Dopamine hypothesis of schizophrenia
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The dopamine hypothesis of schizophrenia or the dopamine hypothesis of psychosis is a model, attributing symptoms of schizophrenia (like psychoses) to a disturbed and hyperactive dopaminergic signal transduction. The model draws evidence from the observation that a large number of antipsychotics have dopamine-receptor antagonistic effects. The theory, however, does not posit dopamine overabundance as a complete explanation for schizophrenia. Rather, the overactivation of D2 receptors, specifically, is one effect of the global chemical synaptic disregulation observed in this disorder.
Some researchers have suggested that dopamine systems in the mesolimbic pathway may contribute to the 'positive symptoms' of schizophrenia (whereas problems with dopamine function in the mesocortical pathway may be responsible for the 'negative symptoms', such as avolition and alogia.) Abnormal expression, thus distribution of the D2 receptor between these areas and the rest of the brain may also be implicated in schizophrenia, specifically in the acute phase. A relative excess of these receptors within the limbic system means Broca's area which can produce illogical language, has an abnormal connection to Wernicke's area, which comprehends language, but does not create it. Note that variation in distribution is observed within individuals, so abnormalities of this characteristic likely play a significant role in all psychological illnesses. Individual alterations are produced by differences within glutamatergic pathways within the limbic system, which are also implicated in other psychotic syndromes. Among the alterations of both synaptic and global structure, the most significant abnormalities are observed in the uncinate fasciculus and the cingulate cortex. The combination of these creates a profound dissymmetry of prefrontal inhibitory signaling, shifted positively towards the dominant side. Eventually, the cingulate gyrus becomes atrophied towards the anterior, due to long-Term Depression (LTD) and Long-Term Potentiation (LTP) from the abnormally strong signals transversely across the brain. This, combined with a relative deficit in GABAergic input to Wernicke's area, shifts the balance of bilateral communication across the corpus callosum posteriorly. Through this mechanism, hemispherical communication becomes highly shifted towards the left/dominant posterior. As such, spontaneous language from Broca's can propagate through the limbic system to the tertiary auditory cortex. This retrograde signaling to the temporal lobes, results in the parietal lobes not recognizing it as internal, resulting in the auditory hallucinations typical of chronic schizophrenia.
In addition, significant cortical grey matter volume reductions are observed in this disorder. Specifically, the right hemisphere atrophies more, while both sides show a marked decrease in frontal and posterior volume. This indicates abnormal synaptic plasticity occurs, where certain feedback loops become so potentiated, others receive little glutaminergic transmission. This is a direct result of the abnormal dopaminergic input to the striatum, thus (indirectly) disinhibition of thalamic activity. The excitatory nature of dopaminergic transmission means the glutamate hypothesis of schizophrenia is inextricably intertwined with this altered functioning. 5-HT also regulates monoamine neurotransmitters, including dopaminergic transmission. Specifically, the 5-HT2A receptor regulates cortical input to the basal ganglia and many typical and atypical antipsychotics are antagonists at this receptor. Several antipsychotics are also antagonists at the 5-HT2C receptor, leading to dopamine release in the structures where 5-HT2C is expressed; striatum, prefrontal cortex, nucleus accumbens, amygdala, hippocampus (all structures indicated in this disease), and currently thought to be a reason why antipsychotics with 5HT2C antagonistic properties improves negative symptoms. More research is needed to explain the exact nature of the altered chemical transmission in this disorder.
