Introduction

Neem plant (Azadirachta indica L.) has been considered as one of the most important medicinal plants worldwide. The medicinal utility of this plant are listed in ancient documents ‘Charak-Samhita’ and ‘Susruta-Samhita’ that are considered as the foundation of the Indian system of natural treatment, Ayurveda (Girish and Shankara Bhat2008). It is considered as ‘Sarvaroga nivarini’ that means the curer of all ailments (Subapriya and Nagini2005). The various parts of neem plant are used for the treatment of several diseases in ayurvedic system of medicine worldwide. The aqueous extract of neem bark has therapeutic potential for controlling gastric hypersecretion and gastroduodenal ulcer (Bandyopadhyay et al.2004), while neem leaf extract has been used to reduce oral infections, plaque index and bacterial count (Pai et al.2004a,[b]).

Medicinal properties of Neem Leaf

Medicinal properties of neem leaf have already been reviewed (Subapriya and Nagini2005). Neem leaf extract (NLE) exhibit anti-inflammatory, anti-hyperglycemic, anti-ulcer, immunomodulatory, antiviral, anti-fungal, anti-bacterial, nematicidal, anti-malarial, insecticidal, anti-mutagenic and anti-oxidant properties (Biswas et al.2002; Sharma et al.2003; Wandscheer et al.2004; Udeinya et al.2004; Siddiqui et al.2004; Subapriya and Nagini2005; Sithisaran et al.2005). The Anti-helminthic activity of NLE has also been reported in ruminents (Al-Rofaai et al.2012). One of the bioactive fractions of neem leaf (nimbolide) has anti-cancer property (Harish Kumar et al.2010). The apoptosis inducing ability of NLE has been investigated in cancer cells (Dharmalingam et al.2011). The NLE induces cytoplasmic granulation and deteriorates oocyte quality suggesting its potential use for female fertility regulation in brown dog ticks (Denardi et al.2010). The anti-fertility properties of neem extracts have been reported in several mammalian species (Mukherjee et al.1999; Subapriya and Nagini2005).

Several studies have been carried out to find the role of neem products in male fertility regulation. The neem bark ethereal extract induces reversible changes in reproductive system of male rats and resulting into male infertility (Raji et al.2003). Neem oil treatment reduces tubular diameter, inhibits spermatogenesis in rat (Shaikh et al.2009) and sperm motility in mice (Yin et al.2004).

The ethanolic NLE induces abnormal head morphology and reduces mean sperm count in murine (Khan and Awasthi2003). The aqueous NLE inhibits motility and viability of human spermatozoa under in vitro culture conditions (Khillare and Shrivastav2003). Neem oil has been used as herbal vaginal contraceptive in human (Sharma et al.1996).

The role of NLE on female fertility regulation remains poorly understood. Few studies suggest that neem oil reduces number of developing follicles in the rat ovary and induces degeneration of oocyte in vitro (Juncia and Williams1993; Dhaliwal et al.1999; Roop et al.2005). The neem oil inhibits implantation in rats and bonnet monkeys and acts as a reversible contraceptive (Upadhyay et al.1990,1994; Garg et al.1998). However, neem oil has unpleasant sharp odour and thus becomes unpalatable. Hence, aqueous NLE could be used as an alternative herbal medicine for fertility regulation in mammals including human since it does not possess an unpleasant characteristic sharp odour of neem (Selvamurthy1997).

Generation of ROS in ovary and oocyte quality

Ovary is a dynamic organ and generates excess amount of reactive oxygen species (ROS) during final stages of folliculogenesis and ovulation, while their effects are neutralized by active enzymatic antioxidant system (Agarwal et al.2005; Fujii et al.2005; Sugino2005). A moderate increase of ROS under physiological range could be beneficial for meiotic resumption from diplotene arrest in mammalian oocytes cultured in vitro (Agarwal et al.2005; Chaube et al.2005; Pandey et al.2010). However, overproduction of ROS in ovary or depletion of enzymatic antioxidant system may results in oxidative stress (Agarwal et al.2005). The increased oxidative stress level can reduce oocyte quality by inducing apoptosis, fertilization and pregnancy rates in mouse as well as in human (Tamura et al.2008).

NLE triggers ROS-mediated granulosa cell and oocyte apoptosis

The mechanism by which NLE has a direct access at the level of mammalian ovary and oocyte remains unclear. Animal studies suggest that NLE reduces ovary weight, ovulation rate (Gbotolorun et al.2008), inhibits folliculogenesis and antrum formation in follicles (Mukherjee et al.1999; Dhaliwal et al.1999; Roop et al.2005). Studies from our laboratory suggest that NLE triggers apoptosis in preovulatory follicles, reduces number of granulosa cells encircling oocyte, induces dispersion of granulosa cells and oocyte apoptosis in majority of ovulated cumulus oocytes complexes (COCs) (Tripathi et al.2012,2013).

