The purpose of this study was to determine whether plasma oxytocin (OT) levels change during human sexual responses and, if so, to demonstrate the temporal pattern of change. Plasma OT levels were measured by RIA before, during, and after private self-stimulation to orgasm in normal men (n = 9) and women (n = 13). Blood samples were collected continuously through indwelling venous catheters. The subjects pressed a signal to indicate the start and finish of orgasm/ejaculation. Objective assessment of sexual arousal and orgasm was obtained by measuring blood-pulse amplitude and electromyographic activity, recorded continuously throughout testing from an anal device containing a photoplethysmograph and electromyograph electrodes connected to a polygraph located in an adjacent room. These measures allowed collection of data from men and women of changes in blood flow and muscle activity in the lower pelvic/pubic area. Plasma OT levels increased during sexual arousal in both women and men and were significantly higher during orgasm/ejaculation than during prior baseline testing. We suggest that the temporal pattern of secretion could be related to smooth muscle contractions of the reproductive system during orgasm.

A cross-sectional study on 220 men, aged 41-93 yr, was conducted to determine whether age-related changes in circulating pituitary and gonadal hormone levels are related to quantitatively assessed changes in sexuality over this age span. The conclusion of most previous studies, that total and free plasma testosterone (T) levels decline with advancing age as gonadotropins increase, was corroborated. These changes were found to roughly parallel a decline in sexual function affecting the level of sexual activity, libido, and potency measures. PRL and estradiol did not change with age, and the age-related decline in free T was greater than that in total T. Decreases in free T and increases in LH manifested significant, but small, correlations with sexual hypofunction. Behavioral variables were also clearly related to LH and to the ratio of free T to LH and estradiol. The data also suggested that aging and hormonal changes were more strongly related to sexual activity and nocturnal erections that to libido (enjoyment, drive, and thoughts). Partial correlation procedures demonstrated that diseases and drugs were not responsible for the hormone-behavior relationships. Declining androgen levels, reduced sexual activity, and decreased sexual interest thus appear to be related sequelae of the aging process in men. Hormonal factors do not completely account for age-related changes in sexuality, although the full explanation of these changes must include a consideration of hormonal factors.

Sexual function and the effects thereon of testosterone enanthate were studied in six hypogonadal men with the objective of delineating the specific components of male sexuality affected by androgen. To obtain a detailed picture of these components, prospective self-report data (from daily logs) of sexual activity and feelings, recordings of all night penile tumescence, and laboratory psychophysiological data were assessed. Double blind placebo experiments with cross-over design were used to compare the effects of placebo and 200-and 400-mg doses of testosterone enanthate. Erectile responses to erotic film and fantasy were not lower in the hypogonadal patients than in normal men and, in fact, were higher on some parameters, especially prolongation of detumescence time after exposure to film or fantasy. Three subjects who kept consistent daily logs had increased frequencies of sexual acts and feelings, orgasms, and spontaneous erections after testosterone administration. Nocturnal penile tumescence and spontaneous daytime erections were reduced in untreated hypogonadal men and were significantly increased after testosterone treatment, but the laboratory-tested erectile responses to film and fantasy were not affected by testosterone. These data and previous findings lead to the conclusion that the major androgen action on male sexuality involves libido factors {i.e. sexual motivation/interest). Though stimulus-bound erections elicited in the laboratory were not reduced in hypogonadal men, spontaneous (sleep or waking) erections were clearly testosterone dependent. (J Clin Endocrinol Metab 57: 557, 1983) T HOUGH appropriately controlled studies have now clearly vindicated the use of testosterone replacement therapy for the sexual deficits of hypogonadal men, the mechanism of this behavioral action of androgen remains to be established. Recent double blind studies presented self-reported prospective data on sexual activity and libido (1-4) and, also, erections occurring spontaneously (1, 4). However, published data on the effects of androgen on physiological responses to controlled stimuli in the laboratory are still lacking. Such data are important in the elucidation of the mode of action of androgen.One problem in the interpretation of the role of androgen in male sexuality arises from the persistent (albeit poorly documented) reports of the retention of erectile function in castrates (for reviews, see Refs. 5 and 6). These appear to contradict our earlier finding (1) that erections from all causes, including overt sexual behavior, waxed and waned within days of the rise and fall of testosterone levels during periodic administration of testosterone enanthate. The solution may have to do with