Evgeny A. Budygin

Optogenetic Interrogation of Dopaminergic Modulation of the Multiple Phases of Reward-Seeking Behavior

Phasic activation of dopaminergic neurons is associated with reward-predicting cues and supports learning during behavioral adaptation. While noncontingent activation of dopaminergic neurons in the ventral tegmental are (VTA) is sufficient for passive behavioral conditioning, it remains unknown whether the phasic dopaminergic signal is truly reinforcing. In this study, we first targeted the expression of channelrhodopsin-2 to dopaminergic neurons of the VTA and optimized optogenetically evoked dopamine transients. Second, we showed that phasic activation of dopaminergic neurons in freely moving mice causally enhances positive reinforcing actions in a food-seeking operant task. Interestingly, such effect was not found in the absence of food reward. We further found that phasic activation of dopaminergic neurons is sufficient to reactivate previously extinguished food-seeking behavior in the absence of external cues. This was also confirmed using a single-session reversal paradigm. Collectively, these data suggest that activation of dopaminergic neurons facilitates the development of positive reinforcement during reward-seeking and behavioral flexibility.

INCREASED PHASIC DOPAMINE SIGNALING IN THE MESOLIMBIC PATHWAY DURING SOCIAL DEFEAT IN RATS

While reward-dependent facilitation of phasic dopamine signaling is well documented at both the cell bodies and terminals, little is known regarding fast dopamine transmission under aversive conditions. Exposure to aggressive confrontation is extremely aversive and stressful for many species including rats. The present study used fast-scan cyclic voltammetry and multiunit recording to determine if aggressive encounters and subsequent social defeat affect burst firing of ventral tegmental area (VTA) dopamine neurons and accumbal dopamine transients in defeated rats. Significant increases in the frequency of transient dopamine release were observed during interactions with an aggressive rat but not with a familiar cage mate. In agreement with voltammetric results, significant increases in burst frequency were detected in the VTA dopamine firing patterns during an aggressive confrontation; however, the number of spikes per burst remained unchanged. We found that neurons with lower burst rates under home cage conditions did not switch from nonbursting to bursting types, while neurons with higher burst levels showed amplified increases in bursting. This study demonstrates for the first time that aggressive confrontations in defeated rats are associated with increases in phasic dopamine transmission in the mesolimbic pathway.

Aversive stimulus differentially triggers subsecond dopamine release in reward regions

Aversive stimuli have a powerful impact on behavior and are considered to be the opposite valence of pleasure. Recent studies have determined some populations of ventral tegmental area (VTA) dopaminergic neurons are activated by several types of aversive stimuli, whereas other distinct populations are either inhibited or unresponsive. However, it is not clear where these aversion-responsive neurons project, and whether alterations in their activity translate into dopamine release in the terminal field. Here we show unequivocally that the neurochemical and anatomical substrates responsible for the perception and processing of pleasurable stimuli within the striatum are also activated by tail pinch, a classical painful and aversive stimulus. Dopamine release is triggered in the dorsal striatum and nucleus accumbens (NAc) core by tail pinch and is time locked to the duration of the stimulus, indicating that the dorsal striatum and NAc core are neural substrates, which are involved in the perception of aversive stimuli. However, dopamine is released in the NAc shell only when tail pinch is removed, indicating that the alleviation of aversive condition could be perceived as a rewarding event.

Neurochemical characterization of the release and uptake of dopamine in ventral tegmental area and serotonin in substantia nigra of the mouse.

