What Have We Learned from Molecular Imaging Studies of Dopamine Neurotransmission in Schizophrenia and in Mediating Reward Behaviors Robert M Kessler, MD Professor of Radiology University of Alabama at Birmingham School of Medicine
Financial Interests The Speaker: Dr. Kessler has consulted with Shire, Merck, PharmoRx Therapeutics, owns shares of PharmoRx Therapeutics, and has received research funding from NIDA, NIA, and NIMH. The Planners: Bita Moghaddam, PhD; Liz Stevenson, JD, MPH; William Wilson, MD; Neisha D Souza, MD; Sean Stanley, MD; Micaela Sandoval, MBA, and Kevin Howden have no relevant financial relationships to disclose.
Objectives of this Presentation Present how the distribution of dopamine D2 and D3 receptors differs across species, in particular the differences between rats, rhesus monkeys, and humans. Discuss the role of dopaminergic neurotransmission including the distribution of dopamine D2/3 receptors in schizophrenia, the relationship of such changes to symptoms, and the effects of different antipsychotic drugs on striatal and extrastriatal D2/3 receptors. The role of dopaminergic neurotransmission in mediating reward behaviors in healthy subjects and subjects with altered reward function.
IMAGING OF CEREBRAL DOPAMINE D2 AND D3 RECEPTORS IN HUMANS, NONHUMAN PRIMATES, AND RATS
Cortical DA D2/3 Receptors
Rhesus Monkey Frontal Cortex and Anterior Cingulate DA D2/3 Receptors
Human Prefrontal Cortex and Anterior Cingulate DA D2/3 Receptors
Human Inferior Temporal Cortex DA D2/3 Receptors
Human Anterior Striatum, Claustrum, Temporal Tip DA D2/3 Receptors
Cortical DA D2/3 Receptors In humans, no DA D3 receptors were detected in cortical regions. Theses results indicate that only very low levels at most are present in human cortex. The distribution and levels of DA D2 receptors in the cingulate and temporal cortex in humans differs from that seen in rhesus monkeys and rats suggesting differences in DA D2 mediated neurotransmission in these regions
Basal Ganglia DA D2 and D3 Receptors
Rat Ventral Striatal DA D3 Receptors
Rhesus Monkey Anterior Striatum Dopamine D2 Dopamine D3
Rhesus Monkey Ventral Striatum/Pallidum DA D2/3 DA D3
Human Ventral Striatal/Pallidal DA D3 Receptors
Basal Ganglia DA D2 and D3 Receptors The distribution of DA D3 receptors in the ventral striatum differs significantly in humans from that seen in rhesus monkeys and rats. In humans there are relatively higher levels of DA D3 receptors seen in the medial aspect of the ventral striatum corresponding to the region of the shell of the nucleus accumbens as well as under the anterior commissure in humans in an area that has been called the substantia innominata by Heimer. In humans this area is a mixture of ventral striatum, ventral pallidum,the extended amygdala and portions of the basal nucleus of Meynert
Thalamic DA 2 and D3 Receptors
Human Anterior Thalamus DA D2/3
Human Thalamus, Midlevel
Human Anterior Nucleus of Thalamus and Mammillothalamic Tract DA D3 Receptors
Rhesus Monkey Mid Striatum/Anterior Thalamus Dopamine D2 Dopamine D3
Rhesus Monkey Thalamus/Mammillary Bodies DA D2/3 DA D3
Thalamic DA D2 and D3 Receptors The highest levels of DA D2 receptors are in the intralaminar system highest anteriorly in the paraventricular nuclei, the parataenial nucleus, and more posteriorly in the parafascicular and centromedian nuclei. These nuclei project from thalamus to ventral striatum, limbic regions and cortex allowing DA D2 mediated neurotransmission to modulate the function of the cortical/striatal/thalamic circuits by feedback mechanisms. The mediodorsal nucleus of the thalamus, particularly the magnocellular portion has moderate levels of DA D2 receptors in man but not seen in rhesus monkeys. The mediodorsal nucleus projects to prefrontal cortex which in humans has about 5 fold lower levels of DA D2 receptors than in rhesus monkeys. There appears to be a shift in the strength of DA D2 receptor mediated modulation of prefrontal/striatal/thalamic circuits in humans compared to rhesus monkeys.
