Evaluation of a Possible Role for the Dopamine D1 and D2 Receptors in the Steroid‐Dependent Suppression of Luteinizing Hormone Secretion in the Seasonally Anoestrous Ewe

This study was undertaken to determine whether dopaminergic suppression of pulsatile luteinizing hormone (LH) secretion during seasonal anoestrus in the ewe is mediated via the dopamine D1 or D2 receptor. This was tested by 1) assessing the response to dopamine D1 and D2 antagonists during seasonal anoestrus, and 2) determining the ability of D1 and D2 agonists to suppress pulsatile LH secretion during the breeding season. In seasonally anoestrous ewes the D2 antagonist pimozide increased LH pulse frequency although this effect did not reach significance (P = 0.07). The D1 antagonist SCH 23390 had no effect on LH pulse frequency. LH pulse amplitude and mean LH were not affected by either treatment. During the breeding season, ovariectomized oestradiol-implanted ewes were injected intracerebroventricularly with vehicle, LY 171555 (dopamine D2 agonist) and SKF 38393 (D1 agonist) with each drug tested at 50 μg and 200 μg. At the higher dose, LY 171555 significantly (P<0.05) reduced LH pulse frequency in the 2 h period immediately after treatment. Mean LH declined at both doses but only in the first hour after treatment. SKF 38393 did not affect LH pulse frequency, pulse amplitude or mean LH. These results suggest that the D1 receptor is not involved in the suppression of pulsatile LH secretion during seasonal anoestrus. Dopaminergic suppression of pulsatile LH secretion is mediated via the D2 receptor but the significance of this neurotransmitter in the seasonal suppression of LH remains to be elucidated.