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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.

Asymmetric mRNA expression was found in preoptic anterior and hypothalamic anterior areas of the two estrogen receptor isoforms and the gonadotropin-releasing hormone. On the right side of these areas, estrogen receptor alpha mRNA expression reached its peak on estrus day, while on the left side the peak was reached on proestrus day. Estrogen receptor beta mRNA expression peaked on both sides on the same day, diestrous-2 day, but at different hours, showing a sustained expression for the next measured hour on the left side, while peaking and dropping abruptly on the right side. Gonadotropin-releasing hormone also peaked on both sides on diestrous-2 day, being the left side peak expression significantly lower than the peak expression at the right side. The side expression differences suggest that different sides of the before mentioned areas may play different roles of endocrine reproductive functions, while differences of expression at different times may suggest interaction between sides for the same functions.