Cats injected with LSD display distinctive behaviors, such as limb flick and abortive grooming, that are extremely rare in normal cats but become dominant after treatment. The frequency of these behaviors depends on the LSD dose, and the effects last a long time after a single injection. Repeated LSD administration leads to tolerance. These behaviors are not caused by various control drugs but are triggered by other indole nucleus hallucinogens. Because these behavioral effects are specific, reliable, easy to score, and quantifiable, they provide an animal model for studying LSD and related hallucinogens.
The hypothesis that hallucinogenic drugs work by suppressing the activity of serotonin-producing neurons in the raphe nuclei was tested by giving LSD to freely moving cats and monitoring both their behavior and raphe neuron activity. Results generally supported the hypothesis, but several mismatches emerged: low LSD doses caused only small decreases in raphe activity yet produced clear behavioral changes; behavioral effects lasted longer than the suppression of raphe firing; and during drug tolerance, raphe neurons remained just as responsive to LSD as they were before tolerance developed.