Mammalian endocrine neurones: Control of phasic activity by antidromic action potentials

Hypothalamic supraoptic neurones were identified by antidromic invasion following stimulation of the pituitary stalk in urethane anaesthetized rats. These endocrine neurones either discharged continuously at low average rates (random) or were intermittently active (phasic). Phasic cells are thought to be vasopressinergic. With phasic neurones, antidromic activation with short trains of stimuli during the quiescent period triggered the 'on-phase' of spike discharge, and the probability of this response increased as the 'off-phase' progressed. Stimulation was ineffective in triggering high frequency firing when applied below the threshold for antidromic invasion, and caused no parallel response of random neurones. Arrest of firing during the 'on-phase' was achieved by stimulation with higher currents and longer pulse trains. These studies demonstrate that the switching process determining the firing pattern of phasic neurones depends on the cells' ongoing activity. Further, they reveal progressive changes in the cells' capacity to alter their electrophysiological state, a capacity that is not expressed in terms of the temporal pattern of spike discharges.