There are two approaches to evaluation of the effects of drugs on steady-state ligand-gated receptor function: (1) concentration-response curves derived from whole-cell recordings and (2) analysis of single-channel recordings. Whole-cell currents are the average response of a large population of channels, whereas single-channel recordings document the gating behavior of a single protein. Analysis of single-channel recordings details the effects of modulators and agonists on channel kinetics at the molecular level. Whole-cell concentration-response analysis is a standardized protocol for classification and interpretation of a mechanism that permits quantitative comparisons with other modulators. Quantitation of charge at the single-channel level can be used as a basis for comparison with whole-cell concentration-response curves. This procedure allows the investigator to compare the effects of a variety of drugs at the molecular level using a well-established set of tools developed for analysis of macroscopic responses.

The effects of 0.1 to 500 mM ethanol on NMDA-activated currents were studied in primary cultures of mouse cortical and hippocampal neurons. In whole-cell recordings the IC50S for inhibition of NMDA-activated currents by ethanol were 129 mM +/- 20 mM in hippocampal neurons and 126 +/- 18 mM in cortical neurons. In single-channel recordings from excised outside-out patches of cortical neurons, ethanol inhibited total charge per minute with an IC50 of 174 +/- 23 mM, which was not significantly different from the IC50S for inhibition of whole-cell current. The reduction in mean open channel lifetime by ethanol was fit by the logistic equation with an apparent IC50 of 340 +/- 28 mM. Analysis of single-channel data indicated that ethanol inhibition of NMDA currents did not involve substantial changes in fast closed state kinetics, changes in open channel conductance, or block of the open channel. At the whole-cell IC50 of ethanol, mean open channel lifetime would decrease by 28% and frequency of opening would decline by 31% to account for the reduction in current. Single-channel data were consistent with ethanol being an allosteric modulator of gating which reduces agonist efficacy.

The effects of ethanol on excitatory amino acid activated ion channels were investigated using patch-clamp recording methods. Intoxicating concentrations of ethanol (5-50 mM) inhibited ion current activated by the glutamate receptor agonist N-methyl-D-aspartate (NMDA) in a concentration-dependent manner (IC50 = 30 mM). The intoxicating potency of different alcohols was correlated with their potency for inhibiting NMDA-activated current, suggesting that alcohol-induced inhibition of NMDA channel function may contribute to the neural and cognitive impairments associated with intoxication. Analysis of mechanism suggests that ethanol inhibits NMDA-activated current by altering gating of the channel, rather than by affecting channel conductance, ion permeance or regulatory sites on the channel. Anesthetic concentrations of ethanol (> 50 mM) produced a concentration-dependent inhibition of kainate- and quisqualate-activated currents (IC50 = 220 mM), suggesting that this inhibition may contribute to the general anesthetic effects of ethanol. This hypothesis is supported by the observations that the general anesthetic agents, trichloroethanol (the active metabolite of chloral hydrate), pentobarbital and halothane, all inhibit kainate- and quisqualate-activated currents.

Bicuculline methiodide (BIC-Mel) (10-100 microM) altered the kinetics of N-methyl-D-aspartate (NMDA) responses in single-channel and whole-cell recordings. The principal effect of BIC-Mel (10-100 microM) on NMDA channels was a dose-dependent decrease in mean channel open time (tau o), accompanied by the introduction of a new closed time (tau B) of 14.0 +/- 3.5 msec (mean +/- standard deviation; n = 14) in closed time distributions, which was independent of BIC-Mel concentration. BIC-Mel (10-100 microM) increased the frequency of NMDA channel opening in a dose-dependent manner, offsetting the decrease in tau o, such that the total time spent in the open state per minute was unchanged, and thus the total charge/min through NMDA channels was unchanged. Similarly, the amplitudes of NMDA whole-cell current responses were not noticeably affected by 10-80 microM BIC-Mel, even though power spectra density analysis of the whole-cell NMDA-stimulated noise revealed changes in the underlying channel kinetics in the presence of BIC-Mel. Taken together, the effects of 10-80 microM BIC-Mel on NMDA responses were consistent with the predictions of the sequential block model; however, the effects of BIC-Mel exhibited no obvious voltage dependence. In addition to the low-dose effects of BIC-Mel, 100 and 200 microM BIC-Mel inhibited whole-cell NMDA responses. The inhibition by 100 microM BIC-Mel was not large, but it was augmented from 15% to 30% by increasing the NMDA concentration from 10 microM NMDA to 20 microM NMDA, indicating that channel activation was necessary for BIC-Mel-mediated inhibition. Preliminary single-channel experiments performed under conditions conducive to trapping of an open channel blocker at its binding site indicated that the effect of BIC-Mel on tau o persisted after the removal of the blocker, consistent with use dependence of the dissociation of BIC-Mel from the NMDA receptor-channel complex.

