The publication by Iizuka and colleagues is an important advance toward unraveling the basic biology of psychosis in general, and schizophrenia in particular. This is because they have found that a pathway known to be genetically associated with schizophrenia can alter the surface expression of dopamine D2 receptors. D2 continues to be the main target for all antipsychotic drugs (including aripiprazole and even the new Lilly glutamate agonists that have a potent affinity for D2High receptors).

In fact, the authors of this excellent study may do well to go one step further by testing whether the downregulation of dysbindin actually increases the proportion of D2 receptors that are in the high-affinity state, namely D2High. This is because all schizophrenia animal models markedly increase the proportion of D2High receptors by 100 to 900 percent (Seeman et al., 2005; Seeman et al., 2006). This generalization holds for animal models based on brain lesions, sensitization by...  Read more

The publication by Iizuka and colleagues is an important advance toward unraveling the basic biology of psychosis in general, and schizophrenia in particular. This is because they have found that a pathway known to be genetically associated with schizophrenia can alter the surface expression of dopamine D2 receptors. D2 continues to be the main target for all antipsychotic drugs (including aripiprazole and even the new Lilly glutamate agonists that have a potent affinity for D2High receptors).

In fact, the authors of this excellent study may do well to go one step further by testing whether the downregulation of dysbindin actually increases the proportion of D2 receptors that are in the high-affinity state, namely D2High. This is because all schizophrenia animal models markedly increase the proportion of D2High receptors by 100 to 900 percent (Seeman et al., 2005; Seeman et al., 2006). This generalization holds for animal models based on brain lesions, sensitization by amphetamine, phencyclidine, cocaine, caffeine or corticosterone, birth injury, social isolation, and more than 15 gene deletions in pathways for glutamate (NMDA), dopamine, GABA, acetylcholine, and norepinephrine. Although the proportion of D2High receptors invariably increases markedly, the total number of D2 receptors is generally unchanged, slightly reduced, or modestly elevated.

This publication for the first time bridges the hitherto wide gap between genetics and the antipsychotic targeting of the main cause of psychotic signs and symptoms, which is excessive D2 activity, presumably that of D2High, the functional component of D2.

References:

Seeman P, Weinshenker D, Quirion R, Srivastava LK, Bhardwaj SK, Grandy DK, Premont RT, Sotnikova TD, Boksa P, El-Ghundi M, O'dowd BF, George SR, Perreault ML, Männistö PT, Robinson S, Palmiter RD, Tallerico T. Dopamine supersensitivity correlates with D2High states, implying many paths to psychosis. Proc Natl Acad Sci U S A. 2005 Mar 1;102(9):3513-8. Epub 2005 Feb 16. Abstract

Seeman P, Schwarz J, Chen JF, Szechtman H, Perreault M, McKnight GS, Roder JC, Quirion R, Boksa P, Srivastava LK, Yanai K, Weinshenker D, Sumiyoshi T. Psychosis pathways converge via D2high dopamine receptors. Synapse. 2006 Sep 15;60(4):319-46. Review. Abstract

The study by Iizuka and colleagues is indeed very interesting. It suggests that one of the most promising risk genes for schizophrenia, the dysbindin gene, may functionally interact with dopamine D2 receptors. The D2 receptor itself is an old candidate in the study of schizophrenia, mostly because until very recently all antipsychotic medication had been directed against D2 receptors. But in addition, PET imaging studies have shown that the density and occupancy of D2 receptors is increased in drug-free and drug-naïve patients with schizophrenia.

How could this increase arise? In a subpopulation of patients it may be due to a polymorphism in the D2 receptor gene, the C957T polymorphism. The C-allele increases mRNA stability and has been found to be associated with schizophrenia, though obviously not all patients carry the C-allele. Iizuka and colleagues found an independent way in which the genetic risk factor dysbindin may upregulate D2 receptor signaling. Because dysbindin is downregulated in the brains of patients with schizophrenia, they used siRNA technology to study...  Read more

The study by Iizuka and colleagues is indeed very interesting. It suggests that one of the most promising risk genes for schizophrenia, the dysbindin gene, may functionally interact with dopamine D2 receptors. The D2 receptor itself is an old candidate in the study of schizophrenia, mostly because until very recently all antipsychotic medication had been directed against D2 receptors. But in addition, PET imaging studies have shown that the density and occupancy of D2 receptors is increased in drug-free and drug-naïve patients with schizophrenia.

