**4. Mechanism of action of antipsychotic drugs**

Almost all antipsychotic drugs have many different pharmacological actions that it is very difficult to relate any one action to a therapeutic effect [45, 46]. Effective antipsychotic drugs share the ability to inhibit the physiological actions of dopamine D2 receptors in the brain [47]. Collectively, the drugs are quite useful in controlling the states of agitation observed/found in acute schizophrenia, mania and some other forms of delirium and in paranoia. Their exact mode of action in these conditions remains unknown but most of them block the action of dopamine on D2 receptors in the mesolimbic system of the brain and this seems crucial to their sedative and antipsychotic properties [48]. These drugs also inhibit the action of dopamine on chemoreceptor trigger zone of the brain and are thus antiemetic. Furthermore, drugs such as haloperidol prevent the action of the dopaminergic nerves that run from the substantia nigra to the corpus striatum. Disruption of physiological action of this system causes Parkinsonism and these drugs may cause various disorders of movement and posture [49].
