Aromatization and androgen stimulation of sexual behavior in male and female rats

Aromatization Is Not Required for the Facilitation of Appetitive Sexual Behaviors in Ovariectomized Rats Treated With Estradiol and Testosterone

Testosterone can be safely and effectively administered to estrogen-treated post-menopausal women experiencing hypoactive sexual desire. However, in the United States and Canada, although it is often administered off-label, testosterone co-administered with estradiol is not a federally approved treatment for sexual arousal/desire disorder, partly because its mechanism is poorly understood. One possible mechanism involves the aromatization of testosterone to estradiol. In an animal model, the administration of testosterone propionate (TP) given in combination with estradiol benzoate (EB) significantly increases sexually appetitive behaviors (i.e., solicitations and hops/darts) in ovariectomized (OVX) Long-Evans rats, compared to those treated with EB-alone. The goal of current study was to test whether blocking aromatization of testosterone to estradiol would disrupt the facilitation of sexual behaviors in OVX Long-Evans rats, and to determine group differences in Fos immunoreactivity within brain regions involved in sexual motivation and reward. Groups of sexually experienced OVX Long-Evans rats were treated with EB alone, EB+TP, or EB+TP and the aromatase inhibitor Fadrozole (EB+TP+FAD). Females treated with EB+TP+FAD displayed significantly more hops and darts, solicitations and lordosis magnitudes when compared to EB-alone females. Furthermore, TP, administered with or without FAD, induced the activation of Fos-immunoreactivity in brain areas implicated in sexual motivation and reward including the medial preoptic area, ventrolateral division of the ventromedial nucleus of the hypothalamus, the nucleus accumbens core, and the prefrontal cortex. These results suggest that aromatization may not be necessary for TP to enhance female sexual behavior and that EB+TP may act via androgenic pathways to increase the sensitivity of response to male-related cues, to induce female sexual desire.

Two aromatase inhibitors inhibit the ability of a third to promote mating in male rats

Aromatase, the enzyme that aromatizes androstenedione (A) to estrone and testosterone (T) to estradiol (E), affects androgen control of male sex behavior in many vertebrates. In male monkeys, rats and quail, E mimics the ability of T to promote mating, and aromatase inhibitors block mating induced by T but not E. Aromatase inhibitors include androgens with different A-rings than T and A, e.g., 1,4,6-androstatriene-3,17-dione (ATD), azoles, e.g., fadrozole, and androgens α-halogenated at carbon 6, e.g., 6α-bromoA, 6α-fluoroA and 6α-fluoroT. 6α-FluoroT is the only 6α-halogenated androgen studied in regard to mating. It promotes mating in male rats and quail and was studied, before it was known to inhibit aromatase, because it cannot be aromatized yet has the same A-ring as T. 6α-FluoroT might promote mating by binding estrogen receptors (ER) directly, i.e., unassisted, or by metabolism to an androgen that binds ER. Since neither process would require aromatase, this study tested both hypotheses by determining how mating induced in castrated male rats by 6α-fluoroT is affected by ATD and fadrozole. Both aromatase inhibitors inhibited the effects of 6α-fluoroT on mating. Thus, 6α-fluoroT does not promote mating by direct ER binding or metabolism to another androgen. Since aromatase underlies a process in which 6α-fluoroT, unlike most nonaromatizable androgens, mimics T effects on male sex behavior, the process must involve a feature that 6α-fluoroT shares with T but not other nonaromatizable androgens. A-ring structure is a candidate. A hypothesis is also offered for how aromatase may participate without aromatizing the androgen.

The role of gonadal steroid receptor activation in the restoration of sociosexual behavior in adult male rats

