Testosterone and estradiol modulate TSH-binding in the thyrocytes of Wistar rats: influence of age and sex.

Age and sex are important factors that influence thyroid pathophysiology. Though sex steroids are known to enhance thyrotropin (TSH) mRNA expression and incidence of thyroid tumours, there is no report on their effects on TSH action under normal physiological conditions. In the present study, the effects of testosterone (T) and estradiol (E2) on thyroidal TSH-receptor (TSH-R) concentration, and TSH-binding to thyrocytes (in vitro) were elucidated in immature and mature Wistar rats. Immature (10 days old) and adult (120 days old) rats of either sex were gonadectomized (GDX) and one group of GDX rats was treated with physiological doses of T and another with E2. Immature GDX rats were supplemented with the steroids for 10 days and adults were supplemented with the steroids for 30 days. While supplementation of steroids to immature rats was begun immediately after surgery, for adult rats it was started 10 days after gonadectomy. The rats were killed at the end of the experimental period. Gonadectomy significantly decreased serum TSH, and TSH-R concentration under in vivo condition and [125I]-TSH binding to thyrocytes under in vitro conditions. Supplementation of T to male and E2 to female GDX rats restored normality of the parameters. Thyrocytes of immature male rats challenged with linearly increasing doses of TSH or T (6.25-800 ng/ml) showed a dose-dependent increase in TSH-binding. However, thyrocytes of immature female rats challenged with T showed a gender-specific response. While there was a linear increase in TSH-binding in thyrocytes of males, a biphasic response was evident in thyrocytes of females. In the case of thyrocytes from adult rats, T induced a dose-dependent change in TSH-binding in males, which reached the peak in response to 12.5 ng T, and diminished thereafter. In contrast, E2 was inhibitory to TSH-binding to thyrocytes of adult male rats. On the other hand, E2 showed a clear gender-specific stimulation of TSH-binding in thyrocytes of females and an inhibition of the same in males. TSH and sex steroids upregulated TSH receptors in immature rats, whereas the effect was biphasic in adult rat thyrocytes. It is concluded from the present study that sex steroids modulate TSH-binding in rat thyrocytes, which may vary according to the age and sex of the animals.     

Testosterone and estradiol differentially regulate TSH-induced thyrocyte proliferation in immature and adult rats

Though sex steroids are found to influence thyroid pathogenesis in human and in animals, their role in normal thyroid growth and thyrocyte proliferation is not yet understood fully. The present study is addressed to know the effect of testosterone and estradiol on the basal and TSH-induced thyrocyte proliferation in immature and adult rats in vitro. The male and female Wistar rats were gonadectomized (GDX) and one group of GDX rats were supplemented with either testosterone or estradiol. After the experimental period, the rats were sacrificed by decapitation and thyroid glands were removed, washed in Hank's Balanced Salt Solution (HBSS), pH 7.4 and digested with the enzyme mixture containing 0.08% collagenase and 0.12% dispase in HBSS. The isolated follicles were washed thrice with Dulbecco's modified Eagle's medium (DMEM) containing 0.5% fetal bovine serum (FBS), and were cultured in Falcon's tissue culture flasks containing 5 ml DMEM with FBS (5%) transferrin (5 microg/ml), hydrocortisone (10(-8) M), somatostatin (10 microg/ml), insulin (10 microg/ml) and glycyl-L-histidyl-L-lysine acetate (10 microg/ml). The cells (2.5 x 10(4)) were exposed to various exponential doses of TSH or testosterone (6.25-800 ng/ml) or estradiol (6.25-800 pg/ml). It is suggested from the present study that both TSH and sex steroids enhance thyrocyte proliferation. The mitogenic effect of TSH is greater than that of sex steroids. Sex steroids modulate TSH-induced cell proliferation in a gender-specific manner.     

Developmental profiles of TSH, sex steroids, and their receptors in the thyroid and their relevance to thyroid growth in immature rats.

