Enhancement by proopiomelanocortin-derived peptides of growth hormone and prolactin secretion by bullfrog pituitary cells

Development of the hypothalamo-hypophyseal system in amphibians with special reference to metamorphosis

In this review article, topics of the embryonic origin of the adenohypophysis and hypothalamus and the development of the hypothalamo-hypophyseal system for the completion of metamorphosis in amphibians are included. The primordium of the adenohypophysis as well as the primordium of the hypothalamus in amphibians is of neural origin as shown in other vertebrates, and both are closely associated with each other at the earliest stage of development. Metamorphosis progresses via the interaction of thyroid hormone and adrenal corticosteroids, of which secretion is enhanced by thyrotropin and corticotropin, respectively. However, unlike in mammals, the hypothalamic releasing factor for thyrotropin is not thyrotropin-releasing hormone (TRH), but corticotropin-releasing factor (CRF) and the major releasing factor for corticotropin is arginine vasotocin (AVT). Prolactin, the release of which is profoundly enhanced by TRH at the metamorphic climax, is another pituitary hormone involved in metamorphosis. Prolactin has a dual role: modulation of the metamorphic speed and the development of organs for adult life. The secretory activities of the pituitary cells containing the three above-mentioned pituitary hormones are elevated toward the metamorphic climax in parallel with the activities of the CRF, AVT, and TRH neurons.

TRH acts as a multifunctional hypophysiotropic factor in vertebrates

Thyrotropin-releasing hormone (TRH) is the first hypothalamic hypophysiotropic neuropeptide whose sequence has been chemically characterized. The primary structure of TRH (pGlu-His-Pro-NH(2)) has been fully conserved across the vertebrate phylum. TRH is generated from a large precursor protein that contains multiple repeats of the TRH progenitor tetrapeptide Gln-His-Pro-Gly. In all tetrapods, TRH-expressing neurons located in the hypothalamus project towards the external zone of the median eminence while in teleosts they directly innervate the pars distalis of the pituitary. In addition, in frogs and teleosts, a bundle of TRH-containing fibers terminate in the neurointermediate lobe of the pituitary gland. Although TRH was originally named for its ability to trigger the release of thyroid-stimulating hormone (TSH) in mammals, it later became apparent that it exerts multiple, species-dependent hypophysiotropic activities. Thus, in fish TRH stimulates growth hormone (GH) and prolactin (PRL) release but does not affect TSH secretion. In amphibians, TRH is a marginal stimulator of TSH release in adult frogs, not in tadpoles, and a major releasing factor for GH and PRL. In birds, TRH triggers TSH and GH secretion. In mammals, TRH stimulates TSH, GH and PRL release. In fish and amphibians, TRH is also a very potent stimulator of alpha-melanocyte-stimulating hormone release. Because the intermediate lobe of the pituitary of amphibians is composed by a single type of hormone-producing cells, the melanotrope cells, it is a suitable model in which to investigate the mechanism of action of TRH at the cellular and molecular level. The occurrence of large amounts of TRH in the frog skin and high concentrations of TRH in frog plasma suggests that, in amphibians, skin-derived TRH may exert hypophysiotropic functions.

Structures and diverse functions of frog growth hormone-releasing peptide (fGRP) and its related peptides (fGRP-RPs): a review

A new Arg-Phe-NH(2) (RFamide) peptide has been discovered in the amphibian hypothalamus. The cell bodies and terminals containing this peptide were localized in the suprachiasmatic nucleus and median eminence, respectively. This peptide was further revealed to have a considerable growth hormone (GH)-releasing activity in vitro and in vivo and hence designated as frog GH-releasing peptide (fGRP). Molecular cloning of cDNA encoding the fGRP precursor polypeptide revealed that it encodes fGRP and its putative gene-related peptides (fGRP-RP-1, -RP-2, and -RP-3). Subsequently, we identified these putative fGRP-RPs as mature peptides and analyzed their hypophysiotropic activities. Only fGRP-RP-2 stimulated the release of GH and prolactin (PRL) in vitro and in vivo. Thus, in addition to fGRP, fGRP-RP-2 acts as a hypothalamic factor on the frog pituitary to stimulate the release of GH and PRL.

Hypothalamic LPXRF-amide peptides in vertebrates: identification, localization and hypophysiotropic activity

Probing undiscovered neuropeptides that play important roles in the regulation of pituitary function in vertebrates is essential for the progress of neuroendocrinology. Recently, we identified a novel hypothalamic neuropeptide with a C-terminal LPLRF-amide sequence in the quail brain. This avian neuropeptide was shown to be located in the hypothalamo-hypophysial system and to decrease gonadotropin release from cultured anterior pituitary. We, therefore, designated this novel neuropeptide as gonadotropin-inhibitory hormone (GnIH). We further identified novel hypothalamic neuropeptides closely related to GnIH in the brains of other vertebrates, such as mammals, amphibians, and fish. The identified neuropeptides possessed a LPXRF-amide (X = L or Q) motif at their C-termini. These LPXRF-amide peptides also were localized in the hypothalamus and other brainstem areas and regulated pituitary hormone release. Subsequently, cDNAs that encode LPXRF-amide peptides were characterized in vertebrate brains. In this review, we summarize the identification, localization, and hypophysiotropic activity of these newly identified hypothalamic LPXRF-amide peptides in vertebrates.