This work tested the hypothesis that gonadal steroid receptor activation was necessary for the restoration of several sociosexual behaviors (such as copulatory behavior, partner preference, 50-kHz vocalizations, and scent marking) in testosterone-treated gonadectomized male rats. Gonadal steroid receptors were blocked by systemic administration of the antiandrogen hydroxyflutamide, the antiestrogen RU 58668, or both antagonists simultaneously in a restoration paradigm. Inhibiting androgen receptors with hydroxyflutamide blocked the restoration of male copulatory behavior, partner preference (time spent with a sexually receptive female over a nonreceptive female), 50-kHz ultrasonic vocalizations, and scent marking. On the other hand, we did not find that blocking estrogen receptors with RU 58668 inhibited the restoration of copulatory behavior or partner preference in testosterone-treated gonadectomized male rats, even though the level of brain nuclear estrogen receptor occupation was significantly reduced to the level found in gonadectomized males. However, the restoration of scent marking and 50-kHz vocalizations were impaired by RU 58668. Blocking both nuclear androgen and estrogen receptors with the two antagonists simultaneously did not have a greater inhibitory effect than treatment with each antagonist alone. Therefore, the activation of nuclear estrogen receptors is necessary for the restoration of some, but not all, sociosexual behaviors, which are also androgen receptor-dependent. Besides nuclear estrogen receptors, there are additional, but unknown, targets of estradiol that play a role in mediating copulatory behavior in adult male rats. Moreover, the signals from multiple gonadal steroid signaling pathways converge in the regulation of some sociosexual behaviors in adult male rats.

The effects of cocaine and nandrolone co-administration on aggression in male rats

1. Cocaine and anabolic-androgenic steroids are among the more commonly abused substances in selected populations. These agents, when used alone or in combination, have been reported to cause aggressive tendencies in both laboratory-based animal models and in human clinical situations. This project, using a resident-intruder paradigm, examined the effects of co-administration of cocaine and a typical anabolic-androgenic steroid, nandrolone decanoate, on the development of aggression in male Sprague-Dawley rats. 2. Dose response studies demonstrated that low dose cocaine (1 mg/kg) produced more aggression in a greater percentage of animals than for either the controls or groups receiving higher doses (up to 20 mg/kg). Initially, high intermittent doses of nandrolone (20 mg twice weekly) produced more aggression; however, low daily doses of nandrolone (2 mg) produced greater levels of aggression following 4 weeks of treatment. 3. Optimal doses of cocaine and nandrolone, when administered together, resulted in aggression scores that were not significantly different from controls or either drug singly. However, a greater percentage of animals receiving both drugs exhibited aggression than did rats receiving either drug alone. 4. These results support the interpretation that the drugs interact to produce unique effects in the development of aggression. However, the complexity and extent of the interactions is great and remains to be fully elucidated.

Neuronal aromatase expression in preoptic, strial, and amygdaloid regions during late prenatal and early postnatal development in the rat

Brain aromatase has been considered to be an important clue in elucidating the actions of androgen on brain sexual differentiation. Using highly specific anti-P450arom antiserum, the regional and subcellular distributions were immunohistochemically evaluated in the preoptic, strial, and amygdaloid regions of developing rat brains. Aromatase-immunoreactive (AROM-I) neurons were classified into three groups. The first, in which immunostaining occurs only during certain pre- or neonatal days (E16-P2), included the anterior medial preoptic nucleus, the periventricular preoptic nucleus, neurons associated with the strial part of the preoptic area, and the rostral portion of the medial preoptic nucleus. The second is a striking AROM-I cell group in the "medial preopticoamygdaloid neuronal arc," which extends from the medial preoptic nucleus to the principal nucleus of the bed nucleus of the stria terminalis and the posterodorsal part of the medial amygdaloid nucleus. The AROM-I neurons appeared by E16, reaching a peak in staining intensity between E18 and P2 and diminishing after the perinatal stage. After P14, a third group of AROM-I neurons emerged in the lateral septal nucleus, the oval nucleus of the bed nucleus of the stria terminalis, and the central amygdaloid nucleus. The second group was thought to be the major aromatization center in developing rat brains, while the center might partly shift to the third group of neurons after the late infantile stage. The distribution and developmental patterns were basically similar in males and females, suggesting that the neonatally prominent aromatase is not induced by male-specific androgen surges occurring around birth. On immunoelectron microscopy, subneuronal aromatase was predominantly localized on the nuclear membrane and endoplasmic reticulum, which appeared to be appropriate for the efficient conversion of androgen into estrogen just prior to binding to the nuclear receptors.

