A Neural Circuit Mechanism for Encoding Aversive Stimuli in the Mesolimbic Dopamine System

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Depression of Mesolimbic Dopamine Transmission and Sensitization to Morphine During Opiate Abstinence

To investigate the role of mesolimbic dopamine (DA) in the mechanism of drug dependence, extracellular DA was monitored by transcerebral dialysis in the caudal nucleus accumbens under basal conditions and after challenge with morphine (5 mg/kg s.c.) in control rats and in rats made dependent on and then deprived of morphine. Withdrawal from morphine resulted in a marked reduction of extracellular DA concentrations from control values at 1, 2, 3, and 5 days of withdrawal. After 7 days of withdrawal, DA output was less, but still significantly, reduced. Challenge with morphine resulted in stimulation of DA output in controls (maximum, 35%), no effect on the first day of withdrawal, and stimulation similar to controls' on days 2 and 7 of withdrawal. On day 5 and, particularly, on day 3 of withdrawal, morphine-induced stimulation of DA output was markedly potentiated (maximum, 100 and 160%, respectively). Changes in the sensitivity of DA transmission to morphine challenge were associated with changes in the behavioral stimulant effects of morphine, with tolerance on day 1 and marked sensitization on days 3 and 5 but also on day 7, when morphine-induced stimulation of transmission was no longer potentiated. The results indicate that repeated morphine administration induces a state of dependence in DA neurons and a short-lasting tolerance followed by an increased sensitivity to its stimulant effects on DA transmission. These changes might play an important role in the development of opiate addiction and in the maintenance of opiate self-administration in dependent subjects.     

Local Perfusion of Nicotine Differentially Modulates Somatodendritic Dopamine Release in the Rat Ventral Tegmental Area After Nicotine Preexposure

We examined the effects of nicotine perfusion into the ventral tegmental area (VTA) on extracellular dopamine (DA) levels in rats using in vivo microdialysis. Local perfusion with nicotine for 80 min (10–100 M) modestly increased (105–131% of basal) the extracellular DA levels in the VTA of rats that had been pretreated with saline for 5 days. In animals that had been pretreated with nicotine for 5 days (0.3 mg/kg, s.c.), perfusion with nicotine for 80 min (10–100 M) dose-dependently increased the extracellular DA levels in the VTA of rats and did so to a greater extent than in saline-pretreated animals (125–171% of basal). Co-perfusion through the dialysis probe with 100 M mecamylamine, a nonselective nicotinic acetylcholine receptor (nAChR) antagonist, or 100 M dihydro--erythroidine, a high affinity and competitive nAChR antagonist, attenuated the enhancement of extracellular DA levels produced by 100 M nicotine alone. These results suggest that local nicotine challenge potentiated the somatodendritic DA release after nicotine preexposure by stimulation of high-affinity nAChRs in the VTA.     

Reward-Induced Phasic Dopamine Release in the Monkey Ventral Striatum and Putamen

In-vivo voltammetry has successfully been used to detect dopamine release in rodent brains, but its application to monkeys has been limited. We have previously detected dopamine release in the caudate of behaving Japanese monkeys using diamond microelectrodes (Yoshimi 2011); however it is not known whether the release pattern is the same in various areas of the forebrain. Recent studies have suggested variations in the dopaminergic projections to forebrain areas. In the present study, we attempted simultaneous recording at two locations in the striatum, using fast-scan cyclic voltammetry (FSCV) on carbon fibers, which has been widely used in rodents. Responses to unpredicted food and liquid rewards were detected repeatedly. The response to the liquid reward after conditioned stimuli was enhanced after switching the prediction cue. These characteristics were generally similar between the ventral striatum and the putamen. Overall, the technical application of FSCV recording in multiple locations was successful in behaving primates, and further voltammetric recordings in multiple locations will expand our knowledge of dopamine reward responses.     

