Neurotransmitter regulation of dopamine neurons in the ventral tegmental area

Differential Roles of Key Brain Regions: Ventral Tegmental Area, Locus Coeruleus, Dorsal Raphe, Nucleus Accumbens, Caudate Nucleus, and Prefrontal Cortex in Regulating Response to Methylphenidate: Insights from Neuronal and Behavioral Studies in Freely Behaving Rats

A total of 3102 neurons were recorded before and following acute and chronic methylphenidate (MPD) administration. Acute MPD exposure elicits mainly increases in neuronal and behavioral activity in dose-response characteristics. The response to chronic MPD exposure, as compared to acute 0.6, 2.5, or 10.0 mg/kg MPD administration, elicits electrophysiological and behavioral sensitization in some animals and electrophysiological and behavioral tolerance in others when the neuronal recording evaluations were performed based on the animals' behavioral responses, or amount of locomotor activity, to chronic MPD exposure. The majority of neurons recorded from those expressing behavioral sensitization responded to chronic MPD with further increases in firing rate as compared to the initial MPD responses. The majority of neurons recorded from animals expressing behavioral tolerance responded to chronic MPD with decreases in their firing rate as compared to the initial MPD exposures. Each of the six brain areas studied-the ventral tegmental area, locus coeruleus, dorsal raphe, nucleus accumbens, prefrontal cortex, and caudate nucleus (VTA, LC, DR, NAc, PFC, and CN)-responds significantly (p < 0.001) differently to MPD, suggesting that each one of the above brain areas exhibits different roles in the response to MPD. Moreover, this study demonstrates that it is essential to evaluate neuronal activity responses to psychostimulants based on the animals' behavioral responses to acute and chronic effects of the drug from several brain areas simultaneously to obtain accurate information on each area's role in response to the drug.

Characterization of the Neurochemical and Behavioral Effects of the Phenethylamine 2-Cl-4,5-MDMA in Adolescent and Adult Male Rats

BACKGROUND

The proliferation of novel psychoactive substances (NPS) in the drug market raises concerns about uncertainty on their pharmacological profile and the health hazard linked to their use. Within the category of synthetic stimulant NPS, the phenethylamine 2-Cl-4,5-methylenedioxymethamphetamine (2-Cl-4,5-MDMA) has been linked to severe intoxication requiring hospitalization. Thereby, the characterization of its pharmacological profile is urgently warranted.

METHODS

By in vivo brain microdialysis in adolescent and adult male rats we investigated the effects of 2-Cl-4,5-MDMA on dopamine (DA) and serotonin (5-HT) neurotransmission in two brain areas critical for the motivational and rewarding properties of drugs, the nucleus accumbens (NAc) shell and the medial prefrontal cortex (mPFC). Moreover, we evaluated the locomotor and stereotyped activity induced by 2-Cl-4,5-MDMA and the emission of 50-kHz ultrasonic vocalizations (USVs) to characterize its affective properties.

RESULTS

2-Cl-4,5-MDMA increased dialysate DA and 5-HT in a dose-, brain area-, and age-dependent manner. Notably, 2-Cl-4,5-MDMA more markedly increased dialysate DA in the NAc shell and mPFC of adult than adolescent rats, while the opposite was observed on dialysate 5-HT in the NAc shell, with adolescent rats being more responsive. Furthermore, 2-Cl-4,5-MDMA stimulated locomotion and stereotyped activity in both adolescent and adult rats, although to a greater extent in adolescents. Finally, 2-Cl-4,5-MDMA did not stimulate the emission of 50-kHz USVs.

CONCLUSIONS

This is the first pharmacological characterization of 2-Cl-4,5-MDMA demonstrating that its neurochemical and behavioral effects may differ between adolescence and adulthood. These preclinical data could help understanding the central effects of 2-Cl-4,5-MDMA by increasing awareness on possible health damage in users.

Neural Mechanisms of Neuro-Rehabilitation Using Transcranial Direct Current Stimulation (tDCS) over the Front-Polar Area

Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation (NIBS) technique that applies a weak current to the scalp to modulate neuronal excitability by stimulating the cerebral cortex. The technique can produce either somatic depolarization (anodal stimulation) or somatic hyperpolarization (cathodal stimulation), based on the polarity of the current used by noninvasively stimulating the cerebral cortex with a weak current from the scalp, making it a NIBS technique that can modulate neuronal excitability. Thus, tDCS has emerged as a hopeful clinical neuro-rehabilitation treatment strategy. This method has a broad range of potential uses in rehabilitation medicine for neurodegenerative diseases, including Parkinson's disease (PD). The present paper reviews the efficacy of tDCS over the front-polar area (FPA) in healthy subjects, as well as patients with PD, where tDCS is mainly applied to the primary motor cortex (M1 area). Multiple evidence lines indicate that the FPA plays a part in motor learning. Furthermore, recent studies have reported that tDCS applied over the FPA can improve motor functions in both healthy adults and PD patients. We argue that the application of tDCS to the FPA promotes motor skill learning through its effects on the M1 area and midbrain dopamine neurons. Additionally, we will review other unique outcomes of tDCS over the FPA, such as effects on persistence and motivation, and discuss their underlying neural mechanisms. These findings support the claim that the FPA could emerge as a new key brain region for tDCS in neuro-rehabilitation.

A structural connectivity atlas of limbic brainstem nuclei

Background

Understanding the structural connectivity of key brainstem nuclei with limbic cortical regions is essential to the development of therapeutic neuromodulation for depression, chronic pain, addiction, anxiety and movement disorders. Several brainstem nuclei have been identified as the primary central nervous system (CNS) source of important monoaminergic ascending fibers including the noradrenergic locus coeruleus, serotonergic dorsal raphe nucleus, and dopaminergic ventral tegmental area. However, due to practical challenges to their study, there is limited data regarding their in vivo anatomic connectivity in humans.

Objective

To evaluate the structural connectivity of the following brainstem nuclei with limbic cortical areas: locus coeruleus, ventral tegmental area, periaqueductal grey, dorsal raphe nucleus, and nucleus tractus solitarius. Additionally, to develop a group average atlas of these limbic brainstem structures to facilitate future analyses.

Methods

Each nucleus was manually masked from 197 Human Connectome Project (HCP) structural MRI images using FSL software. Probabilistic tractography was performed using FSL's FMRIB Diffusion Toolbox. Connectivity with limbic cortical regions was calculated and compared between brainstem nuclei. Results were aggregated to produce a freely available MNI structural atlas of limbic brainstem structures.

Results

A general trend was observed for a high probability of connectivity to the amygdala, hippocampus and DLPFC with relatively lower connectivity to the orbitofrontal cortex, NAc, hippocampus and insula. The locus coeruleus and nucleus tractus solitarius demonstrated significantly greater connectivity to the DLPFC than amygdala while the periaqueductal grey, dorsal raphe nucleus, and ventral tegmental area did not demonstrate a significant difference between these two structures.

Conclusion

Monoaminergic and other modulatory nuclei in the brainstem project widely to cortical limbic regions. We describe the structural connectivity across the several key brainstem nuclei theorized to influence emotion, reward, and cognitive functions. An increased understanding of the anatomic basis of the brainstem's role in emotion and other reward-related processing will support targeted neuromodulatary therapies aimed at alleviating the symptoms of neuropsychiatric disorders.

Organisation of enkephalin inputs and outputs of the central nucleus of the amygdala in mice

The central nucleus of the amygdala (CeA) is a key hub integrating sensory inputs and modulating behavioural outputs. The CeA is a complex structure with discrete subdivisions, high peptidergic heterogeneity and broad CNS afferent and efferent projections. While several neuropeptide systems within the CeA have been examined in detail, less is known about CeA preproenkephalin (ppENK) cells. Here, we used a recently developed transgenic Penk-Cre mouse line to advance our understanding of the efferent and afferent connectivity of ppENK in the CeA. First, to determine the fidelity of Cre expression in Penk-Cre transgenic mice, we conducted RNAscope in the CeA of Penk-Cre mice. Our analysis revealed that 96.6% of CeA Cre+ neurons co-expressed pENK mRNA, and 99.7% of CeA pENK+ neurons co-expressed Cre mRNA, indicating faithful recapitulation of Cre expression in CeA ppENK-expressing cells, supporting the fidelity of the Penk-Cre reporter mouse. Anterograde tracing of CeA^Penk cells showed strong efferent projections to the extended amygdala, midbrain and hindbrain PBN and NTS. Retrograde tracing of Penk afferents to the CeA were more restricted, with primary innervation originating within the amygdala complex and bed nucleus of the stria terminalis, and minor innervation from the parabrachial nucleus and nucleus of the solitary tract. Together, our data provide a comprehensive map of ENKergic efferent and afferent connectivity of the CeA in Penk-Cre mice. Further, we highlight both the utility and limitations of the Penk-Cre mice to study the function of CeA, PBN and NTS ppENK cells.

