Muscarinic Receptors Types 1 and 2 in the Preoptic-Anterior Hypothalamic Areas Regulate Ovulation Unequally in the Rat Oestrous Cycle

Abstract

Muscarinic receptors types 1 (m ₁AChR) and 2 (m ₂AChR) in the preoptic and anterior hypothalamus areas (POA-AHA) were counted, and the effects of blocking these receptors on spontaneous ovulation were analysed throughout the rat oestrous cycle. Rats in each phase of the oestrous cycle were assigned to the following experiments: (1) an immunohistochemical study of the number of cells expressing m ₁AChR or m ₂AChR in the POA-AHA and (2) analysis of the effects of the unilateral blockade of the m ₁AChR (pirenzepine, PZP) or m ₂AChR (methoctramine, MTC) on either side of the POA-AHA on the ovulation rate. The number of m ₂AChR-immunoreactive cells was significantly higher at 09:00 h on each day of the oestrous cycle in the POA-AHA region, while no changes in the expression profile of m ₁AChR protein were observed. The ovulation rate in rats treated with PZP on the oestrous day was lower than that in the vehicle group. Animals treated on dioestrous-1 with PZP or MTC had a higher ovulation rate than those in the vehicle group. In contrast, on dioestrous-2, the MTC treatment decreased the ovulation rate. These results suggest that m ₁AChR or m ₂AChR in the POA-AHA could participate in the regulation of spontaneous ovulation in rats.