Neurophysiological aspects of typical and atypical neuroleptic agents

Transcranial magnetic stimulation (TMS) offers the unique possibility to measure neurophysiological effects of central acting agents in vivo. In detail, TMS allows to detect inhibitory and facilitatory effects at distinct cortical levels. Using this technique, we investigated schizophrenic patients under low-dose haloperidol as well as patients under clozapine medication. Both under haloperidol and clozapine treatment, a significant prolongation of a special neurophysiological parameter called cortical silent period could be detected as compared to untreated schizophrenic patients. This parameter mirrors inhibitory acting processes at the thalamic level. In accordance with recent theories typical and atypical neuroleptics may share a common neurobiological pathway by improving deficient thalamic gating, thereby preventing cortical areas from sensory overload. In contrast, only patients under haloperidol treatment showed a significantly reduced intracortical inhibition. Reduction of this parameter is thought to reflect increased probability for extrapyramidal-motor side-effects. Taken together, TMS seems to be able to detect neurophysiological patterns of conventional and atypical neuroleptic agents and may provide new insights into the neurobiological background of antipsychotic treatment.