PHOTOPERIODIC MODULATION OF THE DOPAMINERGIC CONTROL OF PULSATILE LH-SECRETION IN SHEEP

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.