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Ionic permeability characteristics of the N-methyl-D-aspartate receptor channel.

N-methyl-D-aspartate (NMDA) receptor channels in cultured CA1 hippocampal neurons were studied using patch-clamp techniques. The purpose of the research was to determine the occupancy of the channel by permeant cations and to determine the influence of charged residues in or near the pore. The concentration dependence of permeability ratios, the mole-fraction dependence of permeability ratios, the concentration dependence of the single-channel conductance, and a single-channel analysis of Mg2+ block all independently indicated that the NMDA receptor behaves as a singly-occupied channel. More precisely, there is one permeant cation at a time occupying the site or sites that are in the narrow region of the pore directly in the permeation pathway. Permeability-ratio measurements in mixtures of monovalent and divalent cations indicated that local charges in or near the pore do not produce a large local surface potential in physiologic solutions. In low ionic strength solutions, a local negative surface potential does influence the ionic environment near the pore, but in normal physiologic solutions the surface potential appears too small to significantly influence ion permeation. The results indicate that the mechanism for the high Ca2+ conductance of the NMDA receptor channel is not the same as for the voltage-dependent Ca2+ channel (VDCC). The VDCC has two high affinity, interacting binding sites that provide high Ca2+ selectivity and conductance. The binding site of the NMDA receptor is of lower affinity. Therefore, the selectivity for Ca2+ is not as high, but the lower affinity of binding provides a faster off rate so that interacting sites are not required for high conductance.


INTRODUCTION
The N-methyl-D-aspartate (NMDA) subtype of glutamate receptors is a ligand-gated cation-selective ion channel that is highly permeable to Ca 2+ and is blocked by Mg 2+ in a voltage-dependent manner (Nowak, Bregestovski, Ascher, Herbet, and Prochiantz, 1984;Mayer, Westbrook, and Guthrie, 1984;MacDermott, Mayer, Westbrook, Smith and Barker, 1986;Jahr and Stevens, 1987). These ion permeation properties underlie the role of the NMDA receptor in the induction of long-term potentiation at several excitatory pathways in the central nervous system (review by Bliss and Collingridge, 1993). Although these properties of ion movement through the NMDA receptor channel have been clearly demonstrated, the mechanisms that underlie these properties are controversial.
An important determinant of the permeability characteristics of an ion channel is the number of ions that occupy the narrow region of the pore. This region contains the ion binding sites that are directly in the permeation pathway, and therefore, these sites must be encountered by every ion that passes through the pore. Several pieces of evidence suggest that there may be multiple occupancy of the NMDA receptor pore. When an open-channel blocker apparently senses more than 100% of the voltage drop across the membrane, that indicates that the blocker is coupled to other ions in the permeation pathway (i.e., multiple occupancy). In several cases, it has been reported that blockers of the NMDA receptor may sense up to 100% of the field (Ascher and Nowak, 1988;Mayer, Westbrook, and Vyklicky, 1988; but see Jahr and Stevens, 1990). In addition, when a blocker was applied first to the outside of the channel then the inside, the proportion of the field sensed from the two sides added up to greater than 100%, suggesting that the blocking ion is coupled to the movement of permeant ions (Ascher and Nowak, 1988;Johnson and Ascher, 1990). Other evidence indicates that the open channel blockers, MK-801 and Mg 2+, interact with each other or with Ca 2+ (Mayer and Westbrook, 1987;Ascher and Nowak, 1988;Huettner and Bean, 1988). Although much of the work suggests there may be competition among these ions for a single site, at very low salt concentrations the interaction between MK-801 and Mg 2+ does not seem strictly competitive (Reynolds and Miller, 1988). In low concentrations of salt, it also was found that reversal potential measurements could not be described by a simple single occupancy permeation model (Mayer and Westbrook, 1987).
In many of the cases described above, the results also could be explained by single occupancy of the pore if, in addition, there are local negative charges that influence ion permeation. Local net negative charge in or near the pore can be thought of as producing a local "surface potential." There is controversy about the existence or strength of a local surface potential. It was found that a small local surface potential could explain the increased permeability ratio of Ca ~+ to Na + (Pca/PNa) in low ionic strength solutions. Although multiple occupancy of the pore could produce the result, an equally good explanation was that a small negative surface potential present in low ionic strength solutions elevates the local concentration of Ca 9+ in relation to that of Na + and produces an apparent increase in the permeability ratio (Pca/PNa; Mayer and Westbrook, 1987;Iino, Ozawa, and Tsuzuki, 1990). On the other hand, a much larger negative local surface potential present even in high ionic strength solutions was proposed to explain single-channel current-voltage relations in mixtures of Ca 2+ and Na + (Ascher and Nowak, 1988).