Modafinil Administration to Preadolescent Rat Impairs Non-Selective Attention, Frontal Cortex D2 Expression and Mesolimbic GABA Levels

The misuse of psychostimulants is an increasing behavior among young people, highlighting in some countries the abuse of modafinil (MOD) as a neuropotentiator. However, several clinical trials are investigating MOD as an alternative pharmacological treatment for attentional deficit and hyperactivity disorder (ADHD) in children and adolescents. On the other hand, the early use of psychostimulants and the misdiagnosis rates in ADHD make it crucial to investigate the brain effects of this type of drug in young healthy individuals. The aim of this work was to evaluate the effects of chronic MOD treatment on neurochemicals (γ-aminobutyric acid and glutamate), dopamine receptor 2 (D₂) expression and behavior (non-selective attention "NSA") in the mesocorticolimbic system of young healthy Sprague-Dawley rats. Preadolescent male rats were injected with MOD (75 mg/kg, i.p.) or a vehicle for 14 days (from postnatal day 22 to 35). At postnatal day 36, we measured the GLU and GABA contents and their extracellular levels in the nucleus accumbens (NAc). In addition, the GLU and GABA contents were measured in the ventral tegmental area (VTA) and D₂ protein levels in the prefrontal cortex (PFC). Chronic use of MOD during adolescence induces behavioral and neurochemical changes associated with the mesocorticolimbic system, such as a reduction in PFC D₂ expression, VTA GABA levels and NSA. These results contribute to the understanding of the neurological effects of chronic MOD use on a young healthy brain.

Influence of Selective Dopamine Agonist Ropinirole on Conditioned Place Preference and Somatic Signs of Morphine Withdrawal in Rats

The underlying mechanism of dependence and rewarding effects of morphine is imperative to understand. The primary aim of this study was to investigate whether ropinirole D2/3 agonist affects the rewarding and reinforcing properties of morphine-induced conditioned place preference (CPP) and withdrawal syndromes in rats. On day one, the animals were randomly divided to conduct the pre-test. The morphine (10 mg/kg, i.p.) and/or saline was administered on alternate days in an 8-day CPP session. On day 10, 15 min prior to the post-conditioning test (expression), a single dose of ropinirole (1, 2, and 5 mg/kg, i.p.) was given to rats. In extinction session, ropinirole was injected daily, and CPP was extinguished by repeated testing, with intervals of 3 days. Finally, reinstatement was assessed by administering ropinirole (1, 2, and 5 mg/kg) 15 min before the morphine injection. Morphine dependence was developed by administering increasing doses of morphine (10–50 mg/kg, i.p.). To assess withdrawal symptoms, ropinirole (1, 2, and 5 mg/kg) was injected 15 min before naloxone (2 mg/kg, s.c.) administration. The present study confirms that ropinirole attenuates expression and reinstatement of CPP, while it precipitates the extinction of morphine-induced CPP. Naloxone-precipitated morphine withdrawal symptoms, including wet dog shakes and weight loss, were attenuated although jumping was increased by a single ropinirole injection. Thus, ropinirole was influential in attenuating expression, reducing drug seeking and weakening reinstatement via the dopaminergic system. These findings show that ropinirole might affect neuro-adaptive changes related to dependence.

Oxytocin and the social facilitation of placebo effects

Significant clinical improvement is often observed in patients who receive placebo treatment in randomized double-blind placebo-controlled trials. While a proportion of this "improvement" reflects experimental design limitations (e.g., reliance on subjective outcomes, unbalanced groups, reporting biases), some of it reflects genuine improvement corroborated by physiological change. Converging evidence across diverse medical conditions suggests that clinically-relevant benefits from placebo treatment are associated with the activation of brain reward circuits. In parallel, evidence has accumulated showing that such benefits are facilitated by clinicians that demonstrate warmth and proficiency during interactions with patients. Here, we integrate research on these neural and social aspects of placebo effects with evidence linking oxytocin and social reward to advance a neurobiological account for the social facilitation of placebo effects. This account frames oxytocin as a key mediator of treatment success across a wide-spectrum of interventions that increase social connectedness, thereby providing a biological basis for assessing this fundamental non-specific element of medical care.

Adolescent rats respond differently to methylphenidate as compared to adult rats- concomitant VTA neuronal and behavioral Recordings

Methylphenidate (MPD) is the most widely prescribed psychostimulant used in adolescents and adults to treat attention-deficit/hyperactivity disorder (ADHD). The recreational use of MPD is becoming more prevalent because of its ability to improve cognitive enhancement. The ventral tegmental area (VTA) of the brain is highly associated with reward, cognition and addiction to drugs including psychostimulants like MPD. The VTA neuronal activity was recorded alongside the horizontal behavioral activity from freely behaving non-anesthetized rats. Four adolescent and four adult groups were treated with either saline, 0.6, 2.5 or 10.0 mg/kg MPD. In both adolescent and adult animals, the animals responded to MPD in a dose-dependent manner, such that as the dose of MPD increased, more animals and more VTA unit responded to the drug. The same doses of MPD elicited in some animals behavioral and neuronal sensitization and in other animals behavioral and neuronal tolerance. In the 0.6 and 10.0 mg/kg MPD dose groups there were significant differences between the age groups for how many animals expressed behavioral sensitization and behavioral tolerance to chronic MPD exposure. Additionally, the animal's behavioral response to MPD by excitation or attenuation of activity did not always correlate to the VTA neuronal response, and the age group with significantly higher behavioral responses did not always correlate to the age group with significantly higher VTA neuronal responses for a given MPD dose. These findings differ from similar studies recorded from the prefrontal cortex (PFC), which exhibited behavioral responses continuously directly correlated to PFC responses for increasing MPD doses. This demonstrates that unlike other areas of the brain, there is not a direct relationship between VTA firing and behavioral activity, suggesting that there is input or modulation of this area from elsewhere in the brain. Further investigation is needed to clearly understand the relationship between VTA firing rates and behavioral responses to different MPD doses, especially given the significant differences in response between young and adult animals and the increasing use of the drug in adolescent populations.

Cannabinoids, reward processing, and psychosis

BACKGROUND

Evidence suggests that an overlap exists between the neurobiology of psychotic disorders and the effects of cannabinoids on neurocognitive and neurochemical substrates involved in reward processing.

AIMS

We investigate whether the psychotomimetic effects of delta-9-tetrahydrocannabinol (THC) and the antipsychotic potential of cannabidiol (CBD) are underpinned by their effects on the reward system and dopamine.

METHODS

This narrative review focuses on the overlap between altered dopamine signalling and reward processing induced by cannabinoids, pre-clinically and in humans. A systematic search was conducted of acute cannabinoid drug-challenge studies using neuroimaging in healthy subjects and those with psychosis RESULTS: There is evidence of increased striatal presynaptic dopamine synthesis and release in psychosis, as well as abnormal engagement of the striatum during reward processing. Although, acute THC challenges have elicited a modest effect on striatal dopamine, cannabis users generally indicate impaired presynaptic dopaminergic function. Functional MRI studies have identified that a single dose of THC may modulate regions involved in reward and salience processing such as the striatum, midbrain, insular, and anterior cingulate, with some effects correlating with the severity of THC-induced psychotic symptoms. CBD may modulate brain regions involved in reward/salience processing in an opposite direction to that of THC.

CONCLUSIONS

There is evidence to suggest modulation of reward processing and its neural substrates by THC and CBD. Whether such effects underlie the psychotomimetic/antipsychotic effects of these cannabinoids remains unclear. Future research should address these unanswered questions to understand the relationship between endocannabinoid dysfunction, reward processing abnormalities, and psychosis.

