Role of Midbrain Parabrachial Nucleus in Controlling Electrical Activity of Oxytocin and Vasopressin Secreting Neurones in the Hypothalamic Supraoptic Nucleus.

Abstract

Release of posterior pituitary hormones is known to be controlled by hemodynamic information in addition to factors related to reproductive function such as suckling stimulus by pups during lactation and afferents from the uterine cervix during parturition. The midbrain parabrachial nucleus, which is one of the major sites for the integration of the autonomic function, sends heavy projections to the hypothalamic supraoptic and paraventricular nuclei which contain oxytocin and vasopressin secreting neurones. The present experiments were undertaken to examine the physiological role of these projections. Electrical stimulation of the parabrachial nucleus increased the excitability of 65% of putative oxytocin neurones, and 60% of putative vasopressin neurones. Electrolytic lesion of the parabrachial nucleus itself did not affect the basal firing rate, which suggests that inputs from the parabrachial nucleus to the supraoptic nucleus are not tonically driven. Although the lesion did not change the responsiveness of neurones to hyperosmotic stimulation, it affected the responsiveness of putative vasopressin neurones to the decrease of blood pressure. In addition to the well-known role of neurosecretory neurones in reproductive functions, these neurones may change their electrical activity in response to inputs driven by the parabrachial nucleus and contribute to the control of body fluid balance.