PS231: Hormones and the Brain III

The major vertebrae endocrine glands

• Hypothalamus: control of hormone secretions

• Pituitary gland

  • Anterior pituitary: hormone secretion by thyroid, adrenal cortex and gonads; growth

  • Posterior pituitary: water balance; salt balance

Neuroendocrine communication

• In the hypothalamus

• Neuroendocrine cells fire action potentials to release hormones directly into the blood

• Then affect cells in the anterior pituitary

Level I: the hypothalamus

• Small peptide hormones that increase or decrease the release of hormones from the anterior pituitary gland (left: little green triangles)

• Can be thought of as a special class of neurotransmitters

• Released by neurosecretory cells at the base of the hypothalamus

• These neurons act as endocrine cells, but look like conventional neurons, firing action potentials that cause the release of “releasing hormones”

Level II: the pituitary gland

• Latin for “mucus”

• Sits directly below the hypothalamus, encased in the “sella tursica” (Turkish saddle) bone

• Made up of two anatomically distinct parts:

  • Anterior pituitary (glandular tissue)

  • Posterior pituitary (neural tissue)

IIa: the anterior pituitary

1. Hypothalamic neurons release neurohormones (or “releasing” hormones) into capillaries of the portal vessels (closed blood circuit linking the hypothalamus with the anterior pituitary)

2. Releasing hormones from the portal vessels stimulate or inhibit the release of tropic hormones from distinct anterior pituitary endocrine cells

3. Anterior pituitary “tropic” hormones leave the pituitary gland into the general circulation to act on peripheral endocrine organs (e.g. adrenal glands, thyroid gland, gonads, etc.)

The anterior pituitary contains cells that secrete hormones—it’s an endocrine gland

HPA axis

blood pressure/stress

HPT axis

metabolic rate

HPG axis

reproducition

Anterior pituitary gland associated hormones

Stress response system

CRH → ACTH → Glucocorticoids (e.g. cortisol)

IIb: the posterior pituitary

1. Magnocellular (large) neurons in the hypothalamus (PVN/SON) synthesize oxytocin (OT) and vasopressin (AVP) and transport them to their axon terminals, located in the posterior pituitary—no portal system!

2. Oxytocin and vasopressin are directly released from axon terminals in the posterior pituitary and diffuse into capillaries

3. Oxytocin and vasopressin then leave the posterior pituitary via the bloodstream into the general/systemic circulation to act on peripheral tissues

Posterior Pituitary Gland-Associated Hormones: Oxytocin & Vasopressin

• 9 amino acid peptides (“nonapeptides”)

• Differ by only two amino acids

• Very different functions

Vasopressin

• Water retention

• Vasoconstriction

Oxytocin

• Uterine contractions

• Milk ejection

Oxytocin and Milk Letdown

1. Stimulation of the mother’s nipple by the infant’s suckling response produces brain activity in the mother

2. This brain activity stimulates hypothalamic cells to release oxytocin from the posterior pituitary

3. The oxytocin causes the cells of the mammary glands to contract, thereby releasing milk

4. The baby, rewarded with milk, continues suckling until sated

Anterior pituitary

• Produces and secretes its own hormones

• Glandular tissue

• Connected by network of blood vessels

• Tropic hormones

Posterior pituitary

• Stores and releases hormones made in the hypothalamus

• Neural tissue

• Connected by nerve fibers that extend from the hypothalamus

• Oxytocin and vasopressin

Compare and contrast neural vs. hormonal communication. Explain how neuroendocrine cells at the base of the hypothalamus “blend” neuronal and neuroendocrine mechanisms. Do these neurons fire action potentials? Do they release anything? If so, what do they release and where do they release it?

• Neural communication uses electrical impulses (action potentials) and chemical neurotransmitters through specific neural pathways. They are fast, precise, and very short-lived.

• Hormonal communication uses chemical messengers (hormones) released into the bloodstream and are relatively slow but with long-lasting effects.

• Neuroendocrine receive neuronal signals and fire action potentials but they also release hormones directly into the blood, affecting cells in the anterior pituitary.

How, specifically, does the hypothalamus communicate with the anterior pituitary gland? Which types of hormones are involved, and what do anterior pituitary cells do in response to receiving a hormonal signal from the hypothalamus?

• Neurosecretory cells at the base of the hypothalamus release small peptide hormones that either increase or decrease the release of hormones from the anterior pituitary gland

• Receiving hormones are involved in the process and it leads to either the increase or decrease of the synthesis and secretion of their own hormones

What are oxytocin and vasopressin? What role do each play in the body? What role do they play in the brain? From where are they released into the bloodstream?

Oxytocin:

• Role in the Body (Peripheral Actions):

  • Labor and Childbirth: Stimulates powerful contractions of the uterine muscles.

  • Milk Ejection: Causes the contraction of myoepithelial cells in the mammary glands, leading to the "let-down" of milk for breastfeeding.

• Role in the Brain (Central Actions):

  • Social Bonding: Promotes pair bonding, maternal-infant bonding, and other social attachments.

  • Emotional Connection: Often called the "love hormone" or "cuddle hormone," it is associated with trust, empathy, and sexual arousal.

  • Stress Reduction: Can have anti-anxiety and calming effects.

Vasopressin (also called Antidiuretic Hormone or ADH):

• Role in the Body (Peripheral Actions):

  • Water Retention: Its primary role is to regulate water balance in the body. It acts on the kidneys to increase water reabsorption, thereby concentrating the urine and conserving water. This is crucial for preventing dehydration.

  • Vasoconstriction: At very high concentrations (e.g., during severe blood loss), it causes constriction of blood vessels, which helps to increase blood pressure.