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Selectively bred rat lines were developed in order to study affective disorders. The first experiment investigated the food preferences of selectively bred
Sprague-Dawley male rats that demonstrated high (Swim High-Active, SwHi) or low (Swim Low-Active, SwLo) motor activity in a swim test. SwLo rats may
represent an animal model of atypical depression. A characteristic of atypical depression is overeating, usually with palatable foods. The purpose of this
experiment was to characterize the differences in food preference between SwHi, SwLo, and Swim Non-Selected (SwNS) rats. All three groups were given
marshmallow pellets and standard rodent chow in separate food hoppers and their daily food intake was calculated. Results showed an increased consumption
of marshmallow pellets compared to the standard rodent chow. In addition, the food intake significantly decreased after the marshmallow pellets were removed.
The SwLo rats showed a slight trend of decreased marshmallow pellet consumption and a decreased total food intake compared to the SwHi and SwNS rats.
Therefore, the results suggest that the SwLo rats may not demonstrate the characteristic of atypical depression involving overeating with palatable foods.
The purpose of the second study was to characterize the swim behavior of another selectively bred line of rats that showed ambulatory hyperactivity, the
Monitor Hyper rat (these rats have not previously been tested for swim behavior). Monitor Hyper and SwNS rats were tested in one of three conditions:
non-stressed rats swim tested in high water (50 cm), non-stressed rats swim tested in low water (26 cm), and rats subjected to 30 minutes of noise stress
and tested in low water. Swim tests for all groups were quantified for struggling and floating behavior and compared between the rat lines. Results show
a significant decrease in struggling behavior as a result of noise stress of the Monitor Hyper and SwNS rats and a significant increase in floating behavior of
the Monitor Hyper rats under the low water conditions, compared to the SwNS group. This suggests that the activity of the Monitor Hyper and SwNS rats in
the swim test may be reduced by stress exposure and the activity of the Monitor Hyper rats may also be reduced by low water conditions.
The selectively bred rat lines used in this laboratory were developed in order to develop potential animal models of affective disorders, such as depression.
Sprague-Dawley rats were selectively bred for either high motor activity or low motor activity in a modified version of the Porsolt swim test. Their behavior was
observed in a 15-minute swim test, (a test of behavioral depression in rodents). Swim High-Active (SwHi) rats show high struggling and low floating behavior.
The Swim Low-Active (SwLo) rats exhibit little struggling and much floating, and the Swim Non-Selected (SwNS) rats show moderate struggling and floating
behavior. Struggling is defined as vigorous motor activity in the swim test, in which the paws of the rat break the surface of the water. In contrast, floating is
defined as the rat having no limb movement during the swim test. The SwLo rat may be a model for atypical depression. Atypical depression in humans is
characterized by slow movement, hypoactivity, and hypersomnia. In addition, the atypical depression patient may over eat, but usually with food that is highly
palatable. Therefore, the purpose of this study was to characterize the food preferences between SwHi, SwLo, and SwNS rats, in order to determine if the rat
groups prefer standard rodent chow or a more palatable food, especially the SwLo group.
Another selectively bred rat line is the Monitor Hyper rat, bred based on their hyperactive home-cage ambulatory activity. Young (3-5 months of age) Monitor
Hyper rats are characterized by extreme hyperactivity following exposure to a stressor. On the other hand, older (10-14 months of age) monitor hyper rats
exhibit behavioral depression by showing a prolonged decreased home cage ambulatory activity and decreased food and water intake. Therefore, the Monitor
Hyper rats have the ability to show prolonged depression-like and mania symptoms, thereby possibly possessing the characteristics of bipolar disorder. Two
other selectively bred rat lines are the Swim-Test Susceptible (SUS) and Swim-Test Resistant (RES) lines. The SUS line is selected to exhibit significant
reductions in their swim activity following exposure to a noise stressor. The RES line is selected to show little or no reduction in swim activity after a noise
stressor. However, the Monitor Hyper rats have not been characterized in the swim test. Therefore, the purpose of the second study was to characterize the
Monitor Hyper rat behavior in the swim test and compare their swim test behavior to SwHi, SwLo, and SwNS rats, using the high water condition (50 cm). In
addition, we also wanted to compare the Monitor Hyper rats to SUS and RES rats, by testing them under the same conditions as SUS and RES rats are tested
for selective breeding, using the low water condition (26 cm).
Food Intake:
1. Three groups of male Sprague-Dawley rats were used, SwHi, SwLo, and SwNS, n=12/group, totaling n =36 rats. The rats were approximately 2.5 months of
age and remained group housed(n=2/cage).