Evidence that Neural Aromatization of Androgen Regulates the Expression of Sexual Behaviour in Female Musk Shrews

Abstract The experiments reported on here were conducted to test the hypothesis that sexual behaviour in the female musk shrew (Suncus murinus) is regulated by the neural aromatization of testosterone to oestradiol. In the first experiment ovariectomized animals received subcutaneous hormone implants containing either an aromatizable androgen (testosterone or androstenedione), a non-aromatizable androgen (dihydrotestosterone or methyltrienolone), or cholesterol. Only females that received an aromatizable androgen exhibited significant amounts of sexual behaviour as compared with controls (cholesterol). To examine the role of the oestrogen receptor, the anti-oestrogen, tamoxifen (200 or 400 mug daily) was given to ovary intact or ovariectomized females treated with testosterone. Tamoxifen treatment had significant negative effects both on female sexual behaviour and on the weights of several peripheral tissues as compared with control treatments. A similar set of experiments was conducted to examine the effect of an anti-androgen on female sexual behaviour. The androgen receptor blocker, flutamide, had no effect on sexual behaviour or weights of peripheral tissues. To determine whether flutamide can act as an anti-androgen in this species two final experiments were conducted in male musk shrews. Flutamide treatment in males did affect several measures of sexual behaviour. In summary, these data demonstrate that the oestrogen receptor is involved in the control of female copulatory behaviour. The androgen receptor plays a role in the expression of male, but not female, sexual behaviour. Female musk shrews display copulatory behaviour in advance of follicular development when oestradiol concentrations in plasma are very low. Thus, they may have evolved a strategy of aromatizing peripherally produced androgens in the brain to concentrate the oestrogen required for the expression of sexual behaviour.

Can a single androgen receptor fill the bill?

If the above two hypotheses are correct, they would require at least one more specific nuclear receptor for T, and at least one membrane receptor to account for the very rapid effects induced by androgens on certain target tissues. If this is the case, clearly a single androgen receptor will not fill the bill.

6 alpha-fluorotestosterone: a nonaromatizable androgen inhibitor of aromatase cytochrome P450

In an early survey of steroids which might serve as estrogen precursors, Gual et al. reported that 6 alpha-fluorotestosterone is not aromatized by human placental microsomes. Subsequently, 6 alpha-fluorotestosterone has been used to distinguish between androgen- and estrogen-mediated physiologic effects. We have reexamined the interaction of 6 alpha-fluorotestosterone with human placental and rat ovarian microsomes and with reconstituted purified aromatase cytochrome P450. Under conditions in which testosterone was readily aromatized, no aromatization of 6 alpha-fluorotestosterone was observed using either fluorescence detection of dansyl-estrogens separated by high-performance liquid chromatography or estrogen radioimmunoassay methods. The lack of aromatization is not due to failure of 6 alpha-fluorotestosterone to bind to P450arom, because 6 alpha-fluorotestosterone acts as a competitive inhibitor of the enzyme, and it exhibits a binding affinity similar to that of testosterone. Moreover, the addition of 6 alpha-fluorotestosterone to human placental microsomes elicits a spectral shift indicative of conversion of the heme from a low to a high spin state as observed for androgen substrates, consistent with its binding to the substrate site. The mechanism by which substitution of a fluorine at the 6 alpha-position interferes with the aromatization reaction remains to be determined, but the inhibitory action on estrogen formation may potentiate the androgenic properties of 6 alpha-fluorotestosterone in vivo due to a lowering of estrogen levels.

Effects of intracranial implants of dihydrotestosterone on the reproductive physiology and behavior of male guinea pigs

An experiment was conducted to determine whether the intracranial application of dihydrotestosterone (DHT), a nonaromatizable androgen, would stimulate male guinea pig mating. Of three castrate groups studied, one was a control group in which subjects were implanted both in the medial preoptic area (MPOA) and under the skin with cannulae containing cholesterol (NoDHT). Males in one of the experimental groups received implants of cholesterol in the MPOA plus subcutaneous implants containing DHT (ScDHT). Members of the other experimental group were subcutaneously implanted with cholesterol and simultaneously given intracranial implants of DHT (IcDHT). Compared to either the NoDHT control group of the ScDHT experimental group, greater numbers of males in the IcDHT group displayed mounts (P less than 0.01), intromissions (P less than 0.01), and ejaculations (P less than 0.001). Additionally, the hypothalamic implants of DHT had no significant effects on peripheral target tissues. These data indicate that androgenic stimulation of the guinea pig brain is sufficient to activate masculine sexual behavior in this species.