How could this increase arise? In a subpopulation of patients it may be due to a polymorphism in the D2 receptor gene, the C957T polymorphism. The C-allele increases mRNA stability and has been found to be associated with schizophrenia, though obviously not all patients carry the C-allele. Iizuka and colleagues found an independent way in which the genetic risk factor dysbindin may upregulate D2 receptor signaling. Because dysbindin is downregulated in the brains of patients with schizophrenia, they used siRNA technology to study the molecular consequences of decreased dysbindin levels in cell culture.

They found that downregulation of dysbindin increases D2 receptor density in the outer cell membrane, suppresses dopamine-induced D2 receptor internalization, and increases D2 receptor signaling. The study is very promising but requires further confirmation.

How specific are the observed effects for D2 receptors? Because dysbindin is involved in both membrane trafficking and degradation of synaptic vesicles, knocking down dysbindin in growing cells may affect many physiological processes, one of them being D2 receptor signaling. Does quinpirole treatment differentially affect GTPgS incorporation in siRNA and control cells? This would be a more immediate way of looking at D2 signaling than measuring CREB phosphorylation. And, of course, the most important question is, What will happen in vivo? Maybe the sandy mouse, which carries a deletion in the dysbindin gene, could be of help here. Using these mice for a similar line of experiments may answer this question.

Iizuka and colleagues found an exciting new functional interaction between two major molecules involved in schizophrenia. I believe that these are the kind of interactions we have to look for if we want to understand complex genetic disorders such as schizophrenia.

Kellendonk et al. have reported that transient and selective overexpression of dopamine D2 receptors in the mouse striatum during development has long-term effects on cognitive function mediated by the prefrontal cortex. This is an important study providing further elegant evidence that disturbed function of the subcortical dopamine system may affect dopamine functioning in the entire circuitry and have important adverse behavioral consequences. It is unclear, however, whether this mouse model provides us with new clues about the pathophysiology of schizophrenia. A hyperdopaminergic hypothesis of schizophrenia originated from pharmacological studies showing that dopamine D2 antagonists have antipsychotic efficacy and dopamine agonists, such as amphetamine or apomorphine, can induce psychosis (Randrup and Munkvad, 1974; Snyder, 1972). This hypothesis has been supported recently by clinical data from brain imaging studies...  Read more

Kellendonk et al. have reported that transient and selective overexpression of dopamine D2 receptors in the mouse striatum during development has long-term effects on cognitive function mediated by the prefrontal cortex. This is an important study providing further elegant evidence that disturbed function of the subcortical dopamine system may affect dopamine functioning in the entire circuitry and have important adverse behavioral consequences. It is unclear, however, whether this mouse model provides us with new clues about the pathophysiology of schizophrenia. A hyperdopaminergic hypothesis of schizophrenia originated from pharmacological studies showing that dopamine D2 antagonists have antipsychotic efficacy and dopamine agonists, such as amphetamine or apomorphine, can induce psychosis (Randrup and Munkvad, 1974; Snyder, 1972). This hypothesis has been supported recently by clinical data from brain imaging studies with D2 receptor ligands showing higher presynaptic dopamine terminal activity in at least acutely psychotic patients when challenged with amphetamine or at baseline (Abi-Dargham et al., 2000; Hietala et al., 1994). Accordingly, amphetamine or apomorphine-induced hyperactivity and stereotypy in rodents have been postulated as psychosis-like behaviors and such pharmacological models have been widely used for screening antipsychotic drugs. Currently, all antipsychotic drugs on the market act by reducing D2 signals in brain, most by functioning as antagonists of D2 receptors. It is also clear, however, that although these drugs are beneficial, they do not cure the disease. It is also increasingly clear that although there is considerable evidence about the role of the dopaminergic system in the pathophysiology of schizophrenia, genetic association and linkage between schizophrenia and the genes encoding dopamine receptors or transporter remain weak (Daniels et al., 1995; Kojima et al., 1999). Thus, dopamine abnormalities may not be at the core of pathophysiology. The exploration of genetic models beyond the dopamine system may perhaps prove more fruitful for capturing many aspects of this devastating illness.

