The suprachiasmatic nucleus

Question 1

How was the SCN discovered to be the circadian pacemaker?

Answer 1

SCN lesion in mice ruins their clock

Grafted a SCN with an FRP of 24 hours to the mouse

Mouse regained rhythmicity with FRP of 24 hours

but what if clock -> SCN - > rhythm?

Mice have an FRP that isn’t 24 hours, if that were the case it would overwrite the SCN’s clock but it didn’t

Question 2

Is the SCN organized? How so? What hormones are produced by the SCN?

Answer 2

Ventral and dorsal sections (for the purpose of this course)

Each secton secretes their own hormones,

Ventral SCN: VIP

Dorsal SCN: VP

All other proteins are distributed differently

Question 3

When does Per1 peak in each region of the SCN? Why might this be?

Answer 3

Ventral SCN peaks in early day, troughs in early night

Dorsal SCN peaks in late day and troughs in late late nigiht

Ventral SCN linked to optic nerve, receives light input (will see later)

Question 4

How does jet lag affect per expression in the SCN?

Answer 4

ventral region catches up first (optic nerve)

dorsal region catches up

Fully adjusted within a week but catch-up time depends on region

Question 5

What was noticed when measuring per::luc activity in individual SCN neurons? What does that imply?

Answer 5

Isolated neurons still showed circadian rhythm with a period of around 24 hours

But they each had different phases meaning they communicate in the SCN together (somehow)

Showed that all you need for a circadian rhythm can be contained within a single cell

Question 6

What is cyclohexamide and what did a ‘wedge’ experiment with it prove?

Answer 6

CHX is an inhibitor of protein synthesis

Pulsing the SCN with CHX for less than 6H did nothing, but 12H and up caused a noticeable delay in peaks (always peaked ~20 hours after pulse)

Shows that the clock restarts itself somehow, despite stopping all translation

Measuring individual neurons in this experiment showed that they ‘remembered’ which order they fired in

Strong evidence for interneuronal network

Question 7

What was observed when measuring Cry1 KO and Cry2 KO neurons a) in an SCN slice and b) as dispersed neurons?

What does this prove about synchronicity?

Answer 7

Cry 2 KO neurons kept rhythmicity and phase in slice, and became desynchronous when dispersed (phase remained constant)

Cry 1 KO neurons kept rhythmicity and phase in slice, but lost rhythmicity and synchronicity when dispersed

The cellular network was necessary to stay in phase, not to keep rhythm

Question 8

What did the TTX experiment prove regarding the SCN network?

Answer 8

TTX prevents neurons from firing

Action potentials are necessary for the cells to synchronize, otherwise they’re desynchronous

Question 9

How was it proven that the synchrony of SCN neurons comes from VIP?

Answer 9

A VIP SCN KO host had decreasing synchrony/bioluminescence

A healthy SCN was grafted and amplitude was restored

Something in the healthy SCN rescues wild type and the only difference is VIP

Question 10

What was found when the dorsal and ventral SCNs were cut apart? Why was this result found?

Answer 10

Ventral SCN retained rhythmicity, Dorsal became desynchronous

The ventral SCN secretes VIP

VIP KO shows individual rhythmicity but lack of synchrony