Are separable aromatase systems involved in hormonal regulation of the male brain?

Puberty and depression

Unipolar depression only becomes more common in girls than boys after the age of 13, as a result of an increased incidence of depressive episodes in girls at that time. This article reviews evidence that links multiple dimensions of maturation in the hypothalamo-pituitary-gonadal axis with this phenomenon. Effects of developmental status and timing have been implicated, but few studies have deployed either the measurement strategies or the statistical power needed to provide a satisfactory answer to the question regarding which components of puberty are most responsible.

Sex steroid communication in the ring dove brain during courtship

This review examines possible role of progesterone receptor (PR) and androgen receptor (AR) "cross-talk" in the expression of courtship behaviour in the ring dove (Streptopelia risoria). In doves, although androgen has been mostly associated with aggressive courtship behaviour and progesterone with the initiation of incubation, progesterone administration to courting birds terminates the aggressive component of courtship whilst having no effect on nesting behaviour. Recent results in doves have identified a high density of androgen receptor and progesterone receptor immunoreactivity (AR-ir and PR-ir) in the hypothalamus of both sexes in regions known to be directly involved in courtship and incubation behaviour. Nuclear AR-ir in courting birds is widespread throughout the brain. Nuclear PR-ir is only localized in discrete regions of the preoptic hypothalamus of both sexes. In the anterior and posterior hypothalamus of courting birds an increase number of AR-ir and PR-ir neurons colocalizes (70-90%) in the nucleus preopticus anterior (POA), nucleus preopticus medialis (POM), nucleus preopticus paraventricularis magnocellularis (PPM), nucleus hypothalami lateralis posterioris (PLH), and tuberal hypothalamus (Tu). A lower percentage of colocalization is seen in birds at other stages of the breeding cycle. The high percentage of AR-ir and PR-ir colocalization in the preoptic hypothalamus of courting doves supports previous reports involving progesterone acting in these brain regions to terminate the androgen-dependent aggressive courtship behaviour in male doves. The increase in PR-ir staining intensity in AR-ir neurons in courting birds suggests that this progesterone-dependent termination of aggressive courtship display in males occurs at the receptor level and may be orchestrated by central oestrogen.

Characterization of aromatase activity in the sea bass: effects of temperature and different catalytic properties of brain and ovarian homogenates and microsomes

Two aromatase genes have been discovered in the brain and ovary of some teleosts. However, data on native aromatase enzyme kinetics and thus actual catalytic activity are scarce in fish, impeding comparison of aromatase activity (AA) from different organs within and between species. In the present study, the tritiated water assay was optimized and validated to measure AA in the sea bass using 1 beta-[3H]-androstenedione as a substrate in crude homogenates and microsomes. Optimized assay variables included pH, temperature, buffer strength, incubation time, amount of fresh tissue, substrate, and cofactor concentration. Specificity of the assay was verified by using known inhibitors, inappropriate substrates, and heat-inactivation. Subcellular fractionation revealed ten-fold more activity in the microsomal over the cytosolic fraction. The assay was also validated by comparing results from the direct product isolation method. The validated assay described allows measurement of AA to levels as low as < 10 fmol/mg protein/hr. Sex differentiation is temperature-dependent in the sea bass. It was found that in the physiological range of temperatures where the sea bass can live, 10-30 degrees C, AA is highly dependent on temperature in a linear fashion (brain: r2 = 0.92; P < 0.001; ovary: r2 = 0.94; P < 0.001). When AA levels from brain and ovarian homogenates obtained from the same fish during the spawning season were compared, the respective Michaelis-Menten constant (Km) values were 7.3 nM vs. 4.6 nM, with no significant differences detected between the two tissues. Thus, sea bass aromatase has a very high affinity for androstenedione, similar to what has been found in goldfish, but much higher than other piscine or mammalian aromatases (30-435 nM). In contrast, the brain maximum reaction rate (Vmax 7.8 pmol/mg protein/hr) was four-fold higher (P < 0.001) than the ovarian Vmax (2.1 pmol/mg protein/hr). Consistent results were found using purified microsomes. Although this is the first time that the kinetic parameters are reported for a native piscine aromatase in two different tissues within the same fish, it remains to be determined whether this is a reflection of two distinct isoforms in this particular species.

