HPA Axis and Feedback Loops

What are steroid hormones modified from?

cholesterol

Define hormone

Set of chemical messengers of the endocrine system

Hemocrine

* Signal causes vesicles to dock to cell membrane,
* Hormone released and diffuses into blood stream
* Travels around body and encounters receptors
* Causes an effect in the cell

Paracrine

Hormone released from cell and acts on neighbouring cell (in the same tissue)

Autocrine

Hormone released from cell and acts on the same cell

Synthesis of hormones

Synthesised in, and released from, specialised endocrine glands (eg thyroid hormone by the thyroid gland, cortisol by the adrenal gland)

Name 3 types of hormones

* Peptide and protein hormones
* Small amino acid hormones
* Steroids

What amino acid are small amino acid hormones based on?

tyrosine

What type of hormone are catecholamines

small amino acid hormone

What type of hormone is insulin?

peptide and protein hormone

Where is cortisol produced?

adrenal gland

Storage of peptide hormones

stored in intra-cellular secretory granules

Storage of steroid hormone

not stored, made when required

Describe secretion of hormones

1. Release activated by nervous system, other hormones from other glands, or local environment (metabolites) around cells
2. Hormones released from granules via Ca2+ mechanism (similar to exocytosis at synapse)
3. Leave cells of glands or tissues and diffuse via ECF and enter microcirculation or lymph system (and then blood)

Describe possible circulation methods of hormones

* Bind to proteins in plasma
* Steroids have to be bound to another protein – insoluble
* Catecholamines can circulate unbound


* This acts as a pool of hormone, protecting it from the metabolism to increase half-life

Why can the same hormone have different effects

* Can affect target structures far away from site of release
* Each hormone can affect many types of tissues
* Effects depend on which receptor subtype is expressed

Examples of processes affected by hormones

* Growth
* Metabolism
* Reproduction

What systems control hormone release

* Neuronal control
* Feedback mechanisms
* Tropic hormones
* Circadian release

How do tropic hormones work

Release of some hormones is under the control of the endocrine axis (hormones under control of other hormones)


1. Neurones in the hypothalamus secrete releasing hormones
2. Stimulation/inhibition of secretion of tropic hormones from pituitary
3. Tropic hormones stimulate peripheral endocrine gland, releasing hormone
4. Multi-organ effects e.g. thyroid hormone

Types of time dependant control of hormone release

**Circadian** - Amount released varies throughout the day

**Pulsatile** – Hormone released in bursts at different times

How do peptide hormones act on target cells

Peptide hormones act at cell surface receptors

* Only act on cells with the appropriate receptor, but receptors can be widespread
* Simultaneous actions on widespread organs and tissues
* Activation of cell surface receptor leads to down-stream signalling (activation outside = effects inside cells)

How do steroid hormones act on target cells?

Steroid hormones act at cytoplasmic or nuclear receptor

* Only act on cells with the appropriate receptor, but receptors can be widespread
* Simultaneous actions on widespread organs and tissues
* Lipid soluble so
* Diffuses through membrane
* Binds to receptors in cytosol, activating them
* Move into nucleus and bind to DNA causing transcription/translation of proteins

What does HPA stand for?

Hypothalamus - pituitary - target organ axis

How are posterior lobe hormones released?

1. Hormone generated in neuroendocrine cells in hypothalamus
2. Hormone transported down axon cells
3. Released at axon terminals into circulation in posterior lobe
4. Released into body to act on target tissues

How are anterior lobe hormones released?

1. Neuroendocrine cells release hypothalamic hormone into hypophyseal portal circulation
2. Hormone travels to anterior lobe and affects glandular cells in anterior lobe
3. Anterior lobe hormone released into circulation

what are neuroendocrine cells?

neural cells that produce hormones

Examples of signals which neuroendocrine cells respond to

Stress, cold, trauma, hunger, pain, neural reflex

Function of neuroendocrine cells

reside in the nuclei of the hypothalamus and cause hormone release from posterior and anterior pituitary

What hormones are released from posterior pituitary?

AVP

Oxytocine

Function of oxytocin

1. During labour increase uterus contraction
2. During lactation for milk let-down

Type of feedback loop of oxytocin and how does it occur?

neural

Activation of stretch-sensitive sensory cells (eg stretch during labour) feeds back to hypothalamus via spinal cord, causing release of hormone

Function of vasopressin

1. Acts in kidney to regulate water permeability of the distal tubules
→ increased water reabsorption
→ decreased water excretion
2. Constriction of arterioles
→ increased arterial pressure

Regulation of vasopressin

Secretory stimulus

* Increased plasma osmolarity
* Decreased plasma volume