Comments: There is a very important concept in this study: Testosterone increases libido by stimulating dopamine. 
Libido always comes down to dopamine. Many porn users who have sexual dysfunction or libido problems think somethings has happened to their testosterone levels. No indeed; they have changed their brains and dopamine functioning. They have dopamine dysregulation, not testosterone dysregulation. If there is dopamine dysfunction, as is often the case with heavy porn use, then all the testosterone in the world will not help with erections and libidos.
Putnam SK, Du J, Sato S, Hull EM. Horm Behav. 2001 May;39(3):216-24. Department of Psychology, State University of New York at Buffalo, Buffalo, New York 14260, USA.
The medial preoptic area (MPOA) is an important integrative site for male sexual behavior. We have reported an increase in dopamine (DA) release in the MPOA of male rats shortly before and during copulation. Postcastration loss of copulatory ability mirrored the loss of the precopulatory DA response to an estrous female.
The present study investigated the time courses of restoration, rather than loss, of the MPOA DA response to a receptive female and of copulation in long-term castrates. Male rats were castrated and tested for loss of copulatory ability 21 days later. They then received 2, 5, or 10 daily subcutaneous injections of testosterone propionate (TP, 500 microg) or oil. Microdialysate samples were collected from the MPOA during baseline, exposure to a female behind a barrier, and copulation. Extracellular DA was measured using HPLC-EC. None of the six 2-day-TP-treated animals copulated, nor did they show elevated DA release in the MPOA in the presence of a receptive female. Five of the nine 5-day-TP-treated animals ejaculated; three intromitted without ejaculating; and one failed to copulate, with all but the noncopulating animal showing elevated DA release.
All of the six 10-day-TP-treated animals copulated and also demonstrated an increase in MPOA DA. None of the oil controls copulated or showed an increase in DA release. Therefore, a consistent relationship between MPOA DA release during exposure to a receptive female and the subsequent ability of the male to copulate was observed.
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