Insignificant data: n = 2!

Just thought I’d write an update about my project now that I’m through with the first batch of animals.  I hope this may alleviate some of the frustration it brought me over the weekend, and help me better organize and prepare for the next batch.  To summarize the background, briefly: hypertensive rats are receiving daily oral injections of the CB1 cannabinoid receptor antagonist SR141716 (SR) or its vehicle for 4 weeks, starting at 16 weeks of age, and once a week for the first three I’m recording their blood pressures, heart rates, food and water consumption, and blood glucose.  During the last week of dosing they are outfitted with femoral artery catheters so that on Day 28 I can take direct conscious recordings of blood pressure and heart rate, from which I can determine their baroreflex sensitivity and heart rate variability.  These values are both indices of autonomic function, specifically of  parasympathetic vagus nerve activity, that the brain uses to control blood pressure and heart rate.

I’m happy to report that the last 28 days have yielded me some decent preliminary data!  I dosed only four rats total in this first batch, so each group has two subjects so far.  Of course, that means these data are not yet statistically analyzable, but the trends in each set show promise.  And they remind me of this gem:

Let’s start with body weights of each group.  I weighed animals every day to determine the injection volume each would receive, so body weights were my most frequent measurement and thus my only window into the drug’s effects for most of the study.  Note that because just two animals are in each group, the bars on each point represent the raw values and NOT the standard error.  The points are the average of the two values for that day.

The top graph represents the animals’ body weights each day just prior to dosing (which was always in the afternoon, between 2:30 and 3:30 pm).  The bottom graph represents changes in body weights from each animal’s baseline (i.e. body weight on Day X – Day 0).  So, anything jump out?

To address the most salient feature of each graph, Day 24 was the catheter surgery day.  Surgery is obviously a stressful event and I expected it to arrest weight gain.  However, I did not expect them to lose weight for three consecutive days, and fearing that significant weight loss would affect the conscious recordings, I terminated the experiment early and recorded on Days 26 and 27.  Going forward I will probably allow just one or two full days for recovery before proceeding with the conscious recordings (the primary issue is allowing the anesthesia to clear the body because it dampens autonomic nervous system activity).  Although the weight loss looks precipitous, it is less than 10% of peak body weight, which my advisor believes would not significantly interfere with autonomic function.  I’m skeptical, since that would be like a 200 lb person dropping 20 lbs in 3 days.  But these are rats, not people, so… try to do better next time?  Now that I think about it, they may not have been drinking much water, in addition to eating less (if at all), during those post-surgery days.  That could definitely lead to rapid weight loss irrespective of fat or muscle tissue loss (which would definitely affect blood pressure).

Surgical ramifications aside, there’s definitely an exciting trend.  Rats given the drug gained less weight than their cagemates receiving vehicle.  I think I’ll need to report this as something like “maximum weight gained” or “weight gained by Day 26” to circumvent the effect of the catheter surgery.  Also, rats seemed to slightly lose weight at the start of treatment and did not begin gaining weight until after Day 8.  I believe the initial stress from injections through an oral gavage may account for much of this effect.  Going forward I intend to train rats to receive vehicle injections starting a week prior to their experimental baseline (Day 0).

I’ll stop here for tonight and pick it up tomorrow.  This exercise has been therapeutic.  I’m looking forward to organizing more of this mess as the weekend shockwave fades.


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