A new member of the hypothalamic RF-amide peptide family, LPXRF-amide peptides: structure, localization, and function

Recently, we identified a novel hypothalamic neuropeptide with a C-terminal LPLRF-amide sequence in the quail brain. This avian neuropeptide was shown to inhibit gonadotropin release from the cultured anterior pituitary. This peptide is the first hypothalamic peptide that inhibited gonadotropin release reported in vertebrates. We, therefore, termed it gonadotropin-inhibitory hormone (GnIH). After this finding, we found that GnIH-related peptides were present in the brains of other vertebrates, such as mammals, amphibians, and fish. These GnIH-related peptides possessed a LPXRF-amide (X=L or Q) motif at their C-termini in all investigated animals. Mass spectrometric analyses combined with immunoaffinity chromatography were powerful techniques for the identification of mature endogenous LPXRF-amide peptides. The identified LPXRF-amide peptides were found to be localized in the hypothalamus and brainstem areas, and to regulate pituitary hormone release. Subsequently, cDNAs that encode LPXRF-amide peptides were characterized in vertebrate brains. In this review, we summarize the identification, localization, and function of a new member of the hypothalamic RF-amide peptide family, LPXRF-amide peptides in vertebrates. Recent studies on the receptors for LPXRF-amide peptides will also be reviewed.

Distribution of the mRNAs encoding the thyrotropin-releasing hormone (TRH) precursor and three TRH receptors in the brain and pituitary of Xenopus laevis: effect of background color adaptation on TRH and TRH receptor gene expression

In amphibians, thyrotropin-releasing hormone (TRH) is a potent stimulator of alpha-melanotropin (alpha-MSH) secretion, so TRH plays a major role in the neuroendocrine regulation of skin-color adaptation. We have recently cloned a third type of TRH receptor in Xenopus laevis (xTRHR3) that has not yet been characterized in any other vertebrate species. In the present study, we have examined the distribution of the mRNAs encoding proTRH and the three receptor subtypes (xTRHR1, xTRHR2, and xTRHR3) in the frog CNS and pituitary, and we have investigated the effect of background color adaptation on the expression of these mRNAs. A good correlation was generally observed between the expression patterns of proTRH and xTRHR mRNAs. xTRHRs, including the novel receptor subtype xTRHR3, were widely expressed in the telencephalon and diencephalon, where two or even three xTRHR mRNAs were often simultaneously observed within the same brain structures. In the pituitary, xTRHR2 was expressed selectively in the distal lobe, and xTRHR3 was found exclusively in the intermediate lobe. Adaptation of frog skin to background illumination had no effect on the expression of proTRH and xTRHRs in the brain. In contrast, adaptation of the animals to a white background provoked an 18-fold increase in xTRHR3 mRNA concentration in the intermediate lobe of the pituitary. These data demonstrate that, in amphibians, the effect of TRH on alpha-MSH secretion is mediated through the novel receptor subtype xTRHR3.

Novel neuropeptides related to frog growth hormone-releasing peptide: isolation, sequence, and functional analysis

We previously identified in the bullfrog a novel hypothalamic RFamide peptide (SLKPAANLPLRF-NH(2)) that stimulated GH release in vitro and in vivo and therefore was designated frog GH-releasing peptide (fGRP). Molecular cloning of cDNA encoding the deduced fGRP precursor polypeptide further revealed that it encodes fGRP and its related peptides (fGRP-RP-1, -RP-2, and -RP-3). In this study immunoaffinity purification using the antibody against fGRP was therefore conducted to determine whether these three putative fGRP-RPs exist as mature endogenous ligands in the frog brain. The mass peaks of the isolated immunoreactive substances were detected at 535.78, 1034.14, and 1079.71 m/z ([M+2H](2+)), and their sequences, SIPNLPQRF-NH(2), YLSGKTKVQSMANLPQRF-NH(2), and AQYTNHFVHSLDTLPLRF-NH(2), were revealed by the fragmentation, showing mature forms encoded in the cDNA sequences of fGRP-RP-1, -RP-2, and -RP-3, respectively. All of these fGRP-RPs contained a C-terminal -LPXRF-NH(2) (X = L or Q) sequence, such as fGRP. This study further analyzed hypophysiotropic activities of the identified endogenous fGRP-RPs. Only fGRP-RP-2 stimulated, in a dose-related way, the release of PRL from cultured frog pituitary cells; its threshold concentration ranged from less than 10(-7) M. A similar stimulatory action of fGRP-RP-2 on GH release was evident. It was ascertained that fGRP-RP-2 was also effective in elevating the circulating GH and PRL levels when administered systemically. In contrast, fGRP-RPs did not have any appreciable effect on the release of gonadotropins. Thus, fGRP-RP-2 may act as a novel hypothalamic factor on the frog pituitary to stimulate the release of GH and PRL.