Limitations of [ 123 I] epidepride First it is a SPECT tracer and this effectively limits resolution and quantitation in humans. Second, it has a very slow off-rate from the DA D2 receptor, about 56 minutes in vitro which results in very long imaging times, 12 hours, for quantitation of striatal DA D2 levels. While [123I] epidepride can be used to study extrastriatal receptors, we preferred to use one radioligand for imaging both striatal and extrastriatal DA D2 receptor levels Third, about 30% of cerebellar uptake in humans is specific making definition of a reference region difficult. Fourth, primates showed no sensitivity to d-amphetamine induced DA release in striatum.
Fallypride
[ 18 F]Fallypride Uptake in Human Brain
A Short History of Schizophrenia and Dopamine In the 1950 s and 1960 s it was apparent that chronic abuse of drugs which increased catecholamine release such as amphetamine can produce psychotic symptoms and drugs which deplete cerebral catecholamines such as reserpine decrease psychotic symptoms. In the 1970 s it was shown that the average therapeutic dose of the then available antipsychotic drugs was directly related to the affinity of such drugs for the striatal [ 3 H]haloperidol or [ 3 H]dopamine binding site, i.e. the dopamine (DA) D2 type receptor. Numerous post mortem studies of schizophrenic brain reported increased striatal DA D2 receptor levels in schizophrenic subjects. One large study reported a bimodal distribution of striatal DA D2 levels in schizophrenics with one mode in the normal range and a second mode about twice normal levels (Seeman, 1987).
The Dopamine Hypotheses of Schizophrenia Post mortem studies in schizophrenic brain reported increased DA and HVA levels particularly in the ventral striatum (Bird, 1979), Studies in rodents (Pycock, 1980) reported that lesioning presynaptic frontal cortical DA innervation led to increased striatal DA turnover. Post mortem studies in schizophrenic brain have reported decreased DA innervation in layer 6 of DLPFC and layers 3 and 6 of entorhinal cortex (Akil, 1999, 2000). These observations and others have led to the various DA hypotheses of schizophrenia which postulated increased striatal dopamine release and/or increased striatal DA D2 receptor levels as the pathophysiological abnormality mediating psychosis. The increased striatal DA release in schizophrenic subjects has been postulated to result from decreased frontal cortical DA innervation.
Striatal DA D2/3 Receptor Levels in Schizophrenia Initial PET [ 11 C]N-methylspiperone studies of striatal DA D2 type receptor levels in schizophrenia and psychotic bipolar disorders reported increased striatal levels (Wong, 1986, Pearlson, 1995) but further imaging studies of striatal DA D2/3 receptor levels reported conflicting findings. A metaanalysis of 18 studies found an overall 12% increase in baseline DA D2/3 receptor levels (Weinberger, 2001). Similarly, imaging studies of baseline extracellular DA levels using benzamide radioligands performed prior to and following DA depletion (Abi Dargham, 2000; Kegeles, 2010) have reported significantly increased striatal DA D2/3 receptor levels seen after DA depletion, 6-13%. While mildly increased striatal DA D2/3 receptor levels are seen in schizophrenic subjects, some schizophrenic subjects have levels markedly above the normal range. However, no large imaging study has carefully characterized schizophrenic subjects with normal versus elevated striatal D2 levels.
Studies of Extracellular Striatal Dopamine Levels
Effect of Disease State on Striatal Dopamine Release in Schizophrenia
In schizophrenic subjects the baseline level of extracellular DA as measured by imaging studies of DA D2 receptors performed at baseline and after DA depletion using -MPT was not related to the severity of positive symptoms (r= 0.01, P=0.76) or negative symptoms (r= 0.10, P= 0.20). However, the decrease in positive symptoms seen after 6 weeks of antipsychotic medication was significantly correlated with extracellular DA levels.