The purpose of this study was to resolve these controversies by determining whether the NMDA receptor channel is multiply occupied and by determining the influence of a local surface potential. The concentration dependence of the permeability ratio of two permeant cations, the mole-fraction dependence of the permeability ratio, the shape of the concentration dependence of the single-channel conductance, and a burst analysis of Mg 2+ block all separately indicate that the NMDA receptor behaves as a single-occupancy channel. The influence of local charges in or near the channel was investigated by measuring permeability ratios in different ionic strength mixtures of monovalent and divalent cations. Measurements in a wide range of solutions indicate that a low density of net negative charges influence ion permeation through the NMDA receptor. Only in low ionic strength solutions, not in physiologic solutions, is the permeability ratio of divalent to monovalent cations affected by a local surface potential. Some of this work was previously reported at a scientific meeting (Zarei and Dani, 1992).

Cell Culture
Hippocampal neurons were cultured using standard techniques (adapted from Huettner and Baughman, 1986;Amador and Dani, 1991a). In short, two Sprague-Dawley rats (postnatal day 1-2) were used for each preparation. The brain was removed and was kept in Earl's balanced salt solution (EBSS) throughout the dissection. The CAI region was dissected from the hippocampus, and the isolated CA1 tissue was then transferred into an enzyme solution containing 10 ml of EBSS and 200 U ofpapain for ~ 30 min (Worthington Biochemical Corp., Freehold, NJ). The tissue was washed several times with complete media that also contained 2.5 mg/ml trypsin inhibitor and bovine serum albumin (Sigma Chemical Co., St. Louis, MO). Then, the tissue was gently triturated with the same solution to dissociate the cells. Complete media was composed of Minimum Essential Medium (GIBCO Laboratories, Grand Island, NY), 292 I~g/ml glutamine, 5% heat-inactivated fetal bovine serum (Hyclone Labs, Logan, UT), 50 U/ml penicillin, 50 ~,g/ml streptomycin, 20 mM glucose, and 1 ~,l/ml serum extender (Collaborative Research, Inc., Bedford, MA). The cells were plated onto coverslips that had been coated with collagen and poly-o-lysine and exposed to ultraviolet light for 30 min. After 3 d, the cells were treated for 2 d with 5 IxM cytosine arabinofuranoside. The cells were maintained in complete media in an incubator at 37°C, 5% CO~. Media was changed every 3 d, and the cells were used between 6 to 9 d after plating. Although the cells were healthy for much longer, we used them early and for a short period of time so that the cells did not have extremely long processes.

Patch-Clamp Techniques
Whole-cell and single-channel NMDA receptor currents were measured using standard patchclamp techniques (Hamill, Marty, Neher, Sakmann, and Sigworth, 1981;Dani, 1989). Electrodes were pulled with a two-stage electrode puller (PP-83; Narishige USA, Inc., Greenvale, NY) using Garner Glass 7052 and were coated with Sylgard silicon elastomer (Dow Corning Corp., Midland, MI). The tips of the electrodes were polished immediately before they were used with a microforge (MF-83; Narishige USA, Inc.). Their resistances were between 2 to 5 M~ in standard solutions. Currents were amplified and filtered using an Axopatch 1B voltage clamp with a 4-pole Bessel filter (Axon Instruments, Foster City, CA). Currents were digitally sampled with a 12-bit analog-to-digital converter (TL-1; Axon Instruments). The computer system delivered the holding potentials through a 12-bit digital-to-analog output. Whole-cell currents were sampled every 20 ms, and the currents were measured 2 s after the application of agonist. Singlechannel currents were sampled every 0.1 ms. Various filter settings were used.