Pharmacological and behavioral divergence of ketamine enantiomers: implications for abuse liability

Ketamine, a racemic mixture of (S)-ketamine and (R)-ketamine enantiomers, has been used as an anesthetic, analgesic and more recently, as an antidepressant. However, ketamine has known abuse liability (the tendency of a drug to be used in non-medical situations due to its psychoactive effects), which raises concerns for its therapeutic use. (S)-ketamine was recently approved by the United States' FDA for treatment-resistant depression. Recent studies showed that (R)-ketamine has greater efficacy than (S)-ketamine in preclinical models of depression, but its clinical antidepressant efficacy has not been established. The behavioral effects of racemic ketamine have been studied extensively in preclinical models predictive of abuse liability in humans (self-administration and conditioned place preference [CPP]). In contrast, the behavioral effects of each enantiomer in these models are unknown. We show here that in the intravenous drug self-administration model, the gold standard procedure to assess potential abuse liability of drugs in humans, rats self-administered (S)-ketamine but not (R)-ketamine. Subanesthetic, antidepressant-like doses of (S)-ketamine, but not of (R)-ketamine, induced locomotor activity (in an opioid receptor-dependent manner), induced psychomotor sensitization, induced CPP in mice, and selectively increased metabolic activity and dopamine tone in medial prefrontal cortex (mPFC) of rats. Pharmacological screening across thousands of human proteins and at biological targets known to interact with ketamine yielded divergent binding and functional enantiomer profiles, including selective mu and kappa opioid receptor activation by (S)-ketamine in mPFC. Our results demonstrate divergence in the pharmacological, functional, and behavioral effects of ketamine enantiomers, and suggest that racemic ketamine's abuse liability in humans is primarily due to the pharmacological effects of its (S)-enantiomer.

A Parkinson's disease patient displaying increased neuromelanin-sensitive areas in the substantia nigra after rehabilitation with tDCS: a case report

Previous studies have reported that transcranial direct current stimulation (tDCS) of the frontal polar area (FPA) ameliorated motor disability in patients with Parkinson's disease (PD). Here we report changes in neuromelanin (NM) imaging of dopaminergic neurons before and after rehabilitation combined with anodal tDCS over the FPA for 2 weeks in a PD patient. After the intervention, the patient showed clinically meaningful improvements while the NM-sensitive area in the SN increased by 18.8%. This case study is the first report of NM imaging of the SN in a PD patient who received tDCS.Abbreviations FPA: front polar area; PD: Parkinson's disease; NM: neuromelanin; DCI: DOPA decarboxylase inhibitor; STEF: simple test for evaluating hand function; TUG: timed up and go test; TMT: trail-making test; SN: substantia nigra; NM-MRI: neuromelanin magnetic resonance imaging; MCID: the minimal clinically important difference; SNpc: substantia nigra pars compacta; VTA: ventral tegmental area; LC: locus coeruleus; PFC: prefrontal cortex; M1: primary motor cortex; MDS: Movement Disorder Society; MIBG: ¹²³I-metaiodobenzylguanidine; SBR: specific binding ratio; SPECT: single-photon emission computed tomography; DAT: dopamine transporter; NIBS: noninvasive brain stimulation; tDCS: transcranial direct current stimulation; MAOB: monoamine oxidase B; DCI: decarboxylase inhibitor; repetitive transcranial magnetic stimulation: rTMS; diffusion tensor imaging: DTI; arterial spin labeling: ASL.

Binge eating and psychostimulant addiction

Many of the various factors, characteristics, and variables involved in the addictive process can determine an individual’s vulnerability to develop drug addiction. Hedonic eating, based on pleasure rather than energy needs, modulates the same reward circuits, as do drugs of abuse. According to the last report of the World Health Organization, the worldwide obesity rate has more than doubled since 1980, reaching especially critical levels in children and young people, who are overexposed to high-fat, high-sugar, energy-dense foods. Over the past few decades, there has been an increase in the number of studies focused on how eating disorders can lead to the development of drug addiction and on the comorbidity that exists between the two disorders. Herein, we review the most recent research on the subject, focusing especially on animal models of binge eating disorders and drug addiction. The complex profile of patients with substance use and binge eating disorders requires an integrated response to dually diagnosed patients. Nutritional patterns should be considered an important variable in the treatment of substance use disorders, and future studies need to focus on specific treatments and interventions in individuals who show a special vulnerability to shift from one addiction to the other.

Cocaine- and amphetamine-regulated transcript peptide- and dopamine-containing systems interact in the ventral tegmental area of the zebra finch, Taeniopygia guttata, during dynamic changes in energy status

The mesolimbic dopamine (DA)-pathway regulates food-reward, feeding-related behaviour and energy balance. Evidence underscores the importance of feeding-related neuropeptides in modulating activity of these DA neurons. The neuropeptide, CART, a crucial regulator of energy balance, modulates DA-release, and influences the activity of ventral tegmental area (VTA) DAergic neurons in the mammalian brain. Whether CART- and DA-containing systems interact at the level of VTA to regulate energy balance, however, is poorly understood. We explored the interaction between CART- and DA-containing systems in midbrain of the zebra finch, Taeniopygia guttata, an interesting model to study dynamic changes in energy balance due to higher BMR/daytime body temperature, and rapid responsiveness of the feeding-related neuropeptides to changes in energy state. Further, its midbrain DA-neurons share similarities with those in mammals. In the midbrain, tyrosine hydroxylase-immunoreactive (TH-i) neurons were seen in the substantia nigra (SN) and VTA [anterior (VTAa), mid (VTAm) and caudal (VTAc)]; those in VTA were smaller. In the VTA, CART-immunoreactive (CART-i)-fibers densely innervated TH-i neurons, and both CART-immunoreactivity (CART-ir) and TH-immunoreactivity (TH-ir) responded to energy status-dependent changes. Compared to fed and fasted birds, refeeding dramatically enhanced TH-ir and the percentage of TH-i neurons co-expressing FOS in the VTA. Increased prepro-CART-mRNA, CART-ir and a transient appearance of CART-i neurons was observed in VTAa of fasted, but not fed birds. To test the functional interaction between CART- and DA-containing systems, ex-vivo superfused midbrain-slices were treated with CART-peptide and changes in TH-ir analysed. Compared to control tissues, CART-treatment increased TH-ir in VTA but not SN. We propose that CART is a potential regulator of VTA DA-neurons and energy balance in T. guttata.

Mesolimbic opioid-dopamine interaction is disrupted in obesity but recovered by weight loss following bariatric surgery

Obesity is a growing burden to health and the economy worldwide. Obesity is associated with central µ-opioid receptor (MOR) downregulation and disruption of the interaction between MOR and dopamine D₂ receptor (D₂R) system in the ventral striatum. Weight loss recovers MOR function, but it remains unknown whether it also recovers aberrant opioid-dopamine interaction. Here we addressed this issue by studying 20 healthy non-obese and 25 morbidly obese women (mean BMI 41) eligible for bariatric surgery. Brain MOR and D₂R availability were measured using positron emission tomography (PET) with [¹¹C]carfentanil and [¹¹C]raclopride, respectively. Either Roux-en-Y gastric bypass or sleeve gastrectomy was performed on obese subjects according to standard clinical treatment. 21 obese subjects participated in the postoperative PET scanning six months after bariatric surgery. In the control subjects, MOR and D₂R availabilities were associated in the ventral striatum (r = .62) and dorsal caudate (r = .61). Preoperatively, the obese subjects had disrupted association in the ventral striatum (r = .12) but the unaltered association in dorsal caudate (r = .43). The association between MOR and D₂R availabilities in the ventral striatum was recovered (r = .62) among obese subjects following the surgery-induced weight loss. Bariatric surgery and concomitant weight loss recover the interaction between MOR and D₂R in the ventral striatum in the morbidly obese. Consequently, the dysfunctional opioid-dopamine interaction in the ventral striatum is likely associated with an obese phenotype and may mediate excessive energy uptake. Striatal opioid-dopamine interaction provides a feasible target for pharmacological and behavioral interventions for treating obesity.

Neurochemical and Behavioral Effects of a New Hallucinogenic Compound 25B-NBOMe in Rats

4-Bromo-2,5-dimethoxy-N-(2-methoxybenzyl)phenethylamine (25B-NBOMe) is a hallucinogen exhibiting high binding affinity for 5-HT2A/C serotonin receptors. In the present work, we investigated its effect on dopamine (DA), serotonin (5-HT), acetylcholine (ACh), and glutamate release in the rat frontal cortex, striatum, and nucleus accumbens. Hallucinogenic activity, impact on cognitive and motor functions, and anxiogenic/anxiolytic properties of this compound were also tested. The release of DA, 5-HT, ACh, and glutamate was studied using microdialysis in freely moving animals. Hallucinogenic activity was investigated using head and body twitch response (WDS), cognitive functions were examined with the novel object recognition test (NOR), locomotor activity was studied in the open field (OF), while anxiogenic/anxiolytic effect was tested using the light/dark box (LDB). Neurotoxicity was evaluated with the comet assay. 25B-NBOMe increased DA, 5-HT, and glutamate release in all studied brain regions, induced hallucinogenic activity, and lowered the recognition index (Ri) vs. control in the NOR test. It also decreased locomotor activity of rats in the OF test. The effect of 25B-NBOMe in the NOR test was inhibited by scopolamine. In the LDB test, the time spent in the dark zone was longer in comparison to control and was dose-dependent. In contrast to MDMA, 25B-NBOMe showed subtle genotoxic effect observed in the comet assay.Our findings indicate that 25B-NBOMe shows hallucinogenic activity in the wide range of doses. The changes in neurotransmitter levels may be related to 25B-NBOMe affinity for 5-HT2A receptor. Alterations in the NOR, OF, and LDB indicate that 25B-NBOMe impacts short-term memory, locomotion, and may be anxiogenic.

