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Research Synopsis 3

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Neuroendocrine and Genetic Investigations

Fig 5: GnRH self-priming of LH release in DLS rams and ewes in response to two small injections of GnRH (times indicated by arrows)

GnRH self-priming of LH release from the pituitary

Episodic LH releases are caused by GnRH releases coming from the brain. A sudden rise in GnRH pulse frequency can increase both the frequency and the amplitude of LH pulses. We have shown that, as in the ewe, GnRH will enhance or "self-prime" LH release in the ram if estrogen is present. The LH response to GnRH priming tests is similar in rams and ewes (Fig 5), suggesting that this mechanism is not sexually dimorphic. In young rams, LH releases often occur just minutes apart. The second release of a close pair is usually twice as large as the first. This is likely not due to GnRH priming, whose role in the ram remains an enigma.

Melatonin signaling and reproductive-endocrine rhythms

The timing of daily and yearly endocrine rhythms is set by light-dark information coming to the brain in the form of a melatonin signal. In DLS sheep, the duration of a nocturnal rise in melatonin secretion does not differ between sexes in any season, but the magnitude of the rise is often greater in ewes. This is especially so in the summer (Fig 6), which may explain why rams and ewes differ in the timing of their daily prolactin rhythm in this season. Duration of the nighttime rise in melatonin is known to regulate LH release. DLS sheep are able to produce more frequent LH pulses in the autumn because changes in the duration of the melatonin signal have lessened the negative effect that estrogen has on the brain and GnRH release. Our findings indicate that seasonal changes in melatonin can signal changes in GnRH and LH pulsatility in more direct ways as well.

Fig 6: Nocturnal rise in melatonin in DLS rams and ewes in summer

Fig 7: Breed diversity in sperm production
and libido of rams (D Dorset, FL Finnish Landrace,
S Suffolk, SB Scottish Blackface)

Breed and strain effects on seasonal reproduction

On the prairies (latitude 50ºN), we noted that testis size and testosterone secretion peaked for shorter periods in the summer and autumn in Suffolk and Scottish Blackface rams than in Dorset and Finnish Landrace rams. Testes redevelopment was timed by a rise in FSH secretion in all four breeds. Testosterone was more strongly related to sperm production in the two highly seasonal breeds. Other breed differences were also observed (Fig 7). In southern Québec (latitude 45ºN), we examined adult DLS rams born to dams selected for early lambing ability. Testes of these rams began redeveloping several weeks before those of control DLS rams, although FSH and testosterone were similar between strains.

Pubertal development and prediction of adult reproductive traits

In both Suffolk and DLS rams, FSH and pulsatile LH release increase between birth and 4 months of age. These changes are soon followed by increases in FSH and LH receptors (Fig 8), testosterone secretion and testes mass. The endocrine events leading to pubertal onset closely resemble those that occur in the adult ram during testicular recrudescence. Specific juvenile traits have proven to be useful in predicting testis size in early adulthood. In our studies on Suffolk rams, single traits (testosterone or testis size) at 6 months of age or a combination of traits (body weight and testis size) at 10 months had strong predictive value.

Fig 8: Testicular LH and FSH receptors in DLS ram lambs
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2008 L. Sanford. Email