Somatolactin Research Articles

Close association of gonadotropin-releasing hormone fibers and gonadotropin, growth hormone, somatolactin and prolactin expressing cells in pejerrey, Odontesthes bonariensis

The purpose of this study was to determine if there is any association between immunoreactive (ir) gonadotropin-releasing hormone (GnRH) fibers with different pituitary endocrine cell types in the pejerrey, Odontesthes bonariensis. Using a monoclonal antibody raised against mammalian GnRH (mGnRH) (LRH13), ir-GnRH fibers were observed passing through the pituitary stalk and reaching the three areas of the pituitary gland: rostral (RPD) and proximal pars distalis (PPD) and pars intermedia (PI). Double labeled immunocytochemistry showed ir-GnRH fibers in close association with prolactin (PRL)-producing cells in the RPD, growth hormone (GH)-producing cells in the PPD, gonadotropin (GtH)-producing cells in the PPD and the external border of the PI, and with somatolactin (SL)-producing cells in the PI. Our results show, direct morphological evidences of a close association of GnRH fibers with GH, PRL, GtH and SL-expressing cells. These results would suggest that GnRH has a broad role in the regulation of the secretion of different pituitary hormones.

Effects of Light on Plasma Somatolactin Levels in Red Drum (Sciaenops ocellatus)

The effects of illumination on circulating somatolactin (SL) levels were studied in red drum sampled at various times during a 24-h light–dark cycle (12L:12D) and during a 24-h period of constant light. Plasma SL concentrations were low in red drum sampled during the light phase (0.6±0.05–1.1±0.2 ng/ml). The levels were significantly elevated during the early-dark phase (5.8±1.0 ng/ml) and declined during the late-dark phase (0.39±0.05 ng/ml). In contrast, plasma SL levels remained low (0.4±0.1–1.3±0.4 ng/ml) in fish sampled throughout a 24-h constant light period following a 1-week exposure to the light–dark cycle. Circulating SL levels were also significantly elevated during the early-dark phase in fish maintained under a reversed light–dark cycle. The role of the eyes in mediating the SL response to light was evaluated by comparing circulating SL levels in optic-tract sectioned and enucleated fish with those of intact (control) fish. Plasma SL concentrations were significantly higher in optic-tract sectioned and enucleated fish (6.5±0.9–13.8±1.5 ng/ml) than in control fish (0.6±0.05–3.9±0.7 ng/ml) during both the early-dark phase and the early-light phase of the 24-h light–dark cycle. The absence of significant changes in plasma SL levels between the light and dark phases in optic-tract sectioned and enucleated fish appears to be due to a loss of light perception in these fish. Moreover, intact and sham-operated red drum maintained in constant darkness had dramatically increased plasma SL levels (18.8±2.0–24.8±1.8 ng/ml). The present results are consistent with our earlier findings that plasma SL levels are elevated in fish kept in constant darkness and in a dark-background tank (reduction in overall light levels) during the light phase. Taken together, these results suggest that plasma SL levels are elevated in red drum in the absence of light and in response to low illumination. Interestingly, the integument of the fish became light during the dark phase of the light–dark cycle. In our earlier studies, the increase of plasma SL concentrations was associated with aggregation of melanophores, and direct effects of SL on melanophore aggregation were demonstrated. Overall, our studies with red drum suggest a possible role of elevated SL levels on melanophore aggregation during the dark phase of the 24-h day–night cycle.

Release of glycosylated and non-glycosylated forms of somatolactin by fish pituitary culture in vitro.

