A NEW CONCEPT FOR BRAIN AROMATASE IN SEXUAL DIFFERENTIATION
Koh Shinoda*
Department of Anatomy, Yamaguchi University School of Medicine,
1144 Kogushi, Ube, Yamaguchi 755-8505, Japan
*E-mail :
shinoda@po.cc.yamaguchi-u.ac.jp
The presence of neuronal aromatase has recently been immunohistochemically determined in the rat and monkey brains (Shinoda et al.,'94a; Shinoda et al.,'96), confirming that intracranial estrogen-biosynthesis is essential to development of reproductive neural substrates and functions in mammals. The enzyme has been considered to provide an important clue to account for paradoxical or overlapping actions of androgen and estrogen on the central nervous system (MacLusky and Naftolin,'81; Shinoda,'94a). The assumed aromatase-mediated mechanism, however, has yet to be clarified. It has still remained to be determined if brain masculinization by aromatizable androgens is primarily attributable to actions of intracranially aromatized estrogens via estrogen receptor (not via androgen receptor) as the "aromatization hypothesis" suggested (MacLusky and Naftolin,'81). The present paper will briefly refer to a new concept which we have recently proposed that the brain aromatase is critically involved in sexual differentiation via regulation of sex-steroid receptors.
To sum up outlines of the P450arom immunoreactive (AROM-I) neurons detected in aldehyde-fixed tissue sections of fetus-to-adult Wistar rat brains, they can be classified into at least three different groups (Shinoda et al.,'94a; Shinoda,f94b). The first (fetal type), in which immunostaining occurs only during certain pre- or neonatal days (E13-15 to P7), includes the anterior medial preoptic nucleus (AMPN), the periventricular preoptic nucleus (PVPN), the rostral portion of the medial preoptic nucleus (rMPN), the ventrolateral part of the ventromedial hypothalamic nucleus (vlVMH), and an area close to the dorsolateral periphery of the posterior arcuate nucleus. The second (fetal/neonatal type) is a striking AROM-I cell-group in the medial preoptico-amygdaloid neuronal arc (mPOAM arc) which extends from the medial preoptic nucleus (MPN) to the principal nucleus of the bed nucleus of the stria terminalis (prBST) and the posterodorsal part of the medial amygdaloid nucleus (pdMAm). They appear by E15, reaching a peak in staining intensity between E18-P2 and diminishing after the perinatal stage but still retaining the sufficient immunoreactivity in the young-to-adults. The second group also includes other immunoreactive neurons which are associated with the strial part of the preoptic area (stPA), and those which are sporadically seen in the rostral part (rBST) and intraamygdaloid division (iaBST) of the bed nucleus of the stria terminalis. The third group of AROM-I neurons (young-to-adult type) emerge in the lateral division of the central amygdaloid nucleus (lCAm), the oval nucleus of the bed nucleus of the stria terminalis (ovBST) and the septum including the septo-hippocampal nucleus (SpHip) and the lateral septal nucleus (LS). In addition, AROM-I terminal-like structures are observed clearly only in the islands of Calleja, the pyramidal cell islands adjacent to the ventral surface of the olfactory tubercle, and CA2-CA3 of the hippocampus.
Also in young Japanese male monkeys (Macaca fuscata fuscata), we have recently succeeded to clarify the regional distribution of AROM-I neurons (Shinoda et al.,'96). Striking numbers of the AROM-I neurons were found in the MPN, prBST and MAm as well as in rodent brains. Weak-to-moderate AROM-I cells were localized in the LS and VMH, and disseminated in the basal forebrain including the nucleus accumbens, olfactory tubercle, ventral pallidum and nucleus of the diagonal band. Other solitary AROM-I neurons were often detected in the sixth (and occasionally fifth) layer of the cerebral cortex, especially in the limbic cortex. These results were largely same as those obtained in the rat brain, strongly suggesting that intracranial estrogen formation is essential also in the primate brains including the human brain and that the basic (subcortical) distribution pattern is phylogenetically conserved at least in mammals.
