mGluR5 stimulates gliotransmission in the nucleus accumbens

Although metabotropic glutamate receptor 5 (mGluR5) is essential for cocaine self-administration and drug-seeking behavior, there is limited knowledge of the cellular actions of this receptor in the nucleus accumbens (NAc). Although mGluR5 has the potential to regulate neurons directly, recent studies have shown the importance of mGluR5 in regulating Ca(2+) signaling in astrocytes and, as a consequence, the Ca(2+)-dependent release of excitatory transmitters from these glia. In this study, we demonstrate that activation of mGluR5 induces Ca(2+) oscillations in NAc astrocytes with the correlated appearance of NMDA receptor-dependent slow inward currents detected in medium spiny neurons (MSNs). Photolysis of caged Ca(2+) loaded specifically into astrocytes evoked slow inward currents demonstrating that Ca(2+) elevations in astrocytes are responsible for these excitatory events. Pharmacological evaluation of these glial-evoked NMDA currents shows that they are mediated by NR2B-containing NMDA receptors, whereas synaptic NMDA receptors rely on NR2A-containing receptors. Stimulation of glutamatergic afferents activates mGluR5-dependent astrocytic Ca(2+) oscillations and gliotransmission that is sustained for minutes beyond the initial stimulus. Because gliotransmission is mediated by NMDA receptors, depolarized membrane potentials exhibited during up-states augment excitation provided by gliotransmission, which drives bursts of MSN action potentials. Because the predominant mGluR5-dependent action of glutamatergic afferents is to cause the sustained activation of astrocytes, which in turn excite MSNs through extrasynaptic NMDA receptors, our results raise the potential for gliotransmission being involved in prolonged mGluR5-dependent adaptation in the NAc.     

Timing-dependent limbic-motor synaptic integration in the nucleus accumbens

The nucleus accumbens is a brain region in which limbic and motor inputs converge. How these information modalities shape accumbens output is not clearly understood. Here, we report that synaptic inputs from the prefrontal cortex and limbic structures interact differently depending on their timing. Coincident inputs may result in enhancing information flow through the nucleus accumbens. Responses to asynchronous inputs are affected by their relative order of arrival, with limbic inputs allowing subsequent prefrontal responses, and prefrontal inputs dampening limbic responses. These mechanisms allow for both coincidence detection and input selection in this integrative brain region.     

Dopamine-transporter density in nucleus accumbens of type-1 alcoholics

We have shown that alcoholic patients have a lower number of dopamine transporters in the nucleus accumbens, which mediates the rewarding effects of addictive drugs. Thus, certain dopaminergic agents may be beneficial in the treatment of alcohol withdrawal and in the long-term treatment of alcoholism with selective use.     

Cholinergic interneurons in the nucleus accumbens regulate depression-like behavior

A large number of studies have demonstrated that the nucleus accumbens (NAC) is a critical site in the neuronal circuits controlling reward responses, motivation, and mood, but the neuronal cell type(s) underlying these processes are not yet known. Identification of the neuronal cell types that regulate depression-like states will guide us in understanding the biological basis of mood and its regulation by diseases like major depressive disorder. Taking advantage of recent findings demonstrating that the serotonin receptor chaperone, p11, is an important molecular regulator of depression-like states, here we identify cholinergic interneurons (CINs) as a primary site of action for p11 in the NAC. Depression-like behavior is observed in mice after decrease of p11 levels in NAC CINs. This phenotype is recapitulated by silencing neuronal transmission in these cells, demonstrating that accumbal cholinergic neuronal activity regulates depression-like behaviors and suggesting that accumbal CIN activity is crucial for the regulation of mood and motivation.     

