Modulation of N‐methyl‐D‐aspartic acid receptor desensitization by glycine in mouse cultured hippocampal neurones.

Abstract

1. Responses to N-methyl-D-aspartic acid (NMDA) were recorded from mouse embryonic hippocampal neurones in dissociated culture, using the tight-seal, whole-cell, patch-clamp technique for voltage clamp. A rapid perfusion system, with an exchange time constant of less than 10 ms, was used to apply NMDA under conditions which minimized slow, calcium-sensitive desensitization. With no added glycine, responses to 100 .mu.M-NMDA applied for 1.5 s declined by greater than 90%, due to an additional component of desensitization of time constant 250 ms. 2. Adding glycine to the extracellular solution, over the range 30 nM to 3 .mu.M, both potentiated responses to NMDA and to L-glutamate, and reduced fast desensitization. In the presence of 3 .mu.M-glycine responses to NMDA declined by only 10%. Similar potentiation and reduction of desensitization was obtained with 3 .mu.M concentrations of the glycine analogues D-alanine and D-serine. 3. Analysis of dose-response curves for the action of glycine on responses to 100 .mu.M-NMDA revealed a 3-fold higher potency of glycine for potentiation of peak versus steady-state responses, with concentrations for half-activation of 134 and 382 nM, respectively. The competitive glycine antagonist 7-chlorokynurenic acid produced a similar shift of both the peak and steady-state dose-response curves for glycine, consistent with an equilibrium dissociation constant of 280 nM for interaction of 7-chlorokynurenic acid with the glycine binding site on NMDA receptors. 4. In the presence of 100 nM-glycine, 10 .mu.M-7-chlorokynurenic acid produced nearly complete block of the response to 3 mM-NMDA, suggesting that glycine is absolutely required for activation of the NMDA receptor channel complex. 5. In some neurones responses to NMDA showed essentially no desensitization in the presence of 3 .mu.M-glycine. Under these conditions, 7-chlorokynurenic acid produced a concentration-dependent block of both the initial and equilibrium response to NMDA, with a 4-fold greater sensitivity for block of the steady-state current (IC50 = 2.25 .mu.M) than for block of the peak current (IC50 = 8.96 .mu.M). As a result, in the presence of 7-chlorokynurenic acid, responses to NMDA showed strong desensitization, even in the presence of 3 .mu.M-glycine. 6. Our results show that glycine-evoked potentiation of NMDA receptor activity is accompanied by reduced desensitization. Because responses to NMDA measured at equilibrium require higher concentrations of glycine for expression of activity than responses measured early after the application of agonist, the reduction of desensitization by glycine is an important mechanism for promoting responses to sustained applications of NMDA receptor agonists.