Inhibition of STAT-mediated cytokine responses to chemically-induced colitis prevents inflammation-associated neurobehavioral impairments.

Depression can be associated with chronic systemic inflammation, and production of peripheral proinflammatory cytokines and upregulation of the kynurenine pathway have been implicated in pathogenesis of depression. However, the mechanistic bases for these comorbidities are not yet well understood. As tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO), which convert tryptophan to kynurenine, are rate-limiting enzymes of the kynurenine pathway, we screened TDO or IDO inhibitors for effects on the production of proinflammatory cytokines in a mouse macrophage cell line. The TDO inhibitor 680C91 attenuated LPS-induced pro-inflammatory cytokines including IL-1ß and IL-6. Surprisingly, this effect was TDO-independent, as it occurred even in peritoneal macrophages from TDO knockout mice. Instead, the anti-inflammatory effects of 680C91 were mediated through the suppression of signal transducer and activator of transcription(STAT) signaling. Furthermore, 680C91 suppressed production of proinflammatory cytokines and STAT signaling in an animal model of inflammatory bowel disease. Specifically, 680C91 effectively attenuated acute phase colon cytokine responses in male mice subjected to dextran sulfate sodium (DSS)-induced colitis. Interestingly, this treatment also prevented the development of anxiodepressive-like neurobehaviors in DSS-treated mice during the recovery phase. The ability of 680C91 to prevent anxiodepressive-like behavior in response to chemically-induced colitis appeared to be due to rescue of attenuated dopamine responses in the nucleus accumbens. Thus, inhibition of STAT-mediated, but TDO-independent proinflammatory cytokines in macrophages can prevent inflammation-associated anxiety and depression. Identification of molecular mechanisms involved may facilitate the development of new treatments for gastrointestinal-neuropsychiatric comorbidity.

Subregion-Specific Regulation of Dopamine D1 Receptor Signaling in the Striatum: Implication for L-DOPA-Induced Dyskinesia.

The striatum is the main structure of the basal ganglia. The striatum receives inputs from various cortical areas, and its subregions play distinct roles in motor and emotional functions. Recently, striatal maps based on corticostriatal connectivity and striosome-matrix compartmentalization were developed, and we were able to subdivide the striatum into seven subregions. Dopaminergic modulation of the excitability of medium spiny neurons (MSNs) is critical for striatal function. In this study, we investigated the functional properties of dopamine signaling in seven subregions of the striatum from male mice. By monitoring the phosphorylation of PKA substrates including DARPP-32 in mouse striatal slices, we identified two subregions with low D1 receptor signaling: the dorsolateral portion of the intermediate/rostral part (DL-IR) and the intermediate/caudal part (IC). Low D1 receptor signaling in the two subregions was maintained by phosphodiesterase (PDE)10A and muscarinic M4 receptors. In an animal model of 6-hydroxydopamine (6-OHDA)-induced hemi-parkinsonism, D1 receptor signaling was upregulated in almost all subregions including the DL-IR, but not in the IC. When L-DOPA-induced dyskinesia (LID) was developed, D1 receptor signaling in the IC was upregulated and correlated with the severity of LID. Our results suggest that the function of the striatum is maintained through the subregion-specific regulation of dopamine D1 receptor signaling and that the aberrant activation of D1 receptor signaling in the IC is involved in LID. Future studies focusing on D1 receptor signaling in the IC of the striatum will facilitate the development of novel therapeutics for LID.SIGNIFICANCE STATEMENT Recent progress in striatal mapping based on corticostriatal connectivity and striosome-matrix compartmentalization allowed us to subdivide the striatum into seven subregions. Analyses of D1 receptor signaling in the seven subregions identified two unique subregions with low D1 receptor signaling: the dorsolateral portion of the intermediate/rostral part (DL-IR) and the intermediate/caudal part (IC). Aberrant activation of D1 receptor signaling in the IC is involved in L-DOPA-induced dyskinesia (LID). Previous studies of LID have mainly focused on the DL-IR, but not on the IC of the striatum. Future studies to clarify aberrant D1 receptor signaling in the IC are required to develop novel therapeutics for LID.

