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

The investigation of cholinergic receptor muscarinic 1 activity in the rat ovary with induced ovarian hyperstimulation

Objective

We look at the immunoreactivity of cholinergic receptor muscarinic 1 (CHRM1) in the ovarian tissues of rats with ovarian hyperstimulation syndrome (OHSS) considering the possibility that the muscarinic activity may contribute to the pathophysiology of OHSS.

Materials and Methods

In this study, 14 immature female Wistar Albino rats were divided into two groups at random. The rats were 22 days old. Rats in the control group (n=7) were 22 days old, while those in the OHSS group (n=7) received 10 IU follicle-stimulating hormone subcutaneously over the course of four days and 30 IU human chorionic gonadotropin (hCG) on the fifth day to induce ovarian hyperstimulation. All the rats were sacrificed after all the groups' ovaries and blood samples were collected at the conclusion of the experiment. The left ovarian tissues were kept in aluminum foil at -80 °C, while the right ovarian tissues were kept in 10% formalin. Tissue vascular endothelial growth factor (VEGF), interleukin (IL)-1β, IL-6, IL-10, tumor necrosis factor (TNF)-α and malondialdehyde (MDA) levels were measured by The Enzyme Linked Immunosorbent Assay technique in the ovarian tissues. CHRM1 immunoreactivity was scored immunohistochemically.

Results

Ovarian weight, tissue IL-10, TNF-α, VEGF and MDA levels, and CHRM1 immunoreactivity were significantly increased in the OHSS group.

Conclusion

Increased levels of CHRM1 activity may play a role in the pathophysiology of OHSS. With further studies, the effect of luteinizing hormone and hCG on the ovarian and hypothalamic cholinergic system can be further investigated, and useful information can be obtained in determining OHSS prevention strategies.

Suprachiasmatic nucleus and vagus nerve trigger preovulatory LH and ovulation

In brief

In the proestrus day, the neural and endocrine signals modulate ovarian function. This study shows vagus nerve plays a role in the multisynaptic pathways of communication between the suprachiasmatic nucleus and the ovaries where such neural information determines ovulation.

Abstract

The suprachiasmatic nucleus (SCN) regulates the activity of several peripheral organs through a parasympathetic-sympathetic pathway. Previously, we demonstrated that atropine (ATR) microinjection in the right SCN of rats during proestrus blocks ovulation. In the present study, we analysed whether the vagus nerve is one of the neural pathways by which the SCN regulates ovulation. For this, CIIZ-V strain cyclic rats on the day of proestrus were microinjected with a saline solution (vehicle) or ATR in the right or left SCN, which was followed by ventral laparotomy or ipsilateral vagotomy to the microinjection side. Some animal groups were sacrificed (i) on the same day of the surgery to measure oestradiol, progesterone and luteinizing hormone (LH) levels or (ii) at 24 h after surgery to evaluate ovulation. The left vagotomy in rats microinjected with ATR in the left SCN did not modify ovulation. In rats with ATR microinjection in the right SCN, the right vagotomy increased the levels of steroids and LH on the proestrus and ovulatory response. The present results suggest that the right vagus nerve plays a role in the multisynaptic pathways of communication between the SCN and the ovaries and indicate that such neural information participates in the regulation of the oestradiol and progesterone surge, which triggers the preovulatory peak of LH and determines ovulation.

Roles of the cholinergic system and vagal innervation in the regulation of GnRH secretion and ovulation: Experimental evidence

Reproduction is the biological process that sustains life. It is regulated by a neuro-hormonal mechanism that is synchronized by the interaction among the hypothalamus, hypophysis, and ovaries. Ovulation is regulated by the secretion of the gonadotropin-releasing hormone (GnRH), which stimulates the release of the luteinizing hormone (LH) and follicle-stimulating hormone (FSH). In addition to these neuroendocrine signals, other signals originating from the central nervous system, hypophysis, thyroid, adrenal glands, and the ovary itself are also involved. One of the neurotransmission systems involved in the regulation of ovulation is the cholinergic system, which not only participates in the regulation of reproductive functions but also modulates motor coordination, thermoregulation, and cognitive function. In mammals, the vagus nerve is one of the pathways through which acetylcholine reaches the ovary, and this pathway also participates in the regulation of ovulation. However, this regulation depends on the age of the animal (prepubertal or adult) and its endocrine status. The present review analyzes evidence of the roles of the central and peripheral cholinergic system and vagal innervation in the regulation of GnRH secretion and ovulation as well as their roles in the development and persistence of polycystic ovary syndrome (PCOS).