References:

Abi-Dargham A, Rodenhiser J, Printz D, Zea-Ponce Y, Gil R, Kegeles LS, Weiss R, Cooper TB, Mann JJ, Van Heertum RL, Gorman JM, Laruelle M. Increased baseline occupancy of D2 receptors by dopamine in schizophrenia. Proc Natl Acad Sci U S A. 2000 Jul 5;97(14):8104-9. Abstract

Daniels J, Williams J, Asherson P, McGuffin P, Owen M. No association between schizophrenia and polymorphisms within the genes for debrisoquine 4-hydroxylase (CYP2D6) and the dopamine transporter (DAT). Am J Med Genet. 1995 Feb 27;60(1):85-7. Abstract

Hietala J, Syvalahti E, Vuorio K, Nagren K, Lehikoinen P, Ruotsalainen U, Rakkolainen V, Lehtinen V, Wegelius U. Striatal D2 dopamine receptor characteristics in neuroleptic-naive schizophrenic patients studied with positron emission tomography. Arch Gen Psychiatry. 1994 Feb;51(2):116-23. Abstract

Kojima H, Ohmori O, Shinkai T, Terao T, Suzuki T, Abe K. Dopamine D1 receptor gene polymorphism and schizophrenia in Japan. Am J Med Genet. 1999 Apr 16;88(2):116-9. Abstract

Randrup A, Munkvad I. Pharmacology and physiology of stereotyped behavior. J Psychiatr Res. 1974;11:1-10. Review. No abstract available. Abstract

Snyder SH. Catecholamines in the brain as mediators of amphetamine psychosis. Arch Gen Psychiatry. 1972 Aug;27(2):169-79. Review. No abstract available. Abstract

The development of animal models is a critical need in the realm of schizophrenia research. Current models relying on lesions or pharmacological manipulations may be relatively nonspecific, and thus, less than optimal for unraveling the underlying pathophysiology of the disorder. Models in which specific key candidate genes are up- or down-regulated may be better models because the effects can be more subtle and, as in this study, a very specific behavioral deficit may result. Ultimately, many genes, including DRD2, may be involved in discrete aspects of the illness, and when those gene deficiencies co-occur in certain individuals, schizophrenia may manifest. This study developed and validated a model, but the study itself is a model for how such studies should be done.

View all comments by Stephen J. Glatt

The study by Kellendonk and colleagues from Eric Kandel’s lab at Columbia is a landmark piece of science in a number of respects. Transgenic overexpression of D2 receptors in the mouse striatum is a novel model of how a developmental perturbation in striatal dopaminergic signaling has long-term implications for processing of information through critical brain circuits involved in learning and memory. The model may also have implications for understanding abnormalities of the function of this circuit in schizophrenia. There is ample evidence from clinical and from postmortem studies that cortical-striatal circuits are involved as part of the pathophysiology of schizophrenia. The work of Ann Marie Thierry and colleagues in Paris in the 1970s first drew attention to the fact that cortical function impacted on the striatal dopamine system (Thierry et al., 1973). A ground-breaking study of Pycock et al. (1980) showed that DA...  Read more

The study by Kellendonk and colleagues from Eric Kandel’s lab at Columbia is a landmark piece of science in a number of respects. Transgenic overexpression of D2 receptors in the mouse striatum is a novel model of how a developmental perturbation in striatal dopaminergic signaling has long-term implications for processing of information through critical brain circuits involved in learning and memory. The model may also have implications for understanding abnormalities of the function of this circuit in schizophrenia. There is ample evidence from clinical and from postmortem studies that cortical-striatal circuits are involved as part of the pathophysiology of schizophrenia. The work of Ann Marie Thierry and colleagues in Paris in the 1970s first drew attention to the fact that cortical function impacted on the striatal dopamine system (Thierry et al., 1973). A ground-breaking study of Pycock et al. (1980) showed that DA depletion in the prefrontal cortex affected DA parameters in the striatum, by increasing specifically DA turnover and D2 receptor expression. They were the first to report an inverse relationship between cortical and subcortical DA activity, a finding that has been reproduced in a broad variety of studies in rodents, nonhuman primates, and in humans (e.g., Jaskiw et al., 1990; 1991; Deutch, 1993; Saunders et al., 1998; Bertolino et al., 1999; 2000; Meyer-Lindenberg et al., 2002; 2005). The mechanism of this effect is still uncertain, but likely involves the anatomical connectivity between prefrontal cortex and brainstem DA neurons, which involve a tonic inhibitory brake, such that normal prefrontal cortical function translates into tonic inhibition of DA neurons that project to the striatum (Carr and Sesack, 2000). Thus, prefrontal cortex is in a position to release that brake and increase DA-related reinforcement of environmental stimuli, when circumstances dictate an appropriately DA response, as might be expected during learning and memory. It is tempting to conclude from these various experiments over 30 years that the prefrontal cortex regulates the reward/reinforcement effects of DA neurons based on experiential context. The studies beginning with Thierry showed that when prefrontal function was disturbed, DA activity was no longer appropriately regulated. The study of Kellendonk et al. is consistent in terms of the circuitry involved with these earlier studies, but instead of creating an abnormality at the level of the prefrontal cortex and disrupting regulation of the DA reward system, they changed DA function directly in the striatum and their behavioral readout suggested that abnormal function of prefrontal cortex was a result. The “yin-yang” relationship again was reproduced, but now starting with the yang rather than the yin. The yang-based mechanism of the striatal effect on cortical function and cortical DA turnover is likely complex, including via striatal feedback to mesencephalic DA neurons that project to cortex and via striatal projections through thalamus back to prefrontal cortex.

