Changes in placental progesterone receptors in term and preterm labour.

Currently, preterm labour is associated with increased fetal mortality and morbidity and is often associated with elevated levels of inflammatory cytokines. However, the exact mechanisms that lead to this pathology are not fully elucidated. In the present study evidence was obtained using a specific membrane progesterone receptor (mPR) agonist, Org OD 02-0, that the progestin antagonism of apoptotic effects of a cytokine, IL-1ß, on human placental BeWo cells is mediated through mPRs. Therefore the aim of this study was to determine whether the gene expression of mPRs and all other known progesterone receptors changes in human placentas at term and during labour. Quantitative PCR (qPCR) in clinical samples revealed a 2.8 fold decrease of mPRß in labouring comparing to non-labouring tissues and 4.6 fold higher levels of mPRγ in preterm mPRγ compared to term placentas. The ratio of mPRα to PR-B was increased in term compared to preterm samples, whereas it was decreased in labour compared to non-labour placentas. There was also a high correlation between mPRα and PGRMC1 expression irrespective of pathologies. Collectively, our data indicates that changes in the ratios of progesterone receptors rather than individual fluctuations might affect progesterone signalling at the placental level.

Overwinter fasting and re-feeding in rainbow trout: plasma growth hormone and cortisol levels in relation to energy mobilisation.

This study investigated the roles of cortisol and growth hormone (GH) during a period of fasting in overwintering salmonid fish. Indices of carbohydrate (plasma glucose, liver glycogen), lipid (plasma free fatty acids (FFAs)) and protein metabolism (plasma protein, total plasma amino acids) were determined, together with plasma GH, cortisol and somatolactin (SL) levels at intervals in three groups of rainbow trout (continuously fed; fasted for 9 weeks then fed; fasted for 17 weeks). In fasted fish, a decline in body weight and condition factor was accompanied by reduced plasma glucose and hepatic glycogen and increased plasma FFA. No consistent elevation of plasma GH occurred until after 8 weeks of fasting when plasma GH levels increased ninefold. No changes were observed in plasma total protein and AA until between weeks 13 and 17 when both were reduced significantly. When previously fasted fish resumed feeding, plasma glucose and FFA, and hepatic glycogen levels rapidly returned to control values and weight gain resumed. No significant changes in plasma cortisol levels, related to feeding regime, were evident at any point during the study and there was no evidence that SL played an active role in the response to fasting. The results suggest that overwinter fasting may not represent a significant nutritional stressor to rainbow trout and that energy mobilisation during fasting may be achieved without the involvement of GH, cortisol or SL.

Cloning and gene expression of P450 17alpha-hydroxylase,17,20-lyase cDNA in the gonads and brain of the fathead minnow Pimephales promelas.

P450 17alpha-hydroxylase,17,20-lyase (P450c17) is a key steroidogenic enzyme in the production of androgens and, therefore, is also indispensable for the production of oestrogens (that are produced from the aromatisation of androgens). In this study, P450c17 cDNA was cloned from the ovary of the fathead minnow (FHM) and its gene expression was examined in the gonads and brains of male and female FHM at different stages of gonadal development with a view to developing an understanding of its involvement in the reproductive physiology in this species. The FHM-P450c17 cDNA sequence cloned was 1812 bp in length, with an open reading frame of 1554 nucleotides encoding a protein of 518 amino acids. Amino acid identity of FHM-P450c17 with P450c17s in other animals was up to 81.8% in other teleosts (channel catfish), 62% in elasmobranches (spiny dogfish), 64% in birds (chicken), and up to 48.8% in mammals (human). FHM-P450c17 gene expression occurred in the ovary, testis, and also in the brain (both male and female) at all stages of sexual development studied. Expression in the brain was at least 30-fold lower than in the gonads, but consistent in all fish life stages studied. In the testis, FHM-P450c17 gene expression was negatively correlated with gonadal development, but there was no obvious association between P450c17 gene expression and sexual development in the ovary, or brain (in both males and females). The results from this study demonstrate the expression of P450c17 in the brain for the first time in fish. Enzymatic studies are now needed to investigate the possible role of P450c17 in neurosteroid production in teleosts.