Occurrence of beta-endorphin binding sites in the pituitary of the frog Rana esculenta: effect of beta-endorphin on luteinizing hormone secretion

The possible effect of proopiomelanocortin-derived peptide, beta-endorphin on frog gonadotrope cells was investigated. Binding and internalization of beta-endorphin to pituitary pars distalis cultured cells were visualized by immunofluorescence and analyzed by means of confocal laser scanning microscopy. Using biotinylated endorphin, the time-course of beta-binding showed that this opioid was internalized through receptor-mediated endocytosis, the mechanism in which actin and clathrin were involved; then, the lysosomal degradation program occurred at later stages. The beta-endorphin binding was well antagonized by Naloxone, the opiate receptor antagonist, and up-regulated since more rapid response was obtained in the previously primed cells. The double immunostaining reaction for beta-endorphin and LH beta-subunit revealed that half the beta-endorphin labeled cell population was positively immunostained for LH beta-subunit, and beta-endorphin was able to induce an increasing trend of LH secretion in cultured pars distalis cells. Therefore, it seems that beta-endorphin acts directly on pituitary pars distalis and influences gonadotropin secretion through the interaction with its own receptor.

A novel amphibian hypothalamic neuropeptide: isolation, localization, and biological activity

Neuropeptides similar to the molluscan cardioexcitatory Phe-Met-Arg-Phe-NH2 have been identified in several vertebrates and characterized by the RFa motif at their C terminus (RFa peptides). In this study, we sought to identify an amphibian hypothalamic RFa peptide that may regulate secretion of hormones by the anterior pituitary gland. An acid extract of bullfrog hypothalami was passed through C-18 reversed-phase cartridges, and then the retained material was subjected to HPLC, initially using a C-18 reversed-phase column. RFa immunoreactivity was measured in the eluted fractions by a dot immunoblot assay employing an antiserum raised against RFa. Immunoreactive fractions were subjected to further cation exchange and reversed-phase HPLC purification. The isolated peptide was a novel RFa peptide and shown to have the sequence Ser-Leu-Lys-Pro-Ala-Ala-Asn-Leu-Pro-Leu-Arg-Phe-NH2. The cell bodies and terminals containing this peptide were localized immunohistochemically in the suprachiasmatic nucleus and median eminence, respectively. This RFa peptide stimulated, in a dose-related way, the release of GH from cultured pituitary cells, its threshold concentration ranging between 10(-9) and 10(-8) M. This peptide did not have any appreciable effect on the secretion of PRL and gonadotropins. It was ascertained that the peptide was also effective in elevating the circulating GH level when administered systemically. Thus, the amphibian hypothalamus was revealed to contain a novel functional RFa peptide that stimulates GH release. This peptide was designated frog GH-releasing peptide.

Bullfrog ghrelin is modified by n-octanoic acid at its third threonine residue

We have identified the amphibian ghrelin from the stomach of the bullfrog. We also examined growth hormone (GH)-releasing activity of this novel peptide in both the rat and bullfrog. The three forms of ghrelin identified, each comprised of 27 or 28 amino acids, possessed 29% sequence identity to the mammalian ghrelins. A unique threonine at amino acid position 3 (Thr(3)) in bullfrog ghrelin differs from the serine present in the mammalian ghrelins; this Thr(3) is acylated by either n-octanoic or n-decanoic acid. The frog ghrelin-28 has a complete structure of GLT (O-n-octanoyl)FLSPADMQKIAERQSQNKLRHGNM; the structure of frog ghrelin-27 was determined to be GLT(O-n-octanoyl)FLSPADMQKIAERQSQNKLRHGN; frog ghelin-27-C10 possessed a structure of GLT(O-n-decanoyl)FLSPADMQKIAERQSQNKLRHGN. Northern blot analysis demonstrated that ghrelin mRNA is predominantly expressed in the stomach. Low levels of gene expression were observed in the heart, lung, small intestine, gall bladder, pancreas, and testes, as revealed by reverse transcription polymerase chain reaction analysis. Bullfrog ghrelin stimulated the secretion of both GH and prolactin in dispersed bullfrog pituitary cells with potency 2-3 orders of magnitude greater than that of rat ghrelin. Bullfrog ghrelin, however, was only minimally effective in elevating plasma GH levels following intravenous injection into rats. These results indicate that although the regulatory mechanism of ghrelin to induce GH secretion is evolutionary conserved, the structural changes in the different ghrelins result in species-specific receptor binding.

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.