Annals of the New York Academy of SciencesVolume 839, Issue 1 p. 478-479 Release of Glycosylated and Non-Glycosylated Forms of Somatolactin by Fish Pituitary Culture in Vitro CARLOS PENDON, CARLOS PENDON Departamento de Bioquímica y Biología Molecular, Facultad de Ciencias, Universidad de Cádiz, 11510 Puerto Real, CádizSearch for more papers by this authorANTONIO ASTOLA, ANTONIO ASTOLA Departamento de Bioquímica y Biología Molecular, Facultad de Ciencias, Universidad de Cádiz, 11510 Puerto Real, CádizSearch for more papers by this authorJAUME PÉREZ-SÁNCHEZ, JAUME PÉREZ-SÁNCHEZ Instituto de Acuicultura Torre de la Sal, CSIC, 12595 Torre de la Sal, Castellón, SpainSearch for more papers by this authorMANUEL M. VALDIVIA, MANUEL M. VALDIVIA Departamento de Bioquímica y Biología Molecular, Facultad de Ciencias, Universidad de Cádiz, 11510 Puerto Real, CádizSearch for more papers by this author CARLOS PENDON, CARLOS PENDON Departamento de Bioquímica y Biología Molecular, Facultad de Ciencias, Universidad de Cádiz, 11510 Puerto Real, CádizSearch for more papers by this authorANTONIO ASTOLA, ANTONIO ASTOLA Departamento de Bioquímica y Biología Molecular, Facultad de Ciencias, Universidad de Cádiz, 11510 Puerto Real, CádizSearch for more papers by this authorJAUME PÉREZ-SÁNCHEZ, JAUME PÉREZ-SÁNCHEZ Instituto de Acuicultura Torre de la Sal, CSIC, 12595 Torre de la Sal, Castellón, SpainSearch for more papers by this authorMANUEL M. VALDIVIA, MANUEL M. VALDIVIA Departamento de Bioquímica y Biología Molecular, Facultad de Ciencias, Universidad de Cádiz, 11510 Puerto Real, CádizSearch for more papers by this author First published: 07 February 2006 https://doi.org/10.1111/j.1749-6632.1998.tb10840.xCitations: 5Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Citing Literature Volume839, Issue1TRENDS IN COMPARATIVE ENDOCRINOLOGY AND NEUROBIOLOGY: FROM MOLECULAR TO INTEGRATIVE BIOLOGYMay 1998Pages 478-479 RelatedInformation

Recombinant somatolactin as a stable and bioactive protein in a cell culture bioassay: development and validation of a sensitive and reproducible radioimmunoassay.

A recombinant somatolactin (SL) obtained by cloning and expression of sole SL cDNA was analyzed and used to develop a sensitive and specific RIA. In contrast to native proteins, which tend to dimerize and aggregate immediately after pituitary isolation, the majority of recombinant sole SL (rsSL) remained as a monomeric protein after long-term storage, as shown by size exclusion chromatography and Western blot. Using rsSL as a tracer and standard in the RIA, the minimum detectable dose and the midrange (ED50) of the assay were 0.15 and 1.8-2.1 ng/ml respectively. Intra-and interassay coefficients of variation were 4.3% and 6.5% at ED50 levels. Recombinant gilthead sea bream GH and recombinant trout GH did not show cross-reactivity, whereas a good parallelism between rsSL standard and serial dilutions of plasma and sole pituitary extracts was observed. In order to demonstrate some biological activity of rsSL, the ability of this recombinant product to prime gilthead sea bream phagocytes for in vitro enhancement of mitochondrial activity was examined by a chromogenic assay. A bell-shape dose-response curve was obtained with a maximum at 50 nM (1.2 micrograms/ml), similar to that reported previously for GH. Therefore, taking together all these data, it appears conclusive that rsSL is a long-term stable protein which retains, at least in part, biological activity, providing a useful tool to clarify the physiological role of fish SL.

Effects of Gonadectomy and Steroids on Plasma and Pituitary Levels of Somatolactin in Atlantic Salmon,Salmo salar

