Alternative splicing of the NMDAR1 subunit affects modulation by calcium

Four splice variants of the NR1 receptor subunit, characterized by the presence or absence of cassettes encoding inserts of 21 (Insert 1) and 37 (Insert 2) amino acids were expressed in Xenopus oocytes and studied using voltage-clamp techniques. In 1.8 mM Ca2+, a slow inward current (I-slow), which peaked 20 s after exposure to NMDA was evident when Insert I was present, but not when absent. However, in elevated external Ca2+ medium a similar I-slow was observed in variants missing Insert I. The Ca2+ dependency of I-slow reflected a requirement for intracellular accumulation of Ca2+. The divalent ion permeability of Insert 1 containing and Insert 1 lacking receptor channels expressed alone, as well as in heteromeric assemblies with NR2A and NR2B, was similar for all combinations tested. Thus, the lower Ca2+ dependency for I-slow in oocytes expressing Insert I was not due to higher calcium entry. I-slow was less sensitive to blockers of I-Cl(Ca) than were endogenous calcium-activated chloride currents (I-Cl(Ca)). Also, I-slow was not abolished in Cl--free external medium, when voltage was manipulated such that I-slow was outward-going. Thus, I-slow, while containing a component due to activation of endogenous I-Cl(Ca), is primarily due to current flowing through the receptor ion channel. Development of I-slow was unaffected by PKC or PKA inhibitors. The modulation of the Ca2+ dependency of I-slow by Insert I occurs in a range of Ca2+ concentrations which are physiologically relevant, and may provide an important means of modulation of glutamate transmission under normal and pathological conditions.