Whole-Brain Mapping of Inputs to Projection Neurons and Cholinergic Interneurons in the Dorsal Striatum

The dorsal striatum integrates inputs from multiple brain areas to coordinate voluntary movements, associative plasticity, and reinforcement learning. Its projection neurons consist of the GABAergic medium spiny neurons (MSNs) that express dopamine receptor type 1 (D1) or dopamine receptor type 2 (D2). Cholinergic interneurons account for a small portion of striatal neuron populations, but they play important roles in striatal functions by synapsing onto the MSNs and other local interneurons. By combining the modified rabies virus with specific Cre- mouse lines, a recent study mapped the monosynaptic input patterns to MSNs. Because only a small number of extrastriatal neurons were labeled in the prior study, it is important to reexamine the input patterns of MSNs with higher labeling efficiency. Additionally, the whole-brain innervation pattern of cholinergic interneurons remains unknown. Using the rabies virus-based transsynaptic tracing method in this study, we comprehensively charted the brain areas that provide direct inputs to D1-MSNs, D2-MSNs, and cholinergic interneurons in the dorsal striatum. We found that both types of projection neurons and the cholinergic interneurons receive extensive inputs from discrete brain areas in the cortex, thalamus, amygdala, and other subcortical areas, several of which were not reported in the previous study. The MSNs and cholinergic interneurons share largely common inputs from areas outside the striatum. However, innervations within the dorsal striatum represent a significantly larger proportion of total inputs for cholinergic interneurons than for the MSNs. The comprehensive maps of direct inputs to striatal MSNs and cholinergic interneurons shall assist future functional dissection of the striatal circuits.     

Leptin Acts via Leptin Receptor-Expressing Lateral Hypothalamic Neurons to Modulate the Mesolimbic Dopamine System and Suppress Feeding

The lateral hypothalamic area (LHA) acts in concert with the ventral tegmental area (VTA) and other components of the mesolimbic dopamine (DA) system to control motivation, including the incentive to feed. The anorexigenic hormone leptin modulates the mesolimbic DA system, although the mechanisms underlying this control have remained incompletely understood. We show that leptin directly regulates a population of leptin receptor (LepRb)-expressing inhibitory neurons in the LHA and that leptin action via these LHA LepRb neurons decreases feeding and body weight. Furthermore, these LHA LepRb neurons innervate the VTA, and leptin action on these neurons restores VTA expression of the rate-limiting enzyme in DA production along with mesolimbic DA content in leptin-deficient animals. Thus, these findings reveal that LHA LepRb neurons link anorexic leptin action to the mesolimbic DA system.     

Uptake of Dopamine Released by Impulse Flow in the Rat Mesolimbic and Striatal Systems In Vivo

Abstract: The release of dopamine in the striatum, nucleus accumbens, and olfactory tubercle of anesthetized rats was evoked by electrical stimulation of the mesolimbic dopaminergic pathway (four pulses at 15 Hz or four pulses at 200 Hz). Carbon fiber electrodes were implanted in these regions to monitor evoked dopamine overflow by continuous amperometry. The kinetics of dopamine elimination were estimated by measuring the time to 50% decay of the dopamine oxidation current after stimulation ceased. This time ranged from 64 ms in the striatum to 113 ms in the nucleus accumbens. Inhibition of dopamine uptake by nomifensine (2–20 mg/kg), GBR 12909 (20 mg/kg), cocaine (20 mg/kg), mazindol (10 mg/kg), or bupropion (25 mg/kg) enhanced this decay time by up to +602%. Uptake inhibition also produced an increase in the maximal amplitude of dopamine overflow evoked by four pulses at 15 Hz. This latter effect was larger in the striatum (+420%) than in mesolimbic areas (+140%). These results show in vivo that these uptake inhibitors actually slow the clearance of dopamine released by action potentials and suggest that dopaminergic transmission is both prolonged and potentiated strongly by these drugs, in particular in the striatum.     

Ghrelin and Nicotine Stimulate Equally the Dopamine Release in the Rat Amygdala

The orexigenic peptide ghrelin plays a prominent role in the regulation of energy balance and in the mediation of reward processes and reinforcement for addictive drugs, such as nicotine. Nicotine is the principal psychoactive component in tobacco, which is responsible for addiction and relapse of smokers. Ghrelin and nicotine activates the mesolimbicocortical dopaminergic pathways via growth hormone secretagogue receptors (GHS-R1A) and nicotinic acetylcholine receptors (nAchR), respectively, resulting in the release of dopamine in the nucleus accumbens, the amygdala and the prefrontal cortex. In the present study an in vitro superfusion of rat amygdalar slices was performed in order to investigate the direct action of ghrelin and nicotine on the amygdalar dopamine release. Ghrelin increased significantly the dopamine release from the rat amygdala following electrical stimulation. This effect was inhibited by both the selective GHS-R1A antagonist GHRP-6 and the selective nAchR antagonist mecamylamine. Under the same conditions, nicotine also increased significantly the dopamine release from the rat amygdala. This effect was antagonized by mecamylamine, but not by GHRP-6. Co-administration of ghrelin and nicotine induced a similar increase of amygdalar dopamine release. This stimulatory effect was partially reversed by both GHRP-6 and mecamylamine. The present results demonstrate that both ghrelin and nicotine stimulates directly the dopamine release in the amygdala, an important dopaminergic target area of the mesolimbicocortical pathway.     