Somatolactin (SL) is a hormone recently isolated and characterized from fish pituitaries. Although the functions of SL are still largely unknown, it has been implicated in reproduction. In this study, the effects of gonadal steroids on SL secretion were investigated in Atlantic salmon male parr in two experiments. In both, mature males were gonadectomized in the autumn and implanted with Silastic capsules containing testosterone (T), 11-ketoandrostenedione (11kA), or 17α,20β-dihydroxy-4-pregnene-3-one (20-P), gonadectomized alone, or sham-operated. In addition, immature males were implanted with T or 11kA in experiment 1. After 4–5 weeks pituitaries and plasma were sampled and SL levels measured by radioimmunoassay (RIA). Plasma levels of T, 11-ketotestosterone, and 20-P were also measured by RIA. In experiment 1, initial immature males had lower (0.7±0.4 ng/ml) plasma SL levels than initial mature males (3.3±0.4 ng/ml), whereas pituitary content was not influenced. Gonadectomy significantly reduced both plasma SL levels (experiment 1, sham controls 5.6±0.5 ng/ml, castrated 1.6±0.5 ng/ml; experiment 2, sham controls 6.5±1.1 ng/ml, castrated 3.3±0.4 ng/ml) and the pituitary content of SL (experiment 1, sham controls 1206±187 ng/pituitary, castrated 663±104 ng/pituitary; experiment 2, sham controls 1043±199 ng/pituitary, castrated 629±70 ng/pituitary), suggesting that the testes stimulated the synthesis and release of pituitary SL. Overall, the effects of steroid replacement were inconsistent between the experiments, although in experiment 2 castrated males receiving the highest dose of T had significantly higher plasma SL levels (8.2±1.2 ng/ml) than all other castrated groups (1.8–4.3 ng/ml).

Ontogeny of immunoreactive somatolactin cells in the pituitary of gilthead sea bream (Sparus aurata L., Teleostei).

This is the first report on the identification of somatolactin (SL) cells during the early developmental stages of the teleost fish Sparus aurata. The SL cells were followed from newly hatched until 46 months. SL cells were immunocytochemically identified at light microscopical level with anti-cod SL in the developing pituitary using the peroxidase-antiperoxidase method. SL cells first appeared in newly hatched specimens, in which the pituitary gland lacked the neurohypophysis. They were scarce and located from the middle to the posterior region of the adenohypophysis. As the fish developed, the cells progressively increased in number and surrounded the developing neurohypophysis, which could be distinguised from 12-day-old larvae onwards in the caudal region of the gland. From 51 days onwards, SL cells were found in a discontinuous layer surrounding the neurohypophysis branches that entered the pars intermedia as clustered or isolated cells among non-SL-immunoreactive cells of the pars intermedia, and in the proximal pars distalis. The somatolactin-immunoreactive cells are periodic acid-Schiff-positive only in the adult stages. These data confirm, previous findings concerning the presence of two molecular forms of SL, glycosylated and nonglycosylated, in this species.

Sequence of a cDNA Clone Encoding a Novel Somatolactin in Goldfish,Carassius auratus

As a first step towards the development of a sensitive ribonuclease protection assay to study the regulation of somatolactin (SL) mRNA expression in pituitary cells of goldfish, we have isolated a complementary DNA (cDNA) clone encoding precursor sequence of SL from a cDNA library prepared from goldfish pituitary poly(A)+RNA. The 843-bp goldfish SL (gfSL) cDNA has an open reading frame of 693 nucleotides with two possible start codons of AUG. Amino acid sequence alignment revealed that gfSL has the characteristics of four conserved domains (A, B, C and D) common to all SLs with the domain B being the most conserved region among all the characterized SLs. Similar to other teleost SLs, this gfSL is similarly related but clearly distinct from growth hormone and prolactin of goldfish and other teleosts. However, unlike most other known teleost SLs which have more than 70% amino acid sequence identity to each other, the overall amino acid sequence identity of this novel gfSL with other previously characterized SLs ranges from only 36% to 51%. Moreover, this gfSL contains only six cysteine residues, rather than seven in most other SLs, in conserved positions. Northern blot analysis revealed a single gfSL mRNA transcript of approximately 1 kb in the pituitaries of both sexually regressed and maturing male and female goldfish.

CDNA cloning of eel ( Anguilla anguilla) somatolactin

The nucleotide sequence of the cDNA coding for eel somatolactin (SL), a pituitary hormone belonging to the growth hormone (GH)/prolactin (PRL) family, has been determined. The full-length eel SL cDNA (1213 bp) encodes a protein of 229 amino acids (aa), with a putative signal peptide of 24 aa and a mature protein of 204 aa. Eel SL contains seven Cys residues found to be characteristic of SLs, and two potential N-glycosylation sites. Significant sequence homology between eel and seven other fish SLs (42% aa identity, 63% aa similarity) reveal SL to be highly conserved. A higher sequence identity of SL to GH than PRL is suggested by the comparison of these hormones in eel and chum salmon.