Sex steroids are reported to influence thyroid pathogenesis in human and experimental animals. However, there is no report on this phenomenon during the early developmental period. The mitotic activity of thyrocytes in rats reaches its peak by day 10 postpartum. Thyrocytes actively proliferate in immature rats during the first three postnatal weeks, during which the pre-pubertal rise in serum titers of testosterone and estradiol has been recorded. The aim of the present study was to analyze whether there is a physiological relevance between thyroid growth and sex steroids during the postnatal period. Serum and thyroid tissue hormones (TSH, testosterone, and estradiol) were assayed by liquid phase RIA, and receptors for these hormones were also quantified. The peak rate of thyrocyte proliferation was observed during the second postnatal week in rats. Since the concentrations of sex steroids and their receptors also reached a peak around this period, it is suggested that elevated sex steroids and their receptors in the thyroid might enhance thyrocyte proliferation. A positive correlation between thyroid growth indices and sex steroids and their receptors further strengthens this suggestion. This is a preliminary study, and further experimental study may strengthen this proposal. This is the first report to show the availability of sex steroids and their receptors in the thyroid glands of immature rats under normal conditions.     

Androgen receptors in rat and human prostate

In intact adult rats almost all androgen receptor (AR) sites of the rat ventral prostate (RVP) are occupied by endogenous dihydrotestosterone, and about 80% of these sites are nuclear. Nuclear AR disappears rapidly after castration (half-life of 3 h). The amount of cytosolic AR does not change within the initial 36 h, then markedly decreases during the next 2-5 days. An early and specific action of androgen is a remarkable increase of its own receptor. RVP also contains an estradiol receptor (ER) which rapidly disappears after castration and which, contrary to AR, is predominantly localized in the cytosol of stromal elements. The published procedures for steroid receptors grossly underestimate receptors concentrations in normal (NHP) and hyperplastic (BPH) human prostate. We have recently established a reliable method for the measurement of total AR, and we have found no difference in AR concentrations between NHP and BPH. BPH also contains a progesterone receptor and an elusive ER. Finally, we have used specific immunoglobulins in sex hormone binding plasma protein (SBP) for the demonstration of SBP-like immunoreactivity by the indirect immunofluorescence technique. The specific antigenic material was exclusively localized in the cytoplasm of BPH epithelial cells.     

Synthesis and phosphorylation of androgen receptor of the mouse brain cortex and their regulation by sex steroids during aging

To examine the synthesis and phosphorylation of androgen receptor (AR) and their regulation by sex steroids, adult (24 weeks) and old (65 weeks) male and female mice were gonadectomized and administered with testosterone and estradiol. AR amount, synthesis and phosphorylation were measured in the brain cortex by immunoblotting and immunoprecipitation using antibody raised against rat AR transactivation domain (TAD) which was expressed in E. coli as a fusion protein. We found that the amount of AR was high in adult and declined in old mice of both sexes. Administration of testosterone and estradiol significantly down-regulated the level of AR in old male and adult female. Similarly, the rate of AR synthesis also declined with age. Exogenous treatment of gonadectomized mice with testosterone and estradiol reduced the extent of synthesis significantly in all groups except in old female. No sex-dependent variation was noticed either in the level or synthesis of AR. In contrast, the extent of phosphorylation was higher in old mice of both sexes as compared to their adult counterparts. Testosterone and estradiol supplementation resulted in remarkable increase in AR phosphorylation in all groups. Thus it is evident from our findings that the amount and synthesis of AR decrease but phosphorylation of AR increases in the brain cortex with advancing age of mice and they are regulated by testosterone and estradiol in age and sex-specific manner.     

Is gender difference in postnatal thyroid growth associated with specific expression patterns of androgen and estrogen receptors?

Variations in sex steroids bioavailability were linked to the gender difference in the growth of thyroid glands of neonatal rats. In the present study we tested androgen receptor (AR) and estrogen receptor (ER) concentrations by ligand binding assay, and expression of their genes by RT-PCR and Western blot in the thyroid glands of neonatal rats. AR concentration remained elevated from postnatal day (PND) 10 onwards in males, whereas it decreased by PND 20 in females. AR mRNA and protein expressions were higher in males than females, which increased by PND 10, decreased after PND 15 and reached the nadir by PND 20. ER concentration increased by PND 10 and decreased thereafter in both sex. ERα mRNA expression diminished by PND 15 in both sex; while ERβ mRNA decreased by PND 15 to reach the nadir by PND 20 in males, it was augmented by PND 10 in females to reach the peak by PND 15 and diminished by PND 20. ERα protein expression increased by PND 10 and remained elevated till PND 20 in both sex. ERβ protein expression in males increased by PND 10 and decreased by PND 20, while it remained static up to PND 15 and decreased in females. Testosterone stimulated [³H]-thymidine uptake and the expression of IGF-1 and NIS genes in thyrocytes of both sex in vitro, while estradiol stimulated them in females but not in males. We conclude that androgens influence the growth and differentiation of thyrocytes through augmented expression of AR, IGF-1 and NIS in either sex, whereas estrogen imparts the gender difference, which may be at a level beyond the expression of ERs.     