Stress-Induced Dopamine Release in the Neostriatum: Evaluation of the Role of Action Potentials in Nigrostriatal Dopamine Neurons or Local Initiation by Endogenous Excitatory Amino Acids

Abstract: It has been hypothesized that excitatory amino acids can initiate dopamine release in neostriatum. We examined whether the increase in extracellular dopamine in neostriatum produced by acute stress reflects presynaptic initiation of dopamine release by endogenous excitatory amino acids. Thirty minutes of intermittent tail-shock stress significantly elevated extracellular concentrations of dopamine, glutamate, aspartate, and γ-aminobutyric acid in neostriatum of freely moving rats as measured with in vivo microdialysis. Local infusion of the N -methyl- d -aspartate receptor antagonist 2-amino-5-phosphonovaler-ate or the non- N -methyl- d -aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione via the dialysis probe did not attenuate the stress-induced increase in extra cellular dopamine. In fact, the increase was prolonged in rats treated with specific excitatory amino acid receptor antagonists. Infusion of tetrodotoxin into medial forebrain bundle increased extra cellular glutamate and aspartate in neostriatum yet reduced basal dopamine in extra cellular fluid to below the limit of detection of the assay and eliminated the stress-induced increase in extra cellular dopamine. These findings fail to support the hypothesis that the stress-induced increase in extra cellular dopamine in neostriatum is initiated locally by excitatory amino acids. Rather, the effects of stress on extra cellular dopamine seem to be determined by impulse propagation in dopamine neurons.     

Neurochemical Studies of the Mesolimbic Dopaminergic Pathway: Somatodendritic Mechanisms and GABAergic Neurones in the Rat Ventral Tegmentum

Abstract: The rat ventral tegmentum (containing dendrites and somata of mesolimbic neurones) contained 1.3 μg/g of dopamine, which was reduced to 40% of the control level by reserpine. Slices of ventral tegmentum were able to accumulate and release (elevated potassium or protoveratrine A) exogenous [³H]dopamine. In parallel studies the uptake mechanism in ventral tegmentum was shown to be virtually identical to the nerve terminal uptake of [³H]dopamine by slices of nucleus accumbens. The release of [³H]dopamine was indistinguishable from that observed in substantia nigra, where there is substantial evidence for dendritic mechanisms. Basal adenylate cyclase activity was present, but dopamine-stimulated activity was not detected. A high GABA concentration (7.7 μmol/g) was present in ventral tegmentum, in conjunction with an uptake and a release mechanism for [³H]GABA. GABA and muscimol elicited a small, reproducible efflux of [³H]dopamine, but an interaction between dopamine and [³H]GABA efflux was not observed. The results are in accord with transmitter roles for dopamine and GABA in the somatoden-dritic area of mesolimbic dopaminergic neurons.     

Effects of Emotion Regulation Difficulties on the Tonic and Phasic Cardiac Autonomic Response

Background

Emotion regulation theory aims to explain the interactions between individuals and the environment. In this context, Emotion Regulation Difficulties (ERD) disrupt the physiological component of emotions through the autonomic nervous system and are involved in several psychopathological states.

Objective

We were interested in comparing the influence of a film-elicited emotion procedure on the autonomic nervous system activity of two groups with different levels of emotion regulation difficulties.

Methods

A total of 63 women (undergraduate students) ranging from 18 to 27 (20.7±1.99) years old were included. Using the upper and lower quartile of a questionnaire assessing the daily difficulties in regulating emotions, two groups, one with low (LERD) and one with high (HERD) levels of emotion regulation difficulties, were constituted and studied during a film-elicited emotion procedure. Cardiac vagal activity (HF-HRV) was analyzed during three periods: baseline, film-elicited emotion, and recovery.

Results

The cardiovascular results showed a decrease in HF-HRV from baseline to elicitation for both groups. Then, from elicitation to recovery, HF-HRV increased for the LERD group, whereas a low HF-HRV level persisted for the HERD group.

Conclusions

The HERD group exhibited inappropriate cardiac vagal recovery after a negative emotion elicitation had ended. Cardiac vagal tone took longer to return to its initial state in the HERD group than in the LERD group. Prolonged cardiac vagal suppression might constitute an early marker of emotion regulation difficulties leading to lower cardiac vagal tone.     