In the present report, fast‐scan cyclic voltammetry was used to identify the monoamines that were released by electrical stimulation in mouse brain slices containing ventral tegmental area (VTA), substantia nigra (SN) ‐pars compacta (SNc) and ‐pars reticulata (SNr). We showed that voltammograms obtained in mouse VTA were consistent with detection of a catecholamine, while those in both subregions of the SN were consistent with detection of an indolamine, based on the reduction peak potentials. We used pharmacological blockade and genetic deletion of monoamine transporters to further confirm the identity of released monoamines in mouse midbrain and to assess the control of monoamines by their transporters in each brain region. Inhibition of dopamine and norepinephrine transporters by nomifensine (1 and 10 μm) decreased uptake rates in the VTA, but did not change uptake rates in either subregion of the SN. Serotonin transporter inhibition by fluoxetine (10 μm) decreased uptake rates in the SNc and SNr, but was without effect in the VTA. Selective inhibition of the norepinephrine transporter by desipramine (10 μm) had no effect in any brain region. Using dopamine transporter‐ and serotonin transporter‐knockout mice, we found decreased uptake rates in VTA and SN subregions, respectively. Peak signals recorded in each midbrain region were pulse number dependent and exhibited limited frequency dependence. Thus, dopamine is predominately detected by voltammetry in mouse VTA, while serotonin is predominately detected in mouse SNc and SNr. Furthermore, active uptake occurs in these areas and can be altered only by specific uptake inhibitors, suggesting a lack of heterologous uptake. In addition, somatodendritic dopamine release in VTA was not mediated by monoamine transporters. This work offers an initial characterization of voltammetric signals in the midbrain of the mouse and provides insight into the regulation of monoamine neurotransmission in these areas.

Role of serotonin in cocaine effects in mice with reduced dopamine transporter function

The mesolimbic dopaminergic system, especially the nucleus accumbens, has received attention for its involvement in the reinforcing and addictive properties of cocaine and other drugs of abuse. It is generally accepted that the ability of cocaine to inhibit the dopamine transporter (DAT) is directly related to its reinforcing actions. However, mice with a genetic deletion of the DAT (DAT-KO mice) still experience the rewarding effects of cocaine. These behavioral findings suggest that there is an alternate site for cocaine reinforcement. We demonstrate here that modulation of the serotonergic system in the ventral tegmental area, where the mesolimbic dopamine system originates, is a target of cocaine action. The ultimate effect of this serotonin mechanism in animal models with sustained elevations of dopamine may be a feed-forward enhancement of dopamine levels in the nucleus accumbens.

Optogenetic control of striatal dopamine release in rats

J. Neurochem. (2010) 114, 1344–1352.

Dopamine transporter‐dependent and ‐independent actions of trace amine β‐phenylethylamine

β‐Phenylethylamine (β‐PEA) is an endogenous amine that is found in trace amounts in the brain. It is believed that the locomotor‐stimulating action of β‐PEA, much like amphetamine, depends on its ability to increase extracellular dopamine (DA) concentrations owing to reversal of the direction of dopamine transporter (DAT)‐mediated DA transport. β‐PEA can also bind directly to the recently identified G protein‐coupled receptors, but the physiological significance of this interaction is unclear. To assess the mechanism by which β‐PEA mediates its effects, we compared the neurochemical and behavioral effects of this amine in wild type (WT), heterozygous and ‘null’ DAT mutant mice. In microdialysis studies, β‐PEA, administered either systemically or locally via intrastriatal infusion, produced a pronounced outflow of striatal DA in WT mice whereas no increase was detected in mice lacking the DAT (DAT‐KO mice). Similarly, in fast‐scan voltammetry studies β‐PEA did not alter DA release and clearance rate in striatal slices from DAT‐KO mice. In behavioral studies β‐PEA produced a robust but transient increase in locomotor activity in WT and heterozygous mice. In DAT‐KO mice, whose locomotor activity and stereotypy are increased in a novel environment, β‐PEA (10–100 mg/kg) exerted a potent inhibitory action. At high doses, β‐PEA induced stereotypies in WT and heterozygous mice; some manifestations of stereotypy were also observed in the DAT‐KO mice. These data demonstrate that the DAT is required for the striatal DA‐releasing and hyperlocomotor actions of β‐PEA. The inhibitory action on hyperactivity and certain stereotypies induced by β‐PEA in DAT‐KO mice indicate that targets other than the DAT are responsible for these effects.

