Sex Differences and Hormones Week 7

We are now switching gears from electrical/chemical communication between neurons to a discussion of hormonal communication. In a way, this is a continuation of chemical communication, since hormones carry out similar kinds of effects.

What is a hormone? A hormone is a chemical substance that it released from the endocrine glands (system of ductless glands around the body) into the bloodstream, where it travels to distant target organs to have its effects (often the release of another hormone, or in the case of the nervous system, to have effects on neurons). They are similar to neurotransmitters (the same substance can be both a hormone and a neurotransmitter depending on where it was released). Hormones can travel a long distance in the body, whereas NT’s usually only travel a short distance.

Can hormones affect behavior? Yes, most common example is that of anabolic steroids (derivatives of testosterone) on aggression. Castrating animals (removing their testes) markedly reduces aggression. How do we know that’s due to T? Replace T, observe whether behavior is reinstated. If so, this is considered an “activational” effect of a hormone (temporary effect, lasting only as long as the hormone is present. Generally occurs at puberty and beyond), as opposed to an organizational effect (permanent effect, structural change due to hormone exposure during an early critical period). The whole process of masculinization of the body and the brain is a hormonally-dependent process that is organizational in nature…in many cases, the default pattern is female and hormonal influences are required for male development to occur (in sexually dimorphic species, such as humans). For example, the ability of the hypothalamus to release hormones cyclically in females is due to the absence of T during development. Males will not release hormones cyclically in adulthood even if given implanted ovaries, because the ability of the brain to release hormones cyclically is blocked by the organizational effect of testosterone during development. This is an organizational effect.

One difference between hormones and NT’s is that the effects of hormones are somewhat longer lasting—even activational effects of hormones sometimes that a few days to wear off after removal of the hormone. How is it possible for the effects of a hormone to outlast its presence? Hormones can have transient effects like neurotransmitters by binding to cell surface receptors and altering membrane potential. However, most often, steroid hormones (which are lipid soluble and can both pass through the BBB and the cell membrane) simply pass through the cell body membrane, bind to cytoplasmic receptors inside the cell. There, the steroid-receptor complex enters the nucleus of the cell, binds to regulatory elements on the DNA and alter gene expression. These effects take a while to occur and also to turn off.

The brain is an endocrine organ, controlling the release of the sex steroids (the most relevant hormones for out discussion of sex differences: estrogens (estradiol), progestins (progesterone) and androgens (testosterone)) by releasing hormones directly into the bloodstream. Here’s how it happens

The hypothalamus releases release hormones to the anterior pituitary, which releases stimulating hormones to the peripheral targets (in this case, the gonads, the testes/ovaries) to release their hormones. The cycle works like this:

Hypothalamus à anterior pituitary à testes/ovaries

(release hormones, GnRH) (stimulating hormones, FSH, LH)

The testes/ovaries release E, P, T, (also released from the adrenal gland, in both males and females) whose levels are detected by the hypothalamus, and through a process of negative feedback, can maintain a steady state of hormone levels. The hypothalamus and the anterior pituitary share a blood supply so that the release hormones are not diluted by the general circulation. The posterior pituitary (which releases only oxytocin and vasopressin) share a neural connection with the hypothalamus.

What sex differences are influenced by hormones?

What sexual dimorphisms are present?