While the distribution and developmental patterns of AROM-I neurons in most brain regions are basically similar between males and females, those in the mPOAM arc showing the most intense immunoreactivity in the rat and monkey brains are clearly more prominent in males in number and immunoreactivity than in females, especially after young-to-adult stages (Shinoda,f94b). It is of importance to note that the mPOAM arc, which receives the main and accessory olfactory inputs and projects mainly to the MPN and VMH related to reproductive functions (Simerly and Swanson, e96, e98), is larger in size in males than in females (Hines, et al.,'92). Stimulation of the mPOAM arc can promote copulatory behavior in males and females and/or induce surges of luteinizing hormone and ovulation in females (Beltramino and Taleisnik,'78,'80; Chozick,'86). Furthermore, our recent studies on the rat brain have clarified that neurons in the arc also show sexually dimorphic expression for both a-estrogen receptor (EsR) (Shinoda et al.,'94b) and androgen receptor (AnR) (Nagano et al.,'96). The mPOAM arc which has been considered as the major aromatization center can be regarded as a functional complex serving reproductive functions in both sexes and one of the most important key regions related to hormonally-regulated brain sexual differentiation (Shinoda et al.,'94a). In neonates, expression of EsR in the mPOAM arc is already more prominent in females than in males (Shinoda et al.,'94b), whereas that of AnR is more prominent in males than in female (Nagano et al.,'96). In young-to-adult stages, the sexually dimorphic expressions are further enhanced in accordance with increased expressions of EsR in females or AnR in males during adolescence. With no doubt, the mPOAM arc of the young-to-adult rats has clearly sex-specific patterns for expressions of AROM, EsR and AnR; males show strong AROM and AnR and weak EsR expressions, making a sharp contrast to females which inversely show strong EsR and weak AROM and AnR expressions. These data support the classic hypothesis that brain masculinization is induced by androgen via AnR rather than estrogen and that brain feminization is promoted by estrogen via EsR, contradicting the ''aromatization hypothesis''. Interestingly, neonatal castration in male rats was found to change the male's pattern to the female's one with respect to AROM, EsR and AnR in the mPOAM arc (Shinoda,'96). In order to clarify the regulation of AROM, EsR and AnR by sex steroids, 5a-dihydrotestosterone (5a-DHT), 17b-estradiol or testosterone was administrated in neonatally castrated adult rats (Shinoda,'96). A non-aromatizable androgen, 5a-DHT strongly increases the expressions of AROM and AnR and slightly increase EsR, while an 17b-estradiol slightly increases the expressions of AROM and AnR and strongly decreases the expression of EsR. An aromatizable androgen, testosterone strongly increases the expressions of AROM and AnR, and strongly decreases the expression of EsR in the mPOAM arc, indicating that only testosterone can recovered expression of the male pattern in the neonatally castrated male rats. Thus, some testosterones are considered to be actually aromatized into estradiol and to down-regulate EsR, while others sparing aromatization can act directly on AnR in the mPOAM arc. Hormonally-regulated sexual differentiation is crucially controlled by the ratio and levels of intracellular androgen/estrogen and expressions of their receptors. Neural aromatase is considered to control the stimulation intensity of both sex-steroid receptors by regulating the intraneuronal sex-steroid milieu. The enzyme can locally produce estrogens by the conversion of high levels of aromatizable androgens in the perinatal period and adolescence in males and form a very-locally-high-estrogen-concentration milieu, resulting in the ''auto-down-regulation'' of EsR in the mPOAM arc. A complete male probably requires not only masculinization by enhancement of an AnR-stimulation via the up-regulation of but also defeminization by reduction of an EsR-stimulation via the down-regulation in the limbic arc.
Taken together, brain aromatase, especially in the mPOAM arc, is thought to indirectly regulate the expressions of EsR and AnR by controlling the levels of androgen and estrogen, playing a crucial role in determination of the direction of brain sexual differentiation.
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