D1 Receptor-expressing Medium Spiny Neurons Research Articles

Melanocortin 4 receptors switch reward to aversion

The ability to recognize and avoid noxious stimuli is essential for survival. The factors that determine whether a given stimulus is considered positive or negative are complex and not fully understood. In this issue of the JCI, Klawonn and colleagues demonstrate that melanocortin 4 receptor (MC4R) signaling is critical for proper responses to negative stimuli. Mice lacking MC4R were shown to have a surprising preference for aversive stimuli compared with WT animals. Moreover, the authors provide evidence that avoidance behaviors are mediated by hypothalamic POMC neurons signaling to striatal dopamine D1 receptor-expressing medium spiny neurons. Together, these results provide important insight into the regulation of responses to aversive stimuli.

Dopamine receptors mediate strategy abandoning via modulation of a specific prelimbic cortex–nucleus accumbens pathway in mice

The ability to abandon old strategies and adopt new ones is essential for survival in a constantly changing environment. While previous studies suggest the importance of the prefrontal cortex and some subcortical areas in the generation of strategy-switching flexibility, the fine neural circuitry and receptor mechanisms involved are not fully understood. In this study, we showed that optogenetic excitation and inhibition of the prelimbic cortex-nucleus accumbens (NAc) pathway in the mouse respectively enhances and suppresses strategy-switching ability in a cross-modal spatial-egocentric task. This ability is dependent on an intact dopaminergic tone in the NAc, as local dopamine denervation impaired the performance of the animal in the switching of tasks. In addition, based on a brain-slice preparation obtained from Drd2-EGFP BAC transgenic mice, we demonstrated direct innervation of D2 receptor-expressing medium spiny neurons (D2-MSNs) in the NAc by prelimbic cortical neurons, which is under the regulation by presynaptic dopamine receptors. While presynaptic D1-type receptor activation enhances the glutamatergic transmission from the prelimbic cortex to D2-MSNs, D2-type receptor activation suppresses this synaptic connection. Furthermore, manipulation of this pathway by optogenetic activation or administration of a D1-type agonist or a D2-type antagonist could restore impaired task-switching flexibility in mice with local NAc dopamine depletion; this restoration is consistent with the effects of knocking down the expression of specific dopamine receptors in the pathway. Our results point to a critical role of a specific prelimbic cortex-NAc subpathway in mediating strategy abandoning, allowing the switching from one strategy to another in problem solving.

S36. DIFFERENTIAL ENCODING OF SENSITIZATION AND CROSS SENSITIZATION TO PSYCHOSTIMULANTS AND ANTIPSYCHOTICS IN NUCLEUS ACCUMBENS D1- AND D2- RECEPTOR EXPRESSING MEDIUM SPINY NEURONS

BackgroundNearly half of all individuals diagnosed with schizophrenia abuse addictive substances such as cocaine. Currently, the neurobiological mechanisms in patients with schizophrenia that lead to cocaine abuse are unknown. A possible explanation for the co-morbidity between schizophrenia and addiction is that the rewarding properties of cocaine reverse the diminished motivational drive caused by chronic antipsychotic regimen. Moreover, chronic antipsychotic treatment can sensitize and amplify cocaine rewarding effects and exacerbate psychoses.MethodsThe rewarding properties of cocaine are attributed to the differential effects of dopamine on D1 and D2 receptor-expressing medium spiny neurons (MSNs) in the nucleus accumbens (NAc). Using in vivo Ca2+ miniature microscopic imaging, we characterize the role of D1 and D2 MSN in mono- and a cross- sensitization paradigms. D1- and D2-Cre mice were injected with a Cre dependent calcium indicator (gCaMP6f) and implanted with a gradient index (GRIN) lens above the nucleus accumbens and calcium activity was recorded using a head mounted miniature microscope. Cocaine sensitization was measured after a classic repeated cocaine regiment and antipsychotic and psychostimulant cross-sensitization was measured by a single cocaine injection after chronic pre-treatment with haloperidol.ResultsWe found that both D1-MSN and D2-MSN populations are modulated by initial cocaine experience and further modulated during the expression of cocaine sensitization. A subpopulation of D1-MSN displayed initial activation, but reduced activity during the expression of sensitization. By contrast, the majority of D2-MSNs were suppressed by initial cocaine experience, but became active during the expression of sensitization. Furthermore, activity of D1- and D2-MSNs bidirectionally related with the observed behavioral responses to cocaine. Cross-sensitization following haloperidol treatment led to increased behavioral responses to psychostimulants. Current experiments are set out to investigate the neuronal responses of D1 and D2-MSN during cross sensitization between haloperidol and cocaine.DiscussionCocaine sensitization leads to differential neuronal responses in D1- and D2-MSN and these responses are differentially correlated with the magnitude of the sensitized behavioral response. These results reveal important new insights in the neurobiological processes in the nucleus accumbens that underlie psychostimulant sensitization and provide an important new model for studying the pharmacology of antipsychotic effects on striatal function and its potential role in increasing the susceptibility of schizophrenic patients to developing drug addiction.

