Electron microscopic examination of the catecholaminergic innervation of neurophysin- or vasopressin-containing neurons in the rat hypothalamus

The ultrastructural relationships between neurophysin (NP)- or vasopressin (VP)-containing neurons and catecholamine terminals in the paraventricular nucleus and supraoptic nucleus of the rat were observed by means of a technique combining immunocytochemistry using NP I/II or VP antiserum with autoradiography after [3H]noradrenaline (NA) injection or 5-hydroxydopamine (5-OHDA) uptake. NP- or VP-like immunoreactive nerve cell bodies and fibers received synaptic contacts from a large number of immunonegative axon terminals. The presynaptic elements that innervate the neurosecretory neurons were studied. Axon terminals labeled with [3H]NA or 5-OHDA made synaptic contacts with NP- or VP-like immunoreactive nerve cell bodies and fibers. Furthermore, axodendritic and/or axo/axonic and axosomatic synapses occurred between the same NP- or VP-like immunoreactive neurons. These findings suggest that at least NA- and 5-OHDA-containing neurons play some important role in the control of neurosecretion in the NP- or VP-producing neurons of the rat hypothalamus and that the axon collaterals of NP- and VP-containing neurons make synaptic contacts with the same kind of neurons to form a recurrent collateral circuit.