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
S. Diano et al., 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, Journal of neuroendocrinology, 10(10), 1998, pp. 731-742
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.