Injection of vehicle is not a stressor in Porsolt's swim test

A single injection or chronic injections with vehicle are reported to induce physical changes in the brain, suggesting this treatment may be stressful. Furthermore, preliminary observations indicate that vehicle injections may interact with the behavioral effects of psychoactive compounds. We investigated the behavioral response to an injection in two tests sensitive to stress: Porsolt's swim test of depressive behavior and the resident-intruder test of aggression. Acute or repeated injections of vehicle did not alter immobility in the swim test in "normal" male NIH Swiss mice, isolated mice, or mice previously exposed to continuous fight stress. Behavior in the resident-intruder test was similar in injected and noninjected mice. The present data suggest that in male NIH Swiss mice a vehicle injection does not induced behavioral changes in tests sensitive to stress, although it is shown to alter various physiological parameters indicative of stress.