Light, circadian cycles, and cardiac function

James Kozloski KOZLOSKI_J at a1.mscf.upenn.edu
Tue Apr 26 21:27:52 EST 1994

In article <2p7q10$46g at ariel.ucs.unimelb.EDU.AU>,
dlj at raptor.physiol.unimelb.edu.au (Doug Jones) wrote:

> David Hallowell (st93q84w at Dunx1.OCS.Drexel.Edu) wrote:
> : I'm not sure if this really is the correct place to post this but...I'm
> : interested in testing the relationship between circadian rhythms and
> : cardiac function and the affect that light might have on this relationship
> : for a class project.
> :                          
> : How would I go about doing this?  I have a clue how to test this 
> : generally, but I'm not sure on the specifics: what organism to use, how 
> : to test for changes in cardiac function....
> : It's too hard to use humans since I can't think of anyone that is willing
> : or has the time to undergo light depravation for this experiment.  Can
> : heart rate and/or blood pressure be measured in mice?  Does anyone know of
> : any organisms that might prove to be easiest to study?
> : Any help or insight is great appreciated!
> : David
> You can get some information on alterations in cardiac performance by
> using a Holter Monitor. This records heart beats in time and with a
> decoder can generate alterations in heart intervals over a 24 hour
> period. Also you can use programs which determine the standard
> deviation in the "R-R intervals". This information provides some
> indication of the autonomic influence over the heart during the
> 24 hour period - AND has been used as one index of the potential
> health/dysfunction of the autonomic controls.
> Your local heart association should also be able to provide you with
> some of the results of studies which indicate the potential of these
> measurements to determine subjects as risk of sudden death.
> If you are interested in testing the effects of light, AND IF your
> institution has environmental chambers, you could try having a couple
> of friends spend a weekend (friday night to sunday night) in the
> chamber with a reverse light cycle and see if there are any shifts in
> their heart rate variability.
> Others might be able to suggest experiments with mice or rats.
> Certainly there is the ablility to measure heart rate, and even blood
> pressure from many rodent species.
> Good luck!

I'm working in a circadian biology lab now.  We use hamsters and examine
wheel-running behavior.  I'm not sure how much experience you have with
circadian biology, so I'll tell you the most important bit of advice I can
think of:

In order to measure the effect of light on any expressed behavior of the
animal, you need to first entrain it to a light dark cycle (12:12, 14:10,
etc).  This takes about a week.  Then put the animal in constant dark
conditions.  You should see whatever behavior you are examining (heart rate
cycles for example) start to drift in time. Leave it like this for at least
a week as well. The drifting onset time is due to the animal having an
internal clock which is not set exactly to 24 hrs.  Perhaps its 23.5 hrs.,
in which case it will start its behavior cycle 1/2 an hour earlier each
day.  This will persist so long as the animal is in constant conditions
(complete darkness, no cues from the outside world to reset the clock...) 
You can then, and only then, test the animal's response to light by
exposing it to brief 10 minute pulses, once every week or so.  This should
"phase-shift" the behavior in time, depending on when in the cycle you
administer the pulse.  If you want you can construct a "phase-response
curve" showing how much the animal shifted depending on when during its
cycle you administered the light pulse.  Good luck!!!

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