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 immunocytochemistry 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. TRH neurons were identified by immunocytochemistry. Selective uptake of tritiated serotonin (5-HT) was used to identify serotoninergic elements. TRH-immunoreactive axon terminals were found to be in synaptic contact with TRH-immunoreactive dendrites and with unlabeled dendritic branchlets. There were direct appositions between radiolabeled 5-HT terminals and TRH-immunoreactive dendrites, but differential synaptic contacts between 5-HT axonal elements and TRH neurons were not seen. TRH-immunopositive 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 asymmetrical 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.