Seasonal relationships between dopamine D1 and D2 receptor and equine FSH receptor mRNA in equine ovarian epithelium

Dopamine (DA) blockade during anestrus or early spring transition can facilitate ovarian recrudescence and advance the timing of the first ovulation of the season. Some laboratories have reported variable results using DA antagonists to stimulate follicular growth during the mid-portion of the anestrual period. Differences in DA antagonist efficacy may be due to the FSH secretory status of the anestrous mare and the presence or absence of functional ovarian FSH receptors. We hypothesize that direct ovarian dopaminergic input can affect follicular growth through regulation of FSH receptor (FSHr) populations. To investigate this, the amount of DA D1 and D2 receptor (D1r, D2r) and FSHr mRNA was quantified in ovarian tissues in anestrous and mares expressing estrus at typical intervals that are detected during the breeding season. Ovaries (n=26) were collected from 10 anestrous mares and 13 mares that had initiated estrous cycles (n=8 luteal; n=5 follicular phase). The quantity of D1r and D2r mRNA and FSHr mRNA was determined in cortex of both groups and granulosa/theca (those having initiated estrous cycles) tissues by semi-quantitative polymerase chain reaction using the comparative cycle time method. The reference gene was glyceraldehyde-3-phosphate dehydrogenase. The fold-change for each sample was calculated based on a calibrator sample. Fold-change values for D1r and D2r were the dependent variable and tissue was the independent variable in a one-way ANOVA. Results of fold-change in FSHr were compared by ANCOVA due to unequal sample sizes from each mare. Correlations between receptors within each tissue type were determined. For each receptor type and tissue, correlations between follicular and luteal phases were determined. The fold-change of D1r mRNA was less than D2r mRNA in all tissue types and between seasons. The quantity of D2r message in ovarian cortex was greater (p<0.05) during anestrus than after estrous cycles had been initiated. Fold-change in D2r in granulosa/theca was not different dependant on estrous cycle phase or follicle size. Quantity of FSHr mRNA was less in anestrous ovarian cortex and greater after estrous cycles had been initiated. FSHr mRNA fold-change in the ovarian cortex after estrous cycle initiation was not different between estrous cycle phases, but was greater in smaller (<30mm) follicles compared with larger (>/=30mm) follicles. We have demonstrated an inverse temporal relation between ovarian D2r and FSHr in mares dependant upon season. The functional significance of this relationship deserves further study.