Probes for DRD2

Major depressive disorder (MDD) patients display a common but often variable set of symptoms making successful, sustained treatment difficult to achieve. Separate depressive symptoms may be encoded by differential changes in distinct circuits in the brain, yet how discrete circuits underlie behavioral subsets of depression and how they adapt in response to stress has not been addressed. We identify two discrete circuits of parvalbumin-positive (PV) neurons in the ventral pallidum (VP) projecting to either the lateral habenula or ventral tegmental area contributing to depression. We find that these populations undergo different electrophysiological adaptations in response to social defeat stress, which are normalized by antidepressant treatment. Furthermore, manipulation of each population mediates either social withdrawal or behavioral despair, but not both. We propose that distinct components of the VP PV circuit can subserve related, yet separate depressive-like phenotypes in mice, which could ultimately provide a platform for symptom-specific treatments of depression.

Dopaminergic systems play a major role in reward-related behavior and dysregulation of dopamine (DA) systems can cause several mental disorders, including depression. We previously reported that dopamine D2 receptor knockout (D2R-/-) mice display increased anxiety and depression-like behaviors upon chronic stress. Here, we observed that chronic stress caused myelin loss in wild-type (WT) mice, while the myelin level in D2R-/- mice, which was already lower than that in WT mice, was not affected upon stress. Fewer mature oligodendrocytes (OLs) were observed in the corpus callosum of stressed WT mice, while in D2R-/- mice, both the control and stressed group displayed a decrease in the number of mature OLs. We observed a decrease in the number of active β-catenin (ABC)-expressing and TCF4-expressing cells among OL lineage cells in the corpus callosum of stressed WT mice, while such regulation was not found in D2R-/- mice. Administration of lithium normalized the behavioral impairments and myelin damage induced by chronic stress in WT mice, and restored the number of ABC-positive and TCF4-positive OLs, while such effect was not found in D2R-/- mice. Together, our findings indicate that chronic stress induces myelin loss through the Wnt/β-catenin signaling pathway in association with DA signaling through D2R.

Despite decades of research on neurobiological mechanisms of psychostimulant addiction, the only effective treatment for many addicts is contingency management, a behavioral treatment that uses alternative non-drug reward to maintain abstinence. However, when contingency management is discontinued, most addicts relapse to drug use. The brain mechanisms underlying relapse after cessation of contingency management are largely unknown, and, until recently, an animal model of this human condition did not exist. Here we used a novel rat model, in which the availability of a mutually exclusive palatable food maintains prolonged voluntary abstinence from intravenous methamphetamine self-administration, to demonstrate that the activation of monosynaptic glutamatergic projections from anterior insular cortex to central amygdala is critical to relapse after the cessation of contingency management. We identified the anterior insular cortex-to-central amygdala projection as a new addiction- and motivation-related projection and a potential target for relapse prevention.

Most people exposed to stress do not develop depression. Animal models have shown that stress resilience is an active state that requires broad transcriptional adaptations, but how this homeostatic process is regulated remains poorly understood. In this study, we analyze upstream regulators of genes differentially expressed after chronic social defeat stress. We identify estrogen receptor α (ERα) as the top regulator of pro-resilient transcriptional changes in the nucleus accumbens (NAc), a key brain reward region implicated in depression. In accordance with these findings, nuclear ERα protein levels are altered by stress in male and female mice. Further, overexpression of ERα in the NAc promotes stress resilience in both sexes. Subsequent RNA-sequencing reveals that ERα overexpression in NAc reproduces the transcriptional signature of resilience in male, but not female, mice. These results indicate that NAc ERα is an important regulator of pro-resilient transcriptional changes, but with sex-specific downstream targets.

Contexts exert bi-directional control over relapse to drug seeking. Contexts associated with drug self-administration promote relapse, whereas contexts associated with the absence of self-administration protect against relapse. The nucleus accumbens shell (AcbSh) is a key brain region determining these roles of context. However, the specific cell types, and projections, by which AcbSh serves these dual roles are unknown. Here, we show that contextual control over relapse and abstinence is embedded within distinct output circuits of dopamine 1 receptor (Drd1) expressing AcbSh neurons. We report anatomical and functional segregation of Drd1 AcbSh output pathways during context-induced reinstatement and extinction of alcohol seeking. The AcbSh→ventral tegmental area (VTA) pathway promotes relapse via projections to VTA Gad1 neurons. The AcbSh→lateral hypothalamus (LH) pathway promotes extinction via projections to LH Gad1 neurons. Targeting these opposing AcbSh circuit contributions may reduce propensity to relapse to, and promote abstinence from, drug use.

