Firing patterns of GnRH neurons in vivo. A, GnRH neurons displayed a range of firing activity, from irregular (top), tonic (middle), to phasic (bottom). Action currents exist as large predominantly downward deflections. *Occasional unfiltered high-frequency noise can be seen as deflections of equal upward and downward amplitude. B, The local application of TTX, voltage-gated sodium channel blocker, stopped firing. C, Thirty millimolars of KCl evoked a transient burst of activity from a silent GnRH neuron.