5.2 Chemistry and immunological properties
5.3 One versus two gonadotropins
5.4 Plasma gonadotropin profiles
Gonadotropic hormones (GtH) have been isolated in varying degrees of purity from the pituitary glands of several teleost fishes including some cultivated species such as common carp, Cyprinus carpio (Burzawa-Gérard, 1971, 1974; Idler and Ng, 1979), the chinook salmon, Oncorhynchus tshawytscha (Donaldson et al., 1972b; Pierce, Faith and Donaldson, 1976), cat-fish, Heteropneustes fossilis (Sundararaj and Samy, 1974), sturgeon, Acipenser stellatus (Burzawa-Gérard, Goncharov and Fontaine, 1975a, b), chum salmon, Oncorhynchus keta (Idler, Hwang and Bazar, 1975; Idler, Bazar and Hwang, 1975a, b; Yoneda and Yamazaki, 1976; Yoneda, Yamazaki and Ishihara, 1977; Ng and Idler, 1978b; Idler and Hwang, 1978; Idler and Ng, 1979), rainbow trout, Salmo gairdneri (Breton, Jalabert and Reinaud, 1976), tilapia, Sarotherodon (Tilapia) mossambicus (Farmer and Papkoff, 1977), American plaice, Hippoglossoides platessoides (Ng and Idler, 1978a, 1979), winter flounder, Pseudopleuronectes americanus (Ng and Idler, 1978a, 1979) and another tilapia, Sarotherodon (Tilapia) spirulus (Hyder, Shah and Hartree, 1979). In addition, the pituitary extracts of grass carp, Ctenopharyngodon idella, bighead carp, Aristichthys nobilis and catfish, Heteropneustes fossilis, have been subjected to gel chromatography on Sephadex-G100 and three, protein fractions have been obtained; of these, the second fraction has gonadotropic activity (Sinha, 1969, 1971; Sundararaj, Anand and Sinha, 1972; Nath and Sundararaj, 1977). Hattingh and du Toit (1973) and Haider and Blüm (1977) using pituitary material from mudfish, Labeo umbratus and goldfish, respectively, have separated by polyacrylamide gel electrophoresis two fractions having gonadotropic activities.
The piscine gonadotropins are glycoprotein in nature (Fontaine and Burzawa-Gérard, 1978; Idler and Ng, 1979; Fontaine, 1980). The amino acid composition of gonadotropins from carp (Burzawa-Gérard, 1974; Idler and Ng, 1979), sturgeon (Burzawa-Gérard, Goncharov and Fontaine, 1975b), plaice (Ng and Idler, 1979), rainbow trout (Breton, Jalabert and Reinaud, 1976), and tilapia (Farmer and Papkoff, 1977), shows a broad similarity to the mammalian luteinizing hormone. The gonadotropins from carp, sturgeon and trout are each composed of two subunits, the alpha and beta chains; the amino acid and carbohydrate compositions, the long N-terminal sequences and the C-terminal amino acids of the two subunits of carp GtH have been determined (Jolles et al., 1977). A significant degree of homology between the two subunits of carp GtH with those of mammalian follicle-stimulating hormone (FSH) and luteinizing hormone (LH) has been demonstrated and further, the beta subunit of carp GtH is more closely related to the beta subunit of LH than to that of FSH (Fontaine and Burzawa-Gérard, 1978; Fontaine, 1980).
Tan and Dodd (1978) have studied the immunological relatedness of gonadotropins of 35 species of teleost fishes by using a salmon-salmon homologous radioimmunoassay and a salmon-carp heterologous radioimmunoassay. In homologous radioimmunoassay, most salmon species tested except powan and ayu cross react in a manner identical with that of the standard hormone, while the nonsalmonid species showed no cross reaction. In the heterologous radioimmunoassay, all cyprinids and salmonids except ayu gave inhibition curves parallel to standard carp gonadotropin. Thus, immunological properties of fish gonadotropins do not correspond to known phylogenetic relations of fishes. A study of immunological properties of pituitary GtH and its subunits from fishes to mammals gives additional evidence of homology among piscine and mammalian pituitary glycoproteins indicating evolutionary similarity between subunits of the same type (Fontaine and Burzawa-Gérard, 1978; Burzawa-Gérard, Dufour and Fontaine, 1980). Burzawa-Gérard and Fontaine (1976) have produced a hybrid molecule containing the alpha subunit from bovine GtH and beta subunit from carp which is much more potent in the amphibian spermiation test; interestingly, the reverse hybrid molecule has no effect in either mammal or carp.
