Activation of VTA GABA neurons disrupts reward consumption

The activity of ventral tegmental area (VTA) dopamine (DA) neurons promotes behavioral responses to rewards and environmental stimuli that predict them. VTA GABA inputs synapse directly onto DA neurons and may regulate DA neuronal activity to alter reward-related behaviors; however, the functional consequences of selective activation of VTA GABA neurons remains unknown. Here, we show that in?vivo optogenetic activation of VTA GABA neurons disrupts reward consummatory behavior but not conditioned anticipatory behavior in response to reward-predictive cues. In addition, direct activation of VTA GABA projections to the nucleus accumbens (NAc) resulted in detectable GABA release but did not alter reward consumption. Furthermore, optogenetic stimulation of VTA GABA neurons directly suppressed the activity and excitability of neighboring DA neurons as well as the release of DA in the NAc, suggesting that the dynamic interplay between VTA DA and GABA neurons can control the initiation and termination of reward-related behaviors.     

Electrophysiological effects of orexin-B and dopamine on rat nucleus accumbens shell neurons in vitro

Orexin (ORX) plays a critical role in reward-seeking behavior for natural rewards and drugs of abuse. The mesolimbic dopamine (DA) pathway that projects into the nucleus accumbens (NAc) from the ventral tegmental area is deeply involved in the neural mechanisms underlying reward, drug abuse and motivation. A recent study demonstrated that ORX-immunopositive fibers densely project into the shell of the NAc (NAcSh), suggesting that the NAcSh might be a site of the interaction between the ORXergic and DAergic systems for reward-seeking behavior. Therefore, the electrophysiological effects of ORX-B and DA on NAcSh neurons were examined extracellularly in rat brain slice preparations. ORX-B excited approximately 78% of neurons tested and inhibited 4%, whereas DA excited 50% and inhibited 22% of NAcSh neurons. These excitations and inhibitions persisted during synaptic blockade in a low-Ca²⁺/high-Mg²⁺ solution. DA-induced excitation was attenuated by SCH23390 or sulpiride, whereas DA-induced inhibition was suppressed by sulpiride. Of the neurons that were excited by ORX-B, 71% and 18% were excited and inhibited by DA, respectively. In 63% of neurons that were excited by ORX-B, the simultaneous application of ORX-B and DA increased the firing rate to two times greater than ORX-B alone, whereas, the simultaneous application significantly decreased the neuronal firing rate by 73% in the remaining 37% compared to ORX-B. These results suggest that an interaction between the ORXergic and DAergic systems occurs in the NAcSh and that the NAcSh is involved in the neural mechanisms in which ORX participates in the regulation of reward-seeking behavior.     

Endocannabinoids shape accumbal encoding of cue-motivated behavior via CB1 receptor activation in the ventral tegmentum

Transient increases in nucleus accumbens (NAc) dopamine concentration are observed when animals are presented with motivationally salient stimuli and are theorized to energize reward seeking. They arise from high-frequency firing of dopamine neurons in the ventral tegmental area (VTA), which also results in the release of endocannabinoids from dopamine cell bodies. In this context, endocannabinoids are thought to regulate reward seeking by modulating dopamine signaling, although a direct link has never been demonstrated. To test this, we pharmacologically manipulated endocannabinoid neurotransmission in the VTA while measuring transient changes in dopamine concentration in the NAc during reward seeking. Disrupting endocannabinoid signaling dramatically reduced, whereas augmenting levels of the endocannabinoid 2-arachidonoylglycerol (2AG) increased, cue-evoked dopamine concentrations and reward seeking. These data suggest that 2AG in the VTA regulates reward seeking by sculpting ethologically relevant patterns of dopamine release during reward-directed behavior.     

Hyperpolarization-activated currents control the excitability of principal neurons in the basolateral amygdala

Anxiety is thought to be influenced by neuronal excitability in basolateral nucleus of the amygdala (BLA). However, molecules that are critical for regulating excitability of BLA neurons are yet to be determined. In the present study, we have examined whether hyperpolarization-activated cyclic-nucleotide-gated (HCN) channels, which mediate the depolarizing cation current, can control the neuronal excitability. HCN channel-like activity appeared to be detected in BLA principal neurons. ZD7288, a specific blocker for HCN channels, increased the input resistance of membrane, hyperpolarized resting membrane potential, and enhanced action potential firing in BLA principal neurons. The blockade of HCN channels facilitated temporal summation of repetitively evoked excitatory postsynaptic potentials, suggesting that suppression of HCN channel activity in principal neurons can accelerate the propagation of synaptic responses onto the axon hillock. Thus, our findings have laid foundation for studies to reveal how HCN channel activity in BLA principal neurons regulates anxiety in vivo.     