Relationship of 18F FDOPA Uptake to Treatment Resistance
Striatal Dopaminergic Neurotransmission in Schizophrenia Increased striatal DA release is state dependent being increased with acute psychosis and not significantly different from healthy subjects in stable untreated schizophrenic subjects. Baseline levels of extracellular DA do not predict the level of psychotic symptom indicating that the etiology of psychosis in schizophrenia may be dopaminergic or nondopaminergic. Response to antipsychotic drugs is positively correlated with elevated baseline extracellular dopamine levels and [18F]6- FDOPA uptake, i.e. the greater the elevation the greater the response. Treatment resistant subjects have negligible increases in baseline extracellular dopamine levels and [18F]6-FDOPA uptake On average there are mildly increased striatal DA D2 receptor levels in schizophrenia but with a subset of subjects having markedly elevated levels. The combination of increased DA D2 levels and increased DA release/extracellular levels indicates altered regulation of DA neurotransmission at both the pre and post synaptic levels.
Extrastriatal DA D2/3 Receptors in Schizophrenia PET studies of extrastriatal DA D2 receptors using [11C]FLB457 or [18F]fallypride which examined subregions of thalamus have consistently reported significantly decreased DA D2/3 receptor levels in the medial thalamus, more on the left than right (Talvik, 2003, 2006; Yasuno, 2004; Buchsbaum, 2006; Kessler, 2009). A few studies have reported decreased anterior cingulate and temporal DA D2/3 levels (Suhara, 2002; Buchsbaum, 2006).
Table 1. Binding Potentials for regions of interest sampled in 11 unmedicated schizophrenic subjects and 11 age matched healthy subjects. Region Schizophrenic Normal Significance level R L R L R L Medial Thalamus 4.21±0.58 4.17±0.55 4.30±0.62 4.55±0.58 0.56 0.03 Posterior Thalamus 2.28±0.41 2.40±0.30 2.36±0.32 2.54±0.36 0.43 0.19 Anterior Cingulate 0.79±0.15 0.75±0.15 0.78±0.17 0.77±0.16 0.52 0.55 Substantia Nigra 2.87±0.33 2.75±0.34 2.44±0.22 2.41±0.19 0.0008 0.01 Hippocampus 1.57±0.41 1.68±0.38 1.48±0.26 1.59±0.34 0.70 0.83 Temporal Cortex 1.52±0.33 1.63±0.33 1.59±0.18 1.72±0.24 0.51 0.42 Caudate 30.70±3.30 31.81±4.12 32.25±2.08 32.33±2.13 0.20 0.72 Putamen 36.52±4.36 35.00±4.46 37.02±2.56 36.94±2.77 0.74 0.24 Ventral Striatum 18.80±3.66 19.49±3.49 18.10±2.81 18.36±3.41 0.48 0.45 Amygdala 3.28±0.43 3.20±0.61 3.23±0.27 3.26±0.24 0.75 0.76
Extrastriatal DA Release Estimated using PET [ 11 C]FLB457 Studies in Off Medication Schizophrenic Subjects Studied at Baseline and Following D-Amphetamine using (Slifstein, 2015)
Clusters of Significant Correlation of DA D2 Receptors with SAPS Total Positive Symptoms Score (R. P=0.85; L, P=0.86) Kessler et al. Page 13 NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA
Clusters of Significant Correlations of Delusions with Regional DA D2 Receptor Levels (R, P=0.84; L, P=0.86)
Cluster of Significant Correlation of Hallucinations with Regional DA D2 Receptor Levels (P=0.84)
Relationship of IED Set Shifting to Frontal Cortical DA D2 Receptor Levels in First Episode, Treatment Naïve Schizophrenic Subjects (N=24) and Healthy Subjects (N=20) using [123I]epidepride SPECT (Fagerlund, 2013)
Extrastriatal DA Release in Schizophrenic Subjects (Slifstein, 2015) D-amphetamine studies of DA release in extrastriatal regions were performed using PET [ 11 C]FLB457 studies in unmedicated schizophrenic subjects (N=20) and healthy subjects (N=21). There were widespread decreases in cortical DA release which reached trend level significance in DLPFC and substantia nigra BP ND s. Using V T s as a metric of regional extrastriatal DA D2 levels revealed a significant decrease in dopamine release in the DLPFC. These findings suggest widespread decreases in extrastriatal DA release, particularly in DLPFC. Decreased frontal cortical DA release was correlated with decreased working memory function. The findings in the substantia nigra are in conflict with [18F]6- FDOPA studies which found increased dopamine synthesis. While the reasons for this discrepancy are unclear, methodological issues with [ 11 C]FLB457 PET study estimation of DA D2 levels in substantia nigra may be one factor.