Annals of the New York Academy of SciencesVolume 648, Issue 1 p. 353-354 A Slow Voltage-Dependent Increase in N-Methyl-D-Aspartate Open-Channel Probabilitya J. M. WRIGHT, J. M. WRIGHT Department of Pharmacology Cornell University Ithaca, New York 14853Search for more papers by this authorL. M. NOWAK, L. M. NOWAK Department of Pharmacology Cornell University Ithaca, New York 14853Search for more papers by this author J. M. WRIGHT, J. M. WRIGHT Department of Pharmacology Cornell University Ithaca, New York 14853Search for more papers by this authorL. M. NOWAK, L. M. NOWAK Department of Pharmacology Cornell University Ithaca, New York 14853Search for more papers by this author First published: May 1992 https://doi.org/10.1111/j.1749-6632.1992.tb24580.x a This work was supported by NIH Grant NS24467. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume648, Issue1Neurotoxins and Neurodegenerative DiseaseMay 1992Pages 353-354 RelatedInformation

1. The mechanisms of tetraethylammonium (TEA) antagonism of N-methyl-D-aspartate (NMDA) responses were investigated in cultured mouse cortical neurons by analysing single-channel and whole-cell currents from patch clamp recordings. TEA (1-5 mM) decreased whole-cell NMDA responses. Kainate and quisqualate receptor-mediated responses were unaffected at these TEA concentrations. 2. In whole-cell recordings, increasing the NMDA concentration while keeping the TEA concentration constant resulted in greater inhibition by TEA. Thus, TEA-mediated inhibition of NMDA responses was not due to competitive antagonism, and the greater inhibition by a single dose of TEA as NMDA concentration was elevated indicated some form of non-competitive inhibition. In single-channel recordings, two inhibitory effects were seen in 1-5 mM-extracellular TEA: single-channel conductance (gamma) was decreased, and the frequency of channel events was decreased. These effects were not accompanied by any change in average channel open time. 3. Single-channel current-voltage (I-V) curves obtained in 2, 5, 10 and 30 mM-TEA indicated the decrease in NMDA channel conductance was voltage dependent with larger reduction occurring as patches were hyperpolarized. The data were well fitted by the Woodhull model with the dissociation constant (KD) showing an e-fold increase in inhibition for a 43-45 mV change in membrane potential. The 0 mV KD was 45 mM-TEA decreasing to about 11 mM at -60 mV. The TEA block site appeared to sense approximately 60% of the transmembrane potential field (delta = 0.6) for extracellular application of TEA. 4. The decrease in channel opening frequency seen in TEA was concentration dependent and generally more sensitive to extracellular TEA than the channel block effect. There was a 50% reduction in the number of NMDA channel openings observed in 5 mM-TEA. Increasing either NMDA or glycine concentrations in constant TEA concentration caused an additional decrease in the frequency of NMDA channel opening. In contrast to extracellular TEA, intracellular TEA had no noticeable effect on open-state probability. 5. NMDA single-channel currents were observed at positive potentials after completely replacing pipette Cs+ by 140 mM-TEA-Cl indicating TEA could serve as a current carrier through NMDA channels. Single channel I-V curves obtained with pipettes containing 70 or 140 nM-TEA in place of equivalent amounts of Cs+ were fitted by the Goldman-Hodgkin-Katz (GHK) equation over the range of -80 to +70 mV assuming a permeability of 0.45 compared with a Cs+ permeability of 1.0.(ABSTRACT TRUNCATED AT 400 WORDS)