Effects of chronic alcohol exposure on dopamine uptake in rat nucleus accumbens and caudate putamen

RationaleExisting data strongly suggest that alcohol affects dopamine (DA) neurotransmission in the brain. However, many questions remain about the effects of alcohol on the delicate equilibrium between such neurochemical processes as DA release and uptake. Dysregulation of these processes in the mesolimbic and nigrostriatal systems after chronic alcohol ingestion could be a neuroadaptation contributing to dependence.ObjectivesIn the present study, we have employed an alcohol vapor inhalation model to characterize the effects of chronic alcohol exposure on DA dynamics in rat nucleus accumbens (NAc) and caudate putamen (CP) using fast-scan cyclic voltammetry (FSCV) in brain slices. This method provides a unique view of real-time, spatially resolved changes in DA concentration.ResultsWe found that chronic alcohol exposure enhanced DA uptake rates in rat NAc and CP. These changes would have the effect of down-regulating extracellular DA levels, presumably a compensatory effect related to increased DA release by repeated alcohol exposure. The sensitivity of terminal release-regulating DA autoreceptors was not different in alcohol-exposed rats compared with alcohol-naïve animals.ConclusionsThe DA uptake changes after chronic alcohol exposure documented here using FSCV may be associated with a compensatory response of the DA system aimed at decreasing DA signaling. Alterations in autoreceptor function may require relatively long lasting alcohol exposure.

Optogenetic stimulation of VTA dopamine neurons reveals that tonic but not phasic patterns of dopamine transmission reduce ethanol self-administration.

There is compelling evidence that acute ethanol exposure stimulates ventral tegmental area (VTA) dopamine cell activity and that VTA-dependent dopamine release in terminal fields within the nucleus accumbens plays an integral role in the regulation of ethanol drinking behaviors. Unfortunately, due to technical limitations, the specific temporal dynamics linking VTA dopamine cell activation and ethanol self-administration are not known. In fact, establishing a causal link between specific patterns of dopamine transmission and ethanol drinking behaviors has proven elusive. Here, we sought to address these gaps in our knowledge using a newly developed viral-mediated gene delivery strategy to selectively express Channelrhodopsin-2 (ChR2) on dopamine cells in the VTA of wild-type rats. We then used this approach to precisely control VTA dopamine transmission during voluntary ethanol drinking sessions. The results confirmed that ChR2 was selectively expressed on VTA dopamine cells and delivery of blue light pulses to the VTA induced dopamine release in accumbal terminal fields with very high temporal and spatial precision. Brief high frequency VTA stimulation induced phasic patterns of dopamine release in the nucleus accumbens. Lower frequency stimulation, applied for longer periods mimicked tonic increases in accumbal dopamine. Notably, using this optogenetic approach in rats engaged in an intermittent ethanol drinking procedure, we found that tonic, but not phasic, stimulation of VTA dopamine cells selectively attenuated ethanol drinking behaviors. Collectively, these data demonstrate the effectiveness of a novel viral targeting strategy that can be used to restrict opsin expression to dopamine cells in standard outbred animals and provide the first causal evidence demonstrating that tonic activation of VTA dopamine neurons selectively decreases ethanol self-administration behaviors.

Fast onset of dopamine uptake inhibition by intravenous cocaine.

In vivo voltammetry in the nucleus accumbens of anesthetized rats was used to investigate the time of onset of dopamine uptake inhibition by intravenous cocaine. There is disagreement between behavioral and neurochemical studies concerning the time‐course of cocaine effects. Because of the high temporal resolution of voltammetry, the processes of dopamine release and uptake could be temporally separated to make evaluation of cocaine effects on uptake easier to address. Within 4 s after intravenous cocaine administration (1.5 mg/kg) there was significant inhibition of dopamine uptake that reached a plateau in 20 s. The peak heights of electrically evoked dopamine signals were also rapidly increased by cocaine. The signals returned to baseline values within approximately 1 h. In parallel behavioral studies, locomotor activity was significantly increased within 5–6 s following intravenous infusion of cocaine. Here we demonstrate that intravenous cocaine administration begins inhibiting the uptake of dopamine within a few seconds. This is at least 10‐fold faster than previous neurochemical estimates. The present findings may contribute to the understanding of the neurobiological mechanisms underlying the early behavioral responses to cocaine.