The findings of Kellendonk et al. illustrate how critical prefronto-centric circuitry is, especially during development, for the elaboration of behaviors and biochemical phenomena related to schizophrenia. Their important finding that restoring normal DA function in the striatum did not restore prefrontal cognition indicates that it was no longer a matter of acute excess D2 activity in the striatum that accounted for the cognitive abnormalities. Presumably, in developmentally wiring the circuitry in and out of prefrontal cortex, abnormal information processing through the striatum (which feeds back to prefrontal cortex and presumably formats cortical information for frontally mediated action) changes the wiring diagram, producing more trait-like functional abnormalities. Trait-like changes in prefrontal function and molecular biology related to early developmental perturbations of other prefronto-centric neuronal systems implicated in schizophrenia, for example, temporal-limbic inputs to prefrontal cortex, also have been described (see Lipska and Weinberger, 2000, for review).

Finally, the study has important implications for neurobiologic models of phenomena associated with schizophrenia. PET studies of patients with schizophrenia suggest that, to the extent that striatal DA activity may be increased (Abi-Dargham et al., 1998), it is a state phenomenon, linked to active psychosis. On the other hand, evidence of abnormal cortical DA activity and function is more trait-like and persists between periods of florid psychosis. The persistent changes in cortical function independent of fluctuations in striatal D2 expression may provide some parallels to the clinical phenomena. It is surprising that these animals showed no deficits in activity or in prepulse inhibition of startle, both of which have been interpreted as measures of excess DA activity and as animal correlates of psychosis. The failure to observe these phenomena may be related to species differences—here mice as compared to earlier models with rats—or it may reflect relatively more selective overexpression of D2 receptors in the dorsal striatum. This latter finding is of interest as recent studies from Laurelle and colleagues at Columbia, using high-resolution PET imaging, have found that increased DA activity in patients with schizophrenia may also involve preferentially the dorsal striatum. The changes in DA measures in the cortex also bear interesting relationships to those found in patients with schizophrenia. The transgenic mice showed no change in markers of cortical DA innervation, which has been reported in schizophrenia (Akil et al., 1999), but they did show reduced cortical DA turnover, evidence of which also has been reported in schizophrenia (Weinberger et al., 1988). During D2 overexpression, D1 receptor sensitivity appeared to be increased, but during normalization of D2 overexpression, D1 receptors in prefrontal cortex appeared to be functionally subsensitive. These variations in cortical DA function may correspond to apparent reduced cortical DA activity as a trait characteristic and enhanced cortical DA activity as a correlate of acute psychosis (Winterer and Weinberger, 2004).

References:

Abi-Dargham A, Gil R, Krystal J, Baldwin RM, Seibyl JP, Bowers M, van Dyck CH, Charney DS, Innis RB, Laruelle M. Increased striatal dopamine transmission in schizophrenia: confirmation in a second cohort. Am J Psychiatry. 1998 Jun;155(6):761-7. Abstract

Akil M, Pierri JN, Whitehead RE, Edgar CL, Mohila C, Sampson AR, Lewis DA. Lamina-specific alterations in the dopamine innervation of the prefrontal cortex in schizophrenic subjects. Am J Psychiatry. 1999 Oct;156(10):1580-9. Abstract

Bertolino A, Breier A, Callicott JH, Adler C, Mattay VS, Shapiro M, Frank JA, Pickar D, Weinberger DR. The relationship between dorsolateral prefrontal neuronal N-acetylaspartate and evoked release of striatal dopamine in schizophrenia. Neuropsychopharmacology. 2000 Feb;22(2):125-32. Abstract

Bertolino A, Knable MB, Saunders RC, Callicott JH, Kolachana B, Mattay VS, Bachevalier J, Frank JA, Egan M, Weinberger DR. The relationship between dorsolateral prefrontal N-acetylaspartate measures and striatal dopamine activity in schizophrenia. Biol Psychiatry. 1999 Mar 15;45(6):660-7. Abstract

