Hormones
Fall 2002

A hormone is a chemical secreted by a gland into the blood stream that affects other cells at a distant site.

Hormones do many things (e.g., stress response, sexual development, etc.)
 

Hormones occur in two distinct forms:

1. peptides (e.g. oxytocin)
2. steroids: defined by four ring chemical structure (e.g. testosterone, estradiol) Hormones produce changes in target cells through two mechanisms: 1. bind to membrane receptors that cause ion channels to open
2. bind to intracellular receptors to change gene expression Hormones are released from glands throughout the body (e.g., hypothalamus, pituitary, adrenal gland, ovary, testis, thymus, thyroid, pancreas, etc.)

The pituitary is a very important gland that hangs below the hypothalamus (Figure 15.2). There are two distinct parts:

1. Anterior pituitary (Figure 15.6): Hormone release is stimulated by releasing hormones that the blood carried from the hypothalamus.

2. Posterior pituitary (Figure 15.4): Neurons in the hypothalamus project into the posterior pituitary. Action potentials cause hormones to be released into the blood.

 Hormone levels in the blood are regulated by negative feedback systems. For example, high levels of testosterone cause a reduction in testosterone production. Thus, taking anabolic steroids, an androgen similar to testosterone, causes the body to produce less testosterone and the testes to atrophy.

Hormones are important in both sexual develepment and behavior.
 

Sexual Development

What determines whether someone is a male or female?
There are three ways to answer this question:
 
 
Male
Female
Chromosomes
XY
XX
Internal biology
Testes
Ovaries
External anatomy
Penis
Vagina

During the first few months of fetal development all fetuses look the same regardless of chromosomes (XX and XY). The chromosomes will cause the gonads to develop into testes or ovaries. Hormones, or a lack of hormones, from the gonads will cause a penis or vagina to develop. These permanent changes produced by hormones are called organizational effects (vs. activational effects, see below). These effects occur during fetal months 3 and 4. Prior to this stage the fetus can develop into a male or female regardless of ovaries or testes depending on the presence of testosterone.
 

Testosterone causes the development of male genitals (penis & scrotum)
A lack of testosterone causes female genitals to develop (clitoris, vagina & labia).
  Testosterone does this by going into a cell and causing gene expression which results in the development of a penis. A mismatch between the chromosomes and anatomy can occur by altering testosterone levels. Two conditions where this happens are:
 
 
Adrogen insensitivity
Congenital adrenal hyperplasia
Chromosomes
XY
XX
Internal biology
Testes
Ovaries
External anatomy
Vagina
Penis

Other organizational effects of hormones occur during puberty to cause the development of secondary sex characteristics. Puberty typically begins between 10 and 15 years of age when:

a) The hypothalamus starts releasing gonadotropin releasing hormone (GnRH) which stimulates the:

b) Anterior pituitary to release luteinizing hormone (LH) which stimulates the:

c) Testes & ovaries to release testosterone & estradiol, respectively.

Estradiol causes breast development, broadening hips, etc.(female pubic hair is from androgens released by the adrenal gland)
Testosterone causes penis growth, deep voice, broadening shoulders, etc.
 
  Sexual Behavior

Hormones can also have activational effects. Activational effects are non-permanent effects caused by the presence of a hormone.

Sexual behavior requires both organizational and activational effects. Sexual behavior consists of mounting for male rats and lordosis for female rats. These behaviors require hormones both as a neonate and as an adult.
 
 
Mounting
Lordosis
Neonate (organizational)
Testosterone
No testosterone
Adult (activational)
Testosterone
Estradiol & progesterone

Testosterone has organizational effects on brain circuitry so animals can engage in future mounting. In particular, the sexually dimorphic nucleus in the hypothalamus is five times larger in males compared to females.
 

Non-human animals typically restrict mating behavior to particular times. These times can be regulated by:

1. Season
2. Ovulation
3. Mating may induce ovulation
Do hormones regulate mating in humans? Males: testosterone levels correlate with sex drive and aggression so both of these will decrease.

Females: Effects of estradiol & progesterone on sex drive is less clear. Estradiol & progesterone levels fluctuate drastically every month, but sex drive does not change noticeably. Very little change in female sexual activity during ovulation.

   Homosexuality

Several hypotheses have been put forth to explain male homosexuality:
 
Hypothesis
Data
Homosexuality is a result of childhood experience Bell et al. (1981) found no consistent childhood factors predicting homosexuality
Adult hormone levels cause homosexual behavior No difference in testosterone levels in homosexual & heterosexual men
Homosexuality caused by differences in brain structures LeVay and others have shown differences in the hypothalamus between gay and straight men. This finding has been questioned because the gay men studied had AIDS.
Homosexuality is coded by a personís genes Monozygotic twins have a higher concordance for homosexuality than dizygotic twins

These findings indicate that homosexuality has a biological cause.
 

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