Immunolocalization of estrogen and androgen receptors and steroid concentrations in the stallion epididymis

The presence of steroids and their receptors throughout development, specifically androgen receptor (AR), estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta), in the epididymis of a high estrogen producing species like the stallion has not been determined. Epididymal and testicular samples were collected for analysis of testosterone and estradiol-17beta (E(2)) concentrations and for immunolocalization of AR, ERalpha and ERbeta. The concentration of testosterone in the testis and epididymis were not different among age groups (P>0.05). AR was localized in the principal cells of the caput, corpus and cauda in all four age groups. This lack of change in testosterone concentration and receptor localization suggests that testosterone is important for both development and maintenance of epididymal function. There was an age-related increase in E(2) concentrations in all regions of the epididymis (P<0.05), suggesting that E(2) is also important for adult function. ERbeta was localized in the principal cells of the caput, corpus and cauda in all four age groups, but the localization of ERalpha was regional and age dependent. In peri-pubertal animals, ERalpha immunostaining was most prominent and estradiol was similarly present in all three epididymal regions; this suggests that estradiol also plays a key role in the maturation of the stallion epididymis during the pubertal transition when sperm first arrive in the epididymis. In conclusion, these results suggest that the stallion epididymis is regulated by both androgens and estrogens throughout development and that estradiol is more important to epididymal function in the stallion than previously believed.     

The effect of dexamethasone on disruption of ovarian steroid levels and receptors in female rats

This study was conducted to investigate if the injection of a single dose of dexamethasone may cause disruption of adult female rat gonadal function in terms of plasma and ovarian level of both androgen and estrogen, ovarian morphology, and changes in localization of androgen, estrogen and glucocorticoid receptors. Adult female Long Evans rats (n=50, 250-300 g) were used. At day 0 rats received subcutaneously 1 ml of saline (n=25; control group) or dexamethasone at 0.1 mg/kg (n=25, treated group). Rats were sacrificed in groups of five on days 10, 15, 20, 25 and 30 after injection. Blood samples and one ovary were collected to analyze dexamethasone, 17beta-estradiol (E2), testosterone (T) and androstenedione (A4) concentrations by amplified EIA. The remaining ovary was removed and processed for histopathology and immunocytochemistry. Differences between individual means were analyzed by Pairwise t-test and Bonferroni post test to asses whether values presented statistical significance. Increased E2, T and A4 levels were observed both in plasma and ovary samples in treated group when comparing with control (p< 0.01) at all days post-injection even when dexamethasone was undetectable. Ovarian morphology of treated group showed features compatible with female infertility. Inmmunolocalization of androgen and estrogen receptors showed that both were negative in treated group while controls showed highest positivity (AR +++, ER ++). Glucocorticoid receptor showed higher positivity in dexamethasone treated rats (GR ++) than in controls (GR +). Obtained results showed clear evidence that a single dose of dexamethasone may disrupt gonadal function in rats, and that possibly leads to infertility.     

Sex differences in androgen-regulated expression of cytochrome P450 aromatase in the rat brain

The basis of functional gender differences in adult responsiveness to testosterone (T) is not yet understood. Conversion of T to estradiol by cytochrome P450 aromatase in the medial preoptic area is required for the full expression of male sexual behavior in rats. High levels of aromatase are found in the medial preoptic nucleus (MPN) and in an interconnected group of sexually dimorphic nuclei which mediate masculine sexual behavior. Within this neural circuit, aromatase is regulated by T, acting through an androgen receptor (AR)-mediated mechanism. This arrangement constitutes a feedforward system because T is both the regulator and the major substrate of aromatase. Preoptic aromatase is thus more active in adult males than in females because of normal sex differences in circulating androgen levels. However, the mechanism of enzyme induction also appears to be sexually dimorphic because equivalent physiological doses of T stimulate aromatase to a greater extent in males than in females. Dose-response studies indicate that the sex difference is apparent over a range of circulating T concentrations and constitute a gender difference in T efficacy, but not potency. Sex differences in aromatase correlate with sex differences in nuclear AR concentrations in most regions of the sexually dimorphic neural circuit, but not in MPN. These results suggest that males may have larger populations of target cells in which aromatase is regulated by androgen, but the lack of a gender difference in AR levels in the MPN suggests that differences in post-receptor mechanisms could also be involved. Measurements of aromatase mRNA in androgen-treated gonadectomized rats demonstrate that sex difference in regulation is exerted pretranslationally. Taken together these results demonstrate a sexually dimorphic mechanism that could potentially limit the action of T in females, and may relate to the enhanced expression of T-stimulated sexual behaviors in males.     

