Metabolic hypothesis

1
Metabolic hypothesis

• Suppression of reprod. axis during CU is due to
  a change in energy balance / signal within the
  body.
• candidates metabolic fuels (fat, protein,
  glucose), insulin, leptin

2
Possible role for metabolic fuels

• starved hamsters resume estrous cycles if glucose
  is placed in drinking water (dont require fat,
  protein)
• ad lib fed hamsters stop cycling when
  administered drugs that remove immediate
  metabolic fuels (glucose, fatty acids)
• food-restricted females exhibit one or two LH
  pulses after eating their one daily meal (but not
  at other times during day)

3
Metabolic hypothesis, contd.

• role for insulin? studies contradictory
• LH levels in monkeys rise after infusion of
  insulin
• LH secretion in pigs increases after insulin
  infusion
• LH pulses dont change in underfed lambs infused
  with insulin in brain

4
Leptin as a metabolic signal

• leptin is hormone produced by fat cells
• secretion increases in response to improvement in
  metabolic status
• amount of circulating leptin proportional to
  amount of body fat
• mechanism acts on brain to cause cessation of
  feeding behavior

5
Leptin, contd.

• first identified due to genetic mutation in ob
  mouse gt hyperphagic, obese
• another mouse (db mouse) insensitive to leptin gt
  hyperphagic, obese
• both strains of mice have reproductive disorders
• give ob mouse leptin gt stops eating reproductive
  function resumes