Androgen receptor immunolocalization in brains of courting and brooding male and female ring doves (Streptopelia risoria)

Nuclear androgen receptors (ARs) were localized immunocytochemically in the brains of courting and brooding male and female ring doves (Streptopelia risoria). AR immunoreactivity (AR-ir) in courting birds was localized in cell nuclei in the telencephalon, diencephalon, and mesencephalon. In the anterior hypothalamus, high density of AR-ir was concentrated in several nuclei including the nucleus lateralis hypothalami, nucleus periventricularis magnocellularis, nucleus preopticus anterior, nucleus preopticus medialis, and nucleus preopticus paraventricularis magnocellularis. In the posterior hypothalamus, areas showing high density of AR-ir included the nucleus lateralis hypothalami posterioris, nucleus medialis hypothalami posterior, nucleus ectomamillaris, nucleus mamillaris lateralis, and nucleus tuberis. No sex differences in the density or localization of AR-ir were observed. Compared to brains from courting birds, AR-ir density was either extremely low or absent in most brain regions of brooding birds. It is concluded that in the dove, central ARs are closely associated with the sexual stages of the reproductive cycle.

Subcellular localization and kinetic properties of aromatase activity in rat brain

The conversion of testosterone to estradiol is catalyzed by cytochrome P450 aromatase. In situ aromatization is required for the full expression of the effects of testosterone in the brain. This study examined the subcellular distribution and reaction kinetics of aromatase in the adult rat brain. Preoptic area, hypothalamus and amygdala were homogenized in isotonic sucrose buffered with potassium phosphate. Tissue homogenates were fractionated by ultracentrifugation. Aromatase activity was measured using a previously validated 3H2O assay. Marker enzymes were measured to identify organelles in the different subcellular fractions. Aromatase activity in all 3 tissues was enriched 10-fold in microsomes, but not in other subcellular fractions. The addition of either a NADPH-generating system or 1 mM NADPH to the reaction mixture stimulated aromatase activity in all subcellular fractions, whereas NADH was only minimally effective. In general, substrate affinity constants were equivalent in all brain areas and subcellular fractions (approximately 10 nM) suggesting that one predominant catalytic form of the enzyme is present in the rat brain. One week after castration, aromatase activity was significantly reduced in all subcellular fractions of preoptic area and in the whole homogenate and microsomal fraction of the hypothalamus. Castration did not significantly alter aromatase activity in any subcellular compartment of amygdala. To more critically evaluate its subcellular localization, aromatase activity was measured in purified synaptosomes. Aromatase activity was not enriched in these preparations suggesting that it is not substantially associated with nerve terminals in rat brain.

Brain formation of oestrogen in the mouse: sex dimorphism in aromatase development

Steroid sex hormones have an organizational role in gender-specific brain development. Aromatase, converting testosterone (T) to oestradiol-17 beta (E2), is a key enzyme in the brain and the regulation of this enzyme is likely to determine availability of E2 effective for neural differentiation. In rodents, oestrogens are formed very actively during male perinatal brain development. This paper reviews work on the sexual differentiation of the brain aromatase system in vitro. Embryonic day 15 mouse hypothalamic culture aromatase activity (AA: mean Vmax = 0.9 pmol/h/mg protein) is several times greater than in the adult, whereas apparent Km is similar for both (approximately 30-40 nM). Using microdissected brain areas and cultured cells of the mouse, sex differences in hypothalamic AA during both early embryonic and later perinatal development can be demonstrated, with higher E2 formation in the male than in the female. The sex differences are brain region-specific, since no differences between male and female are detectable in cultured cortical cells. AA quantitation and immunoreactive staining with an aromatase polyclonal antibody both identify neuronal rather than astroglial localizations of the enzyme. Kainic acid eliminates the gender difference in hypothalamic oestrogen formation indicating, furthermore, that this sex dimorphism is neuronal. Gender-specific aromatase regulation is regional in the brain. Oestrogen formation is specifically induced in cultured hypothalamic neurones of either sex by T, since androgen has no effect on cortical cells. Androgen is clearly involved in the growth of hypothalamic neurones containing aromatase. It appears that differentiation of the brain involves maturation of a gender-specific network of oestrogen-forming neurones.

Synergism between androgens and estrogens in the induction of aromatase and its messenger RNA in the brain