Complete solution changes were achieved by using large inflow pipettes. These inflow pipettes (KG-33; Garner Glass, Claremont, CA) had wide openings (380-1~m inner diam) and narrow walls and were arranged in a row and glued together (see Amador and Dani, 1991b;Vernino, Amador, Luetje, Patrick, and Dani, 1992). These pipettes were mounted on a hydraulic manipulator that allowed rapid solution changes. Fast solution changes, however, were not critical for measuring steady state currents after the application of agonist. For whole-cell experiments, measurements in test solutions were always bracketed by measurements in a control solution. Control solutions contained the same composition as the test solutions except the main charge carrying ion was Cs ÷. In all experiments, the liquid junction potentials were measured, and the holding potentials were corrected before plotting the data. Depending on the solutions, the liquid junction potentials varied from 0 to 3 rnV. All the experiments were conducted at room temperature, 23°C.

Solutions
A wide range of electrolyte solutions were used to study whole cells and outside-out membrane patches. The osmolalities of all the solutions were measured with a vapor pressure osmometer (Wescor, Inc., Logan, UT) and adjusted to appropriate values by adding sucrose. The pH of the solutions was adjusted to 7.4 using the hydroxide of the main cation. Unless stated otherwise, the bath was (mM) 150 NaCI, 3 KCI, 2 CaClz, 10 HEPES, 10 glucose, 50 I~M picrotoxin, 1 ~M strychnine, 0.5 o~M tetrodotoxin and 200 ~M DIDS. The external solutions in the inflow pipettes had the same channel blockers (without DIDS) as the bath solution (ultra pure salts, Johnson Matthey, Inc., Wayne, PA). On the day of the experiment, 20 ~M NMDA (Cambridge Research Biochemical, Inc., Wilmington, DE) and 10 I~M glycine were added to the external solutions for whole-cell recordings and 1-3 ~M NMDA and glycine were added for singlechannel recordings.
The cations used in the permeability-ratio experiments were dimethylammonium (DMA ÷) and Cs +. These cations were chosen because their permeabilities were different enough to produce easily measured shifts in the reversal potentials. The external solution was (raM) 500 or 150 DMA +, 10 HEPES plus blockers. The internal solution was (mM) 500 or 150 CsCI, 0.1 CaCI2, 10 HEPES, 5 Na-ATP, 5 EGTA. The solutions for experiments at a monovalent cation concentration of 50 or 10 mM were obtained by diluting the 150 mM solutions and, then, adjusting the osmolalities to 0.3 osMo with sucrose. The HEPES concentrations were adjusted to 5 raM, and the concentration of blockers were adjusted to the concentrations used in the original bath solution. Unless stated otherwise these external solutions were nominally free from divalent cations.
Ammonium was used as the permeant ion to measure the concentration dependence of the single-channel conductance. Ammonium was chosen because it has a high conductivity through the NMDA receptor channel, which produces large single-channel currents that could be accurately measured even in low salt concentrations. Because low concentrations of divalent cations can influence the single-channel currents carried by monovalent cations (Dani and Eisenman, 1987;Jahr and Stevens, 1990), the external solution bathing the patch was nominally free from divalent cations: 150 or 400 mM NH4CI, 20 HEPES plus blockers. The internal solution contained (raM) 135 or 385 NH4CI, 15 EGTA, 20 HEPES. When the pH of these solutions was adjusted with NH4OH, the final concentrations of NH~ became 150 and 400 raM. The solutions used for measurements in 50, 20, and 7 mM NH4CI were prepared by diluting the 150 mM solutions as explained in the permeability-ratio experiments. In addition, at 7 mM NH4C1 we also added 0.5 mM EGTA and EDTA to the external solution to be certain that divalent cations were not influencing the results.
Permeability ratios of divalent cations to monovalent cations were measured using solutions that were similar to those described above, but 0.5 mM Ca 2+ was added to the external solution of 150 mM DMA ÷. That solution was diluted to 50 and 10 mM as explained above, and the internal solutions were the same as the internal CsCI solutions described above. Permeability ratios were also measured in external mixtures ofCs + with Ba 2+ or Ca 2+. A solution of 150 mM CsC1, 10 mM HEPES plus blockers was mixed with an identical solution that also contained either 30 mM Ba ~÷ or Ca 2+. By mixing appropriate ratios of these two solutions, external solutions were created that contained 150 mM CsCI, 10 HEPES plus 1.5 or 15 or 30 mM BaCI2 or CaCI2. The internal solution was (mM) 135 CsCI, 0.1 CaCI2, 10 HEPES, 5 Na-ATP, 15 EGTA, 15 EDTA. For a second set of experiments, these solutions were diluted to 10 mM as described above and the osmolalities were adjusted to 0.3 osMol with sucrose. The bath solutions in these experiments contained 150 mM CsCI, 10 mM HEPES, 0.5 mM CaCI2 plus blockers.
