Direct pathways to the supraoptic nucleus from the brainstem and the main olfactory bulb are activated at parturition in the rat

Sensory input from female reproductive structures is paramount for the co-o rdination of neuroendocrine changes at parturition. Using a retrograde trac er (fluorescent latex microspheres) in combination with Fos (as an indicato r of neuronal activation) and tyrosine hydroxylase (to identify catecholami nergic neurons) immunocytochemistry we identified cells within the brainste m and main olfactory bulb that project to the supraoptic nucleus, and which become significantly activated at parturition (compared to virgin rats and rats on the day of expected parturition). Within the A2/C2 region in the n ucleus tractus solitarii, 60% of the projecting activated cells were catech olaminergic, as were 59% of such cells in the A1/C1 region of the ventrolat eral medulla. This suggests that oxytocin and vasopressin neurons within th e supraoptic nucleus are stimulated at parturition via afferent inputs from the brainstem, but the input is not exclusively noradrenergic. Within the mitral layer of the main olfactory bulb, cells that projected to the suprao ptic nucleus were significantly activated, suggesting that the olfactory sy stem may regulate supraoptic nucleus cell firing at parturition. The preopt ic area, organum vasculosum of the lamina terminalis and medial amygdala co ntained cells that projected to the supraoptic nucleus but these projection s were not significantly activated at parturition, although non-projecting cells in these regions were. On the expected day of parturition, but before birth, projections from the organum vasculosum of the lamina terminalis to the supraoptic nucleus became significantly activated. These findings provide evidence of direct afferent pathways to the supraopt ic nucleus from the brain stem and olfactory bulbs that are activated at pa rturition. (C) 2000 IBRO. Published by Elsevier Science Ltd. All rights res erved.