Neuronal Nitric Oxide Synthase Regulates Cerebellar Parallel Fiber Slow EPSC in Purkinje Neurons by Modulating STIM1-Gated TRPC3-Containing Channels.

Responding to burst stimulation of parallel fibers (PFs), cerebellar Purkinje neurons (PNs) generate a convolved synaptic response displaying a fast excitatory postsynaptic current (EPSC_Fast) followed by a slow EPSC (EPSC_Slow). The latter is companied with a rise of intracellular Ca²⁺ and critical for motor coordination. The genesis of EPSC_Slow in PNs results from activation of metabotropic type 1 glutamate receptor (mGluR1), oligomerization of stromal interaction molecule 1 (STIM1) on the membrane of endoplasmic reticulum (ER) and opening of transient receptor potential canonical 3 (TRPC3) channels on the plasma membrane. Neuronal nitric oxide synthase (nNOS) is abundantly expressed in PFs and granule neurons (GNs), catalyzing the production of nitric oxide (NO) hence regulating PF-PN synaptic function. We recently found that nNOS/NO regulates the morphological development of PNs through mGluR1-regulated Ca²⁺-dependent mechanism. This study investigated the role of nNOS/NO in regulating EPSC_Slow. Electrophysiological analyses showed that EPSC_Slow in cerebellar slices of nNOS knockout (nNOS^-/-) mice was significantly larger than that in wildtype (WT) mice. Activation of mGluR1 in cultured PNs from nNOS^-/- mice evoked larger TRPC3-channel mediated currents and intracellular Ca²⁺ rise than that in PNs from WT mice. In addition, nNOS inhibitor and NO-donor increased and decreased, respectively, the TRPC3-current and Ca²⁺ rise in PNs. Moreover, the NO-donor effectively decreased TRPC3 currents in HEK293 cells expressing WT STIM1, but not cells expressing a STIM1 with cysteine mutants. These novel findings indicate that nNOS/NO inhibits TRPC3-containig channel mediated cation influx during EPSC_Slow, at least in part, by S-nitrosylation of STIM1.