Distinct Role of Dopamine in the PFC and NAc During Exposure to Cocaine-Associated Cues.

BACKGROUND: Dopamine neurotransmission plays a critical role in reward in drug abuse and drug addiction. However, the role of dopamine in the recognition of drug-associated environmental stimuli, retrieval of drug-associated memory, and drug-seeking behaviors is not fully understood. METHODS: Roles of dopamine neurotransmission in the prefrontal cortex (PFC) and nucleus accumbens (NAc) in the cocaine-conditioned place preference (CPP) paradigm were evaluated using in vivo microdialysis. RESULTS: In mice that had acquired cocaine CPP, dopamine levels in the PFC, but not in the NAc, increased in response to cocaine-associated cues when mice were placed in the cocaine chamber of an apparatus with 2 separated chambers. The induction of the dopamine response and the development of cocaine CPP were mediated through activation of glutamate NMDA (N-methyl-D-aspartate)/AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor signaling in the PFC during conditioning. Activation of dopamine D1 or D2 receptor signaling in the PFC was required for cocaine-induced locomotion, but not for the induction of the dopamine response or the development of cocaine CPP. Interestingly, dopamine levels in the NAc increased in response to cocaine-associated cues when mice were placed at the center of an apparatus with 2 connected chambers, which requires motivated exploration associated with cocaine reward. CONCLUSIONS: Dopamine neurotransmission in the PFC is activated by the exposure to the cocaine-associated cues, whereas dopamine neurotransmission in the NAc is activated in a process of motivated exploration of cues associated with cocaine reward. Furthermore, the glutamate signaling cascade in the PFC is suggested to be a potential therapeutic target to prevent the progression of drug addiction.

Obligatory roles of dopamine D1 receptors in the dentate gyrus in antidepressant actions of a selective serotonin reuptake inhibitor, fluoxetine.

Depression is a leading cause of disability. Current pharmacological treatment of depression is insufficient, and development of improved treatments especially for treatment-resistant depression is desired. Understanding the neurobiology of antidepressant actions may lead to development of improved therapeutic approaches. Here, we demonstrate that dopamine D1 receptors in the dentate gyrus act as a pivotal mediator of antidepressant actions in mice. Chronic administration of a selective serotonin reuptake inhibitor (SSRI), fluoxetine, increases D1 receptor expression in mature granule cells in the dentate gyrus. The increased D1 receptor signaling, in turn, contributes to the actions of chronic fluoxetine treatment, such as suppression of acute stress-evoked serotonin release, stimulation of adult neurogenesis and behavioral improvement. Importantly, under severely stressed conditions, chronic administration of a D1 receptor agonist in conjunction with fluoxetine restores the efficacy of fluoxetine actions on D1 receptor expression and behavioral responses. Thus, our results suggest that stimulation of D1 receptors in the dentate gyrus is a potential adjunctive approach to improve therapeutic efficacy of SSRI antidepressants.

Long-Term Citalopram Treatment Alters the Stress Responses of the Cortical Dopamine and Noradrenaline Systems: the Role of Cortical 5-HT1A Receptors.

BACKGROUND: Cortical dopamine and noradrenaline are involved in the stress response. Citalopram, a selective serotonin reuptake inhibitor, has direct and indirect effects on the serotonergic system. Furthermore, long-term treatment with citalopram affects the dopamine and noradrenaline systems, which could contribute to the therapeutic action of antidepressants. METHODS: The effects of long-term treatment with citalopram on the responses of the dopamine and noradrenaline systems in the rat prefrontal cortex to acute handling stress were evaluated using in vivo microdialysis. RESULTS: Acute handling stress increased dopamine and noradrenaline levels in the prefrontal cortex. The dopamine and noradrenaline responses were suppressed by local infusion of a 5-HT1A receptor agonist, 7-(Dipropylamino)-5,6,7,8-tetrahydronaphthalen-1-ol;hydrobromide, into the prefrontal cortex. The dopamine response was abolished by long-term treatment with citalopram, and the abolished dopamine response was reversed by local infusion of a 5-HT1A receptor antagonist, (Z)-but-2-enedioic acid;N-[2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl]-N-pyridin-2-ylcyclohexanecarboxamide into the prefrontal cortex. On the other hand, long-term treatment with citalopram reduced the basal noradrenaline levels (approximately 40% of the controls), but not the basal dopamine levels. The noradrenaline response was maintained despite the low basal noradrenaline levels. Signaling from the 5-HT1A receptors and α2-adrenoceptors was not involved in the decrease in the basal noradrenaline levels but partially affected the noradrenaline response. CONCLUSIONS: Chronic citalopram treatment differentially suppresses the dopamine and noradrenaline systems in the prefrontal cortex, and the dopamine stress response was preferentially controlled by upregulating 5-HT1A receptor signaling. Our findings provide insight into how antidepressants modulate the dopamine and noradrenaline systems to overcome acute stress.