Cannabinoid receptor 1-expressing neurons in the nucleus accumbens

Endocannabinoid signaling critically regulates emotional and motivational states via activation of cannabinoid receptor 1 (CB1) in the brain. The nucleus accumbens (NAc) functions to gate emotional and motivational responses. Although expression of CB1 in the NAc is low, manipulation of CB1 signaling within the NAc triggers robust emotional/motivational alterations related to drug addiction and other psychiatric disorders, and these effects cannot be exclusively attributed to CB1 located at afferents to the NAc. Rather, CB1-expressing neurons in the NAc, although sparse, appear to be critical for emotional and motivational responses. However, the cellular properties of these neurons remain largely unknown. Here, we generated a knock-in mouse line in which CB1-expressing neurons expressed the fluorescent protein td-Tomato (tdT). Using these mice, we demonstrated that tdT-positive neurons within the NAc were exclusively fast-spiking interneurons (FSIs). These FSIs were electrically coupled with each other, and thus may help synchronize populations/ensembles of NAc neurons. CB1-expressing FSIs also form GABAergic synapses on adjacent medium spiny neurons (MSNs), providing feed-forward inhibition of NAc output. Furthermore, the membrane excitability of tdT-positive FSIs in the NAc was up-regulated after withdrawal from cocaine exposure, an effect that might increase FSI-to-MSN inhibition. Taken together with our previous findings that the membrane excitability of NAc MSNs is decreased during cocaine withdrawal, the present findings suggest that the basal functional output of the NAc is inhibited during cocaine withdrawal by multiple mechanisms. As such, CB1-expressing FSIs are targeted by cocaine exposure to influence the overall functional output of the NAc.Cannabinoid receptor type 1 (CB1) has been extensively implicated in a variety of psychological and psychiatric disorders, including drug addiction (1, 2). Recent studies suggest that CB1 within the nucleus accumbens (NAc), a key component of the brain reward circuit, plays a particularly important role in the development and maintenance of cocaine-induced behavioral alterations (3). Compared with the extensive expression of CB1 in the striatum, the mRNA and protein levels of CB1 within the NAc are sparse, leading to the notion that CB1 at afferent terminals projecting to the NAc are largely responsible for intra-NAc, CB1-dependent, cocaine-induced behaviors (4–6). However, a recent study primarily targeting CB1-expressing neurons demonstrates that inhibiting the expression of CB1 within the NAc antagonizes cocaine-induced reward responses (7). This and other results (8) suggest that CB1-expressing neurons in the NAc, although sparse, are critical for cellular and behavioral alterations induced by cocaine and other drugs of abuse.To examine these putative CB1-expressing neurons within the NAc, we generated a knock-in mouse line in which CB1-expressing neurons expressed the fluorescent protein td-Tomato (tdT). Our results show that tdT-positive neurons within the NAc were exclusively fast-spiking interneurons (FSIs). These FSIs were not only electrically connected with each other but exerted extensive inhibitory control on nearby medium spiny neurons (MSNs), the principal neurons in the NAc, via monosynaptic connections. Furthermore, the membrane excitability of these neurons became significantly up-regulated throughout short- and long-term withdrawal from repeated exposure to cocaine. These results suggest that CB1-expressing FSIs within the NAc are neural substrates targeted by cocaine exposure and influence the overall functional output of the NAc.     

Ethanol exposure decreases glutamate uptake in the nucleus accumbens

BACKGROUND: An increased level of extracellular glutamate is a key neurochemical feature associated with ethanol exposure and withdrawal. METHODS: In the current study, extracellular levels of glutamate and glutamate transport in the nucleus accumbens were assayed 24 hr after repeated ethanol exposure (1 g/kg ip daily for 7 days) with use of in vivo no-net-flux microdialysis and in vitro [(3)H]glutamate uptake, respectively. RESULTS: Microdialysis revealed higher extracellular glutamate concentrations in the nucleus accumbens of rats that were given ethanol. The increase in basal extracellular glutamate levels was accounted for in part by a decrease in the in vivo probe recovery of glutamate. Moreover, an in vitro accumbens slice preparation measuring [(3)H]glutamate uptake revealed that Na(+)-dependent [(3)H]glutamate uptake was significantly reduced 24 hr after 7 days of repeated ethanol exposure. The ethanol-induced deficit in glutamate uptake was not associated with decreased total tissue levels of the transporters GLAST or GLT1. The in vivo and in vitro ethanol-induced changes in glutamate levels and uptake returned to control levels 14 days after discontinuing 7 days of repeated ethanol exposure. CONCLUSIONS: These results suggest that the previously reported increases in extracellular glutamate induced by ethanol exposure may be due in part to deficits in glutamate transport.     