Homeostatic regulation of basal ganglia circuit for flexible behavior

Behavioral inflexibility has been reported in various psychiatric disorders including drug addiction and schizophrenia. Considerable evidence has demonstrated a critical role for the basal ganglia in the flexibility of behavioral strategies. These processes are guided by the activity of two discrete neuron types, dopamine D1- or D2-receptor expressing medium spiny neurons (D1-/D2-MSNs) in the basal ganglia circuit. We used a reversible neurotransmission blocking technique to examine the role of D1- and D2-MSNs in the acquisition and reversal learning of a place discrimination task in the IntelliCage. We demonstrated that D1- and D2-MSNs do not mediate the acquisition of the task, but that suppression of activity in D2-MSNs impairs reversal learning and increased perseverative errors. Additionally, global knockout of the dopamine D2L receptor isoform produced a similar behavioral phenotype to D2-MSN-blocked mice. We also showed that D2L receptors are necessary for visual discrimination and reversal learning. These results suggest that D2L receptors and D2-MSNs have a critical role in the homeostatic regulation of basal ganglia circuit for flexible behaviors.

Fyn Signaling Is Compartmentalized to Dopamine D1 Receptor Expressing Neurons in the Dorsal Medial Striatum.

The tyrosine kinase Fyn plays an important role in synaptic plasticity, learning, and memory. Here we report that Fyn is activated in response to 15 min D1 receptor (D1R) but not D2 receptor (D2R) stimulation specifically in the dorsomedial striatum (DMS) of mice but not in the other substriatal regions, the dorsolateral striatum (DLS), and the nucleus accumbens (NAc). Once activated Fyn phosphorylates its substrate GluN2B, and we show that GluN2B is phosphorylated only in the DMS but not in the other striatal regions. Striatal neurons are divided into D1R expressing medium spiny neurons (MSNs) and D2R expressing MSNs. Thus, to explore the cell-type specificity of this signaling pathway in the DMS, we developed a Cre-dependent Flip Excision (FLEX) approach to knockdown Fyn in D1R MSNs or D2R MSNs, and proved that the D1R-dependent Fyn activation is localized to DMS D1R MSNs. Importantly, we provide evidence to suggest that the differential association of Fyn and GluN2B with the scaffolding RACK1 is due to the differential localization of Fyn in lipid rafts.Our data further suggest that the differential cholesterol content in the three striatal regions may determine the region specificity of this signaling pathway. Together, our data show that the D1R-dependent Fyn/GluN2B pathway is selectively activated in D1R expressing MSNs in the DMS, and that the brain region specificity of pathway depends on the molecular and cellular compartmentalization of Fyn and GluN2B.

Parvalbumin Interneurons of the Mouse Nucleus Accumbens are Required For Amphetamine-Induced Locomotor Sensitization and Conditioned Place Preference.

To determine the requirement for parvalbumin (PV) expressing GABAergic interneurons of the nucleus accumbens (NAc) in the behavioral adaptations induced by amphetamine (AMPH), we blocked synaptic vesicle release from these neurons using Cre-inducible viral expression of the tetanus toxin light chain in male and female PV-Cre mice. Silencing PV+ interneurons of the NAc selectively inhibited the expression of locomotor sensitization following repeated injections of AMPH and blocked AMPH-induced conditioned place preference (CPP). AMPH induced significantly more expression of the activity-dependent gene Fos in both D1 and D2 dopamine receptor-expressing medium spiny neurons (MSNs) of the NAc of PV+ interneuron silenced mice, suggesting a function for PV+ interneuron-mediated MSN inhibition in the expression of AMPH-induced locomotor sensitization and CPP. These data show a requirement for PV+ interneurons of the NAc in behavioral responses to AMPH, and they raise the possibility that modulation of PV+ interneuron function may alter the development or expression of psychostimulant-induced behavioral adaptations.

