Macropodid marsupial testicular gonadotrophin receptors and their use in assays for marsupial gonadotrophins

The hypothalamic-pituitary-ovarian axis and manipulations of the oestrous cycle in the brushtail possum

The main purpose of this review is to provide a comprehensive update on what is known about the regulatory mechanisms of the hypothalamic-pituitary-ovarian axis in the brushtail possum, and to report on the outcomes of attempts made to manipulate by hormonal means, these processes in the possum. Over the last 15 years, several unique features of possum reproductive physiology have been discovered. These include an extended follicular phase despite elevated concentrations of FSH during the luteal phase, and early expression of LH receptors on granulosa cells of small antral follicles, suggesting a different mechanism for the selection of a dominant follicle. The use of routine synchronisation protocols that are effective in eutherians has failed to be effective in possums, and so the ability to reliably synchronise oestrus in this species remains a challenge.

Follicle-stimulating hormone in the brushtail possum (Trichosurus vulpecula): purification, characterization, and radioimmunoassay

Follicle-stimulating hormone (FSH) was purified from brushtail possum (Trichosurus vulpecula) pituitary glands by using the following purification techniques: fractional ammonium sulfate precipitation, triazinyl-dye affinity chromatography, hydrophobic interaction chromatography, and gel filtration. A yield of 18 micrograms of FSH per gram of pituitary, with a recovery of 12%, was obtained from 1400 glands (20.3 g wet weight). The purified FSH activity per gram of protein was 1320 times more potent than the initial pituitary homogenate. Contamination with possum luteinizing hormone (LH) was < 0.02%. The amino acid composition of possum FSH was similar to that of ovine FSH. Amino-terminal sequencing for 11 cycles indicated that the alpha subunit has the same sequence as ovine FSH except for residue 7, where the possum FSH alpha subunit contains isoleucine compared to the ovine subunit which contains threonine. The beta subunit has two substitutions in the first 11 residues and does not contain the N terminal serine that is found in ovine FSH. Amino acid sequencing did not detect any contaminating proteins. Possum FSH bound possum and bovine testicular receptors with similar affinities. It was also able to stimulate in vitro cAMP production by Chinese hamster ovary cells which express recombinant FSH receptors. In the receptor assays and the bioassay possum FSH has about 21% of the potency of ovine FSH (USDA-oFSH-19-SIAFP-RP2). An RIA was developed for possum FSH using 125I-possum FSH and an antiserum raised against human FSH. The RIA has a sensitivity of 0.3 ng/ml, a 50% displacement of 2.7 ng/ml, and a cross reactivity of 0.05% against possum LH. Plasma FSH levels in male possums (10.4 +/- 2.4 ng/ml, n = 4) were higher (P < 0.05) than levels in females (1.0 +/- 0.1 ng/ml, n = 4). Five days after gonadectomizing these possums the plasma FSH levels increased (P < 0.05) to 27.1 +/- 0.2 ng/ml in the males and to 6.6 +/- 2.0 ng/ml in the females. In summary, we have purified and partially characterized possum FSH. We have also set up an RIA for the hormone and shown that males have higher levels than females and that plasma FSH increases after gonadectomy.

Testosterone-LH response and episodic secretion in the male marsupial, Dasyurus viverrinus

The pattern of testosterone-LH secretion, feedback mechanism, and periodicity of release were investigated in the male marsupial eastern quoll. Testosterone secretion is controlled directly by LH and in the wild, the secretion of both are synchronous and show a major peak at breeding time. An inverse relationship between the secretion of LH and testosterone occurs after castration which supports the hypothesis that a negative feedback mechanism exists between the gonads and the pituitary gland as in other mammals. These hormones fluctuate in a cyclic manner over 24 hr.

Specific gonadotrophin binding sites in the bullfrog testis

We demonstrated the presence of specific binding sites for bullfrog luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in a testicular crude plasma membrane fraction of the bullfrog, Rana catesbeiana. The equilibrium analysis of the binding showed that a straight line was fitted to the Scatchard plot of LH and a curvilinear line to that of FSH, suggesting the presence of single-type binding sites for LH and the presence of multiple-type binding sites or negative cooperactivity of the binding for FSH. Competition experiments showed that bullfrog LH could replace the specific binding of bullfrog FSH and vice versa. Binding affinity of bullfrog LH to radioiodinated bullfrog FSH binding sites was as high as that of intact bullfrog FSH to the same sites. In contrast, binding properties of bullfrog FSH to radioiodinated bullfrog LH binding sites were not simple: the competition curve obtained with FSH against the binding of bullfrog LH had an extremely low slope value and was not parallel to that obtained with LH. These results suggest that differentiation of LH and FSH receptors is not complete in this species, although it has been reported that these hormones have separate actions on the testis. Competition experiments further showed that bullfrog testicular LH and FSH receptors possessed higher affinities for gonadotrophins of homologous or closely related animal species compared with phylogenically distant groups. Species specificity of the hormone-receptor interaction may cause species specificity of the gonadotrophin action.

The embryological development and cytodifferentiation of the anterior pituitary in the marsupial, Isoodon macrourus

Light and electron microscopy were used to study the development of the anterior pituitary gland from fetal stages to the end of pouch life in the marsupial I. macrourus. The early morphological development of the anterior pituitary in I. macrourus follows a similar pattern of events to that described for eutherians. Rathke's and Seessel's pouches were present in 10 1/2 day old embryos. At birth these pouches had formed a multi-chambered vesicle which was still connected to the stomodeum by a thin cord of tissue. A small number of granules (200-400 nm dia.) were found in cells at birth. These cells could not be classified on ultrastructural features but alcian blue-periodic acid Schiffs-orange G staining suggested one cell type was possibly a presumptive thyrotroph. There were no capillaries in the pars distalis at birth. The cords connecting Rathke's and Seessel's pouches to the stomodeum were located at the site of the periosteal bud of the developing basisphenoid which commenced to ossify at 7 days. At this stage presumptive thyrotrophs, gonadotrophs, and somatotrophs could be distinguished using alcian blue-periodic acid Schiffs-orange G staining. However, five cell types could be categorised at the same age using ultrastructural characteristics alone. Precise names for these cells are unavailable but two closely resemble presumptive mammotrophs and thyrotrophs described for another marsupial M. eugenii. By 13 days after birth the anterior pituitary of I. macrourus had become vascular and acidophils were concentrated in a posterior zone. There was little gross morphological change from 13 to 66 days after birth by which the time weaning has occurred. Cilia were seen in cells of the anterior pituitary and mitosis of granulated cells was observed from birth onwards. There is a considerable range of variation in pituitary cytogenesis amongst marsupials, and its functional significance awaits further investigation.