Undernutrition upregulates fumarate hydratase in the rat nucleus accumbens

Previous comparative studies of fumarate hydratase (FH) protein density revealed that the enzyme was overexpressed in the striatum of rodents that are less influenced by rewarding stimuli, from cocaine to food. Therefore, we recently proposed FH as a potential striatal biomarker of brain reward deficiency and addiction vulnerability. This work has been focused to investigate FH activity in the Nucleus Accumbens (NAc) of undernourished rats, taking into account that malnutrition has been related to increased responsiveness to food and drug reward. To this end, we have studied adult female Wistar rats severely food restricted from the 16th day of intrauterine life until adulthood. Animals were sacrificed to dissect the NAc and obtain mitochondrial and cytosolic fractions after homogenisation and centrifugation. FH activity was measured by conversion of malate to fumarate, and protein levels were compared by Western blot analysis when fractions showed differences in activity. Undernutrition did not change cytosolic FH activity but led to a marked increase of mitochondrial FH activity (72 %) and protein content (50 %) in the NAc. This change was in the opposite direction that one would predict if it was related to addiction vulnerability of some kind, but strongly suggests that mitochondrial FH needs to be at some optimal level for normal reward responsiveness.     

Muscarinic regulation of dopamine and glutamate transmission in the nucleus accumbens

Cholinergic transmission in the striatum functions as a key modulator of dopamine (DA) transmission and synaptic plasticity, both of which are required for reward and motor learning. Acetylcholine (ACh) can elicit striatal DA release through activation of nicotinic ACh receptors (nAChRs) on DA axonal projections. However, it remains controversial how muscarinic ACh receptors (mAChRs) modulate striatal DA release, with studies reporting both potentiation and depression of striatal DA transmission by mAChR agonists. This study investigates the mAChR-mediated regulation of release from three types of midbrain neurons that project to striatum: DA, DA/glutamate, and glutamate neurons. We found that M₅ mAChRs potentiate DA and glutamate release only from DA and DA/glutamate projections from the midbrain. We also show that M₂/M₄ mAChRs depress the nAChR-dependent mechanism of DA release in the striatum. These results suggest that M₅ receptors on DA neuron terminals enhance DA release, whereas M₂/M₄ autoreceptors on cholinergic terminals inhibit ACh release and subsequent nAChR-dependent DA release. Our findings clarify the mechanisms of mAChR-dependent modulation of DA and glutamate transmission in the striatum.Increases in dopamine (DA) concentration in the nucleus accumbens (NAc) are required for reward seeking, motivation, and motor control (1). DA is released from the axonal projections of midbrain DA neurons to the NAc and the dorsal striatum, where it regulates synaptic plasticity to influence striatal microcircuitry (2). Traditionally, the firing of DA neurons was thought to be the main factor controlling DA release in the striatum. However, DA release can also be evoked by activation of neurons within the striatum. Synchronous activation of striatal cholinergic interneurons (CINs) is sufficient to trigger DA release in the NAc (3) and dorsal striatum (4) through activation of nicotinic acetylcholine receptors (nAChRs) on DA axonal projections. Thus, nAChRs exert powerful control over DA transmission in the striatum.Muscarinic acetylcholine receptors (mAChRs) are also known to modulate striatal DA transmission. Earlier studies show that muscarinic agonists potentiate DA efflux in the striatum (5–8). However, other studies reported that mAChR agonists depress DA transients measured with fast-scan cyclic voltammetry (FSCV) and evoked by electrical stimulation in the striatum (9–11). Thus, the role of mAChRs in modulating striatal DA transmission remains controversial and unclear. In this study, we address this issue by systematically investigating the effect of muscarinic agonists and endogenous acetylcholine (ACh) on DA transmission evoked by classical methods (electrical stimulation), as well as the selective stimulation of DA neuron axonal projections (optogenetic stimulation), using electrochemical and electrophysiological recordings.Different mAChR subtypes have been detected in the striatum (12, 13). G_i/o-coupled M₂ and M₄ receptors are mainly found on presynaptic terminals of CINs (14, 15), and M₂ on glutamatergic terminals (13), where they inhibit neurotransmitter release. G_q/11-coupled M₁ receptors are expressed in spiny projection neurons and have been implicated in endocannabinoid production (16). Interestingly, the G_q/11-coupled M₅ receptors are the only mAChRs subtype found in midbrain DA neurons (12, 17). Although direct anatomic evidence for the presence of M₅ receptors in DA axons is lacking, functional studies implicate M₅ mAChRs in either facilitating (8, 10) or depressing (11) DA release. In the past, studies in this field have been hampered by the lack of selective muscarinic agonists and antagonists (18). To overcome this limitation, this study takes advantage of mutant mouse lines carrying genetic deletions of selected mAChR subtypes (19) to determine their specific role in modulating DA transmission in the striatum. Our findings are highly relevant for the development of novel mAChR subtype-selective drugs to modulate dopaminergic signaling for therapeutic purposes.     