Dysregulation of dopamine neurotransmission in the nucleus accumbens in immobilization-induced hypersensitivity.

Cast immobilization causes sensory hypersensitivity, which is also a symptom of neuropathic pain and chronic pain. However, the mechanisms underlying immobilization-induced hypersensitivity remain unclear. The present study investigated the role of dopamine neurotransmission in the nucleus accumbens shell (NAcSh) of rats with cast immobilization-induced mechanical hypersensitivity using in vivo microdialysis. Cast immobilization of the hind limb decreased the paw withdrawal threshold (PWT). Mechanical stimulation of the cast-immobilized hind limb induced a decrease in dopamine in the NAcSh, and this decrease was associated with the upregulation of presynaptic D2-like receptors. A D2-like receptor antagonist infused into the NAcSh reversed the decrease in PWT in rats with cast immobilization, whereas a D2-like receptor agonist infused into the NAcSh induced a decrease in PWT in control rats. In addition, the expression of the D2 receptor (Drd2) mRNA in the NAcSh was increased by cast immobilization. Importantly, systemic administration of the D2-like receptor antagonist reversed the decrease in PWT in rats with cast immobilization. As dopamine levels regulated by presynaptic D2-like receptors did not correlate with the PWT, it is presumed that the D2-like receptor antagonist or agonist acts on postsynaptic D2-like receptors. These results suggest that immobilization-induced mechanical hypersensitivity is attributable to the upregulation of postsynaptic D2-like receptors in the NAc. Blockade of D2-like receptors in the NAcSh is a potential therapeutic strategy for immobilization-induced hypersensitivity.

SLITRK1-mediated noradrenergic projection suppression in the neonatal prefrontal cortex.

SLITRK1 is an obsessive-compulsive disorder spectrum-disorders-associated gene that encodes a neuronal transmembrane protein. Here we show that SLITRK1 suppresses noradrenergic projections in the neonatal prefrontal cortex, and SLITRK1 functions are impaired by SLITRK1 mutations in patients with schizophrenia (S330A, a revertant of Homo sapiens-specific residue) and bipolar disorder (A444S). Slitrk1-KO newborns exhibit abnormal vocalizations, and their prefrontal cortices show excessive noradrenergic neurites and reduced Semaphorin3A expression, which suppresses noradrenergic neurite outgrowth in vitro. Slitrk1 can bind Dynamin1 and L1 family proteins (Neurofascin and L1CAM), as well as suppress Semaphorin3A-induced endocytosis. Neurofascin-binding kinetics is altered in S330A and A444S mutations. Consistent with the increased obsessive-compulsive disorder prevalence in males in childhood, the prefrontal cortex of male Slitrk1-KO newborns show increased noradrenaline levels, and serotonergic varicosity size. This study further elucidates the role of noradrenaline in controlling the development of the obsessive-compulsive disorder-related neural circuit.

Voluntary exercise is motivated by ghrelin, possibly related to the central reward circuit.

