Dj. Tortonese et Ga. Lincoln, PHOTOPERIODIC MODULATION OF THE DOPAMINERGIC CONTROL OF PULSATILE LH-SECRETION IN SHEEP, Journal of Endocrinology, 143(1), 1994, pp. 25-32
This study was conducted to investigate whether the photoperiodic regu
lation of the seasonal changes in pulsatile LH secretion in the ram in
volves changes in the activity of inhibitory hypothalamic dopaminergic
(DA) pathways. To test this hypothesis, a series of experiments was c
arried out in Soay rams in which the effects of a DA-D-2 receptor anta
gonist (sulpiride) or a DA-D-2 receptor agonist (bromocriptine) on the
pulsatile secretion of LH were determined under both long and short d
ays. In each experiment blood samples were collected every 10 min for
8 h starting at the time of vehicle, sulpiride or bromocriptine inject
ions to assess concentrations of LH. Sulpiride (0.59 mg/kg, s.c.) admi
nistered to rams under long days induced an immediate and sustained in
crease in the secretion of LH that lasted for approximately 4 h (P < 0
.05; ANOVA); this LH response reflected both a rise in mean concentrat
ions (0.247 +/- 0.03 vs. 0.452 +/- 0.1 mu g/l) and an increase in the
frequency of LH pulses (0.5 +/- 0.5 vs. 2.33 +/- 0.42 pulses/8 h; P <
0.01). In contrast, under short days sulpiride had no effect. Bromocri
ptine (0.06 mg/kg, s.c.) administered to rams under long days, when LH
concentrations were low, was without effect, but when given to rams u
nder short days significantly (P < 0.05) suppressed mean LH concentrat
ions (0.627 +/- 0.08 vs. 0.320 +/- 0.02 mu g/l) and LH pulse frequency
(4.86 +/- 0.46 vs. 2.43 +/- 0.37 pulses/8 h). In an additional experi
ment, pimozide (total dose: 0.16 mg/kg, i.m.), a DA antagonist less sp
ecific for DA-D-2 receptors than sulpiride, was ineffective in modifyi
ng LH secretion in sexually inactive rams exposed to long days. These
results are consistent with the hypothesis that an inhibitory dopamine
rgic system is involved in the regulation of pulsatile LH secretion in
the ram. The induced changes in LH pulse frequency under long days (i
ncreased by sulpiride) and under short days (decreased by bromocriptin
e) indicate that, under both photoperiods, DA acts within the hypothal
amus, via a specific DA-D-2 receptor, to influence pulsatile GnRH secr
etion. A photoperiodic-induced activation of this inhibitory system ma
y therefore represent the mechanism whereby long days suppress LH secr
etion and lead to the sexually inactive state characteristic of the no
n-breeding season.