Background
The reproductive system is governed by the hypothalamic-pituitary-gonadal axis (HPG), wherein a pulsatile release of GnRH from the hypothalamus stimulates anterior pituitary gonadotropes to release LH and FSH, leading to steroid production from the ovaries. The regulation of HPG axis is quite complex, involving several intrinsic factors (estrogens, progesterone, inhibin, activin, etc) [
1] as well as extrinsic factors (neurotransmitters, neuropeptides, stress, etc) [
2]. However, any abnormality that prevents or interferes with the function of these factors may culminate into reproductive endocrine anomalies. One of the most prevalent reproductive endocrinopathies is polycystic ovarian syndrome (PCOS), affecting 6–10% of women worldwide [
3]. The key features of PCOS include hyperandrogenemia, oligo−/anovulation and peripheral cyst formation in ovaries [
4]. In addition, PCOS is a disorder underpinning neuroendocrine abnormalities, characterized by increased GnRH and LH:FSH ratio [
5]. However, in spite of its widespread occurrence and profound implications, the etiology of this disease remains poorly understood.
While reduced norepinephrine, dopamine and serotonin has been reported in the serum of PCOS women [
6], their levels in GnRH regulatory regions of the brain are unknown due to the obvious difficulties in obtaining these tissues from patients. A tissue-specific understanding of the neurotransmitters would help us gain an insight into the pathogenesis of PCOS. Thereby, the objective of the current study was to evaluate the status of GnRH-regulatory neurotransmitters in a PCOS rat model.
To address the above objective, Letrozole, an aromatase inhibitor, was used to induce PCOS in rats [
7]. Evaluation of the neurotransmitter levels was performed from hypothalamus, pituitary as well as from hippocampus and frontal cortex. The reasons for selecting these areas of brain mainly include i) presence of GnRHR in the described tissues, which contributes to regulation of reproduction and reproductive behaviour [
8,
9]; ii) active steroidogenesis occurring in these regions [
10,
11] and iii) them being important sites of neurotransmitter synthesis [
12]. The rates of neurotransmitter synthesis and clearance were monitored by estimating the activities of neurotransmitter metabolizing enzymes. Gene expression analysis of specific neurotransmitter receptors that profoundly influence pulsatile release of GnRH/LH and other reproductive processes was performed in PCOS and normal rats.
Discussion
PCOS is a very common endocrine disorder in women of reproductive age and is characterized by increased androgen production and abnormal gonadotropin secretion, resulting in chronic anovulation. To understand the etiopathology of PCOS, the present study employed a letrozole-treated rat model, which exhibits hormonal, reproductive and metabolic signs similar to the human PCOS condition [
7,
25,
26]. Furthermore, it was observed currently that the PCOS rats had a high serum LH:FSH ratio, a characteristic feature of PCOS. This, we believe, must stem from increased transcription of the
GnRH in the hypothalamus and
GnRHR in the pituitary, as also observed by Kauffman and group [
26]. Thus, letrozole-induced PCOS rat model possesses similar neuroendocrine traits as seen in PCOS women, making it a favourable model for use in PCOS research.
A number of studies using dual-label immunohistochemistry and in situ hybridization have shown that several neurotransmitter and neuropeptide receptors are expressed in GnRH neurons and they directly regulate GnRH, LH and FSH release [
2]. The effect of serotonin on GnRH neurons is biphasic in nature wherein activation of 5-HT2A receptor increases GnRH neuronal activity via PKC (Protein kinase C) pathway, while activation of 5-HT1A receptor suppresses GnRH neuronal firing through adenylate cyclase [
27,
28]. Serotonin content was found significantly reduced in all brain tissues of PCOS animals as compared to control, which can be well correlated with the observed decrease in TDC activity (serotonin synthesis) and heightened MAO activity. Also, the expression of 5HT1A receptor was decreased in PCOS animals. Based on this, and the above-cited references, increased GnRH and LH release in PCOS may result, at least partially, from reduced inhibition of GnRH by serotonin.