Effect of optogenetic manipulation of accumbal medium spiny neurons expressing dopamine D2 receptors in cocaine-induced behavioral sensitization.

Repetitive exposure to addictive drugs causes synaptic modification in the mesocorticolimbic dopamine (DA) system. Dopamine D1 receptors (D1R) or D2 receptors (D2R) expressed in the medium spiny neurons (MSNs) of the nucleus accumbens (NAc) play critical roles in the control of addictive behaviors. Optogenetic activation of D2R-expressing MSNs (D2R-MSNs) in the NAc previously demonstrated that these neurons play a key role in withdrawal-induced plasticity. Here, we examined the effect of optogenetic inhibition of D2R-MSNs in the NAc on cocaine-induced behavioral sensitization. Adeno-associated viral vectors encoding archaerhodopsin (ArchT) were delivered into the NAc of D2-Cre transgenic mice. Activation of ArchT produced photoinhibition of D2R-MSNs and caused disinhibition of neighboring MSNs in the NAc. However, such optogenetic silencing of D2R-MSNs in the NAc invivo affected neither the initiation nor the expression of cocaine-induced behavioral sensitization. Similarly, photoinhibition of NAc D2R-MSNs in the NAc during the drug withdrawal period did not affect the expression of cocaine-induced behavioral sensitization. More detailed analysis of the effects of optogenetic activation of D2R-MSNs suggests that D2R-MSNs in the NAc exert important modulatory effects on neighboring MSN neurons, which may control the balanced output of NAc MSNs to control addictive behaviors.

Chronic Nicotine Alters Corticostriatal Plasticity in the Striatopallidal Pathway Mediated By NR2B-Containing Silent Synapses.

Smoking is the leading cause of preventable death in the United States and success rates for quitting remain low. High relapse rates are attributed to pervasive nicotine-reinforced associative learning of incentive cues that is highly resistant to extinction. Why such learning is so persistent is poorly understood but may arise as a consequence of neuroadaptations in synaptic plasticity induced by chronic nicotine. We used whole-cell patch clamp recording to investigate the effect of chronic nicotine (cNIC) on synaptic plasticity in dopamine D2 receptor-expressing medium-spiny neurons in the indirect, striatopallidal pathway in dorsolateral striatum. Mice exposed to cNIC exhibited long-term potentiation in response to high-frequency stimulation instead of the expected depression. cNIC decreased baseline AMPA/NMDA ratio, arising from increased NMDA currents enriched in the NR2B subunit with a concomitant upregulation of NMDA-only, silent synapses. These data demonstrate that cNIC can increase silent synapses in MSNs, as observed with cocaine and opiates, and alter the regulation of corticostriatal plasticity. Prior work has characterized cocaine- and morphine-induced upregulation of silent synapses in the ventral striatum; we show it can occur in the dorsal striatum, a region associated with later stages of addiction, craving, and cue-induced relapse.

Suppression of inhibitory G protein signaling in forebrain pyramidal neurons triggers plasticity of glutamatergic neurotransmission in the nucleus accumbens core

Cocaine and other drugs of abuse trigger long-lasting adaptations in excitatory and inhibitory neurotransmission in the mesocorticolimbic system, and this plasticity has been implicated in several key facets of drug addiction. For example, glutamatergic neurotransmission mediated by AMPA receptors (AMPAR) is strengthened in medium spiny neurons (MSNs) in the NAc core and shell during withdrawal following repeated in vivo cocaine administration. Repeated cocaine administration also suppresses inhibitory signaling mediated by G protein-gated inwardly rectifying K+ (GIRK) channels in pyramidal neurons of the prelimbic cortex, an important source of glutamatergic input to the NAc core that has been implicated in cocaine-seeking and behavioral sensitization. Here, we tested the hypothesis that suppression of GIRK channel activity in forebrain pyramidal neurons can promote plasticity of glutamatergic signaling in MSNs. Using novel conditional knockout mouse lines, we report that GIRK channel ablation in forebrain pyramidal neurons is sufficient to enhance AMPAR-dependent neurotransmission in D1R-expressing MSNs in the NAc core, while also increasing motor-stimulatory responses to cocaine administration. A similar increase in AMPAR-dependent signaling was seen in both D1R- and D2R-expressing MSNs in the NAc core during withdrawal from repeated cocaine administration in normal mice. Collectively, these data are consistent with the premise that the cocaine-induced suppression of GIRK-dependent signaling in glutamatergic inputs to the NAc core contributes to some of the electrophysiological and behavioral hallmarks associated with repeated cocaine administration.

