The type 1 inositol (1,4,5)-trisphosphate receptor (InsP₃R1) plays a critical role in Ca²⁺ signaling in cells. Neuronal and nonneuronal isoforms of the InsP₃R1 differ by alternative splicing in the coupling domain of the InsP₃R1 (SII site) (Danoff et al., 1991). Deletion of 107 amino acids from the coupling domain of the InsP₃R1 results in epileptic-like behaviors in opisthotonos (opt) spontaneous mouse mutant (Street et al., 1997). Using Spodoptera frugiperda cells expression system, we compared single-channel behavior of recombinant InsP₃R1-SII(+), InsP₃R1-SII(−), and InsP₃R1-opt channels in planar lipid bilayers. The main results of our study are: 1) the InsP₃R1-SII(−) has a higher conductance (94pS) and the InsP₃R1-opt has a lower conductance (64pS) than the InsP₃R1-SII(+) (81pS); 2) the bell-shaped Ca²⁺-dependence peaks at 200–300nM Ca²⁺ for all three InsP₃R1 isoforms; 3) the bell-shaped Ca²⁺-dependence is wider for the InsP₃R1-SII(+) and narrower for the InsP₃R1-SII(−) and InsP₃R1-opt; 4) the apparent affinity for ATP is sixfold lower for the InsP₃R1-SII(−) (1.4mM) and 20-fold lower for the InsP₃R1-opt (5.3mM) than for the InsP₃R1-SII(+) (0.24mM); 5) the InsP₃R1-SII(−) is approximately twofold more active than the InsP₃R1-SII(+) in the absence of ATP. Obtained results provide novel information about the molecular determinants of the InsP₃R1 function.