Stimulation of nicotinic receptors in the suprachiasmatic nucleus results in a higher number of growing follicles and ova shed.

Abstract

What is the central question of this study? What is the role of the nicotinic system of the suprachiasmatic nucleus (SCN) in the regulation of follicular growth and ovulation? What is the main finding and its importance? The stimulation of the nicotinic system of the pro-oestrus rat SCN results in an increase in the number of ova shed, in the number of growing ovarian follicles and in the secretion of oestradiol. The timing of the preovulatory luteinizing hormone surge that leads to ovulation depends to a large extent on a functional circadian clock that is localized in the suprachiasmatic nucleus (SCN). The activities of the SCN are regulated by several neurotransmitter systems, including the muscarinic system. Given that acetylcholine binds to muscarinic (mAChRs) and nicotinic (nAChRs) receptors, in the present study, we analysed the effects of unilaterally stimulating nAChRs in the left or right SCN. Stimulation treatment was administered in rats in pro-oestrus at 09.00 or 19.00h by injecting 0.3µl of a nicotine solution (200µm). The effects of the stimulation were assessed by evaluating the number of ova shed, the number of ovarian follicles, and the levels of oestradiol and progesterone in serum 24h after treatment. We observed that regardless of the time (4h after lights on, 09.00h, or immediately after lights off, 19.00h) or the side of the SCN treated, the unilateral microinjection of nicotine resulted in a higher number of ova shed and higher number of growing follicles in the ovaries as well as higher oestradiol serum levels. When the nicotine microinjection treatment failed to reach the SCN, the oestradiol levels in serum were similar to those of animals treated with vehicle solution. Based on the current results, we suggest that during pro-oestrus, the nicotinic neuronal information in the SCN modulates follicular growth and ovulation in a stimulatory manner.