The stimulus effect of 5,6,7,8-tetrahydro-1,3-dioxolo[4,5- g]isoquinoline is similar to that of cocaine but different from that of amphetamine

Abstract

5,6,7,8-Tetrahydro-1,3-dioxolo[4,5- g]isoquinoline (TDIQ) is a conformationally restricted phenylalkylamine related in structure to amphetamine and N-methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane (MDMA) that does not act as a locomotor stimulant. To further evaluate this agent, a group of six rats was trained to discriminate 5.0 mg/kg of TDIQ from vehicle and tests of stimulus generalization were conducted to define the stimulus. The TDIQ stimulus (ED 50=0.9 mg/kg) failed to generalize to the central stimulants (+)amphetamine, methylphenidate or (−)ephedrine but, curiously, generalized to cocaine (ED 50=1.5 mg/kg). When administered to rats ( n=5) trained to discriminate 1.0 mg/kg of (+)amphetamine from vehicle, TDIQ produced a maximum of 7% (+)amphetamine-appropriate responding, whereas when administered to rats ( n=7) trained to discriminate 4.0 mg/kg of (−)ephedrine from vehicle, TDIQ produced a maximum of 57% drug-appropriate responding. Administration of MDMA to TDIQ-trained animals resulted in 76% TDIQ-appropriate responding. Tests of stimulus generalization were also conducted with fenfluramine, nisoxetine, clenbuterol, imipramine and buspirone, and tests of antagonism were conducted with haloperidol and R(+)SCH-23390 using the TDIQ-trained animals. Results were inconclusive in that these agents either failed to completely substitute for or failed to completely antagonize the TDIQ stimulus. Nevertheless, the generalization seen with cocaine, the partial generalization seen with (−)ephedrine, MDMA, nisoxetine, clenbuterol and buspirone and the partial antagonism seen with haloperidol suggest that TDIQ might be acting through a mixed mechanism that involves adrenergic, dopaminergic and/or serotonergic systems. Given that TDIQ is an agent that seems to differentiate among the stimuli produced by amphetamine, methylphenidate, ephedrine and cocaine, it is proposed that further tests be undertaken, using animal models of cocaine abuse, to evaluate the potential usefulness of TDIQ as pharmacotherapy in cocaine dependence.