Carr DB, Sesack SR. Projections from the rat prefrontal cortex to the ventral tegmental area: target specificity in the synaptic associations with mesoaccumbens and mesocortical neurons. J Neurosci. 2000 May 15;20(10):3864-73. Abstract

Deutch AY. Prefrontal cortical dopamine systems and the elaboration of functional corticostriatal circuits: implications for schizophrenia and Parkinson's disease. J Neural Transm Gen Sect. 1993;91(2-3):197-221. Review. Abstract

Jaskiw GE, Karoum F, Freed WJ, Phillips I, Kleinman JE, Weinberger DR. Effect of ibotenic acid lesions of the medial prefrontal cortex on amphetamine-induced locomotion and regional brain catecholamine concentrations in the rat. Brain Res. 1990 Nov 26;534(1-2):263-72. Abstract

Jaskiw GE, Weinberger DR, Crawley JN. Microinjection of apomorphine into the prefrontal cortex of the rat reduces dopamine metabolite concentrations in microdialysate from the caudate nucleus. Biol Psychiatry. 1991 Apr 1;29(7):703-6. No abstract available. Abstract

Lipska BK, Weinberger DR. To model a psychiatric disorder in animals: schizophrenia as a reality test. Neuropsychopharmacology. 2000 Sep;23(3):223-39. Review. Abstract

Meyer-Lindenberg A, Miletich RS, Kohn PD, Esposito G, Carson RE, Quarantelli M, Weinberger DR, Berman KF. Reduced prefrontal activity predicts exaggerated striatal dopaminergic function in schizophrenia. Nat Neurosci. 2002 Mar;5(3):267-71. Abstract

Meyer-Lindenberg A, Kohn PD, Kolachana B, Kippenhan S, McInerney-Leo A, Nussbaum R, Weinberger DR, Berman KF. Midbrain dopamine and prefrontal function in humans: interaction and modulation by COMT genotype. Nat Neurosci. 2005 May;8(5):594-6. Epub 2005 Apr 10. Abstract

Pycock CJ, Kerwin RW, Carter CJ. Effect of lesion of cortical dopamine terminals on subcortical dopamine receptors in rats. Nature. 1980 Jul 3;286(5768):74-6. No abstract available. Abstract

Saunders RC, Kolachana BS, Bachevalier J, Weinberger DR. Neonatal lesions of the medial temporal lobe disrupt prefrontal cortical regulation of striatal dopamine. Nature. 1998 May 14;393(6681):169-71. Abstract

Thierry AM, Blanc G, Sobel A, Stinus L, Golwinski J. Dopaminergic terminals in the rat cortex. Science. 1973 Nov 2;182(4111):499-501. No abstract available. Abstract

Weinberger DR, Berman KF, Illowsky BP. Physiological dysfunction of dorsolateral prefrontal cortex in schizophrenia. III. A new cohort and evidence for a monoaminergic mechanism. Arch Gen Psychiatry. 1988 Jul;45(7):609-15. Abstract

Winterer G, Weinberger DR. Genes, dopamine and cortical signal-to-noise ratio in schizophrenia. Trends Neurosci. 2004 Nov;27(11):683-90. Review. Abstract

I read the paper by Simpson et al. from Kandel's group with much interest. It seems that the dopamine hypothesis of schizophrenia has many lives and appears and reappears in many forms. This latest reincarnation combines hyperdopaminergia with the neurodevelopmental hypothesis of the disorder. My initial enthusiasm, however, waned upon closer reading of the paper.

It seems that the various conclusions reached are not wholly supported by the results. The prefrontal cognitive deficits of the D2 mice seem to be extremely subtle. It is difficult to infer specific impairments of working memory performance solely from acquisition effects. The D2 mice require more trials to reach criteria, but how do the mice perform once these criteria are met? To be sure, schizophrenia patients present with learning impairments, but their working memory deficits are persistent and ever present. It is interesting that high-order “executive functions” as measured by attentional set-shifting (e.g., intra- and extra-dimensional shifts) are spared in these mice, given that these depend on the rodent...  Read more

I read the paper by Simpson et al. from Kandel's group with much interest. It seems that the dopamine hypothesis of schizophrenia has many lives and appears and reappears in many forms. This latest reincarnation combines hyperdopaminergia with the neurodevelopmental hypothesis of the disorder. My initial enthusiasm, however, waned upon closer reading of the paper.

