Phasically firing neurons in long-term cultures of the rat hypothalamic supraoptic area: pacemaker and follower cells.

The firing of a proportion of hypothalamic neurons in long-term cultures from the supraoptic nucleus was characterized by a phasic discharge pattern. Burst duration and cycle time varied considerably between different cultures, but were similar in neurons of the same culture. The instantaneous frequency of firing was highest at the onset of bursts. The phasic activity of the majority of cells was synaptic in origin. In these follower cells, current injection through the recording electrode had no effect on burst duration and cycle time, and their activity was abolished by synaptic isolation. In contrast, the activity of bursting pacemaker cells persisted during applications of Co2+ and was a function of membrane potential. In both pacemaker and follower cells, action potentials and slow oscillations of membrane potential were abolished by application of tetrodotoxin.