Serotonin’s Role in the Biology of Ejaculation

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It is well known that anti-depressants can have sexual side effects. They can lead to problems with desire, arousal, ejaculation, and orgasm. These sexual problems are most associated with the use of two drug classes—selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRI).

Very early on in the development of SSRIs, it was discovered that these medications can cause difficulties with ejaculation. In fact, these medications are actually sometimes prescribed for men who have problems with premature ejaculation. Why can antidepressants have such profound effects on sexual health, particularly for men? It has nothing to do with mood. Instead, it has to do with the role of serotonin and other neurotransmitters in arousal and ejaculation.

Serotonin and Ejaculation

Sex is, in many ways, a function of the mind. Physical arousal can take place by direct stimulation. It can also take place without any direct stimulation. Sex therapists often recommend "sexy thoughts" as an intervention, because thinking about sex can be a turn on in and of itself. How does that work? The same way that many thoughts are processed—through the production of various neurotransmitters. That's how nerve cells connect through each other. They release and respond to neurotransmitters. Such neurotransmitters include serotonin, norepinephrine, and dopamine.

Much of our understanding of the role of neurotransmitters and brain structures in arousal and orgasm comes from animal research. However, there have been human studies as well. Through research, scientists have learned that there are several areas of the brain associated with male sexual function. In particular, the medial preoptic area of the hypothalamus is very important. The MPOA, as it is sometimes called, is where sexual stimuli from various parts of the body come together for processing. It's also the part of the brain that initiates several types of sexual response. The brain sends signals down the spinal cord that cause the body to become first aroused and then orgasm. Parts of the amygdala and the parietal cortex are also important in the control of ejaculation.

Neurotransmitters are the mechanism for these signals. Studies in rats have shown that it is possible to cause rats to orgasm just by activating certain neurotransmitter receptors in the brain. (Receptors are the places where neurotransmitters bind. There are multiple types of receptors that can respond to each, or a combination, of the various transmitters. Not all types are equally distributed through the brain.)

In humans, serotonin is the neurotransmitter most clearly associated with ejaculation. SSRIs work by preventing cells from reabsorbing serotonin. This means that signals caused by serotonin are extended, lasting for a longer period of time. Chronic use of SSRIs has been shown to extend the time between erection and ejaculation in men. That is why it is prescribed as a treatment for premature ejaculation. Interestingly, studies in rats have shown that where the serotonin is changed its effects. When serotonin is injected into some parts of the rat brain, it causes a delay in ejaculation. In other areas of the brain, it causes ejaculation to take place.

Dopamine also plays a role in ejaculation, although its role has not been as thoroughly explored as that of serotonin. Studies in rats suggest that dopamine stimulation can cause ejaculation to take place. In humans, there is also research to support this. Schizophrenics who are treated with antipsychotic drugs that block a particular type of dopamine receptor (D2-like receptors) are likely to find it difficult, or impossible, to ejaculate. Those same drugs have been tested in men with premature ejaculation. Similar to SSRIs, they seem to extend the time between arousal and ejaculation. In addition, there is a small amount of data suggesting that mutations in dopamine receptors may make some men more likely to experience premature ejaculation.

Understanding Ejaculation

Ejaculation is defined as the forceful propulsion of semen from the body. It takes place in two phases. The first phase is emission. That is when the various components of the semen, including sperm, are secreted from various glands and organs. The second phase is expulsion. That is when intense contractions of the muscles in the genital region cause semen to be pushed out of the penis.

Of note, some men who have had certain types of prostate cancer surgery may experience "dry" ejaculation. This is because their bodies no longer produce the fluid components of semen. Approximately one-third of seminal fluid comes from the prostate. Other glands that contribute to seminal fluid production can also be affected by cancer surgeries.

In men, the term orgasm is often used as a synonym for ejaculation. However, ejaculation and orgasm are not the same thing. Although for most men, orgasm primarily takes place at the time of ejaculation, that is not always the case. Some men who are multi-orgasmic may have multiple orgasms with only one ejaculation. Other men are not able to ejaculate at all. This condition is known as anejaculation.

Separating Erection and Ejaculation

It is important to note that an erection is not required for ejaculation to occur. Ejaculation without erection is not common, but it is possible. The most frequent example of this takes place in adolescent boys while they sleep—so-called "nocturnal emissions" or "wet dreams." Nocturnal emissions can occur with or without the presence of an erection. Ejaculation without erection can also be stimulated using vibration of the penis in men with certain types of spinal cord injury. This technique is also sometimes used to collect sperm for assisted reproduction procedures.

Unsurprisingly, the neurological and other physiological pathways that lead to erection and ejaculation are related, but they are not the same. That's why the PDE-5 inhibitors used to help men with erectile dysfunction do not have a significant effect on their ability to ejaculate. They affect the flow of blood into and out of the penis. They do not affect the release of the components of semen or the muscle contractions of expulsion.

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