Context coding in the mouse nucleus accumbens modulates motivationally relevant information

Neural activity in the nucleus accumbens (NAc) is thought to track fundamentally value-centric quantities linked to reward and effort. However, the NAc also contributes to flexible behavior in ways that are difficult to explain based on value signals alone, raising the question of if and how nonvalue signals are encoded in NAc. We recorded NAc neural ensembles while head-fixed mice performed an odor-based biconditional discrimination task where an initial discrete cue modulated the behavioral significance of a subsequently presented reward-predictive cue. We extracted single-unit and population-level correlates related to the cues and found value-independent coding for the initial, context-setting cue. This context signal occupied a population-level coding space orthogonal to outcome-related representations and was predictive of subsequent behaviorally relevant responses to the reward-predictive cues. Together, these findings support a gating model for how the NAc contributes to behavioral flexibility and provide a novel population-level perspective from which to view NAc computations.

Neural activity in the nucleus accumbens is thought to track value-centric quantities such as current or future expected reward, reward prediction errors, etc. This study shows that neural ensembles in nucleus accumbens encode a context signal that modulates subsequent stimulus-outcome associations, supporting a circuit-level gating model for behavioral flexibility.     

Nucleus accumbens neurons are innately tuned for rewarding and aversive taste stimuli, encode their predictors, and are linked to motor output

The nucleus accumbens (NAc) is a key component of the brain's reward pathway, yet little is known of how NAc cells respond to primary rewarding or aversive stimuli. Here, naive rats received brief intraoral infusions of sucrose and quinine paired with cues in a classical conditioning paradigm while the electrophysiological activity of individual NAc neurons was recorded. NAc neurons (102) were typically inhibited by sucrose (39 of 52, 75%) or excited by quinine (30 of 40, 75%) infusions. Changes in firing rate were correlated with the oromotor response to intraoral infusions. Most taste-responsive neurons responded to only one of the stimuli. NAc neurons developed responses to the cues paired with sucrose and quinine. Thus, NAc neurons are innately tuned to rewarding and aversive stimuli and rapidly develop responses to predictive cues. The results indicate that the output of the NAc is very different when rats taste rewarding versus aversive stimuli.     

Sources contributing to the average extracellular concentration of dopamine in the nucleus accumbens

Mesolimbic dopamine neurons fire in both tonic and phasic modes resulting in detectable extracellular levels of dopamine in the nucleus accumbens (NAc). In the past, different techniques have targeted dopamine levels in the NAc to establish a basal concentration. In this study, we used in vivo fast scan cyclic voltammetry (FSCV) in the NAc of awake, freely moving rats. The experiments were primarily designed to capture changes in dopamine caused by phasic firing - that is, the measurement of dopamine 'transients'. These FSCV measurements revealed for the first time that spontaneous dopamine transients constitute a major component of extracellular dopamine levels in the NAc. A series of experiments were designed to probe regulation of extracellular dopamine. Lidocaine was infused into the ventral tegmental area, the site of dopamine cell bodies, to arrest neuronal firing. While there was virtually no instantaneous change in dopamine concentration, longer sampling revealed a decrease in dopamine transients and a time-averaged decrease in the extracellular level. Dopamine transporter inhibition using intravenous GBR12909 injections increased extracellular dopamine levels changing both frequency and size of dopamine transients in the NAc. To further unmask the mechanics governing extracellular dopamine levels we used intravenous injection of the vesicular monoamine transporter (VMAT2) inhibitor, tetrabenazine, to deplete dopamine storage and increase cytoplasmic dopamine in the nerve terminals. Tetrabenazine almost abolished phasic dopamine release but increased extracellular dopamine to ～500?nM, presumably by inducing reverse transport by dopamine transporter (DAT). Taken together, data presented here show that average extracellular dopamine in the NAc is low (20-30?nM) and largely arises from phasic dopamine transients.     

Circadian activity rhythms and phase-shifting of cultured neurons of the rat suprachiasmatic nucleus

The mammalian suprachiasmatic nucleus (SCN) is the major endogenous pacemaker that coordinates various daily rhythms including locomotor activity and autonomous and endocrine responses, through a neuronal and humoral influence. In the present study we examined the behavior of dispersed individual SCN neurons obtained from 1- to 3-day-old rats cultured on multi-microelectrode arrays (MEAs). SCN neurons were identified by immunolabeling for the neuropeptides arginine-vasopressin (AVP) and vasoactive intestinal polypeptide (VIP). Single SCN neurons cultured at low density onto an MEA can express firing rate patterns with different circadian phases. In these cultures we observed rarely synchronized firing patterns on adjacent electrodes. This suggests that, in cultures of low cell densities, SCN neurons function as independent pacemakers. To investigate whether individual pacemakers can be influenced independently by phase-shifting stimuli, we applied melatonin (10 pM to 100 nM) for 30 min at different circadian phases and continuously monitored the firing rate rhythms. Melatonin could elicit phase-shifting responses in individual clock cells which had no measurable input from other neurons. In several neurons, phase-shifts occurred with a long delay in the second or third cycle after melatonin treatment, but not in the first cycle. Phase-shifts of isolated SCN neurons were also observed at times when the SCN showed no sensitivity to these phase-shifting stimuli in recordings from brain slices. This finding suggests that the neuronal network plays an essential role in the control of phase-shifts.     

