Abstract

Hyperactive animals were sensitive and hypoactive animals relatively resistant to the changes in locomotor activity produced by various central nervous system stimulants and depressants. The poor response of hypoactive animals to stimulants could not be improved by adjustment in dosage, suggesting a physiological incapacity to respond.

The locomotor response of the animal to stimulants or depressants was poorly correlated with the intensity of the same animal’s response to other qualitative changes produced by the drugs, such as ataxia, bizarre behavior and the like. Each drug action appeared to be affected on an independent basis, suggesting considerable biological variation in the manifold responses of the individual animal to drug, and not just between animals.

Although female animals as a group showed a greater locomotor response to CNS stimulants and depressants than did male animals, this could be accounted for entirely in terms of their higher level of activity prior to therapy. When pre-and post-treatment activity counts were correlated, there were no significant differences between male and female animals in the slope or intercepts for locomotor responses produced by stimulant or depressant drugs.

The locomotor activity counts of male rats and their responses to drugs were found to be relatively stable from the second to fifth month of life.

The implications of these findings in drug comparisons are discussed.