Dopaminergic treatment weakens medium spiny neuron collateral inhibition in the parkinsonian striatum.

The striatal medium spiny neurons (MSNs) are critical to both motor and cognitive functions. A potential regulator of MSN activity is the GABAergic collateral axonal input from neighboring MSNs. These collateral axon terminals are further under the regulation of presynaptic dopamine (DA) receptors that may become dysfunctional when the intense striatal DA innervation is lost in Parkinson's disease (PD). We show that DA D1 receptor-expressing MSNs (D1-MSNs) and D2 receptor-expressing MSNs (D2-MSNs) each formed high-rate, one-way collateral connections with a homotypic preference in both normal and DA-denervated mouse striatum. Furthermore, whereas the homotypic preference, one-way directionality and the basal inhibitory strength were preserved, DA inhibited GABA release at the D2-MSN→D2-MSN collateral synapse in a supersensitive manner in the DA-denervated striatum. In contrast, for D1-MSN-originated collateral connections, whereas D1 agonism facilitated D1-MSN→D1-MSN collateral inhibition in the normal striatum, this presynaptic D1R facilitation of GABA release was lost in the parkinsonian striatum. These results indicate that in the parkinsonian striatum, dopaminergic treatment can presynaptically weaken the D2-MSN→D2-MSN collateral inhibition and disinhibit the surrounding D2-MSNs, whereas the D1-MSN→D1-MSN collateral inhibition is weakened by the loss of the presynaptic D1 receptor facilitation, disinhibiting the surrounding D1-MSNs. Together, these newly discovered effects can disrupt the MSN circuits in the parkinsonian striatum and may contribute to dopaminergic treatment-induced aberrant motor and nonmotor behaviors in PD.NEW & NOTEWORTHY With the use of a large database, this study establishes that neighboring homotypic striatal spiny projection neurons have a 50% chance to form one-way collateral inhibitory connection, a substantially higher rate than previous estimates. This study also shows that dopamine denervation may alter presynaptic dopamine receptor function such that dopaminergic treatment of Parkinson's disease can weaken the surround inhibition and may reduce the contrast of the striatal outputs, potentially contributing to dopamine's profound motor and nonmotor behavioral effects.

GPR88 in A2AR Neurons Enhances Anxiety-Like Behaviors.

GPR88 is an orphan G-protein-coupled receptor highly expressed in striatal dopamine D1 (receptor) R- and D2R-expressing medium spiny neurons. This receptor is involved in activity and motor responses, and we previously showed that this receptor also regulates anxiety-like behaviors. To determine whether GPR88 in D2R-expressing neurons contributes to this emotional phenotype, we generated conditional Gpr88 knock-out mice using adenosine A2AR (A2AR)-Cre-driven recombination, and compared anxiety-related responses in both total and A2AR-Gpr88 KO mice. A2AR-Gpr88 KO mice showed a selective reduction of Gpr88 mRNA in D2R-expressing, but not D1R-expressing, neurons. These mutant mice showed increased locomotor activity and decreased anxiety-like behaviors in light/dark and elevated plus maze tests. These phenotypes were superimposable on those observed in total Gpr88 KO mice, demonstrating that the previously reported anxiogenic activity of GPR88 operates at the level of A2AR-expressing neurons. Further, A2AR-Gpr88 KO mice showed no change in novelty preference and novelty-suppressed feeding, while these responses were increased and decreased, respectively, in the total Gpr88 KO mice. Also, A2AR-Gpr88 KO mice showed intact fear conditioning, while the fear responses were decreased in total Gpr88 KO. We therefore also show for the first time that GPR88 activity regulates approach behaviors and conditional fear; however, these behaviors do not seem mediated by receptors in A2AR neurons. We conclude that Gpr88 expressed in A2AR neurons enhances ethological anxiety-like behaviors without affecting conflict anxiety and fear responses.

