Forced Swimming Stress Induced Alterations in Ingestive Behavior in Rats

NAGARAJA H. S.* AND P. S. JEGANATHAN
Department of Physiology,
Kasturba Medical College,
Mangalore  - 575 001
(Received on June 26, 2001)

 

 

Keywords: stress, swimming, body weight, food intake

 

INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

INTRODUCTION

 

A stressful stimulus can be considered as any event which precipitates a significant activation of the sympathoadrenal medullary system and a measurable change in the behavior (1).  Various types of acute and chronic stressors were employed in different stress studies by various authors.  The different types of acute and chronic stressors in experimental animals include immobilization, tail shock, cold stress, forced swimming stress etc.  When animals were subjected to acute stress, a wide range of physiological alterations take place (2).  A short single exposure of stress can produce profound changes in animal's stress responses and behavior (3).  Chronic stress models are in the form of repeated exposure on a daily basis to the same stressor.  Chronic stress has been linked to major physiological and psychological illness in humans.  One predominant feature of this type of stress regimen is the finding that repeated stress lead to habituation or adaptation (4).  Chronic or repeated stress is known to cause a wide range of physiological, psychological and neuroendocrine changes (5).  Consequences of chronic exposure to stressors are dependent on particular characteristics of the stressors such as intensity and duration.  Regularity of exposure to chronic stress has been shown to be an important factor in determining its influence on physiological variables such as corticosterone, glucose and free fatty acids (6).

 

In small laboratory animals, swimming has been widely used for studying the physiological manifestations and capacity of the organism to respond to stress (7).  For the study of exercise physiology, swimming has a number of advantages over other types of exercises (8).  In the forced swimming stress, which is commonly employed for investigating in physiology of physical exercise, the involvement of emotional components cannot be always ruled out (9).

 

Available literatures indicate that exposure of rats to different types of stress bring about a significant change in the ingestive behavior (10, 11, 12, 13).  Rats subjected to immobilization stress for 13 days showed a decrease in food intake and body weight but without any change in their water intake and urine output (14).  Rats exposed to 16 hours of cold stress showed a decrease in food intake and increase in daily urine output (15).  Psychological stress appeared to induce changes in renal excretory function both in animals and man (16).  Despite the widespread use of the forced swim test as an animal model of depression, etc., the physiological changes that accompany testing in this procedure have received little attention with the exception of serum coricosterone and prolactin measurements (17, 18, 19).

 

As the information about the forced swim stress (of varying durations) on the maintenance of body fluid balance is lacking, and also it is unclear whether an acute stress would produce the same effect on the ingestive behavior of rats as the chronic stress, the present work was attempted to study the influence of acute and chronic type of forced swimming stress for varying durations (1 day, 7 days, 14 days and 21 days) on the ingestive behavior of rats and also to delineate the mechanisms involved in bringing out these changes in the physiological parameters.
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METHODS

Adult male albino rats of Wistar strain with the body weight ranging from 200-275 g were used for this study.  Rats were kept individually in metabolic cages (20 x 60 cm) under standard laboratory conditions.  The food pellets (Lipton India Ltd.) were kept in the food chamber provided in the cage and both food and water were available ad libitum.

Control group consisted of 10 rats, kept under standard laboratory conditions, left undisturbed in their home cages without any stress exposure.  In the experimental group, the rats were forced to swim in plastic tubs (height: 60 cm, diameter: 30 cm) containing water maintained at room temperature (28°C). The water level in the tub was 30 cm from the bottom. The swimming session lasted for 15 minutes daily in all the experimental groups.  Forced swimming stress was studied in four subgroups of 10 animals each.  In the first subgroup (one day stress group), the stress period lasted for 15 minutes only in the first day.  In the second subgroup (7 days stress group), the stress period was for 7 days.  In the third subgroup (14 days stress group) it lasted for 14 days and the fourth subgroup of animals for 21 days (21 days stress group).

 

Food intake, whole body weight, water intake and urine output were monitored daily and the data collected. 24-hour food consumption was measured by weighing what was left behind in the cage, of the food provided on the previous day.  Food intake was indexed to body weight in control as well as experimental groups. 24-hour water intake was measured by measuring the volume of water left in the water bottle and 24-hour urine output was measured by collecting the urine in the metabolic cages.  From the blood samples collected by decapitating the animals at the end of the stress durations, blood sugar level was determined calorimetrically.  All the experiments were carried out between 10 AM to 12 Noon to minimize the circadian variability.

 

Data collected were computed for mean values ±  SEM.  Percentage changes in the various parameters in control and stress groups were compared using Mann Whitney 'U' test.  Comparison within the subgroups were performed using repeated measures of ANOVA followed by Bonferroni 't' test where post hoc tests were warranted.  P value less than 0.05 was considered statistically significant.
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RESULTS

There was a significant decrease in body weight up to 14 days after forced swimming stress (P<0.001, Mann-Whitney 'U' test).  A gradual decrease in body weight was seen up to 7 days (P<0.01, Bonferroni 't' test) from the one-day stress group.  Decrease in body weight continued up to 14 days, compared to the control groups body weight for the same duration.  After 21 days stress, there was a significant increase in the body weight compared to the control (P<0.001, Mann-Whitney 'U' test), one day (P<0.05), 7 days (P<0.001) and 14 days stress (P<0.01, Bonferroni 't' test).  During the same period the control group showed a gradual increase in body weight from first day to 21st day (P<0.001 Bonferroni 't' test) (Table I and II).