We previously reported that voluntary exercise contributed to the amelioration of abnormal feeding behavior with a concomitant restoration of ghrelin production in a rat model of obesity, suggesting a possible relationship between exercise and appetite-regulating hormones. Ghrelin is known to be involved in the brain reward circuits via dopamine neurons related to motivational properties. We investigated the relevance of ghrelin as an initiator of voluntary exercise as well as feeding behavior. The plasma ghrelin concentration fluctuates throughout the day with its peak at the beginning of the dark period in the wild-type (WT) mice with voluntary exercise. Although predominant increases in wheel running activity were observed accordant to the peak of plasma ghrelin concentration in the WT mice, those were severely attenuated in the ghrelin-knockout (GKO) mice under either ad libitum or time-restricted feeding. A single injection of ghrelin receptor agonist brought about and reproduced a marked enhancement of wheel running activity, in contrast to no effect by the continuous administration of the same drug. Brain dopamine levels (DAs) were enhanced after food consumption in the WT mice under voluntary exercise. Although the acceleration of DAs were apparently blunted in the GKO mice, they were dramatically revived after the administration of ghrelin receptor agonist, suggesting the relevance of ghrelin in the reward circuit under voluntary exercise. These findings emphasize that the surge of ghrelin plays a crucial role in the formation of motivation for the initiation of voluntary exercise possibly related to the central dopamine system.

Long-term administration of citalopram reduces basal and stress-induced extracellular noradrenaline levels in rat brain.

RATIONALE: Panic disorders are commonly treated with selective serotonin reuptake inhibitors (SSRIs). However, the effect of SSRIs on noradrenaline systems in the brain has not been fully elucidated at the present time. OBJECTIVES: The effects of long-term administration of citalopram, an SSRI, on basal as well as stress-induced extracellular noradrenaline levels in the basolateral nucleus of the amygdala (BLA) and the locus coeruleus (LC) were determined. In addition, the responsiveness of noradrenaline transporters and alpha2-adrenoceptors were determined after long-term administration of citalopram. MATERIALS AND METHODS: Brain microdialysis was used to assess the extracellular levels of noradrenaline in conscious rats. Desipramine and clonidine were used to functionally evaluate the noradrenaline transporter and alpha2-adrenoreceptor, respectively. RESULTS: In rats treated daily for 14 days with citalopram (10 mg kg(-1) day(-1) s.c.), dialysate noradrenaline levels showed remarkable decreases in both the BLA and the LC to about 25 and 45% of controls, respectively. The stress-induced increase of noradrenaline was almost completely abolished in the BLA, but was relatively stable in the LC. The effect of local application of desipramine tended to be suppressed only in the LC. The effect of local application of clonidine was enhanced only in the BLA. CONCLUSION: The present results indicate that chronic administration of citalopram strongly decreases the extracellular levels of noradrenaline in the brain. The anti-panic effect of citalopram might be due to sensitization of the alpha2-adrenoceptors leading to suppression of the stress response through noradrenergic activity. This mechanism is specific for the BLA.

Tonic inhibition by orphanin FQ/nociceptin of noradrenaline neurotransmission in the amygdala.

The present microdialysis study investigated whether nociceptin/orphanin FQ exerts a tonic inhibition of the release of noradrenaline in the basolateral nucleus of the amygdala in awake rats. The non-peptide competitive nociceptin/orphanin FQ (N/OFQ) peptide receptor antagonist J-113397 (20 mg/kg i.p.) induced an increase in the release of noradrenaline to about 150-200%. The increase was strongly suppressed by local infusion of an endogenous N/OFQ peptide receptor agonist, nociceptin/orphanin FQ (1 microM) via retrograde microdialysis, into the basolateral nucleus of the amygdala. Local infusion of nociceptin/orphanin FQ (1 microM) itself reduced noradrenaline release in the basolateral nucleus of the amygdala to about 70% of basal levels. These results indicate that a large part of basal release of noradrenaline in the basolateral nucleus of the amygdala is under tonic inhibitory control by endogenous nociceptin/orphanin FQ through the N/OFQ peptide receptors localized within the basolateral nucleus of the amygdala.

The spontaneously hypertensive rat/Izm (SHR/Izm) shows attention deficit/hyperactivity disorder-like behaviors but without impulsive behavior: therapeutic implications of low-dose methylphenidate.