Nucleus accumbens dopamine D2-receptor expressing neurons control behavioral flexibility in a place discrimination task in the IntelliCage.

Considerable evidence has demonstrated a critical role for the nucleus accumbens (NAc) in the acquisition and flexibility of behavioral strategies. These processes are guided by the activity of two discrete neuron types, dopamine D1- or D2-receptor expressing medium spiny neurons (D1-/D2-MSNs). Here we used the IntelliCage, an automated group-housing experimental cage apparatus, in combination with a reversible neurotransmission blocking technique to examine the role of NAc D1- and D2-MSNs in the acquisition and reversal learning of a place discrimination task. We demonstrated that NAc D1- and D2-MSNs do not mediate the acquisition of the task, but that suppression of activity in D2-MSNs impairs reversal learning and increased perseverative errors. Additionally, global knockout of the dopamine D2L receptor isoform produced a similar behavioral phenotype to D2-MSN-blocked mice. These results suggest that D2L receptors and NAc D2-MSNs act to suppress the influence of previously correct behavioral strategies allowing transfer of behavioral control to new strategies.

Phosphoproteomics of the Dopamine Pathway Enables Discovery of Rap1 Activation as a Reward Signal In Vivo

Dopamine (DA) type 1 receptor (D1R) signaling in the striatum presumably regulates neuronal excitability and reward-related behaviors through PKA. However, whether and how D1Rs and PKA regulate neuronal excitability and behavior remain largely unknown. Here, we developed a phosphoproteomic analysis method to identify known and novel PKA substrates downstream of the D1R and obtained more than 100 candidate substrates, including Rap1 GEF (Rasgrp2). We found that PKA phosphorylation of Rasgrp2 activated its guanine nucleotide-exchange activity on Rap1. Cocaine exposure activated Rap1 in the nucleus accumbens in mice. The expression of constitutively active PKA or Rap1 in accumbal D1R-expressing medium spiny neurons (D1R-MSNs) enhanced neuronal firing rates and behavioral responses to cocaine exposure through MAPK. Knockout of Rap1 in the accumbal D1R-MSNs was sufficient to decrease these phenotypes. These findings demonstrate a novel DA-PKA-Rap1-MAPK intracellular signaling mechanism in D1R-MSNs that increases neuronal excitability to enhance reward-related behaviors.

Glutamate Receptors within the Mesolimbic Dopamine System Mediate Alcohol Relapse Behavior

Glutamatergic input within the mesolimbic dopamine (DA) pathway plays a critical role in the development of addictive behavior. Although this is well established for some drugs of abuse, it is not known whether glutamate receptors within the mesolimbic system are involved in mediating the addictive properties of chronic alcohol use. Here we evaluated the contribution of mesolimbic NMDARs and AMPARs in mediating alcohol-seeking responses induced by environmental stimuli and relapse behavior using four inducible mutant mouse lines lacking the glutamate receptor genes Grin1 or Gria1 in either DA transporter (DAT) or D1R-expressing neurons. We first demonstrate the lack of GluN1 or GluA1 in either DAT- or D1R-expressing neurons in our mutant mouse lines by colocalization studies. We then show that GluN1 and GluA1 receptor subunits within these neuronal subpopulations mediate the alcohol deprivation effect, while having no impact on context- plus cue-induced reinstatement of alcohol-seeking behavior. We further validated these results pharmacologically by demonstrating similar reductions in the alcohol deprivation effect after infusion of the NMDAR antagonist memantine into the nucleus accumbens and ventral tegmental area of control mice, and a rescue of the mutant phenotype via pharmacological potentiation of AMPAR activity using aniracetam. In conclusion, dopamine neurons as well as D1R-expressing medium spiny neurons and their glutamatergic inputs via NMDARs and AMPARs act in concert to influence relapse responses. These results provide a neuroanatomical and molecular substrate for relapse behavior and emphasize the importance of glutamatergic drugs in modulating relapse behavior. Here we provide genetic and pharmacological evidence that glutamate receptors within the mesolimbic dopamine system play an essential role in alcohol relapse. Using various inducible and site-specific transgenic mouse models and pharmacological validation experiments, we show that critical subunits of NMDARs and AMPARs expressed either in dopamine neurons or in dopamine receptor D1-containing neurons play an important role in the alcohol deprivation effect (the increase in alcohol intake after a period of abstinence) while having no impact on context- plus cue-induced reinstatement of alcohol-seeking responses. Medications targeting glutamatergic neurotransmission by selective inactivation of these glutamate receptors might have therapeutic efficacy.

