Stress Reworks Dopamine Systems via Glucocorticoids
19 January 2013. Stressful events contribute to the development of psychiatric illnesses, but what are the transducers of this connection? Two studies published January 18 in Science uncover actions by stress hormones on dopamine signaling in mice. One study, led by François Tronche of Université Pierre et Marie Curie in Paris, France, finds that stress-induced glucocorticoids act on neurons receiving dopamine messages. The second study, led by Akira Sawa of Johns Hopkins University in Baltimore, Maryland, examines stress-induced consequences in mice engineered to model a genetic predisposition for psychiatric illness—disruption of the DISC1 gene. In these mice, stress-induced glucocorticoids tamped down dopamine release through epigenetic mechanisms. Both studies illuminate circuit-specific consequences of the body-wide release of glucocorticoids during stressful events.
Although these studies do not set out to model a specific psychiatric disorder, they grapple with mechanisms behind the adolescence or early adult onset of many of these, including schizophrenia. Because stress can precipitate illness onset, researchers have been studying the cumulative effects of stress on mouse behavior. Different stress regimens in mice can produce various adverse outcomes, including immobility in the forced swim test (taken as a sign of despair), anxiety-like behaviors, or impaired social interactions (Russo et al., 2012). Recent optogenetic studies in rodents have linked stress-induced depression-like behaviors to dopamine signaling (see SRF related news story), but the new studies address how dopamine systems become dysregulated by stress in the first place.
Glucocorticoids are steroid hormones released by the adrenal glands during stress. Glucocorticoid receptors are found throughout the brain, and when activated, they move to the cell’s nucleus to work as transcription factors, where they can fine-tune gene expression. But that isn’t their only mode of action, as laid out in an accompanying perspective piece by Bruce McEwen (McEwen, 2013). Though the precise steps between glucocorticoid receptor activation and changes to dopamine systems remain murky, the results suggest that glucocorticoids may offer some therapeutic clues to psychiatric disorders.
The agony of social defeat
In the first study, first author Jacques Barik and colleagues use the social defeat paradigm, in which mice are repeatedly attacked by another mouse. After 10 days of this, these “socially defeated” mice show signs of social aversion, spending less time interacting with a novel mouse. To figure out where in the brain glucocorticoids were acting to produce this behavior, the researchers engineered mice to lack glucocorticoid receptors in dopamine-containing neurons of the ventral tegmental area (VTA); these mice, however, still developed social aversion after social defeat. But when the researchers tested a different group of mice—missing glucocorticoid receptors in dopamine-receiving neurons (specifically, those with D1a receptors), these mice did not develop social aversion after social defeat, looking very much like “undefeated” mice. Interestingly, anxiety-like and despair-like behaviors remained in these mice, suggesting they are mediated by separate neural circuitry.
The ablation of glucocorticoid receptors from dopamine-receiving neurons trickled back to the dopamine-containing neurons of the VTA, too. The researchers found signs of abnormal dopamine neuron activity, with less burst-firing recorded in the VTA, and less dopamine release measured by microdialysis in the nucleus accumbens, a target of the VTA. The researchers suggest that this might involve a feedback loop from dopamine-receiving neurons in the nucleus accumbens that project back to the VTA. Next, the researchers tested whether suppressing dopamine release in control mice could protect them from developing social aversion in response to social defeat. It did: while saline-injected mice exhibited the expected social aversion, mice injected with quinpirole (a D2R agonist that lowers dopamine neuron activity through autoreceptors) spent more than twice the amount of time interacting with another mouse.
In the second study, first author Minae Niwa and colleagues focused on the lingering effects of an episode of stress in adolescence in a mouse model of susceptibility to mental illness. Mice were engineered to carry a dominant-negative form of human DISC1, a gene fingered by a chromosomal translocation found in a family beset by mental illness (see SRF related news story). Sawa and colleagues have proposed that the DISC1 disruption produces a truncated protein that binds to normal copies of the protein, thus interfering with their usual function. Sawa’s group has found schizophrenia-related phenotypes in mice expressing this dominant-negative DISC1 (DN-DISC1) in hippocampus and cortex (see SRF related news story), but the new study expresses the construct more widely throughout the brain via a prion protein promoter.
