Download Sexual Differentiation of the Brain: Hormonal Influences and Structural Differences and more Study notes Biology in PDF only on Docsity! Sexual Differentiation of the Brain Sexual Dimorphism – structural differences between the sexes Sexual dimorphisms found in many regions of the brain. We will focus on sexual differentiation of the hypothalamic-pituitary axis and the role of steroid hormones in sexual differentiation. In mammals the default is female, with differentiation toward masculine patterns of gonadotropin secretion and behavior occurring in the male as a result of exposure to hormones of testicular origin during development. In mammals differentiation includes: 1) Defeminization - suppression of female behavioral and cyclic pattern of gonadotropin secretion. 2) Masculinization – enhancement of male characteristics Landmark study Phoenix, Goy, Gerall, and Young 1959 These authors proposed that testicular steroids could permanently alter the developing nervous system to make it more likely to display masculine behaviors and less likely to display feminine behaviors. • This irreversible effect was organizational and not activational. • Exposure of female guinea pigs to androgens in utero reduces their receptivity or female behavior in adulthood. • What is different about males and females that cause them to behave differently? Answer: Brains are structurally different. Aromatization hypothesis Aromatization hypothesis was originally developed through research with rats. Observations: 1) 5α-DHT is less effective than either testosterone or estradiol to induce defeminization. 2) The developing brain is a site of androgen to estrogen conversion 3) Inhibition of estrogen formation from androgen or the ability of estrogen to bind to the estrogen receptor impairs sexual differentiation of the brain. 4) Estrogen is 1000 times more potent than testosterone. 5) A single injection of estradiol benzoate postnatally masculinizes the volume of the SDN-POA of the female rat. 6) Further evidence that the actions of androgens are not critical in the developing brain came from Tfm male rats. These rats have 85 – 90% fewer androgen receptors than normal littermates, yet gonadotropin release and sex behavior do differentiate with testosterone. O O OH OH HO Testosterone Estradiol 5α - DHT Aromatase5α - Reductase Critical Periods: From: MacLusky and Naftolin, 1981 Animal Gestation or Incubation Critical Period (after conception) Rat 20 – 22 18 – 27 days Mouse 19 – 20 Postnatal Guinea Pig 63 – 70 30 – 37 days Sheep 145 30 – 90 days Rhesus monkey 146 – 180 40 – 60 days Zebra finch 12 - 14 posthatching