Testosterone dose-dependency of sexual and nonsexual behaviors in the gonadotropin-releasing hormone antagonist-treated male rat

The testosterone dose-dependency of several mating and nonmating behaviors was examined in the male rat, chemically castrated with a GnRH antagonist analog. Graded doses of testosterone enanthate (TE) were given to male rats to reinstate behaviors abolished by GnRH antagonist treatment. GnRH antagonist treatment alone markedly lowered serum LH, FSH and T concentrations and ventral prostate and testis weights. Open field behaviors were not significantly affected by GnRH antagonist treatment or castration. Scent-marking behavior was markedly suppressed by both castration and GnRH antagonist and restored by the lowest dose of TE (0.05 mg). All measures of male sexual behavior were impaired by GnRH antagonist treatment and castration and restored by the lowest dose of TE (0.05 mg). The doses of TE required to restore normal ventral prostate weights and testis weights were higher than those required to maintain scent marking and mating behaviors. No direct behavioral effects of the GnRH antagonist, other than those that can be explained by GnRH antagonist-induced suppression of testosterone were observed. The finding that sexual and nonsexual behaviors in the male rat have different testosterone requirements from those maintaining spermatogenesis and fertility may have significant implications for contraception.

Effects of ATD on male sexual behavior and androgen receptor binding: a reexamination of the aromatization hypothesis

The aromatization hypothesis asserts that testosterone (T) must be aromatized to estradiol (E2) to activate copulatory behavior in the male rat. In support of this hypothesis, the aromatization inhibitor, ATD, has been found to suppress male sexual behavior in T-treated rats. In our experiment, we first replicated this finding by peripherally injecting ATD (15 mg/day) or propylene glycol into T-treated (two 10-mm Silastic capsules) or control castrated male rats. In a second experiment, we bilaterally implanted either ATD-filled or blank cannulae into the medial preoptic area (MPOA) of either T-treated or control castrated male rats. With this more local distribution of ATD, a lesser decline in sexual behavior was found, suggesting that other brain areas are involved in the neurohormonal activation of copulatory behavior in the male rat. To determine whether in vivo ATD interacts with androgen or estrogen receptors, we conducted cell nuclear androgen and estrogen receptor binding assays of hypothalamus, preoptic area, amygdala, and septum following treatment with the combinations of systemic T alone. ATD plus T, ATD alone, and blank control. In all four brain areas binding of T to androgen receptors was significantly decreased in the presence of ATD, suggesting that ATD may act both as an androgen receptor blocker and as an aromatization inhibitor. Competitive binding studies indicated that ATD competes in vitro for cytosol androgen receptors, thus substantiating the in vivo antiandrogenic effects of ATD. Cell nuclear estrogen receptor binding was not significantly increased by exposure to T in the physiological range. No agonistic properties of ATD were observed either behaviorally or biochemically. Thus, an alternative explanation for the inhibitory effects of ATD on male sexual behavior is that ATD prevents T from binding to androgen receptors.

Activation of sexual behavior in male rats by combined subcutaneous and intracranial treatments of 5 alpha-dihydrotestosterone

When given peripherally, 5 alpha-dihydrotestosterone, the major androgenic metabolite of testosterone, is relatively less effective than testosterone in activating sexual behavior of castrated male rats. In order to test the possible central nervous system effects of dihydrotestosterone more directly, we castrated Long-Evans rats, gave them a behaviorally subthreshold dose of dihydrotestosterone placed subcutaneously in Silastic capsules (ScDHT), and then additionally treated the rats with intracranial implants of crystalline dihydrotestosterone (IcDHT, N = 12), testosterone (IcT, N = 12), or cholesterol (IcCHOL, N = 10) placed in the medial preoptic area. The peripheral ScDHT treatment maintained sexual organ weights of castrated males at levels comparable to those of intact males, but did not in itself significantly activate mating behavior. The addition of IcT or IcDHT to this treatment regimen significantly increased the number of males displaying mounting behavior, intromissions, and ejaculatory behavior (P less than 0.05) compared to males with IcCHOL implants. There were no significant differences between the group given IcT and the group given IcDHT. Results of this study support the hypothesis that the nonaromatizable androgen 5 alpha-dihydrotestosterone can act in the rat brain to influence male sexual behavior. In addition, these data lead us to suggest that the relative ineffectiveness of dihydrotestosterone versus testosterone when given systemically may reflect differences in bioavailability of these hormones to the brain following such treatment.