Exposure to exogenous 17beta-oestradiol disrupts p450aromB mRNA expression in the brain and gonad of adult fathead minnows (Pimephales promelas).

Oestrogens are key regulators in sexual differentiation and development in higher vertebrates. P450 aromatase (p450arom) is the steroidogenic enzyme responsible for the synthesis of oestrogens from aromatisable androgens. Effects of endocrine disrupting chemicals on steroidogenic enzyme gene expression have received little attention so far, yet it is potentially a major pathway for sexual disruption. In this 14-day study the effects of exogenous 17beta-oestradiol (E2) at environmentally relevant concentrations were assessed on gene expression of p450aromB in the gonad and brain of maturing male and female fathead minnows (FHM). Exposure to E2 resulted in an oestrogenic response as shown by a dose-dependent induction of plasma vitellogenin (VTG) in female and male fish and a dose-dependent inhibition of testis growth. There was an effect of exposure to E2 on p450aromB mRNA expression in the gonads; E2 up-regulated p450aromB mRNA expression in the testis and ovary in a dose-response manner after 14 days of exposure. In male brain, p450aromB mRNA concentrations were significantly reduced in fish exposed to 100 and 320 ng E2/l on day 4, but on day 14 were elevated in males exposed to both 32 and 100 ng E2/l. No effects of E2 on p450aromB mRNA expression occurred in the brain of females. The results of this study show that concentrations of E2 found in the environment can have disruptive effects on key steroidogenic enzyme pathways that control sexual development in fish.

Molecular characterisation of a putative Atlantic salmon (Salmo salar) odorant receptor.

The olfactory system of fish is extremely important as it is able to recognise and distinguish a vast array of odorous molecules that are involved in behaviours paramount to survival. This is achieved by the activation of a diverse multigene family of G-protein coupled receptors through odorous ligand binding. Using molecular techniques, the nucleotide sequence of the cDNA coding for an Atlantic salmon (Salmo salar) odorant receptor (ASOR1) has been determined. The full-length cDNA (1260 nt) encodes a protein of 320 amino acid residues, including one potential N-linked glycosylation site, within the short extracellular amino terminal of the receptor. Hydrophobicity analysis revealed seven hydrophobic regions within the amino acid sequence, corresponding to possible positions of the transmembrane domains characteristic of the G-protein coupled receptor superfamily. Several conserved motifs unique to odorant receptors were also present. Through characterisation of this receptor, we hope to increase the understanding of the mechanisms underlying olfaction in salmonid species.

Follicle-stimulating hormone and its alpha and beta subunits in rainbow trout (Oncorhynchus mykiss): purification, characterization, development of specific radioimmunoassays, and their seasonal plasma and pituitary concentrations in females.

Gonad development in fish, as in mammals, is regulated by two gonadotropins (GTHs), FSH and LH. The function of LH in fish has been clearly established; however, the function(s) of FSH is less certain. The lack of specific and sensitive assays to quantify FSH and its alpha and beta subunits has hindered studies to assess physiological function. In this study, gel filtration chromatography, ion exchange chromatography, and HPLC were employed to purify FSH and its subunits from pituitary glands of rainbow trout (Oncorhynchus mykiss), and the identities of the isolates were confirmed by amino acid analysis. Polyclonal antibodies were raised against the free GTHalpha2 and free FSHbeta subunits to develop specific RIAs. The sensitivities of the intact FSH, GTHalpha2, and FSHbeta assays were 1 ng/ml, 0.2 ng/ml, and 0.1 ng/ml, respectively, and the cross-reaction of these molecules with each other and with intact LH in the heterologous assays was <10.4% throughout. Pituitary and plasma samples diluted in parallel with the standards in all three assays and spiked sample recoveries were >90% throughout. Measurement of plasma and pituitary concentrations of intact FSH in female rainbow trout confirmed the established seasonal profiles. Concentrations of free GTHalpha2 subunit were elevated both in the plasma and in the pituitary in females at ovulation (maximum concentrations: 34.93 +/- 6.3 ng/ml in plasma; 37.63 +/- 5.79 microg/pituitary). In both the plasma and the pituitary, free FSHbeta subunit was present throughout the reproductive cycle but at very low concentrations when compared with both free GTHalpha2 and intact FSH. The presence of free GTHalpha2 subunit in the plasma similarly occurs in mammals, but its functional significance in fish has yet to be established.