Dual Nitrergic/Cholinergic Control of Short-Term Plasticity of Corticostriatal Inputs to Striatal Projection Neurons.

The ability of nitric oxide and acetylcholine to modulate the short-term plasticity of corticostriatal inputs was investigated using current-clamp recordings in BAC mouse brain slices. Glutamatergic responses were evoked by stimulation of corpus callosum in D1 and D2 dopamine receptor-expressing medium spiny neurons (D1-MSNs and D2-MSN, respectively). Paired-pulse stimulation (50 ms intervals) evoked depressing or facilitating responses in subgroups of both D1-MSNs and D2 MSNs. In both neuronal types, glutamatergic responses of cells that displayed paired-pulse depression were not significantly affected by the nitric oxide donor S-nitroso-N-acetylpenicillamine (SNAP; 100 μM). Conversely, in D1-MSNs and D2-MSNs that displayed paired-pulse facilitation, SNAP did not affect the first evoked response, but significantly reduced the amplitude of the second evoked EPSP, converting paired-pulse facilitation into paired-pulse depression. SNAP also strongly excited cholinergic interneurons and increased their cortical glutamatergic responses acting through a presynaptic mechanism. The effects of SNAP on glutamatergic response of D1-MSNs and D2-MSN were mediated by acetylcholine. The broad-spectrum muscarinic receptor antagonist atropine (25 μM) did not affect paired-pulse ratios and did not prevent the effects of SNAP. Conversely, the broad-spectrum nicotinic receptor antagonist tubocurarine (10 μM) fully mimicked and occluded the effects of SNAP. We concluded that phasic acetylcholine release mediates feedforward facilitation in MSNs through activation of nicotinic receptors on glutamatergic terminals and that nitric oxide, while increasing cholinergic interneurons’ firing, functionally impairs their ability to modulate glutamatergic inputs of MSNs. These results show that nitrergic and cholinergic transmission control the short-term plasticity of glutamatergic inputs in the striatum and reveal a novel cellular mechanism underlying paired-pulse facilitation in this area.

Neural circuits: Consumption control.

Dopamine D1 receptor-expressing medium spiny neurons in the medial nucleus accumbens shell that project to the lateral hypothalamus mediate rapid control of feeding behaviour in mice.

Loss of Striatonigral GABAergic Presynaptic Inhibition Enables Motor Sensitization in Parkinsonian Mice

Degeneration of dopamine (DA) neurons in Parkinson's disease (PD) causes hypokinesia, but DA replacement therapy can elicit exaggerated voluntary and involuntary behaviors that have been attributed to enhanced DA receptor sensitivity in striatal projection neurons. Here we reveal that in hemiparkinsonian mice, striatal D1 receptor-expressing medium spiny neurons (MSNs) directly projecting to the substantia nigra reticulata (SNr) lose tonic presynaptic inhibition by GABAB receptors. The absence of presynaptic GABAB response potentiates evoked GABA release from MSN efferents to the SNr and drives motor sensitization. This alternative mechanism of sensitization suggests a synaptic target for PD pharmacotherapy.

Reacquisition of cocaine conditioned place preference and its inhibition by previous social interaction: Neurochemical and electrophysiological correlates in the nucleus accumbens corridor.

The nucleus accumbens has long been a major target for studies on the rewarding effects of drugs of abuse or physiological reinforcers, whereas the brain regions medial of the medial accumbens shell have received less attention. We investigated if counterconditioning with dyadic (i.e., one-to-one) social interaction, a strong inhibitor of the subsequent reacquisition of cocaine conditioned place preference (CPP), differentially modulates the activity of the diverse brain regions oriented along a mediolateral corridor reaching from the interhemispheric sulcus to the anterior commissure, i.e., the nucleus of the vertical limb of the diagonal band, the medial septal nucleus, the major island of Calleja, the intermediate part of the lateral septal nucleus, and the medial accumbens shell and core. EGR1 activation was predominantly found in dynorphin-labeled cells, i.e., presumably D1 receptor-expressing medium spiny neurons (D1-MSNs), with D2-MSNs (immunolabeled with an anti-DRD2 antibody) being less affected. Cholinergic interneurons or GABAergic interneurons positive for parvalbumin, neuropeptide Y or calretinin were not involved in these CPP-related EGR1 changes. Glial cells did not show any EGR1 expression either. Cocaine conditioning increased the spike frequency of neurons in the septal nuclei, whereas social interaction conditioning increased the spike frequency in the nucleus accumbens compared to saline control animals. In addition, social interaction conditioning decreased the amount of active neuron clusters in the nucleus accumbens. The present findings could be of relevance for the therapy of impaired social interaction in substance use disorders, depression, psychosis, and autism spectrum disorders.