The spontaneously hypertensive rat (SHR) has been used as a genetic animal model of attention deficit/hyperactivity disorder (ADHD). SHR/Izm is derived from stroke-resistant SHR as SHR/NIH and SHR/NCrl but from 22nd to 23rd generation descendants of the SHR/NIH ancestor and therefore may show different behavioral phenotypes compared to other SHR sub-strains. In this study, ADHD-like behaviors in SHR/Izm were evaluated compared to Wistar rats. SHR/Izm showed high locomotor activity in the habituation phase in a novel environment, although locomotor activity in the initial exploratory phase was low. In a behavioral test for attention, spontaneous alternation behavior in the Y-maze test was impaired in SHR/Izm. However, impulsive behavior in the elevated-plus maze test, which is designed to detect anxiety-related behavior but also reflects impulsivity for novelty seeking, was comparable to Wistar rats. Hyperactivity and inattention, detected as ADHD-like behaviors in SHR/Izm, were ameliorated with methylphenidate at a low dose (0.05mg/kg, i.p.). Therefore, SHR/Izm represents a unique animal model of ADHD without anxiety-related impulsive behavior.

Acute effects of resveratrol to enhance cocaine-induced dopamine neurotransmission in the striatum.

Resveratrol is known as an activator of SIRT1, which leads to the deacetylation of histone and non-histone protein substrates, but also has other pharmacological profiles such as the inhibition of monoamine oxidase (MAO)-A and MAO-B. Resveratrol was previously demonstrated to potentiate the rewarding effects of chronic cocaine via activation of SIRT1. However, the role of resveratrol in cocaine responses in the acute phase remains unexplored. Therefore, we investigated the acute effects of resveratrol on cocaine-stimulated dopamine neurotransmission by analyzing protein phosphorylation in neostriatal slices. Treatment with resveratrol (50µM for 30min) enhanced cocaine-induced increases in the phosphorylation of DARPP-32 at Thr34 and GluA1 at Ser845, postsynaptic substrates for dopamine/D1 receptor/PKA signaling, and a cocaine-induced decrease in the phosphorylation of tyrosine hydroxylase at Ser40, a presynaptic substrate for dopamine/D2 receptor signaling. The inhibition of both MAO-A and MAO-B by clorgyline and pargyline, respectively, enhanced the effects of cocaine on DARPP-32 phosphorylation. The acute effect of resveratrol on cocaine-induced DARPP-32 phosphorylation was occluded with inhibition of MAO-A and MAO-B. In behavioral studies, resveratrol (40mg/kg, s.c.) enhanced the increase in locomotor activity induced by acute cocaine administration (10mg/kg, i.p.). Thus, this study provides pharmacological evidence that acute resveratrol enhances cocaine-induced dopamine neurotransmission and behavioral responses, presumably via mechanisms involving the inhibition of dopamine catabolism by MAO-A and MAO-B. Resveratrol may be useful to treat dysregulated dopamine neurotransmission, but it may enhance the risk of developing drug addiction.

Food reward-sensitive interaction of ghrelin and opioid receptor pathways in mesolimbic dopamine system.

Ghrelin is a stomach-derived orexigenic peptide. The goal of the study was to investigate the roles of mu and kappa opioid receptors in systemic ghrelin-mediated regulation of the mesolimbic dopamine system. To evaluate the interaction of systemic ghrelin with values of food reward, rats were exposed to food removal, regular food or palatable food after systemic ghrelin administration. Extracellular dopamine levels were quantified in the nucleus accumbens (NAc) and receptor-specific compounds were infused into the ventral tegmental area (VTA) using dual-probe microdialysis. Consumption of regular or palatable food without systemic ghrelin administration induced an increase in dopamine levels in the NAc via activation of mu opioid receptors in the VTA. Systemic ghrelin administration (3 nmol, i.v.) followed by no food induced a decrease in dopamine levels via activation of kappa opioid receptors in the VTA. Systemic ghrelin administration followed by consumption of regular food induced an increase in dopamine levels via preferential activation of mu opioid receptors, whereas systemic ghrelin administration followed by consumption of palatable food suppressed the increase in dopamine levels via preferential activation of kappa opioid receptors. Thus, natural food reward and systemic ghrelin activate mu and kappa opioid receptor pathways in the VTA, respectively, resulting in opposite influences on dopamine release in the NAc. Furthermore, systemic ghrelin induces switching of the dominant opioid receptor pathway for highly rewarding food from mu to kappa, resulting in suppression of the mesolimbic dopamine system. These novel findings might provide insights into the neural pathways involved in eating disorders.