Serotonin/dopamine interaction: Electrophysiological and neurochemical evidence

The interaction between serotonin (5-HT) and dopamine (DA) in the central nervous system (CNS) plays an important role in the adaptive properties of living animals to their environment. These are two modulatory, divergent systems shaping and regulating in a widespread manner the activity of neurobiological networks and their interaction. The concept of one interaction linking these two systems is rather elusive when looking at the mechanisms triggered by these two systems across the CNS. The great variety of their interacting mechanisms is in part due to the diversity of their neuronal origin, the density of their fibers in a given CNS region, the distinct expression of their numerous receptors in the CNS, the heterogeneity of their intracellular signaling pathway that depend on the cellular type expressing their receptors, and the state of activity of neurobiological networks, conditioning the outcome of their mutual influences. Thus, originally conceptualized as inhibition of 5-HT on DA neuron activity and DA neurotransmission, this interaction is nowadays considered as a multifaceted, mutual influence of these two systems in the regulation of CNS functions. These new ways of understanding this interaction are of utmost importance to envision the consequences of their dysfunctions underlined in several CNS diseases. It is also essential to conceive the mechanism of action of psychotropic drugs directly acting on their function including antipsychotic, antidepressant, antiparkinsonian, and drug of abuse together with the development of therapeutic strategies of Alzheimer's diseases, epilepsy, obsessional compulsive disorders. The 5-HT/DA interaction has a long history from the serendipitous discovery of antidepressants and antipsychotics to the future, rationalized treatments of CNS disorders.

Orexin-A/Hypocretin-1 Controls the VTA-NAc Mesolimbic Pathway via Endocannabinoid-Mediated Disinhibition of Dopaminergic Neurons in Obese Mice

Disinhibition of orexin-A/hypocretin-1 (OX-A) release occurs to several output areas of the lateral hypothalamus (LH) in the brain of leptin knockout obese ob/ob mice. In this study, we have investigated whether a similar increase of OX-A release occurs to the ventral tegmental area (VTA), an orexinergic LH output area with functional effects on dopaminergic signaling at the mesolimbic circuit. By confocal and correlative light and electron microscopy (CLEM) morphological studies coupled to molecular, biochemical, and pharmacological approaches, we investigated OX-A-mediated dopaminergic signaling at the LH-VTA-nucleus accumbens (NAc) pathway in obese ob/ob mice compared to wild-type (wt) lean littermates. We found an elevation of OX-A trafficking and release to the VTA of ob/ob mice and consequent orexin receptor-1 (OX1R)-mediated over-activation of dopaminergic (DA) neurons via phospholipase C (PLC)/diacylglycerol lipase (DAGL-α)-induced biosynthesis of the endocannabinoid 2-arachidonoylglycerol (2-AG). In fact, by retrograde signaling to cannabinoid receptor type 1 (CB1R) at inhibitory inputs to DA neurons, 2-AG inhibited GABA release thus inducing an increase in DA concentration in the VTA and NAc of ob/ob mice. This effect was prevented by the OX1R antagonist SB-334867 (30 mg/Kg, i.p.), or the CB1R antagonist AM251 (10 mg/Kg, i.p.) and mimicked by OX-A injection (40 μg/Kg, i.p.) in wt lean mice. Enhanced DA signaling to the NAc in ob/ob mice, or in OX-A-injected wt mice, was accompanied by β-arrestin2-mediated desensitization of dopamine D2 receptor (D2R) in a manner prevented by SB-334867 or the D2R antagonist L741 (1.5 mg/Kg, i.p.). These results further support the role of OX-A signaling in the control of neuroadaptive responses, such as compulsive reward-seeking behavior or binge-like consumption of high palatable food, and suggest that aberrant OX-A trafficking to the DA neurons in the VTA of ob/ob mice influences the D2R response at NAc, a main target area of the mesolimbic pathway, via 2-AG/CB1-mediated retrograde signaling.

Progress in opioid reward research: From a canonical two-neuron hypothesis to two neural circuits

Opioid abuse and related overdose deaths continue to rise in the United States, contributing to the national opioid crisis in the USA. The neural mechanisms underlying opioid abuse and addiction are still not fully understood. This review discusses recent progress in basic research dissecting receptor mechanisms and circuitries underlying opioid reward and addiction. We first review the canonical GABA-dopamine neuron hypothesis that was upheld for half a century, followed by major findings challenging this hypothesis. We then focus on recent progress in research evaluating the role of the mesolimbic and nigrostriatal dopamine circuitries in opioid reward and relapse. Based on recent findings that activation of dopamine neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) is equally rewarding and that GABA neurons in the rostromedial tegmental nucleus (RMTg) and the substantia nigra pars reticula (SNr) are rich in mu opioid receptors and directly synapse onto midbrain DA neurons, we proposed that the RTMg→VTA → ventrostriatal and SNr → SNc → dorsostriatal pathways may act as the two major neural substrates underlying opioid reward and abuse. Lastly, we discuss possible integrations of these two pathways during initial opioid use, development of opioid abuse and maintenance of compulsive opioid seeking.

Role of the Medial Orbitofrontal Cortex and Ventral Tegmental Area in Effort-Related Responding

The posterior subdivision of the medial orbitofrontal cortex (mOFC-p) mediates the willingness to expend effort to reach a selected goal. However, the neural circuitry through which the mOFC-p modulates effort-related function is as yet unknown. The mOFC-p projects prominently to the posterior ventral tegmental area (pVTA). Therefore, we analyzed the role of the mOFC-p and interactions with the pVTA in effort-related responding using a combination of behavioral, pharmacological, and neural circuit analysis methods in rats. Pharmacological inhibition of the mOFC-p was found to increase lever pressing for food under a progressive ratio (PR) schedule of reinforcement. These findings provide further support for a modulation of effort-related function by the mOFC-p. Then, we investigated effects of disconnecting the mOFC-p and pVTA on PR responding using unilateral pharmacological inhibition of both areas. This asymmetric intervention was also found to increase PR responding suggesting that the mOFC-p controls effort-related function through interactions with the pVTA. Possibly, a reduced excitatory mOFC-p drive on pVTA gamma-aminobutyric acid (GABA)ergic relays disinhibits VTA dopamine neurons which are known to support PR responding. Collectively, our findings suggest that the mOFC-p and pVTA are key components of a neural circuit mediating the willingness to expend effort to reach a goal.

Dissecting the Role of GABA Neurons in the VTA versus SNr in Opioid Reward

Opioid reward has traditionally been thought to be mediated by GABA-induced disinhibition of dopamine (DA) neurons in the VTA. However, direct behavioral evidence supporting this hypothesis is still lacking. In this study, we found that the μ opioid receptor (MOR) gene, Oprm1, is highly expressed in GABA neurons, with ∼50% of GABA neurons in the substantia nigra pars reticulata (SNr), ∼30% in the VTA, and ∼70% in the tail of the VTA (also called the rostromedial tegmental nucleus) in male rats. No Oprm1 mRNA was detected in midbrain DA neurons. We then found that optogenetic inhibition of VTA DA neurons reduced intravenous heroin self-administration, whereas activation of these neurons produced robust optical intracranial self-stimulation in DAT-Cre mice, supporting an important role of DA neurons in opioid reward. Unexpectedly, pharmacological blockade of MORs in the SNr was more effective than in the VTA in reducing heroin reward. Optogenetic activation of VTA GABA neurons caused place aversion and inhibited cocaine, but not heroin, self-administration, whereas optogenetic activation of SNr GABA neurons caused a robust increase in heroin self-administration with an extinction pattern, suggesting a compensatory response in drug intake due to reduced heroin reward. In addition, activation of SNr GABA neurons attenuated heroin-primed, but not cue-induced, reinstatement of drug-seeking behavior, whereas inhibition of SNr GABA neurons produced optical intracranial self-stimulation and place preference. Together, these findings suggest that MORs on GABA neurons in the SNr play more important roles in opioid reward and relapse than MORs on VTA GABA neurons.SIGNIFICANCE STATEMENT Opioid reward has long been believed to be mediated by inhibition of GABA interneurons in the VTA that subsequently leads to disinhibition of DA neurons. In this study, we found that more μ opioid receptors (MORs) are expressed in GABA neurons in the neighboring SNr than in the VTA, and that pharmacological blockade of MORs in the SNr is more effective in reducing heroin reward than blockade of MORs in the VTA. Furthermore, optogenetic activation of VTA GABA neurons inhibited cocaine, but not heroin, self-administration, whereas activation of SNr GABA neurons inhibited heroin reward and relapse. These findings suggest that opioid reward is more likely mediated by stimulation of MORs in GABA afferents from other brain regions than in VTA GABA neurons.