Reacquisition of cocaine conditioned place preference and its inhibition by previous social interaction preferentially affect D1-medium spiny neurons in the accumbens corridor.

We investigated if counterconditioning with dyadic (i.e., one-to-one) social interaction, a strong inhibitor of the subsequent reacquisition of cocaine conditioned place preference (CPP), differentially modulates the activity of the diverse brain regions oriented along a mediolateral corridor reaching from the interhemispheric sulcus to the anterior commissure, i.e., the nucleus of the vertical limb of the diagonal band, the medial septal nucleus, the major island of Calleja, the intermediate part of the lateral septal nucleus, and the medial accumbens shell and core. We also investigated the involvement of the lateral accumbens core and the dorsal caudate putamen. The anterior cingulate 1 (Cg1) region served as a negative control. Contrary to our expectations, we found that all regions of the accumbens corridor showed increased expression of the early growth response protein 1 (EGR1, Zif268) in rats 2 h after reacquisition of CPP for cocaine after a history of cocaine CPP acquisition and extinction. Previous counterconditioning with dyadic social interaction inhibited both the reacquisition of cocaine CPP and the activation of the whole accumbens corridor. EGR1 activation was predominantly found in dynorphin-labeled cells, i.e., presumably D1 receptor-expressing medium spiny neurons (D1-MSNs), with D2-MSNs (immunolabeled with an anti-DRD2 antibody) being less affected. Cholinergic interneurons or GABAergic interneurons positive for parvalbumin, neuropeptide Y or calretinin were not involved in these CPP-related EGR1 changes. Glial cells did not show any EGR1 expression either. The present findings could be of relevance for the therapy of impaired social interaction in substance use disorders, depression, psychosis, and autism spectrum disorders.

Tonic Inhibition of Accumbal Spiny Neurons by Extrasynaptic α4βδ GABAAReceptors Modulates the Actions of Psychostimulants

Within the nucleus accumbens (NAc), synaptic GABAA receptors (GABAARs) mediate phasic inhibition of medium spiny neurons (MSNs) and influence behavioral responses to cocaine. We demonstrate that both dopamine D1- and D2-receptor-expressing MSNs (D-MSNs) additionally harbor extrasynaptic GABAARs incorporating α4, β, and δ subunits that mediate tonic inhibition, thereby influencing neuronal excitability. Both the selective δ-GABAAR agonist THIP and DS2, a selective positive allosteric modulator, greatly increased the tonic current of all MSNs from wild-type (WT), but not from δ(-/-) or α4(-/-) mice. Coupling dopamine and tonic inhibition, the acute activation of D1 receptors (by a selective agonist or indirectly by amphetamine) greatly enhanced tonic inhibition in D1-MSNs but not D2-MSNs. In contrast, prolonged D2 receptor activation modestly reduced the tonic conductance of D2-MSNs. Behaviorally, WT and constitutive α4(-/-) mice did not differ in their expression of cocaine-conditioned place preference (CPP). Importantly, however, mice with the α4 deletion specific to D1-expressing neurons (α4(D1-/-)) showed increased CPP. Furthermore, THIP administered systemically or directly into the NAc of WT, but not α4(-/-) or α4(D1-/-) mice, blocked cocaine enhancement of CPP. In comparison, α4(D2-/-) mice exhibited normal CPP, but no cocaine enhancement. In conclusion, dopamine modulation of GABAergic tonic inhibition of D1- and D2-MSNs provides an intrinsic mechanism to differentially affect their excitability in response to psychostimulants and thereby influence their ability to potentiate conditioned reward. Therefore, α4βδ GABAARs may represent a viable target for the development of novel therapeutics to better understand and influence addictive behaviors.