Figure 3. Direct and indirect mechanisms involved in the control of magnocellular neurosecretory cells (MNCs) firing frequency by nitric oxide (NO). NO controls the firing rate of the MNCs in order to prevent over-secretion of the neurohypophysial hormones through an indirect (increase in synaptic inputs) and/or a direct pathway by S-nitrosylation of hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels (see text). Traces shown on each side of the neurohypophysis are action potentials recorded directly from the MNCs under the conditions indicated. Increased firing rate (blue trace), induced by a hypertonic solution, is associated with an increased VP/OT secretion (blue arrow). NO decreases firing rates (red traces) in both basal and osmotic stress conditions. Voltage and time scales are the same for all records.