Response of rat paraventricular neurones with central projections to suckling, haemorrhage or osmotic stimuli.

In lactating, urethane-anaesthetized female rats extracellular recordings were made from paraventricular nucleus (PVN) neurones that were antidromically activated following electrical stimulation of the neurohypophysis, amygdala or nucleus tractus solitarius/vagal complex (NTS/VC). Of the PVN units, 98 projected to the neurohypophysis but none of these neurosecretory neurones were found to simultaneously project to extrahypothalamic areas. From the firing patterns and the response of these neurons to suckling, haemorrhage or osmotic stimuli both 'vasopressinergic' and 'oxytocinergic' neurones were identified. We found 43 PVN units to project to the NTS/VC and 22% of tested neurones were activated by osmotic or haemorrhage stimuli; no phasic activity was associated with this activation. The suckling stimulus failed to elicit any response from these units. Upon testing the PVN units that projected to the amygdala (n = 35), it was found that haemorrhage and suckling stimuli were without effect, while the osmotic stimulus activated one of 6 units tested. Thus, the extrahypothalamic PVN projections examined in this study were not associated with the suckling reflex response, although there is evidence for their limited involvement in neural response to osmotic or haemorrhage stimuli.