Ventral Tegmental Area Microinjected-SKF38393 Increases Regular Chow Intake in 18 Hours Food-Deprived Rats

INTRODUCTION

Ventral Tegmental Area (VTA) dopamine neurons play an important role in reward mechanisms of food intake, and VTA dopamine receptors exist on the terminal of glutamatergic and GABAergic neurons and regulate Gamma-Aminobutyric Acid (GABA) and glutamate release. To our knowledge, no evidence indicates any role for VTA D1 dopamine receptors in regular chow intake.

METHODS

In this paper, different dose of SKF38393, a D1 receptor agonist, was microinjected in VTA of 18-h food deprived-conscious rats and food intake was measured.

RESULTS

Our results revealed that VTAmicroinjected SKF383993 increased regular chow intake in a dose-dependent manner. The SKF3833 stimulatory effect persisted over 2 h post-injection. The results showed that the SKF38393, at doses less than 5 μg, did not affect locomotor activities.

CONCLUSION

VTA D1-like and/or serotonergic receptors may be involved in regulatory pathways. the current study suggests that VTA D1-like and/or serotonergic receptors not only affects food reward but is also involved in regulatory mechanisms of regular feeding.

Morphine selectively disinhibits glutamatergic input from mPFC onto dopamine neurons of VTA, inducing reward

Ventral tegmental area (VTA) dopamine (DA) neurons presynaptic glutamate release plays a very important role in the mechanism of morphine. Previously, a study from our lab found that morphine disinhibited glutamatergic input onto the VTA-DA neurons, which was an important mechanism for the morphine-induced increase in the VTA-DA neuron firing and related behaviors (Chen et al., 2015). However, what source of glutamatergic inputs is disinhibited by morphine remains to be elucidated. Using optogenetic strategy combined with whole-cell patch-clamp, qRT-PCR, immunofluorescence and chemical genetic approach combined with behavioral methods, our results show that: 1) morphine promotes glutamate release from glutamatergic terminals of medial prefrontal cortex (mPFC) neurons projecting to VTA-DA neurons but does not on those from glutamatergic terminals of the lateral hypothalamus (LH) neurons projecting to VTA-DA neurons; 2) different response of glutamatergic neurons projecting to VTA-DA neurons from the mPFC or the LH to morphine is related to the expression of GABA_B receptors at terminals of these neurons; 3) inhibition of projection neurons from the mPFC to the VTA significantly reduces morphine-induced locomotor activity increase and conditioned place preference but inhibition of projection neurons from the LH to the VTA does not. These results suggest that morphine selectively promotes glutamate release of the glutamatergic input from mPFC onto VTA-DA neurons by removing the inhibition of the GABA_B receptors in this glutamatergic input from mPFC.

Blunted highs: Pharmacodynamic and behavioral models of cannabis tolerance

Acute exposure to cannabis comes with neurocognitive impairment, leading to increased risk of human error and injury. Evidence however indicates that such acute effects are less prominent in chronic users, suggesting cannabis tolerance. Models of cannabis tolerance stress the importance of neurobiological or behavioral adaptations following repeated cannabis exposure. The pharmacodynamic model relates neuroadaptive changes in the brain to a blunted response to cannabis. Downregulation of CB1 receptors in chronic cannabis users has been associated with a normalization of dopaminergic output from the ventral tegmental area to the mesolimbic circuit, and a reduction of impairment during acute cannabis exposure. Such neuroadaptions are absent in occasional users, who show strong increments of dopamine and glutamate levels in the striatum, a loss of functional connectivity within the mesolimbic circuit and neurocognitive impairments when exposed to cannabis. Evidence for a behavioral model of cannabis tolerance that poses that users can have volitional control to overcome functional impairment during cannabis intoxication is relatively weak, and at best shows limited control over a limited number of behavioral functions. Cannabis tolerance is most likely to occur in users that consume high doses of cannabis continuously, at a high pace, for a prolonged period of time. Knowledge on frequency, dose and duration of cannabis use that is needed to achieve, maintain or lessen tolerance however is very limited, but will be of importance in the context of cannabis therapeutics and in legal settings when evaluating the impact of cannabis exposure on human function.

Inhibitory transmission in the bed nucleus of the stria terminalis in male and female mice following morphine withdrawal

The United States is experiencing an opioid crisis imposing enormous fiscal and societal costs and driving the staggering overdose death rate. While prescription opioid analgesics are essential for treating acute pain, cessation of use in individuals with a physical dependence induces an aversive withdrawal syndrome that promotes continued drug use to alleviate/avoid these symptoms. Additionally, repeated bouts of withdrawal often lead to an increased propensity for relapse. Understanding the neurobiology underlying withdrawal is essential for providing novel treatment options to alleviate physiological and affective components accompanying the cessation of opiate use. Here, we administered morphine and precipitated withdrawal with naloxone to investigate behavioral and cellular responses in C57BL/6J male and female mice. Following 3 days of administration, both male and female mice demonstrated sensitized withdrawal symptoms. Since the bed nucleus of the stria terminalis (BNST) plays a role in mediating withdrawal-associated behaviors, we examined plastic changes in inhibitory synaptic transmission within this structure 24 hours following the final precipitated withdrawal. In male mice, morphine withdrawal increased spontaneous GABAergic signaling compared with controls. In contrast, morphine withdrawal decreased spontaneous GABAergic signaling in female mice. Intriguingly, these opposing GABAergic effects were contingent upon activity-dependent dynamics within the ex vivo slice. Our findings suggest that male and female mice exhibit some divergent cellular responses in the BNST following morphine withdrawal, and alterations in BNST inhibitory signaling may contribute to the expression of behaviors following opioid withdrawal.

Role of the COMT gene Val158Met polymorphism in mental disorders: A review

The Val158Met polymorphism of the COMT gene is functional, easily detectable, and significantly related to metabolism of catecholamines, which underlie pathogenesis of a significant number of mental disorders. Evidence for the role of this polymorphism in schizophrenia, substance dependence, bipolar disorder, obsessive-compulsive disorder, anorexia nervosa and attention deficit hyperactivity disorder is summed up in this review article. The results make it unlikely that the COMT gene plays an important role in these mental disorders, although a minor effect can not be excluded. Future studies on the COMT gene in mentally ill subjects should be stratified by clinical subtypes of the disorder, gender and ethnicity. Studies of endophenotypes instead of the complex disorder seem to be another promising research strategy. Gene-gene and gene-environment interactions should also be considered. The COMT gene is probably not “a gene for” any mental disorder, but the Val158Met polymorphism appears to have pleiotropic effects on human behavior.

The Response of Ventral Tegmental Area Dopaminergic Neurons to Bupropion: Excitation or Inhibition?

Introduction:

Antidepressants can modulate brain monoamines by acting on pre-synaptic and postsynaptic receptors. Autoreceptors can reduce the monoamines effect on the somatodendritic or pre-synaptic regions despite its postsynaptic counter effects. The direct effect of some antidepressants is related to its temporal and spatial bioavailability in the vicinity of these receptors (still a matter of controversies). This research evaluated the direct effect of acute bupropion on the Ventral Tegmental Area (VTA) dopaminergic neuronal firing rate.

Methods:

Male Wistar rats were divided into intracerebroventricular and microiontophoretic groups with 14 subgroups (n=5 in each subgroup). Amounts of 1, 0.5, 0.1, 0.01, 0.001, and 0.0001 mol of bupropion (5 μL/3 min) were microinfused to the first group and then the ejected amounts of bupropion at -500, -300, -150, -50 nA of electrical currents (1 mol, pH=4.5, 5 min) were applied to the second group. The control and sham subgroups were studied in each group, too. The units with stable firing rates were extracted, and the effect of bupropion was evaluated statistically with a P value less than 0.05 as the level of significance.

