Spontaneous and reflex activity of paraventricular nucleus units in cycling and lactating rats.

Extracellular recording of paraventricular neurones after electrical stimulation of the neurohypophysis reveals three cell types: type I cells, invaded by an antidromic action potential; type II cells, whose evoked response is orthodromic; and type III cells, which do not respond to stimulation. During the oestrous cycle of the rat, type I cells are the only ones in the paraventricular nucleus (PVN) whose spontaneous activity (expressed in number of action potentials per second) varies. It is significantly higher during lactation (3.6 "/- 0.6) and pro-oestrus (3.4 "/- 0.5) than during oestrus (2.0 +/- 0.5), metoestrus (1.2 +/- 0.3) and dioestrus (1.5 +/- 0.4). There are significantly more phasic units during prooestrus. The mean spontaneous firing rate of type II and III cells, remain unchanged. The reflex activity of those neurones was studied. Vaginal dilatation, which evokes a release of oxytocin31, increases the activity of a small percentage of type I cells, but there are no differences in this response at different stages of the cycle and lactation. Silent and phasic type I units are never activated. Only the evoked activity of type III cells is affected by the endocrine state. These results are discussed in the light of the known or hypothetical roles of these different cells.