Molecular characterization of the first non-mammalian p73 cDNA.

The p53 gene is believed to be mutated or deficient in over 50% of human tumours, and is therefore considered to be instrumental in the process of carcinogenesis. Recently in humans, homologues of p53 (such as p73 and p63) have been isolated. In our studies in fish, we have been isolating tumour suppressor genes with a view to their potential use to study genotoxins in the aquatic environment. In this paper, we report the characterisation of the first non-mammalian p73 cDNA, isolated from barbel (Barbus barbus), a freshwater cyprinid fish indigenous to UK rivers. The deduced barbel p73 amino acid sequence has a high homology with human p73 alpha: the proteins are 641 and 636 aa in length, respectively, and there is a 72% identity over the entire sequence length of the protein (over 90% in the putative DNA binding domain). The level of conservancy for p73 is considerably higher across class (from man to fish), than for p53 and it may therefore have particular value in studies on environmental mutagenesis. Northern analysis showed expression of three p73 mRNA transcripts/homologues. The patterns of p73 tissue expression in the barbel differed from the expression of p53 mRNA, suggesting specific functional roles for the two genes.

Fish p53 as a possible biomarker for genotoxins in the aquatic environment.

The p53 gene is a tumour suppressor gene which has a fundamental role in cell cycle control and division, and in mammals certain genotoxic agents induce specific mutations in p53, leading to tumourigenesis. Fish have been investigated as models for studying carcinogens, but as yet very little data exists that links exposure to specific chemicals with the aetiology of tumours found in wild populations. In this study, p53 was sequenced from five species of fish with a view to the possible use of mutations in the highly conserved domains of p53 to identify genotoxins in the aquatic environment. A 0.8 kb fragment of the cDNA encompassing the conserved DNA-binding domain of p53 was sequenced in three Oncorhynchus salmonid fish: coho (O. kisutch), chum (O. keta), and chinook (O. tshawytscha) and full-length p53 cDNAs were sequenced in the puffer fish (Tetraodon miurus) and the barbel (Barbus barbus). The full-length puffer fish and barbel p53 cDNAs were 1834 bp and 1790 bp in length, encoding a 367 aa protein and a 369 aa protein, respectively. The deduced aa sequences of the p53 cDNA in the Oncorhynchus salmon shared a 100% identity in the five conserved regions (I-V). Comparisons of the deduced aa sequences for puffer fish and barbel p53 with other fish p53s revealed a high homology within the conserved DNA binding domain (68-86% for puffer fish and between 66-88% for barbel). "Conserved" domain I was not highly conserved in fish, as it is in mammals, and, therefore, conserved domains II-V are most likely to provide the valuable sequences in fish p53 for use in mutational studies to fingerprint genotoxins in the aquatic environment.

Studies on the GH/SL gene family: cloning of African lungfish (Protopterus annectens) growth hormone and somatolactin and toad (Bufo marinus) growth hormone.