Recent evidence on a variety of animal models of psychosis, such as sensitization of animal behaviour by amphetamine, or phencyclidine (PCP, Angel Dust), or excess steroids, or by removing various genes (COMT, DBH, GPRK6, RGS9, RIIbeta), or making brain lesions in newborn animals, or delivering animals abnormally by Caesarian section, all induce a marked behavioural supersensitivity to dopamine and a marked rise in the number of dopamine D2 receptors in the high-affinity state for dopamine. This latter work implies that there are multiple genes and neuronal pathways that can lead to psychosis and that all these multiple psychosis pathways converge via the high-affinity state of the D2 receptor, the common target for all antipsychotics, typical or atypical. Combined with less inhibitory signalling from the thalamus and other basal ganglic structures, from hyoptrophy the abnormal activation of the cingulate cortex, specifically around Broca's and Wernicke's areas, abnormal D2 agonism can facilitate the self-reinforcing, illogical patterns of language found in such patients. In schizophrenia, this feedback loop has progressed, which produced the widespread neural atrophy characteristic of this disease. Patients on neuroleptic or antipsychotic medication have significantly less atrophy within these crucial areas. As such, early medical intervention is crucial in preventing the advancement of these profound deficits in bilateral communication at the root of all psychotic disorders. Advanced, chronic schizophrenia can not respond even to clozapine, regarded as the most potent antipsychotic, as such, a cure for highly advanced schizophrenia is likely impossible, so the value of early intervention cannot be stressed enough.
Evidence for the dopamine hypothesis
Amphetamine, cocaine and similar drugs increase levels of dopamine in the brain and can cause symptoms which resemble those present in psychosis, particularly after large doses or prolonged use. This is often referred to as "amphetamine psychosis" or "cocaine psychosis," but may produce experiences virtually indistinguishable from the positive symptoms associated with schizophrenia. Similarly, those treated with dopamine enhancing levodopa for Parkinson's disease can experience psychotic side effects mimicking the symptoms of schizophrenia. Up to 75% of patients with schizophrenia have increased signs and symptoms of their psychosis upon challenge with moderate doses of methylphenidate or amphetamine or other dopamine-like compounds, all given at doses at which control normal volunteers do not have any psychologically disturbing effects.
Some functional neuroimaging studies have also shown that, after taking amphetamine, patients diagnosed with schizophrenia show greater levels of dopamine release (particularly in the striatum) than non-psychotic individuals. However, the acute effects of dopamine stimulants include euphoria, alertness and over-confidence; these symptoms are more reminiscent of mania than schizophrenia.
A group of drugs called the phenothiazines, including antipsychotics such as chlorpromazine, has been found to antagonize dopamine binding (particularly at receptors known as D2 dopamine receptors) and reduce positive psychotic symptoms. This observation was subsequently extended to other antipsychotic drug classes, such as butyrophenones including haloperidol. The link was strengthened by experiments in the 1970s which suggested that the binding affinity of antipsychotic drugs for D2 dopamine receptors seemed to be inversely proportional to their therapeutic dose. This correlation, suggesting that receptor binding is causally related to therapeutic potency, was reported by two laboratories in 1976.
Genetic evidence has suggested that there may be genes, or specific variants of genes, that code for mechanisms involved in dopamine function, which may be more prevalent in people experiencing psychosis or diagnosed with schizophrenia. Dopamine related genes linked to psychosis in this way include COMT, DRD4, and AKT1.
Evidence against the dopamine hypothesis
Further experiments, conducted as new methods were developed (particularly the ability to use PET scanning to examine drug action in the brain of living patients) challenged the view that the amount of dopamine blocking was correlated with clinical benefit. These studies showed that some patients had over 90% of their D2 receptors blocked by antipsychotic drugs, but showed little reduction in their psychoses. This primarily occurs in patients who have had the psychosis for ten to thirty years. At least 90-95% of first-episode patients, however, respond to antipsychotics at low doses and do so with D2 occupancy of 60-70%. The antipsychotic aripiprazole occupies over 90% of D2 receptors, but this drug is both an agonist and an antagonist at D2 receptors.
Furthermore, although dopamine-inhibiting medications modify dopamine levels within minutes, the associated improvement in patient symptoms is usually not visible for at least several days, suggesting that dopamine may be indirectly responsible for the illness.
Similarly, a new generation of antipsychotic drugs (called the atypical antipsychotics) were found to be just as effective as older typical antipsychotic drugs in controlling psychosis, but more effective in controlling the negative symptoms, despite the fact that they have lower affinity for dopamine receptors than for various other neurotransmitter receptors. More recent work, however, has shown that atypical antipsychotic drugs such as clozapine and quetiapine bind and unbind rapidly and repeatedly to the dopamine D2 receptor. All of these drugs exhibit inverse agonistic effects at the 5-HT2A/2C receptors, meaning serotonin abnormalities are also involved in the complex constellation of neurologic factors predisposing one to the self reinforcing language-based psychological deficits found in all forms of psychosis.
