Ultrastructural features of presumptive vasopressinergic synapses in the hypothalamic magnocellular secretory nuclei of the rat

Despite convincing physiological evidences for vasopressin (VP) autoregulation in the supraoptic (SON) and paraventricular (PVN) nuclei, the morphological demonstration of VP synapses has lagged behind. The present work investigates the possible existence of such synapses in the SON and PVN of the rat. Electron microscopy of sections immunostained with VP antibody (1:5,000) and conjugated with avidin-biotin demonstrated presynaptic terminals containing neurosecretory granule (NSG)-like bodies, 80-100 nm in diameter. The terminals formed axodendritic, axosomatic and axoaxonic synapses, though the postsynaptic elements remained largely unidentified. Other ultrastructural features of synaptic specialization were evident. The NSG-like bodies exhibited a varying and dynamic relationship to the presynaptic membrane, suggesting their involvement in synaptic mechanisms.