Androgen action in the brain and spinal cord for the regulation of male sexual behaviors
Introduction
Testosterone and 5α-dihydrotestosterone (DHT) are biologically important androgens derived from Leydig cells of the testes or its enzymatically converted steroid in the target cells, while dehydroepiandrosterone (DHEA) is a relatively weak androgen secreted from the zona reticularis of the adrenal cortex. Androgens have a broad spectrum of physiological and behavioral functions: development and maintenance of the male reproductive system, musculo-skeletal system, fatty tissue, hair follicle, kidney, liver, and clinical involvement in several diseases including prostate cancer/hypertrophy and hypogonadism. Androgens also play a key role in the sexual differentiation of the nervous system, regulation of reproductive behavior, neuroendocrine responses, and aggressive behavior [1•, 2]. Androgens exert these effects by binding to and activating an androgen receptor (AR) in target cells [3]. AR-expressing neurons have a widespread distribution in the brain and spinal cord, and the AR is involved in the control of CNS masculinization and its dysfunction, such as androgen insensitive syndrome and spinal and bulbar muscular atrophy [4]. However, these nuclear receptors do not explain all aspects of androgen function in the nervous system [5, 6]. In this review, we focused on androgen action on sexual dimorphism of the brain and spinal cord and discussed recently published data regarding the mechanism underlying sexual behavior in experimental animals and humans.
Section snippets
Androgen in sexual differentiation and function in the male brain
Although the involvement of genetic factors independent from testosterone including the testis-determining gene SRY has been mentioned [7], sexual differentiation of the brain is basically dependent on the presence or absence of testosterone [8•, 9, 10]. In males, testosterone secreted from the testes organizes the developing brain into masculinized characters corresponding to the phenotype of the external genitalia; in females, the absence of testis allows for feminized characters. This
Sexual differentiation in the male spinal cord
Onuf's nucleus, located in the ventral horn of the sacral spinal cord of many mammals, including humans, is well accepted as a sexually dimorphic nucleus, which innervates the perineal muscles involved in copulatory behavior: males have significantly more motoneurons in this nucleus than females [46, 47, 48]. A similar sexually dimorphic distribution of serotonergic fibers and terminals in this region was also reported [49]. The spinal nucleus of the bulbocavernosus (SNB) of rats is homologous
Conclusion
It is needless to say that there are sexual influences at all aspects of the neuronal system from genetic to behavioral levels, together with the structure. There have been tremendous advances in our understanding of the neuronal mechanism of sexual dimorphism of the phenotype and expression of behavior. Future development and investigation will pave the way for a deep understanding why males and females behave differently.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
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