It seems that the various conclusions reached are not wholly supported by the results. The prefrontal cognitive deficits of the D2 mice seem to be extremely subtle. It is difficult to infer specific impairments of working memory performance solely from acquisition effects. The D2 mice require more trials to reach criteria, but how do the mice perform once these criteria are met? To be sure, schizophrenia patients present with learning impairments, but their working memory deficits are persistent and ever present. It is interesting that high-order “executive functions” as measured by attentional set-shifting (e.g., intra- and extra-dimensional shifts) are spared in these mice, given that these depend on the rodent medial frontal cortex and are modulated by dopamine as well (Birrell and Brown, 2000; Tunbridge et al., 2004). Thus, contrary to what has been reported, these mice show normal behavioral flexibility. We are thus left with mice whose prefrontal function, at least behaviorally, is relatively intact.

A more pressing issue is the controls that were used for the experiments. The authors did not compare D2 mice (carrying both transgenes) with mice carrying the same two transgenes but who did not at any time express the D2 receptor. Instead the authors compared the D2R expressing mice with their littermates who carried no transgene or either the CamKII or D2 transgenes alone. They state that these groups showed no differences, but their control groups were of nine mice, so there is a potential lack of power to detect any differences between these groups. It will be of interest to know whether any of the other striatal D2 overexpressing lines that were created show similar phenotypes. Lacking this information, we cannot be sure that the subtle effect on behavior is not due to the disruption of another gene by the random insertion of the D2 transgene.

This paper is a natural extension of many years of work showing the balance between cortical and subcortical dopamine systems (Grace, 1991). A brief transient overexpression of striatal D2Rs during development does seem to affect DA function long into adulthood. This mouse model also reflects the long-used strategy of probing those systems thought to underlie the pathophysiology of schizophrenia. These models are of great benefit, but whether they shed any light on the cause or etiology of the disorder is an open question. One would hope that with these now sophisticated genetic tools and the identification of several reliable susceptibility genes (NRG1, DTNBP1, DISC1), more etiologically relevant mouse models can be created.

References:
Birrell JM and Brown VJ. Medial frontal cortex mediates perceptual attentional set shifting in the rat. J Neurosci. 2000 Jun 1;20(11):4320-4. Abstract Grace AA. Phasic versus tonic dopamine release and the modulation of dopamine system responsivity: a hypothesis for the etiology of schizophrenia. Neuroscience. 1991;41(1):1-24. Abstract Tunbridge EM, Bannerman DM, Sharp T, Harrison PJ. Catechol-o-methyltransferase inhibition improves set-shifting performance and elevates stimulated dopamine release in the rat prefrontal cortex. J Neurosci. 2004 Jun 9;24(23):5331-5. Abstract

Comment by Tomiki Sumiyoshi and Philip Seeman
Kellendonk et al. report various behavioral and neurochemical findings from transgenic mice expressing an increased number of dopamine (DA)-D2 receptors in the striatum, labeled by 3H-spiperone. These mice showed deficits in some aspects of working memory, a cognitive domain associated with the prefrontal cortex function.

This study was prompted by the landmark hypothesis that DA supersensitivity in some of the subcortical brain regions, such as the striatum, constitutes a neurochemical basis for psychotic symptoms of schizophrenia (e.g., van Rossum, 1966; Seeman et al., 2005). Conventionally, dysregulation of DA-related behaviors, including enhanced locomotor activity and stereotypy, as well as disrupted prepulse inhibition, have been thought to reflect psychosis-related symptoms. However, the D2 receptor transgenic mice did not demonstrate alterations in any...  Read more

Comment by Tomiki Sumiyoshi and Philip Seeman
Kellendonk et al. report various behavioral and neurochemical findings from transgenic mice expressing an increased number of dopamine (DA)-D2 receptors in the striatum, labeled by 3H-spiperone. These mice showed deficits in some aspects of working memory, a cognitive domain associated with the prefrontal cortex function.

This study was prompted by the landmark hypothesis that DA supersensitivity in some of the subcortical brain regions, such as the striatum, constitutes a neurochemical basis for psychotic symptoms of schizophrenia (e.g., van Rossum, 1966; Seeman et al., 2005). Conventionally, dysregulation of DA-related behaviors, including enhanced locomotor activity and stereotypy, as well as disrupted prepulse inhibition, have been thought to reflect psychosis-related symptoms. However, the D2 receptor transgenic mice did not demonstrate alterations in any of these behavioral measures, although an in vitro assay indicated reduced DA-induced adenylate cyclase activity in these animals. To follow the behavioral changes after challenging the mice with amphetamine or other DA-agonists would have conveyed more information on whether the up-regulated D2 receptors are actually functional.

