ULTRASTRUCTURAL ANALYSIS OF THE INNERVATION OF TRH-IMMUNOREACTIVE NEURONAL ELEMENTS LOCATED IN THE PERIVENTRICULAR SUBDIVISION OF THE PARAVENTRICULAR NUCLEUS OF THE RAT HYPOTHALAMUS

A combination of electron microscopic immunocyotchemistry and autoradiography was employed to examine the synaptic organization of thyrotropin-releasing hormone (TRH) neurons in the periventricular subdivision of the paraventricular nucleus of the rat hypothalamus. THR neurons were identified by immunocyotchemistry. Selective uptake of tritriated serotin (5-HT) was used to identify serotininergic elements. TRH-immunoreactive axon terminals were found to be in synaptic contact with TRH-immunoreactive dendrites and with unlabeled dendritic branchets. There were direct appositions between radiolabeled 5-HT terminals and TRH-immunoreactive dendrites, but differential synaptic contacts between 5-HT axonal elements and THR neurons were not seen. TRH-immunopossitive cell bodies and dendrites received a very intense innervation by unlabeled axon terminals or axonal varicosities showing morphologically defined synaptic junctions. These were mostly of the symmetrical variety and different types could be distinguished. The findings substantiate the view that TRH neurons of the periventricular subvision of the paraventricular nucleus may be influenced by TRH axons, serotoninergic fibers and a large number of unidentified nerve terminals. © 1990.