Upregulation of the dorsal raphe nucleus-prefrontal cortex serotonin system by chronic treatment with escitalopram in hyposerotonergic Wistar-Kyoto rats.

Wistar-Kyoto (WKY) rats are sensitive to chronic stressors and exhibit depression-like behavior. Dorsal raphe nucleus (DRN) serotonin (5-HT) neurons projecting to the prefrontal cortex (PFC) comprise the important neurocircuitry underlying the pathophysiology of depression. To evaluate the DRN-PFC 5-HT system in WKY rats, we examined the effects of escitalopram (ESCIT) on the extracellular 5-HT level in comparison with Wistar rats using dual-probe microdialysis. The basal levels of 5-HT in the DRN, but not in the PFC, in WKY rats was reduced as low as 30% of Wistar rats. Responses of 5-HT in the DRN and PFC to ESCIT administered systemically and locally were attenuated in WKY rats. Feedback inhibition of DRN 5-HT release induced by ESCIT into the PFC was also attenuated in WKY rats. Chronic ESCIT induced upregulation of the DRN-PFC 5-HT system in WKY rats, with increases in basal 5-HT in the DRN, responsiveness to ESCIT in the DRN and PFC, and feedback inhibition, whereas downregulation of these effects was induced in Wistar rats. Thus, the WKY rat is an animal model of depression with low activity of the DRN-PFC 5HT system. The finding that chronic ESCIT upregulates the 5-HT system in hyposerotonergic WKY rats may contribute to improved understanding of mechanisms of action of antidepressants, especially in depression with 5-HT deficiency.

Regional distribution and the dynamics of n-decanoyl ghrelin, another acyl-form of ghrelin, upon fasting in rodents.

n-Decanoyl ghrelin (D-ghrelin), a member of ghrelin-derived peptides, is found in plasma and the stomach; however, there have so far been no studies describing its dynamics. A D-ghrelin-specific radioimmunoassay was established to examine the tissue distribution and the kinetics of D-ghrelin in mice. The effect of D-ghrelin on food intake was also examined and compared to n-octanoyl ghrelin (O-ghrelin). D-ghrelin was detected throughout the gastrointestinal tissue and plasma with highest level in the stomach. An immunofluorescent study revealed the co-localization of D- and O-ghrelin in the same stomach cells. Upon fasting, the levels of D-ghrelin in the stomach and plasma significantly increased, while that of O-ghrelin in the stomach declined. D-ghrelin increased the 2 h food consumption in mice as O-ghrelin does. These findings indicate that D-ghrelin is mainly produced in the stomach to work in concert with O-ghrelin. The different kinetics of D- and O-ghrelin in the stomach upon fasting implies the possibility of D-ghrelin-specific bioregulation.

Hypotension-induced dopamine release in prefrontal cortex is mediated by local glutamatergic projections at the level of nerve terminals.

In a previous study it was shown that nitroprusside-induced hypotension strongly enhances the release of dopamine (DA) in the prefrontal cortex (PFC). In the present study we have further investigated the mechanism involved in this effect. Glutamate receptor antagonists were infused into the ventral tegmental area (VTA) or PFC, while DA release was measured in the ipsilateral PFC and hypotension was applied by intravenous infusion of nitroprusside. Infusion into the VTA of neither a NMDA receptor antagonist (CPP), nor a non-NMDA antagonist (DNQX) affected the hypotension-induced increase of DA in the PFC. Intracortical infusion of CPP also failed to affect significantly, whereas local infusion of DNQX inhibited the hypotension-enhanced release of DA dose-dependently. The stimulation of DA release was relatively small in the VTA as well as in the nucleus accumbens when compared with the response in the PFC. It is concluded that DA released from mesocortical neurons can be modulated by two different mechanisms: first, by glutamate afferents to the VTA that modify the nerve-impulse flow of DA neurons; and, second, by glutamate afferents to the PFC that act at the level of the DA nerve terminals. The behaviour context (arousal or stress versus hypotension) determines which type of interaction predominates.