Unreliability of the rat stomach fundus as a predictor of hallucinogenic activity in substituted phenethylamines.

A series of three isomeric 2,4,5-substituted monoethoxy dimethoxy phenylisopropylamines were compared for their contractile effect in the rat fundus as potential antagonists to the effect of serotonin in the fundus. The three isomers were also evaluated for their discriminative stimulus properties in rats that had been trained to discriminate injections of saline from LSD tartrate (0.08 mg/kg). The drug discrimination studies revealed that the 2,5-dimethoxy-4-ethoxy substitution was most potent in rats, consistent with the reported clinical activity of this isomer in man. By contrast, of the three isomers examined, this was the weakest in eliciting a contraction in the fundus. None of the compounds antagonized serotonin induced contractions, and it was not possible to determine pA2 values. Questions are raised about the determination of pA2 values for partial agonists and it is concluded that the fundus is not a reliable model for prediction of hallucinogenic activity of phenethylamines.