2 TYPES OF STEADY-STATE DESENSITIZATION OF N-METHYL-D-ASPARTATE RECEPTOR IN ISOLATED HIPPOCAMPAL-NEURONS OF RAT

1. Whole-cell voltage-clamp recordings were made from rat isolated hippocampal neurones. Aspartate (Asp) and/or glycine (Gly) were applied by a method in which the external solution could be changed within 30 ms and thereafter held constant. 2. Asp and Gly applied together at maximal concentrations (5 mM and 10-mu-M, respectively) evoked an inward current due to activation of N-methyl-D-aspartate (NMDA) receptors. The current peaked and then declined to a steady state during the application. The time constant of desensitization (tau) was about 1 s when the agonists were applied soon after the onset of whole-cell recording. The desensitization became more rapid (tau = 0.3 s) and more complete during the first 15 min of recording, and thereafter remained stable; the amplitude of the peak response did not change throughout. In solutions containing 10-mu-M-Gly, Asp had an apparent K(d) of 51-mu-M at the peak of response and 20-mu-M measured at the steady state. The steady-state current was 14% of the peak current. 3. Asp was applied after a conditioning exposure of the cell of Gly (from 1 to 50-mu-M), together with the same Gly concentration. The maximum current evoked by the application of Asp was increased while increasing Gly in the conditioning solution, with no change in the apparent K(d) for Asp at the peak of Asp-activated response. 4. Various concentrations of Asp (plus 10-mu-M-Gly) were applied after a conditioning exposure to Asp (which alone was without effect). The maximum current induced by Asp applications was only 28% of that observed without conditioning Asp application, but the apparent K(d) was unchanged (about 57-mu-M). 5. Test solution containing maximal concentrations of Asp and Gly was applied after conditioning exposure to both Asp (varying concentrations) and Gly (10-mu-M). Complete desensitization was caused by 200-mu-M-Asp. The apparent K(d) for Asp to induce desensitization (8.7-mu-M) was less than the K(d) as an agonist (51-mu-M). 6. Test solution containing maximal concentrations of Asp and Gly was applied after conditioning exposure to both Gly (varying concentrations) and Asp (5 mM). Complete desensitization was caused by 1-mu-M-Gly. The apparent K(d) for Gly to induce desensitization (120 nM) was less than the K(d) as a co-agonist (about 1-mu-M). 7. The results imply that Asp alone induces desensitization of the NMDA receptor manifested as a reduced maximal response with no change in apparent K(d), whereas a combination of both co-agonists induces a desensitization of the NMDA receptor which is accompanied by an increase in affinity of receptor for both co-agonists.