The number of gonadotropins present in the pituitary glands of fishes has often been discussed, debated and is still controversial. Earlier studies favoured the one hormone hypothesis (Burzawa-Gérard, 1971, 1974; Donaldson et al., 1972b; Fontaine, 1975, 1980), since the single gonadotropin isolated from salmon and carp pituitaries could induce complete gametogenesis including oocyte maturation and spermiation in the hypophysectomized recipients (Donaldson, 1973; Billard, Burzawa-Gérard and Breton, 1970; Sundararaj et al., 1976). In 1975 Idler and associates (Idler, Bazar and Hwang, 1975a, b) using affinity chromatography have reported evidence of more than one sex-specific gonadotropin in chum salmon pituitary glands. Breton, Prunet and Reinaud (1978) have confirmed the above observation in chinook salmon, where gonadotropin isolated from the male glands is more effective on the testis than on the ovary, while the reverse is true for the female-specific gonadotropin. Sundararaj and Samy (1974) have adopted purification procedures used for isolating mammalian gonadotropin where ammonium sulphate at 50 percent saturation is known to precipitate the mammalian LH, while full saturation is required for precipitation of FSH (Papkoff et al., 1965; Papkoff, Gospodarowicz and Li, 1967) to catfish, Heteropneustes fossilis, pituitary material. An LH-like preparation with the. capacity to induce maturation and ovulation in catfish has been obtained; a precipitate obtained after full saturation is not active in catfish ovulation assay. It was, however, not tested for vitellogenic activity in catfish. Farmer and Papkoff (1977) have prepared a gonadotropin from the pituitary glands of tilapia, Sarotherodon (Tilapia) mossambicus by following the purification procedures used for isolation of mammalian gonadotropins. They have reported an LH-like gonadotropin which stimulates in vitro testosterone production in rat Leydig cells; the biological activity of another preparation, which fractionates identically to mammalian and nonmammalian FSH, is not reported. Their work indicates that if fish pituitary produces two gonadotropins corresponding to FSH and LH of mammals, the material isolated represents teleost LH. Hyder, Shah and Hartree (1979) isolated from Sarotherodon (Tilapia) spirulus pituitary glands a potent gonadotropic fraction capable of inducing both spermatogenesis and Leydig cell activity which by analogy with mammals would contain the FSH fraction; the other fraction was not equally potent.
The glycoprotein nature of piscine gonadotropin has been used by Idler and associates to retain it on concanavalin A-Sepharose in some of the recent purification procedures (see Campbell and Idler, 1976, 1977; Ng and Idler, 1978a, b, 1979; Idler and Ng, 1979). Recently, Ng and Idler (1979) and Idler and Ng (1979) have isolated from the pituitary glands of common carp, chum salmon, American plaice and the winter flounder two gonadotropins, the vitellogenic and the maturational hormones. The fraction low in carbohydrate content, lacking the affinity to concanavalin A-Sepharose, produces yolk incorporation into the ovary of winter flounder and trout oocytes in vitro, whereas the carbohydrate rich fraction, which is absorbed on concanavalin A-Sepharose induces maturation arid ovulation. Further, the carbohydrate rich fraction also has vitellogenic activity in the female flounder (Idler and Ng, 1979). The vitellogenic hormone is distinguishable from the maturational hormone in. the composition of its protein and carbohydrate moieties. Antisera to the two gonadotropic fractions from the chum salmon pituitaries capable of blocking gonadotropic actions in vivo have been prepared (Ng, Campbell and Idler, 1980). The question now is to determine whether or not the male also produces two gonadotropins like the female.
The plasma gonadotropin profiles have been determined in rainbow trout, brook trout, common carp, Atlantic salmon, Salmo salar, and sockeye salmon, Oncorhynchus nerka, by using specific radioimmunoassays (see also section 2 on Reproductive cycles and environmental cues). In salmonids generally GtH is secreted in gradually increasing amounts to stimulate gonadal recrudescence and a sharp increase in GtH levels occurs at the time of spermiation or ovulation (Billard, Richard and Breton, 1977; Fostier et al., 1978). Elevated levels of plasma GtH at the time of spermiation and ovulation are also reported in other species (Peter and Crim, 1979).
The responsiveness of the gonad to GtH varies with the time of the day (de Vlaming and Vodicnik, 1977; Cook and Peter, 1980) and this may have some relation with the daily fluctuations in the plasma levels of GtH. Circadian rhythmic variations in plasma levels of GtH have been reported in goldfish (Breton et al., 1972b; Peter and Hontela, 1978; Hontela and Peter, 1978). The circadian rhythms in GtH levels are nearly absent or amplitudes smaller in sexually regressed female goldfish than in recrudescing and gravid goldfish. The circadian rhythms in GtH levels appear to be important in stimulating gonadal activity. Exposure of goldfish to stimulatory conditions of long photoperiod and warm temperature elevates plasma GtH levels (Gillett, Billard and Breton, 1977; Gillet, Breton and Billard, 1978) and also causes large circadian fluctuations (Hontela and Peter, 1978). The uptake of injected GtH from the site of injection to the circulatory system in goldfish is higher at 20°C than at 12°C. Further, warmer temperatures are associated with an increase in the metabolic clearance rate of the injected GtH in goldfish (see Cook and Peter, 1980). Thus, the above findings imply that the poor success in the hypophysation of major Indian and Chinese carps (see Chaudhuri, 1976) may be in part due to warm ambient temperatures.
The morphology and physiology of the gonads, the ovary and the testis, the target organs of gonadotropic hormone(s), as well as regulation of their function by gonadotropins are discussed in the next five sections.