Immunocytochemical and ultrastructural characterization of prolactin, growth hormone, and somatolactin cells from the Mediterranean yellowtail (Seriola dumerilii, Risso 1810).

Prolactin (PRL), growth hormone (GH), and somatolactin (SL) are structurally related pituitary hormones that belong to a peptide family. Whereas growth hormone and prolactin are present in the hypophysis of all vertebrates, somatolactin, a recently discovered hormone, has been found only in fish. It has been demonstrated immunocytochemically in a few teleost species; ultrastructurally, cells producing this hormone have been characterized only in one species of salmon. In this paper, we identify and characterize ultrastructurally the cells producing these three hormones in Mediterranean yellowtail (Seriola dumerilii). Pituitaries from adult specimens were dissected out and processed for electron microscopy. The immunogold technique was performed in some ultrathin sections using fish primary antibodies. PRL cells had round, peripherally distributed, very electron-dense, homogeneous secretory granules of variable size. GH cells had dense, round secretory granules with a conspicuous scalloped membrane, which were grouped in the cell area near the neurohypophysis. SL cells had round, polymorphic, or very irregularly shaped secretory granules, the last seeming to arise from the fusion of various secretory granules. The population of secretory granules varied greatly from one cell to another. In all cases, immunogold labeling was seen exclusively in the secretory granules. Exocytosis was observed in all cell types. Some of the PRL, GH, and SL cells showed involutive features. PRL, GH, and SL, although structurally and functionally related, are secreted by ultrastructurally different cells in the pituitary of M. yellowtail.

Immunocytochemical and ultrastructural characterization of prolactin, growth hormone, and somatolactin cells from the Mediterranean yellowtail (Seriola dumerilii, Risso 1810)

Background Prolactin (PRL), growth hormone (GH), and somatolactin (SL) are structurally related pituitary hormones that belong to a peptide family. Whereas growth hormone and prolactin are present in the hypophysis of all vertebrates, somatolactin, a recently discovered hormone, has been found only in fish. It has been demonstrated immunocytochemically in a few teleost species; ultrastructurally, cells producing this hormone have been characterized only in one species of salmon. In this paper, we identify and characterize ultrastructurally the cells producing these three hormones in Mediterranean yellowtail (Seriola dumerilii). Methods Pituitaries from adult specimens were dissected out and processed for electron microscopy. The immunogold technique was performed in some ultrathin sections using fish primary antibodies. Results PRL cells had round, peripherally distributed, very electrondense, homogeneous secretory granules of variable size. GH cells had dense, round secretory granules with a conspicuous scalloped membrane, which were grouped in the cell area near the neurohypophysis. SL cells had round, polymorphic, or very irregularly shaped secretory granules, the last seeming to arise from the fusion of various secretory granules. The population of secretory granules varied greatly from one cell to another. In all cases, immunogold labeling was seen exclusively in the secretory granules. Exocytosis was observed in all cell types. Some of the PRL, GH, and SL cells showed involutive features. Conclusions PRL, GH, and SL, although structurally and functionally related, are secreted by ultrastructurally different cells in the pituitary of M. yellowtail. Anat. Rec. 247:395–404, 1997 © 1997 Wiley-Liss, Inc.

Isolation, Characterization, and Radioimmunoassay of Atlantic Halibut Somatolactin and Plasma Levels during Stress and Reproduction in Flatfish