Because brain maturation continues into the teenage years (see SRF related news story), this is considered a vulnerable time for brain development, when stressful events may have lasting consequences. The researchers found that housing the DN-DISC1 mice alone for three weeks during adolescence produced behavioral abnormalities likened to symptoms of psychiatric disorders: impaired paired pulse inhibition, longer immobility in the forced swim test, and increased locomotion (including after an acute injection of methamphetamine). These outcomes required a combination of the genetic risk and stressful environment: controls, control mice with the same schedule of social isolation, or DN-DISC1 mice without social isolation did not develop these behaviors.
Looking for neural changes that might account for these behaviors, the researchers found evidence for disturbed dopamine signaling in one pathway: the frontal cortex contained decreased dopamine levels and decreased tyrosine hydroxylase (TH), an enzyme needed to make dopamine; these changes were not apparent, however, in the caudate putamen or nucleus accumbens, which are subcortical destinations for dopamine-releasing projections. These disturbances were put right by a dose of mifepristone, a glucocorticoid receptor blocker, thus implicating the stress hormone in creating the dopamine disturbances. Further experiments found that DN-DISC1 mice receiving a stint of social isolation had elevated methylation of the TH gene—an epigenetic mark that suppresses transcription—but only in those VTA neurons projecting to the cortex, not those headed to the nucleus accumbens.
This methylation pattern was not easily undone, lasting well into mouse adulthood. But a dose of mifepristone normalized methylation of the TH gene, indicating regulation by glucocorticoids. The researchers suggest that their mice may model psychotic depression, an illness reported to respond to glucocorticoid receptor blockers (Flores et al., 2006).
Together, the studies offer up some tangible leads on where stress acts in the brain to produce adverse outcomes, for mice at least. Given the complex interactions of glucocorticoids in the brain, more remains to be discovered, but the circuit-specificity of glucocorticoid action can already inform ideas about how individuals may cope with stressful events.—Michele Solis.
Barik J, Marti F, Morel C, Fernandez SP, Lanteri C, Godeheu G, Tassin JP, Mombereau C, Faure P, Tronche F. Chronic stress triggers social aversion via glucocorticoid receptor in dopaminoceptive neurons. Science. 2013 Jan 18. Abstract
Niwa M, Jaaro-Peled H, Tankou S, Seshadri S, Hikida T, Matsumoto Y, Cascella N.G. Kano S, Ozaki N, Nabeshima T, Sawa A. Adolescent Stress–Induced Epigenetic Control of Dopaminergic Neurons via Glucocorticoids. Science. 2013 Jan 18. Abstract
Comments on Related News
Related News: Modeling Schizophrenia Phenotypes—DISC1 Transgenic Mouse DebutsComment by: David J. Porteous, SRF Advisor
, Kirsty Millar
Submitted 2 August 2007
Posted 2 August 2007
Several genetic studies point to involvement of DISC1 in major psychiatric illness, including schizophrenia and bipolar disorder, but to date the only causal variant that has been definitively identified is the translocation between human chromosomes 1 and 11 that co-segregates with major mental illness in a large Scottish family and which directly disrupts the DISC1 gene (Millar at al., 2000). It has been speculated that a truncated form of DISC1 may be expressed from the translocated allele and, if so, that this could exert a dominant-negative effect, but there is no such evidence from studies of the translocation cases. Rather, the evidence from studies of lymphoblastoid cell lines carrying the translocation suggests that haploinsufficiency is the most likely disease mechanism in this family (Millar et al., 2005). The unresolvable caveat to this, of course, is that it has not been possible to determine whether this is true also for the brain. Moreover, it is far from certain that any productive product from the translocation chromosome would be a perfectly truncated protein encoded by all the remaining exons, as opposed to an exon-skip isoform, with or without a hybrid protein component borrowing sequence information from chromosome 11. What does seem likely from other human studies is that additional genetic mechanisms, including missense mutations, altered expression, and possibly also copy number variation, play a role in the generality of DISC1 as a risk factor.
