***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: Studies in nonhuman primates documented that appropriate stimulation of dopamine ( DA ) D1 receptors in the dorsolateral prefrontal cortex ( DLPFC ) is critical for working memory processing . The defective ability of patients with schizophrenia at working memory tasks is a core feature of this illness . It has been postulated that this impairment relates to a deficiency in mesocortical DA function . In this study , D1 receptor availability was measured with positron emission tomography and the selective D1 receptor antagonist [11C]NNC 112 in 16 patients with schizophrenia ( seven drug-naive and nine drug-free patients ) and 16 matched healthy controls .  [11C]NNC 112 binding potential ( BP ) was significantly elevated in the DLPFC of patients with schizophrenia ( 1.63 ± 0.39 ml/gm ) compared with control subjects ( 1.27 ± 0.44 ml/gm ; p = 0.02 ) . In patients with schizophrenia , increased DLPFC [11C]NNC 112 BP was a strong predictor of poor performance at the n-back task , a test of working memory . These findings confirm that alteration of DLPFC D1 receptor transmission is involved in working memory deficits presented by patients with schizophrenia . Increased D1 receptor availability observed in patients with schizophrenia might represent a compensatory ( but ineffective ) upregulation secondary to sustained deficiency in mesocortical DA function Molecular imaging has been used to estimate both drug-induced and tonic dopamine release in the striatum and most recently extrastriatal areas of healthy humans . However , to date , studies of drug-induced and tonic dopamine release have not been performed in the same subjects . This study performed positron emission tomography ( PET ) with [18F]fallypride in healthy subjects to assess ( 1 ) the reproducibility of [18F]fallypride and ( 2 ) both D-amphetamine-induced and alpha-methyl-p-tyrosine (AMPT)-induced changes in dopamin release on [(18)F]fallypride binding in striatal and extrastriatal areas . Subjects underwent [18F]fallypride PET studies at baseline and following oral D-amphetamine administration ( 0.5 mg/kg ) and oral AMPT administration ( 3 g/70 kg/day over 44 h ) . Binding potential ( BP ) ( BP(ND ) ) of [18F]fallypride was calculated in striatal and extrastriatal areas using a reference region method . Percent change in regional BP(ND ) was computed and correlated with change in cognition and mood . Test-retest variability of [18F]fallypride was low in both striatal and extrastriatal regions . D-Amphetamine significantly decreased BP(ND ) by 8 - 14 % in striatal subdivisions , cau date , putamen , substantia nigra , medial orbitofrontal cortex , and medial temporal cortex . Correlation between change in BP(ND ) and verbal fluency was seen in the thalamus and substantia nigra . In contrast , depletion of endogenous dopamine with AMPT did not effect [18F]fallypride BP(ND ) in both striatum and extrastriatal regions . These findings indicate that [18F]fallypride is useful for measuring amphetamine-induced dopamine release , but may be unreliable for estimating tonic dopamine levels , in striatum and extrastriatal regions of healthy humans [11C]PE2I is a novel positron emission tomography ( PET ) radiotracer for the dopamine transporter ( DAT ) . The reproducibility and reliability of [11C]PE2I measurements , especially in the small DAT-rich brain regions , is unknown and of critical importance to the interpretation of the data . Five healthy volunteers were scanned twice during the same day using [11C]PE2I and the HRRT PET scanner . Methods based on metabolite-corrected arterial plasma curve and reference region were used to estimate distribution volumes ( VT ) and binding potential ( BP ) . Within-subject and between-subject variabilities were compared . [11C]PE2I accumulated in the DAT-rich striatum and the midbrain . Equilibrium of specific binding appeared late in the striatum , whereas it was reached earlier in the midbrain . Plasma metabolite analysis showed that the potentially brain-penetrant 4-hydroxymethyl metabolite represented 15 % to 20 % of total plasma radioactivity . VT and BP measurements were associated with low within-subject variability . Measurement of DAT binding in small brain regions , including the substantia nigra , is reproducible and reliable using [11C]PE2I and high-resolution research tomograph . A scanning time of more than 70 mins is required for the striatum , while less is sufficient for DAT quantification in the midbrain . The previously suggested involvement of the potentially brain-penetrant radioactive metabolite in the quantification should be further studied 
    Output:
    
Evidence for other measures /regions is limited because of the small number of studies and in some instances inconsistent findings , although significant differences were reported for D2/D3 receptors in the cingulate and uncus , for D1 receptors in the prefrontal cortex and for dopamine transporter availability in the thalamus .
There is a relative paucity of direct evidence for cortical dopaminergic alterations in schizophrenia , and findings are inconclusive .