A Physiological Perspective by PEhomepage.com Editorial Team
Meston and Frohlich conducted a broad overview of the field concerning sexual functioning as a means of understanding the way the body is influenced by endocrine, neurotransmitters, and central nervous system circumstances during male and female sexual functions. For purposes of this overview, only male sexual responses are discussed. The field commonly defines sexual desire as “the broad interest in sexual objects or experiences.” Generally individuals provide information by self-report concerning frequency of sexual activity, sexual thoughts, fantasies, dreams, and the amount of interest in having sexual relations.
In the endocrine system, studies have shown testosterone has an effect on sexual interest and activity in men who have been castrated. When testosterone is replaced, sexual interest and activity generally returns within a few weeks. Testosterone has not, however, been shown to help males achieve erection if they are otherwise healthy. Estrogen is also part of the endocrine system and has been shown to have little effect on the sexual desire or interests of men. Some studies have shown some effectiveness when giving large amounts of estrogen to sex offenders or men who have uncontrollable sexual urges.
Progesterone has not been fully studied, although one study did note a decrease in sexual desire in 4 men who were given intramuscular injections of progesterone treatments. The lack of controlled studies involving progesterone makes it difficult to make any definitive statements about its role in male sexuality. Prolactin, on the other hand, in high amounts in the male system has been demonstrated to decrease sexual interest. Erectile dysfunctions (ED) have also been reported in men with abnormally high and low amounts of prolactin. Cortisol in high amounts can show a variety of symptoms including depression, insomnia, and decreased libido in males. Men given synthetic pheromones may increase a male’s willingness to engage in sexual intercourse and romantic “petting,” although more studies need to be done to determine the exact reasons for these activities. The same study regarding pheromones did not find, however, an increase in masturbation.
With respect to neurotransmitters, nitric oxide (NO) is a critical component for men to experience an erection. NO works in cooperation with guanylate cyclase and guanosine triphosphate in order to facilitate erection. The medication sildenafil to treat ED derives from studies in NO and its affect in producing erection. Serotonin is another neurotransmitter that has been extensively studied with respect to its affect on male sexuality. Many studies demonstrate that selective serotonin reuptake inhibitors (SSRIs) can increase intragavinal ejaculation latency time, thus treating premature ejaculation (PE). Treatment with these same antidepressants, however, has the risk of decreasing male sexual desire, as well as other side effects. Medications to treat Parkinson’s Disease (such as levodopa) act on the neurotransmitter dopamine and have been reported in conjunction with increased sexual desire. In addition, levodopa has also been shown to help men achieve erection. In contrast, antipsychotic medications act to decrease dopamine levels and have been shown to both increase and decrease a man’s ability to achieve erection. Meston and Frohlich report that drug dosage and the length of time between the drug’s introduction in the patient to the time that behaviors are observed seem to be factors in measuring how much effect dopamine have on male sexual functioning. Cocaine has been shown to increase dopamine activity and is “commonly belived to enhance sexual pleasure.” Increased and prolonged use of cocaine has been reported in connection with decreased sexual functioning and enjoyment.
Epinephrine is also a neurotransmitter, yet studies do not demonstrate a marked change in men before masturbation. Viewing erotic films does not seem to increase this neurotransmitter either. On the other hand, norepinephrine (NE) does increase, according to several studies, during sexual activity. In men, higher NE levels were seen when men were aroused and had achieved erection. NE also increased up to 12 fold at orgasm. Due to these studies, and others, demonstrating the importance of NE on male sexual functioning, studies are now being conducted with new classes of antidepressants tha tact solely on NE levels, without the action on serotonin. Some researchers feel that NE may be useful in treating ED as well as anorgasmia (absence of orgasm). Opiods are also neurotransmitters and researchers have learned much about them through studying individuals who abuse narcotics. Abusing opiates, like heroin, can lead to loss of libido, ED, and anorgasmia if the person is able to achieve erection. Acetylcholine, along with an intestinal peptide facilitated erection. Finally, increasing histamines can assist in achieving erection and increasing libido.
In the central nervous system (CNS), the brainstem region has been implicated in assisting SSRI-induced anorgasmia in both males and females. The hypothalamus has been studied primarily in animals and caution should be exercised when extrapolating animal studies into complex human dynamics. In animals who had lesions in the hypothalamus, animals were unable to recognize sexual partners and disinterested in sexual behavior. Finally, the forebrain’s medial amygdale is suspected in control of sexual motivation in men. Studies that applied electrical stimulation to the hippocampus resulted in erections and even some reports of orgasm. Researchers note caution when applying these findings to normal healthy men since subjects in those studies were “experiencing severe neurological and psychiatric conditions.”
Source: Meston, Cindy M. and Frohlich, Penny F. “The Neurobiology of Sexual Function.”
Archives of General Psychiatry (57), 2000, p. 1012-1030.
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