Cortical D2 dopamine receptor (Drd2) have mostly been examined in the context of cognitive function regulation and neurotransmission modulation of medial prefrontal cortex by principal neurons and parvalbumin positive, fast-spiking, interneurons in schizophrenia. Early studies suggested the presence of D2 receptors in several cortical areas, albeit with major technical limitations. We used combinations of transgenic reporter systems, recombinase activated viral vectors, quantitative translatome analysis, and high sensitivity in situ hybridization to identify D2 receptor expressing cells and establish a map of their respective projections. Our results identified previously uncharacterized clusters of D2 expressing neurons in limbic and sensory regions of the adult mouse brain cortex. Characterization of these clusters by translatome analysis and cell type specific labeling revealed highly heterogeneous expression of D2 receptors in principal neurons and various populations of interneurons across cortical areas. Transcript enrichment analysis also demonstrated variable levels of D2 receptor expression and several orphan G-protein-coupled receptors coexpression in different neuronal clusters, thus suggesting strategies for genetic and therapeutic targeting of D2 expressing neurons in specific cortical areas. These results pave the way for a thorough re-examination of cortical D2 receptor functions, which could provide information about neuronal circuits involved in psychotic and mood disorders.

Background

Mu opioid receptors (MORs) are central to pain control, drug reward and addictive behaviors, but underlying circuit mechanisms have been poorly explored by genetic approaches. Here we investigate the contribution of MORs expressed in GABAergic forebrain neurons to major biological effects of opiates, and also challenge the canonical disinhibition model of opiate reward.

Methods

We used Dlx5/6-mediated recombination to create conditional Oprm1 mice in GABAergic forebrain neurons. We characterized the genetic deletion by histology, electrophysiology and microdialysis, probed neuronal activation by c-Fos immunohistochemistry and resting state-functional magnetic resonance imaging, and investigated main behavioral responses to opiates, including motivation to obtain heroin and palatable food.

Results

Mutant mice showed MOR transcript deletion mainly in the striatum. In the ventral tegmental area (VTA), local MOR activity was intact, and reduced activity was only observed at the level of striatonigral afferents. Heroin-induced neuronal activation was modified at both sites, and whole-brain functional networks were altered in live animals. Morphine analgesia was not altered, neither was physical dependence to chronic morphine. In contrast, locomotor effects of heroin were abolished, and heroin-induced catalepsy was increased. Place preference to heroin was not modified, but remarkably, motivation to obtain heroin and palatable food was enhanced in operant self-administration procedures.

Conclusions

Our study reveals dissociable MOR functions across mesocorticolimbic networks. Thus beyond a well-established role in reward processing, operating at the level of local VTA neurons, MORs also moderate motivation for appetitive stimuli within forebrain circuits that drive motivated behaviors.

Molecular identification and characterization of fear controlling circuitries is a promising path towards developing targeted treatments of fear-related disorders. Three-color in situ hybridization analysis was used to determine whether somatostatin (Sst), neurotensin (Nts), corticotropin releasing factor (Crf), tachykinin 2 (Tac2), protein kinase c delta (Prkcd), and dopamine receptor 2 (Drd2) mRNA co-localize in male mouse amygdala neurons. Expression and co-localization was examined across capsular (CeC), lateral (CeL), and medial (CeM) compartments of the central amygdala. The greatest expression of Prkcd and Drd2 were found in CeC and CeL. Crf was expressed primarily in CeL while Sst, Nts, and Tac2 expressing neurons were distributed between CeL and CeM. High levels of co-localization were identified between Sst, Nts, Crf, and Tac2 within the CeL while little co-localization was detected between any mRNAs within the CeM. These findings provide a more detailed understanding of the molecular mechanisms that regulate the development and maintenance of fear and anxiety behaviors.

Significance Statement Functional and behavioral analysis of central amygdala microcircuits has yielded significant insights into the role of this nucleus in fear and anxiety related behaviors. However, precise molecular and locational description of examined populations is lacking. This publication provides a quantified regionally precise description of the expression and co-expression of six frequently examined central amygdala population markers. Most revealing, within the most commonly examined region, the posterior CeL, four of these markers are extensively co-expressed suggesting the potential for experimental redundancy. This data clarifies circuit interaction and function and will increase relevance and precision of future cell-type specific reports.

Repetitive behaviors are diagnostic for autism spectrum disorder (ASD) and commonly observed in other neurodevelopmental disorders. Currently, there are no effective pharmacological treatments for repetitive behavior in these clinical conditions. This is due to the lack of information about the specific neural circuitry that mediates the development and expression of repetitive behavior. Our previous work in mouse models has linked repetitive behavior to decreased activation of the subthalamic nucleus, a brain region in the indirect and hyperdirect pathways in the basal ganglia circuitry. The present experiments were designed to further test our hypothesis that pharmacological activation of the indirect pathway would reduce repetitive behavior. We used a combination of adenosine A1 and A2A receptor agonists that have been shown to alter the firing frequency of dorsal striatal neurons within the indirect pathway of the basal ganglia. This drug combination markedly and selectively reduced repetitive behavior in both male and female C58 mice over a six-hour period, an effect that required both A1 and A2A agonists as neither alone reduced repetitive behavior. The adenosine A1 and A2A receptor agonist combination also significantly increased the number of Fos transcripts and Fospositive cells in dorsal striatum. Fos induction was found in both direct and indirect pathway neurons suggesting that the drug combination restored the balance of activation across these complementary basal ganglia pathways. The adenosine A1 and A2A receptor agonist combination also maintained its effectiveness in reducing repetitive behavior over a 7-day period. These findings point to novel potential therapeutic targets for development of drug therapies for repetitive behavior in clinical disorders.