In all calculations and for graphs that showed theoretical lines, the activities of cations were always used. The following method was used to determine activity coefficients. The ionic strength of the solution was calculated and was used to look up the activity coefficient from tables of that electrolyte. The tables were from the CRC Handbook of Chemistry and Physics, Edition 65, 1984, and from the Journal of Physical Chemistry Reference Data, Vol. 6, 1977. To obtain the single ion activity coefficients for divalent cations we followed the convention of Butler (1968), who suggests that a divalent cation's activity coefficient is given by ~/++ = (,/)3. In the cases were calculations were made using activities, we give the activity of each cation in parenthesis after the concentration (e.g., 150 (112) mM CsC1).
Because the solution in the patch pipette was often very different from the cytoplasmic electrolyte, > 8 min were allowed for the pipette electrolyte to diffuse throughout the cell. By following the shift in the reversal potential, it was found that a steady value was reached after 6 min in large cells, indicating that the internal exchange of electrolyte was sufficiently complete in < 6 min with the smaller cells that were used in the experiments described in this paper.

Occupancy of Ion Binding Sites Directly in the Permeation Pathway of the NMDA Receptor Pore
Permeability-ratio experiments were used to determine whether the NMDA receptor channel is singly or multiply occupied by permeant cations directly in the permeation pathway. If the reversal potential of two permeant ions remains constant as a function of concentration, that is strong evidence that the channel is singly occupied (see Levitt, 1986;Dani, 1989). Reversal potentials were measured using Cs + as the internal permeant cation and DMA + as the external permeant cation. The ionic concentrations of Cs ÷ and DMA + were changed together from 10 to 500 mM, and the reversal potentials were measured at each concentration ( Fig. 1). An analysis of variance showed that the measured reversal potentials did not change significantly as a function of concentration (P > 0.05): the permeability ratio remained constant, Pcs/PDMA =-4.8. These results indicate that the NMDA receptor channel behaves as though there is a single cation at a time occupying the site(s) directly in the permeation pathway over the concentration range that was studied.
Mole-fraction experiments were conducted to confirm the conclusion of the permeability-ratio experiments. If the reversal potential varies monotonically as the mole fraction of two permeant cations is varied, then the result indicates that the  channel is singly occupied (Neher, 1975). Reversal potentials were measured with 150 mM Cs + as the internal permeant cation and with different mole fractions of DMA+/Cs + as the external permeant cations. When the mole fraction of the DMA + was changed from 0 up to 1, the reversal potential changed monotonically (Fig. 2), indicating that the NMDA receptor channel behaves as though it is singly occupied by a permeant cation directly in the permeation pathway.

Concentration Dependence of the Single-Channel Conductance
The single-channel conductance of the NMDA receptor was measured in five different concentrations of NH~. In these experiments, equal concentrations of NH~ bathed both sides of the membrane. The concentration of NH~ was changed from 7 (6.4) mM up to 400 (272) raM, and Fig. 3 shows examples of single-channel currents. In each concentration, the mean zero-voltage conductance of the NMDA receptor channel was determined (Fig. 4). Then, the experimental data were fitted  . Concentration dependence of the zero-voltage single-channel conductance. The single-channel conductance is plotted versus the symmetrical activity of NH~. The zero-voltage conductance was determined from the slope of the currentvoltage relation through zero. Conductances determined from separate patches or from extremes in the slope used to describe the data allowed us to estimate the error, which is showed as a bar when it is greater than the symbol. The insert is an Eadie-Hofstee plot of conductance versus conductance divided by NH~ activity. The solid theoretical line represents a single-site model based on Eq. 1 with a ~m~* of 90 pS and a KM of 12 mM. The dashed theoretical curve represents a single-site model that also includes a small negative charge density of 0.02 charges per nmL The negative charge produces a negative local potential that influences the concentration of NH~ next to the channel only in low concentrations. The same theoretical lines are shown both in the insert and in the main graph.