The lungfishes (lobe-finned fish) occupy a unique position in vertebrate phylogeny, being regarded as the closest extant relatives to the tetrapods. The putative pituitary hormone somatolactin (SL) has hitherto been found only in teleost fishes, and the presence of this protein in tetrapods or lobe-finned fishes has not been ascertained. It was therefore of interest to determine the structure of SL in the African lungfish (Protopterus annectens), as this information would be useful for designing probes to facilitate the detection of SL genes in amphibians and other tetrapods. The structural relationships between SL, growth hormone (GH), and prolactin (PRL) strongly suggest that these proteins evolved from a common ancestor. To obtain a more complete picture of the evolution of these hormones in lungfish, African lungfish GH has been cloned and sequenced. The cDNA sequence of a toad (Bufo marinus) GH was determined to facilitate maximum parsimony analysis of GH sequences. Cladistic analysis confirmed that lungfish and amphibian GH sequences form a clade distinct from the GH sequences of ray-finned fishes. A distance matrix analysis of SL sequences indicated that lungfish SL had the lowest primary sequence identity with goldfish SL (47%) and the highest with flounder SL (66%). The detection of SL in a lungfish indicates that the gene duplication within the SL/GH/PRL family, which gave rise to SL, must have occurred in a common ancestor of the ray-finned fishes (Actinopterygii) and the lungfishes (Sarcopterygii) and tetrapods.

Identification and modulation of a growth hormone-binding protein in rainbow trout (Oncorhynchus mykiss) plasma during seawater adaptation.

A soluble protein that specifically bound 125I-human growth hormone (hGH) was identified in rainbow trout plasma, using HPLC-gel filtration. The binding affinity of the protein for hGH was 1.2 x 10(9)M-1. 125I-rainbow trout GH (tGH) was also able to bind to the protein albeit with a lower affinity (6.6 x 10(7)M-1) than hGH. Crosslinking experiments using 125I-hGH revealed two specific bands of 150 and 130 kDa. The complex 125I-hGH-BP could be precipitated by a monoclonal anti-GH receptor antibody, suggesting a close relationship between the plasma GH-BP and the GH receptor. A fourfold increase in the hGH binding to the GH-BP was shown 48 h after transfer of the fishes from freshwater to seawater. The increase in binding was related to a high binding capacity without significant changes in binding affinity. These results suggest a potential role of this related GH-BP as an index of GH effects during seawater adaptation in salmonids.

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 17 alpha,20 beta-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).

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.

Isolation and characterisation of mRNA encoding the salmon- and chicken-II type gonadotrophin-releasing hormones in the teleost fish Rutilus rutilus (Cyprinidae).

The complementary DNAs (cDNA) encoding the [Trp7,Leu8]-gonadotrophin-releasing hormone (salmon-type GnRH; sGnRH:GeneBank accession no. u60667) and the [His5,Trp7,Tyr8]-GnRH (chicken-II-type GnRH; cGnRH-II: GeneBank accession no. u60668) precursor in the roach (Rutilus rutilus) were isolated and sequenced following reverse transcription and rapid amplification of cDNA ends (RACE). The sGnRH and cGnRH-II precursor cDNAs consisted of 439 and 628 bp, and included open reading frames of 282 and 255 bp respectively. The structures of the encoded peptides were the same as GnRHs previously identified in other vertebrates. The sGnRH and cGnRH-II precursor cDNAs, including the non-coding regions, had 88.6 and 79.9% identity respectively, to those identified in goldfish (Carassius auratus). However, significant similarity was not observed between the non-coding regions of the GnRH cDNAs of Cyprinidae and other fish. The presumed third exon, encoding partial sGnRH associated peptide (GAP) of roach, demonstrated significant nucleotide and amino acid similarity with the appropriate regions in the goldfish, but not with other species, and this may indicate functional differences of GAP between different families of fish. cGnRH-II precursor cDNAs from roach had relatively high nucleotide similarity across this GnRH variant. Cladistic analysis classified the sGnRH and cGnRH-II precursor cDNAs into three and two groups respectively. However, the divergence between nucleotide sequences within the sGnRH variant was greater than those encoding the cGnRH-II precursors. Consistent with the consensus developed from previous studies, Northern blot analysis demonstrated that expression of sGnRH and cGnRH-II was restricted to the olfactory bulbs and midbrain of roach respectively. This work forms the basis for further study on the mechanisms by which the tapeworm, Ligula intestinalis, interacts with the pituitary-gonadal axis of its fish host.

Pronounced seasonal rhythms in plasma somatolactin levels in rainbow trout.