The excitatory neurotransmitter glutamate is now also thought to be associated with schizophrenia. Phencyclidine (also known as PCP or "Angel Dust") and ketamine, both of which block glutamate (NMDA) receptors, are known to cause psychosis at least somewhat resembling schizophrenia, further suggesting that psychosis and perhaps schizophrenia cannot fully be explained in terms of dopamine function, but may also involve other neurotransmitters.
Similarly, there is now evidence to suggest there may be a number of functional and structural anomalies in the brains of some people diagnosed with schizophrenia, such as changes in grey matter density in the frontal and temporal lobes. It appears, therefore, that there are multiple causes for psychosis and schizophrenia, including gene mutations and anatomical lesions.
Psychiatrist David Healy has argued that drug companies have inappropriately promoted the dopamine hypothesis of schizophrenia as a deliberate and calculated simplification for the benefit of drug marketing.
Relationship with glutamate
Research has shown the importance of glutamate receptors, specifically N-methyl-D-aspartate receptors (NMDARs), in addition to dopamine in the etiology of schizophrenia. Mice with only 5% of the normal levels of NMDAR’s expressed schizophrenic like behaviors seen in animal models of schizophrenia while mice with 100% of NMDAR’s behaved normally. Schizophrenic behavior in low NMDAR mice has been effectively treated with antipsychotics that lower dopamine. NMDAR’s and dopamine receptors in the prefrontal cortex are associated with the cognitive impairments and working memory deficits commonly seen in schizophrenia. Rats that have been given a NMDAR antagonist exhibit a significant decrease in performance on cognitive tasks. Rats given a dopamine antagonist (antipsychotic) experience a reversal of the negative effects of the NMDAR antagonist. Glutamate imbalances appear to cause abnormal functioning in dopamine. When levels of glutamate are low dopamine is overactive and results in the expression schizophrenic symptoms.
People with Schizophrenia also have a higher incidence of nicotine use than those without schizophrenia. The high rate of use may be a form of self-medication that reduces negative symptoms by increasing the activity of glutamate and lowering the activity of dopamine. Elevated activation in the prefrontal cortex from nicotine use can increase focus and cognitive performance which are impaired in people with schizophrenia. Activation of nicotinic acetylcholine receptors in mice has been shown to increase glutamate levels in the prefrontal cortex and may help to decrease the cognitive deficits seen in schizophrenia. In addition, nicotine is thought to help regulate dopamine release. Nicotine acts as an agonist to nicotinic acetylcholine receptors which then increases the release of dopamine. However, prolonged use of nicotine causes a diminished response to nicotine thus lowering dopamine levels long term. Nicotine use may be higher in people with schizophrenia because it helps to balance levels of glutamate and dopamine in the brain causing a reduction in some negative symptoms.
- McIntosh AM Maniega SM Lymer GK McKirdy J Hall J Sussman JE Bastin ME Clayden JD et al. (2008). "White matter tractography in bipolar disorder and schizophrenia". Biological Psychiatry 64 (12): 1088–1092. doi:10.1016/j.biopsych.2008.07.026. PMID 18814861.
- Haznedar MM Buchsbaum MS Hazletta EA Shihabuddina L Newa A Sieverasa LJ (2004). "Cingulate gyrus volume and metabolism in the schizophrenia spectrum". Schizophrenia Research 71 (2–3): 249–262. doi:10.1016/j.schres.2004.02.025. PMID 15474896.
- Schlaug G Marchina S Norton A (2009). "Evidence for plasticity in white-matter tracts of patients with chronic Broca's aphasia undergoing intense intonation-based speech therapy". Annals of the New York Academy of Sciences 1169 (1): 385–94. Bibcode:2009NYASA1169..385S. doi:10.1111/j.1749-6632.2009.04587.x. PMC 2777670. PMID 19673813.
