Direct projections form catecholaminergic neurons in the caudal ventrolateral medulla to vasopressin-containing neurons in the supraoptic nucleus: a triple-labeling electron microscope study in the rat.

Direct projections form the A1 catecholaminergic cell group in the caudal ventrolateral medulla (CVLM) to arginine-vasopressin-containing (AVP-containing) neurosecretory neurons in the hypothalamic supraoptic nucleus (SON) were examined electron microscopically in the rat by a triple-labeling technique in which an anterograde tract-tracing method of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) was combined with immunocytochemistry for tyrosine hydroxylase (TH) and that for AVP. In the SON, TH-like immunoreactive axon terminals which were labeled with WGA-HRP injected in the CVLM were in synaptic contact with neuronal profiles with AVP-like immunoreactivity. The results indicate that AVP-containing neurons in the SON receive monosynaptic catecholaminergic synaptic input from the CVLM.