Antonella Bona-Gallo

Annual cycles in levels of pituitary and plasma gonadotropin, gonadal steroids, and thyroid activity in the Chinese cobra (Naja naja)

Abstract Adult Chinese cobras, Naja naja , were purchased monthly from a snake dealer in Hong Kong (6–8 animals per month for each sex, total of 290 snakes) and sacrificed after 1 day of acclimation in the laboratory. Pituitary and plasma gonadotropin (Gn), plasma thyroxine (T 4 ), sex steroids in plasma and testes, and ovarian Gn binding were examined in relation to the annual reproductive cycle. Changes in plasma Gn did not always correspond to pituitary Gn content, and a marked annual cycle in plasma T 4 was evident, despite a lack of change in thyroid weight or epithelial cell height. Although Gn levels were generally elevated during the reproductive season, gonadal activity did not always show the expected relationship to plasma Gn levels. In males, a winter peak in plasma Gn occurred despite low pituitary content; this increase in the plasma coincided with the initiation of testis growth. However, in summer, the testes showed full regression in weight, spermatogenesis, and testosterone (T) secretion, almost 2 months before a decline in plasma Gn was evident. Increased plasma T 4 might be implicated in this apparent dissociation between testis function and circulating Gn levels. Both plasma and testicular T increased only at the time of maximum testicular weight; they did not show the expected increase at the time when spermatogenesis was initiated. In females, plasma Gn showed a bimodal annual cycle but ovarian activity was compressed into a single relatively brief period of about 2 months in spring. A pronounced winter rise in plasma Gn, despite low pituitary Gn levels, was not reflected in detectable ovarian changes. Increases in plasma estradiol (E) and T occurred during the vitellogenic phase; plasma E dropped rapidly after ovulation, but T remained partially elevated until after oviposition. In contrast, plasma progesterone (PRO) did not rise until after ovulation; it then remained fully elevated until after oviposition. Thus, both T and PRO levels along with high Gn binding indicated that the corpora lutea remain functional until oviposition occurs. Steroid levels were reduced to baseline values in animals with atretic follicles. Plasma and pituitary Gn rose during the vitellogenic period and remained elevated until oviposition. Unlike the male, plasma T 4 levels were maximal during the active reproductive period (vitellogenesis and ovulation). The ability of ovarian tissues to bind radiolabeled gonadotropin was lowest in the previtellogenic stages, especially when plasma Gn was highest. Corpora atretica also had significantly reduced binding. We postulate that the ovarian cycle of this snake represents a progressive differentiation in the functional state of the ovary which is induced by relatively constant high levels of a single gonadotropin; thus, functional transitions in ovarian tissue rather than alterations in the nature or concentration of gonadotropins in the circulation are responsible for the changing pattern of ovarian steroid secretion.

Effects of chicken and mammalian gonadotropin-releasing hormones (GnRH) on in vivo pituitary gonadotropin release in amphibians and reptiles

The ability of mammalian and chicken gonadotropin-releasing hormones (GnRH) and their agonistic analogs to stimulate in vivo gonadotropin release were tested in a frog (Rana catesbeiana), snake (Naja naja), and turtle (Sternotherus odoratus). In the frog, chicken and mammalian GnRH were equipotent in stimulating the release of FSH and LH. Attendant increases in plasma androgen and the occurrence of spermiation confirmed the release of biologically active gonadotropin. Neither of the GnRH preparations or their agonists produced significant changes in plasma hormones in either of the reptiles. In light of comparable data for the actions of these GnRH preparations in mammals and birds, it appears that species specificity in the response to different GnRHs does not correlate well with the nature of the homologous hypothalamic GnRH molecule.

Biochemical and immunological characterization of pituitary hormones from the ostrich (Struthio camelus)

Abstract Fractionation of pituitary glands of the ostrich (Struthio camelus) resulted in the preparation of highly purified growth hormone (GH), prolactin (PRL), follicle-stimulating hormone (FSH), and luteinizing hormone (LH) in relatively high yields; thyrotropin (TSH) was partially purified. Hormones were identified by a variety of biological and immunological tests. Biochemical and immunological characterizations were performed to study the purity of each preparation and to allow comparison with other species of hormones, with special emphasis on other avian hormones. Similarities to the corresponding hormones of other tetrapods and even fish are apparent in the various biochemical features. THe ostrich hormones show closer affinities to other avian hormones than to mammalian or reptilian species in some respects (especially immunological), but notable biochemical differences are also evident among the few avian species examined.

Gonadotropin specificity of in vitro testosterone secretion by fish testes

Highly purified gonadotropin preparations from representatives of all four tetrapod classes and several fish were used to examine the specificity of in vitro testosterone production by minced testes from four species of teleost fish, representing three families and two orders. No consistent phylogenetic patterns emerged in either hormonal (FSHLH) or phylogenetic specificity in the steroidogenic response of the teleost testis. Testes of Gillichthys mirabilis (Family Gobiidae) were highly specific for all species of LH (FSHs were inactive) and they also showed a certain degree of species specificity; mammalian and amphibian LHs were much more potent than reptilian and avian LHs. In contrast, testes of Cichlasoma citrinellum and Sarotherodon mossambicus (Family Cichlidae) lacked gonadotropin specificity (most preparations of FSH and LH were about equipotent), and in C. citrinellum all species of tetrapod hormones were active within approximately the same dose range. Gonadotropins from the sturgeon (Chondrostei) were the most potent in both G. mirabilis and C. citrinellum. In Salmo gairdneri (Family Salmonidae) there was specificity for amphibian (bullforg) LH, but not for mammalian (ovine) LH. Steroid secretion by the fish testes was temperature dependent, but relative potencies of ovine LH, bullfrog LH, and sturgeon gonadotropin were not altered by incubation temperatures between 10 and 30° in G. mirabilis. As a group, the teleosts examined here differ from mammals, birds, and reptiles in their hormonal specificity (or consistent lack there of) for testicular androgen secretion; they most closely resemble the Amphibia in their high degree of interspecific variability in response to FSH and LH. The lack of predictability in this aspect of specificity as well as in species specificity in the potencies of individual types of hormones of different origins precludes generalizations about the evolutionary pattern of gonadotropin-testis interactions.