The evidence in support of DISC1 as an important biological determinant across a spectrum of major mental illness has now received a further boost from the study in PNAS by Hikida et al. The Sawa lab describes a transgenic approach where a truncated human DISC1 protein is expressed from a CAMKII promoter. The truncation was designed to mimic the hypothetical truncation arising from the Scottish translocation. This forebrain-specific promoter confers preferential expression of the transgene at neonatal stages, as distinct from the expression of the endogenous protein, which is abundant from embryonic development into adulthood. This model therefore permits investigation of the effect of the truncated protein in the forebrain within a specific developmental window, against a background of endogenous mouse DISC1 expression. Since there is no evidence for production of a truncated protein from the translocated allele, the relevance of this model to psychiatric illness remains unclear. However, on the positive side and from a functional perspective, dominant-negative effects as a consequence of expressing the truncated protein have already been demonstrated in cultured cells, altering the subcellular distribution of DISC1 and interaction with DISC1 partner proteins. Moreover, expression of the truncated form of DISC1 mimics downregulation of DISC1 in vivo, inhibiting migration of neurons in the developing mouse cortex (Kamiya et al., 2005). Thus, this model has the genuine potential to enhance our understanding of the biology of DISC1.
This is, in fact, the third study describing mice expressing modified DISC1 alleles. In the first study, Gogos and colleagues (Kioke et al., 2006) studied the effects of a modified DISC1 allele carrying a spontaneous 25 bp deletion in exon 6 that is present in all 129 mouse strains (Koike et al., 2007; see SRF related news story). This allele additionally has an artificial stop codon in exon 8 and a downstream polyadenylation signal. After back-crossing this mutagenised version of the 129 allele onto a C57Bl6 background, they reported a deficit in an assay of working memory in both heterozygous and homozygous mutants, but other standard behavioral tests were unaltered or unreported, and there were no anatomical, electrophysiological, or pharmacological studies included. In the second study, one led by the Roder laboratory, Toronto, we described two mouse strains with missense mutations in exon 2, Q31L and L100P (Clapcote et al., 2007). Reductions in brain volume, deficits in a range of standard behavioral tests, and responses to pharmacological treatments were reported, which can be summarized as consistent with the 100P mutants displaying schizophrenia-like phenotypes and the 31L mutants, mood disorder-like phenotypes. There is a frustrating dearth of consistent biomarkers for schizophrenia, but one of the best replicated findings is by brain imaging of enlarged ventricles in schizophrenia (also, but less markedly, in bipolar disorder). Adding to the observations of Clapcote et al., arguably the most striking claim by Hikida et al. is for altered ventricular brain volume and reduced brain laterality following neonatal transgenic overexpression of truncated DISC1. Additionally, behavioral tests were reported that overlap in part with those reported earlier by Clapcote et al. That three studies all describe behavioral abnormalities consistent with modeling components of schizophrenia is reassuring. That there are clear differences, too, between the phenotypes should come as no surprise either, given the important differences in terms of genetic lesion and developmental expression. Other mouse models are in the pipeline and they, too, will be welcomed. Indeed, this is very much what is needed for the field to move forward. But we should do so with some caution, paying careful attention to the specific nature of the models, what they can and cannot tell us about DISC1 biology, and what they may or may not tell us about the human condition. Although none of these models relates directly to a known causal variant, it appears that the mouse models concur with the human genetic studies in suggesting that there are likely to be several routes by which DISC1 can perturb brain function leading to characteristics of human mental illness. What we need now is for the human genetic studies to catch up with the mouse so that defined pathognomic variants can be modeled.
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Related News: Modeling Schizophrenia Phenotypes—DISC1 Transgenic Mouse Debuts
Comment by: John Roder
Submitted 2 August 2007
Posted 2 August 2007
A new mouse model from the Sawa lab strengthens the evidence for the candidate gene DISC1 playing a role in psychosis and mood disorders. This important paper is the first to address one potential disease mechanism, that of a dominant-negative effect. Expression of the C-terminal deletion of human DISC1—which represented the original rearrangement found by the Porteous group in the Scottish families with schizophrenia and depression—in transgenic mice driven by the α CaMKII promoter, first described by Mark Mayford when a postdoctoral fellow in the Kandel lab, leads to mice showing behaviors consistent with schizophrenia and depression, with enlarged lateral ventricles. Since the Sawa group expressed the human C-terminal truncation in mouse with no change in mouse DISC1 levels, they feel this supports a dominant-negative mechanism. More direct experiments are required. For example, create a null mutant mouse for DISC1 and express the full-length and truncated human DISC1 under the influence of their own promoter in transgenic mice using human BACs. Full-length human DISC1 would, hopefully, rescue the null. If so, then a mixture of full-length and truncated DISC1 proteins could be tried. No rescue by the mixture of full-length and truncated proteins would suggest a dominant-negative mechanism.