Mechanisms of Metabonomic for a Gateway Drug: Nicotine Priming Enhances Behavioral Response to Cocaine with Modification in Energy Metabolism and Neurotransmitter Level

Nicotine, one of the most commonly used drugs, has become a major concern because tobacco serves as a gateway drug and is linked to illicit drug abuse, such as cocaine and marijuana. However, previous studies mainly focused on certain genes or neurotransmitters which have already been known to participate in drug addiction, lacking endogenous metabolic profiling in a global view. To further explore the mechanism by which nicotine modifies the response to cocaine, we developed two conditioned place preference (CPP) models in mice. In threshold dose model, mice were pretreated with nicotine, followed by cocaine treatment at the dose of 2 mg/kg, a threshold dose of cocaine to induce CPP in mice. In high-dose model, mice were only treated with 20 mg/kg cocaine, which induced a significant CPP. ¹H nuclear magnetic resonance based on metabonomics was used to investigate metabolic profiles of the nucleus accumbens (NAc) and striatum. We found that nicotine pretreatment dramatically increased CPP induced by 2 mg/kg cocaine, which was similar to 20 mg/kg cocaine-induced CPP. Interestingly, metabolic profiles showed considerable overlap between these two models. These overlapped metabolites mainly included neurotransmitters as well as the molecules participating in energy homeostasis and cellular metabolism. Our results show that the reinforcing effect of nicotine on behavioral response to cocaine may attribute to the modification of some specific metabolites in NAc and striatum, thus creating a favorable metabolic environment for enhancing conditioned rewarding effect of cocaine. Our findings provide an insight into the effect of cigarette smoking on cocaine dependence and the underlying mechanism.     

Neurochemical Studies of the Mesolimbic Dopaminergic Pathway: Glycinergic Mechanisms and Glycinergic-Dopaminergic Interactions in the Rat Ventral Tegmentum

Abstract: The rat ventral tegmentum (containing somata and dendrites of mesolimbic dopaminergic neurones) contained 1.3 μmnol/g wet weight of glycine. Slices of ventral tegmentum accumulated exogenous [³H]glycine by an energy-, temperature- and sodium-dependent mechanism. The uptake was mediated by two different transport systems; one system with relatively low affinity for glycine ( K_m ∼400 μ m ) and the other a higher affinity for glycine ( K_m ∼ 10 μ m ). Small amino acid analogues of glycine inhibited the uptake process, the most potent being taurine and β-alanine (47% and 44% inhibition, respectively, at 1 m m ). Release of exogenous [³H]glycine by elevated potassium and by protoveratrine A was calcium-dependent and tetrodotoxin-sensitive. Glycine (500 μ m -2 m m ) potentiated the protoveratrine A-induced release of exogenous [³H]dopamine from slices of ventral tegmentum; this potentiation was blocked by strychnine (10 μ m ). A convulsant dose of strychnine elevated the concentration of 3,4-dihydroxyphenylacetic acid in the ventral tegmentum. Glycine is likely to be a transmitter in the ventral tegmentum and to have a role regulating the activity of somatodendritic regions of mesolimbic dopaminergic neurones.     

Acute High Fat Diet Consumption Activates the Mesolimbic Circuit and Requires Orexin Signaling in a Mouse Model

Overconsumption of palatable energy-dense foods has negative health implications and it is associated with obesity and several eating disorders. Currently, little is known about the neuronal circuitries activated by the acute ingestion of a rewarding stimulus. Here, we used a combination of immunohistochemistry, pharmacology and neuronal tracing analyses to examine the role of the mesolimbic system in general, and the orexin neurons in particular, in a simple experimental test in which naïve mice are allowed to spontaneously eat a pellet of a high fat diet (HFD) for 2 h. We found that acute HFD activates c-Fos expression in several reward-related brain areas, including the ventral tegmental area (VTA), nucleus accumbens, central amygdala and lateral hypothalamic area. We also found that: i- HFD-mediated orosensory stimulation was required for the mesolimbic pathway activation, ii- acute HFD differentially activates dopamine neurons of the paranigral, parabrachial pigmented and interfascicular sub-regions of the VTA, and iii- orexin neurons of the lateral hypothalamic area are responsive to acute HFD. Moreover, orexin signaling blockade, with the orexin 1 receptor antagonist SB-334867, reduces acute HFD consumption and c-Fos induction in the VTA but not in the other mesolimbic nuclei under study. Finally, we found that most orexin neurons responsive to acute HFD innervate the VTA. Our results show that acute HFD consumption recruits the mesolimbic system and that the full manifestation of this eating behavior requires the activation of orexin signaling.     

