Intrafollicular and Systemic Dopamine, Noradrenaline and Adrenaline Concentrations in Cycling Mares

SIMPLE SUMMARY: This study provides new evidence on the physiological changes of catecholamines in follicular fluid during the follicular growth in the mare. Both dopamine and epinephrine increase in the follicular fluid with the advance of follicular development, although norepinephrine decreases. These changes could be related to the existence of systemic, autocrine and/or paracrine mechanisms of synthesis, metabolism and interconversion of catecholamines for the regulation of follicular growth and development. ABSTRACT: In some species, catecholamines in follicular fluid (FF) are related to local physiological events responsible for the regulation of ovarian functions and oocyte maturation. The aim of the present study was to determine and compare intrafollicular and systemic concentrations of dopamine (DA), noradrenaline (NA) and adrenaline (AD) in cycling mares. Sixty ovaries were collected during breeding season from 30 mares raised for slaughterhouse meat production, with clinically normal reproductive tracts, were evaluated. Blood samples were collected prior to slaughter. Follicles were classified into three categories in relation to size: small (20–30 mm; n = 20), medium (≥31–40 mm; n = 20) and large (≥41 mm; n = 20). Follicular fluid (FF) samples were extracted from each follicle. Intrafollicular DA, NA and AD concentrations were significantly higher than the systemic concentrations (p < 0.05). Intrafollicular DA concentrations were higher in medium than small and large follicles (p < 0.05). Intrafollicular NA concentrations were higher in small than medium and large follicles (p < 0.05). Intrafollicular AD concentrations were higher in large than small and medium follicles (p < 0.05). Follicle diameter was significantly and negatively correlated with NA and AD (p < 0.05). A significant correlation of the same hormone concentration in FF and in systemic fluid was observed (p < 0.05). In summary, the FF can serve as an intraovarian catecholamine-storing compartment, with the ability to release neurotransmitters in a regulated way. These results provide novel insights into the neuronal nature of the follicle, suggesting the involvement of catecholamines in normal ovarian functions in mares.