Endogenous cannabinoids mediate long-term synaptic depression in the nucleus accumbens

Do endocannabinoids (eCBs) participate in long-term synaptic plasticity in the brain? Using pharmacological approaches and genetically altered mice, we show that stimulation of prelimbic cortex afferents at naturally occurring frequencies causes a long-term depression of nucleus accumbens glutamatergic synapses mediated by eCB release and presynaptic CB1 receptors. Translation of glutamate synaptic transmission into eCB retrograde signaling involved metabotropic glutamate receptors and postsynaptic intracellular Ca(2+) stores. These findings unveil the role of the eCB system in activity-dependent long-term synaptic plasticity and identify a mechanism by which marijuana can alter synaptic functions in the endogenous brain reward system.     

Glycine receptors regulate dopamine release in the rat nucleus accumbens

BACKGROUND: The mesolimbic dopamine (DA) system seems to be centrally involved in regulating reward-related behavior and consequently has been implicated in addictive processes, such as alcoholism and drug addiction. This DA system has also been implicated in psychosis and in regulating hedonia/anhedonia, important components of mania and depression. Given the potentially great importance of the mesolimbic DA system for several psychiatric disorders, it is of major interest to delineate the mechanisms and dynamics underlying DA regulation and release. Recently strychnine-sensitive glycine receptors (GlyR) have attracted some interest in this matter. METHODS: Western blot and in vivo microdialysis (couplied to high-pressure liquid chromatography with electrochemical detection), as well as reversed microdialysis, in awake, freely moving, adult male Wistar rats. RESULTS: Here we demonstrate by means of Western blot that alpha GlyR subunit proteins are expressed in the rat nucleus accumbens (nAc), a major target of the mesolimbic DA system. We further show that reversed microdialysis of the competitive GlyR antagonist strychnine into the nAc concentration-dependently (2-200 microM) and in a reversible manner decreases accumbal extracellular DA levels. Conversely, reversed microdialysis of the agonist glycine increases accumbal DA levels in some rats but not others. The strychnine-induced depression of the accumbal DA levels is antagonized by simultaneous local perfusion of glycine. CONCLUSIONS: The present results indicate that GlyRs in the nAc are tonically activated and of importance for regulating extracellular DA levels. The possibility of pharmacologically interfering with GlyRs to combat psychiatric disorders, in which the mesolimbic DA system is implicated, such as alcoholism, drug addiction, and psychosis, should be explored.     

Coregulation of ethanol discrimination by the nucleus accumbens and amygdala

BACKGROUND: Activation of GABA(A) receptors in the amygdala or nucleus accumbens produces discriminative stimulus effects that substitute fully for those of systemically administered ethanol. This study was conducted to determine if GABA(A) receptors in the amygdala and nucleus accumbens interactively modulate ethanol discrimination. METHODS: Male Long-Evans rats were trained to discriminate between intraperitoneal injections of ethanol (1 g/kg) and saline on a 2-lever drug discrimination task. The rats were then surgically implanted with bilateral injection cannulae aimed at the nucleus accumbens and the amygdala. RESULTS: Infusion of the GABA(A) agonist muscimol in the nucleus accumbens resulted in full substitution for systemically administered ethanol. Concurrent infusion of the GABA(A) antagonist bicuculline in the amygdala shifted the muscimol substitution curve in the nucleus accumbens 10-fold to the right. CONCLUSIONS: These results indicate that blockade of GABA(A) receptors in the amygdala significantly reduces the potency of the GABA(A) agonist in the nucleus accumbens. This suggests that the ethanol-like stimulus effects of GABA(A) receptor activation in the nucleus accumbens are modulated by GABA(A) receptor activity in the amygdala. These data support the hypothesis that the addictive stimulus properties of alcohol are mediated by GABAergic transmission in a neural circuit involving the amygdala and nucleus accumbens.     