Novel Endogenous 1,2,3,4-Tetrahydroisoquinoline Derivatives: Uptake by Dopamine Transporter and Activity to Induce Parkinsonism

Abstract: We designed as candidate metabolites and synthesized two 1-benzyl-1,2,3,4-tetrahydroisoquinoline derivatives containing a dopamine moiety: 1-(3',4'-dihydroxybenzyl)-1,2,3,4-tetrahydroisoquinoline (3',4'-DHBnTIQ) and 1-benzyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (6,7DHBnTIQ). Both were detected in mouse brain as endogenous amines by gas chromatography/mass spectrometry. 3',4'DHBnTIQ induced parkinsonism in mice when chronically administered intraperitoneally, whereas 6,7DHBnTIQ did not despite the structural similarity of the two compounds. This difference may be related to cellular uptake: In rat striatal synaptosomes, these compounds were intracellularly transported by the dopamine transporter with K _m values of 6.14 and 7.82 µ M and V _max values of 214.3 and 112.2 pmol/min/mg of protein, respectively. Thus, endogenous 3',4'DHBnTIQ may be actively transported into dopaminergic neurons and accumulated there, contributing at least in part to the induction of idiopathic Parkinson's disease.     

Regulation of the Vesamicol Receptor in Cholinergic Synaptic Vesicles by Acetylcholine and an Endogenous Factor

Cholinergic synaptic vesicles obtained from Torpedo electric organ have an active transport system for acetylcholine (ACh). Linked to ACh transport is a cytoplasmically oriented receptor for the inhibitory drug (-)-trans-2-(4-phenylpiperidino)cyclohexanol (vesamicol, formerly AH5183). Storage of freshly isolated vesicles for several days leads to more vesamicol binding. This can be induced immediately by hyposmotic lysis of the vesicles, which reseal to form right-side-out ghosts. The increased drug binding was due to a twofold increase in the affinity and a 20% increase in the amount of the receptor expressed, probably as a result of the release of an endogenous factor. Binding of vesamicol to ghosts was specifically inhibited by exogenous ACh acting with a dissociation constant of 18 mM. This suggests that the vesamicol binding site probably is linked to a low-affinity ACh binding site that is different from the higher affinity transport binding site. Equilibrium and kinetic attempts to determine whether exogenous ACh acts on the outside or the inside of the ghost membrane to inhibit vesamicol binding failed because of rapid equilibration of exogenous ACh across the ghost membrane. It is argued that the endogenous factor released by hyposmotic lysis might be ACh. Potential roles for such a transmembrane signal regulating the vesamicol receptor are discussed.     

Phencyclidine-Induced Dysregulation of Dopamine Response to Amphetamine in Prefrontal Cortex and Striatum

Phencyclidine (PCP) administration in rodents has been used to model aspects of schizophrenia. One aspect of such treatment has been the enhancement of amphetamine-induced increase of dopamine in the prefrontal cortex and striatum. To further characterize this mechanism rats were treated for 2 weeks with continuous PCP (15 mg/kg per day via Alzet minipump). Rats were implanted with a microdialysis probe into the prefrontal cortex (PFC) or striatum. Amphetamine was administered locally via the dialysis probe during one collection period and changes in extracellular dopamine were monitored. The effect of local administration of the dopamine uptake blocker nomifensine was also measured. Amphetamine (10 M) and nomifensine (10 M) increased the level of dopamine in both the PFC and striatum. PCP administration did not alter the response to amphetamine or nomifensine in the PFC, but reduced this response about 2-fold in striatum. To examine effects of continuous PCP administration on dopamine autoreceptor function, release of [³H]dopamine in response to electrical stimulation and in the presence of a dopamine agonist or antagonist was tested in striatal and prefrontal cortical tissue. Autoreceptor responses were similar in control and PCP-treated tissues. We conclude that the brain region-specific enhancement of dopamine release by peripheral amphetamine administration in rats after PCP is not likely mediated by alterations in the dopamine autoreceptors or changes in the dopamine transporter. The selective local responses of amphetamine indicates heterogeneous regional effects of continuous PCP on NMDA receptor function; effects that influence both regional excitatory responses and the overall dynamics of tonic excitatory/inhibitory inputs to the PFC and striatum.