Abstract:
Recent work has implicated stimulatory kisspeptin neurons in the arcuate nucleus (ARC) as important
for seasonal changes in reproductive function in sheep, but earlier studies support a role for
inhibitory A15 dopaminergic (DA) neurons in the suppression of GnRH (and LH) pulse frequency
in the nonbreeding (anestrous) season. Because A15 neurons project to the ARC, we performed
three experiments to test the hypothesis that A15 neurons act via ARC kisspeptin neurons to inhibit
LH in anestrus: 1) we used dual immunocytochemistry to determine whether these ARC neurons
contain D2 dopamine receptor (D2-R), the receptor responsible for inhibition of LH in anestrus; 2)
wetested the ability of local administration of sulpiride, a D2-R antagonist, into theARCto increase
LH secretion in anestrus; and 3) we determined whether an antagonist to the kisspeptin receptor
could block the increase in LH secretion induced by sulpiride in anestrus. In experiment 1, 40% of
this ARC neuronal subpopulation contained D2-R in breeding season ewes, but this increased to
approximately 80% in anestrus. In experiment 2, local microinjection of the two highest doses (10
and 50 nmol) of sulpiride into the ARC significantly increased LH pulse frequency to levels 3 times
that seen with vehicle injections. Finally, intracerebroventricular infusion of a kisspeptin receptor
antagonist completely blocked the increase in LH pulse frequency induced by systemic administration
of sulpiride to anestrous ewes. These results support the hypothesis that DA acts to inhibit
GnRH (and LH) secretion in anestrus by suppressing the activity of ARC kisspeptin neurons.