It is established that testosterone (T) increases aromatase activity (AA) in the quail brain and that this induction of AA represents a limiting factor in the activation of male copulatory behavior. This action of T presumably results from an induction of aromatase synthesis since the number of aromatase-immunoreactive (ARO-ir) cells increases and, in parallel, there is an increase in aromatase mRNA as measured by reverse transcriptase-polymerase chain reaction (RT-PCR) technology. The specific role of androgenic and estrogenic metabolites of T in this induction is not yet clear but product-formation assays suggest that both types of compounds synergize to increase AA. The exact role of androgens and estrogens in the induction of aromatase was examined by studying both the aromatase protein by immunocytochemistry and the aromatase mRNA by RT-PCR in castrated quail that had been treated with T or its androgenic metabolite, 5 alpha-dihydrotestosterone (DHT) or its estrogenic metabolite, estradiol-17 beta (E2) or both DHT and E2 simultaneously. A specific quantitative PCR technique using a modified aromatase as internal standard was developed for this purpose. T increased the number of ARO-ir cells in all brain areas and increased the concentration of ARO mRNA in the preoptic area-anterior hypothalamus (POA-aHYP) and in the posterior hypothalamus (pHYP). E2-treated birds had more ARO-ir cells than castrates in the posterior part of the medial preoptic nucleus (POM), in the bed nucleus stria terminalis (BNST) and tuber. Their aromatase mRNA concentration was significantly increased in the POA-aHYP but this effect did not reach significance in the pHYP. DHT by itself had no effect on either the number of ARO-ir cells (all brain regions considered) or the concentration of aromatase mRNA. DHT, however, synergized with E2, both in inducing ARO-ir neurons and in increasing aromatase mRNA concentration. This synergism was shown to be statistically significant in several brain areas. These data demonstrate that both androgens and estrogens regulate aromatase at the pretranslational level. Because the percentage increase in the number of ARO-iR cells was in general very similar to the increase in aromatase mRNA concentration, these data also suggest that these steroids regulate aromatase mostly by changing its mRNA synthesis or catabolism.

Aromatase: neuromodulator in the control of behavior

Estrogens are required for both the organization of the brain in early development and adult behavior. Two approaches have been used in our laboratory to study the behavioral role of brain aromatase. First, brain metabolism of testosterone (T) has been related to behavior in the same individual using a well established neuroendocrine model, the ring dove, in which estradiol-17 beta (E2) has specific effects on brain mechanisms of male behavior. Aromatase in preoptic area (POA) (a) has a high activity (Vmax) and strong substrate binding affinity (Km < 5 nM), (b) is regulated by both androgens and estrogens, and the type of regulation differs according to brain area, (c) is influenced by products of an endogenous inactivating pathway, 5 beta-reduction; 5 beta-dihydrotestosterone and other 5 beta-reduced metabolites appear to be non-genomic regulators of the brain aromatase. Preoptic aromatase activity is also influenced by photoperiod and socio-sexual stimuli. The codistribution of regulated aromatase activity and estrogen receptor cells is found to be T-dependent. Our second approach has been to relate the aromatase system to developmental sex differences in brain structure and behavior of the Mongolian gerbil. Neonatal gerbil aromatase is relatively active in the POA, but has a weaker T substrate-binding affinity (Km = 30 nM) than the dove. T acting via its metabolite, E2, masculinizes the sexually dimorphic area of the hypothalamus; the differentiating effect is asymmetric. We suggest that the regulation of the brain aromatase system may be lateralized during steroid-sensitive periods of development.

Immunocytochemical localization of androgen receptors in the male songbird and quail brain

The distribution of androgen receptors was studied in the brain of the Japanese quail (Coturnix japonica), the zebra finch (Taeniopygia guttata), and the canary (Serinus canaria) by immunocytochemistry with a polyclonal antibody (AR32) raised in rabbit against a synthetic peptide corresponding to a sequence located at the N-terminus of the androgen receptor molecule. In quail, androgen receptor-immunoreactive cells were observed in the nucleus intercollicularis and in various nuclei of the preoptic-hypothalamic complex, namely, the nucleus preopticus medialis, the ventral part of the nucleus anterior medialis hypothalami, the nucleus paraventricularis magnocellularis, the nucleus ventromedialis hypothalami, and the tuberal hypothalamus. In the two songbird species, labeled cells were also observed in various nuclei in the preoptic-hypothalamic region, in the nucleus taeniae, and in the nucleus intercollicularis. Additional androgen receptor-immunoreactive cells were present in the androgen-sensitive telencephalic nuclei that are part of the song control system. These immunoreactive cells filled and outlined the boundaries of the hyperstriatum ventrale, pars caudalis, nucleus magnocellularis neostriatalis anterioris (both in the lateral and medial subdivisions), and nucleus robustus archistriatalis. The immunoreactive material was primarily present in cell nuclei but a low level of immunoreactivity was also clearly detected in cytoplasm in some brain areas. These studies demonstrate, for the first time, that androgen receptors can be detected by immunocytochemistry in the avian brain and the results are in general agreement with the binding data obtained by autoradiography with tritiated dihydrotestosterone. Immunocytochemical methods offer several advantages over autoradiography and their use for the study of the androgen receptor will greatly facilitate the analysis of steroid-sensitive systems in the avian brain.

