Sex and Hormones:

• Steroid hormones: 4 carbon rings, derived from cholesterol. They work in 3 ways:
  • Bind to membrane receptors like neurotransmitters
  • Enter cells and activate certain kinds of proteins in the cytoplasm
  • Bind to chromosomes where they activate/inactivate specific genes
  • Sex hormones are a specific type of steroids released by gonads and adrenal glands: estrogens, progesterone, androgens
  • Androgens: a group including testosterone and other “male hormones” (men have higher levels of them)
  • Estrogens: including estradiol, are “female hormones” (higher lvls in females)
  • Progesterone: mostly female hormone, prepares uterus for implantation of a fertilized ovum and promotes maintenance of pregnancy
  • Sex hormones affect brain, genitals and other organs
  • At first they thought men and women were different because of the sex-linked genes (= genes that androgens/estrogens activate), they activate the differences one sees (bodily differences, ex. breasts). Sex hormones increase/decrease apoptosis (cell death) in different parts of  the brain in men and women. Genes are also responsible for differences (ex. Y-chromosome)

Organizing effects of sex hormones:

• Organizing effects of sex hormones: occur mostly at a sensitive stage of development (shortly before birth) and determine whether the brain and body will develop female/male characteristics
• Activating effects: occur at any time in life, when a hormone temporarily activates a particular response

Sex differences in the gonads:

• Male Y chromosome includes the SRY (= sex-determining region on the Y chromosome) gene, which causes the primitive gonads (Muellerian ducts and Wolffian ducts) to develop into testes
• Testosterone increases the growth of testes, and turns Wolffian ducts into seminal vesicles (stores semen) and vas deferens (duct from testis into penis)
• Muellerian inhibiting hormone: causes degeneration of Muellerian ducts (= precursers to female oviducts, uterus and upper vagina)
• Female gonads develop into ovaries, their Wolffian ducts degerate and Muellerian ducts develop and mature.
• Sexual difference depends mainly on the level of testosterone during a sensitive period -> for humans: 3^rd and 4^th month of pregnancy
• Nature’s default: every mammal is female, add testosterone and the individual becomes male (but estradiol is necessary too for behavior or some brain differentiation.

Sex differences in the Hypothalamus:

• Sex hormones bind to receptors in specific areas of the hypothalamus, amygdala and other brain areas.
• Sexually dimorphic nucleus: larger in men than in women -> controls male sexual behavior
• Testosterone is aromatized into estradiol in early development after entering a neuron in the hypothalamus (they are chemically similar). In sensitive period many mammals have alpha-fetoprotein in their bloodstream (not present in adults), it binds with estrogen and blocks it from leaving the bloodstream and entering the cells that are developing in this early period. Testosterone isn’t blocked by it, enter the cells freely and are converted to estradiol there. -> a large amount of estradiol masculanize rodents (they over power the alpha fetoprotein)

Sex differences in the Cerebral Cortex and Cognition:

• Men tend to have more white matter than women
• Women have a greater density of neurons in the part of the temporal lobe which is for language
• Men and women tend to perform better in certain subject, it is more likely to be linked to the organizational aspect than the activating aspect of hormones
• Those differences are not limited to our species, the difference however is more HOW they do it
• Little is known why there are such differences

Activating Effects of Sex Hormones:

Hormones don’t CAUSE external behavior, they alter the activity in various brain areas to change the way the brain responds to various stimuli, they also change sensitivity in the genital areas

-       Rodents:

• Sex hormones activate sexual behavior partly by enhancing sensations
• Estrogens increase the sensitivity of the pudendal nerve (transmits tactile stimulation from the pubic area to the brain)
• Sex hormones also bind to receptors that increase responses of certain areas of the hypothalamus (ex. ventromedial nucleus, medial preoptic area (MPOA), anterior hypothalamus)
• The sexually dimorphic nucleus (part of the anterior hypothalamic area) is larger in males than in females, importance of it is still unclear
• Testosterone and estradiol prepare the MPOA and other areas to release dopamine (mostly during sexual activity, the more they release the more likely is the male to copulate)
• A moderate amount of dopamine stimulates type D1 and D5 receptors, which facilitate erection of the penis in the male and sexually receptive postures in the female, a high amount stimulates type D2 receptors which leads to an orgasm (it’s the same burst of dopamine like the “rush” people get from drugs)
• Serotonin inhibits sexual activity partly by blocking dopamine release

-       Humans:

• Sex hormones don’t only effect sexual behavior (ex. testosterone can decrease pain and anxiety, estrogen stimulates growth of dendritic spines in the hippocampus)
• Men:
  • Excitement is highest when testosterone levels are highest (about age 15-25)
  • Hormone oxytocin is realeased in large amounts during orgasm (more than 3x the normal amount)
  • Low testosterone usually mean low sexual activity, however it isn’t the basis for impotence, its most common cause is impaired blood circulation, or neurological problems
  • Erections: testosterone increases the release of nitric oxide (NO), which facilitates the hypothalamic neurons important for sexual behavior, and also increases blood flow to the penis
• Women:
  • The hypothalamus and pituitary interact with the ovaries to produce the menstrual cycle
  • At the end of the menstrual period the anterior pituitary releases follicle-stimulating hormone (FSH, promoted the growth of a follicle in the ovary)
  • The follicle nurtures the ovum and produces several types of estrogen, including estradiol
  • Toward the middle of the menstrual cycle the follice builds up more and more receptors for FSH -> it produces more estradiol -> increased release of FSH and luteinizing hormone (LH, from anterior pituitary), combines, they cause the follicle to release an ovum
  • Corpus luteum (the remnant of the follicle) releases progesterone -> prepares the uterus for the implantation of a fertilized ovum, it also inhibits the release of LH
  • Toward the end of the cycle the levels of LH, FSH, estradiol and progesterone decline
  • Menstruation -> linings of the uterus is cast off and cycle begins again
    • Pregnancy: levels of estradiol and progesterone increase gradually throughout pregnancy, those high levels cause fluctuating activity at the serotonin 3 receptor (responsible of nausea)
    • Birth control pills: interfere with the usual feedback cycle between the ovaries and the pituitary
    • Changes in hormones alter sexual interest: at the periovulatory period (midway point, when ovulation occurs, maximum fertility, increased estrogen levels), women initiate more sexual activity

Parental Behavior:

• Late in pregnancy the female secretes large amounts of estradiol, prolactin (for milk production and behavior like retreaving the young back to the nest, if fathers raise the children too it’s important for them too) and oxytocin (for maternal behavior, sexual arousal, social attachment, enhancement of learning)
• In addition to changing hormones, the female also changes her pattern of hormone receptors
• Hormones increase the mothers’ attention to their young after delivery, they act by increasing activity in the medial preoptic are and anterior hypothalamus
• Hormone vasopressin is synthesized by the hypothalamus and secreted by the posterior pituitary gland (animals who have a lot of it stay together as a couple to raise their young)
• Animals are sensitive to their young’s odor: the infants release some chemicals that stimulate the mother’s vomeronasal organ (responds to pheromones), it stimulates aggressive behavior, but since the mother went through pregnancy it doesn’t effect her, if the female hasn’t gone through pregnancy she rejects the young until she’s used to their smell
• In humans hormones aren’t necessary to prepare the mother for the baby, except for milk