PHYSIOLOGICAL CORRELATES OF THE FORCED SWIM TEST IN RATS

Rats were tested in the forced swim test to evaluate the effects of duration of exposure (0, 5, 15, or 25 min), and water temperature (0, 35, 30, 25, or 20-degrees-C), on a variety of physiological measures. Serum corticosterone, glucose, lactate, phosphorus levels, and the anion gap (a measure of acid-base status) were increased significantly, whereas carbon dioxide and potassium levels were consistently decreased by testing, as was the potassium/phosphorus ratio; creatinine, triglycerides, and magnesium were not altered significantly in any study. Effects on prolactin, amylase, lipase, cholesterol, alkaline phosphatase, sodium, and chloride were inconsistent. Levels of serum corticosterone were increased at each duration of testing, and the increments were significantly higher than the previous duration. Corticosterone levels were also increased in proportion to decreasing water temperature, but the increments were not significantly different from the previous or following temperatures. Glucose levels were increased at every duration and at every water temperature with the exception of the coldest water temperature. Lactate and phosphorus levels and the anion pp were all increased, whereas carbon dioxide levels decreased after 5 min of immersion. Potassium levels did not decrease until some time after 5 min of testing. Immobility times were marginally correlated with corticosterone levels (r = -0.38) but were highly correlated with serum carbon dioxide (r = 0.59), potassium (r = 0.67), and phosphorus levels (r = -0.73); the correlation between immobility times and the ratio of potassium/phosphorus was 0.82. The potassium/phosphorus ratio accounted for 67% of the variance in immobility. These high correlations were interpreted in terms of acid-base changes associated with testing. The results shed new light on the physiological changes underlying the immobility response in the forced swim test. Although traditional stress measures such as corticosterone and glucose are reliable physiological stress measures, changes in serum potassium and phosphorus appear to reflect behavioral responses to water immersion stress more closely than these other measures,