Control of seasonal breeding and coat development in the goat
The aim of this work was to study the interaction of melatonin and
prolactin in the control of seasonal breeding and coat development in goats.
British Saanen goats (n=5) were maintained for 8 weeks in long days
(20L: 40) followed by 8 weeks exogenous melatonin. Group treatment was
staggered over the year in an attempt to dissociate breeding season advance
from advanced coat growth. To assess the effect of prolactin suppression, the
20L:4D treatment from January was repeated in consecutive years but followed
in March by melatonin (Year 1) or bromocriptine (Year 2). To study further
the interaction of melatonin and prolactin on coat development and
reproductive advance, goats (n=5) were maintained as follows: 1) natural
photoperiod and temperature, Jul-Juni 2) 16L: 8D, natural temperature, Jul-Deci
3) 16L:8D, 17°C, Jul-Decj 4) 8L:16D, 8°C, Dec-Jun. To study melatonin
suppression of prolactin, the secretagogues arginine vasopressin, serotonin
and thyrotropin releasing hormone were given to goats (n=6) maintained in
sequence under 8L:16D, 20L:4D and simulated short days (20L:4D+melatonin).
Depending on long-day Imelatonin treatment onset, the breeding season was
advanced from November to May, August or September. With 20L:4D followed by
melatonin or bromocriptine the spring prolactin rise was delayed. A
corresponding delay in winter coat moulting was observed only under melatonin.
However, a winter secondary to primary (SIP) follicle ratio was retained with
both treatments. Prolactin may determine coat structure, whilst melatonin
influences the duration of fibre retention.
Under conditions of summer or winter solstice hold, the seasonal
prolactin variation was inhibited and initial SIP follicle ratios were
retained. A tendency for a spontaneous change in prolactin secretion
suggested existence of an endogenous rhythm. Maintained seasonal temperature
did not influence melatonin secretion. Dusk and dawn prolactin peaks
corresponded with the rise and fall in nocturnal melatonin.
Basal prolactin concentrations and peak responses to the secretagogues
altered with photoperiod, being low under 8L:16D and increased under 20L:4D.
Exogenous melatonin under 20L:4D did not suppress prolactin to short-day
concentrations. Invol vement of another factor mediating photoperiodic changes
in prolactin secretion is suggested.