Somatolactin (SL), a recently identified teleost pituitary hormone which is a member of the growth hormone/prolactin family, was isolated from pituitary tissue of Atlantic halibut (Hippoglossus hippoglossus). Pituitary proteins were extracted in ammonium bicarbonate (pH 7.8), fractionated using gel filtration chromatography, and purified using reversed-phase high-performance liquid chromatography. Halibut SL was identified on the basis of molecular size (determined by gel electrophoresis and mass spectroscopy), cross-reactivity of the putative hormone with antisera to cod SL, and N-terminal amino acid sequence. Polyclonal antibodies to purified halibut SL were raised in rabbits, and a radioimmunoassay (RIA) was developed for measurement of plasma concentrations of SL using purified halibut SL as a standard. The RIA was tested in several flatfish species including Pacific halibut (Hippoglossus stenolepis), English sole (Pleuronectes vetulus), and rock sole (Lepidopsetta bilineata). The assay was specific for SL as indicated by absence of cross-reactivity with Atlantic halibut growth hormone, prolactin, and GTH α subunit. Dilutions of plasma and pituitary extracts from Pacific halibut, English sole, and rock sole were parallel to the Atlantic halibut SL standard curve, indicating that the assay is valid for a range of flatfish species. Using halibut SL antiserum, SL was localized in the pars intermedia of English sole pituitary, where it has been identified in previously examined teleost species. The RIA was used to measure plasma levels of SL in Atlantic halibut and English sole during reproductive development, and in English sole subjected to various types of environmental stressors, including handling and crowding. In both sole and halibut, plasma SL concentrations remained relatively constant throughout gonadal development, but dropped during or following ovulation. Plasma SL levels in English sole tended to increase in response to acute stress, in parallel with plasma cortisol levels.

Studies on the physiology of somatolactin secretion in red drum and Atlantic croaker

The effects of exposure to water of different salinities and calcium concentrations, and to various backgrounds and illuminations on somatolactin (SL) levels in juvenile red drum (Sciaenops ocellatus) were investigated using a recently developed red drum SL radioimmunoassay. Plasma SL concentrations were also monitored in wild-caught Atlantic croaker (Micropogonias undulatus) during gonadal recrudescence. No correlations were obtained between plasma SL concentrations and different salinities, external calcium concentrations or reproductive condition in these sciaenid fishes. Plasma SL concentrations remained low (<1 ng ml-1) in red drum 1h, 1 day and 1 week after exposure to full strength seawater (salinity 37%, calcium 1290 ppm), half strength seawater (salinity 18%, calcium 744 ppm), fresh water (salinity 1%, calcium 260 ppm) or soft water (salinity 0%, calcium 0 ppm). Circulating levels of SL did not change significantly in wild-caught croaker at the onset of vitellogenesis. However, by the end of ovarian recrudescence (late-yolk globule stage), plasma SL levels were significantly lower than those observed in females with immature (perinucleolar) oocytes. In contrast, plasma SL levels showed marked differences in red drum exposed to various backgrounds and illuminations. Plasma SL was lower in red drum kept in a light background (<1 ng ml-1) than in those kept in a black background or in the dark (1.4-6.9 ng ml-1). The highest plasma SL concentration (4–30 ng ml-1) was obtained in red drum kept in a black background without illumination. These results suggest that SL is involved in the adaptation of the red drum to various backgrounds and illumination levels. SL may not have an important role during the reproductive cycle and adaptation to water of different salinities and calcium concentrations in sciaenid fishes.

Ontogeny of somatolactin mRNA in the gilthead sea bream Sparus aurata

Expression of somatolactin (SL) gene during embryonal and early stages of larval development of the teleost Sparus aurata was determined by Northern blot analysis. Poly(A+)RNA was prepared from a pool of embryos collected at early and late stages or from larvae collected on different days after hatching. When hybridized to Sparus aurata SL cDNA, SL specific mRNA was seen both in embryos and in larvae. Levels of SL increased from day 1 onwards and reached the highest levels on day 21. Re-hybridization with Sparus aurata growth hormone (GH) cDNA revealed GH specific mRNA first on day 6 post-hatching. Levels of GH increased to maximal levels on day 10 and then decreased on days 15 and 21, thereby confirming the pattern of GH expression (Funkenstein and Cohen 1996). The patterns of SL and GH gene expression are different in gilthead sea bream during embryonal and larval development. Furthermore, the presence of SL transcript in embryos, prior to pituitary gland development, suggests the possibility that SL mRNA of maternal origin may be present in the oocyte.