Results:

The highest amount of bupropion in the intracerebroventricular application could excite 42% of the neurons and inhibit 56% of them, but the highest amount of microiontophoretic application of bupropion could inhibit 97.5% of the neurons. The neuronal response to bupropion was dose-dependent in all treated groups.

Conclusion:

The dual effects of intracerebroventricular bupropion on the VTA dopaminergic neurons but solo inhibitory effect of its microiontophoretic application reflect the intra-VTA and extra-VTA heterogenic cellular and molecular control over the dopaminergic outflow that can be mediated by different receptors. The dopamine autoreceptors on the VTA dopaminergic neurons have complex modulatory effects on the dopaminergic response.

Motor Imagery Training With Neurofeedback From the Frontal Pole Facilitated Sensorimotor Cortical Activity and Improved Hand Dexterity

To develop a real-time neurofeedback system from the anterior prefrontal cortex (aPFC) using functional near-infrared spectroscopy (fNIRS) for motor rehabilitation, we investigated the effects of motor imagery training with neurofeedback from the aPFC on hand dexterity and cerebral hemodynamic activity during a motor rehabilitation task. Thirty-one right-handed healthy subjects participated in this study. They received motor imagery training six times for 2 weeks under fNIRS neurofeedback from the aPFC, in which they were instructed to increase aPFC activity. The real group subjects (n = 16) were shown real fNIRS neurofeedback signals from the aPFC, whereas the sham group subjects (n = 15) were shown irrelevant randomized signals during neurofeedback training. Before and after the training, hand dexterity was assessed by a motor rehabilitation task, during which cerebral hemodynamic activity was also measured. The results indicated that aPFC activity was increased during the training, and performance improvement rates in the rehabilitation task after the training was increased in the real group when compared with the sham group. Improvement rates of mean aPFC activity across the training were positively correlated with performance improvement rates in the motor rehabilitation task. During the motor rehabilitation task after the training, the hemodynamic activity in the left somatosensory motor-related areas [premotor area (PM), primary motor area (M1), and primary somatosensory area (S1)] was increased in the real group, whereas the hemodynamic activity was increased in the supplementary motor area in the sham group. This hemodynamic activity increases in the somatosensory motor-related areas after the training correlated with aPFC activity during the last 2 days of motor imagery training. Furthermore, improvement rates of M1 hemodynamic activity after the training was positively correlated with performance improvement rates in the motor rehabilitation task. The results suggest that the aPFC might shape activity in the somatosensory motor-related areas to improve hand dexterity. These findings further suggest that the motor imagery training using neurofeedback signals from the aPFC might be useful to patients with motor disability.

Expression of neural markers of gustatory signaling are differentially altered by continuous and intermittent feeding patterns

Food intake patterns are regulated by signals from the gustatory neural circuit, a complex neural network that begins at the tongue and continues to homeostatic and hedonic brain regions involved in eating behavior. The goal of the current study was to investigate the short-term effects of continuous access to a high fat diet (HFD) versus limited access to dietary fat on the gustatory neural circuit. Male Sprague-Dawley rats were fed a chow diet, a HFD (56% kcal from fat), or provided limited, daily (2 h/day) or limited, intermittent (2 h/day, 3 times/week) access to vegetable shortening for 2 weeks. Real time PCR was used to determine mRNA expression of markers of fat sensing/signaling (e.g. CD36) on the circumvallate papillae, markers of homeostatic eating in the mediobasal hypothalamus (MBH) and markers of hedonic eating in the nucleus accumbens (NAc). Continuous HFD increased mRNA levels of lingual CD36 and serotonin signaling, altered markers of homeostatic and hedonic eating. Limited, intermittent access to dietary fat selectively altered the expression of genes associated with the regulation of dopamine signaling. Overall, these data suggest that short-term, continuous access to HFD leads to altered fat taste and decreased expression of markers of homeostatic and hedonic eating. Limited, intermittent access, or binge-like, consumption of dietary fat led to an overall increase in markers of hedonic eating, without altering expression of lingual fat sensors or homeostatic eating. These data suggest that there are differential effects of meal patterns on gustatory neurocircuitry which may regulate the overconsumption of fat and lead to obesity.

Methylenedioxypyrovalerone (MDPV) impairs working memory and alters patterns of dopamine signaling in mesocorticolimbic substrates

Knowledge remains limited about how chronic cathinone exposure impacts dopamine systems in brain reward circuits. In the present study, a binge-like MDPV exposure that impaired novel object recognition (NOR) dysregulated dopamine markers in mesocorticolimbic substrates of rats, with especially profound effects on D1 and D2 receptor's and VMAT gene expression. Our data suggested that dopamine receptivity was reduced in the NAc but increased in the PFC and dopamine-producing VTA. The MDPV-induced impairment of NOR was prevented by a D1 receptor antagonist, suggesting that chronic MDPV exposure produces site-specific dysregulation of dopamine markers in the mesocorticolimbic circuit and memory deficits in the NOR test that are influenced by D1 receptors.

The Endogenous Opioid System: Role and Dysfunction Caused by Opioid Therapy

The endogenous opioid system is comprised of a wide array of receptors and ligands that are present throughout the central and peripheral nervous system, the gastrointestinal tract, and the immune system. This explains the multitude of physiological functions it is responsible for including analgesia, mood regulation, and modulation of the stress response. It also plays a pivotal role in modulating the brain's reward center with behavioral and social implications on mood disorders and addiction. Exogenous opioid therapy hijacks the endogenous system and alters its functions contributing to an imbalance that is responsible for the pathogenesis of several disease states.

Love is analogous to money in human brain: Coordinate-based and functional connectivity meta-analyses of social and monetary reward anticipation

Both social and material rewards play a crucial role in daily life and function as strong incentives for various goal-directed behaviors. However, it remains unclear whether the incentive effects of social and material reward are supported by common or distinct neural circuits. Here, we have addressed this issue by quantitatively synthesizing and comparing neural signatures underlying social (21 contrasts, 207 foci, 696 subjects) and monetary (94 contrasts, 1083 foci, 2060 subjects) reward anticipation. We demonstrated that social and monetary reward anticipation engaged a common neural circuit consisting of the ventral tegmental area, ventral striatum, anterior insula, and supplementary motor area, which are intensively connected during both task and resting states. Functional decoding findings indicate that this generic neural pathway mediates positive value, motivational relevance, and action preparation during reward anticipation, which together motivate individuals to prepare well for the response to the upcoming target. Our findings support the common neural currency hypothesis by providing the first meta-analytic evidence to quantitatively show the common involvement of brain regions in both social and material reward anticipation.

NMDA Receptor-Dependent Cholinergic Modulation of Mesolimbic Dopamine Cell Bodies: Neurochemical and Behavioral Studies

Substance abuse disorders are devastating, costly, and difficult to treat. Identifying the neurochemical mechanisms underlying reinforcement promises to provide critical information in the development of effective treatments. Several lines of evidence suggest that striatal dopamine (DA) release serves as a teaching signal in reinforcement learning, and that shifts in DA release from the primary reward to reward-predicting stimuli play a critical role in the self-administration of both natural and non-natural rewards. However, far less is known about the reinforcing effects of motivationally neutral sensory stimuli, or how these signals can facilitate self-administration behavior. Thus, we trained rats ( n = 7) to perform a visual stimulus-induced instrumental task, which involved lever pressing for activation of a stimulus light. We then microinfused vehicle (phosphate buffered saline), carbachol (acetylcholine receptor agonist), or carbachol in the presence of an N-methyl-d-aspartate (NMDA) receptor-specific drug (NMDA itself, or the antagonist, AP5) into the ventral tegmental area (VTA). This enabled us to directly evaluate how chemical modulation of dopamine cell bodies affects the instrumental behavior, as well as the nature of extracellular dopamine transients recorded in the nucleus accumbens shell (NAc shell) using fast-scan cyclic voltammetry (FSCV). Intra-VTA infusion of carbachol enhanced the magnitude and frequency of dopamine transients in the NAc shell and potentiated active lever responding without altering inactive lever responding, as compared to infusion of vehicle. Coinfusion of carbachol with AP5 abolished dopamine transients recorded in the NAc and attenuated active lever responding without altering inactive lever responding. Finally, coadministration of carbachol and NMDA into the VTA restored both lever pressing and dopaminergic signals recorded in the striatum. Together, these results suggest that acetylcholine and glutamate synergistically act at dopamine cells in the VTA to modulate VTA-NAc shell dopaminergic output, and this underlies motivation to lever press for a motivationally neutral visual stimulus.

