Genetic variability in plasma melatonin concentrations in ewes results from variations in pineal weight. This study investigated whether it is due to a difference in the number of pinealocytes, or in their size. Two groups of lambs were assigned before birth to being extremes (18 High and 21 Low) by calculating their genetic value on the basis of the melatonin concentrations of their parents. Lambs were bled from 1 week of age until 14 weeks of age. Pineal gland, brain and pituitary weights, length and width of the brain, and length of the hypothalamus were recorded. A significant effect (ANOVA) of genetic group (P<0.05) and age (P<0.05) was detected on mean nocturnal plasma melatonin concentrations, as soon as the first week after birth (mean +/- s.e.m.; High: 51.7 +/- 10.7 vs Low: 31.9 +/- 3.2 pg/ml). There was no difference between the two genetic groups in any of the brain parameters measured, but the pineal glands of the High group were heavier and contained significantly more pinealocytes (High: 27.8 +/- 2.4 vs Low: 21.0 +/- 2.4 x 10(6); P<0.05) than those in the Low group. The mean size of pinealocytes did not differ between the two genetic groups. Thus, the genetic variability in nocturnal plasma melatonin concentrations in sheep is expressed by 1 week of age and higher levels of secretion are the consequence of larger pineal glands containing a greater number of pinealocytes.
A Gomez Brunet, A Gomez Brunet, B Malpaux, A Daveau, C Taragnat and P Chemineau
J Santiago-Moreno, A Gómez-Brunet, A Toledano-Díaz, R Salas-Vega, F Gómez-Guillamón and A López-Sebastián
This work examines the effect of testosterone secretion and photoperiod on seasonal changes in horn growth and sperm variables in the Iberian ibex (Capra pyrenaica), here used as a model for polygynous wild bovids. The hypothesis that high levels of testosterone provide an endocrine signal that inhibits horn growth in autumn was tested by assessing the effect of cyproterone acetate (CA), an anti-androgen, administered in October – coinciding with the period of natural increases in plasma testosterone concentrations – under different photoperiodic conditions (natural photoperiod and artificial long days). The persistence of horn growth during autumn in all ibexes held under the long-day photoperiodic conditions clearly shows that horn growth regulation in the mating season is primarily modulated by day length and not by a fall in testosterone concentration. A retrospectively designed second experiment involving testosterone propionate (TP) administration in April (when horns are growing) was then undertaken to confirm that high levels of testosterone do not inhibit horn growth. Overall, the results strongly suggest that the rise in testosterone secretion during the autumn mating season does not act as an endocrine signal for the arrest of horn growth, although the rate of horn growth before the mating season may be related to springtime testosterone levels. A direct relationship was seen between the rate of horn growth and the incidence of sperm abnormalities. Neither CA treatment in October nor TP administration in April affected the studied sperm variables. By contrast, CA treatment plus artificial long days in autumn had a negative effect on sperm motility and sperm morphology.