Hypothalamo-hypophyseal axis means the hormones that are released from the hypothalamus to the pituitary gland will stimulate the release of pituitary hormones into the blood to the target organs.
Review the Pituitary Gland before you continue reading.
Not mentioned in the anatomy of the pituitary gland, the hypothalamus has many types of nuclei. However, the ones that we will be touching on are the paraventricular, supraoptic and arcuate nuclues. Each of these nuclei has 2 types of neurons:
1) Parvocellular neuron (small neuron) –> Anterior pituitary
2) Magnocellular neuron (large neuron) –> Posterior pituitary
which are responsible for stimulating the pituitary glands.
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Anterior pituitary
The anterior pituitary hormonal mechanism is a rather simple mechanism as compared to the posterior pituitary.
The axons of the parvocellular neurosecretory neurons of the nuclei (Paraventricular, supraoptic, arcuate etc) project to the median eminence, at the base of the brain, via axonal transport where their neurosecretory nerve terminals release peptides into the blood vessels.
The median eminence is where the portal vessels (that connects to the anterior pituitary) arises. The median eminence is of great physiological importance, as it is integral to the hypophyseal portal system, which connects the hypothalamus with the anterior lobe of the pituitary gland. It is in this structure that the secretions of the hypothalamus (releasing and inhibiting regulatory hormones) collect before entering the portal system.It is outside the blood-brain-barrier, so the capillaries are more permeable.
The median eminence will then release the hypothalamus regulatory hormones into the portal vessels, which reaches the anterior pituitary, and excites the hormone secreting cell. The various hormones will then be produced and secreted into the systemic circulation.
| HYPOTHALAMUS HORMONES | ANT PITUITARY HORMONES | CELL TYPE | |
| Corticotrophin-Releasing-Hormone (CRH) | Corticotrophin (ACTH) | Corticotrope | Basophilic |
| Thyrotropin-Releasing-Hormone (TRH) | Thyroid-Stimulating Hormone (TSH) | Thyrotrope (Glycoprotein) |
Basophilic |
| Luteinizing-Hormone-Releasing-Hormone (LHRH) | Luteinizing Hormone (LH) | ||
| Growth-Hormone-Releasing-Hormone (GHRH) | Growth Hormone (GH) | Somatotrope | Acidophilic |
| Somatostatin (SS/GHIH) | Inhibit GH & TSH | ||
| Dopamine | Prolactin | Lactotrope | Acidophilic |
| Gonadotrophin-Releasing-Hormone (GnRH) | LH, FSH | Gonadotrope (Glycoprotein) |
Basophilic |
Tropic hormones (All except Prolactin): Act on target endocrine gland (promote vascularity, growth and stimulate hormone secretion)
Nontropic hormones (Prolactin): mammary gland is exocrine, not endocrine.
All hormones are peptides except dopamine which is an amine.
All glycoprotein hormones (TSH, LH, FSH) consist of alpha & beta subunits. However, the alpha subunit is also present in the placental glycoprotein hormone: human chorionic gonadotrophin (hCG). Therefore it’s action is similar to LH.
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Posterior Pituitary
The posterior pituitary differs from the anterior pituitary, that the hormones have already been produced beforehand and stored. It will be released once it is stimulated by the hypothalamus.
The hormones of the posterior pituitary are:
1) Vasopressin/ AVP/ ADH
2) Oxytocin
Synthesis:
They are synthesized as a larger precursor molecule in the cell bodies of the neurons of the hypothalamus. The precursor molecules are:
1) prepropressophysin (vasopressin)
2) preprooxyphysin (oxytocin)
These hormones will be transported along the axon (axoplasmic flow) and stored in the nerve endings in the posterior pituitary gland as free hormones.
Release:
This time, the neuron involved are the magnocellular neurosecretory neurons, where the nerve terminals will release neuropeptides into the posterior pituitary. This is a nervous stimulation, direct into the posterior pituitary. Once stimulated, the posterior pituitary will release it’s stored hormones (vasopressin/oxytocin) into the systemic circulation.
Vasopressin
Actions of vasopressin/ADH:
1) Increase water reabsorption in the distal nephron (distal convulated tubule & collecting duct).
By increasing water permeability. ADH will bind to the V2 receptors – cAMP – increase water channels (Aquaporins). This hormone is useful in preventing excessive uring formation and retention of water in the body.
When there is less water in the body, the plasma osmolality will be increased (more concentrated), detected by the osmoreceptor neuron, stimulating the thirst centre and causes a person to drink excessively (polydipsia). However, with vasopressin release, there will be reabsorption of water, therefore the plasma osmolality will drecrease, hence preventing polydipsia.
2) Increase vasoconstriction
Binds to V1 receptor — DAG, IP3 — Ca2+ – generalised arteriolar constriction (increase TPR) and venoconstriction (increase venous return). Main purpose is to regulate blood pressure. However, this effect is not as sensitive and effective as osmoregulation. Therefore, the main function of ADH is still antidiuresis.
It’s part of the feedback mechanism, whereby a fall in blood volume will decrease stretch on atrial walls, decreasing the stimulation of baroreceptors, decrease discharge of vagal afferents, decrease inhibition of vasomotor centre in medulla oblongata and therefore increase stimulation of hypothalamic neurones secreting vasopressin from anterior pituitary.
3) Stimulate ACTH secretion from anterior pituitary (in synergism with CRH from hypothalamus)
4) In large doses, gylcogenolysis in liver
5) Inhibit renin release
Factors controlling vasopressin secretion
| STIMULATIN G |
INHIBITING |
| *Increase plasma osmolality | Decrease plasma osmolality |
| *Decrease ECF volume | Increase ECF volume |
| Mental/Physical stress | |
| Nausea/Vomiting | |
| Angiotensin 2 | |
| Opioid peptides | |
| Nicotine | Alcohol |
What happens where there is excess in vasopressin?
Also known as SIADH : Syndrome of Inappropriate ADH Secretion
Inability to control blood pressure, oedema formation.
Deficiency in vasopression
Causes Diabetes Insipidus (Insipid: Tasteless)
Water diuresis – polyuria – increase plasma osmolality – thirst centre – thirst – polydipsia.
Oxytocin
Actions of oxytocin:
1) Contraction of uterine smooth muscles (Oxytocic action)
- Pregnant – parturition (child birth)
Increase of oxytocin receptors in uterus – prostaglandins – uterine contractions – dilation of cervix, distention of vagina – fetal descent – stretch receptors in birth canal – positive feedback – more oxytocin.
Therefore drugs that causes uterine contractions are called Oxytocics.
- Nonpregnant – Sperm transport
2) Contraction of myoepithelial cells surround the alveoli of lactating mammary gland
- milk ejection
Sucking reflex – hypothalamus – decrease dopamine – anterior pituitary (prolactin: glandular epithelial cells, acinus milk synthesis & secretion) posterior pituitary (oxytocin: milk ejection)
3) Contraction of smooth muscle of vas deferens in males (ejaculation)
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Intermediate Lobe
Besides the ant & post pituitary hormones is the intermediate lobe, which do release it’s own hormones as well.
1) Pro-opiomelanocortin (POMC)
Hydrolysed into polypeptide fragments : Opioid peptide , Melanin-Stimulating-Hormone (Darken skin), ACTH.
ACTH in anterior pituitary also has some MSH activity: Hyperpigmentation in excess, and pallor in deficiency.
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For the actions of LH, FSH & Prolactin: Refer Reproductive System lectures.
For the actions of growth hormones: Refer Growth hormones & growth factors.