Brain activity of anandamide: a rewarding bliss?

Anandamide is a lipid mediator that acts as an endogenous ligand of CB1 receptors. These receptors are also the primary molecular target responsible for the pharmacological effects of Δ9-tetrahydrocannabinol, the psychoactive ingredient in Cannabis sativa. Several studies demonstrate that anandamide exerts an overall modulatory effect on the brain reward circuitry. Several reports suggest its involvement in the addiction-producing actions of other abused drugs, and it can also act as a behavioral reinforcer in animal models of drug abuse. Importantly, all these effects of anandamide appear to be potentiated by pharmacological inhibition of its metabolic degradation. Enhanced brain levels of anandamide after treatment with inhibitors of fatty acid amide hydrolase, the main enzyme responsible for its degradation, seem to affect the rewarding and reinforcing actions of many drugs of abuse. In this review, we will provide an overview from a preclinical perspective of the current state of knowledge regarding the behavioral pharmacology of anandamide, with a particular emphasis on its motivational/reinforcing properties. We will also discuss how modulation of anandamide levels through inhibition of enzymatic metabolic pathways could provide a basis for developing new pharmaco-therapeutic tools for the treatment of substance use disorders.

Resting State Functional Connectivity of Dorsal Raphe Nucleus and Ventral Tegmental Area in Medication-Free Young Adults With Major Depression

Background: This study has, for the first time, investigated the dorsal raphe nucleus (DRN) and ventral tegmental area (VTA) resting state whole-brain functional connectivity in medication-free young adults with major depression (MDD), at baseline and in relationship to treatment response. Method: A total of 119 subjects: 78 MDD (24 ± 4 years.) and 41 Healthy Controls (HC) (24 ± 3 years) were included in the analysis. DRN and VTA ROIs anatomical templates were used to extract resting state fluctuations and used to derive whole-brain functional connectivity. Differences between MDD and HCs were examined, as well as the correlation of baseline Hamilton Depression and Anxiety scale scores to the baseline DRN and VTA connectivity. The relationship to treatment response was examined by investigating the correlation of the percentage decrease in depression and anxiety scale scores with baseline connectivity measures. Results: There was a significant decrease (p = 0.05; cluster-wise corrected) in DRN connectivity with the prefrontal and mid-cingulate cortex in the MDD group, compared with the HC group. DRN connectivity with temporal areas, including the hippocampus and amygdala, positively correlated with baseline depression scores (p = 0.05; cluster-wise corrected). VTA connectivity with the cuneus-occipital areas correlated with a change in depression scores (p = 0.05; cluster-wise corrected). Conclusion: Our results indicate the presence of DRN-prefrontal and DRN-cingulate cortex connectivity abnormalities in young medication-free depressed subjects when compared to HCs and that the severity of depressive symptoms correlates with DRN-amygdala/hippocampus connectivity. VTA connectivity with the parietal and occipital areas is related to antidepressant treatment associated with a decrease in depressive symptoms. Future studies need to be carried out in larger and different age group populations to confirm the findings of the study.

Acute and chronic methylphenidate administration in intact and VTA-specific and nonspecific lesioned rats

Methylphenidate (MPD) is a psychostimulant used for the treatment of ADHD and works by increasing the bioavailability of dopamine (DA) in the brain. As a major source of DA, the ventral tegmental area (VTA) served as the principal target in this study as we aimed to understand its role in modulating the acute and chronic MPD effect. Forty-eight male Sprague-Dawley rats were divided into control, sham, electrical lesion, and 6-OHDA lesion groups. Given the VTA's implication in the locomotive circuit, three locomotor indices-horizontal activity, number of stereotypy, and total distance-were used to measure the animals' behavioral response to the drug. Baseline recording was obtained on experimental day 1 (ED 1) followed by surgery on ED 2. After recovery, the behavioral recordings were resumed on ED 8. All groups received daily intraperitoneal injections of 2.5 mg/kg MPD for six days after which the animals received no treatment for 3 days. On ED 18, 2.5 mg/kg MPD was re-administered to assess for the chronic effect of the psychostimulant. Except for one index, there was an increase in locomotive activity in all experimental groups after surgery (in comparison to baseline activity), acute MPD exposure, induction with six daily doses, and after MPD re-challenge. Furthermore, the increase was greatest in the electrical VTA lesion group and lowest in the 6-OHDA VTA lesion group. In conclusion, the results of this study suggest that the VTA may not be the primary nucleus of MPD action, and the VTA plays an inhibitory role in the locomotive circuit.

Dopamine and addiction: what have we learned from 40 years of research

Among the neurotransmitters involved in addiction, dopamine (DA) is clearly the best known. The critical role of DA in addiction is supported by converging evidence that has been accumulated in the last 40 years. In the present review, first we describe the dopaminergic system in terms of connectivity, functioning and involvement in reward processes. Second, we describe the functional, structural, and molecular changes induced by drugs within the DA system in terms of neuronal activity, synaptic plasticity and transcriptional and molecular adaptations. Third, we describe how genetic mouse models have helped characterizing the role of DA in addiction. Fourth, we describe the involvement of the DA system in the vulnerability to addiction and the interesting case of addiction DA replacement therapy in Parkinson's disease. Finally, we describe how the DA system has been targeted to treat patients suffering from addiction and the result obtained in clinical settings and we discuss how these different lines of evidence have been instrumental in shaping our understanding of the physiopathology of drug addiction.

Diazepam reverses increased anxiety-like behavior, social behavior deficit, and dopamine dysregulation following withdrawal from acute amphetamine

Psychostimulants such as amphetamine (AMPH) increase dopamine (DA) release from ventral tegmental area (VTA) neurons, which is associated with their acute reinforcing actions. This positive state is followed by a negative affective state during the withdrawal period each time the drug is taken (i.e., opponent process theory). AMPH withdrawal is accompanied by symptoms of anxiety and depression, which are associated with DA system dysfunction in humans and animal models. Most studies have focused on the negative affective state after withdrawal from chronic drug administration; yet, this negative state appears even after a drug is taken for the first time in both humans and rodents. In rats, withdrawal from a single dose of AMPH (2 mg/kg) increases forced swim test immobility and decreases the number of spontaneously active VTA DA neurons up to 48 h post-withdrawal. In the current study, acute AMPH withdrawal was found to increase anxiety-like behavior in the elevated plus maze (EPM), reduce social cage time in the three-chambered social approach test (SAT), and attenuate VTA population activity. The effects of diazepam, a drug commonly used to treat anxiety disorders, were tested on anxiety-like and social behavior as well as VTA DA neuron activity following acute AMPH withdrawal. A single (5 mg/kg) dose of diazepam circumvented the neurobehavioral effects induced by acute AMPH withdrawal, as demonstrated by increased open arm time and social cage time as well as normalized VTA DA activity comparable to controls, suggesting that these neurobehavioral effects of acute AMPH withdrawal reflect an anxiety-like state.

A review of nonhuman primate models of early life stress and adolescent drug abuse

Adolescence represents a developmental stage in which initiation of drug use typically occurs and is marked by dynamic neurobiological changes. These changes present a sensitive window during which perturbations to normative development lead to alterations in brain circuits critical for stress and emotional regulation as well as reward processing, potentially resulting in an increased susceptibility to psychopathologies. The occurrence of early life stress (ELS) is related to a greater risk for the development of substance use disorders (SUD) during adolescence. Studies using nonhuman primates (NHP) are ideally suited to examine how ELS may alter the development of neurobiological systems modulating the reinforcing effects of drugs, given their remarkable neurobiological, behavioral, and developmental homologies to humans. This review examines NHP models of ELS that have been used to characterize its effects on sensitivity to drug reinforcement, and proposes future directions using NHP models of ELS and drug abuse in an effort to develop more targeted intervention and prevention strategies for at risk clinical populations.

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ELS has long-lasting neurobiological and behavioral consequences.

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ELS is a major risk factor for the initiation of adolescent drug use.

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Sex differences are apparent in the consequences of ELS, including drug use.

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Nonhuman primate models of ELS are critical for understanding ELS effects on neurobiology and risk for adolescent drug use.

The galanin receptor-3 antagonist, SNAP 37889, inhibits cue-induced reinstatement of alcohol-seeking and increases c-Fos expression in the nucleus accumbens shell of alcohol-preferring rats

INTRODUCTION

This study aimed to investigate the effects of the galanin-3 receptor antagonist, SNAP 37889, on c-Fos protein expression after cue-induced reinstatement of alcohol-seeking in the brains of alcohol-preferring rats.

