Calcium‐mediated modulation of N‐methyl‐D‐aspartate (NMDA) responses in cultured rat hippocampal neurones.

1. Agonist‐independent (inactivation) and agonist‐induced (desensitization) refractory states of N‐methyl‐D‐aspartate (NMDA) receptors were studied on cultured rat hippocampal neurones using whole‐cell and inside‐out patch‐clamp techniques and a fast perfusion system. 2. Shortly after whole‐cell formation, application of 100 microM NMDA in the presence of 10 microM glycine and 0.2 mM [Ca2+]o induced membrane currents that desensitized by 23%. Repeated application of NMDA at 30 s intervals resulted in a progressive increase in the degree and rate of onset of NMDA receptor desensitization. 3. Test responses to NMDA recorded in the presence of 0.2 mM [Ca2+]o were reversibly inactivated by 60% following a train of ten 1 s applications of NMDA delivered at 0.5 Hz in the presence of 2 mM [Ca2+]o; similar results were obtained with 2 mM [Sr2+]o and 2 mM [Ba2+]o. In the presence of Ca2+ or Sr2+, desensitization during the train of responses to NMDA increased by 14 and 19% respectively, while with Ba2+ there was no increase in desensitization. 4. In the presence of 0.2 mM [Ca2+]o at a holding potential of ‐60 mV, or in the presence of 2 mM [Ca2+]o at a holding potential of +50 mV, a train of ten applications of NMDA failed to induce either inactivation or an increase in desensitization of test responses to NMDA. These results suggest an important role for [Ca2+]o in the induction of both inactivation and desensitization of NMDA receptors. 5. Increasing the intracellular calcium concentration, [Ca2+]i, via repeated activation of voltage‐gated Ca2+ channels, resulted in a reversible inactivation of test responses to NMDA by 35% but failed to increase desensitization. In neurones dialysed with intracellular solution containing 2.5 mM Ca2+ NMDA receptor desensitization was similar to that in neurones dialysed with 10 nM Ca2+. 6. Block of NMDA receptor‐channels by 2 mM [Mg2+]o during the train application of NMDA prevented the induction of both inactivation and desensitization. In contrast 3 mM [Mg2+]i was ineffective. 7. The magnitude of both inactivation and desensitization of NMDA receptors was not affected by intracellular dialysis of ATP, the non‐hydrolysable ATP analogue 5'‐adenylylimido‐diphosphate (AMP‐PNP), different Ca2+ chelators (EGTA or BAPTA), the Ca(2+)‐activated protease inhibitor (leupeptin), dithiothreitol, or the phosphatase inhibitors (okadaic acid and a calcineurin inhibitor). 8. Application of 2.5 mM Ca2+ to the cytoplasmic side of inside‐out patches induced inactivation of NMDA responses similar in magnitude to the inactivation seen in whole‐cell recording.(ABSTRACT TRUNCATED AT 400 WORDS)