Effects of Hypothalamic Factors on Somatolactin Secretion from the Organ-Cultured Pituitary of Rainbow Trout

The profile of daily release of somatolactin (SL) and effects of hypothalamic factors on SL secretion from the organ-cultured pituitary of rainbow trout were examined. The daily release of SL was relatively high (340–380 ng/pituitary/day) for the first 2 days and then decreased. After Day 5, the SL release was maintained at a low level (30–50 ng/pituitary/day) until the end of the experiment at Day 7. The secretory patterns of SL differed from those of growth hormone (GH) and prolactin (PRL); GH secretion was consistently high (15–20 μg/pituitary/day), whereas PRL secretion was low (10 ng/pituitary/day) during the experiment. SL release was not stimulated by calcium ionophore on Days 2 and 6, suggesting that SL release was maximal. Dopamine and epinephrine, added separately to the medium, inhibited SL release. In contrast, serotonin, corticotropin-releasing factor, and gonadotropin-releasing hormone stimulated the dopamine-inhibited SL release. Thus, SL secretion is concluded to be under hypothalamic control and regulated by mechanisms different from those affecting PRL and GH secretion.

Cloning and Expression of Somatolactin, a Pituitary Hormone Related to Growth Hormone and Prolactin from Gilthead Seabream,Sparus aurata

A pituitary hormone, somatolactin (SL), belonging to the GH/PRL family, is produced in the intermediate lobe of the teleost pituitary. The function of this protein is uncertain. Clones coding for SL were isolated and sequenced from a gilthead seabream pituitary cDNA expression library. The nucleotide sequence of the larger cDNA isolated was 1.5 kb containing a 0.8-kb 3′-untranslated region and two potential polyadenylation signals (AATAAA). The mature polypeptide is composed of 207 amino acids, and a signal peptide of 24 residues was also found in the SL precursor. A potential N-glycosylation site Asn-Lys-Thr was identified in gilthead seabream SL. A comparison of the SL amino acid sequences of several fishes indicated that seven cysteine residues are characteristically present in all the SLs so far isolated. Six of those residues are present in homologous positions in SL and GHSparus aurataproteins. SL and GH fromS. auratashowed a 43% homology at the nucleotide level and 22% identity at the amino acid level. Expression of recombinant SL (rSL) inEscherichia coliand isolation from inclusion bodies led to a monomeric form of SL identical in electrophoretic mobility to one of the two forms of the native SL secreted from gilthead seabream pituitaries culturedin vitro.Further, a native glycosylated modified SL secretedin vitroas shown by N-glycosidase treatment was identified. Specific anti-SL antibodies that discriminate well against gilthead seabream GH and PRL in immunoblotting were also raised against rSL.

In vitro activation of fish phagocytic cells by GH, prolactin and somatolactin.

The activation of rainbow trout (Oncorhyndus mykiss) phagocytic cells by chum salmon GH, prolactin (PRL) and somatolactin (SL) was investigated in vitro. Rainbow trout kidney leucocytes were cultured in RPMI 1640 medium containing 1, 10, 50 or 100 ng of each hormone/ml and the production of superoxide anion was measured using reduction of nitroblue tetrazolium (NBT) and ferricytochrome C. Macrophages incubated with 10-100 ng GH or PRL/ml showed significantly enhanced production of superoxide anion in both the NBT and ferricytochrome C tests compared with control macrophages (without hormone). SL did not induce enhanced production of superoxide anion in macrophages. The phagocytic cells treated with GH or PRL also showed increased phagocytic activity and phagocytic index. However, the cells treated with SL showed no enhancement of phagocytic activity or index. These results indicate that GH and PRL stimulate macrophages in vitro.

Bacterial Production and Purification of the Fish Pituitary Hormone Somatolactin

Somatolactin, a pituitary hormone belonging to the growth hormone/prolactin family, is produced in the intermediate lobe of teleost pituitary. To date, the functions of this new hormone and the target tissues are unknown. ASolea senegalensissomatolactin (ssSL) cDNA has previously been cloned and isolated. Here we have inserted this cDNA into a pET-3a plasmid in order to produce recombinant ssSL inE. coliBL21 (DE3) cells. The protein induced was isolated from inclusion bodies by a solubilization–renaturation procedure originally developed to generate native disulfide bonds, to get putative active proteins. The recombinant somatolactin was further purified to homogeneity by gel filtration on FPLC. The estimated molecular weight of 26 kDa by sodium dodecyl sulfate–polyacrylamide gel electrophoresis agrees well with the molecular mass calculated from the translated cDNA sequence and with native somatolactin (SL). The recombinant protein showed electrophoretic mobility identical to that of one of the native forms of SL secretedin vitroby cultured pituitaries from sole. Another native SL expressed inS. senegalensisrepresented a glycosylated modified hormone as shown byN-glycosidase treatment. Further, recombinant SL was recognized by an anti-native SL antibody and used to generate polyclonal sera reactive with the native pituitary hormone. To date, this represents the first recombinant SL protein isolated in sufficient quantities for biophysical and biochemical investigation and for studies on its physiological actions.

