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) an d 37 (Insert 2) amino acids were expressed in Xenopus oocytes and stud ied using voltage-clamp techniques. In 1.8 mM Ca2+, a slow inward curr ent (I-slow), which peaked 20 s after exposure to NMDA was evident whe n Insert I was present, but not when absent. However, in elevated exte rnal Ca2+ medium a similar I-slow was observed in variants missing Ins ert I. The Ca2+ dependency of I-slow reflected a requirement for intra cellular 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 f or 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 a bolished in Cl--free external medium, when voltage was manipulated suc h that I-slow was outward-going. Thus, I-slow, while containing a comp onent due to activation of endogenous I-Cl(Ca), is primarily due to cu rrent flowing through the receptor ion channel. Development of I-slow was unaffected by PKC or PKA inhibitors. The modulation of the Ca2+ de pendency of I-slow by Insert I occurs in a range of Ca2+ concentration s which are physiologically relevant, and may provide an important mea ns of modulation of glutamate transmission under normal and pathologic al conditions.