In addition to serotonin, the role of catecholamines is also known in GnRH regulation. Norepinephrine has been shown to rapidly increase GnRH mRNA levels in ovariectomised rats [
29]. It is also responsible for the pre-ovulatory LH surge through the α- and β-adrenergic receptors. Propranolol, an α-adrenergic receptor blocker stimulates NE-induced LH release, while treatment of β-antagonist blocked the release of pre-ovulatory LH surge [
30,
31]. This indicates that the stimulatory effect of norepinephrine on LH release is mediated by β-adrenergic receptors while α-adrenergic receptor inhibits LH release. Moreover, α-adrenergic receptor is also involved in steroid mediated feedback regulation of GnRH [
32]. In the case of epinephrine, although reports on GnRH regulation are rare, a positive relation is implied, where the former stimulates release of GnRH and LH, also through the α-adrenergic receptor [
33,
34]. Whereas both epinephrine and norepinephrine were reduced in the brain of PCOS rats, the GnRH and LH were still elevated, pointing towards the involvement of other regulatory factors in this outcome.
Unlike norepinephrine and epinephrine, dopamine is a major suppressor for GnRH release [
35]. It also inhibited the firing and anteroventral paraventricular (AVPV)-evoked GABA/glutamate postsynaptic currents in the GnRH neurons in vitro mediated by D1 and D2-like receptors in male and female mice [
35]. Recent study in ewes also suggests that D2 dopamine receptor not only affects the GnRH release but also GnRH and GnRHR gene expression in hypothalamus. Also, LH pulse frequency increases upon local injection of Sulpride (D2R antagonist) in ewes, reflecting the potency of D2R in inhibiting GnRH and LH pulsatility [
36]. In addition to its influence on GnRH/LH, it has an inhibitory effect on prolactin release. A positive association between PCOS, low dopamine and hyperprolactinemia has been suggested [
37]. Further, hyperprolactinemia exerts an inhibitory effect on the gonadotrophs [
38]. In the present case, dopamine content in brain of PCOS rats was significantly decreased along with reduced expression of
D2R, which may result into hypersecretion of prolactin in PCOS condition. Supporting our data, many studies suggest the role of reduced dopaminergic tone in increased LH release in PCOS [
39,
40]. Additionally, treatment with bromocriptine, a D2 receptor agonist, can restore normal menstrual cycle and ovulation in PCOS women [
39].
GABA is the major inhibitory neuron of the central nervous system. GABAB1 knockout mice demonstrated abnormal estrus cyclicity and reduced fecundity with significantly increased GnRH release as well as GnRH pulse frequency [
41], whereas GABAA knockdown mice had normal estrus cycle and puberty onset [
42]. In addition, treatment of GABA or muscimol, a GABAA/C receptor agonist, to cultured anterior pituitary cells results into increased secretion of LH through Ca2+ release [
43]. However, when cultured pituitary cells were incubated with baclofen, a GABAB agonist, GnRH-induced LH release was inhibited while basal LH secretion did not change [
43,
44]. This suggests that GABAA/C stimulate basal LH secretion whereas GABAB suppresses GnRH-induced LH release. Reduced signalling of GABA through GABAB1 observed in our system may have contributed to an increased GnRH/LH pulse. Interestingly, a study in prenatally androgenised mouse model of PCOS demonstrated increased GABA innervations to GnRH neurons [
45]. This disparity in the result is likely due to the fact that Moore and group have used arcuate nucleus for their study while we have used whole hypothalamus, which includes many such nuclei. Also, this group has used prenatally androgenised mouse model of PCOS [
45] and in utero androgen exposure can lead to epigenetic changes, which could result in developmental alterations in neural circuits.
In contrast to GABA, Glutamate is the major excitatory neurotransmitter for GnRH release. GnRH neurons express both ionotropic glutamate receptors (AMPA, Kainate and NMDA) and metabotropic glutamate receptors. However, reports describing the role of metabotropic glutamate receptors in GnRH regulation are scanty [
46]. NMDA receptor antagonist-MK801 abolished endogenous pulses of GnRH secretion whereas pulsatile release of GnRH was not affected in the presence of 6,7-dinitroquinoxaline-2,3-dione (kainate receptor blocker) [
47]. In addition, mRNA and protein expression study has revealed the presence of vesicular glutamate transporter in gonadotrophs of anterior pituitary and a stimulatory role for glutamate in LH release was also documented [
48,
49]. In PNA-induced PCOS mouse model, no effect of glutamate was observed in GnRH pulsatility [
45]. However, significantly high glutamate levels and NMDA receptor expression in PCOS animals were observed in the current study, suggesting direct overstimulation of GnRH and LH release. Further, the activities of GAD and GDH were significantly decreased in PCOS rats while that of GABA-T was markedly elevated suggesting that in PCOS condition the flux of reaction is towards the glutamate and not towards GABA.
