No evidence for prolactin’s involvement in the post-ejaculatory refractory period

In many species, ejaculation is followed by a state of decreased sexual activity, the post-ejaculatory refractory period. Several lines of evidence have suggested prolactin, a pituitary

hormone released around the time of ejaculation in humans and other animals, to be a decisive player in the establishment of the refractory period. However, data supporting this hypothesis

is controversial. We took advantage of two different strains of house mouse, a wild derived and a classical laboratory strain that differ substantially in their sexual performance, to

investigate prolactin’s involvement in sexual activity and the refractory period. First, we show that there is prolactin release during sexual behavior in male mice. Second, using a

pharmacological approach, we show that acute manipulations of prolactin levels, either mimicking the natural release during sexual behavior or inhibiting its occurrence, do not affect sexual

activity or shorten the refractory period, respectively. Therefore, we show compelling evidence refuting the idea that prolactin released during copulation is involved in the establishment

of the refractory period, a long-standing hypothesis in the field of behavioral endocrinology.

Sexual behavior follows the classical sequence of motivated behaviors, terminating with an inhibitory phase after ejaculation: the post-ejaculatory refractory period (PERP)1. The PERP is

highly conserved across species and includes a general decrease in sexual activity and also inhibition of erectile function in humans and other primates2. This period of time is variable

across and within individuals and is affected by many factors, such as age3,4 or the presentation of a new sexual partner5,6. The PERP is thought to allow replacement of sperm and seminal

fluid, functioning as a negative feedback system where by inhibiting too-frequent ejaculations an adequate sperm count needed for fertilization is maintained7,8.

Several lines of evidence have suggested the hormone prolactin (PRL) to be a key player in the establishment of the PERP9,10. PRL is a pleiotropic hormone, first characterized in the context

of milk production in females, but for which we currently know several hundred physiological effects in both sexes11,12. The association of PRL to the establishment of the PERP in males is

based on several observations. First, it was shown that PRL is released around the time of ejaculation in humans and rats13,14,15,16,17,18,19,20,21. Anecdotally, no PRL release has been

observed in a subject with multiple orgasms22. Second, chronically abnormal high levels of circulating PRL are associated with decreased sexual drive, anorgasmia, and ejaculatory

dysfunctions23,24. Finally, removal of PRL-producing pituitary tumors or treatment with drugs that inhibit PRL release reverse sexual dysfunctions25,26. Taking these observations into

consideration, it has been hypothesized that the PRL surge around the time of ejaculation plays a role in the immediate subsequent decrease of sexual activity, the hallmark of the PERP. In

fact, this idea is widespread in behavioral endocrinology textbooks27 and the popular press (https://en.wikipedia.org/wiki/Refractory_period; https://www.humanitas.net/treatments/prolactin).

PRL is primarily produced and released into the bloodstream from the anterior pituitary11,28 and consistent with its functional diversity, PRL receptors are found in most tissues and cell

types of the body29,30. Therefore, PRL may depress sexual activity directly, via PRL receptors present in the male reproductive tract. In fact, PRL has been shown to impact the function of

accessory sex glands and to contribute to penile detumescence31. PRL can also affect central processing, as it can reach the central nervous system either via circumventricular regions

lacking a blood–brain barrier32 or via receptor-mediated mechanisms33, binding its receptors which have widespread distribution, including in the social brain network34. Hence, circulating

PRL can impact the activity of neuronal circuits involved in the processing of socio-sexual relevant cues and thus sexual performance. Circulating PRL reaches the central nervous system on a

timescale that supports the rapid behavioral alterations that are observed immediately after ejaculation (in