Photoperiodic modulation of the dopaminergic control of pulsatile LH secretion in sheep

This study was conducted to investigate whether the photoperiodic regulation of the seasonal changes in pulsatile LH secretion in the ram involves changes in the activity of inhibitory hypothalamic dopaminergic (DA) pathways. To test this hypothesis, a series of experiments was carried out in Soay rams in which the effects of a DA-D2 receptor antagonist (sulpiride) or a DA-D2 receptor agonist (bromocriptine) on the pulsatile secretion of LH were determined under both long and short days. In each experiment blood samples were collected every 10 min for 8 h starting at the time of vehicle, sulpiride or bromocriptine injections to assess concentrations of LH. Sulpiride (0.59 mg/kg, s.c.) administered to rams under long days induced an immediate and sustained increase in the secretion of LH that lasted for approximately 4 h (P < 0.05; ANOVA); this LH response reflected both a rise in mean concentrations (0.247 +/- 0.03 vs. 0.452 +/- 0.1 microgram/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. Bromocriptine (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 under short days significantly (P < 0.05) suppressed mean LH concentrations (0.627 +/- 0.08 vs. 0.320 +/- 0.02 microgram/l) and LH pulse frequency (4.86 +/- 0.46 vs. 2.43 +/- 0.37 pulses/8 h).(ABSTRACT TRUNCATED AT 250 WORDS)