Immunocytochemical identification of growth hormone (GH) cells in the pituitary of three anuran species using an antiserum against purified bullfrog GH

Cosecretion of prolactin and growth hormone by dispersed pituitary cells of the adult bullfrog, Rana catesbeiana

The coexistence of prolactin (PRL) and growth hormone (GH) was previously demonstrated in newly hatched bullfrog (Rana catesbeiana) tadpoles, whereas in adult bullfrogs, there were no cells containing both PRL and GH. However, a cell blot assay with enzymatically dispersed adult pituitary cells demonstrated the existence of cells secreting both PRL and GH. The number of cells secreting both PRL and GH was reduced by a protein synthesis inhibitor, cycloheximide, but not by an RNA synthesis inhibitor, actinomycin D. In situ hybridization and immunostaining of intact pituitary glands revealed the existence of GH mRNA in some of the PRL-immunoreactive cells and of PRL mRNA in some of the GH-immunoreactive cells. We propose that dispersion of the pituitary cells triggered the translation of GH mRNA in the PRL cells and/or of PRL mRNA in the GH cells.

Molecular cloning of newt prolactin (PRL) cDNA: effect of temperature on PRL mRNA expression

A partial prolactin (PRL) cDNA was specifically PCR amplified from a cDNA library constructed from pituitary mRNAs of the newt (Cynops pyrrhogaster) and cloned into plasmid vectors. One clone thus obtained contained a 739-bp insert encoding the C-terminal amino acid sequence of the mature hormone molecule. Using this clone as a probe, the full-length newt PRL cDNA was screened from the cDNA library. The PRL cDNA clone thus obtained consisted of 1024 bp encoding the entire sequence of the mature PRL molecule in addition to its signal peptide. The amino acid sequence of newt PRL deduced from its nucleotide sequence showed higher homologies with those PRL sequences of tetrapod animals than with those of teleosts. Northern blot analysis revealed the newt PRL mRNA size to be approximately 1 kb. In situ hybridization using the newt PRL cDNA as a probe revealed that the pituitary region expressing PRL mRNA corresponded to that immunoreactive with antiserum against PRL. PRL mRNA levels in the pituitary of newts subjected to room and low temperatures were determined by Northern analysis employing the PRL cDNA as a probe. PRL mRNA levels were significantly higher in the pituitaries of newts subjected to 10 degrees than in those of newts kept at 23 degrees. Likewise, immunoassayable plasma PRL levels were higher in animals subjected to 10 degrees than in those kept at 23 degrees.

Insulin-like growth factor I in the anterior pituitary of the clawed frog Xenopus laevis: immunocytochemical and autoradiographic indication for a paracrine action and corelease with prolactin

Insulin-like growth factor I (IGF-I) and its receptor are present in human and rat anterior pituitary. However, few data exist on the potential presence of IGF-I or its receptor in the non-mammalian pituitary and the cellular sites of IGF-I production have not been identified in any species. Thus, we investigated the anterior pituitary of the clawed frog Xenopus laevis which is widely used to study growth and differentiation. The study was performed with antisera against mammalian insulin-like growth factor I (IGF-I), prolactin (PRL) and growth hormone (GH) using immunohistochemical and immunocytochemical techniques. IGF-I binding was determined by in-vitro receptor autoradiography. The PRL-and GH-immunoreactive cells exhibited distinct distribution patterns. Neither at the light nor the electron microscopical level any colocalization of PRL-and GH-immunoreactivities was apparent. The PRL-immunoreactive cells exhibited round granules of medium electron density (mean diameter: 312 nm) and the GH-immunoreactive cells spherical granules of medium electron density (mean diameter: 165 nm). By the use of serial semithin sections IGF-I-immunoreactivity was exclusively located in PRL-immunoreactive cells. At the ultrastructural level, IGF-I-immunoreactivity was confined to the secretory granules in coexistence with PRL-immunoreactivity using the double labelling immunogold technique. Specific IGF-I binding sites were localized throughout the pituitary. The results provide evidence for a concomitant release of PRL and IGF-I and suggest autocrine/paracrine actions of IGF-I in the anterior pituitary.

