Hypothalamic and pituitary hormones

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Numerous hormones have an effect upon or are produced by the hypothalamus or the pituitary. These will be discussed in turn, but it must be remembered that the hormones do not work in isolation; rather, they form part of an integrated system of whole body control.

Melatonin

As indicated previously, one of the major means of controlling the hypothalamus is via the hormone melatonin, which is produced by the pineal gland. Melatonin is secreted during the hours of darkness and, due to its antigonadotrophic nature, it dominates the system and suppresses hypo-thalamic activity during the periods of its secretion (Burns et al., 1982; Argo et al., 1991). This response to daylength results in melatonin secretion being divided into two phases: photophase during the daytime and scotophase during the hours of darkness (Grubaugh, 1982). Within this pattern, secretion is episodic in fashion, with episodes of greater frequency and amplitude being evident during the hours of darkness. Additionally, melatonin is secreted in a circadian fashion, maintaining a 24-h pattern of release despite exposure to continuous daylight or darkness. Photorefractoriness to continual long days does occur; seasonal cycles in scrotal width and output of spermatozoa continue despite continual treatment of 16 h light, 8 h dark (Clay et al., 1987). Melatonin secretion, therefore, demonstrates an inherent rhythm which is modified by external factors, such as daylength.

The exact site and mode of action of melatonin is unclear but its effect is upon GnRH production. It is possible that it acts at the level of the median eminence, where the split between GnRH-associated protein (GAP) and GnRH occurs, allowing the release of GnRH (Strauss et al., 1979).

Gonadotrophin-releasing hormone

Gonadotrophin-releasing hormone (GnRH) is a decapeptide initially joined to a 56 amino acid peptide called GAP as part of a larger molecule. As a dual structure it is stored, as granules, within the median eminence. When the environment is appropriate it splits into GnRH and GAP and is discharged as such down the hypophyseal portal vessels to the anterior pituitary, where it binds to receptors on the gonadotrophe of the pituitary (Strauss et al., 1979). Eighty per cent of the GnRH so released passes along this route; the remaining 20% passes back through the median eminence and on to the higher centres of the brain driving sexual behaviour (Pozor et al., 1991). GnRH is secreted in both a pulsatile and continuous (tonic) fashion. Pulse frequency during periods of sexual rest are 1-2 per day, both within and outside the breeding season. However, within the breeding season the tonic levels of GnRH are significantly elevated in the absence of melatonin inhibition. Sexual activity increases pulse frequency to two per hour or more (Irvine and Alexander, 1993).

The action of GnRH upon the pituitary results directly in the release of two main gonadotrophic hormones in the male: LH and FSH (Blue et al., 1991; Seamens et al., 1991). The pituitary may also secrete prolactin, which, as will be evident later, may be involved in the control of stallion reproduction. Exogenous treatment with GnRH results in elevated levels of LH and FSH; conversely, active immunization against GnRH has been shown to suppress LH and FSH production (Schanbacher and Pratt, 1985).

Luteinizing hormone and follicle-stimulating hormone

LH and FSH secretion occurs as a direct result of GnRH stimulation of the anterior pituitary. They are both glycoproteins, consisting of two subunits, a and P. The a subunit is species specific and is the same for LH, FSH, thyroid-stimulating hormone (TSH) and equine chorionic gonadotrophin (eCG). The P subunit differs for each of the hormones and determines the biological function of the hormone (Alexander and Irvine, 1993).

LH, previously known as interstitial cell-stimulating hormone (ICSH), is secreted in a pulsatile fashion (Thompson et al., 1985, 1986; Clay et al., 1988). During the breeding season the level of tonic LH secretion is reported to be 4-7 ng ml-1 with a normal pulse frequency of 0.8-0.9 pulses per hour, these pulses having an amplitude of 2-4.5 ng ml-1 and a duration of 43-48 min (Blanchard et al., 1990). However, there are considerable differences between stallions, in addition to which a circadian rhythm has been reported, with higher concentrations in general being observed during the hours of daylight. This circadian rhythm in LH secretion is mimicked by similar changes in testosterone secretion (Lang et al., 1995). The relatively high tonic levels of

LH in stallions during the breeding season mean that the pulses of LH secretion are not always detected in blood collected from the jugular vein (Thompson et al., 1985). The site of action for LH is the testes, and LH levels are positively correlated with daily spermatozoan production (DSP) per gram of testes (Blanchard et al., 1990).

FSH, in common with LH, is produced in a pulsatile fashion, in response to the pulsatile release of GnRH. FSH tonic level of secretion is reported to be 1-14 ng ml-1 during the breeding season, accompanied by a pulse frequency of 0.7-1.0 h-1, a pulse amplitude of 5-7 ng ml-1 and a duration of 31-55 min (Blanchard et al., 1990). However, FSH shows a greater variability in secretion patterns than both LH and GnRH (Thompson et al., 1985; Clay, 1988). Each pulse of FSH has been reported to be associated with an episode of LH, though other workers have failed to demonstrate such a close association (Blanchard et al., 1990). Seasonal variation in FSH levels is less dramatic than is the case with LH (Harris et al., 1983; Johnson and Thompson, 1983; Thompson et al., 1986). It is evident, therefore, that the observed variation in testicular function with season is more likely to be due to a variation in secretion of LH rather than FSH (Johnson and Thompson, 1983).

A feedback mechanism is known to exist that controls LH and FSH production in the stallion and which originates from the testes. In the absence of testes (for example, in geldings), LH levels are naturally high; however, the administration of dihydrotestosterone to such animals results in a depression in LH secretion and eventually in LH production (Thompson et al., 1979b). This can be further demonstrated by the treatment of stallions with exogenous testosterone. Such treatment results in a reduction in circulating FSH levels (Ashley et al., 1986). Two other hormones, activin and inhibin, which originate in the testes are also known to be involved; they have, respectively, an activating and inhibitory role on FSH production (Amann, 1993b).

Prolactin

Prolactin is a single-chain protein, made up of 199 amino acids (Nett, 1993). It is produced by the anterior pituitary under the control of dopamine, originating from the hypothalamus (Johnson and Becker, 1987). In stallions, the variation in prolactin concentration is governed mainly by: season, the highest levels being seen during periods of long daylength (Thompson et al., 1986; Thompson and Johnson, 1987; Evans et al., 1991); age, with levels increasing until maturity at 5 years of age; and sexual activity (Thompson and Johnson, 1987; Rabb et al., 1989).

The function that prolactin plays in the stallion is unclear. In other animals, prolactin is associated with the function of the accessory glands and Leidig cells, enhancing the effect of LH on spermatogenesis. Such an association has not been found as yet in the stallion, though treatment with bromocriptine is reported to decrease the volume of seminal plasma, and a link has been established between sexual activity, and elevated concentrations of prolactin and cortisol (Nett, 1993; Thomson et al., 1996).

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