Module 9: Systems Neuroscience

Localizing circadian clocks in the mammalian brain

who pioneered the use of running wheels and for what?

Curt Richter

• study daily rhythms in rates and made seminal observations on free-running rhythms

what is “fishing expedition” or “unbiased mapping”?

attempting to localize circadian clocks by making lesions in various brain regions, and removing various body organs

why has unbiased mapping found discoveries but also failed?

the brain has many regions and there may be redundancies in brain function, so that a particular function may not dependent on any single region

what did Robert Moore suggest to research in locating the circadian clock?

because it can be entrained by light, you would need to identify which visual pathways (from the retina) were necessary for photic entrainment

retinal ganglion cells

• output neurons of the retina and their axons form the optic nerves

what did Moore find out when he was making lesions in the visual system?

• optic nerves were necessary

• optic tracts lesions did not affect LD entrainment

• pathway is most likely optic chiasm

tritiated amino acid autoradiography method

made for mapping connections between different brain regions

• involves attaching weakly radioactive isotope of hydrogen to amino acids

retinohypothalamic tract

very short pathway exiting the optic chiasm and innervating a small structure immediately above the chiasm, aptly named the suprachiasmatic nucleus (SCN)

which area of the brain first receives input from the retina?

the SCN

What did Zucker find about SCN lesions?

• completely eliminates behavioral rhythms in sighted rates in LD and blind rats in DD

what did Moore find about rats with ablated SCN

the daily peak of corticosterone secretion that normally occurred early in the dark period was absent

what was the key implicated from Zucker and Moore’s studies on SCN ablation?

SCN is critical for the expression of circadian rhythms and the brain had no developmental ability to compensate for its loss

what were the effects of SCN ablation on sleep?

• slept same amount each 24h day

• circadian rhythm was replaced by 3h ultradian rhythm

• sleep homeostasis is not affected

what does electrical stimulation mimic in rat and hamsters SCN

phase shift effect of light pulses

what does intra-SCN of NPY (chemical stimulation) in hamster mimic?

phase shifting of arousal

what study suggested that the SCN could oscillate without neural connections with the rest of the brain?

• Inouye and Kawamura created a circular cut around the SCN

• severing all axonal outputs and inputs

• no axonal connections with the rest of the brain

what did Ralpha and Menaker find when transplanting SCN?

rhythms that reappeared were characteristic of the donor genotype

what are the evidence that the SCN is a master circadian pacemaker?

• lesions eliminate free-running light-entrainable rhythms

• stimulation shifts rhythms

• neurons oscillate at single cell and population levels

• transplants restore rhythms, transferring characteristics of the donor animal to the recipient

Robert Moore

• strategic search

  • first identify the visual pathway necessary for photic entrainment - this should point to the circadian pacemaker

  • did succeed

Curt P. Richter

• ‘unbiased’ search (AKA ‘fishing expedition’)

  • remove brain nuclei and glands, measure circadian rhythms, continue until tissue necessary for rhythms is found

  • came close, but did not succeed

what happens when you cut the optic nerve?

rhythms free-run, therefore retina is site of photoreceptors

what happens when you cut the optic tract?

rhythms still entrain, therefore entrainment pathway must be at optic chiasm

how is ablation different from cutting?

it uses energy to destroy tissue instead of physically separating it with a sharp object

how does the ablation method show localization of function in mammalian brain?

tells us how the organism behaves without the structure, but not what the structure does

what does the stimulation method show for localization of function in mammalian brain?

if SCN neurons are stimulated, do rhythms shift?

neural recording method

• does SCN oscillate

• can it oscillate independent of the rest of the brain

neural transplant method

• if SCN are replaced after SCN lesion, are rhythms restored?
  if so, do restored rhythms have same properties of rhythms in the donor animal?

what are the four things SCN neurons oscillate?

• glucose metabolism

• electrophysiology

• secretary activity

• gene expression