Ultrastructural studies of medullary synaptic inputs to vasopressin-immunoreactive neurons in the supraoptic nucleus of the rat hypothalamus.

Direct projections from the A1/C1 catecholaminergic cell group in the caudal ventrolateral medulla oblongata to neurons containing vasopressin (VP) in the supraoptic nucleus (SON) were studied electron microscopically by a double-labeling technique which combines anterograde tracing after injection of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) into the A1/C1 cell group with VP immunocytochemistry. WGA-HRP-labeled axon terminals originating from neurons in the A1/C1 cell group were found to make synaptic contacts with VP-immunoreactive cell bodies and dendrites in the SON, most often forming axo-dendritic synapses. This indicates that VP-containing neurosecretory neurons in the SON receive monosynaptic catecholaminergic input from the A1/C1 cell group.