Unilaterally blocking the muscarinic receptors in the suprachiasmatic nucleus in proestrus rats prevents pre-ovulatory LH secretion and ovulation

Abstract

BackgroundThe suprachiasmatic nucleus (SCN) and the cholinergic system of various regions of the hypothalamus participate in the regulation of gonadotropin-releasing hormone (GnRH) and gonadotropin secretion, which are necessary for the occurrence of ovulation. In the present study, our goal was to analyse the effects of unilaterally blocking the muscarinic receptors in the SCN on ovulation and steroid secretion.MethodsCyclic rats were randomly allotted to one of the experimental groups. Groups of 8–14 rats were anaesthetized and microinjected with 0.3 μl of saline or a solution of atropine (62.5 ng in 0.3 μl of saline) into the left or right SCN at 09.00 or 19.00 h during diestrus-1 or on the proestrus day. The rats were euthanized on the predicted day of oestrus, and evaluated ovulation and levels of progesterone and oestradiol. Other groups of 10 rats were microinjected with atropine into the left or right SCNs at 09.00 h on the proestrus day, were euthanized eight h later, and luteinizing hormone (LH) was measured.ResultsAt 09.00 or 19.00 h during diestrus-1, atropine microinjections into the SCNs on either side did not modify ovulation. The atropine microinjections performed at 09.00 h of proestrus into either side of the SCN blocked ovulation (right SCN: 1/9 ovulated vs. 9/10 in the saline group; left SCN: 8/14 ovulated vs. 10/10 in the saline group). The LH levels at 17.00 h in the rats that were microinjected with atropine at 09.00 h of proestrus were lower than those of the controls. In the non-ovulating atropine-treated rats, the injection of synthetic LH-releasing hormone (LHRH) restored ovulation. Atropine treatment at 19.00 h of proestrus on either side of the SCN did not modify ovulation, while the progesterone and oestradiol levels were lower.ConclusionBased on the present results, we suggest that the cholinergic neural information arriving on either side of the SCN is necessary for the pre-ovulatory secretion of LH to induce ovulation. Additionally, the regulation of progesterone and oestradiol secretion by the cholinergic innervation of the SCN varies with the time of day, the day of the cycle, and the affected SCN.

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

“…In the rat, the participation of the muscarinic cholinergic system in regulating ovulation varies within the oestrous cycle and demonstrates a circadian rhythm (Domínguez et al., ). In the morning of pro‐oestrus (4 h after lights on), the systems regulating GnRH and LH secretion require a muscarinic signal from the SCN (Vieyra et al., ). According to Earnest and Turek (), acetylcholine may play an important role in the mechanism by which light regulates circadian rhythms and seasonal reproductive cycles.…”

“…According to van den Hurk, Dijkstra, and de Jong () and Moffatt‐Blue, Sury, and Young (), the exposure of female hamsters to a short‐day light photoperiod delayed ovarian growth and disrupted ovarian folliculogenesis and steroidogenesis, and such effects were not observed in animals exposed to a long photoperiod. There were no differences in the oestrous cycle length in rats of our strain maintained on a 12:12 light–dark cycle (Pastelín el al., ) and those maintained under a 14:10 light–dark cycle (Domínguez, Zipitría, Aguilar, & Riboni, ; Vieyra et al., ). Sprague–Dawley rats maintained on a 12:12 light–dark cycle ovulated 7.6 ± 0.5 ova (Gbotolorun, Inikori, Bamisi, Osinubi, & Okanlawon, ), similar to the number of ova released by the control rats (8.9 ± 2.7 ova shed) used in this study maintained on a 14:10 light–dark cycle.…”

“…Previously, we showed that in rats with unilateral blockade of muscarinic receptors (mAChRs), changes in progesterone and oestradiol levels depended on the hormone measured, the time of day of treatment, and the side of the SCN treated, suggesting that the neuroendocrine mechanisms regulating hormone secretion exhibit asymmetry. Additionally, progesterone and oestradiol regulation of secretion is modulated by SCN signals after the preovulatory LH surge release occurs (Vieyra et al., ).…”

“…The present study was performed using methodologies previously described by Vieyra et al. (). In brief, this study was performed using 3‐month‐old CIIZ‐V strain virgin female rats, weighing 230–260 g, from our own breeding stock.…”

“…The same treatment in the left SCN only blocked ovulation in 43% of treated rats. These results suggest a difference in the SCN sensitivity of the muscarinic systems; the results also indicate that the regulation of ovulation by the SCN is asymmetric and that the muscarinic system stimulates GnRH and luteinizing hormone (LH) secretion and therefore ovulation (Vieyra et al., ).…”