Increased sensitivity to cocaine by cholinergic cell ablation in nucleus accumbens

Chronic exposure to cocaine causes long-lasting behavioral changes associated with cocaine reinforcement and addiction. An important neural substrate for cocaine addiction is the nucleus accumbens (NAc), which receives dopaminergic input from the ventral tegmental area. Although the neural circuit of the NAc is controlled by several other neurotransmitters, their involvement in cocaine addiction remains elusive. In this investigation, we ablated cholinergic interneurons from the adult NAc with immunotoxin-mediated cell targeting and examined the role of acetylcholine transmitter in adaptive behavioral changes associated with cocaine reinforcement and addiction. Acute exposure to cocaine induced abnormal rotation in unilaterally cholinergic cell-eliminated mice. This abnormal turning was enhanced by repeated exposure of cocaine. In bilaterally cholinergic cell-eliminated mice, chronic cocaine administration induced a prominent and progressive increase in locomotor activity. Moreover, these mice showed robust conditioned place preference with a lower dose of cocaine, compared with wild-type littermates. This investigation demonstrates that acetylcholine in the NAc plays a key role in both acute and chronic actions of cocaine.     

Acetylcholine enhancement in the nucleus accumbens prevents addictive behaviors of cocaine and morphine

Drug addiction poses serious social, medical, and economic problems, but effective treatments for drug addiction are still limited. Cocaine and morphine elevate dopamine levels in the nucleus accumbens (NAc), and the overwhelming actions of dopamine are implicated in reinforcement and addiction of abusive drugs. In our previous studies, we reported the regulatory role of acetylcholine (ACh) in the NAc function by selectively ablating the NAc cholinergic neurons with use of immunotoxin-mediated cell targeting. These studies indicated that ACh and dopamine acted convergently but oppositely on the NAc circuit and that cholinergic cell ablation enhanced long-lasting behavioral changes of cocaine addiction. In this investigation, we showed that immunotoxin-mediated ablation of the NAc cholinergic neurons enhanced not only the sensitivity to morphine in conditioned place preference but also negative reinforcement of morphine withdrawal in conditioned place aversion. Remarkably, acetylcholinesterase (AChE) inhibitors that act on the brain AChE suppressed both cocaine- and morphine-induced conditioned place preference and blocked the induction and persistence of cocaine-evoked hyperlocomotion. Importantly, this inhibition was abolished by ablation of the NAc cholinergic neurons. These results demonstrate that centrally active AChE inhibitors prevent long-lasting behavioral abnormalities associated with cocaine and morphine addictions by potentiating the actions of ACh released from the NAc cholinergic neurons. Centrally active AChE inhibitors could thus be approached as novel and potential therapeutic agents for drug addiction.     

Changes in dopamine transporter and c-Fos expression in the nucleus accumbens of alcohol-tolerant rats

BACKGROUND: We have shown that neurochemical functions of 5-HT3 receptors in regulating dopamine (DA) release in the nucleus accumbens (ACC) after alcohol exposure compensate for the dysfunction of serotonergic activity to restore the original properties in processing alcohol tolerance, and that the development of alcohol dependence may be mediated by ACC 5-HT3 receptors. In the present study, the effects of chronic alcohol consumption on the functions of the dopamine transporter (DAT) and the expression of c-Fos proteins were investigated using in vivo brain microdialysis and immunocytochemistry. METHODS: Perfusion of cocaine and 1-(2-Bis-(4-fluorophenyl) methoxy) ethyl)-4-(3-phenylpropyl) piperizine (GBR 12909) through the microdialysis probe membrane increased the extracellular levels of DA in ACC of alcohol-treated rats that had developed alcohol tolerance by drinking 10% EtOH for 30 days. RESULTS: The magnitudes of DA reuptake or DAT inhibitors, cocaine, and GBR 12909 that induced DA availability in the ACC were significantly higher in alcohol-treated rats than in controls. When compared with control rats, the alcohol-treated rats exhibited higher levels of DA and its metabolite, DOPAC, in the ACC. Increased expression of the c-Fos-like protein was found in the ACC of alcohol-treated rats. These results show that (1) chronic alcohol consumption desensitizes or decreases the DAT of DA terminals in the ACC and that (2) EtOH causes cellular hyperexcitability of ACC dopaminergic neurons with increased Fos expression during alcohol tolerance. CONCLUSION: The findings suggested that an abnormality of the dopaminergic neurons in the ACC that are involved with DAT dysfunction is associated with the development of alcohol tolerance.     

