Neuroendocrinology

Question 1

What are neuroendocrine cells (including some examples)?

Answer 1

Neuroendocrine cells are neurosecretory cells that release signal molecules (hormones) from their synaptic terminals into the blood.
They are controlled via synaptic transmission from presynaptic neurons.

Examples include:

• chromaffin cells of the adrenal medulla
• hypothalmic magnocellular (big) neurons
• hypothalmic parvocellular (small) neurons

Question 2

Describe the embryology of the pituitary gland.

Answer 2

The pituitary begins as two simultaneous invaginations, one downwards from the neural ectoderm and one upwards from the oral ectoderm. The upward invagination is called Rathke’s Pouch, and it is pinched off, while the downward growth is aligned with it.

This structure eventually becomes the pituitary gland with a posterior and anterior lobe.

Question 3

Describe the separate blood and hormone supplies of the two lobes of the pituitary.

Answer 3

The pituitary gland has two (more or less) independent arterial supplies.

The inferior hypophyseal artery supplies the posterior pituitary, where it divides into a capillary bed, and this is contacted by axons of magnocellular neurosecretory cells in the hypothalamus. They can secrete hormones into it, and then it is drained off by the inferior vein. Thus, the hormones secreted by the hypothalamus through the posterior pituitary can then enter the systemic circulation.

The anterior lobe is supplied by the superior hypophyseal artery indirectly. First, it vascularises the floor of the hypothalamus (the base of the hypothalamus is called the median eminence). It divides into a capillary bed in the median eminence, and then this is drained off by the venules which become a portal system. These portal veins continue the blood supply to the anterior pituitary, where they again revascularise into a capillary bed in the anterior pituitary. Secretions of the anterior pituitary can be secreted into this bed, where it then drains off via the superior hypophyseal vein into the systemic circulation.
In this case, parvocellular neurosecretory cells, via their axonal terminals, release their hormones into the capillary bed of the median eminence; these signals are carried into the capillary bed of the anterior lobe, where they act locally.

Question 4

List the hormones that are released from the different lobes of the pituitary gland and where they act.

Answer 4

POSTERIOR LOBE:

• oxytocin: uterine muscles and mammary glands
• ADH: kidney tubules

ANTERIOR LOBE:

• TSH: thyroid
• ACTH: adrenal cortex
• FSH and LH: testes or ovaries
• Growth Hormone (GH): the entire body
• Prolactin (PRL): mammary glands (in mammals)
• Endorphins: pain receptors in the brain

Question 5

Describe ADH as an example of a neuroendocrine reflex.

Answer 5

ADH’s main function is osmoregulation, which means control of body water, specifically the concentration of body fluids.
If body water is low (solute concentration is high), this is sensed and ADH is released, where it stimulates water retention in the kidney.

Osmoreceptors in the hypothalamus sense the solute concentration rising. they will stimulate neurosecretory cells in the relevant hypothalamic nuclei. These release ADH into the posterior pituitary and hence into the circulation to act.

Question 6

What are some growth hormone functions?

Answer 6

• it has a role in growth and development (anabolic)

- it couples growth to nutritional status

Question 7

What are some ways in which the secretion of growth hormone is controlled?

Answer 7

We have hormonal control:

• ghrelin (‘hunger hormone’ secreted by endocrine cells of the stomach)
• GH-Releasing hormone (hypothalamic neurosecretory cells)
• somatostatin/GHIH (hypothalamic neurosecretory cells)

We also have negative feedback:

• by the GH in the circulation
• by IGF-1 (released by the liver in response to GH)

Question 8

List some other factors that affect GH secretion.

Answer 8

STIMULATORY:
(- GHRH)
(- Ghrelin)
- Hypoglycemia
- Decreased fatty acids
- Fasting
- Exercise, sleep
- Stress

INHIBITORY:
(- Somatostatin (GHIH))
(- GH)
- Hyperglycemia
- Increased fatty acids
(- IGF-1)

Question 9

Describe GH action in the body.

Answer 9

• stimulates production of IGF-1 by liver
• increases lipolysis: raises free fatty acids (FFA)
• increases gluconeogenesis: raises blood sugar
• increases amino acid uptake into muscle, protein synthesis and lean body mass
• stimulates chondrocytes: linear growth
• stimulates somatic growth: increased organ/tissue size

Question 10

What is acromegaly?

Answer 10

It is a condition in which a person has excess growth hormone.
It is most commonly due to pituitary adenoma: an increase in GH-secreting somatrophs. Less commonly, it can also be due to a tumour somewhere that secretes GH.

The excess GH leads to insulin resistance, so many patients will have impaired glucose tolerance and hyperinsulinemia.
It also causes the growing of bones in your face, hands and feet.