Along with all the above-stated neurotransmitters, the role of acetylcholine in GnRH regulation is also emerging. In cultured GT1–7 cell line, acetylcholine stimulates GnRH release through activation of nicotinic receptor whilst an inhibitory effect of acetylcholine on GnRH activity was mediated by muscarinic receptor activation [
50]. Also, in GT1–7 cells, acetylcholine treatment activates M2 muscarinic receptor that further reduces forskolin-induced cAMP production followed by suppression of GnRH release [
51]. Similarly, treatment of exogenous acetylcholine to cultured anterior pituitary cells resulted in decreased response of GnRH-induced LH release. This response was counteracted by muscarinic receptor antagonist atropine [
52]. Currently, activity of acetylcholine esterase (AChE), a hydrolytic enzyme of acetylcholine, was found elevated in the hypothalamus and pituitary of PCOS rats along with decreased expression of M2 muscarinic acetylcholine receptor (M2AChR), thus, suggesting reduced levels of acetylcholine in PCOS condition which may contribute to increased GnRH and LH pulse frequency in PCOS women.
It should be noted that along with neurotransmitters, HPG axis is governed by several neuropeptides of the like of kisspeptin, a major factor which directly or through its interaction with steroids and neurotransmitters, stimulates the release and expression of GnRH/LH [
53,
54]. Various immnohistochemical studies have demonstrated co-localization of GABAB, NMDAR glutamate receptor and D2 dopamine receptor in kisspeptin neurons. Furthermore, antagonists of GABAB and D2R dopamine receptors increase Kisspeptin-mediated GnRH and LH release [
55,
56], whereas treatment with MK801-NMDA receptor antagonist blocks kisspeptin-dependent reinstatement of LH surge [
57]. Data from our lab has revealed significant increases in expression of both
Kiss1 and its receptor
Gpr-54 in the hypothalamus of PCOS rats (manuscript under preparation) that also falls in line with a previous study [
26]. Thus, alteration of neurotransmitters and neuropeptide together are likely to be responsible for the increased GnRH and LH pulsatility in PCOS condition.
Besides the regulation of endocrine axis, neurotransmitters are also implicated in several psychiatric manifestations. The vast majority of anti-depressants include inhibitors of monoamine oxidase and serotonin reuptake transporters (SSRI), indicating the role of serotonin, dopamine and norepinephrine in mood regulation [
58‐
61]. Glutamate and GABA are also emerging candidates for depression and anxiety disorders [
62,
63]. Alterations in acetylcholine signalling have also been shown to lead to symptoms of depression and anxiety wherein overactive or hyper-responsive muscarinic cholinergic system has been documented [
58]. All these references suggest that alteration in neurotransmitter profile seen in letrozole-induced PCOS model may result into development of depression and anxiety-like symptoms (manuscript communicated). In light of these references and our data, the occurrence of depression, anxiety and other mood disorders, which affect upto 40% of PCOS women [
64,
65], can be linked to an altered neurotransmitter profile. We have indeed observed symptoms of depression and anxiety in behavioural experiments on letrozole-induced PCOS rat model (manuscript communicated).
Current study clearly demonstrated severe neurotransmitter modulation in letrozole-induced PCOS rat model. Likewise, in a study, rat treated with letrozole demonstrated decreased norepinephrine and dopamine content in hippocampus and frontal cortex [
66]. However, the concentration and duration of letrozole treatment in that study was much higher as compared to our study. Furthermore, the possibility that the currently observed changes in neurotransmitters are due to other interactions of letrozole can be ruled out by our previous study wherein testosterone propionate-induced PCOS rat model also demonstrated similar neurotransmitter profile [
67]. This thereby strengthens the result of present study indicating that neurotransmitter modulation is a pivotal attribute of PCOS condition.