METHODS

Eighteen alcohol-preferring rats were trained to self-administer 10% v/v ethanol in the presence of response-contingent cues, which was followed by extinction. Rats were then treated with SNAP 37889 (30 mg/kg, i.p.) or vehicle, before being tested for cue-induced reinstatement. Administration of SNAP 37889 reduced cue-induced reinstatement of ethanol-seeking behaviour. To examine the effect of SNAP 37889 and cue-induced reinstatement on neuronal activation, c-Fos expression was measured in subregions of the medial prefrontal cortex and nucleus accumbens.

RESULTS

SNAP 37889 administration increased c-Fos immunoreactivity in the nucleus accumbens shell, but was without effect in the nucleus accumbens core and the medial prefrontal cortex. Dual-label Fos/tyrosine hydroxylase immunohistochemistry was used to examine the effects of SNAP 37889 on dopamine neurons in the ventral tegmental area; however, no differences between SNAP 37889 and vehicle-treated rats were found.

CONCLUSIONS

These data support previous findings of galanin-3 receptor involvement in cue-induced reinstatement of alcohol-seeking behaviour, and provide novel evidence that the ability of galanin-3 receptor antagonism to attenuate cue-induced reinstatement relates to activation of the nucleus accumbens shell.

Effects of Transcranial Direct Current Stimulation (tDCS) Over the Frontal Polar Area on Motor and Executive Functions in Parkinson's Disease; A Pilot Study

Parkinson's disease (PD) is a neurodegenerative disorder with motor and non-motor symptoms due to degeneration of dopaminergic neurons. The current pharmacological treatments induce complications associated with long-term use. However, current stimulation techniques for PD treatment, such as deep brain stimulation (DBS), are too invasive. In this context, non-invasive brain stimulation including transcranial direct current stimulation (tDCS) may be a safe and effective alternative treatment for PD. We previously reported that anodal tDCS over the frontal polar area (FPA) improved motor functions in heathy subjects. Therefore, in the present study, effects of tDCS over the FPA on motor and cognitive functions of PD patients were analyzed. Nine PD patients (3 men and 6 women) participated in this cross over study with three tDCS protocols; anodal, cathodal or sham tDCS over the FPA. Each tDCS protocol was applied for 1 week (5 times/week). Before and after each protocol, motor and cognitive functions of the patients were assessed using Unified PD Rating Scale [UPDRS (part III: motor examination)], Fugl Meyer Assessment set (FMA), Simple Test for Evaluating hand Function (STEF) and Trail Making Test A (TMT-A). The results indicated that anodal stimulation significantly decreased scores of motor disability in UPDRS-III compared with sham and cathodal stimulation, and significantly increased scores of motor functions in FMA compared with sham stimulation. Furthermore, anodal stimulation significantly decreased time to complete a motor task requiring high dexterity in STEF compared with those requiring low and medium levels of dexterity. In addition, anodal stimulation significantly decreased time to complete the TMT-A task, which requires executive functions, compared with sham stimulation. To the best of our knowledge, this is the first clinical research reporting that tDCS over the FPA successfully improved the motor and non-motor functions in PD patients. These findings suggest that tDCS over the FPA might be a useful alternative for the treatment of PD patients.

The novel atypical antipsychotic cariprazine demonstrates dopamine D2 receptor-dependent partial agonist actions on rat mesencephalic dopamine neuronal activity

Aim

Cariprazine, a dopamine D₃‐preferring D₃/D₂ receptor partial agonist, is FDA approved for the treatment of schizophrenia and acute manic or mixed episodes of bipolar disorder. This study used in vivo electrophysiological techniques in anesthetized rats to determine cariprazine's effect on dopaminergic cell activity in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc).

Methods

Extracellular recordings of individual dopaminergic neurons were performed after oral or intravenous administration of cariprazine, the D₃ receptor antagonist SB 277011A, the D₂ receptor antagonist L741,626, and/or the D₃ receptor agonist PD 128,907.

Results

Acute oral treatment with cariprazine significantly increased and chronic cariprazine significantly decreased the number of spontaneously firing dopaminergic neurons in the VTA, but not in the SNc. Intravenous administration of cariprazine partially but significantly inhibited dopaminergic neuronal firing in both regions, which was prevented by L741,626 but not SB 277011A. In both VTA and SNc, cariprazine, SB 277011A, and L741,626 significantly antagonized the suppression of dopamine cell firing elicited by PD 128,907.

Conclusions

Cariprazine significantly modulates the number of spontaneously active VTA dopamine neurons and moderately suppresses midbrain dopamine neuronal activity. The contribution of dopamine D₂ receptors to cariprazine's in vivo effects is prevalent and that of D₃ receptors is less apparent.

Overexpression of Thioredoxin-1 Blocks Morphine-Induced Conditioned Place Preference Through Regulating the Interaction of γ-Aminobutyric Acid and Dopamine Systems

Morphine is one kind of opioid, which is currently the most effective widely utilized pain relieving pharmaceutical. Long-term administration of morphine leads to dependence and addiction. Thioredoxin-1 (Trx-1) is an important redox regulating protein and works as a neurotrophic cofactor. Our previous study showed that geranylgeranylaceton, an inducer of Trx-1 protected mice from rewarding effects induced by morphine. However, whether overexpression of Trx-1 can block morphine-induced conditioned place preference (CPP) in mice is still unknown. In this study, we first examined whether overexpression of Trx-1 affects the CPP after morphine training and further examined the dopamine (DA) and γ-aminobutyric acid (GABA) systems involved in rewarding effects. Our results showed that morphine-induced CPP was blocked in Trx-1 overexpression transgenic (TG) mice. Trx-1 expression was induced by morphine in the ventral tegmental area (VTA) and nucleus accumbens (NAc) in wild-type (WT) mice, which was not induced in Trx-1 TG mice. The DA level and expressions of tyrosine hydroxylase (TH) and D1 were induced by morphine in WT mice, which were not induced in Trx-1 TG mice. The GABA level and expression of GABA_BR were decreased by morphine, which were restored in Trx-1 TG mice. Therefore, Trx-1 may play a role in blocking CPP induced by morphine through regulating the expressions of D1, TH, and GABA_BR in the VTA and NAc.

Genetic inhibition of neurotransmission reveals role of glutamatergic input to dopamine neurons in high-effort behavior

Midbrain dopamine neurons are crucial for many behavioral and cognitive functions. As the major excitatory input, glutamatergic afferents are important for control of the activity and plasticity of dopamine neurons. However, the role of glutamatergic input as a whole onto dopamine neurons remains unclear. Here we developed a mouse line in which glutamatergic inputs onto dopamine neurons are specifically impaired, and utilized this genetic model to directly test the role of glutamatergic inputs in dopamine-related functions. We found that while motor coordination and reward learning were largely unchanged, these animals showed prominent deficits in effort-related behavioral tasks. These results provide genetic evidence that glutamatergic transmission onto dopaminergic neurons underlies incentive motivation, a willingness to exert high levels of effort to obtain reinforcers, and have important implications for understanding the normal function of the midbrain dopamine system.

Novelty seeking mediates the effect of DRD3 variation on onset age of amphetamine dependence in Han Chinese population

The dopamine receptor D3 (DRD3) gene, one of the candidate genes for amphetamine dependence (AD), is involved in the mesolimbic dopaminergic system, implicated as the underlying mechanism of addiction. Our case-control study aimed to investigate whether the DRD3 gene is associated with the susceptibility to AD and specific personality traits in AD patients. A total of 1060 unrelated Han Chinese subjects (559 AD patients and 501 controls) were screened using the same assessment tool and genotyped for eight DRD3 polymorphisms. All patients met the DSM-IV-TR criteria for AD, and personality traits of 539 were assessed using a Tridimensional Personality Questionnaire. Furthermore, AD individuals were divided into four clinical subgroups based on gender and psychosis status, to reduce the clinical heterogeneity. We found that the ATA haplotype combination for SNPs rs324029, rs6280, and rs9825563, respectively, was significantly associated with total AD patients (p = 0.0003 after 10,000 permutations). Similar results were observed in the both male and non-psychosis subgroup but not in other subgroups. In addition, DRD3 rs9825563 may influence onset age of drug use, partially mediated by novelty seeking in the non-psychosis AD group. In conclusion, DRD3 is a potential genetic factor in the susceptibility to AD and is associated with onset age of drug use through interaction with novelty seeking in a specific patient group in the Han Chinese population.