A Comparative Analysis of Somatolactin-Related Immunoreactivity in the Pituitaries of Four Neopterygian Fishes and One Chondrostean Fish: An Immunohistochemical Study

An antiserum to cod somatolactin (SL) was used for immunohistochemical screening of the pars intermedia of two teleosts (Oreochromis mossambicusandGymothorax meleagris), two holostean fishes (Lepisosteus osseusandAmia calva), and a chondrostean fish (Acipenser fulvescens) for SL-immunopositive (SL-IR) cells. As expected, a subset of the epithelial cells in the pars intermedia ofO. mossambicus(tilapia) was immunopositive for SL, and the remainder of the epithelial cells was immunopositive for α-MSH-specific antiserum (α-MSH-IR). SL-IR was not detected in any of the epithelial cells in the pars intermedia of the moray eelG. meleagris.To determine whether SL-IR could be detected in nonteleost fishes, immunohistochemical analyses were done on the pituitaries of two holostean fishes and one chondrostean fish. In the pars intermedia of the gar,L. osseus,a subset of cells was immunopositive for both SL and α-MSH. The majority of the pars intermedia cells was immunopositive for α-MSH only. However, in the pars intermedia of the bowfin,A. calva,all of the epithelial cells indicated the presence of both SL and α-MSH. Finally, no SL-positive cells were detected in the pars intermedia of the sturgeon,A. fulvescens.

Immunocytochemical Localization of the Pit-1 Protein in the Pituitary of the Rainbow Trout (Oncorhynchus mykiss)

Pit-1 is a pituitary-specific transcription factor responsible for activating the growth hormone (GH) gene family. The localization of Pit-1 protein in the pituitary of rainbow trout was studied by immunocytochemical analysis. Fish Pit-1 was found to be extensively localized in the nuclei of cells in the proximal pars distalis (PPD), rostral pars distalis (RPD), and pars intermedia (PI). Sagittal sections were immunocytochemically double stained with antibodies against Pit-1, GH, prolactin (PRL), and somatolactin (SL). The results showed that Pit-1 protein was present in the GH-containing cells of the PPD, the PRL-containing cells of the RPD, and the SL-containing cells of the PI.

Cell Biology of Somatolactin

Somatolactin (SL) is a novel pituitary protein, isolated for the first time from the Atlantic cod. The corresponding proteins have been identified in several toleost species, but not in other classes of vertebrates. Comparison of amino acid sequence has revealed SL molecules to be related to growth hormone (GH) and prolactin (PRL) in teleosts and other vertebrates, suggesting that SL is a new member of the GH/PRL family. Unlike GH and PRL, SL can exist in either glycosylated or nonglycosylated form, depending on the species; most teleosts possess glycosylated SLs, except for salmonids whose SLs are simple proteins. The SL-producing cells are located in the pars intermedia bordering the neurohypophysis and are distinct from melanocyte-stimulating hormone (MSH)-producing cells. The SL cells are PAS positive in most teleosts but chromophobic in salmonids, which may reflect the glycosylation status of SL. Its biochemical and molecular features have become increasingly clear, whereas its physiological significance is still poorly understood. Several possible roles for SL have been suggested, including roles in maturation, calcium regulation, stress response, acid-base regulation, fat metabolism, and background adaptation. Although direct evidence is lacking for any of the proposed functions, this involvement in acid-base regulation appears most probable, since other proposed biological events linked to SL should more or less affect the acid-base status in fish. More detailed studies are needed to define the function of SL.