Inhibitory role of sex steroid in the regulation of ovarian follicle-stimulating hormone receptors during pregnancy.

The present study was conducted to determine the role of sex steroids in the regulation of FSH receptors in pregnant rats. In the normal physiological condition, FSH bindings per unit ovarian weight (density of binding) and per 2 ovaries (total binding) increased during days 14-21 gestation. Scatchard plot analyses of the binding suggested that the increase in FSH binding was due to an increase in the number of FSH-binding sites. The plasma FSH concentration in pregnant rats was stable during the receptor change. In contrast, the plasma estradiol-17 beta concentration continuously increased from gestation day 14 to 21, and the testosterone level showed a large peak on gestation day 18. Estradiol-17 beta (one silastic plate containing 13 mg crystal)-implanted pregnant rats during 14-21 days of gestation induced significant decreases in the total FSH binding and ovarian weight on gestation day 21. Estradiol administration increased the plasma estradiol level 2.3-fold but did not change the FSH level. Testosterone or 5 alpha-dihydrotestosterone, a nonaromatizable androgen, did not influence the binding level under the same dose treatment. In contrast, continuous treatment with aminoglutethimide (2 plates containing 20 mg crystal), an inhibitor of adrenocortical steroidogenesis, for 7 days significantly increased the total FSH binding without a significant change in the ovarian weight. The plasma titers of estradiol and testosterone in pregnant rats treated with aminoglutethimide were reduced by 37% and 51%, respectively. Aminoglutethimide did not influence plasma FSH levels. These results suggest that circulating estradiol acts as a negative factor in the regulation of ovarian FSH receptors, at least during the second half of pregnancy. Other factor(s) that is (are) independent of sex steroids and FSH may contribute to FSH receptor induction.     

Stereological analysis of thyroid mast cells in rats after exposure to extremely low frequency electromagnetic field and the following "off" field period

Influence of extremely low frequency electromagnetic field (ELF-EMF) on thyroid gland mast cells was investigated on male Mill Hill rats. Animals were exposed to EMF (50 Hz, 50 microT to 500 microT, 10 V/m) from 24 hours after birth, 7 hours/day, 5 days/week for three months when a part of animals (group I) was sacrificed, while the rest of them were subjected to recovery evaluation and sacrificed after one (group II), two (group II) and three (group IV) weeks following the exposure. Stereological analysis on toluidine blue-stained paraffin sections showed increased volume density of degranulated mast cells in all groups and, except in group III, and numerical density as well, implicating the sensitivity of thyroidal mast cells to power frequency EMFs. Since in our previous investigations, morphofunctional alterations of thyroid gland in rats exposed to ELF-EMF were found the contribution of released mast cell mediators to these changes could be presumed.     

Thymic sensitivity to sex hormones develops post-natally; an in vivo and an in vitro study

In vitro thymic organ cultures were used to examine the effects of the sex hormones estradiol and dihydrotestosterone on thymocytes. In contrast with the marked loss of cortical thymocytes seen in vivo with these hormones, no effect was apparent in vitro even at concentrations up to 10(-6) M. The glucocorticoid dexamethasone caused severe depletion in vivo and in vitro. Thymic androgen and estrogen receptors were determined; in the newborn animals up to 2 wk of age, receptor levels were barely detectable. The possibility of indirect modulation of thymic function by steroids in vivo was investigated by culturing thymic lobes in media containing serum from animals treated with these hormones. Only sera from dexamethasone-injected animals caused changes in cell size, number, viability, or phenotype in the culture system. The mechanism for the previously reported effects of sex steroids on the neonatal thymus therefore remains to be elucidated.     

