Seasonal variation and opioidergic regulation of growth hormone release incyclic, ovariectomized, and pregnant pony mares

Modulation of reproductive functions is one of the multiple effects of grow th hormone (GH). To investigate effects of reproductive functions on GH rel ease in the horse, plasma CH concentrations in ovary-intact (n = 7) and ova riectomized (n = 8) mares during the anovulatory and breeding seasons and i n pregnant mares (n = 6) at various stages of gestation were determined. To analyze an opioidergic regulation of GH release, repeated blood samples we re taken over 3 h, and mares were injected with the opioid antagonist nalox one (0.5 mg/kg i.v.) or saline. GH was determined by RIA with an antiserum raised against porcine GH and equine GH as standard. In ovariectomized and ovary-intact, cyclic mares, CH concentrations were low and not different be tween the two groups in November and December. CH concentrations increased significantly (P < 0.05) in cyclic mares during May and lune but were not a ffected by stage of the cycle and were low in ovariectomized mares. In preg nant mares, plasma CH concentrations remained high throughout pregnancy and did not decrease during winter but increased significantly (P < 0.05) post partum. Naloxone induced a significant GH release in ovary-intact mares; th is response was most pronounced (P < 0.05) during the breeding season. Nalo xone did not affect CH in ovariectomized mares. During pregnancy, naloxone induced a significant release of GH around Day 280 (P < 0.05) but not at ot her times of pregnancy. In conclusion, GH release is influenced by season. The seasonal changes depend on ovarian factors, are absent in ovariectomize d mares, and can be modulated by pregnancy. GH release is regulated at leas t in part by opioidergic pathways.