 

 

Table I click to view full image

Table I: Body weight, food intake, blood sugar level, water intake and urine output in forced swimming stress.

 

 

Table II click to view full image

Table II: Percentage changes in the different parameters after forced swimming stress.

There was no significant change in the food intake after one day and 7 days of stress exposure (Table I).  But forced swimming stress increased food intake after two weeks of stress exposure compared to control group for the same duration (P<0.05, Mann Whitney 'U' test).  Food intake decreased significantly after 21 days stress exposure, compared to one-day stress subgroups (P<0.05, Mann Whitney 'U' test) (Table II).  There was a significant decrease in the blood sugar level in all the subgroups, compared to control group (P<0.001, Mann Whitney 'U' test and Bonferroni 't' test) (Table I and II).  More significant decrease was seen up to 7 days stress exposure.  But from one day stress to 21-day stress, there was a gradual increase in blood sugar level (P<0.01, Mann-Whitney 'U' test), but this blood sugar level after 21st day was significantly less than control group for the same duration. 

A significant increase in the water intake was observed after forced swimming stress, throughout the duration of stress (P<0.001, Mann-Whitney 'U' test and Bonferroni 't' test) (Table I and II).  Control animals showed gradual increase in water intake from day one to 21 days (P<0.001, Bonferroni 't' test), compared to the stressed animals for the same duration.  Urine output also decreased significantly in the initial period of' exposure to the stressor up to 7 days (P<0.01, Mann-Whitney 'U' test), but later increased significantly after 14 days stress exposure compared to one day stress subgroup (P<0.05, Bonferroni 't' test) (Table I and II).
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DISCUSSION

Ingestive behavior undergoes significant changes when experimental animals like rats are exposed to different kinds of stress.  The forced swimming stress involves immersing the rat in a tub of water from which it cannot escape.  This test procedure itself is physiologically stressful because it results in increased serum corticosterone and prolactin levels (18, 19).  The observed change in body weight after forced swimming stress is in accordance with other reports already published (12, 20).  The effect of stress appeared to be more marked in the initial period of exposure to the stress up to a period of 7 days.  The body weight regained gradually after 14 days stress and 21 days stress exposure.  This recovery back to normalcy after two weeks of stress could be attributed to the habituation to the stressful stimulus when the stress period was prolonged.  In the present study, we observed no change in the food intake in animals up to 7 days of stress exposure.  In spite of having normal food intake, the body weight decreased in animals, up to 14 days of' stress.  Control rats displayed a normal growth curve, where as the experimental animals showed a persistent decrease in their body weight, and this was in spite of improvement in food intake over a period of 14 days.

 

Stress is well known to trigger CRH secretion, which could exert suppressive effect on the food intake in animals (21).  The forced swimming stress might have suppressed CRH secretion from hypothalamus, which might explain the normal food intake, observed in this study up to 14 days in stressed rats.  However, stress is also known to increase protein catabolism.  Normal rats subjected to food deprivation can still maintain growth and minimise weight loss by increasing the efficiency of utilization of whatever food was taken during the 24-hour period (21, 22).  Stress may hamper this mechanism.  Therefore rats subjected to forced swimming continued to lose weight initially, in spite of normal food intake.

 

Forced swimming produced a decrease in water intake in rats throughout the 21 day schedule of stress duration, But the urine volume decreased only in the initial period of exposure to stress.  The urine volume did not change up to 21 days stress period after the initial decrease up to 7 days; but the water intake decreased throughout the swimming stress duration.  This could be due to the possibility of dehydration after swimming stress in animals, which is in accordance with other reports already published (23).  At the same time this observation is contradictory to certain other reports where there was increased water intake and urine output after stress in rats (15, 24).  Stress induced decrease in urine volume may be because of stress induced decrease in water intake and also may be due to a possible antidiuretic effect of stress per se (14, 25).  In the present study, food intake and urine output changed only slightly after stress whereas water intake was reduced significantly resulting in a state of mild dehydration after swimming stress.

 

Response to stress is highly contradictory with regard to blood sugar levels.  Studies related to stressful events and fluctuations in blood sugar have shown responses ranging from slight decrease, relative increase and no change in blood sugar (26, 27). In the present study, it was observed that stress produced severe hypoglycemia. This observation confirms the previous reports by Rodnick et al., (28) who observed a decrease in blood sugar level after physical training by either swimming or running.  Some stressful stimuli like swimming are known to enhance insulin sensitivity in rats (29).  It is possible that the increased insulin sensitivity after forced swimming caused the lower level of blood sugar in rats.

 

The response to stress was maximally observed after 7 days of stress exposure in the various parameters studied.  When the stress period was prolonged to 14-21 days, there was a gradual recovery back to near normal in the body weight, water intake and urine output.  From the forgoing it can be observed that an acute stress like swimming, produces significant change in ingestive behavior where as when the stress becomes chronic the changes induced became non significant.  This type of behavior of the animals to prolonged stress could be attributed to habituation of the hypothalamo-pituitary-adrenal axis (30).  For a given stress, the habituation may depend upon several factors such as, the intensity of stress, the inter-session time interval, inter-individual variability etc.  Since habituation developed following exposure to the same stressor, it is likely to result from behavioral familiarization to that particular type of stress.  On the contrary, no habituation response was observed in the blood sugar level after forced swimming stress possibly, because of variety of several other factors influencing carbohydrate metabolism in situ.
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REFERENCES

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