The influence of steroid hormones on competing sexual and ingestive behavior in the male rat

Water replete rats allowed restricted access to a sweet nonnutritive solution (0.2% Acesulfame-K) spend about one third of their time drinking it. This ingestive response is markedly inhibited if the male rat is simultaneously presented with an estrous female, but not an anestrous female or another male, despite the fact that there is sufficient time for both sexual and ingestive behaviors to occur. Castration and the subsequent decline in sexual behavior is accompanied by an increase in Acesulfame ingestion in the presence of a receptive female. Treatment with testosterone reverses both these effects. Similarly treatment of castrate males with DHT and estradiol (the active metabolites of testosterone) maintains both full sexual behavior and suppression of the ingestive response. However, the steroid requirements for sexual activity do not correspond completely with those for the sexually-induced suppression of ingestive behavior. Treatment of castrate males with estradiol alone maintains mounting behavior (but no intromissions or ejaculations) but does not suppress ingestive behavior in the presence of a receptive female--indeed under these suboptimal hormone conditions sexual behavior appears to be reduced in the presence of Acesulfame. Steroid hormones, therefore, have at least two effects upon sexual behavior. They enable certain aspects of sexual behavior such as intromissions and ejaculations, and also alter the animal's priority of response to two competing (ingestive and sexual) stimuli.

Inhibition of central nervous system aromatase activity: a mechanism for fenarimol-induced infertility in the male rat

Fenarimol (alpha-(2-chlorophenyl)-alpha(4-chlorophenyl)-5-pyrimidine-methanol), a pyrimidine carbinol agricultural fungicide, was previously reported to cause a dose-related decrease in fertility in rats (K. S. Hirsch, E. R. Adams, D. G. Hoffman, J. K. Markham, and N. V. Owen (1986), Toxicol. Appl. Pharmacol. 86, 391-399). Based on the results of a number of reproduction studies (K. S. Hirsch, E. R. Adams, D. G. Hoffman, J. K. Markham, and N. V. Owen (1986), Toxicol. Appl. Pharmacol. 86, 391-399), the infertility appeared to be associated with an impairment of male sexual behavior. When [14C]fenarimol was administered to the dam, high concentrations of radioactivity were observed in the neonatal hypothalamus, which functions in the development and subsequent expression of male sexual behavior. In the present studies fenarimol exhibited neither antiandrogenic nor antiestrogenic activities. The compound did, however, prevent the increase in nuclear estrogen receptors in the brain which normally occurs in the male during the early postnatal period. These results suggested that fenarimol might be acting to inhibit estrogen biosynthesis (via the aromatase enzyme complex) within the central nervous system. [3H]Testosterone was administered to neonatal rats, and the tritiated metabolites were isolated. Testosterone and dihydrotestosterone (17 beta-hydroxy-5 alpha-androstan-3-one) concentrations were similar in all treatment groups. Tritiated estrogens were detected in the brain cell nuclei from control neonates but not in neonates exposed to fenarimol. Fenarimol was also observed to inhibit rat ovarian aromatase activity in vitro. These data indicate that the decrease in male sexual behavior and the infertility associated with exposure to fenarimol were, most likely, due to inhibition of aromatase activity within the central nervous system.

Organization and activation of behavior in quail: role of testosterone metabolism

In quail, the hypothalamus enzymatically transforms testosterone (T) into estradiol (E2), 5 alpha-dihydrotestosterone (5 alpha-DHT), and 5 beta-dihydrotestosterone (5 beta-DHT). During the embryonic life, the 5 beta-reductase activity is very high, which probably protects the brain of males from being behaviorally demasculinized by their endogenous T. 5 beta androstanes are inactive androgens. The decrease of 5 beta reductase with age during sexual maturation corresponds to a potentiation of the effects of T as shown by experiments that compared the effects of T and 5 alpha-DHT in adult and young quail. T metabolism is also involved in the activation of male behavior in the adult. T aromatization is probably essential for behavioral activation, but nonaromatizable androgens such as methyltrienolone, and to some extent 5 alpha-DHT, can also stimulate sexual behavior in castrates. These enzymatic activities show a clear neuroanatomical localization and are sexually dimorphic. Males produce more active metabolites (E2, 5 alpha-DHT) than females, which could explain the male's greater sensitivity to T treatments. It thus appears that T metabolism is involved in the differentiation and activation of behavior in quail.