The aqueous NLE decreases catalase activity that results in the accumulation of hydrogen peroxide (H2O2) in rat ovary (Chaube et al.2006; Tripathi et al.2013). The increased level of H2O2 acts as an upstream signal to induce p53 and Bax protein expression (Chaube et al.2006; Tripathi et al.2013). The overexpression of Bax protein modulates mitochondrial membrane potential and increases cytochrome c release in cell cytoplasm (Chaube et al.2006; Tripathi et al.2013). A rise in cytochrome c concentration induces DNA fragmentation and thereby granulosa cell apoptosis (Chaube et al.2006; Tripathi et al.2013).

The granulosa cell apoptosis results in the disruption of gap junctions between encircling somatic cell and oocyte inside the follicular microenvironment. Reduced cell-cell communication deprives oocyte from nutrients, maturation-enabling factors and survival factors inside the preovulatory follicle and reduces oocyte quality by inducing susceptibility towards apoptosis (Tripathi et al.2013). The granulosa cell apoptosis also reduces estradiol 17-β level required for development and maturation of oocytes in the ovary. The hypo-estrogenic condition inside the follicular microenvironment may affect development and maturation of oocytes and trigger generation of ROS and mitochondria-caspase-mediated pathway (Chaube et al.2006; Tripathi et al.2012,2013). Hence, the follicular oocyte is unable to achieve meiotic competency and become susceptible towards apoptosis after ovulation (Chaube et al.2006; Tripathi et al.2012,2013).

Quercetin is one of the major bioactive flavonoids and constitute to 6 to 48% (w/w) of NLE (Subapriya and Nagini2005; Sithisarn and Gritsanapan2008). Quercetin inhibits antioxidant systems (thioredoxin or glutathione) that increases ROS level and thereby apoptosis (Pelicano et al.2004; Kuo et al.2007; Jeong et al.2009). Studies from our laboratory suggest that quercetin induces cell shrinkage, membrane leakage, cytoplasmic granulation and cytoplasmic fragmentation in treated oocytes prior to degeneration (Chaube et al.2006; Tripathi et al.2012,2013). These morphological apoptotic changes were associated with the increased H2O2 level, overexpression of Bax protein, caspase-3 activation and DNA fragmentation (Chaube et al.2006; Tripathi et al.2012,2013). Based on these studies, we propose that NLE and its bioactive ingredient such as quercetin induce ROS-mediated granulosa cell apoptosis followed by oocyte apoptosis. The NLE-induced oocyte apoptosis deteriorates oocyte quality that finally reduces reproductive outcome (Chaube et al.2006; Tripathi et al.2012,2013).

Future prospects

NLE is ecologically friendly, safe and biologically active botanical substance that induces apoptosis in follicular granulosa cells as well as in oocyte that deteriorates oocyte quality after ovulation. The apoptosis inducing property of NLE makes it a potential candidate for the development of reversible herbal contraceptive for the control of female fertility in mammals. The apoptotic inducing property of NLE may also be used to control the rodent population in the field that damage almost 30% crops worldwide. NLE could be mixed in common edible food of rodents and pellets can be made. These pellets can be kept in rodent prone areas for their easy access. This neem product formulation could replace various harmful rodenticides used frequently by the farmers that can affect human health directly or indirectly by passing on to next trophic level.

Conclusions

NLE reduces catalase activity and increases the accumulation of H2O2 inside follicular microenvironment of the mammalian ovary. The increased H2O2 level induces granulosa cell apoptosis through mitochondria-caspase-mediated pathway. Granulosa cell apoptosis reduces estradiol 17-β level in ovary and induces disruption of gap junctions between granulosa cells and oocytes. Hence, oocyte is deprived of cell signaling molecules, nutrients and survival factors required for the achievement of meiotic competency of oocyte just prior to ovulation. Under this condition, pro-apoptotic factors are upregulated thus making oocytes susceptible towards apoptosis after ovulation (Figure 1). Thus, NLE-induced oocyte apoptosis deteriorates oocyte quality and limits reproductive outcome in mammals.

Figure 1
figure 1

A schematic diagram showing the possible mechanism of NLE action at the level of follicle in mammalian ovary. NLE inhibits catalase activity and induces the accumulation of H2O2 inside the follicular microenvironment of the ovary. The increased H2O2 level induces granulosa cell apoptosis through mitochondria-caspase-mediated pathway. Granulosa cell apoptosis leads to disruption of gap junction, interruption in the transfer of signal molecules from cumulus cells to the oocyte and reduction of estradiol 17-β level further deprive oocyte from nutrients and survival factors. The H2O2 also enters directly in the oocyte and trigger oocytes apoptosis. Under this condition, pro-apoptotic factors are upregulated and oocyte becomes susceptible towards apoptosis. NLE induced apoptosis leads to the deterioration in the oocyte quality and limit reproductive outcome in mammals.

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Authors’ information

Author SKC is the Principal Investigator of a project entitled “Studies on neem (Azadirachta indica L.) leaf extract-induced egg apoptosis in rat” funded by Department of Science and Technology, Ministry of Science and Technology, Government of India, New Delhi.