The importance of testicular hormones in nonhuman males

Peasants have known that castration modifies the behavior of domestic animals for centuries. Bulls were turned into oxen for making them more manageable and males of some other species were castrated for improving the taste of their meat. This shows that both behavioral and physiological consequences of testicular products in non-human animals have been known for a long time. Despite the age-old popular knowledge, it was only about 100 years ago that the first experimental study of the behavioral consequences of testicular removal in a mammal was published. Male rats were castrated and their copulatory behavior was found to disappear after some time (Steinach, 1894). The disappearance of sexual behaviors after castration has been confirmed in many mammalian species, including the human as mentioned in the previous paragraphs. A question that immediately posed itself to the scientists performing these studies was what the testicles did in order to maintain sexual behavior. Those who had castrated human males for thousands of years had never asked that interesting question, curiously enough. In contrast, a professor at the Universität Göttingen in Germany had not only asked the question but had also obtained the answer. In a study performed in roosters it had been found that extirpation of the testicles eliminated mating behaviors. However, if the testicles simply were moved from their normal location to the abdominal cavity rather than being removed from the animal, there was no decline in sexual behaviors. All nervous and vascular connections to the testicles had been cut during the process. A post-mortem histological examination revealed that the moved testicles had revascularized, but there was no indication of a re-established nerve supply. The conclusion was, logically, that the testicles produced some blood-borne factor that stimulated sexual behavior (Berthold, 1849). This suggestion was confirmed in rats in a now classic study in which injections of a testicular extract were found to restore copulatory behavior in castrated males (Nissen, 1929). The blood-borne factor emanating from cells in the testicle was identified in 1935. It turned out to belong to a group of lipids called steroids and was given the name testosterone. Soon after, it was shown that injections of testosterone, either free or in the form of the propionate ester, activated copulatory behavior in castrated male rats (Shapiro, 1937; Moore and Price, 1938; Stone, 1939). The capacity of testosterone to restore copulatory behavior in castrated males was rapidly confirmed in many different species of mammals (Beach, 1948). A clear picture of the endocrine control of copulatory behavior in male mammals had been obtained. There were no data contradicting the notion that testicular hormones, more specifically testosterone, were necessary. Furthermore, several studies revealed that the hormone acted directly within the brain when reactivating copulatory behavior in castrated male rats. Implants in the medial preoptic area were able to restore behavior. These implants had no effect on peripheral target tissues, showing that the hormone did not leak out into the circulation in physiologically significant amounts (e.g. Davidson, 1966). Accordingly, its action must have been within the brain.

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