Steroid and Sex Hormones: Organizing and Activating Effects

Steroid Hormones

Steroid hormones are a type of hormone consisting of four rings made of carbon atoms. They are derived from cholesterol and have three primary modes of action:

• Binding to membrane receptors
• Binding to and activating proteins in the cytoplasm
• Acting on receptors linked to chromosomes

Sex Hormones

Gonadal steroid hormones are produced by the gonads, which include the ovaries and testes. These hormones include:

• Androgens (e.g., testosterone, dihydrotestosterone)
• Estrogens
• Progesterone

It is important to note that gonads can produce both types of hormones, with the aromatase enzyme mediating conversions between them.

Organizing Effects of Sex Hormones

Organizing effects refer to the hormonal effects on the brain and genitals that occur during early developmental stages. These effects are long-lasting, often indefinitely. Examples include injections of androgens in females or estrogens in males during the early sensitive stage, leading to masculinization, feminization, or lordosis.

The difference in the appearance of external genitals between females and males stems from gradients of testosterone during the early sensitive period. In humans, this period occurs between three to four months of pregnancy.

Activating Effects of Sex Hormones

Activating effects are hormonal effects on the brain and genitals that occur later in development, at any time in life. These effects are mediated by testosterone or estradiol and are temporary. Nature's default tendency is to design mammals as female.

Sex Differences in the Brain (Hypothalamus)

During early development, sex hormones bind to receptors in the amygdala, hypothalamus, and other areas. The anterior hypothalamus contains the sexually dimorphic nucleus (SDN), which controls male sexual behavior and is larger in males than females.

In male rodents, testosterone exerts some of its organizing effects by entering neurons in the hypothalamus and being converted into estradiol, which helps to masculinize males. Estradiol does not masculinize females because alpha-fetoprotein in rodents binds to estrogen but not testosterone, controlling downstream effects.

Clinical Considerations

• Removal of testes tends to decrease male sexual activity.
• Impotence (erectile dysfunction) can be caused by circulatory issues, psychological tension, drugs, or neurological problems, not necessarily by changes in testosterone levels.
• If an ovum is fertilized, levels of estradiol and progesterone increase during pregnancy. These hormones affect serotonin type 3 receptor (5HT-3R) activity, which is linked to nausea.
• During the periovulatory period, women experience changes in sexual behavior and perceptions due to high estrogen levels, influencing their responses to videos, work, and faces.

Oxytocin

Oxytocin is released by the posterior pituitary gland. It plays a role in:

• Uterine contractions
• Stimulating the release of milk from mammary glands
• Promoting anxiolytic behavior after sexual pleasure (orgasm)