Neuro: Neurohormones Flashcards
List some principle endocrine organs of the body.
- Testes. Release testosterone which is very important for spermatogenesis.
- Oestrogen and progesterone are released from the ovaries - involved in oogenesis.
- The pancreas release insulin, glucagon (important for glucose regulation), somatostatin
- The adrenal glands contain a cortex and medulla. The adrenal cortex releases steroid hormones such as aldosterone, cortisol as well as sex hormones. The
adrenal medulla releases adrenaline and noradrenaline. - Parathyroid glands release parathyroid hormone
- Thyroid gland releases thyroxine
- The hypothalamus and the pituitary. Packed with neurohormones - neurotransmitters which are released from neurones directly into the blood circulation.
Describe the two main control systems of the body (compare and contrast).
The body has two main control system: the endocrine system and the nervous system.
ENDOCRINE SYSTEM:
- mediators travel within blood vessels
- utilises chemical mediators (hormones)
- slow communication
- effects can be long-lasting
NERVOUS SYSTEM:
- signalling along the nerve fibres
- transmission of electrical impulses
- fast communication
- effects are generally short-acting
Describe the different types of hormones.
Proteins & Peptide hormones:
- Vary considerably in size
- Can be synthesised as a large precursor and processed prior to secretion (e.g. HG, somatostatin, insulin)
- Can be post-translationally modified (e.g. glycosylation) and then released into blood circulation
- Can have multiple subunits synthesised independently and assembled (e.g. FSH, LH, TSH)
- These peptides come from genes which are responsible for the production of big proteins, the big proteins become cleaved via enzymes
Amino acid derivates
- Mostly tyrosine derived
- There are neurotransmitters (e.g. monoamines) that can also act as a neurohormone (so can also be released in the blood circulation)
- E.g. adrenaline, noradrenaline, dopamine
Steroid hormones
- Steroid is a class of lipids derived from cholesterol
- Includes cortisol (stress hormone), aldosterone, testosterone, progesterone, oestrogen
- Steroid hormones can act on the brain (not released from neurones but from periphery and affect CNS)
How are hormone signals sent to both parts of the pituitary
With the anterior pituitary, we have the hypophyseal portal circulation. Hormones are released into these blood vessels, which transport them to the anterior pituitary, where they act.
With the posterior pituitary, we have neurones called magnocellular neurons, which project from the hypothalamus directly into your posterior pituitary. The hormones are released into the PP, where they travel through the capillary network and get released straight into the blood circulation.
Where are neurohormones produced?
- Most neurohormones are neuropeptides and are all derived from genes. So they are produced in the cell body.
- From transcription and translation there is production of proteins which are packed into vesicles and undergo post-translational modification e.g. cleavage in the Golgi apparatus. They will then be packed into vesicles.
- The vesicles get transported along the axons to the synaptic terminals. In the synaptic terminals, the neurohormones will get released directly into the capillary network (blood circulation).
- Calcium entry is usually important in order to drive the release of neurohormones.
- So therefore the neurohormones are synthesised in the hypothalamus, but are released in the posterior pituitary.
List the hormones that control the pituitary, and what effect they have.
ANTERIOR PITUITARY:
Corticotrophin releasing hormone (CRH) - a peptide that controls the release of adrenocorticotrophin (ACTH)
Thyrotrophin releasing hormone (TRH) - a peptide that controls the release of thyroid stimulating hormone (TSH) and prolactin (PRL)
Gonadotrophin releasing hormone (GnRH) - a peptide that controls the release of luteinising hormone (LH) and follicle-stimulating hormone (FSH)
Growth hormone releasing hormone (GHRH) - a peptide that controls the release of growth hormone (GH)
Somatostatin - a peptide that inhibits the release of GH, gastrin vasoactive intestinal polypeptide (VIP), glucagon, insulin, TSH and PRL
Dopamine (DA) - a monoamine that inhibits the release of prolactin
POSTERIOR PITUITARY:
Vasopressin - has an antidiuretic effect
Oxytocin - acts on the uterus to induce uterine contraction, and acts on the mammary glands to induce milk ejection
List the specialised cells of the anterior pituitary and what they release.
- Gonadotrophs secrete LH and FSH in response to GnRH.
- Somatotrophs control GH secretion in response to GHRH
- Corticotrophs control ACTH secretion in response to CRH
- Thyrotrophs regulate TSH secretion in response to TRH
- Lactotrophs control prolactin secretion in response to TRH, somatostatin and dopamine
As a recap, describe the role of ACTH in the release of cortisol.
Hypothalamic neurones release corticotrophin-releasing hormone (CRH) to stimulate pituitary corticotrophs to release ACTH into the circulation.
ACTH stimulates the production of glucocorticoid (cortisol) and sex hormone from the zona fasciculata of the adrenal cortex.
Cortisol provides negative feedback to the hypothalamus and pituitary, reducing the amount of CRH and ACTH released.
What happens to the pattern of cortisol secretion throughout the day?
- Varies along the day as it undergoes circadian rhythm.
- High levels in the blood, saliva, urine detected in the morning.
- Throughout the day levels decrease and become very low in evening.
- This circadian rhythm must be taken into account when considering cortisol replacement therapy as a clinical treatment.
As a recap, describe the regulation of TSH and thyroid secretion by negative feedback.
- Thyrotropic releasing hormone (TRH) from the hypothalamus stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH).
- TSH acts on the thyroid to increase T3/T4 secretion
- T3 is the most potent thyroid hormone, and targets the tissues containing a deiodinase enzyme (DI) to convert T4 to T3.
- The pituitary also expresses deiodinase to convert T4 to T3 to facilitate negative feedback.
As a recap, what happens when you have too much or too little thyroxine?
If you have too much thyroxine, it can lead to hyperthyroidism, tachycardia, anxiety, increased metabolic rate, etc.
If you have too little thyroxine, it can lead to weight gain, low energy, cognitive impairment, depression, etc.
Describe the synthesis and transport of the neurohormones of the posterior pituitary.
- The neurohormones of the posterior pituitary are vasopressin and oxytocin.
- Synthesised in the supraoptic and paraventricular nuclei in the hypothalamus. They’re transported to the terminals of the nerve fibres located in the posterior pituitary.
- Structurally, they’re quite similar (only a 2 amino acid difference in a 9 amino acid structure), yet they have very different functions.
Describe vasopressin.
- Also called anti-diuretic hormone (ADH). It increases water retention and blood pressure (vasoconstrictive effect). Its release is stimulated by changes in the activity of the osmoreceptor complex in the hypothalamus.
- Controls plasma osmolarity by regulating water excretion and drinking behaviour
- Stimulates vascular smooth muscle contraction in the distal tubules of the kidney to reduce loss of water and raise blood pressure
Describe oxytocin.
- Normally undetectable, but elevated during parturition, lactation and mating
- Released in response to peripheral stimuli of the cervical stretch receptors and suckling at the breast. It may also be involved in responses to stroking, caressing, grooming.
- Regulates contraction of smooth muscles (e.g. uterus during labour, myoepithelial cells lining the mammary duct, contraction of reproductive tract during sperm ejaculation)
Describe the two-way interaction between the kidneys and the hypothalamus.
The kidneys secrete renin (in response to low BP, etc.). The renin converts angiotensinogen to angiotensin I. Angiotensin I is converted to angiotensin II.
Angiotensin II is detected by the subfornical organ in the brain. Activation of the subfornical organ will give the signal to stimulate magnocellular neurones found in the hypothalamus to release ADH (vasopressin) directly into the blood circulation in the posterior pituitary. This will induce thirst and also ADH will act on the kidney to induce water retention.