Specificity to gonadotropins in the response of in vitro estrogen secretion by fish ovaries.

Gonadotropin preparations from three classes of tetrapods (amphibian, avian, and mammalian) and a chondrosteian and teleost fish were used to investigate the species specificity and hormonal (FSH/LH) specificity of in vitro steroid (estradiol-17 beta) production by the teleost ovary. Results for ovaries from one species of gobiid, Gillichthys mirabilis, and two cichlids, Cichlasoma citrinellum and Sarotherodon mossambicus, revealed a general lack of species specificity in the response to tetrapod gonadotropins, but the piscine, especially sturgeon, gonadotropins were much more potent than any of the tetrapod hormones. All three species of teleost ovaries responded to both types of tetrapod gonadotropins (FSH and LH), but the extent of hormonal specificity was variable. The gobiid ovary showed the highest LH specificity (potencies of FSHs = 7-14% of LH); in the two cichlids the potency of FSHs ranged from 11 to 100% of the respective LHs. In general, the specificity of the ovarian steroidogenic response to gonadotropins parallels that observed for testosterone secretion in the males of the same three fish, but the differential actions of the tetrapod hormones (both species and hormonal specificity) are more exaggerated for the testes.

Biological and binding activities of pituitary hormones from the ostrich, Struthio camelus.

Biological and binding activities of adenohypophysial hormones purified from the ostrich (ost), Struthio camelus, were compared to those of the corresponding hormones derived from mammalian and other avian species. The potency of ostrich prolactin was comparable to those of other avian preparations and slightly less active than the ovine hormone when tested in the pigeon crop-sac assay, but ostrich growth hormone (GH) was more potent than several other avian preparations and was comparable to mammalian GH in the rat tibia bioassays. Marked discrepancies were evident in the activities of both ostrich gonadotropins (ostGn) when they were tested in a variety of in vivo and in vitro bioassays and radioreceptor assays (RRAs). Both the ostrich follicle-stimulating hormone (ostFSH) and luteinizing hormone (ostLH) were among the most potent tested thus far in in vivo bioassays for total gonadotropin in a lizard and a cockerel; a high sialic acid content may account for these high potencies. OstFSH was also the most potent nonmammalian Gn tested in two FSH specific mammalian bioassays, an in vivo (ovarian augmentation) and an in vitro (cAMP production) rat bioassay; in fact, ostFSH behaved more like a mammalian than an avian hormone in these assays. However, the binding activity of ostFSH was not unlike that of other avian FSH preparations when tested in either mammalian or nonmammalian FSH-RRA systems. OstLH was more potent than other avian preparations in an in vitro mammalian bioassay, but not in avian or amphibian LH bioassays; species specificity was pronounced among these LH assays. Binding activities of ostLH in mammalian and avian LH-RRAs were generally consistent with potencies in the two species of bioassays. However, a marked discrepancy was apparent in the behavior of ostLH in FSH-RRAs. Although ostLH had very low FSH activity when tested by radioimmunoassay, by bioassay, or by FSH-RRA with avian gonads, it was equipotent to ostFSH in competing for FSH-binding sites on the mammalian gonad; in this respect it was more like turkey than chicken LH. The ability of ostLH to antagonize the biological activities of ostFSH in the stimulation of cAMP by rat seminiferous tubules confirms that ostLH binds to the same functional receptors as FSH on the rat testis, even though it does not induce the characteristic physiological response associated with FSH.

Subunits of an avian (ostrich) follicle-stimulating hormone and their hybridization with subunits of mammalian gonadotropins

Follicle-stimulating hormone (FSH) from the ostrich, Struthio camelus, was dissociated by methods previously used to prepare subunits from mammalian gonadotropins. Two chemically dissimilar subunits were obtained from the ostrich FSH and these resembled the alpha- and beta-subunits of mammalian FSH in amino acid composition. The subunits were relatively inactive in radioimmunoassay, radioreceptorassay, and bioassay when tested alone, but significant activity was regenerated upon their recombination. Incubations of mixtures of one of the ostrich subunits with the opposing subunit of ovine FSH also resulted in a significant regeneration of binding and biological activity in FSH assays. These hybrid recombinants demonstrated that the species specificity of the FSH molecule is conferred entirely by the source of the beta-subunit; in fact, hybrids formed between ovine LH-alpha and either ovine or ostrich FSH-beta were more potent in FSH assays than were those containing even two homologous FSH subunits. In contrast, there was little regeneration of LH bioactivity when FSH subunits (from ostrich or sheep) were combined with the subunits of ovine LH, although some LH binding activity was obtained when the mixture contained either one of the LH subunits; i.e., even LH-alpha + FSH-beta showed significant enhancement of binding activity for LH receptors. Thus, the subunits of avian FSH show biochemical and functional homologies to those of mammalian FSH, but there is only limited interchangeability with ovine LH subunits.