Ultrastructural Studies of Nerve Projections from the Anteroventral Periventricular Nucleus to Vasopressin-Containing Neurons in the Hypothalamic Paraventricular Nucleus of the Rat

Nerve projections from the anteroventral periventricular nucleus (AVPV) to the vasopressin (VP) containing magnocellular and parvocellular neurosecretory neurons in the paraventricular nucleus (PVN) of the rat and its synaptic inputs to VP containing neurons were examined. A double labeling technique was used by combining anterograde tracing after iontophoretic injection of wheat germ agglutinin-coupled horseradish peroxidase (WGA-HRP) with VP immunocytochemistry. WGA-HRP-labeled and unlabeled axon terminals were found to make synaptic contacts with VP-like immunoreactive cell bodies and processes in the PVN. This indicates a direct synaptic influence of AVPV on the secretory activity of the VP-containing neurons in the hypothalamic PVN of the rat.