Dihydrotestosterone stimulates mounting behavior but not lordosis in female rats

The ability of androgens to stimulate masculine sexual behavior is thought to depend on the aromatization of such androgens to estrogens. In this scheme, reduced androgens such as dihydrotestosterone (DHT) which cannot be aromatized, are thought to exert major peripheral but little or no central nervous system influences on the display of masculine sexual behavior. Further, an early report that DHT can induce lordosis, an estrogen (E) dependent behavior, led to a notion that DHT may effect behavior through metabolic intermediates such as 5 alpha-androstane-3 beta, 17 beta-diol (ADIOL) which then binds to estrogen receptors eliciting the E-dependent lordotic response. The present study reexamined and compared the relative effectiveness of a range of DHT dosages in stimulating a characteristic masculine (mounting) and feminine (lordosis) sexual behavior. Adult ovariectomized rats were randomly assigned to either 250 micrograms or 1 mg daily injections of DHT or DHTP. Other animals received OIL injections or crystalline DHT delivered by two different lengths (20 mm or 40 mm) of Silastic capsules. Animals were tested once weekly (for 5 weeks) for mounting behavior (20 minute test). Then animals were tested thrice (once weekly) for lordosis 4 hrs after the addition of 500 micrograms Progesterone (P). Finally, all females were tested for lordotic potential to respond to 10 micrograms EB plus P. 1 mg DHT or DHTP dosages and the 40 mm-Silastic condition significantly increased mounting behavior over that of lower dosages and OIL controls. A significant correlation existed between mounting frequency and circulating level of serum DHT. Treatment with DHTP was not different than DHT in eliciting mounting behavior.(ABSTRACT TRUNCATED AT 250 WORDS)

Methyltrienolone (R1881) is not aromatized by placental microsomes or rat hypothalamic homogenates

The in vitro conversion of the synthetic steroidal androgen methyltrienolone (17 beta-hydroxy-17 alpha- methylestra -4,9,11-trien-3-one) to an estrogen was investigated. First, with a placental microsome preparation a 10-fold molar excess of methyltrienolone did not affect the aromatization of testosterone. Therefore, methyltrienolone did not compete with testosterone in this very active aromatase system, nor did it inhibit the aromatization. Second, while the placental aromatase very efficiently converted testosterone to estrogens, it did not convert methyltrienolone [3H] to estrogenic (phenolic) products. Third, homogenates of rat hypothalamic tissue were also unable to aromatize methyltrienolone, although they converted small quantities of testosterone to estradiol and 19-hydroxyandrostenedione to estrone. This lack of aromatization of methyltrienolone is consistent with observations of additional behavioral and in vivo biochemical experiments.

The genetics of hormonal influences on male sexual behavior of mice and rats

This review focuses on the intersection of genes and hormones as they relate to the development of male sexual behavior. Three major hypotheses are discussed: (1) Some differences in adult male sexual behavior are due to gene differences that influence brain differentiation. Genes that influence brain differentiation may do so by affecting the elaboration of testosterone (i.e., H-Y antigen) or the sensitivity to testosterone (i.e., Tfm mutation and autosomal variations) during neonatal and/or prenatal life. (2) Some differences in male sexual behavior are due to gene differences that influence adult levels of testosterone or sensitivity to testosterone and its metabolites. (3) There is a gene(s) on the Y chromosome that influences the development of sexual behavior that is associated with the arousal mechanism. A possible hormonal mechanism of this Y chromosomal gene(s) is discussed.

Hormonal replacement and sexuality in men

Only in the last few years has the scientific study of hormonal replacement therapy for hyposexuality begun in earnest with the advent of appropriately controlled experiment studies. Dose-response relationships can be demonstrated between testosterone (T) and sexual measures, but these have not yet been investigated in detail. Some aspects of sexual function are maintained in the presence of androgen levels well below the normal range, but preliminary evidence suggests that within a normal population high levels of T are correlated with more vigorous responses to visual erotic stimuli. Though T (and to a greater extent free T) declines with aging in parallel with the decline of sexual function, these hormonal changes contribute only to a minor extent to the behavioural change. Some non-aromatizable androgens may be less effective in stimulating sexual behaviour than T, but initial data on effects of dihydrotestosterone suggests that the capacity of an androgen to be aromatized (converted to oestrogen) is not a requirement for its sexual action. While T apparently increases the incidence of all types of male sexual activity, recent data contradict the belief that it directly facilitates the erectile mechanism in men, even though erection frequency is greatly reduced in untreated hypogonadal men. At the present juncture, it appears that the initial action of T may be on libido factors which lead in turn to the stimulation of other aspects of sexuality. Specifically, we propose that androgen acts through stimulating genital sensations and/or other pleasurable awareness of sexual response rather than directly through cognitive processes such as sexual imagery.