using the Michaelis-Menten (M-M) single-site equation: where ~/0 is the measured conductance, ~/max is the maximum conductance, KM is the M-M dissociation constant for NH~, and [NH4]b is the bulk activity of NH~. The conductance of a channel with multiple occupancy can reach a maximum and, then, decrease at higher concentrations of the permeant cation. The single-channel conductance of the NMDA channel, however, begins to level off at higher concentrations of NH~ as expected when a single site is becoming saturated (Fig. 4). This behavior is consistent with the earlier permeability-ratio and mole-fraction experiments. However, at 7 mM NH~, the conductance of the channel is higher than predicted by simple, single-site kinetics. In the inset of Fig. 4, the data are recast as an Eadie-Hofstee plot that more clearly shows the deviation at the lower concentrations. This type of deviation is often explained by assuming that the channel is multiply occupied, but that cannot be the case here. An equally good explanation for the deviation is that at very low electrolyte concentrations a local negative surface potential attracts NH~ to the pore and increases the effective local concentration of the permeant cation (see Dani and Eisenman, 1987;Green and Andersen, 1991). The solid line in Fig. 4 shows the behavior of a simple, single-site description of conductance (Eq. 1). The dotted line shows that by introducing a small surface potential (described below) the single-site model more closely describes the deviation to a higher conductance that is seen in 7 (6.4) mM NH~. This result lead us to investigate more thoroughly the influence of local net negative charges in or near the NMDA receptor channel.

Interactions Between the Blocking Ion, Mg a+, and the Permeant Ion, Cs +
The previous experiments showed that the NMDA receptor behaves as a singleoccupancy channel for permeant monovalent cations. Here we provide evidence that the interaction between Cs + and the blocking ion, Mg z+, is consistent with single occupancy. It is known that external Mg 2+ blocks by entering deeply into the pore like a cork in a bottle (Mayer et al., 1984;Nowak et al., 1984). If there were multiple sites, after Mg 2+ binds to its blocking site other ions binding to a more external site would trap Mg 2+ in the pore, enhancing the block as is seen for the multiple sites of the CaZ+-activated K + channel (Cecchi, Wolff, Alvarez, and Latorre, 1987;Neyton and Miller, 1988). . This protocol was repeated in pure 200 mM Cs + (Fig. 5 B). To have the same amount of Mg 2+ near the channel at 200 mM Cs +, we corrected the activity of Mg 2+ for the influence of ionic strength in the bulk solution as described in the methods and for the small difference in the surface potential, which is -1.9 mV in 50 mM CsC1 and -0.5 in 200 mM CsCI (from Fig. 4). Thus, 10.5 (2.6) tzM Mg 2+ was added to the 200 mM Cs +, and Fig. 5 B shows that there is less rapid block than was seen at the lower ionic strength, Fig. 5  patch). The raw data and the burst analysis (Fig. 5, C and D) indicate that the block by Mg 2÷ is diminished by increasing the concentration of Cs ÷. This result is not like the increased block at higher concentrations of the permeant ion that is seen with multi-site channels (Cecchi et al., 1987;Neyton and Miller, 1988). Rather, the result is consistent with a single-occupancy pore. Cs + competes with Mg 2+ for occupancy of the pore; thus, as the Cs ÷ concentration is elevated the Mg 2+ block is less apparent.

Local Net Negative Charge Influences Ion Permeation
The influence of local net negative charge on the ionic environment near the channel can be studied using different ionic strength solutions. In high ionic strength solutions, the charges near the pore are screened by the electrolytes (reviews by McLaughlin, 1977;Green and Andersen, 1991). In low ionic strength solutions, however, the charges are not screened as well. Therefore, the net negative charge produces a larger potential that more strongly attracts cations toward the pore. Divalent cations are attracted more strongly than are monovalent cations, causing the ratio of divalent cations to monovalent cations to increase in lower ionic strength solutions. In that case, adding permeant divalent cations into a solution will cause a We investigated the influence of the local net negative charges in or near the NMDA receptor by measuring reversal potential shifts in mixtures of monovalent and divalent cations that were of different ionic strength.