Selective filtering of excitatory inputs to nucleus accumbens by dopamine and serotonin

The detailed mechanisms by which dopamine (DA) and serotonin (5-HT) act in the nucleus accumbens (NAc) to influence motivated behaviors in distinct ways remain largely unknown. Here, we examined whether DA and 5-HT selectively modulate excitatory synaptic transmission in NAc medium spiny neurons in an input-specific manner. DA reduced excitatory postsynaptic currents (EPSCs) generated by paraventricular thalamus (PVT) inputs but not by ventral hippocampus (vHip), basolateral amygdala (BLA), or medial prefrontal cortex (mPFC) inputs. In contrast, 5-HT reduced EPSCs generated by inputs from all areas except the mPFC. Release of endogenous DA and 5-HT by methamphetamine (METH) and (±)3,4-methylenedioxymethamphetamine (MDMA), respectively, recapitulated these input-specific synaptic effects. Optogenetic inhibition of PVT inputs enhanced cocaine-conditioned place preference, whereas mPFC input inhibition reduced the enhancement of sociability elicited by MDMA. These findings suggest that the distinct, input-specific filtering of excitatory inputs in the NAc by DA and 5-HT contribute to their discrete behavioral effects.

The nucleus accumbens (NAc), a major node of classic mesolimbic reward circuitry, plays a critical role in a variety of adaptive and pathological motivated behaviors by integrating information carried by inputs from a broad range of brain areas with distinct, yet overlapping functions (1–6). Output from the NAc is provided by medium spiny neurons (MSNs), the activity of which strongly depends on excitatory inputs from these brain areas, most prominently the ventral hippocampus (vHip), periventricular thalamus (PVT), basolateral amygdala (BLA), and medial prefrontal cortex (mPFC) (3, 7–11). The NAc is also a behaviorally important target for two of the brain’s major neuromodulatory systems, dopamine (DA) and serotonin (5-HT) (1, 5, 6, 12–14). DA release in the NAc, whether caused by drugs of abuse or optogenetic stimulation, is powerfully reinforcing and plays a critical role in shaping operant responses (1, 4–6, 15–17). In contrast, unlike DA release, release of 5-HT in the NAc, generated either pharmacologically or optogenetically, is not acutely reinforcing but can powerfully influence sociability (18, 19).The robust differences in the behavioral consequences of DA and 5-HT release in the NAc suggest that these neuromodulators must influence MSN activity in, perhaps profoundly, different ways. Yet little is known about the detailed mechanisms by which these neuromodulators accomplish this task. Because of the importance of excitatory input in controlling MSN activity and the fact that both DA and 5-HT are well established to modulate excitatory synaptic transmission in the NAc (18, 20–23), we hypothesized that an important mechanism by which these neuromodulators might distinctly influence MSN activity is by differentially filtering incoming information from major input structures. Specifically, we hypothesized that DA and 5-HT would depress excitatory synaptic transmission in distinct, input-specific manners. Because of methodological limitations prior to the advent of optogenetics, virtually all previous work examining DA and 5-HT modulation of excitatory transmission in the NAc used bulk electrical stimulation of unknown inputs.Consistent with our hypothesis, exogenously applied DA and 5-HT, as well as release of endogenous DA and 5-HT, depressed excitatory synaptic transmission in distinct, input-specific manners. Input-specific optogenetic inhibition of excitatory inputs to the NAc revealed input-specific effects on conditioned place preference and sociability assays, which are affected by NAc release of DA and 5-HT, respectively. Together, these results provide evidence that the input-specific filtering of excitatory input from distinct brain regions contributes to the behavioral effects of DA and 5-HT release in the NAc and provides a foundation for further work elucidating the neural mechanisms by which modulation of NAc activity influences motivated behaviors.     

Targeting presynaptic H3 heteroreceptor in nucleus accumbens to improve anxiety and obsessive-compulsive-like behaviors

Anxiety commonly co‐occurs with obsessive-compulsive disorder (OCD). Both of them are closely related to stress. However, the shared neurobiological substrates and therapeutic targets remain unclear. Here we report an amelioration of both anxiety and OCD via the histamine presynaptic H3 heteroreceptor on glutamatergic afferent terminals from the prelimbic prefrontal cortex (PrL) to the nucleus accumbens (NAc) core, a vital node in the limbic loop. The NAc core receives direct hypothalamic histaminergic projections, and optogenetic activation of hypothalamic NAc core histaminergic afferents selectively suppresses glutamatergic rather than GABAergic synaptic transmission in the NAc core via the H3 receptor and thus produces an anxiolytic effect and improves anxiety- and obsessive-compulsive-like behaviors induced by restraint stress. Although the H3 receptor is expressed in glutamatergic afferent terminals from the PrL, basolateral amygdala (BLA), and ventral hippocampus (vHipp), rather than the thalamus, only the PrL– and not BLA– and vHipp–NAc core glutamatergic pathways among the glutamatergic afferent inputs to the NAc core is responsible for co-occurrence of anxiety- and obsessive-compulsive-like behaviors. Furthermore, activation of the H3 receptor ameliorates anxiety and obsessive-compulsive-like behaviors induced by optogenetic excitation of the PrL–NAc glutamatergic afferents. These results demonstrate a common mechanism regulating anxiety- and obsessive-compulsive-like behaviors and provide insight into the clinical treatment strategy for OCD with comorbid anxiety by targeting the histamine H3 receptor in the NAc core.

