Abstract
Searching for the role of prolactin (PRL) in controlling gonadotropic axis activity
in sheep, we studied the effects of prolonged, intracerebroventricular (icv) PRL infusion
on luteinizing hormone (LH) secretion and catecholaminergic activity in the hypothalamic
infundibular nuclei/median eminence (IN/ME) in sexually active ewes during the periovulatory
period. Three groups of animals received the following treatments: 1) icv infusion
of PRL at a dose of 200 µg/day (Lower dose, n = 5); 2) icv infusion of PRL at a dose
of 400 µg/day (Higher dose, n = 6), and 3) icv infusion of the vehicle (control, n
= 5). Each dose of PRL was infused in a pulsatile manner, 4 × 50 µg/h and 4 × 100
µg/h, in 30-min intervals, respectively, during four consecutive days before oncoming
ovulation. The estrous behavior of ewes following treatments was also monitored as
a determinant of the GnRH/LH surge. Two series of blood collections were made in every
ewe, the first on the day preceding the infusion (day 0 of the experiment), the second
on the day after the infusion (day 5 of the experiment). In addition, on day 5 of
the experiment, perfusions of the IN/ME were made by the push-pull method, either
in control or lower dose-treated animals. It was shown that a significant (p < 0.01,
p < 0.001) increase in tonic LH secretion during the periovulatory period remained
in ewes irrespective of the kind of infusion. No statistical differences were found
in LH pulse frequency, amplitude, or in the length of the pulse when compared with
values from day 0 and 5 of the experiment within each group. A significant (p < 0.001)
increase in IN/ME perfusate concentrations of dopamine and noradrenaline metabolites
was noted in PRL-treated ewes in comparison with those in the control. The estrous
behavior in PRL-treated animals was delayed for a few days, 3.80 ± 0.80 days at the
lower dose (p < 0.01), and 2.83 ± 0.98 days at the higher dose (p < 0.05) in comparison
with the control, 0.20 ± 0.20 days. These data indicate that maintenance of an increased
PRL concentration within the central nervous system (CNS) for a few days before oncoming
ovulation has no inhibitory effect on tonic LH secretion. A few-day shift of the preovulatory
GnRH/LH surge, as determined by estrous behavior, might, however, be a consequence
of the PRL-induced increase in catecholamine turnover in the IN/ME.
Key words
Prolactin - LH - dopamine - noradrenaline - estrous behavior
References
Neuroendocrine control of the ovine estrous cycle. Knobil E, Neill JD The Physiology of Reproduction. 2nd ed., Vol. 2. New York; Raven
Press 1994: 659-710 in vitro by anterior pituitaries from lactating and cycling rats.
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A receptors in the control of GnRH and β-endorphin release, and catecholaminergic activity
in the preoptic area in anestrous ewes.
Exp Clin Endocrinol Diabetes.
2002;
110
336-342
t.misztal@ifzz.pan.pl