Reversal potentials were measured in solutions that had the ratio of the cations fixed while the concentrations were varied. The internal solution was pure Cs + and the external solution was a mixture of DMA + and Ca 2+. Fig. 6 A shows examples of the measurements in three different concentrations of the cations. One concentration consisted of 150 mM Cs + inside and 150 (113) mM DMA + plus 0.5 (0.12) mM Ca 2+ outside the cell. Those solutions were diluted by a factor of 3 and by a factor of 15 to obtain the other two solutions. Because the ratio of DMA + to Ca 2+ in the external bulk solution remained unchanged, the reversal potential should have remained constant unless a local surface potential altered the ratio of the cation concentrations next to the pore. The open bars in Fig. 6 B show that the reversal potential is the same at 150 (113) mM DMA ÷ plus 0.5 (0.12) mM Ca 2+ and 50 (40) mM DMA ÷ plus 0.17 (0.057) mM Ca 9+ but in 10 mM (9) DMA ÷ plus 0.033 (0.016) mM Ca 2+ the reversal potential is more positive. The result is consistent with a small local negative potential increasing the ratio of the Ca 2+ concentration to that of the DMA + concentration next to the pore only in the low ionic strength solution.
The extended Goldman-Hodgkin-Katz (GHK) equation (Lewis, 1979;Lewis and Stevens, 1979) was used to calculate the permeability ratios, Pca/POMA, from the reversal potential (Erev) measurements:  Fig. 1. When experimental Erev values were substituted into Eq. 2, the apparent Pca/PDM A was significantly increased in the 10 mM electrolyte solution. As was shown earlier (Figs. 1 and 2) and will be shown below (Fig. 7) permeability ratios are constant as activity is changed. Therefore, the apparent increase in Pca/PDMA is caused by local net negative charges in or near the pore. In 10 mM DMA +, the local charges are not effectively screened by the low ionic strength. Therefore, Ca 2+ is concentrated at the pore more than DMA +. Because the surface ratio of the activities, [Ca2+]s/[DMA+]s, is greater than the bulk ratio of the activities, [Ca~+]b/[DMA+]b, Ca 2+ causes a larger shift in the reversal potential at low ionic strength. The differences between the surface activities of the cations and the bulk activities can be approximated using the Boltzmann's equation:

Pc~ (1 + e ~'~,iRr) [ Pc~ ) ~PDMA [Cs]iezFE'~/RT --
where [X]~ and [X]b represent surface and bulk activity of ions respectively, and represents the potential created by the net negative charges in or near the NMDA pore. ~ can be crudely approximated by using Grahame's equation (Grahame, 1947): where cr is the charge density in electronic charges per nm 2, G is a constant with a value of 2.7 (nm2/charge) (mol/liter) 1/2, Xi is the activity of the ith ion species in the bulk solution, and • is the potential created by the charge. This approach only roughly approximates the likely dynamic situation of discreet dipoles and charged amino acids distributed in or near the NMDA receptor pore (see McLaughlin, 1977;Dani, 1986). However, the underlying principles are similar, and thus the approach provides a basis for analysis and discussion. The analysis indicates there is a low negative charge density of 0.02 charges per nmL In Fig. 6 B, the predicted reversal potentials are indicated (*) based on a single Pca/PDMA of 58 and a local negative potential that varies with ionic strength and ranges from about -7 in the 10 mM solution to -0.5 mV in the 150 mM solution.
To verify these conclusions, a more thorough set of permeability-ratio measurements was made in external mixtures of Cs + with Ca z+ or Ba 2+ and with pure Cs + inside. Cs + was used instead of DMA + and both Ca 2+ and Ba z+ were used over a wider concentration range to be certain that the previous conclusions did not depend on the species of permeant cations that were used. As always, reversal potentials were or Ba e+ (Fig. 7 B). Fig. 7 C shows that the permeability ratios, Pca/Pcs and PBa/Pcs, are concentration independent. A single permeability ratio of ~ 12 describes the Erev measurements over the full range of Ca 2+ or Ba 2+ activities that were tested. This result provides further evidence that even when considering monovalent and divalent cations the NMDA receptor behaves as a single-occupancy channel. Then, reversal potentials were measured again after all the solutions were diluted 15 times to obtain 10 (9.0, 8.9, 8.8) mM Cs + with 0.1 (0.053), 1 (0.50), or 2 (0.95) mM Ca 2+ (Fig. 8 A) or Ba 2+ (Fig. 8 B). Because the ratio of divalent to monovalent cations is held constant in the bulk solution, the reversal potentials would be equal in the high and low ionic strength solutions if there were no local surface potential. In produce the solid line. In the low ionic strength of 10 mM, the same constant permeability ratio was used but the surface potential became significant (~ -7 mV), producing the dashed line. The same value of a low charge density could describe all the results, including the single-channel conductances obtained in NH~ (Figs. 3 and 4) and the permeability measurements in mixtures of DMA + and Ca 2+ (Fig. 6). The conclusions of all of these experiments are in quantitative agreement that there is a low density of net negative charge influencing the NMDA receptor pore.