- Nakamura M McCarley RW Kubicki M Dickey CC Niznikiewicz MA Voglmaier MM Seidman LJ Maier SE et al. (2005). "Fronto-temporal disconnectivity in schizotypal personality disorder: a diffusion tensor imaging study". Biological Psychiatry 58 (6): 468–478. doi:10.1016/j.biopsych.2005.04.016. PMC 2768055. PMID 15978550.
- Friston KJ The disconnection hypothesis, 1998
- Harvey I Ron MA Du Boulay G Wicks D Lewis SW Murray RM (1993). "Reduction of cortical volume in schizophrenia on magnetic resonance imaging". Psychological Medicine 23 (3): 591–604. doi:10.1017/S003329170002537X. PMID 8234567.
- Carlsson M., Carlsson A. (1990). "Schizophrenia: A Sub cortical Neurotransmitter Imbalance Syndrome?". Schizophrenia Bulletin 16 (3): 425–430. doi:10.1093/schbul/16.3.425.
- Seeman, P.; Weinshenker, D.; Quirion, R.; Srivastava, K.; Bhardwaj, K.; Grandy, K.; Premont, T.; Sotnikova, D.; Boksa, P.; El-Ghundi, M.; O'Dowd, B. F.; George, S. R.; Perreault, M. L.; Männistö, P. T.; Robinson, S.; Palmiter, R. D.; Tallerico, T. (Mar 2005). "Dopamine supersensitivity correlates with D2High states, implying many paths to psychosis" (FREE FULL TEXT). Proceedings of the National Academy of Sciences of the United States of America 102 (9): 3513–3518. Bibcode:2005PNAS..102.3513S. doi:10.1073/pnas.0409766102. ISSN 0027-8424. PMC 548961. PMID 15716360.
- Gur RE Maany V Mozley PD Swanson C Wilker W Ruben C (1998). "Subcortical MRI Volumes in Neuroleptic-Naive and Treated Patients With Schizophrenia". American Journal of Psychiatry 155 (12): 1711–1717. doi:10.1176/ajp.155.12.1711. PMID 9842780.
- Arinami T Gao M Hamaguchi H Toru M (1997). "A functional polymorphism in the promoter region of the dopamine D2 receptor gene is associated with schizophrenia". Human Molecular Genetics 6 (4): 577–582. doi:10.1093/hmg/6.4.577. PMID 9097961.
- Whitford TJ Kubicki M Schneiderman JS O'Donnell LJ King R Alvarado JL Khan U et al. (2010). "Corpus callosum abnormalities and their association with psychotic symptoms in patients with schizophrenia". Biological Psychiatry 68 (1): 70–77. doi:10.1016/j.biopsych.2010.03.025. PMC 2900500. PMID 20494336.
- McEvoy JP Lieberman JA Stroup TS Davis SM Meltzer HY Rosenheck RA Swartz MS et al. (2006). "Effectiveness of Clozapine Versus Olanzapine, Quetiapine, and Risperidone in Patients With Chronic Schizophrenia Who Did Not Respond to Prior Atypical Antipsychotic Treatment". American Journal of Psychiatry 163 (4): 600–610. doi:10.1176/appi.ajp.163.4.600. PMID 16585434.
- Lieberman, J. A.; Kane, J. M.; Alvir, J. (1987). "Provocative tests with psychostimulant drugs in schizophrenia". Psychopharmacology 91 (4): 415–433. doi:10.1007/BF00216006. PMID 2884687.
- Curran, C.; Byrappa, N.; McBride, A. (Sep 2004). "Stimulant psychosis: systematic review" (FREE FULL TEXT). The British journal of psychiatry : the journal of mental science 185 (3): 196–204. doi:10.1192/bjp.185.3.196. ISSN 0007-1250. PMID 15339823.
- "Dextroamphetamine-induced arousal in human subjects as a model for mania". Psychological Medicine 16 (2): 323–329. May 1986. doi:10.1017/S0033291700009132. ISSN 0033-2917. PMID 3726006.
