The major steroids influencing normal sexual function in males are the androgens secreted by the testes mainly as testosterone with much smaller quantities of androstenedione. The adrenal cortex also manufactures and secretes androgens but this amounts only to 2 per cent of the total. Of the testosterone, 95 per cent is bound to plasma protein; only the unbound fraction is an active virilizing hormone.

The development of the masculine musculature, bone growth, genitals, and pubic and axillary hair is androgen dependent. The mechanism of the hormonal masculinization of the brain in rodents involves the aromatization of testosterone, but in humans the role of aromatization is uncertain. (31>

Androgens and male sexual behaviour

The possible involvement of androgens in adult human male sexual behaviour has been examined many times (see Levin (32) for references). Removal of testosterone by castration usually leads to a decrease in sexual activity and drive in the majority of subjects within 12 months. There is, however, large individual variations, and some castrates retain sexual activity and interest for years.(33) Factors such as adrenal androgens, the availability of sexual partners, and attitude to the operation influence the response. In castrates and in hypogonadal males, replacement of testosterone restores sexual interest and activity.

Everitt and Bancroft(34) proposed two systems in the brain that control erections. The first is an androgen-dependent system subserving sexual desire and arousability and assessed by measuring spontaneous erections during sleep (nocturnal penile tumescence). Nocturnal penile tumescence is impaired in androgen insufficiency and can be restored by androgen replacement. The second system, which is androgen independent, is involved in the erectile response to visual erotic stimuli. It is claimed to be unaffected in hypogonadal men.(34) More recently, however, Bancroft(35) reported that the original proposal was an oversimplification resulting from the use of a single measure of penile response. When other measures are taken, the erectile responses to visual erotic stimuli are affected by androgens in terms of the duration, degree of rigidity, and speed of the detumescence, all measures which had not been used in the earlier studies. Despite these laboratory findings, androgen replacement in androgen-deficient men affects nocturnal penile tumescence more than the erections due to visual erotic stimuli.

Gonadotrophin secretion and control

The control of gonadotrophin secretion in the adult male is by hormonal negative-feedback circuits: luteinizing hormone by testosterone and follicle-stimulating hormone by inhibin (secreted from the Sertoli cells). In the adult male oestrogens are produced by the testis. They exert powerful negative feedback on gonadotrophin secretion (100 times more potent than testosterone) but their amounts are very small and they are not thought to play any significant physiological role. Prolactin is secreted in the male, but it has no proven reproductive function. However, pathologically high levels of prolactin (hyperprolactinaemia) are accompanied by disturbances of sexual function, especially erectile dysfunction. The cause is unknown.

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