Peptidergic and catecholaminergic synaptic contacts onto nucleus preopticus medianus neurons projecting to the subfornical organ in the rat.

The nucleus preopticus medianus (POMe) is known to be a key site in regulation of cardiovascular and body fluid homeostasis. To clarify the regulation mechanism to the POMe, the innervation pattern of synapses made by axon terminals immunoreactive to beta-endorphin, neuropeptide Y and tyrosine hydroxylase onto POMe neurons projecting to the subfornical organ (SFO) was investigated in the rat. After injection of a retrograde tracer, wheat germ agglutinin-conjugated horseradish peroxidase-colloidal gold complex, into the SFO, many neurons were retrogradely labeled in the POMe, more frequently in its dorsal part. Electron microscopy of the POMe revealed that beta-endorphin- and tyrosine hydroxylase-immunoreactive axon terminals formed predominantly axo-somatic synapses, and neuropeptide Y-immunoreactive axon terminals formed more axo-dendritic than axo-somatic synapses with retrogradely labeled neurons. The present localization patterns of POMe neurons retrogradely labeled from the SFO and the type of synapses of axon terminals immunoreactive to three neurochemical markers on these neurons were compared to those of POMe neurons retrogradely labeled from the paraventricular hypothalamic nucleus demonstrated in our previous report.