In this article, we reviewed and analysed the literature of mechanism of cognitive and psychotic disorders in schizophrenia, dementia and Parkinson's disease. Despite of opposite neurotransmitter disturbances in schizophrenia and Parkinson's disease, there is common origin in development of psychotic symptoms. The main risk factors in producing psychotic symptoms are age, cognitive impairment and general disease severity. The key neurotransmitter disturbance of dementia is hypofunction of glutamatergic and cholinergic transmitter systems. An hypofunction of the NMDA glutamate receptors can produce excessively stimulation corticolimbic dopaminergic and serotonergic neurons and appearance of psychosis. We suggest glutamate NMDA receptors can modulate activity of dopaminergic, serotonergic and cholinergic transmitter systems. Successful pharmacological approaches in therapy of cognitive and psychotic disorders in Parkinson's disease may be normalization conditions of glutamatergic and cholinergic transmitter systems.