MONOSYNAPTIC PATHWAY BETWEEN THE ARCUATE NUCLEUS EXPRESSING GLIAL TYPE-II IODOTHYRONINE 5'-DEIODINASE MESSENGER-RNA AND THE MEDIAN-EMINENCEPROJECTIVE TRH CELLS OF THE RAT PARAVENTRICULAR NUCLEUS

Recent evidence suggests that the thyroid regulation of thyrotropin-re leasing hormone (TRH)-containing neurons in the paraventricular nucleu s of the hypothalamus involves the activation of other hypothalamic ne ural circuits. For example, the arcuate nucleus and not the paraventri cular nucleus contains the highest enzyme activity of 5'-deiodinase ty pe II, an enzyme that is pivotal for the local synthesis of T3, This e xperiment was undertaken to demonstrate whether a monosynaptic pathway exists between the arcuate nucleus and those TRH cells of the paraven tricular nucleus that are neuroendocrine, i.e. project to the external layer of the median eminence. A specific cRNA probe derived from the coding region of deiodinase type II was used for the in situ hybridiza tion histochemistry which was combined with immunocytochemistry for a specific marker of glial cells, glial fibrillary acidic protein (GFAP) , The hybridization signals were present within the hypothalamus in th e arcuate nucleus-median eminence region and in the periventricular ar ea. The periventricular labeling was localized to the ependymal layer of the third ventricle and no hybridization product was detected in th e paraventricular nucleus and other hypothalamic nuclei adjacent to th e third ventricle. Within the median eminence, numerous cells containi ng the hybridization product were located in the internal layer adjace nt to the floor of the third ventricle and in the external layer adjac ent to the surface of the brain. In the dorso- and ventromedial region s of the arcuate nucleus, deiodinase type II mRNA-containing cells wer e also detected, Numerous type II deiodinase mRNA-containing cells in the median eminence and arcuate nucleus were also found to be immunopo sitive for GFAP, The abundance of arcuate cells expressing the hybridi zation product was lower than those in the periventricular region or i n the median eminence, The anterograde tracer, Phaseolus vulgaris leuc oagglutinin, was injected into the medial parts of the arcuate nucleus where the in situ hybridization experiment detected deiodinase type I I mRNA. Simultaneously with the anterograde tracing, the retrograde tr acer, Fluoro-Gold, was injected into either the median eminence or the general circulation. Light and electron microscopic double and triple immunolabeling experiments on vibratome sections of colchicine-pretre ated animals revealed that arcuate fibers innervate TRH cells within t he parvicellular region of the paraventricular nucleus. Populations of these TRH cells receiving afferents from the arcuate nucleus were als o retrogradely labelled from either the median eminence or the general circulation indicating their direct role in the regulation of thyrotr opin secretion from the anterior pituitary, The majority of arcuate nu cleus efferents on TRH cells were found to establish symmetrical synap tic connections. The present results provided direct evidence of a mon osynaptic pathway between the hypothalamic site of local thyroid hormo ne production, the arcuate nucleus, and neuroendocrine TRH cells in th e paraventricular nucleus. This signalling modality may play an import ant role in thyroid feedback on TRH cells. Since the arcuate nucleus i s involved in the regulation of central mechanisms controlling diverse homeostatic functions, including reproduction and feeding, the pathwa y described in this study may also carry integrated signals related to reproduction and ingestion to TRH-producing cells.