The cholinergic system of the preoptic-anterior hypothalamic areas regulates the ovarian follicular population in an asymmetric way.

Atropine implants in the preoptic-anterior hypothalamic areas (POA-AHA) block ovulation. The blocking effects depend on the side of POA-AHA and the day of the estrous cycle in which the implants are inserted. Since ovulation is the result of the growth and differentiation of ovarian follicles, the purpose of this study was to analyze the changes in follicular and atresia population in the ovaries of non-ovulating rats resulting from the unilateral atropine implants in the POA-AHA. Groups of cyclic rats were implanted with atropine or cholesterol (sham treatment group) in the left (diestrus-1, diestrus-2) or the right side (estrus, diestrus-1) of the POA-AHA. The animals were sacrificed on the expected proestrus or estrus day, and the follicular population was counted and the follicles measured in both ovaries. Atropine implants inserted in the left POA-AHA on diestrus-2 resulted in lower follicular growth and atresia in the ipsilateral ovary (left one). No apparent effects were observed in the right ovary. Atropine implants inserted in the right POA-AHA on estrus day resulted in fewer numbers of small follicles in the ipsilateral ovary (right) and a greater number of pre-ovulatory ones. Present results suggest that acetylcholine, via muscarinic receptors of the POA-AHA, regulates ovarian follicular fate in an asymmetric way, and that its actions fluctuate during the estrous cycle. In addition, each ovary seems to respond differently to the POA-AHA's muscarinic signal surge on estrus and diestrus-2 days.