Screening hallucinogenic drugs: systematic study of three behavioral tests.

The effects of several hallucinogenic and non-hallucinogenic drugs have been studied on three behavioral tests proposed as useful indexes of hallucinogenic activity: "head-twitching" in mice, defecation in an open-field, and suppression of responding on a differential reinforcement of low rates (DRL) schedule of reinforcement. According to the original propositions, after administration of hallucinogenic agents the frequency of head-twitches would increase in mice, the defecation of rats in an open field would decrease without consistent change in ambulation, rearing and grooming, and the responding of rats on a DRL schedule would yield a typical cumulative record pattern. It was found that the head-twitch test was sensitive to mescaline and LSD-25, but not to delta9-THC or to myristicin and elemicin. Besides, the data on interobserver agreement suggested there is a high degree of subjectivity involved in assessing this response. In the open-field test, non-hallucinogenic drugs such as chlorpromazine and apomorphine fell into the hallucinogenic pattern proposed. In addition, the post-injection interval selected seemed to critically affect defecation scores. The DRL "hallucinogenic" pattern occurred nonspecifically after administration of hallucinogenic and non-hallucinogenic drugs. It was concluded that the three tests have limited value for screening purposes.