2. Each cage was supplied with a food hopper containing 5001 purina chow and a food hopper containing marshmallow pellets (Bio-Serv Laboratories), and water
ad libitum.
3. Food hoppers were placed on opposite sides of the cage.
4. Each food hopper was weighed daily and food intake was calculated.
5. The body weight of each rat was measured weekly.
6. From day 1-7, the cages were supplied with 5001 purina chow and marshmallow pellets, then on day 16, the food hopper carrying the marshmallow pellets
was emptied and re-filled with 5001 purina chow.
7. Food intake measurements continued for 7 more days.
8. A 3 (rat line), 2 (food), 14 (days) analysis of variance (ANOVA) with repeated measures (ANOVA) was used to analyze the daily food intake.
Swim Test:
1. Two groups of male Sprague-Dawley rats were used, Monitor Hyper rats (n= 24) and SwNS rats (n= 18), totaling n= 42 rats. The rats were approximately 6-7
months of age and were chosen from group housing.
2. Each rat line was equally divided into one of the following conditions: non-stressed swum in high water (50 cm), non-stressed swum in low water (26 cm), and
30 minutes of “white noise” stress (95 db loudness) swum in low water.
3. The non-stressed high water group rats were dropped into a tank of high water (50 cm high). Flotation devices were wrapped around these rats.
4. The non-stressed low water group rats were dropped into a tank of low water. No flotation devices were used on these rats.
5. The stressed low water group were subjected to 30-min of noise stress and dropped into a tank of low water (26 cm high). Before these rats were dropped into
the tank, the rats were sham-wrapped with a flotation device, as an added stressor (pseudo-flotation device).
6. After dropping the rats into the tank, swim behavior was scored for struggling and floating for 15 minutes.
7. A T-test of struggling and floating times in the high water condition and a 2 (rat line), 2 (stressor), 1 (struggling or floating) univariate analysis of variance were
used to analyze the struggling and floating times in the low water condition.
1. Overall consumption of the marshmallow pellets was greater than consumption of the standard rodent chow.
2. Total food intake significantly decreased after the marshmallow pellets were removed.
3. The SwLo rats showed a slight trend of decreased marshmallow pellet consumption and a decreased total food intake compared to the SwHi and SwNS rats (p = .107).
4. Struggling behavior of the Monitor Hyper and SwNS rats significantly decreased as a result of noise stress (p = .033)
5. Floating behavior of the Monitor Hyper rats significantly increased under the low water conditions, compared to the control group (p = .004).
Based on the results, it can be concluded that the SwHi, SwLo, and SwNS rats preferred the marshmallow pellets over the standard rodent chow, by consuming
a greater amount of marshmallow pellets. After the marshmallow pellets were replaced with the standard rodent chow the total food intake decreased. This shows
that the rats greatly preferred the more palatable food and consumed less total chow when the marshmallow pellets were not present. The SwLo rats showed a
slight trend of decreased marshmallow pellet and total food intake compared to the SwHi and SwNSrats. Therefore, the SwLo rats may not exhibit the
characteristic of atypical depression involving overeating with palatable foods.
The struggling behavior of the Monitor Hyper and SwNS rats decreased after exposure to noise stress. The swim behavior of the Monitor Hyper rats in the high
water condition showed no difference from the control group, so it can be concluded that the swim behavior of the Monitor Hyper rats are not similar to either the
SwHi or SwLo groups. The Monitor Hyper rats demonstrated increased floating behavior (swim activity reduction) in comparison to the SwNS rats during the low
water conditions. The Monitor Hyper rats demonstrated reduced swim activity when non-stressed and after stress exposure under low water conditions, so their
swim behavior does not show similarities to the Swim-test Susceptible (SUS) or Swim-test Resistant (RES) rats. In the future, a study could be completed to
investigate if other forms of stress, such as foot shock, also affect the swim behavior of the Monitor Hyper rats.
This material is based upon work supported by the following organizations: Howard Hughes Medical Institute under Grant No.52003727,
MARC U*STAR Program at the University of Maryland, Baltimore County by the National Institute of General Medical Sciences, and the National
Institutes of Health (NIGMS/NIH) under National Research Service Award GM 08663.
I observed and characterized the food preference behavior and swim behavior different selectively bred rats. Selectively bred rats are bred depending on certain behavior traits.
animal handling, behavioral observation, rat swim test
selectively bred rat lines, swim behavior, food preference
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