DISCUSSION
This research provides two pieces of information about the NMDA receptor channel. First, five different approaches indicate that the NMDA receptor behaves as a single-occupancy channel for both monovalent and divalent cations. That conclusion can be defined more completely. Only one cation at a time occupies a binding site or sites that are located directly in the permeation pathway. These binding sites are located in the narrow portion of the pore so that every cation encounters these sites during permeation, and cations cannot easily pass by each other in this region. Other binding sites are likely to exist in the water-filled pore, but those are in wider regions and not directly in the permeation pathway of every cation. Many of these widely separated sites may be occupied by counter ions. The site(s) in the narrow region of the pore, however, govern the permeation properties and are singly occupied owing to electrostatic repulsion between cations. Furthermore, elevated Cs + diminishes blockade of the channel by Mg 2+, indicating that Cs + is not occupying a site external to the Mg 2+ blocking site. This result (Fig. 5) suggests that Mg 2+ and Cs ÷ are competing for the same site (or electrostaticaUy indistinguishable sites). Second, measurements in a wide range of ionic strength solutions indicate that permeability ratios are only weakly affected by charged residues in or near the NMDA receptor channel. In physiologic solutions, the potential produced by local charges does not strongly influence ion permeation. In low ionic strength solutions, however, local net negative charges produce a meaningful negative potential that elevates the concentration of cations near the channel. The results indicate that electrolytes must either bind to or screen local charges in physiologic solutions so that the ionic environment near the pore is not dramatically different from bulk solution. The same, very low charge density was used to describe all the data. That estimate of the charge density depends on the models, but the general conclusions are not dependent on the models or equations used for the analysis. Given the limitations and approximate nature of the surface-charge approach (see McLaughlin, 1977;Dani, 1986), the theoretical description of the data is adequate for our purposes.

Impact on Previous Results
Previous studies of the NMDA receptor have suggested that permeation is affected either by multiple occupancy or by a local negative surface potential. Our results provide a more thorough interpretation of previous results. We are in complete agreement with earlier results that estimated the permeability ratio of Ca 2+ to Na + or Cs + (Pca/P+) to be ~ 12 (calculated using activities, Mayer and Westbrook, 1987;Iino et al., 1990). Those authors also found that PcJP+ is apparently larger in low ionic strength solutions, just as we have shown under more varied conditions. That result is not due to multiple occupancy but, rather, is due to the influence of a local negative potential that is significant only in low ionic strength solutions. That interpretation is reinforced by another result. In higher ionic strength solutions (100 or 150 mM monovalent salts), the reversal potential responds to added external Ca 2+ in a way that can be described by a single binding site without the need to invoke a surface potential or multiple occupancy (our Fig. 9; Mayer et al., 1987). Contrary to these findings, it has been suggested that a large negative surface potential in physiologic solutions may explain the parallel shift in the outward limb of current-voltage relationships caused by increasing external Ca 2+ (Ascher and Nowak, 1988). In retrospect, we know that a large surface potential need not be invoked and that the apparently parallel current-voltage relationships arise because Ca 2÷ shifts the reversal potential. A related issue is that Ca 2+ may alter the local surface potential by binding to the NMDA receptor at sites other than directly in the permeation pathway (Ascher and Nowak, 1988). Binding to external sites by Ca ~÷ could change the charge density that influences the ionic environment around the channel. Such a mechanism is consistent with our work and is supported by the finding that elevated Ca 2+ decreases Zn 2+ binding and decreases Zn ~+ modulation of the receptor (Mayer, Vyklicky, and Westbrook, 1989). Studies using cloned NMDA receptor subunits are consistent with our conclusion that there is no significant local surface potential influencing the ionic environment in physiologic solutions. Studies with cloned channels expressed in Xenopus oocytes indicate