The effect of rapid and depot testosterone and estradiol on spatial performance in water maze

Men and women differ in some cognitive functions including spatial abilities. These differences seem to be affected by sex steroids, but the results are controversial. The aim of this work is to describe the effects of rapid or depot testosterone and estradiol on spatial memory in rats. Thirty-two adult male Wistar rats were divided into 6 groups. Five groups were gonadectomized, and one group was left as control. Castrated groups received sterile oil, testosterone isobutyras, testosterone propionate, estradiol dipropionate or estradiol benzoate. We evaluated spatial performance (escape latency, overall improvement, and time in the quadrant after platform removal) of the rats in a spatial water maze. Animals receiving exogenous sex steroids showed higher plasma concentrations of the particular hormones. Experimental groups improved during the acquisition spatial trials in the water maze. No significant differences between the groups during probe trial were found. In overall improvement, the testosterone depot and estradiol depot groups showed less improvement in comparison to the control groups (P<0.05). No differences in respect to administered hormones were found in corresponding receptor gene expression in hippocampus. In conclusion, exogenous testosterone affects spatial memory of adult castrated males.     

Endogenous C19-steroids and oestradiol levels in human primary breast tumour tissues and their correlation with androgen and oestrogen receptors

Endogenous levels of testosterone, 5 alpha-dihydrotestosterone (5 alpha-DHT), androstenedione and oestradiol as well as levels of androgen (AR) and oestrogen (ER) receptors were measured in human primary breast tumour samples. The purification procedure developed allowed simultaneous quantitation of the four steroids, by radioimmunoassay, in small samples with adequate precision, sensitivity and accuracy. The majority of the tumours analysed contained detectable levels of the four steroids in the homogenate or cytosol fractions. There was no significant correlation between steroid content of the tissue and the age of the patient for any of the four steroids. A positive correlation (r = 0.71) was found between the levels of 5 alpha-DHT and testosterone in tumours. In general, tissue steroid concentrations decreased with an increase in dedifferentiation. Fifty-two per cent of the tumours analysed for receptor content were found to be ER positive, and a similar proportion were AR positive. No relationship was observed between AR status and age although receptor concentration was significantly (P = 0.004) higher in post-menopausal women when only receptor positive tumours were evaluated. The mean values for AR and ER were higher in tumours containing both receptors than in tumours showing either receptor alone; there was, however, no significant relationship between concentrations of the two receptors. No correlation was observed between tumour AR or ER status and any of the four steroids measured in either fraction. In addition, the ratio between the combined levels of 5 alpha-DHT and testosterone compared to oestradiol in the same tumour, only showed a maximum value of 40. Thus, in vivo these two androgens are unlikely to influence oestrogen action in human primary breast tumours by interfering with the association of oestradiol with its receptor.     

Estradiol restores diabetes-induced reductions in sex steroid receptor expression and distribution in the vagina of db/db mouse model

Sex steroid hormones and receptors play an important role in maintaining vaginal physiology. Disruptions in steroid receptor signaling adversely impact vaginal function. Limited studies are available investigating the effects of diabetic complications on steroid receptor expression and distribution in the vagina. The goals of this study were to investigate type 2 diabetes-induced changes in expression, localization and distribution of estrogen (ER), progesterone (PR) and androgen receptors (AR) in the vagina and to determine if estradiol treatment ameliorates these changes. Eight-week-old female diabetic (db/db) mice (strain BKS.Cg-m+/+ Lepr^db/J) were divided into two subgroups: untreated diabetic and diabetic animals treated with pellets containing estradiol. Control normoglycemic littermates were subcutaneously implanted with pellets devoid of estradiol. At 16 weeks of age, animals were sacrificed, vaginal tissues excised and analyzed by Western blot and immunohistochemical methods. Diabetes produced marked reductions in protein expression of ER, PR, and AR. Diabetes also resulted in marked differences in the distribution, staining intensity and proportion of immunoreactive cells containing these steroid receptors in the epithelium, lamina propria and muscularis. Treatment of diabetic animals with estradiol restored receptor protein expression and distribution similar to those levels observed in control animals. This study demonstrates that type 2 diabetes markedly reduces steroid receptor protein expression and distribution in the vagina. Estradiol treatment of diabetic animals ameliorates these diabetes-induced changes.     

