Medullary afferents to neurosecretory neurons of the rat supraoptic nucleus

The hypothalamic supraoptic nucleus (SON) in the rat contains neurons that synthesize either vasopressin (VP) or oxytocin (OT) and receives inputs from medullary nuclei. While VP neurons receive inputs from noradrenergic cells in the caudal ventrolateral medulla (cVLM), no pathway from medullary nuclei had yet been described that would account for the excitation of OT neurons produced by gastric stimuli. Retrograde tracer experiments revealed a direct pathway from the nucleus tractus solitarius (NTS) to the SON, made up of axons arising from noradrenergic and non-noradrenergic neurons. Immunocytochemistry experiments identified putative transmitter substances in non-noradrenergic neurons as inhibin-$ beta$ and somatostatin-28 (SS-28). During intracellular recording experiments, SS-28 hyperpolarized and reduced the spontaneous activity of SON neurons, an effect that was reproduced by the fragment SS-28(1-12), but not by SS-14.In the majority of cases, discrete in vivo electrical stimulation of the caudal NTS caused primarily an enhancement of the electrical activity of VP and OT neurons in the SON. Experiments were also undertaken to study the organization of afferents to VP and OT neurons. Transient bilateral blockade of neuronal function in the VLM abolished the caudal NTS-evoked excitation of VP neurons, but not of OT neurons. This data suggests that stimuli such as hypovolemia may be relayed from the NTS to supraoptic VP neurons via a synaptic relay in the cVLM. Oxytocin neurons would receive direct NTS input that may transmit visceral or somatic stimuli