Neurotransmitter interactions in schizophrenia--therapeutic implications.

The search for new and improved antipsychotic agents has escalated during the past five years. The era of searching for non-toxic copies of clozapine has been followed by several different lines of research, some of which pursue the traditional dopamine track, although at a higher level of sophistication, whereas others focus on other neurotransmitters, such as serotonin and glutamate. Emerging knowledge about the interactions between different neurotransmitters in complex neurocircuits opens up possibilities for achieving antipsychotic activity by interfering with many different neurotransmitters. Most intriguing is the finding in animal experimental models, indicating that it should be possible to alleviate psychotic conditions by stabilizing rather than paralyzing neurocircuits, thus avoiding the risk of motor and mental side effects of the currently used drugs. Among these new classes dopaminergic stabilizers and 5-HT2A receptor antagonists seem to offer most promise at present. In a longer perspective, drugs interfering with glutamate function via different mechanisms may also turn out to be useful, especially in the control of negative symptoms.

Neurotransmitter aberrations in schizophrenia: new perspectives and therapeutic implications.

The dopamine hypothesis has dominated schizophrenia research for decades but is now yielding to a more diversified view, where the interaction of several neurotransmitters in complex circuitries is under scrutiny. Especially, glutamatergic and serotonergic mechanisms are attracting attention. However, the role of dopamine also needs further exploration and may still turn out to have novel therapeutic applications. In the present minireview an attempt is made to integrate preclinical and clinical data on neurotransmitter aberrations in schizophrenia and to discuss their therapeutic implications.

Are the disparate pharmacological profiles of competitive and un-competitive NMDA antagonists due to different baseline activities of distinct glutamatergic pathways? (Hypothesis).

Corticostriatal glutamatergic neurons impinging on the so-called "direct" striato-thalamic pathways appear to act as a driving force with respect to psychomotor functions, whereas corticostriatal glutamatergic neurons projecting to the "indirect" striato-thalamic route appear to mediate inhibition of the thalamus and thus act as a "brake" with respect to psychomotor functions. The GABAergic striatal projection neurons pertaining to the "direct" pathway mediating behavioural stimulation appear to be phasically activated, whereas GABAergic striatal projection neurons pertaining to the "indirect" pathway mediating suppression of behaviour must be assumed to display a high tonic activity. Such an organization could explain some of the behavioural differences between competitive and un-competitive NMDA antagonists, since the binding of competitive NMDA antagonists is inhibited by glutamate, whereas the binding of un-competitive NMDA antagonists is enhanced by the presence of NMDA receptor agonists, a phenomenon called use/agonist dependence.