Opioidergic and dopaminergic effects on LH and prolactin release in pony mares at different times of the year.

The effects of dopaminergic and opioidergic systems on LH and prolactin release in Pony mares were investigated. Experiments were performed in intact mares during the non-breeding and the breeding seasons and in ovariectomized mares in November, March and May. Mares were treated with the dopamine D2-antagonist sulpiride, the opioid antagonist naloxone and naloxone plus sulpiride and saline. Naloxone alone and in combination with sulpiride increased plasma LH concentrations in intact anovulatory mares and in cyclic luteal phase mares, whereas sulpiride alone had no effect. None of the treatments influenced LH release in follicular phase mares. Naloxone administration significantly increased LH release in ovariectomized mares at all times of the year, the effect being most pronounced in March. Sulpiride administration increased plasma prolactin concentrations at all times of the year and was most pronounced in cyclic mares, whereas naloxone administration did not affect prolactin secretion. These results confirm that there are opioidergic pathways that regulate LH release and undergo seasonal changes in mares. No dopaminergic regulation of LH release or interactions between dopamine and opioids was demonstrated.