Oxytocin is known as the “hormone of love”. Endogenous oxytocin arouses feelings of pleasure, peace and security when in the company of a partner. The release of endogenous oxytocin from the pituitary gland into the bloodstream is triggered by sexual stimuli such as hugging, touching, and genital and nipple stimulation in both genders, and its plasma level is correlated with the levels of arousal and lubrication, reaching a peak during orgasm. The release of endogenous oxytocin decreases fearfulness and works as an anxiolytic agent, diminishing the level of anxiety through inhibiting fear responses in the amygdala, which contains substantial numbers of oxytocin receptors. The release of endogenous oxytocin from the brain during intimate touching or sexual activity with a partner has been suggested to have a vital role in sexual monogamy in men and women.
Ecstasy [(3,4-methylenedioxymethamphetamine (MDMA)] is a recreational psychoactive drug and is often called the “love pill”. Research has shown that ecstasy stimulates endogenous oxytocin activity via activation of serotonin 5-HT1A receptors resulting in an increase in feelings of love, empathy and connection to others.
A rise in endogenous oxytocin results in an increase of plasma endorphins, natural pain-killers that can diminish pain in women who suffer dyspareunia, due to anxiety or a lack of trust in their partner during the first stages of their relationship. Despite these, research has suggested that endogenous oxytocin may not be high before the commencement of sexual activity and it may not be the main trigger of sexual drive and desire preceding the initiation of sexual activity. According to this, the level of endogenous oxytocin increases after the woman receives appropriate stimulation and starts enjoying the sexual activity. This claim is supported by data from self-report studies indicating that some women may enjoy sexual activity and reach orgasm when sexual stimulation and intercourse occur, although they may not be the initiator of the sexual activity[53,54].
Higher plasma concentrations of oxytocin have been shown in people who have fallen in love as well as during the transition to parenthood. A magnetic resonance imaging study of 10 women and 7 men (mean age 21.4 years) has shown that brain areas involved in the formation of romantic attachment are rich in oxytocin receptors. The same brain regions are activated in new parents with great parental-infant attachment and new lovers in prolonged romantic relationships. These reports suggest that parent-child attachments and romantic bonds may share some fundamental mechanisms mediated by the oxytocinergic system, though it is not evident in the literature.
Postpartum loss of sexual desire, arousal and orgasm have been reported across many studies and have been shown to remain as long as one year to many years after childbirth. Research suggests that changes in sexual function in postpartum women may not be only because of physical changes during the transition to motherhood, but may also be due to psychological and neuroendocrine alterations during and after childbirth. Neuroimaging assessments of seven mothers have shown changes in the prefrontal-limbic system during the transition to motherhood, including the amygdala, which is responsible for the expression of oxytocin receptors, suggesting that the amygdala may be less responsive to sexual images and stimuli in postpartum women. Another suggested alteration is that the brain may not release the expected amounts of endogenous oxytocin during sexual activity in postpartum women, and this may result in decreased self-reported feelings of sexual desire in these women.
A modest body of evidence suggests that any factor that can interfere with the release of endogenous oxytocin can cause sexual dysfunction in postpartum women. Among the various factors contributing to sexual problems in postpartum women[60-62] is the use of intravenous synthetic oxytocin during labour and birth. This factor is not subject to the standard mechanisms regulating endogenous oxytocin and affects the normal behaviors of the amygdala[63,64].
Considering the low levels of endogenous oxytocin in women experiencing sexual problems, and the different mechanisms of action of intranasal and intravenous synthetic oxytocin, researchers have attempted to address the sexual problems of women by using an intranasal spray of synthetic oxytocin which was supposed to deliver lower doses of synthetic oxytocin to the body compared with intravenous synthetic oxytocin administered during labour. A case report by Anderson-Hunt and Dennerstein showed copious vaginal transudate and a subsequent intense sexual desire two hours after the use of intranasal spray of synthetic oxytocin to facilitate breastfeeding. However, findings of their report may not be generalised to the entire population as they studied only one woman for a short period of time. Another study showed that intranasal administration of synthetic oxytocin improved attachment-related behaviors, such as eye gazing, interpersonal trust, compassion and positive communication.
The use of intranasal synthetic oxytocin in men has been shown to result in a remarkable increase in their endogenous oxytocin levels together with increased secretion of catecholamines when they were engaged in sexual activity in a laboratory setting. Nevertheless, no further evidence in the literature supports the use of synthetic oxytocin for female sexual dysfunction.
As mentioned earlier, there are mixed reports regarding the impact of oxytocin on romantic relationships. Some studies have indicated links between plasma oxytocin and positive communication, affiliation, emotional support and love[69,70], but others have shown associations between elevated peripheral oxytocin and post-conflict anxiety and decreased levels of forgiveness in romantic couples[45,71]. These results, however, should be interpreted with caution due to controversy about the reliability of plasma oxytocin levels as a peripheral proxy for central concentrations.
A comprehensive review of animal studies on the effect of neuropeptides on the regulation of the brain, social cognitive processing and associated social behaviors has suggested a link between the oxytocinergic system and dopamine which promotes sexual behaviors such as pair bonding and sexual arousal. This association may also contribute to an expectancy of future reward and the sexual arousal reward that are naturally expected later, as shown in rodents.
When synthetic oxytocin is administered intranasally, it proceeds through the fluid-filled perineural channels created by the cells ensheathing the olfactory receptor neurons. It then travels through the cribriform plate in the skull and reaches the CNS. In their study on primates, Chang et al showed increased levels of endogenous oxytocin in cerebrospinal fluid (CSF) after synthetic oxytocin spray inhalation, supporting the likelihood of central effects of synthetic oxytocin.
Unlike intranasal oxytocin, when intravenous synthetic oxytocin is administered, the blood–brain barrier inhibits it from reaching the brain and it therefore does not function as the “hormone of love”. Other animal studies have reported that synthetic oxytocin may reach the brain, but it may act differently from the endogenous oxytocin and have different effects on the body[76,77]. They have shown that there is not always a correlation between peripheral and cerebral levels of oxytocin, suggesting that the two systems may be controlled independently and that intravenous synthetic oxytocin does not essentially raise oxytocin levels within the brain. Research on male prairie voles has shown inhibitory effects of synthetic oxytocin on pulsatile secretion of endogenous oxytocin that may last year.