Extrastriatal DA Neurotransmission in Schizophrenia There are decreased medial thalamic DA D2 receptor levels in schizophrenics, some studies suggesting decreased anterior cingulate and temporal cortical levels, and increased nigral DA D2 levels consistent with a previous post mortem study (Owen, 1984). Decreased amphetamine induced DA release is seen in frontal cortex consistent with post mortem findings of decreased DA innervation. Temporal cortical and ventral striatal DA D2 receptor levels are highly positively correlated with positive symptoms. No relationships were observed between negative symptoms and extrastriatal DA D2 levels. Level of executive function was negatively correlated with frontal cortical DA D2 levels which may reflect receptor upregulation related to decreased DA release.
Dopaminergic Neurotransmission in Schizophrenia, What Has Imaging Taught Us. Overall, increased striatal DA release is seen in acutely symptomatic schizophrenic subjects but not stable schizophrenic subjects. Increased DA synthesis is also reported in the nigra. The combination of increased striatal and likely nigral DA release and increased DA D2 levels indicates dysregulation of pre and postsynaptaic DA neurotransmission in the nigrostriatal projection particularly during psychotic episodes. The levels of baseline extracellular DA levels do not predict the level of positive symptoms, but both baseline levels and [18F]6F-DOPA uptake levels do predict therapeutic response. These findings suggest that psychotic symptoms may be dopaminergic or nondopaminergic. Decreased frontal cortical DA release has been reported in a single study. This needs replication and its relationship to increased striatal DA release needs to be defined.
The decreased DA D2 receptor levels seen in the medial thalamus, likely reflect changes in the anterior, paraventricular, and parafascicular intralaminar nuclei, which project from thalamus to ventral striatum, limbic regions and frontal cortex, and to some degree to changes in the magnocellular mediodorsal thalamus. This distribution of thalamic DA D2 receptors allows modulation of both feedforward and feedback modulation of frontal cortical/striatal/thalamic circuits believed to involved in schizophrenia. Total SAPS scores are significantly correlated with lateral and superior temporal cortical DA D2 levels, and delusions by anterior temporal D2 levels. Hallucinations, however, are strongly correlated with ventral striatal but not cortical DA D2 levels. The temporal cortex differs from other cortical regions in humans and temporal cortex in rhesus monkeys having relatively high DA D2 levels in layer 1 of cortex. Executive function is inversely correlated with frontal cortical DA D2 levels. This correlation may reflect upregulation of DA D2 receptors due to decreased DA release which has been correlated with working memory deficits.
Antipsychotic Drugs and Dopamine D2 Receptor Occupancy First generation antipsychotic drugs must block 60-65% of striatal DA D2 receptors to produce clinically significant antipsychotic effects. DA D2 receptor occupancy above above 80% is associated with a high incidence of extrapyramidal side effects (EPS). Second generation antipsychotic drugs which include clozapine, olanzapine, risperidone, and quetiapine have been widely adopted as they are believed to have a lower incidence of EPS and in the case of clozapine have therapeutic efficacy in a significant fraction of treatment resistant subjects. The mechanism(s) of action of second generation antipsychotic drugs was unclear.
