The final level in the hypothalamic-pituitary-gonadal axis in the stallion is the testes. Within the testes there are two major cell populations or target organs: the Leidig cells and the Sertoli cells. These two cell populations have two major roles: steroid hormone production and the production of spermatozoa. A close feedback association must exist between these various functional structures within the testes. The exact nature of their control is unclear in the horse, much evidence being extrapolated from other mammals. However, the primary endocrine function of the testes is the production of testosterone by the Leidig cells. Other hormones are also produced, as secondary endocrine products, their importance becoming increasingly evident with current research (Roser, 1997). These secondary hormones include activin and inhibin, produced by the Sertoli cells, as well as oxytocin, produced by the Leidig cells, and oestrogens, the production site of which is unclear but is likely also to be the Leidig cells (Roser, 1997).
Testosterone is the major androgenic steroid produced by the Leidig cells of the testes (Savard and Goldziecher, 1960; Oh and Tamaoki, 1970). Its effect is both local, within the testis, and systemic. Its local effect is on the neighbouring Sertoli cells, where it controls the final stages of spermatogenesis, the process being initially driven by FSH. Its effect upon spermatozoan production is apparently one of decreasing the rate of germinal cell degeneration and increasing the production of spermatogonia, rather than by decreasing the length of the spermatogenic cycle (Setchell, 1982; Johnson, 1991a). Testosterone is essential to drive spermatogenesis in the adult stallion (Amann, 1981b). Its passage to the Sertoli cells is active, rather than passive, via attachment to androgen-binding protein (ABP). This active transfer is evident from the elevated concentrations of testosterone bathing the seminiferous tubules (up to 70 mg g-1 of testicular parenchyma) (Johnson and Thompson, 1987), which is significantly higher than that in blood serum (300 pg ml-1) (Johnson and Thompson, 1983). Its major systemic functions include: the development and maintenance of the male genitalia; the exhibition of characteristic stallion behaviour, including libido; and differential muscle growth. The average concentration of circulating testosterone varies considerably with season (Pickett and Voss, 1972) (Fig. 3.25).
Figure 3.25 shows that testosterone concentrations are at their highest during the spring and summer months and lowest during the autumn and winter (Berndtson et al., 1974; Johnson and Thompson, 1983). If Fig. 3.25 is compared with Figs 3.19-3.23, it is evident that testosterone concentrations are correlated with sexual behaviour and reproductive efficiency (Byers et al., 1983). This variation in plasma testosterone concentrations is evident at the testes level as well as the whole body plasma level (Berndtson et al., 1983). Testosterone concentrations not only vary with season but also show a diurnal variation (Sharman, 1976; Pickett et al., 1989): testosterone plasma
concentrations were significantly higher at 6.00 hours and 18.00 hours, the average increase being 100% (Pickett et al., 1989). Sexual activity shows a similar diurnal variation and it may be postulated that, in the wild, mating activity is increased at dusk and dawn, as at these times the horse is at least risk from predators. Testosterone levels are also reported to vary with testis size and to increase with age. It has been postulated that this may account for the variation in spermatozoan production evident between stallions within a single breeding season (Berndtson and Jones, 1989).
Superimposed upon these major variations in testosterone concentration are changes associated with the episodic nature of testosterone release. In general, testosterone pulses are linked with LH pulses; therefore, testosterone characteristically shows a continuous level of tonic secretion, with the occasional superimposed episodic increase as a result of an LH pulse. This continuous tonic level of testosterone secretion between episodes in the absence of LH pulse secretion and the considerable variation in the response seen to LH pulses indicate that pulsatile LH release may not be the sole requirement for testosterone release (Squires et al., 1977; Thompson et al., 1985; Thompson, 1992; Roser, 1997), though elevation of endogenous LH concentration by administration of GnRH or the use of human chorionic gonadotrophin (hCG) does result in elevated testosterone concentrations (Roser, 1995).
Inhibin and activin are closely related glycoproteins, sharing common sub-units termed a, Pa or Pb. Activin is a combination of the two P subunits and inhibin is a combination of the a plus one of the P subunits. The exact configuration of the form of activin and inhibin produced by the stallion's testis is yet to be ascertained (Bardin, 1989). Both inhibin and activin are produced by the Sertoli cells and act upon the anterior pituitary, inhibiting (inhibin) and activating (activin) its function. Both hormones act primarily upon FSH production, modulating the response of the pituitary to GnRH stimulation (Amann, 1993b; Roser, 1997).
Oxytocin is a neuropeptide, synthesized as part of a larger molecule, neuro-physin (OT-NP) (Nett, 1993). Oxytocin is known to be produced by the Leidig cells of the testis of the ram and the rat and to pass into the interstitial fluid (Knickerbocker et al., 1988; Pickering et al., 1989). From here it passes to the Sertoli cells and on to the lumen of the seminiferous tubules; it seems that it may also be secreted into the lumen of the seminiferous tubules by the rete testis. Its function is unclear, but it may well be involved in the contraction of the seminiferous tubules and hence the movement of spermatozoa towards the epidi-dymis and beyond. Although these conclusions have been drawn from work in animals other than the horse, it is likely that there is a similar function for oxytocin in the horse's testis. Burns et al. (1981) demonstrated a link between the level of sexual activity and oxytocin concentrations in stallion's plasma.
Dihydrotestosterone is a derivative of testosterone, produced in, and released from, the Leidig cells. Its function is likely to be as an additional negative feedback on pituitary function. It also has an effect on the development of male characteristics, but to a lesser extent than testosterone.
The stallion's testis contains significantly higher concentrations of oestrogens than would be expected from work carried out on other mammals (Ganjam and Kenney, 1975). The major oestrogens are oestradiol and oestrone plus the two unique equine oestrogens: equilin and equilenin. It is not clear where their site of production is, but Amann (1993b) suggests that the Leidig cells are responsible for some of the oestrogen production. The function of these oestrogens is also unclear. However, the fact that concentrations of oestrogen in venous return from the testis are elevated, coupled with the discovery that the administration of oestrogen in the presence of testosterone results in an enhanced decline in LH and FSH production, indicates that oestrogen may act as a negative inhibitor of hypothalamic-pituitary function acting in union with testosterone (Thompson et al., 1979b; Seamens et al., 1991). Seminal plasma concentrations of total oestrogens (4447 pg ml-1) are even higher than those observed in blood serum (2497 pg ml-1). If specifically oestrone sulphate concentrations are measured, it becomes apparent that the majority of oestrogens in seminal plasma are oestrone sulphate (4116 pg ml-1), with total free oestrogens a minor component (330.5 pg ml-1). The concentration of oestradiol 17p has been reported by some workers to be higher in seminal plasma (73.4 ± 87.4 pg ml-1) than in blood plasma (40.0 ± 27.6 pg ml-1) (Claus et al., 1992). This difference is disputed by others (Landeck, 1997). It is interesting to note that the work by Landeck (1997) suggested that oestradiol 17p is higher in the spermatozoan fraction of semen than in seminal plasma.
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