Endocrinology

Hormones

  • Produced by endocrine cells

  • Occurs in low concentrations

  • Transported in the blood plasma

  • Transport messages

  • Regulatory influence on function of other cells

Neurohormones

  • Produced by neurons

Hypothalamo-hypophysial system

  • Most important organ in endocrine secretion

  • Interface between brain and endocrine system

  • Controled by the CNS

Hormones can be secreted by epithelial (nonneural) endocrine cells. These epithelial glands are stimulated by other hormones.

Other hormones are secreted by neurosecretory cells in neurosecretory cells in neurosecretory glands, that are stimulated by synaptic input from a typical neuron.

Differences between neurohormones and neurotransmitters

  • Neurohormones

    • Transported in the blood over large distances

      • Affect large parts of the body

    • Slower response due to traveling in blood + proteins need to be synthesised

  • Neurotransmitters

    • Are only transported in the synaptic cleft between two neurons

Hormones and neurohormones are degraded in the liver and kidneys.

3 classes of hormones

  • Steroid hormones

    • Synthesised from cholesterol on demand

      • Not stored in the body

    • Lipid-soluble

      • Can reach receptors inside the cell

  • Peptide and protein hormones

    • Synthesised from amino acids

      • By transcription of DNA

    • Stored in vessels

      • Secreted on demand

    • Water-soluble

      • Bind on receptors outside the cells

    • E.g. insulin, growth hormones

  • Amine hormones

    • Modified amino acids

    • E.g. adrenaline, dopamine, melatonin

Note that hormones only bind to target cells with specific receptor molecules, and many cells have multiple receptors so they can react to different kinds of hormones.

Hormones can be non-polar (hydrophobic)

  • can penetrate cell membrane to bind with intracellular molecules

Hormones can also be polar (hydrophilic)

  • bind to cell-surface receptors

When a hormone binds to its specific receptor, the hormone-receptor complex interacts with the DNA on the target cell.

  • Alters the gene expression

    • Gives modulated protein synthesis

      • The new proteins execute physiological work

3 important control pathways

  • Synergism

    • One hormone causes enhanced responses to other hormones

  • Permissiveness

    • Presence of one hormone is required for another hormone to function

  • Antagonism

    • One hormone counteracts the effects of another hormone

Mammalian stress response

  • Heart rate and breath increases

  • Cognitive abilities and alertness are sharpened

  • Stored energy is released

  • Oxygen and nutrients are transported to the CNS and specific tissues

    • E.g. locomotory muscles in case of fleeing

Nutrition

Storage and release of nutrients are closely regulated by hormones

  • Insulin

    • Peptide hormone

      • Produced in the pancreas

    • Released during high glucose levels + amino acid levels

    • Stimulate glucose uptake from the blood to muscles and fat cells

    • Promotes formation of glycogen + use of glucose

    • Inhibits gluconeogenesis (breakdown of glycogen into glucose)

  • Glucagon

    • Peptide hormone

      • Produced in the pancreas

    • Released during low glucose levels

    • Stimulates production of glucose + release into blood stream

    • Stimulates breakdown of glycogen into glucose

    • Stimulates formation of new glucose from non-carbohydrate molecules

  • Epinephrine

    • Also involved in stress response and exercise

    • Increases blood glucose levels by promoting gluconeogenesis

  • Glucocorticoids

    • E.g. cortisone, cortisol, corticosterone, depending on the organism)

    • Stress hormones

    • Secretion is controlled by the HPA axis

Hormones in invertebrates

Metamorphosis in insects is controlled by hormones

3 hormones involved:

  • PTTH

    • Protein hormone

    • Produced in the brain and transported via the haemolymph into the thorax

  • Ecdysone

    • Steroid hormone

  • Juvenile hormone (JH)

    • Fatty acid derive

PTTH stimulates prothoracic glands to produce ecdysone

  • Is then changed to into 20E (active form)

    • Stimulates stimulates epidermis enzymes

      • Digest the old cuticle and synthesises a new cuticle

  • 20E and JH are soluble in lipids

    • Can penetrate the cell membrane and bind to intercellular receptors

When 20E acts on the epidermis and the hemolymph while levels of JH are high, then a new larvae stage is produced. However, in the last larvae stage, the corpora allata, which produces JH becomes inactive, hence levels of JH sharply decrease. Under low levels or the absence of JH but at simultaneously high levels of 20E, the epidermis produces pupae or adult structures instead of a new larvae stage, allowing the next developmental step. Interestingly, in the adult stage, the corpora allata becomes active again and starts producing new JH which is then needed for mating and reproduction.

Cytokines are important in the control cell development, cell differentiation and immune response. These peptides or proteins are secreted by many types of cells and are produced on demand. Cytokines are also involved in the development of new blood cells (e.g. during wound healing, in growing animals or after menstruation). Interleukins are a subclass of cytokines. They are particularly big cytokines of the immune response. Pheromones are chemical signals for with-species communication and they are produced within an animal and then released to the environment for example to give information about food sources, about mating behaviour or the presence of predators. Kairomones are also chemical signals that are released into the environment. However, other species use the info rather than individuals of the same species. Examples are predator-prey-interactions. Allomones are chemical signals that predators use to attract prey, while Synomones are chemical signals that e.g. one species uses to warn another species.