IB Biology SL

Revision Notes

6.6.2 Hormones Continued

Leptin

  • Leptin is a hormone that is secreted by fat storage cells known as adipose cells
  • The concentration of leptin in the blood is controlled by:
    • Food intake
    • The amount of adipose tissue in the body
  • As we eat (food intake is high), adipose cells fill up with lipids
    • As adipose cells fill up, they secrete more leptin
    • This leptin circulates in the blood and targets groups of cells in the hypothalamus that are responsible for controlling appetite
    • It does this by binding to receptors in the membranes of these cells
    • This inhibits appetite and causes the sensation of hunger to be suppressed (stopped)
  • If we are short of food (food intake is low), the lipid reserves in adipose cells are used up and the adipose cells become empty again
    • As adipose cells empty and shrink, they secrete less leptin
    • This stimulates appetite and causes the sensation of hunger to return

Testing Leptin

  • As leptin inhibits appetite and causes the sensation of hunger to be suppressed, it was once thought that clinical obesity could be ‘cured‘ or controlled by injecting patients with leptin
  • However, large scale clinical trials (to test whether this could be an effective treatment for obesity in humans) found it to be ineffective

Reasons for the failure to control obesity with leptin injections

  • Obese humans often have very high concentrations of leptin in their blood
  • It seems as though their bodies have become resistant to the effects of the hormone
    • It appears the target cells in the hypothalamus become resistant to leptin (even at high concentrations) and therefore fail to respond to it
    • This leads to a lack of appetite inhibition, causing a continuous sensation of hunger and excessive food intake as a result of this
  • This means that injection of extra leptin fails to control obesity

Melatonin

  • Many physiological processes and behavioural patterns occur in regular, daily rhythms in organisms throughout the plant and animal kingdoms
    • For example, many animal species are only active for a specific part of the 24-hour cycle (e.g. nocturnal animals are only active at night)
  • Humans are adapted to live in a 24-hour cycle and many aspects of our physiology and behaviour (including physical activity, sleep, body temperature and secretion of hormones) follow specific and regular cycles throughout the 24-hour period
    • These cycles are known as circadian rhythms
  • In humans, many circadian rhythms are controlled (or at least partially controlled) by the hormone melatonin
  • Melatonin is secreted by the pineal gland, which is located in the brain
  • Melatonin secretion increases in the evening (in response to darkness) and decreases at dawn (in response to light)
  • Although melatonin affects many aspects of human physiology and behaviour, one of the main circadian rhythms it controls is our sleep-wake cycle
    • Increasing melatonin levels lead to feelings of tiredness and promote sleep
    • Decreasing melatonin levels lead to the body’s preparation for waking up at the end of the night and staying awake during the day
  • In addition, experiments have suggested that increased melatonin at night contributes to the night-time drop in core body temperature in humans
  • Melatonin receptors have also been found in the kidney, which suggests that increased melatonin at night contributes to the night-time decrease in urine production in humans

Uses of Melatonin

  • Jet lag is the term used to describe the various symptoms a person can experience after crossing multiple time zones during a flight (usually three or more time zones)
  • The symptoms can include:
    • Difficulty in remaining awake during the day
    • Difficulty in sleeping during the night
    • General fatigue
    • Irritability
    • Headaches
    • Indigestion
  • Jet lag occurs because the body’s circadian rhythms are still set to the timing of day and night in the country (i.e. in the time zone) from which the person has just departed, rather than the country or time zone they have just arrived in
  • Jet lag usually only last for a few days as the body adjusts to the new day and night regime
  • Melatonin tablets are sometimes taken to prevent or reduce jet-lag symptoms
    • The tablets are normally just before going to sleep
    • Some clinical trials have shown this use of melatonin to be effective in promoting sleep and reducing other jet lag symptoms
    • However, the safe and appropriate use of this medication still needs more testing
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