Production of a recombinant newt growth hormone and its application for the development of a radioimmunoassay

Complementary DNA (cDNA) encoding newt (Cynops pyrrhogaster) growth hormone (nGH) was cloned from a cDNA library constructed from mRNAs of newt pituitary glands and was expressed in Escherichia coli. Based on Northern blot analysis using the cDNA as a probe, the nGH mRNA was estimated to be 940 bases in length. The recombinant nGH (nGHr) had a molecular mass of 22 kDa as determined by SDS-PAGE and possessed considerable bioactivity as determined in a Xenopus cartilage assay. Using the nGHr, we produced a polyclonal antibody against nGHr. Western blot analysis of newt anterior pituitary gland homogenates revealed that this antiserum specifically detected a single 22-kDa band, and histological studies of newt pituitary gland sections showed that the cells that reacted immunologically by the anti-nGHr antiserum corresponded to those stained by an antiserum against rat GH. A radioimmunoassay (RIA) that is specific and sensitive for nGH was developed, employing the antiserum thus produced. The sensitivity of the RIA was 57 +/- 7 pg/100 microl assay buffer. Interassay and intraassay coefficients of variation were 1.22 and 2.70%, respectively. Serial dilutions of plasma and pituitary homogenate of C. pyrrhogaster yielded dose-response curves that were parallel to the standard curve. Plasma from hypophysectomized newts showed no cross-reactivity. Moreover, displacement curves obtained using pituitary homogenates of the sword-tailed newt (C. ensicauda) and the crested newt (Triturus carnifex) were also parallel to the standard curve. Mammalian and frog GHs and prolactins (PRLs), as well as newt PRL, showed no inhibition of binding, even at relatively high doses, in this RIA. The RIA was used to measure GH released from newt pituitaries in vitro. Enhancement of GH release by 10(-7) M thyrotropin-releasing hormone was observed in cultures of newt pituitaries.

Effects of the two somatostatin variants somatostatin-14 and [Pro2, Met13]somatostatin-14 on receptor binding, adenylyl cyclase activity and growth hormone release from the frog pituitary

Two isoforms of somatostatin from frog brain have been recently characterized, namely somatostatin-14 (SS1) and [Pro2, Met13]somatostatin-14 (SS2). The genes encoding for the precursors of these two somatostatin variants are expressed in hypothalamic nuclei involved in the control of the frog pituitary. The aim of the present study was to investigate the effect of SS1 and SS2 on adenohypophysial cells. Autoradiographic studies using [125I-Tyr, D-Trp8] SS1 as a radioligand revealed that somatostatin binding sites are evenly distributed in the frog pars distalis. The SS2 variant was significantly (P < 0.01) more potent than SS1 in competing with the radioligand (IC50= 1.2 +/- 0.2 and 5.6 +/- 0.6 nM, respectively). Both SS1 and SS2 induced a modest but significant reduction in cAMP formation in dispersed distal lobe cells but did not affect spontaneous growth hormone (GH) release. Synthetic human GRF (hGRF) induced a significant increase in cAMP accumulation and GH release in this system. Both SS1 and SS2 inhibited the stimulatory effects of hGRF on cAMP formation and GH secretion. These data show that the SS1 and SS2 variants can regulate adenohypophysial functions. The fact that GH cells are exclusively located in the dorsal area of the frog adenohypophysis, while somatostatin receptors are present throughout the pars distalis, indicates that the two somatostatin isoforms may control the secretion of pituitary hormones additional to GH in amphibians.

Growth hormone inhibits the hypophysectomy-induced expression of galanin in hypothalamic neurons of the toad (Bufo arenarum hensel)

The expression of the neuropeptide galanin was analyzed by immunohistochemistry in magnocellular and preoptic hypothalamic neurons of toads following hypophysectomy (HPX) and pars distalectomy (PDX). There was a marked increase in the galanin-like immunoreactive expression in magnocellular hypothalamic cells 3 days after HPX, followed by a decrease to normal levels after 7 days. No changes in the expression of galanin were detected after PDX in these neurons when compared to controls. Moreover, 7 days after HPX or PDX the number of cells expressing galanin was significantly increased in the preoptic area, where numerous intraependymal cells were intensely immunoreactive. The hypophysis grafts into the hind limb in HPX or PDX animals prevented increased galanin-like immunoreactivity in preoptic cells but not in magnocellular neurons. Similarly, PDX toads given growth hormone showed no GAL-LI in the intraependymal preoptic cells. These results suggest the presence of a region regulation of galanin expression in the preoptic area by hypophyseal hormones, in particular growth hormone.