Area-specific hormonal regulation of brain aromatase

In vitro experiments were conducted to compare the steroid regulation of aromatase in preoptic and posterior hypothalamic areas. Kinetic analysis of aromatase activity in the preoptic area (POA) and posterior hypothalamus (AHP) of castrated and intact doves indicate that both areas have a similar, high substrate affinity (apparent Km less than 15 nM), but castration decreases the Vmax to a greater extent in POA than AHP. This differential effect was confirmed using a single substrate (10 nM) concentration representing the Km of the enzyme. Comparison of the effects of non-aromatisable androgens, methyltrienolone (R1881) and 5 alpha-dihydrotestosterone (DHT), on aromatase activity in 20-day castrates showed that neither R1881 nor DHT induced aromatase activity in POA. This was confirmed in 40-day castrates which also showed a reduced inductive effect of testosterone (T) on the brain enzyme activity. R1881 specifically increased aromatase activity in AHP, but DHT did not affect either area. The non-aromatisable androgens influenced androgen-dependent vocal behaviour to the same extent. Oestradiol (E2) increased aromatase activity in both POA and AHP. We conclude that non-aromatisable androgens affect hypothalamic, but not preoptic oestrogen formation. Aromatase activity in the male preoptic area associated with behaviour is specifically sensitive to the aromatisable androgen, T and E2. The results suggest that hormonal regulation of the aromatase differs locally within androgen target areas of the brain.

Inhibition of hypothalamic aromatase activity by 5 Beta-dihydrotestosterone

Abstract A variable amount of circulating testosterone that reaches brain cells is converted to biologically inactive 5beta-reduced metabolites, namely, 5beta-dihydrotestosterone (5beta-DHT) and 5beta-androstane-3alpha,17beta-diol (5beta,3alpha-diol). In avian species, the production of inactive 5beta-DHT and 5beta,3alpha-diol is highest during embryonic and post-hatching life. In the present study, we have investigated the possibility that 5beta-reduction may not only correspond to a steroid inactivation pathway, but that 5beta-reduced metabolites of testosterone may exert direct inhibitory effects on enzymatic pathways producing biologically active steroids. When added to hypothalamic homogen-ates prepared from adult male doves, 5beta-DHT but not 5beta,3alpha-diol inhibits the activity of the aromatase enzyme, which converts testosterone to 17beta-oestradiol. During the first days after hatching, when the production of 5beta-reduced metabolites is high, the hypothalamic aromatase is also inhibited by 5beta-DHT. We conclude that a high 5beta-reductase activity during sensitive periods for sexual differentiation may protect the avian brain from the differentiating effects of circulating androgens by inhibiting the production of oestrogen.

Developmental sex differences in brain aromatase activity are related to androgen level

Sex differences in the metabolism of testosterone (T) in the developing brain of quail were examined using an in vitro microassay. During each developmental stage (day before hatching, hatching and 2 days after hatching) aromatase activity was higher in hypothalamic areas than in a control neostriatal area. There was no sex difference in oestradiol-17 beta (E2) formation in the late embryonic brain or at hatching. But aromatase activity in the male preoptic-anterior hypothalamic area was 50% higher than in females by day 2. No regional differences in brain 5 beta-reductase activity were detected at any of the developmental stages sampled. There was a sex difference in production of catabolic 5 beta-reduced metabolites. Male 5 beta-reductase activity declined continuously from high embryonic levels in all areas, whereas female enzyme activity showed an increase at hatching. In contrast to plasma progesterone, levels of T were higher in the male than in the female by day 1 after hatching. We suggest that elevated circulating T in the male after hatching may account for the sexual dimorphism in brain aromatase activity.

Is androgen-dependent aromatase activity sexually differentiated in the rat and dove preoptic area?

Aromatase activity is higher in the male than in the female anterior hypothalamic-preoptic area (POA) in both the avian and the rodent adult brain. This sex difference is abolished after castration of the male and restored by androgen treatment. Gonadectomy has no effect on POA aromatase in the female. The aim of this study was to find out whether sex dimorphism in adult POA aromatase is only due to a sex difference in circulating gonadal hormones or dependent upon sexual differentiation of the brain. Aromatase activity was measured in vitro in microdissected POA samples using a sensitive radiometric assay. We examined the effects of gonadectomy and testosterone treatment on enzyme activity in adult rats and doves of both sexes. We also studied the effects of neonatal gonadectomy and hormone substitution in male and female rats. The results suggest that levels of POA aromatase in the adult depend primarily on gonadal activity, but that mechanisms involved in the regulation of aromatase activity and enzyme induction may be sex-specific and could result from sexual differentiation of the brain in early life. Further work will be required to determine the developmental stage when this occurs and the exact mechanism(s) responsible for increased sensitivity of the adult male POA to the inductive effect of testosterone.