Changes in the neuroendocrine regulation of adaptive behavior in rats subjected to stress in the late prenatal ontogenesis]

Female rats were subjected from the 14th to 17th day of pregnancy to immobilisation under conditions of 1-hour daily bright illumination. On the 20th day, contents of testosterone and estradiol was decreased with no sex difference in their level in the pregnant female rats' blood and in the amniotic fluid. Sex differences were flattened in 1-month litter both in the androgen and oestrogen contents and in adaptive behaviour. In prenatally stressed males, motor activity was inhibited and anxiety was enhanced as compared with the control. At one and a half months of age, the consequences of the prenatal stress disappeared and then reappeared after sexual maturation but with an opposite trend. Adaptive behaviour of adult males was less flexible and revealed no age-dependent oscillations inherent in the control animals. Prenatal stress while preventing the maximal raise of sex steroids in the blood shifts sex differentiation of the adaptive behaviour toward demasculinization.     

Low sex steroids, high steroid receptors: Increasing the sensitivity of the nonreproductive brain

Male aggressive behavior is generally regulated by testosterone (T). In most temperate breeding males, aggressive behavior is only expressed during the reproductive period. At this time circulating T concentrations, brain steroid receptors, and steroid metabolic enzymes are elevated in many species relative to the nonreproductive period. Many tropical birds, however, display aggressive behavior both during the breeding and the nonbreeding season, but plasma levels of T can remain low throughout the year and show little seasonal fluctuation. Studies on the year-round territorial spotted antbird (Hylophylax n. naevioides) suggest that T nevertheless regulates aggressive behavior in both the breeding and nonbreeding season. We hypothesize that to regulate aggressive behaviors during the nonbreeding season, when T is at its minimum, male spotted antbirds increase brain sensitivity to steroids. This can be achieved by locally up-regulating androgen receptors (ARs), estrogen receptors (ERs), or the enzyme aromatase (AROM) that converts T into estradiol. We therefore compared mRNA expression of AR, ERalpha, and AROM in free- living male spotted antbirds across reproductive and nonreproductive seasons in two brain regions known to regulate both reproductive and aggressive behaviors. mRNA expression of ERalpha in the preoptic area and AR in the nucleus taeniae were elevated in male spotted antbirds during the nonbreeding season when circulating T concentrations were low. This unusual seasonal receptor regulation may represent a means for the year-round regulation of vertebrate aggressive behavior via steroids by increasing the brain's sensitivity to sex steroids during the nonbreeding season.     

Effect of neonatal castration on sex steroid receptor levels in the hypophysis of male rats

The content of estradiol and testosterone cytosolic and nuclear receptors has been studied in the pituitary body of adult male rats gonadectomized on day 1-3 after birth (long-term castrates) or in adulthood (short-term castrates). Intact male rats and long- and short-term castrates had the same level of cytosolic and nuclear estrogen receptors. The number of cytoplasmic and nuclear testosterone-binding sites was identical in the pituitary body of adult intact mice and long-term castrates. Contrastingly, the concentrations of androgen cytosolic and nuclear receptors were significantly lower in neonatally castrated males compared to intact adult animals. The results obtained indicate that nuclear testosterone receptors in the pituitary body mediate negative feedback effect of androgen on the release of luteinizing hormone and that the formation of thin mechanism occurs within the first days of life.     

Female-specific target sites for both oestrogen and androgen in the teleost brain

To dissect the molecular and cellular basis of sexual differentiation of the teleost brain, which maintains marked sexual plasticity throughout life, we examined sex differences in neural expression of all subtypes of nuclear oestrogen and androgen receptors (ER and AR) in medaka. All receptors were differentially expressed between the sexes in specific nuclei in the forebrain. The most pronounced sex differences were found in several nuclei in the ventral telencephalic and preoptic areas, where ER and AR expression were prominent in females but almost completely absent in males, indicating that these nuclei represent female-specific target sites for both oestrogen and androgen in the brain. Subsequent analyses revealed that the female-specific expression of ER and AR is not under the direct control of sex-linked genes but is instead regulated positively by oestrogen and negatively by androgen in a transient and reversible manner. Taken together, the present study demonstrates that sex-specific target sites for both oestrogen and androgen occur in the brain as a result of the activational effects of gonadal steroids. The consequent sex-specific but reversible steroid sensitivity of the adult brain probably contributes substantially to the process of sexual differentiation and the persistent sexual plasticity of the teleost brain.