Sex Hormones

Female Rats and Lordosis Response

  • Hormonal Influence on VMH

    • The ventromedial hypothalamus (VMH) is critical for the lordosis response in female rats.

    • Estrogen's Role:

      • Stimulates production of progesterone receptors, aiding the lordosis response.

      • Increases dendritic trees of neurons in the VMH.

Role of Estrogen and Progesterone

  • Hormonal Interaction

    • Both estrogen and progesterone are essential to evoke the lordosis response.

  • Neural Pathways:

    • VMH → Periaqueductal gray → Spinal cord

    • Sensory information about male mounting triggers lordosis in a female rat in estrus.

Male Rats and Arousal

  • Pheromonal Activation

    • Male arousal is partly activated by pheromones detected by the vomeronasal organ (VNO).

  • Neural Processing:

    • Information flows from the VNO to the medial amygdala and medial preoptic area (mPOA).

    • Testosterone influences the mPOA and medial amygdala.

Testosterone and Sexual Behavior in Male Rodents

  • Essential Hormones

    • Testosterone is critical for sexual behavior in male rats.

    • Castration Effects:

      • Reduces copulatory scores.

      • Testosterone treatment can restore copulation.

  • Activational Effects

    • Testosterone has an immediate activational effect on sexual behavior.

Neurotransmitters and Erections

  • Involvement in Sexual Response

    • Norepinephrine, serotonin, and nitric oxide contribute to the erectile response in males.

  • Testosterone Influence:

    • Acts on mPOA and medial amygdala for sexual behavior regulation.

Testosterone in Circulation

  • Activation of Mating Behavior

    • Testosterone must be present in circulation to activate typical mating behaviors in both men and women.

    • A low dose can enhance sexual interest and activity; higher doses do not increase intensity.

  • Activational Effects of Hormones

    • Both androgens and estrogens exhibit activational effects on sexual behavior.

Organizational Effects of Testosterone

  • Testicular Secretions and Masculinization

    • Early testicular secretions lead to masculine behavior in adulthood.

    • Masculinization of the developing genitalia and brain occurs during sensitive developmental periods.

Genetic Determinants of Sexual Differentiation

  • Sex Chromosomes

    • The presence of sex chromosomes directs sexual differentiation in mammals.

    • An X chromosome results in females, while a Y chromosome leads to males.

Gonadal Development

  • Differentiation Timeline

    • Indifferent gonads begin to differentiate within 1-2 months.

    • SRY Gene:

      • Found on the Y chromosome and responsible for testes development; absence leads to ovaries.

Hormonal Systems in Male Development

  • Testosterone and AMH Roles

    • Testosterone promotes the wolffian system's development, while anti-Müllerian hormone (AMH) causes regression of the müllerian system.

Development of Male and Female Structures

  • Male Development

    • Wolffian ducts develop into epididymis, vas deferens, and seminal vesicles; müllerian ducts shrink.

  • Female Development

    • Müllerian ducts develop into fallopian tubes, uterus, and vagina; wolffian ducts shrink.

Secondary Masculinization Effects

  • Additional Structures

    • Testosterone also leads to masculinization of external structures like the scrotum and penis.

    • Role of 5α-reductase in converting testosterone to dihydrotestosterone (DHT) required for male genitalia development.

Sexual Differentiation Sequence

  • Chronological Development

    1. Y chromosome directs indifferent gonads to develop as testes.

    2. Absence of Y leads to ovaries development.

    3. Hormones from fetal testes masculinize the body and brain.

    4. Fetal ovaries do not secrete androgens, leading to feminine development.

Androgen Insensitivity Syndrome (AIS)

  • AIS Mechanism

    • Occurs in XY fetuses with a defective androgen receptor gene.

    • Tissues do not respond to circulating testosterone, leading to female development externally.

    • Testes remain internal, and external female characteristics thrive, including breast development at puberty.

  • Case Study:

    • Caster Semenya’s case illustrates AIS complexities in gender identity and sports.

Other Sexual Differentiation Disorders

  • Turner’s Syndrome and CAH

    • Turner’s syndrome involves a single X chromosome, leading to female development.

    • Congenital adrenal hyperplasia causes females to be exposed to excessive androgens, leading to variable development outcomes.

Sexual Dimorphism in the Brain

  • Sexually Dimorphic Nucleus

    • Marked differences in appearance and structure between male and female brains.

  • SDN-POA

    • The sexually dimorphic nucleus of the preoptic area is larger in males and crucial for reproductive behavior.

    • Lesions can disrupt ovulatory and copulatory behaviors.

Factors Influencing Sexual Orientation

  • Neural Structures and Behavior

    • The interstitial nuclei of the anterior hypothalamus (INAH-3) size varies between genders and sexual orientations.

    • Lesbians may experience more prenatal androgen exposure than heterosexual women, but little evidence supports links between androgens and sexual orientation in men.

Fraternal Birth Order Effect**

  • Increased likelihood of homosexuality in right-handed males with older brothers, thought to be influenced by the maternal immune response.

Biology vs. Socialization in Gender Identity

  • Influences on sexual orientation are thought to stem from both biological and social learning factors.

  • Observations of homosexual behavior in other species suggest a biological component.