the differential potency of MK-801 stereoisomers determined at recombinant NMDA receptors was confirmed at native receptors in vitro and in vivo. Since the proton sensor is tightly linked both structurally and functionally to channel gating, we examined whether blocking molecules that interact in the channel pore with the gating machinery can differentially sense protonation of the receptor. Blockers capable of remaining trapped in the pore during agonist unbinding showed the strongest dependence on extracellular pH, appearing more potent at acidic pH values that promote channel closure. Determination of pK(a) values for channel blockers suggests that the ionization of ketamine but not of other blockers can influence its pH-dependent potency. Kinetic modelling and single channel studies suggest that the pH-dependent block of NR1/NR2A by (-)MK-801 but not (+)MK-801 reflects an increase in the MK-801 association rate even though protons reduce channel open probability and thus MK-801 access to its binding site. Allosteric modulators that alter pH sensitivity alter the potency of MK-801, supporting the interpretation that the pH sensitivity of MK-801 binding reflects the changes at the proton sensor rather than a secondary effect of pH. These data suggest a tight coupling between the proton sensor and the ion channel gate as well as unique subunit-specific mechanisms of channel block.
Dravid
Shashank M
SM
Department of Pharmacology, Emory University School of Medicine, Rollins Research Centre, 1510 Clifton Road, Atlanta, GA 30322-3090, USA. smdravid@pharm.emory.edu
Erreger
Kevin
K
Yuan
Hongjie
H
Nicholson
Katherine
K
Le
Phuong
P
Lyuboslavsky
Polina
P
Almonte
Antoine
A
Murray
Ernest
E
Mosely
Cara
C
Barber
Jeremy
J
French
Adam
A
Balster
Robert
R
Murray
Thomas F
TF
Traynelis
Stephen F
SF
D000818:Animals; D002460:Cell Line; D016291:Dizocilpine Maleate / Q000494:pharmacology; D004594:Electrophysiology; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D015640:Ion Channel Gating; D007473:Ion Channels / Q000037:antagonists & inhibitors* / Q000187:drug effects / Q000502:physiology; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018408:Patch-Clamp Techniques; D011522:Protons; D051381:Rats; D017207:Rats, Sprague-Dawley; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors* / Q000187:drug effects / Q000502:physiology; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors; D013237:Stereoisomerism; D014982:Xenopus laevis
D018691:Excitatory Amino Acid Antagonists; D007473:Ion Channels; D011522:Protons; D016194:Receptors, N-Methyl-D-Aspartate; D011994:Recombinant Proteins; D016291:Dizocilpine Maleate