Immunocytochemical Evidence for in vivo Internalization of Thyroliberin into Rat Pituitary Target Cells

Abstract

The in vivo internalization of thyrotropin-releasing hormone (TRH) was studied by using a semiquantitative immunoelectron microscopic method. Pituitary glands of normal male rats or of rats intravenously injected with 100 ng TRH and sacrificed after 5–60 min were used. Ultrathin sections were obtained by cryoultramicrotomy of fixed pituitary glands. Pituitary cellular types were identified by appropriate antiserums. An antiserum specifically directed against TRH was used. TRH-like immunoreactivity due to endogenous TRH was observed in thyrotropes and prolactin cells, but never in somatotropes, gonadotropes or corticotropes. At the subcellular level, the reaction was detected within the cytoplasmic matrix, the secretory granules, and the nucleus but only occasionally at the plasma membrane. After in vivo injection of TRH, the immunocytochemical reaction (1) was still restricted to thyrotropes and prolactin cells, (2) increased with time elapsed after injection up to 15–30 min and then returned to basal intensity in cytoplasm, secretory granules, and nucleus, and (3) became very frequent at the plasma membrane. These data (1) provide evidence for endogenous TRH within thyrotropes and prolactin cells, i.e., in physiological target cells for TRH, and (2) support the hypothesis that normal TRH target cells can, in vivo, internalize exogenous as well as endogenous TRH into several subcellular compartments including the nucleus.

Evidence for a Direct Action of Neuropeptide Y in the Rat Pituitary Gland

“…Moreover, the subcellular distribution resembled that previously observed with neuropeptides such as GnRH [23], TRH [28], CRF [24], GRF [27] and somatosta tin [26] in adenohypophyseal cells as well as that for TRH and NPY in the parenchymal cells of the frog intermediate lobe [29]. The presence of NPY-like immunoreactivity at the plasma membrane is difficult to observe, possibly because the molecule is rapidly internalized.…”

“…38], TRH [28,29], CRF [16,24], somatostatin [25,26] and GRF [27], The biological significance of NPY-like immuno reactivity over the secretory granules is unclear. However, based on immunocytochemistry, such a localization has al ready been reported for GnRH [23], TRH [28], CRF [241. GRF [2] and somatostatin [26].…”

Neuroendocrine Aspects of Aging: Experimental Data

“…An increased density of receptors for TRH has been found in the ade nohypophysis of old rats, concomitant with a marked accumulation of TRH in the gland, and these alterations are not associated with changes in the pituitary concen tration of TSH, suggestive that the increased availability of TRH in the adenohypophysis of old rats is not affect ing TSH production [55]. Further studies are needed to investigate possible age-related changes in the intracellu lar processing of TRH in the adenohypophysis [57,58] as well as in the degradation of TRH by specific pituitary enzymes [59,60]. Moreover, variant forms of TSH have been described in the serum and pituitary gland of old rats [61,62], as shown for gonadotrophins.…”

Growth hormone activity in mitochondria depends on GH receptor Box 1 and involves caveolar pathway targeting

“…Samples were cryoprotected in 0.4 M sucrose/phosphate buffer solution for 2 h at room temperature and frozen in a cold gradient of fuming nitrogen (Biogel, CFPO, Saint Priest, France) to À6-C. Samples were then immersed in liquid nitrogen [47]. Ultrathin frozen sections of 80-nm thickness were obtained using a dry sectioning method at À120-C with an Ultracut S ultramicrotome (Leica, Vienna, Austria).…”