Body weight, caloric monitoring, activity and motor function, food and water intake
General. Experiments described here used a Comprehensive Lab Animal
Monitoring System (CLAMS) (Columbus Instruments, Columbus, OH) that
consists of individual live-in cages for mice that allow automated, non-invasive
data collection. Each cage is an indirect open circuit
calorimeter that provides measures of oxygen consumption and carbon
dioxide production. The system also provides concurrent measurement
of ingestive behavior (food, water consumption) and activity.
A total of sixteen mice (8 male, 8 female) from each of 16 strains were
tested. Groups of 8 individually housed mice were tested in each
experiment. In all cases mice of the same gender were tested over the same
3-day period for each of the 16 strains. Data for male and female mice of
a given strain were collected consecutively within a 7 day period for 11 of
the 16 strains and within a 2-3 week period for the remaining 5 strains.
All mice were maintained in the testing facility for at least 48 hours
prior to the experiment. All experiments started between 8:30-10:30 am and
continued for 3 consecutive 24-hour periods with a 12:12 light:dark cycle.
Body weights were determined just before and after testing.
Immediately upon completion of the experiments the mice were prepared for
shipment to the laboratory of Dr. Thomas Hampton (MouseSpecifics, Inc., Boston, MA.) for
non-invasive measurement of the electrocardiogram.
CLAMS records date and time-stamped data on a fixed sampling schedule for each mouse for multiple variables. In the configuration used for these experiments data for each mouse were recorded every 41 minutes such that each mouse had 105 samples recorded over the 72-hour period of the experiment. In all cases data were recorded over three transitions from light to dark and three from dark to light. The daily averages over 3 days of testing were submitted and are available unless specified otherwise. Further relevant details related to sampling are described in association with the different measurements.
Calorimetric monitoring. The cages in which animals were housed are indirect open circuit calorimeters. The system compares oxygen and carbon dioxide gas concentrations by volume at the inlet and outlet ports of the cage chamber through which ambient air flows at a constant rate (0.60 l/min). The difference in concentration between the two ports and the flow information is used to calculate oxygen consumption, carbon dioxide production and the Respiratory Exchange Ratio (RER). Heat production is also estimated with standard formulas using oxygen consumption and the RER. Data reported here were collected with an 8-cage system that utilizes single sensors for oxygen and carbon dioxide. Consequently, gas measures must be taken sequentially from each of the 8 chambers on a fixed cycle. The cycle time between samples/measures for any given chamber is therefore a function of the time required for the sample to reach the sensors and be measured and the number of chambers in operation.
Activity. Two arrays of infrared beams (2.5 cm inter-beam distance) surround each cage. One array is situated 3.2 cm above the floor of the cage and a second array at 7 cm above the floor. This configuration provided three measures of activity: 1) Total activity - any movement producing a beam break in the horizontal plane, 2) Ambulatory activity - movement producing sequential horizontal beam breaks of different beams (i.e. subset of total), 3) Rearing - produced by vertical movement when the mouse stands on it's hind legs to break elevated beam array. For these experiments beam breaks were monitored continuously and summed and recorded each time a metabolic measure was taken (i.e. every 41 min.).1 Activity measurements are expressed in normalized units of beam/breaks per minute.
Food and water consumption. A computer continuously monitors a food container that rests on an electronic weigh scale.2 The cumulative consumption is monitored and stored each time a metabolic measure is recorded.3 Individual water columns containing pressure transducers supplied drinking water to each individual mouse. Drinking by the mouse reduced pressure in the column and was converted to volume.
1 Because of this the correlation of activity with metabolic measures is poor because the latter measures are "snapshots" taken at the end of the period for which activity is summed.
2 Feeder design is such that spilled food is captured to avoid overestimating consumption and to minimize the amount of food the animal can remove from the receptacle for "storage" in their living space.
3 The measurement of water consumption was the measure most prone to inaccuracies. Mice frequently "played" with sipper tubes resulting in water loss not related to consumption. When this behavior resulted in clearly inaccurate data the values have not been reported. Also, mice were not acclimatized to the individual CLAMS cages prior to the experiment and frequently it would take some time before they began drinking. It was assumed that this was due to being unfamiliar with the drinking apparatus that was different than that of their home cages.