Anxiety disorders and obsessive-compulsive disorder (OCD) are disabling psychiatric conditions and the major contributors to global burden of nonfatal illness (1). OCD is characterized by recurrent thoughts (obsessions) and/or repetitive behaviors (compulsions) that are aimed at reducing the anxiety caused by obsessions (2, 3), indicating a close correlation between anxiety and OCD. Indeed, anxiety disorders have been reported epidemiologically as the most frequent comorbid conditions with OCD (3, 4). Therefore, common pathologies may be present in anxiety disorders and OCD, and elucidation of the shared neural substrates will lead to greater insight into their pathophysiology and treatment.The nucleus accumbens (NAc) is a main component of the ventral striatum and a pivotal node in limbic basal ganglia loop, whose dysfunction may result in psychiatric diseases such as anxiety and OCD (5, 6). Accumulating experimental and clinical evidence indicates that the NAc, particularly the core compartment, holds a key position in motivation, emotion, and cognition and is strongly implicated in the psychopathology and treatment of anxiety and OCD. It has been reported that trait anxiety and OCD risk are positively correlated with the volume of NAc (7, 8). Functional neuroimaging reveals that the NAc activation correlates positively with the severity of human anxiety and obsessive-compulsive symptoms in OCD patients (9, 10). More importantly, deep brain stimulation (DBS) targeting the NAc core has been found to improve obsessive-compulsive symptoms and decrease ratings of anxiety in patients suffering from treatment-resistant OCD or depression (11, 12). Therefore, NAc core may be a potential common neural substrate for the clinical and neuropathological overlap between anxiety and OCD.The NAc core receives dense glutamatergic projections from the limbic system, including the prefrontal cortex, basolateral amygdala (BLA), and ventral hippocampus (vHipp), and integrates cognitive and affective information to instigate motivational approach behaviors (13, 14). In addition, the NAc core is regulated by various neuromodulators, such as orexin, serotonin, and histamine, from several brain regions (15–17). Among them, central histamine is synthesized and released by the histaminergic neurons restrictedly concentrated in the tuberomammillary nucleus (TMN) of the hypothalamus and serves as a general modulator for whole-brain activity via the mediation of histamine H1 to H4 receptors (18, 19). Accordingly, the aberrant histamine signaling is closely associated with sleep, motor, cognitive, and psychiatric conditions (18, 20, 21). In the clinic, drugs targeting the presynaptic H3 receptor have been used for prescribed treatment of various psychiatric and neurologic disorders (22). Interestingly, a high density of the H3 receptor has been found in NAc (23, 24). Therefore, in the present study, we create a transgenic rat strain expressing Cre recombinase in histidine decarboxylase (HDC, the histamine-synthesizing enzyme) neurons and employ anterograde axonal tract tracings, whole-cell patch clamp recordings, optogenetic and chemogenetic manipulation, and behavioral tests to explore the role of hypothalamic histaminergic afferents and the H3 receptor in the NAc core in regulation of anxiety and obsessive-compulsive-like behaviors. We find that optogenetic activation of hypothalamic TMN–NAc core histaminergic projections produces an anxiolytic effect and ameliorates obsessive-compulsive-like behaviors induced by restraint stress, which is due to H3 receptor–mediated suppression of glutamatergic transmission in a common prelimbic prefrontal cortex (PrL)–NAc core pathway.     

