Genetic factors have been implicated in the sensitivity of mice to ehlorpromazine, t{t;rF (1962) using a shuttle-box found that 90~ of C57BL/6J mice were completely inactive following a dose of ehlorpromazine which inactivated only 7 ~ of C3HeB/J mice. MEIE~ et al. (1963) using a different test found more prolonged depression of activity in C57BL/6 than in C3t{/An, a result consistent with HUFF'S finding.
Variations in avoidance learning between these strains have been reported (KI~ and ~AVaO~ATIS 1956; ROYCE and CovI~GTO~ 1960; WINSTG~ 1963; COLLINS 1964). Strain C3H has generally been found to be more efficient in avoidance learning. It seemed of interest to determine whether genetic variations in avoidance learning and sensitivity to a tranquillizing drug were related. The more rapid learning of C3H mice might be attributed to a higher anxiety which in turn might make them more resistant to a drug like chlorpromazine. However, it is not certain that the locomotor activities which served as indices for effects of drugs in the two cited papers were induced by anxiety. In fact, ~etivity in a field is often taken to indicate less rather than more timidity. The observed differences in drug sensitivity may reflect variability in exploratory drive rather than in anxiety.
The present experiment compares these two strains on a modified nondiscriminated shock avoidance task, while observing simultaneously the effect of graded doses of ehlorpromazine. The procedure can be considered as a modification of that of SIDHA~ (1953). A third strain, RF/J, was added to the study because of an impression, proved to be erroneous, that it was unusually efficient in learning to avoid shock. To distinguish between effects upon avoidance-motivated behavior and activity otherwise motivated, yoked controls were shocked simultaneously with the experimentals.