Functional Aspects of Hypothalamic Asymmetry

Anatomically, the brain is a symmetric structure. However, growing evidence suggests that certain higher brain functions are regulated by only one of the otherwise duplicated (and symmetric) brain halves. Hemispheric specialization correlates with phylogeny supporting intellectual evolution by providing an ergonomic way of brain processing. The more complex the task, the higher are the benefits of the functional lateralization (all higher functions show some degree of lateralized task sharing). Functional asymmetry has been broadly studied in several brain areas with mirrored halves, such as the telencephalon, hippocampus, etc. Despite its paired structure, the hypothalamus has been generally considered as a functionally unpaired unit, nonetheless the regulation of a vast number of strongly interrelated homeostatic processes are attributed to this relatively small brain region. In this review, we collected all available knowledge supporting the hypothesis that a functional lateralization of the hypothalamus exists. We collected and discussed findings from previous studies that have demonstrated lateralized hypothalamic control of the reproductive functions and energy expenditure. Also, sporadic data claims the existence of a partial functional asymmetry in the regulation of the circadian rhythm, body temperature and circulatory functions. This hitherto neglected data highlights the likely high-level ergonomics provided by such functional asymmetry.

Clock control of mammalian reproductive cycles: Looking beyond the pre-ovulatory surge of gonadotropins

Several aspects of the physiology and behavior of organisms are expressed rhythmically with a 24-h periodicity and hence called circadian rhythms. Such rhythms are thought to be an adaptive response that allows to anticipate cyclic events in the environment. In mammals, the circadian system is a hierarchically organized net of endogenous oscillators driven by the hypothalamic suprachiasmatic nucleus (SCN). This system is synchronized by the environment throughout afferent pathways and in turn it organizes the activity of tissues by means of humoral secretions and neuronal projections. It has been shown that reproductive cycles are regulated by the circadian system. In rodents, the lesion of the SCN results on alterations of the estrous cycle, sexual behavior, tonic and phasic secretion of gonadotropin releasing hormone (GnRH)/gonadotropins and in the failure of ovulation. Most of the studies regarding the circadian control of reproduction, in particular of ovulation, have only focused on the participation of the SCN in the triggering of the proestrus surge of gonadotropins. Here we review aspects of the evolution and organization of the circadian system with particular focus on its relationship with the reproductive cycle of laboratory rodents. Experimental evidence of circadian control of neuroendocrine events indispensable for ovulation that occur prior to proestrus are discussed. In order to offer a working model of the circadian regulation of reproduction, its participation on aspects ranging from gamete production, neuroendocrine regulation, sexual behavior, mating coordination, pregnancy and deliver of the product should be assessed experimentally.

Integration of Circadian and Metabolic Control of Reproductive Function

Optimal fertility in humans and animals relies on the availability of sufficient metabolic fuels, information about which is communicated to the brain via levels of the hormones leptin and insulin. The circadian clock system is also critical; this input is especially evident in the precise timing of the female-specific surge of GnRH and LH secretion that triggers ovulation the next day. Chronodisruption and metabolic imbalance can both impair reproductive activity, and these two disruptions exacerbate each other, such that they often occur simultaneously. Kisspeptin neurons located in the anteroventral periventricular nucleus of the hypothalamus are able to integrate both circadian and metabolic afferent inputs and use this information to modulate the timing and magnitude of the preovulatory GnRH/LH surge. In an environment in which exposure to high caloric diets and chronodisruptors such as artificial night lighting, shift work, and transmeridian travel have become the norm, the implications of these factors for couples struggling to conceive deserve closer attention and more public education.