It is also crucial to determine if there is a shift of D2 receptors to the high-affinity state, or functional state (D2High) (Seeman et al., 2005), in this animal model of schizophrenia. It is argued that D2High sites may be more relevant to psychotic symptoms than the total density of D2 receptors measured by conventional binding methods, such as that used by Kellendonk et al. with 3H-spiperone as a ligand (Seeman et al., 2005; Sumiyoshi et al., 2005). In fact, increased proportions of D2High have been reported in various animal models of psychosis, including those based on the neurodevelopmental hypothesis of schizophrenia (Seeman et al., 2005; Sumiyoshi et al., 2005).

Kellendonk et al. found that the extra D2 receptors in the striatum were associated with the cognitive disturbances. Since it has been found that overexpression of the catechol-O-methyl transferase (COMT) gene also impairs cognitive function (Chen et al., 2005), further research is needed to determine if the cognitive deficits result from overexpression of these specific genes and not just any gene.

References:

van Rossum JM. The significance of dopamine-receptor blockade for the mechanism of action of neuroleptic drugs. Arch Int Pharmacodyn Ther. 1966 Apr;160(2):492-4. No abstract available. Abstract

Seeman P, Weinshenker D, Quirion R, Srivastava LK, Bhardwaj SK, Grandy DK, Premont RT, Sotnikova TD, Boksa P, El-Ghundi M, O'dowd BF, George SR, Perreault ML, Mannisto PT, Robinson S, Palmiter RD, Tallerico T. Dopamine supersensitivity correlates with D2High states, implying many paths to psychosis. Proc Natl Acad Sci U S A. 2005 Mar 1;102(9):3513-8. Epub 2005 Feb 16. Abstract

Sumiyoshi T, Seeman P, Uehara T, Itoh H, Tsunoda M, Kurachi M. Increased proportion of high-affinity dopamine D2 receptors in rats with excitotoxic damage of the entorhinal cortex, an animal model of schizophrenia. Brain Res Mol Brain Res. 2005 Oct 31;140(1-2):116-9. Epub 2005 Jul 28. Abstract

Chen J, Lipska BK, Weinberger DR. New genetic mouse models of schizophrenia: Mimicking cognitive dysfunction by altering susceptibility gene expression. Neuropsychopharmacology. 2005; 30 (Suppl 1):S12-13.

I agree with Dr Weinberger's comments about the work of Kellendonk et al. In this sense, the cortical, frontal-striatal connections are well-known circuits involved in the development of schizophrenia.

Dr. Weinberger, in 1992, reported studies from limbic-prefrontal circuits, connections involved in schizophrenia pathophysiology (Weinberger et al., 1992). This work used an inverse experimental methodology (of corroborating the existing relationship between frontal cortex and the striatum) from the methodology commonly used (search for the line-activation in frontal cortex, then see the results in the striatum).

The most outstanding part of the study is one dedicated to the developmental approach. Thus, in the article, it was clear that restoring the normal DA function in the striatum did not restore cognitive functioning. As this article demonstrates, developmental approaches are excellent for the understanding of the neurobiology of schizophrenia.

References:

Weinberger DR, Berman KF, Suddath R, Torrey EF. Evidence of dysfunction of a prefrontal-limbic network in schizophrenia: a magnetic resonance imaging and regional cerebral blood flow study of discordant monozygotic twins. Am J Psychiatry. 1992 Jul;149(7):890-7. Abstract

View all comments by Patricia Estani

First, Zhang and colleagues examine multiple polymorphisms in the D2 receptor gene and find that none of the "standard" ones that have been linked to clinical characteristics actually affected D2 receptor density in prefrontal cortex or striatum. However, they find that two other, previously unstudied polymorphisms altered the relative expression of short versus long isoforms of the D2 receptor, likely reflecting presynaptic and postsynaptic D2 receptors, respectively. These findings could provide a basis for understanding several perplexing effects in the literature, such as opposing effects of D2 receptor drugs on cognition in individuals with low and high working memory ability, who are shown here to have differential pre- versus postsynaptic D2 receptor function.

