Multiple transmitters in the brain can increase or decrease sexual desire and function
Neurotransmitters involved in sexual excitation include brain dopamine, melanocortin, oxytocin, vasopressin, and norepinephrine, whereas brain opioids, serotonin, prolactin, and endocannabinoids function as sexual inhibitors. Inhibitory transmitters are activated normally during sexual refractoriness but also from primary aversion or secondary avoidance disorders.1 Drugs or conditions that reduce brain dopamine levels, increase the action of brain serotonin, or enhance brain opioid pathways have been shown to inhibit sexual desire, while those that increase hypothalamic and mesolimbic dopamine or decrease serotonin release have been shown to stimulate sexual desire.1
Estradiol and progesterone can impact sexual function and desire
In addition to the neurotransmitters, hormones are important modulators of female sexual function. Decreasing levels of circulating estrogen after menopause lead to physiologic, biologic, and clinical changes in the urogenital tissues, such as decreased elastin, thinning of the epithelium, reduced vaginal blood flow, diminished lubrication, and decreased flexibility and elasticity. These changes result in the symptoms of genitourinary syndrome of menopause (GSM), which affects as many as half of all menopausal women.5,6 In clinical trials, dyspareunia and vaginal dryness are the most bothersome GSM symptoms reported.7
The role of hormonal regulation in sexual dysfunction among premenopausal women is not yet fully understood, but we do know that estradiol has been shown to improve sexual desire, progesterone tends to dampen sexual desire, and that testosterone at physiological levels has been shown in most studies to have a neutral effect on sexual desire in a well-estrogenized patient.8
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Experience and behavior modulate or reinforce sexual dysfunction
The most common psychological factors that trigger or amplify female sexual dysfunction are depression, anxiety, distraction, negative body image, sexual abuse, and emotional neglect.9 Contextual or sociocultural factors, such as relationship discord, life-stage stressors (the empty nest syndrome or anxiety and sleep deprivation from a new baby), as well as cultural or religious values that suppress sexuality, also should be considered.9 Experience-based neuroplasticity (changes in brain pathways that become solidified by negative or positive experiences) may elucidate how a multimodal approach, utilizing medical and psychological treatment, can be beneficial for patients, particularly those with hypoactive sexual desire disorder (HSDD).1
New and emerging approaches to managing female sexual dysfunction
Three agents, one of which has been available for prescription for some time, one that is newly available, and one in the pipeline, are or may soon be in the gynecologist's armamentarium.
Flibanserin
Medications that target excitatory pathways or blunt inhibitory pathways are in development, and one, flibanserin (Addyi), has been US Food and Drug Administration (FDA)-approved for the treatment of acquired, generalized HSDD in premenopausal women.1,10 Flibanserin is a nonhormonal, centrally acting, postsynaptic serotonin 1A receptor agonist and a serotonin 2A receptor antagonist that is taken daily at bedtime (100 mg); several weeks are usually needed before any effects are noted.1 It is not approved for postmenopausal women and has a boxed warning about the risks of hypotension and syncope; its use is contraindicated in women who drink alcohol, in those who have hepatic impairment, and with the use of moderate or strong CYP3A4 inhibitors.11
Also keep in mind that flibanserin is only available through a Risk Evaluation and Mitigation Strategy program, so clinicians who wish to prescribe it must enroll in and complete training to become certified providers.9
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Prasterone
Prasterone (Intrarosa), a once-daily intravaginal dehydroepiandrosterone (DHEA) product, is a prohormone that increases local estrogen and testosterone and has the advantage of improved sexual function, desire, arousal, lubrication, orgasm, satisfaction, as well as pain at sexual activity.12 It was approved by the FDA in November 2016 to treat moderate to severe dyspareunia and has been available for prescribing since July 2017. Its cost is comparable to topical estrogen products, with a $25 copay program.
Because prasterone is not an estrogen, it does not have the boxed warning that all estrogen products are mandated by the FDA to have. This may make it more acceptable to patients, who often decline to use an estrogen product after seeing the boxed warning on the package. The Centers for Medicare and Medicaid Services (CMS) does not have prasterone on its list of potentially hazardous drugs for the elderly. However, keep in mind that because its label is for dyspareunia and not specifically for GSM, CMS considers it a drug of choice--in other words, like sildenafil (Viagra), a lifestyle choice and not for treatment of a medical condition. As such, at the present time, Medicare does not cover it.
Bremelanotide
Late-stage trials of bremelanotide, a melanocortin receptor agonist, are underway. Its mechanism of action is somewhat like that of flibanserin in that both drugs increase dopamine and norepinephrine levels. The advantage of bremelanotide is that it is used as needed. It is dosed subcutaneously (1.75 mg) and it can be used as often as a woman would like to use it. The FDA is expected to consider it for approval in about a year. Unpublished data from poster sessions at recent meetings show that, in a phase 3 study of 1,247 premenopausal women with HSDD (who had already been screened for depression and were found to have a physiologic condition), improvements in desire, arousal, lubrication, and orgasm were shown with bremelanotide. About 18% of women stopped using the drug because of adverse effects (nausea, vomiting, flushing, or headache) versus 2% for placebo. Like flibanserin, it is expected to be approved for premenopausal women only.