Hampered long-term depression and thin spine loss in the nucleus accumbens of ethanol-dependent rats

Alcoholism involves long-term cognitive deficits, including memory impairment, resulting in substantial cost to society. Neuronal refinement and stabilization are hypothesized to confer resilience to poor decision making and addictive-like behaviors, such as excessive ethanol drinking and dependence. Accordingly, structural abnormalities are likely to contribute to synaptic dysfunctions that occur from suddenly ceasing the use of alcohol after chronic ingestion. Here we show that ethanol-dependent rats display a loss of dendritic spines in medium spiny neurons of the nucleus accumbens (Nacc) shell, accompanied by a reduction of tyrosine hydroxylase immunostaining and postsynaptic density 95-positive elements. Further analysis indicates that “long thin” but not “mushroom” spines are selectively affected. In addition, patch-clamp experiments from Nacc slices reveal that long-term depression (LTD) formation is hampered, with parallel changes in field potential recordings and reductions in NMDA-mediated synaptic currents. These changes are restricted to the withdrawal phase of ethanol dependence, suggesting their relevance in the genesis of signs and/or symptoms affecting ethanol withdrawal and thus the whole addictive cycle. Overall, these results highlight the key role of dynamic alterations in dendritic spines and their presynaptic afferents in the evolution of alcohol dependence. Furthermore, they suggest that the selective loss of long thin spines together with a reduced NMDA receptor function may affect learning. Disruption of this LTD could contribute to the rigid emotional and motivational state observed in alcohol dependence.Alcohol addiction is a major public health problem in the Western world. In the United States alone, about 15% of adults have an alcohol-related disorder at some point in their life, and alcohol abuse costs the economy over $220 billion per y in medical care and productivity loss (1). A general consensus has emerged on drug addiction as a substance-induced, aberrant form of neural plasticity (2, 3). The nucleus accumbens (Nacc) plays a central role in the neural circuits that are responsible for goal-directed behaviors (4, 5) and in addictive states. Its activity is heavily modulated by glutamate- (GLU) and dopamine- (DA) containing projections that originate in cortical and limbic regions and converge on a common postsynaptic target: the medium spiny neuron (MSN). Furthermore, DA modulates GLU inputs to Nacc neurons (6, 7), both by directly influencing synaptic transmission and by modulating voltage-dependent conductances (8). Accordingly, interactions between DA and GLU are involved in drug-induced locomotor stimulation and addiction (9, 10) and may represent useful potential therapeutic targets (11, 12). In the distal portion of the dendrites of MSNs a significant subpopulation of spines shows a particular synaptic architecture, called the “striatal microcircuit” or “synaptic triad” (13, 14), which is characterized by a double, discrete, and reciprocal interaction between DA and GLU afferents: The former establishes synaptic contact on the spine neck, whereas the latter reaches the head (13). This classical, widely accepted picture has been integrated with the coexistence of DA and GLU on the same neurons (15), but because this phenomenon appears to regress with growth in vitro (16) and its role is unclear at present (17), in the present study GLU and DA will be considered as originating from cortex and ventral tegmental area (VTA), respectively.At present, little information is available concerning the effects produced by ethanol withdrawal in dependent rats (18), although a selective increase in the density of mushroom-type spines following chronic intermittent ethanol intake has recently been reported on the basal dendrites of layer V neurons of the rodent prefrontal cortex (19). In addition, reduced expression of tyrosine hydroxylase (TH) has been demonstrated in the ventral striatum of rats maintained on a chronic ethanol-containing diet (20), and a decrease of neurofilament protein immunoreactivity in the VTA (21) has been reported. Thus, in the present work, we sought to investigate possible alterations produced by ethanol withdrawal on mesocorticolimbic transmission by exploring critical elements whose presence is strictly correlated with DAergic and GLUergic function, respectively: TH- and dopamine transporter (DAT)-positive fibers and postsynaptic density 95 (PSD-95). Spine density, morphology, and morphometry of MSNs in the Nacc shell were also investigated to obtain structural insights into pre- and postsynaptic elements of the triad simultaneously. Although considered impossible until recently (22), we have developed a new method (23) that allows visualizing the finest morphological details of spinous neurons (Golgi-Cox staining) together with the immunofluorescent neuronal elements under study. By exploiting this novel approach, we are able to visualize (in the same slice) spine morphology, TH- and DAT-positive fibers, and PSD-95–positive elements to gather information on DA and GLU transmission. Notably, because recent work suggests (24, 25) a potential relationship between spine shape, synaptic function, and morphological rearrangements of the spines as forms of developmental or experience-dependent plasticity (26), we performed patch-clamp experiments in Nacc shell slices obtained from ethanol-withdrawn rats to evaluate whether long-term depression (LTD) formation and its underlying synaptic currents are modified by experimental conditions.