- Creese I, Burt DR, Snyder SH (April 1976). "Dopamine receptor binding predicts clinical and pharmacological potencies of antischizophrenic drugs". Science 192 (4238): 481–3. Bibcode:1976Sci...192..481C. doi:10.1126/science.3854. PMID 3854.
- Seeman, P.; Lee, T.; Chau-wong, M.; Wong, K. (1976). "Antipsychotic drug doses and neuroleptic/dopamine receptors". Nature 261 (5562): 717–719. Bibcode:1976Natur.261..717S. doi:10.1038/261717a0. PMID 945467.
- Arguello, A.; Gogos, A. (Jun 2008). "A signaling pathway AKTing up in schizophrenia". The Journal of Clinical Investigation (Free full text) 118 (6): 2018–2021. doi:10.1172/JCI35931. ISSN 0021-9738. PMC 2391280. PMID 18497888.
- R. Thompson, The Brain, ISBN 0-7167-1462-0
- Diaz, Jaime. How Drugs Influence Behavior. Englewood Cliffs: Prentice Hall, 1996.
- Richtand, M.; Welge, A.; Logue, D.; Keck Pe, R.; Strakowski, M.; Mcnamara, K. (Aug 2007). "Dopamine and serotonin receptor binding and antipsychotic efficacy". Neuropsychopharmacology (Free full text) 32 (8): 1715–1726. doi:10.1038/sj.npp.1301305. ISSN 0893-133X. PMID 17251913.
- Williams J Spurlock G McGuffin P Mallet J Nöthen MM Gill M Aschauer H et al. (1996). "Association between schizophrenia and T102C polymorphism of the 5-hydroxytryptamine type 2a-receptor gene. European Multicentre Association Study of Schizophrenia (EMASS) Group". The Lancet 347: 1294–6. doi:10.1016/s0140-6736(96)90939-3. PMID 8622505.
- Berg KA Harvey JA Spampinato U Clarke WP (2005). "Physiological relevance of constitutive activity of 5-HT2A and 5-HT2C receptors". Trends in Pharmacological Science 26 (12): 625–630. doi:10.1016/j.tips.2005.10.008. PMID 16269190.
- "Daring to Think Differently about Schizophrenia". New York Times, February 24, 2008.
- Mohn, Amy; Gainetdinov Caron Koller (20 August 1999). "Mice with reduced NMDA receptor expression display behaviors related to schizophrenia". Cell 98 (4): 427–436. doi:10.1016/S0092-8674(00)81972-8. Retrieved 30 November 2013.
- Verma, Anita; Moghaddam (1 January 1996). "NMDA receptor antagonists impair prefrontal cortex function as assessed via spatial delayed alternation performance in rats: Modulation by dopamine". JOURNAL OF NEUROSCIENCE 16 (1): 373–379. Retrieved 30 November 2013.
- Javitt, Daniel (2007). "Glutamate and schizophrenia: Phencyclidine, N-methyl-D-aspartate receptors, and dopamine-glutamate interactions". INTERNATIONAL REVIEW OF NEUROBIOLOGY 78: 69–+. doi:10.1016/S0074-7742(06)78003-5. Retrieved 2 December 2013.
- Dalack, Gregory; Healy Meador-Woodruf (November 1998). "Nicotine dependence in schizophrenia: Clinical phenomena and laboratory findings". AMERICAN JOURNAL OF PSYCHIATRY 155 (11): 1490–1501. doi:10.1176/ajp.155.11.1490. Retrieved 30 November 2013.
- Lambe, Evelyn; Picciotto Aghajanian (February 2003). "Nicotine induces glutamate release from thalamocortical terminals in pr". NEUROPSYCHOPHARMACOLOGY 28 (2): 216–225. doi:10.1038/sj.npp.1300032. Retrieved 30 November 2013.
- Zhou, Fu-Ming; Liang Dani (December 2001). "Endogenous nicotinic cholinergic activity regulates dopamine release in the striatum". NATURE NEUROSCIENCE 4 (12): 1224–1229. doi:10.1038/nn769. Retrieved 29 November 2013.