The aim of this study was to establish whether there are seasonal or age-related changes in circulating levels of somatolactin (SL) in rainbow trout (Onchorhynchus mykiss). SL levels were determined in blood sampled at monthly intervals over a 2-year period from a population of rainbow trout maintained under a natural daylength and temperature regime (North-West England, latitude 54 degrees 20' N). SL levels displayed a distinct circannual cycle, with peak levels in summer (17-20 micrograms/l) and lowest levels occurring in winter (0.2-2 micrograms/l). This variation in SL levels was closely correlated with water temperature (P < 0.001) but was out of phase with changes in daylength. Plasma SL levels were significantly higher in both mature male and female fish compared with immature fish. Plasma prolactin levels were determined to provide information on a hormone structurally related to SL, and also because mammalian prolactin is known to show distinct seasonal cycles. However, trout prolactin, in contrast to SL, did not show a pronounced seasonal pattern, although prolactin levels were significantly, but inversely, correlated with water temperature (P < 0.001).

Immunocytochemical study of the somatolactin cells in the pituitary of Pacific salmon, Oncorhynchus nerka, and O keta at some stages of the reproductive cycle.

Cells of the intermediate lobe of salmonids, homologous to the PAS-positive cells of other teleost species, cannot be differentiated by normal staining techniques, but can be immunostained with an antiserum against cod somatolactin (SL). Immunocytochemical techniques were applied to pituitary sections of two Pacific salmon, Oncorhynchus nerka and O. keta. Samples of immature or early maturing fish were collected in the Pacific Ocean and from mature spawning fish from hatcheries near Seattle and Willard (Washington). SL cells were rather small and moderately immunoreactive in immature fish. They were slightly enlarged in the early stages of gonadal development and more often contacted the basal lamina through processes with terminal swellings, suggesting granule release into perivascular spaces. In spawning fish, cells were enlarged and frequently more granulated, showing a wide contact with the basal lamina and a proximodistal transport of granules. In addition, large and more or less degranulated cells were noted, also indicating an active release of SL granules. Spawning females tended to have more SL cells than equivalent males. The gradual stimulation of SL synthesis and release during sexual maturation suggests that SL may be involved in the control of some steps of reproduction as previously shown by the increase in SL plasma levels in maturing coho salmon.

Immunoreactive somatolactin cells in the pituitary of young, migrating, spawning and spent chinook salmon, Oncorhynchus tshawytscha.

Immunocytochemical techniques using an antiserum to cod somatolactin (SL) demonstrated the presence of SL cells in the intermediate lobe of the pituitary in Oncorhynchus tshawytscha. The cells were small in yearling fish. Two groups of maturing fish were studied. In the spring run salmon collected in April and May during the upstream migration, the SL cells appeared stimulated. In September, during spawning, SL cell stimulation was maximal with indices of hypertrophy and degranulation often more marked in females than in males. In the other group, salmon of the fall run collected in the Pacific Ocean in August had well developed gonads, large gonadotropes and abundant SL cells. In spawning salmon (September) the SL cells were stimulated, mainly in females. However, the final stimulation was less intense than in spring run spawning fish. The SL cells were smaller, without evident granule release, but still abundant in spent salmon of the fall run caught at the end of November. Various factors (time spent in rivers before spawning, starvation, decalcification, stress, hypothalamic influences) were considered which might explain differences between spring and fall run salmon. These observations suggest that SL may play a role in the control of gonadal maturation in chinook salmon as it may also do in sockeye and chum salmon previously studied, and that SL cells may be sensitive to the ambient salinity.

Plasma somatolactin concentrations in salmonid fish are elevated by stress.