that ion permeation is controlled mainly by uncharged residues in transmembrane region M2 and not by nearby negatively charged residues (Mori, Masaki, Yamakura, and Mishina, 1992;Burnashev, Schoepfer, Monyer, Ruppersberg, Gunther, Seeburg, and Sakmann, 1992). A particular amino acid (asparagine 598) has a dominant role in controlling Ca ~÷ permeability of NMDA receptors composed of subunit NR1. This same amino acid in a homologous subunit, NR2, is predominant in determining the Mg 2÷ blockade of cloned NMDA receptors (Burnashev et al., 1992). Consistent with our results, a group of six negatively charged amino acids that flank transmembrane region M2 do not influence Mg 2+ or MK-801 blockade of the channel in physiological solutions. Charged residues bracketing the membrane spanning region are located in a position to produce local potentials that could influence ion selectivity and permeation (see Dani, 1986). Because they do not, it is likely that the residues are at least partially screened or balanced by bound counter ions. It is also possible that the residues are either not directed into the pore or are not all expressing their charge because the pK of an amino acid is shifted by the presence of other charged amino acids.
Another issue that can be re-examined is the mechanism for the interaction between open channel blockers of the NMDA receptor (Mayer and Westhrook, 1987;Ascher and Nowak, 1988;Huettner and Bean, 1988). Much of the interaction between Mg 2+ and the permeant ion Ca 2+ can be explained by competition for a single site, consistent with the apparent competition between Mg ~+ and Cs + that we saw in Fig. 5. However, in low ionic strength solutions, it seems that the interaction between the large organic blocker MK-801 and Mg 2+ is not competitive (Reynolds and Miller, 1988). They seem to be occupying two different sites that interact, which would imply the channel can have multiple occupancy of sites in the permeation pathway. Several circumstances may underlie this finding. Because MK-801 is large, it may not reach the primary binding site for small cations. Therefore, MK-801 finds a low energy configuration external to that site. Mg 2+, however, reaches the primary site where it lingers while blocking the channel. Because electrostatic interactions reach especially far in very low ionic strength solutions, Mg 2+ at its site speeds the off rate of MK-801 from its site by electrostatic repulsion. This low energy position for MK-801 may not be available to or may not be directly in the permeation pathway for small permeant ions: e.g., a small ion may occupy the position, but other permeant ions could pass by without being strongly influenced. The very large voltagedependencies of some open channel blocks may be explained in the same manner, but we are now investigating that issue.

Conclusions about Ion Permeation in the NMDA Receptor Channel
The narrow region of the pore, where ions cannot easily pass by each other, holds one cation at a time. It may be that this narrow region is very short and, thus, provides only one main binding site, as is the case for the nicotinic acetylcholine receptor (Dani, 1989). Alternatively, electrostatic repulsion may allow only single occupancy of the multiple sites in this region.
The mechanism for the high Ca 2+ conductance of the NMDA receptor is not the same as for the voltage-dependent Ca z+ channel (VDCC). The VDCC has two strongly interacting very high affinity sites for Ca 2+. The high affinity sites allow greater selectivity for Ca 2+ and electrostatic repulsion between these sites speeds Ca z+ off its site and through the channel (Almers and McCleskey, 1984;Hess and Tsien, 1984). For the NMDA receptor, the affinity of the site is not as high. Thus, the selectivity is not as great, but high conductance does not require electrostatic repulsion arising from multiple occupancy to speed the ion off of its site.
In physiologic solutions, the ionic environment near the pore is not strongly influenced by a local negative potential. In low ionic strength solutions, the influence of local charges becomes important. Although we might anticipate that local charges within the channel influence the permeation process in some way, those charges do not alter the general ionic environment near the pore under normal conditions. We thank Drs. Craig Jahr, Robin Lester, Marc Rogers, and Ms. Martha Skender for reading and commenting on the manuscript. We thank Dr. Ramon Latorre for helpful discussions about the single-channel measurements with Mg z+ present. The work was supported by the Whitaker Foundation and by NIH grant NS 21229.