Second Generation Atypical Antipsychotic Drugs Clozapine is the archetypal atypical antipsychotic. At therapeutic doses, only 25-60% of striatal DA D2 receptors are blocked by clozapine unlike typical antipsychotic drugs. it has been hypothesized that clozapine s atypicality may be mediated by its high affinity for the 5HT2A receptor and its low affinity for the DA D2 receptor and that clozapine s therapeutic effects were mediated by selective blockade of limbic and cortical DA D2 receptors. However, initial studies of extrastriatal DA D2 receptor occupancy by clozapine produced contradictory results (Pilowsky, 1997; Talvik, 2001; Xiberas, 2001). To examine this issue we undertook PET [18F]fallypride studies of occupancy of striatal and extrastriatal DA D2 receptors by clozapine as well as olanzapine, risperidone and quetiapine.
REGION PERCENT OCCUPANCY Haloperidol Olanzapine Risperidone Putamen 76.5 ± 8.2 69.2 ± 10.2 66.9 ± 11.0 Ventral Striatum 75.5 ± 8.2 70.9 ± 6.9 70.3 ± 6.1 Medial Thalamus 78.2 ± 6.2 71.0 ± 9.0 72.5 ± 10.0 Amygdala 75.6 ± 8.1 72.4 ± 6.7 75.3 ± 5.5 Temporal Cortex 70.0 ± 5.5 67.5 ± 7.1 72.0 ± 4.8 Ventral Midbrain/ Substantia Nigra 59.3 ± 9.2 *40.2 ±12.2 *46.1 ± 9.4 *Significantly different from regional value for Haloperidol by 2-tail t-test, p o < 0.05
REGION PERCENT OCCUPANCY Haloperidol Clozapine Quetiapine Putamen 76.5 ± 8.1 51.6 ± 12.8 33.5 ± 14.5 Ventral Striatum 75.5 ± 8.2 50.4 ± 14.2 34.4 ± 17.8 Medial Thalamus 79.2 ± 6.2 58.6 ± 11.6 40.0 ± 13.7 Amygdala 75.6 ± 8.1 54.6 ± 10.8 43.2 ± 11.3 Temporal Cortex 70.0 ± 5.5 *63.5 ± 8.2 46.9 ± 14.0 Ventral Midbrain/ Substantia Nigra 59.3 ± 9.2 18.5 ± 12.1 34.3 ± 12.9 * Not significantly different from the regional value for haloperidol
75 A Clozapine OCCUPANCY (%) 50 25 0 Putamen Ventral Striatum Substantia Nigra Thalamus Amygdala Temporal Cortex 75 B Quetiapine OCCUPANCY (%) 50 25 0 Putamen Ventral Striatum Substantia Nigra Thalamus Amygdala Temporal Cortex
Mechanisms of Action of Second Generation Antipsychotic Drugs Pharmacological studies suggest that a high ratio of 5HT2A/D2 affinities and a low affinity for the DA D2 receptor are associated with an atypical antipsychotic profile (Roth, 2003). Drugs having a high ratio of 5HT2A/D2 affinities, olanzapine, risperidone, and clozapine show sparing of nigral DA D2 occupancy at therapeutic doses. Drugs having a low DA D2 receptor affinity, clozapine and quetiapine, show preferential occupancy of cortical versus striatal DA D2 receptors. There appear to be multiple paths to an atypical profile of antipsychotic effects, i.e. antipsychotic effects with a lower incidence of extrapyramidal motor effects. Clozapine has both mechanisms unlike the other drugs examined. In addition, clozapine has affinity for multiple other receptors including serotonergic, cholinergic, noradrenergic, etc. which may also mediate its effects.
