This investigation sought to tease apart serotonin releasing effects from neurotoxic effects to serotonin axons by comparing p-chloroamphetamine (pCA), p-methylthioamphetamine (MTA), 1-(m-chlorophenyl)piperazine (mCPP) and MDMA in synaptosomes and (in MTA and pCA only) through in vivo microdialysis studies. Rat synaptosomes were derived from hippocampus and striatum of CRL:CD(SD)BR strain. Serotonin (5-HT) release in rat synaptosomes pre-loaded with tritiated 5-HT was assessed after 1, 3, 10 or 30 mcM MTA, 1 or 30 mcM mCPP and from 10 to 100 mcM pCA and MDMA (not fully specified). Dopamine (DA) release was assessed using tritiated DA in striatal DA synaptosomes. Synaptosomes were also superfused to prevent the effects of uptake inhibition, and another study measured radiolabeled citalopram binding for the 4 compounds described above. In vivo dialysis measured 5-HT release in the hippocampal area after central administration of 10 or 100 mcM or 1 mM pCA or MTA through reverse dialysis; serotonin content was assessed via HPLC in microdialysate and in brain. The effects of MTA, pCA or the two combined on radiolabeled citalopram in brain was also examined. MDMA and pCA were much more potent 5-HT releasers in synaptosomes than MTA or mCPP, and MTA showed maximal release of 5-HT at 1 mcM, with smaller amounts of release at higher doses. MTA did not release DA in synaptosomes at 3 mcM, but did so in a concentration-dependent manner at 10 and 100 mcM. When compared with the other substances, mCPP did not appear to release much DA. A study measuring release of tritiated 5-HT after a shorter period of superfusion found that MTA and mCPP still released less 5-HT in synaptosomes than pCA. Release of endogenous (not pre-loaded) 5-HT by 10 mcM mCPP, MTA, or pCA found that mCPP did not release 5-HT, and that MTA released less 5-HT than pCA. Citalopram binding assays found that pCA least active in altering citalopram binding (behaving as if it were a releaser or ÒsubstrateÓ for the SERT). mCPP was most active in altering citalopram binding, and MTA and MDMA took intermediate positions re binding, though both were more active on 5-HT uptake than on citalopram binding. Yet microdialysis studies found that MTA released hippocampal 5-HT at all concentrations tested and was more potent than pCA. Treatment with MTA did not affect hippocampal 5-HT, whereas pCA reduced hippocampal 5-HT by 64%. The authors explain different results in synaptosome and microdialysis studies as a demonstration of the independence of serotonin release and serotonin neurotoxicity after substituted amphetamines. The authors propose that releasing synaptosomal 5-HT may relate to degree of interaction with and induced release of 5-HT from within the vesicular pool, and that ability to release synaptosomal 5-HT might serve as a marker of potential serotonergic neurotoxicity.