25 January 2009. It has long been recognized that dopamine dysregulation may contribute to symptoms of schizophrenia. What is not clear is how this abnormality affects schizophrenia onset and symptoms. Oliver Howes and colleagues at the Institute of Psychiatry at King’s College London have published a study in the January Archives of General Psychiatry that examined brain dopamine function in people at high risk of schizophrenia who show early signs of developing the illness. Patients with prodromal signs of schizophrenia showed dopamine "overactivity" in the striatum relative to control subjects, as did patients with diagnosed schizophrenia. Within the potentially prodromal group dopamine overactivity was related to the severity of prodromal symptoms and impairment in verbal fluency.
Increases in striatal dopamine activity are believed to underlie schizophrenia psychotic, or “positive,” symptoms (see Current Hypothesis by A. Abi-Dargham and related SRF news story for reviews). Dopamine in the striatum comes from two sources: the substantia nigra supplies the dorsal striatum via the nigrostriatal pathway, and the ventral tegmental area feeds the ventral striatum via the mesolimbic dopamine pathway.
Initial success with antipsychotic medications that act on the dopamine system have supported the hypothesis that dysregulated dopamine contributes to schizophrenia symptoms, and more recent imaging studies utilizing positron emission tomography (PET) have further validated this idea by enabling the visualization of striatal dopamine. Based on imaging studies, both dopamine synthesis and dopamine release seem to be magnified in the striatum in schizophrenia. The degree of dopamine increase appears to be related to psychotic symptom severity. Despite these findings, it is not clear that elevated dopamine is a cause or consequence of psychotic symptoms (for review, see Howes et al., 2007).
Prodromal schizophrenia is a phase that occurs prior to the onset of schizophrenia. Understanding the cause of symptoms could help to identify interventions directed at this early stage, which can precede the onset of hallucinations and delusions by several years (see related SRF news story). Prodromal symptoms can include anxiety, social isolation, as well as problems with concentration and attention.
The present study used 18Fdopa striatal PET imaging in 24 individuals meeting at-risk-mental-state (ARMs) criteria and compared dopaminergic activity in these subjects to activity in the brains of healthy volunteers and to people with schizophrenia. Prior research has shown that individuals who meet ARMs criteria have a risk of developing psychosis ranging from 15 percent to 54 percent after six months to one year (Haroun et al., 2006; Miller et al., 2002).
Dopamine activity was measured by using the imaging data to calculate what is known as a Ki value. Ki values represent the conversion of 18Fdopa by dopamine decarboxylase to F-fluoro-dopamine, similar to the conversion of L-dopa to dopamine, so the Ki value reflects the rate of dopamine synthesis and storage. When measurements were taken of the entire striatum, the mean Ki value was increased by 6.3 percent (effect size 0.75; (t34 = 2.2, P = .04) in the prodromal group and by 10.6 percent (effect size, 1.25; t17 = 2.5, P = 0.02) in the group with schizophrenia relative to the control group. When increases in a subdivision of the dorsal striatum known as associative striatum—important for integrating information from different regions of the cortex—were measured, even greater elevations were seen relative to controls: 7.3 percent in the prodromal group and 13.9 percent in the group with schizophrenia. Significant changes relative to controls were not seen in the sensorimotor region of the dorsal striatum or in the ventral (limbic) striatum. There were no statistically significant differences between the prodromal group and the group with schizophrenia for increases found in either the whole or the associative striatum.
Howes and coworkers also assessed the functional significance of these dopamine changes, examining whether a relationship exists between striatal dopamine levels and measures of psychopathology or neuropsychological performance. In the prodromal group, whole striatal Ki values positively correlated with several measures of schizophrenia symptom severity, including total comprehensive assessment of ARMs score (CAARMs; r = 0.48, P = 0.02) and positive and negative symptoms of schizophrenia score (PANSS) (r = 0.49, P = 0.01). The investigators also obtained positive correlations for the associative striatum using these measurements, although they found no relationship between measurements of depression or anxiety and any striatal dopamine activity, using the Hamilton Depression Rating Score or Hamilton Anxiety Rating Scale Score. Verbal fluency measures negatively correlated with striatal dopamine activity; these measurements were either significant or showed a trend for significance. Interestingly, in the group with schizophrenia there was no relationship between whole striatal Ki values and PANSS, CAARMs, or the Hamilton rating scores for depression and anxiety. Howes told SRF that “this may reflect a different stage of the illness: dopamine dysregulation may increase up to the onset of illness but not change further, although symptoms may fluctuate.”
According to the investigators, these data support the role of excessive subcortical dopamine as a precursor to schizophrenia, contributing potentially to both prodromal symptoms as well as changes in cognitive function, specifically verbal fluency. Interestingly, a recent report has linked the development of language dysfunction to the onset of hallucinations, suggesting that the two are related (Defreitas et al., 2008). The authors noted that decreases in dopamine activity in associative striatum were negatively correlated with verbal fluency, whereas changes in limbic striatal activity were not. Associative striatum may regulate verbal ability via connections with the prefrontal cortex (Haber, 2003; Middleton and Strick, 2000). This study supports the idea that associative striatal inputs may regulate verbal deficits, possibly through inputs to and from prefrontal cortical regions.
In their conclusions the authors noted that since all people with prodromal symptoms do not necessarily develop schizophrenia, increased striatal dopamine activity may confer a vulnerability to the onset of schizophrenia psychotic symptoms rather than indicate that a person will necessarily develop schizophrenia. In addition, Howes told SRF that, “Our finding that dopamine function is dysregulated in people at risk of developing schizophrenia provides a neurobiological rationale for the use of interventions that reduce the dopamine elevation in these people; by reducing the dopamine elevation such interventions may prevent the onset of schizophrenia in vulnerable individuals.”—Alisa Woods.
Howes OD, Montgomery AJ, Asselin MC, Murray RM, Valli I, Tabraham P, Bramon-Bosch E, Valmaggia L, Johns L, Broome M, McGuire PK, Grasby PM. Elevated striatal dopamine function linked to prodromal signs of schizophrenia. Arch Gen Psychiatry. 2009 Jan;66(1):13-20. Abstract