The preliminary finding that plasma levels of somatolactin (SL) were markedly elevated following stress caused by confinement in chinook salmon (Oncorhynchus tshawytscha) prompted a more detailed study of SL dynamics during stress. SL levels have been determined in the plasma of rainbow trout (Oncorhynchus mykiss) during exposure to acute (0-30 min) and short (0-24 h) periods of stress resulting from handling and confinement. The results show that SL levels increase rapidly within minutes following the onset of stress, reach a peak between 1 and 2 h, decline over the next 3 h, and then show an additional increase again by 24 h. During acute stress caused by confinement, the increase in plasma SL levels occurred within 2 min, thus showing a more rapid response than cortisol. This suggests that the response is mediated directly by the hypothalamus and is not a result of a feedback mechanism. The results also demonstrate that SL secretion in response to stress is at least partially under genetic control. In the short-term stress experiment, progeny of fish selected as high responders or low responders to stress, based on the magnitude of the plasma cortisol levels induced by stress, were used, and these fish showed similarly accentuated or attenuated release of SL following stress. These results clearly demonstrate that non-specific environmental stress causes rapid activation of SL-secreting cells in the pars intermedia, suggesting that this hormone has an important role in the adaptive response of fish to stress.

Isolation of prolactin and growth hormone from the pituitary of the holostean fish Amia calva.

Pituitaries from adult male and female Amia calva (Order Holostei) were acid extracted and fractionated by gel filtration column chromatography and reversed-phase high performance liquid chromatography. This two-step isolation procedure yielded homogeneous pools of Amia prolaction (PRL) and growth hormone (GH). The amino acid composition of both purified polypeptides was determined. Primary sequence analysis of the first 22 positions at the N-terminal of Amia PRL revealed that this region has 63% sequence identity with eel PRL-1. The N-terminal region of Amia PRL lacks the disulfide bridge which is characteristic of tetrapod PRLs. Primary sequence analysis of the first 24 positions at the N-terminal of Amia GH revealed that this region has 62% sequence identity with eel GH and 54% sequence identity with both blue shark GH and sea turtle GH. Based on N-terminal analysis, it appears that Amia PRL and GH are more closely related to teleost PRLs and GHs than they are to tetrapod PRLs and GHs.

Plasma somatolactin levels in coho salmon (Oncorhynchus kisutch) during smoltification and sexual maturation.

Somatolactin (SL) is a novel pituitary hormone recently characterized in several fish species. Structural analyses have shown that SL belongs to the growth hormone/prolactin family, and that it is a highly conserved protein. SL is synthesized by the periodic acid/Schiff-positive cells in the pars intermedia, but has an as yet unidentified function(s). We have recently developed a homologous radioimmunoassay for coho salmon SL and measured plasma levels of SL during two stages of the coho salmon life cycle, smoltification and sexual maturation. During smoltification, plasma levels of SL changed almost in parallel with plasma levels of thyroxine; levels increased as morphological indices of smoltification appeared and decreased as smoltification was completed. Following this period, SL levels remained low until the spring prior to spawning. In a separate study, plasma levels of SL were measured in sexually maturing coho salmon that remained in fresh water throughout their life cycle. During the year of sexual maturation, plasma levels of SL gradually increased from the spring onward, reaching peak levels at the time of spawning in November and December. These data are similar to those previously reported for sexually maturing coho salmon that were maintained in seawater prior to spawning (Rand-Weaver et al. 1992). Therefore, increases in plasma SL levels occurred in sexually maturing fish irrespective of whether they were maintained in fresh water or seawater. Peak levels at spawning were higher than those observed during smoltification. Possible roles for SL in metabolism and reproduction are discussed.

Somatolactin stimulates in vitro gonadal steroidogenesis in coho salmon, Oncorhynchus kisutch.

Somatolactin (SL), a newly discovered pituitary hormone of the teleost pars intermedia, is structurally similar to prolactin and growth hormone. The function(s) of SL are not yet established, although evidence suggests that it may play a role in reproduction. Possible steroidogenic activity of coho salmon SL was tested and compared with gonadotropin I (GTH I) in incubations of ovarian follicles or testicular fragments. SL stimulated production of 11-ketotestosterone and testosterone by testicular fragments, and production of estradiol by ovarian follicles in a concentration-dependent manner. However, the steroidogenic activity of SL was considerably less than that of GTH I. These results suggest that SL may play a role in regulation of gonadal function in salmon.