The Porteous group has shown no detectable DISC1 protein in lymphoblasts from the patients, and put forward the following implicit model. The C-terminal truncation of DISC1 makes the protein unstable and sensitive to degradation, a plausible alternative notion. In my opinion both are likely in different schizophrenia patients with perturbations in DISC1, most of which are alterations other than the C-terminal truncation. Some have SNPs that lead to as yet uncharacterized disease. It has been shown by the Sawa lab that mice with a partial RNAi knockdown of DISC1 show perturbations in brain development, and if aged to 8-12 weeks these mice might have shown behavioral and neuropathological hallmarks of schizophrenia and depression. There is currently no null mutation in the mouse that would address this issue, although DISC1 is one of the genes being targeted in the NIH knockout mouse project. Fortunately, there are now several mouse models—the more the better. The Gogos lab has a 25bp deletion in exon 6 that removes some, but not all isoforms. The Roder lab used a reverse genetic screen of an ENU archive to generate two missense mutants in non-conserved amino acids. The phenotypes of all these lines are nicely summarized in the Sawa paper. This work represents a step forward in our understanding of the DISC1 gene.
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Related News: Studies Dissect Depression’s Circuitry
Comment by: Anthony Grace, SRF Advisor (Disclosure)
Submitted 16 January 2013
Posted 17 January 2013
I recommend the Primary Papers
The Nature articles on the role of dopamine and depression add greatly to our understanding of this transmitter system in affective disorders. While on the surface such studies may be viewed as being opposite in nature, both are highly consistent with results from our lab and others. We (Valenti et al., 2011) and others have shown that strong, acute stressors can activate dopamine neuron activity; however, when measured after long bouts of inescapable stress or following an incubation period, the activity of these neurons is markedly depressed (Moore et al., 2001; Valenti et al., 2012; Chang and Grace, 2013). These studies suggest that dopamine system activation during the stressor may be a precedent for dopamine system downregulation following termination of the stressor. Indeed, in the uncontrollable chronic stress model and in the learned helplessness model of depression (Chang and Grace, 2012; Belujon et al., 2012), we have found that dopamine neuron population activity depression correlates with behavioral indices of depressive-like behavior in rats.
The data from the Han and Deisseroth laboratories are actually highly consistent with these data. Thus, Han showed that phasic activation of dopamine neurons potentiates the effects of stress on subsequent depression (consistent with our studies showing dopamine activation during the initial stress events), whereas restoring dopamine activity during the depressed condition in Deisseroth's paper relieves the symptoms due to dopamine system downregulation.
Therefore, in our opinion, each phase of the depression process may have a dopamine component: an activation during the induction phase and an attenuation during symptom expression. Taken together, these findings provide unique insights into the process and expression of depression.
Belujon, P., Dollish, H.D. and Grace, A.A. (2012) Ketamine restores activity of the dopamine system selectively in rats exhibiting learned helplessness in an animal model of depression. Program No. 774.02.2012. Neuroscience Meeting Planner, New Orleans, LA. Society for Neuroscience, 2012. Online.
Chang, C.H. and Grace, A.A. (2012) Chronic mild stress induces anxiety-like behavior and down-regulation of dopamine system activity in rats. Program No. 774.13.2012. Neuroscience Meeting Planner, New Orleans, LA. Society for Neuroscience, 2012. Online.
Chang, C.-H. and Grace, A.A. (2013) Amygdala beta noradrenergic receptors modulate delayed down-regulation of dopamine activity following restraint. Journal of Neuroscience (in press).
Moore, H., Rose, H.J.. and Grace, A.A. (2001) Chronic cold stress reduces the spontaneous activity of ventral tegmental dopamine neurons. Neuropsychopharmacology 24: 410-419. Abstract
Valenti, O., Lodge, D.J. and Grace, A.A. (2011) Aversive stimuli alter ventral tegmental area dopamine neuron activity via a common action in the ventral hippocampus. Journal of Neuroscience 31: 4280-4289. Abstract
Valenti, O., Gill, K.M. and Grace, A.A. (2012) Different stressors produce excitation or inhibition of mesolimbic dopamine neuron activity: Response alteration by stress pre-exposure. European Journal of Neuroscience 35: 1312-1321. Abstract
View all comments by Anthony Grace