Dopamine and Reward Behaviors in Healthy Subjects Reward motivation/reward energy expenditure Novelty/sensation seeking behaviors Impulsivity Failure of response inhibition and Delay Discounting Hedonic response to rewards
Reward Related Energy Expenditure (Treadway, 2012)
Relationship between Ventral Midbrain DA D2/3 Autoreceptors and Novelty seeking Zald. Neuroscience,2008
Correlations between DA D2/3 Receptor Levels, DA Release and Trait Impulsivity (Buckholtz, Science, 329:532, 2011)
Path analysis demonstrates that the inverse correlation between ventral midbrain DA D2/3 and trait impulsivity is mediated by disinhibition of ventral striatal DA release.
Hedonic Response to Reward Believed to be mediated by ventral striatal opioid neurotransmission. While opioid neurotransmission is likely the primary mediator of the hedonic response, DA release mediates opioid release in humans (Colisanti, 2012; Mick, 2014) in multiple brain regions. This is consistent with human studies showing correlations between ventral striatal DA release and the hedonic response to amphetamine administration (Drevets,; Volkow, 1999)
DA and Reward Function in Drug Addicted Subjects In subjects abusing psychostimulants, alcohol and opioids, and extreme obesity there are decreased striatal/ventral striatal DA D2/3 receptor levels and decreased DA release, but increased impulsivity. These findings differ from those seen in healthy subjects but suggest an inverse U shaped curve of D2 mediated neurotransmission versus impulsivity. These findings have led to the hypothesis that that decreased ventral striatal DA release (Casey, 2014) and/or decreased ventral striatal DA D2 receptor levels may be risk factors for substance abuse (Volkow, 1999, 2002; Gould, 2017; Wiers, 2016, Trilieff, 2014). In humans there is an inverse relationship between striatal DA D2 receptor levels and the rewarding effects of psychostimulants (Volkow, 1999). Alternatively in the development of addiction, there may be an evolution of changes in DA neurotransmission with increased striatal DA release leading to impulsive use of drugs of abuse, and/or palatable foods leading to downregulation of DA release and subsequently decreased D2 receptor levels which mediates addictive behaviors (Kessler, 2014,2016).
DA Release in Normal Weight, Overweight and Mildly Obese Subjects versus Extremely Obese Subjects Age covaried correlation coefficients between regional d-amphetamine induced DA release and BMI were positive for all regions, except the right temporal cortex (r=-0.17), ranging up to r = 0.581 (P=0.023) in the right putamen and r = 0.568 (P=0.027) in the left substantia nigra. Correlation coefficients of 0.411 and 0.391 were seen in the right ventral striatum and caudate which approached a trend level. This is in contrast to a study of amphetamine induced DA release in extremely obese female subjects, BMI>45, who had decreased DA release (van de Giessen, 2014).
Correlation of D-Amphetamine Induced DA Release with BMI in R Putamen
Fitted plots of reward sensitivity versus BMI (N=366) with broken lines giving 95% confidence intervals (Davis, 2008).
Human Substantia Nigra DA D2/3 DA D3
Cluster of differences in correlations of substantia nigra DA D2 receptor levels with DA D2 receptor levels in other brain regions
Cluster of differences in correlations of left medial thalamic DA D2 receptor levels with DA D2 receptor levels in other brain regions
Cluster of differences in correlations of left medial thalamic DA D2 receptor levels with DA D2 receptor levels in other brain regions
As the substantia nigra/vta provides dopaminergic innervation to the caudate and medial thalamus and as the major projection of the ventral striatum is to the mediodorsal nucleus of thalamus, these findings suggest abnormal regulation of DA D2 mediated neurotransmission in both the limbic/ventral striatal/medial thalamic limbic circuit and in the DLPFC/caudate/thalamic circuits in schizophrenia.
Rat Midbrain Human Midbrain
Human Rat
Relationship of Striatal Dopamine Release to Symptoms of Schizophrenia
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