Nitric oxide as intracellular modulator: internal production of NO increases neuronal excitability via modulation of several ionic conductances

Nitric oxide (NO) has been shown to regulate neuronal excitability in the nervous system, but little is known as to whether NO, which is synthesized in certain neurons, also serves functional roles within NO-producing neurons themselves. We investigated this possibility by using a nitric oxide synthase (NOS)-expressing neuron, and studied the role of intrinsic NO production on neuronal firing properties in single-cell culture. B5 neurons of the pond snail Helisoma trivolvis fire spontaneous action potentials (APs), but once the intrinsic activity of NOS was inhibited, neurons became hyperpolarized and were unable to fire evoked APs. These striking long-term effects could be attributed to intrinsic NO acting on three types of conductances, a persistent sodium current (INaP), voltage-gated Ca currents (ICa) and small-conductance calcium-activated potassium (SK) channels. We show that NOS inhibitors 7-nitroindazole and S-methyl-l-thiocitrulline resulted in a decrease in INaP, and that their hyperpolarizing and inhibiting effects on spontaneous spiking were mimicked by the inhibitor of INaP, riluzole. Moreover, inhibition of NOS, soluble guanylate cyclase (sGC) or protein kinase G (PKG) attenuated ICa, and blocked spontaneous and depolarization-induced spiking, suggesting that intrinsic NO controlled ICa via the sGC/PKG pathway. The SK channel inhibitor apamin partially prevented the hyperpolarization observed after inhibition of NOS, suggesting a downregulation of SK channels by intrinsic NO. Taken together, we describe a novel mechanism by which neurons utilize their self-produced NO as an intrinsic modulator of neuronal excitability. In B5 neurons, intrinsic NO production is necessary to maintain spontaneous tonic and evoked spiking activity.