Further, the presynaptic receptor is thought to regulate phasic dopamine signaling via its autoreceptor functions (in addition to controlling glutamate release in corticostriatal terminals via the heteroreceptors alluded to in the article). Thus, based on current evidence, it is expected that these...  Read more

First, Zhang and colleagues examine multiple polymorphisms in the D2 receptor gene and find that none of the "standard" ones that have been linked to clinical characteristics actually affected D2 receptor density in prefrontal cortex or striatum. However, they find that two other, previously unstudied polymorphisms altered the relative expression of short versus long isoforms of the D2 receptor, likely reflecting presynaptic and postsynaptic D2 receptors, respectively. These findings could provide a basis for understanding several perplexing effects in the literature, such as opposing effects of D2 receptor drugs on cognition in individuals with low and high working memory ability, who are shown here to have differential pre- versus postsynaptic D2 receptor function.

Further, the presynaptic receptor is thought to regulate phasic dopamine signaling via its autoreceptor functions (in addition to controlling glutamate release in corticostriatal terminals via the heteroreceptors alluded to in the article). Thus, based on current evidence, it is expected that these polymorphisms should affect not only working memory, but also positive and negative reinforcement learning processes thought to depend on phasic dopamine signaling, and which are clearly implicated in the development of addictive habits.

Finally, these polymorphisms are in linkage disequilibrium with some of the other standard D2 SNPs that have been associated with alcohol abuse, schizophrenia, and sensitivity to reinforcement, and therefore may provide a more direct mechanistic explanation for these prior associations.

The paper by Allen et al. is a tremendously useful addition to the fields of schizophrenia research, psychiatric genetics, and medical genetics. By efficiently summarizing a tremendous amount of work, Allen et al. have endeavored to provide a "state-of-the-art" summary that most of us, as individuals, struggle to accomplish; they have largely succeeded in their attempt. This manuscript, and the continual availability of the SZGene database, should long serve as invaluable resources for the increasingly complex task of building polygenic models of risk for schizophrenia. Furthermore, these methods, which were initially implemented in the AlzGene database, have clearly generalized quite successfully to SZGene and thus, should be easy enough to scale up to cover many other psychiatric disorders as well. In this way, the contribution to psychiatric genetics, and possibly other disorders outside of psychiatry, is crystalline.

Aside from the database, the contribution of the recent manuscript to the field of schizophrenia research is also tremendous. As pointed out by the...  Read more

The paper by Allen et al. is a tremendously useful addition to the fields of schizophrenia research, psychiatric genetics, and medical genetics. By efficiently summarizing a tremendous amount of work, Allen et al. have endeavored to provide a "state-of-the-art" summary that most of us, as individuals, struggle to accomplish; they have largely succeeded in their attempt. This manuscript, and the continual availability of the SZGene database, should long serve as invaluable resources for the increasingly complex task of building polygenic models of risk for schizophrenia. Furthermore, these methods, which were initially implemented in the AlzGene database, have clearly generalized quite successfully to SZGene and thus, should be easy enough to scale up to cover many other psychiatric disorders as well. In this way, the contribution to psychiatric genetics, and possibly other disorders outside of psychiatry, is crystalline.

Aside from the database, the contribution of the recent manuscript to the field of schizophrenia research is also tremendous. As pointed out by the authors, several of the significantly associated genes identified by their meta-analyses were never before studied in this manner, so a whole new set of top candidate genes was identified. This work also served to confirm the results of prior meta-analyses from my group and others, which is always reassuring. Application of the HuGENet criteria to grading the detected associations is useful as a heuristic, but it must be kept in mind that that while these criteria reflect a consensus, they also reflect a moving target. One difficulty in implementing grades (especially the "overall" grade) is analogous to difficulties often encountered in meta-analyses when rating the quality of studies, and that is the ambiguity of ratings. Thus, on a seven-point quality scale (or a three-letter-grade scale), a score can be arrived at by a variety of combinations of flaws or strengths, but similar scores may not (often do not) reflect identical strengths and weaknesses of the graded studies. For example, I, for one, am not certain that having a relatively low number of minor alleles reflected in a meta-analytic result (especially if it is a rare variant) is as big a decrement as the pooled OR dropping from significance when the initial study is omitted.

Nevertheless, I reiterate that the use of this heuristic grading system is helpful, but should be taken with a grain of salt. Overall, the paper and its conclusions are a great contribution to this field and warrant mass attention. The ultimate question, not yet addressed here but apparently on the horizon, is how well the emerging GWASs detect these "positive control" associations, or we might say how well these hypothesis-driven results stack up against new candidates to emerge from the high-throughput generation of novel hypotheses....