Firing-rate models for neurons with a broad repertoire of spiking behaviors

Capturing the response behavior of spiking neuron models with rate-based models facilitates the investigation of neuronal networks using powerful methods for rate-based network dynamics. To this end, we investigate the responses of two widely used neuron model types, the Izhikevich and augmented multi-adapative threshold (AMAT) models, to a range of spiking inputs ranging from step responses to natural spike data. We find (i) that linear-nonlinear firing rate models fitted to test data can be used to describe the firing-rate responses of AMAT and Izhikevich spiking neuron models in many cases; (ii) that firing-rate responses are generally too complex to be captured by first-order low-pass filters but require bandpass filters instead; (iii) that linear-nonlinear models capture the response of AMAT models better than of Izhikevich models; (iv) that the wide range of response types evoked by current-injection experiments collapses to few response types when neurons are driven by stationary or sinusoidally modulated Poisson input; and (v) that AMAT and Izhikevich models show different responses to spike input despite identical responses to current injections. Together, these findings suggest that rate-based models of network dynamics may capture a wider range of neuronal response properties by incorporating second-order bandpass filters fitted to responses of spiking model neurons. These models may contribute to bringing rate-based network modeling closer to the reality of biological neuronal networks.     

Coordinated accumbal dopamine release and neural activity drive goal-directed behavior

Intracranial self-stimulation (ICSS) activates the neural pathways that mediate reward, including dopaminergic terminal areas such as the nucleus accumbens (NAc). However, a direct role of dopamine in ICSS-mediated reward has been questioned. Here, simultaneous voltammetric and electrophysiological recordings from the same electrode reveal that, at certain sites, the onset of anticipatory dopamine surges and changes in neuronal firing patterns during ICSS are coincident, whereas sites lacking dopamine changes also lack patterned firing. Intrashell microinfusion of a D1, but not a D2 receptor antagonist, blocks ICSS. An iontophoresis approach was implemented to explore the effect of dopamine antagonists on firing patterns without altering behavior. Similar to the microinfusion experiments, ICSS-related firing is selectively attenuated following D1 receptor blockade. This work establishes a temporal link between anticipatory rises of dopamine and firing patterns in the NAc shell during ICSS and suggests that they may play a similar role with natural rewards and during drug self-administration.     

Seasonal changes in intrinsic electrophysiological activity of song control neurons in wild song sparrows

Song behavior and its underlying neural substrate can change seasonally in adult songbirds. To test whether environmental cues induce seasonal changes in electrophysiological characteristics of song control neurons, we measured in vitro intrinsic neuronal activity in the song control nucleus RA of adult male song sparrows (Melospiza melodia) in both the fall non-breeding and spring breeding seasons. We found that RA neurons in spring-captured birds show a more than threefold increase in spontaneous firing rate compared to those from fall-captured birds. We conclude that environmental cues are sufficient to induce seasonal changes in electrophysiological properties of song control neurons, and that changes in these properties may underlie seasonal changes in song behavior.     

Nicotinic activation of mesolimbic neurons assessed by rubidium efflux in rat accumbens and ventral tegmentum

Dopaminergic mesolimbic neurons, with cell bodies in the ventral tegmental area (VTA) projecting to the nucleus accumbens (NAc), have been shown to be involved in the development of drug dependence. The application of nicotine to either the VTA or NAc produces an increase in dopamine release; however, the positive reinforcement produced by the systemic injection of nicotine is primarily due to stimulation of nicotinic acetylcholine receptors (nAChRs) in the VTA. Because the brain levels of nicotine would likely be the same in both brain areas, the nAChRs in the NAc may be less sensitive than those in the VTA. This study was undertaken to make a direct comparison of the native nAChRs in intact slices of NAc and VTA by measuring nicotine-stimulated efflux of (86)Rb(+) in a superfusion assay. The potency of nicotine and several other agonists was similar in both brain areas, but nicotine was somewhat more efficacious in the NAc. The effects of treatment duration, calcium and nicotinic antagonists were also determined. The results suggest that the predominant effect of nicotine in the VTA following systemic administration is due to differences in neuronal circuitry or firing patterns rather than inherent differences in the two nAChR populations.     

Positive Reinforcement Mediated by Midbrain Dopamine Neurons Requires D1 and D2 Receptor Activation in the Nucleus Accumbens

The neural basis of positive reinforcement is often studied in the laboratory using intracranial self-stimulation (ICSS), a simple behavioral model in which subjects perform an action in order to obtain exogenous stimulation of a specific brain area. Recently we showed that activation of ventral tegmental area (VTA) dopamine neurons supports ICSS behavior, consistent with proposed roles of this neural population in reinforcement learning. However, VTA dopamine neurons make connections with diverse brain regions, and the specific efferent target(s) that mediate the ability of dopamine neuron activation to support ICSS have not been definitively demonstrated. Here, we examine in transgenic rats whether dopamine neuron-specific ICSS relies on the connection between the VTA and the nucleus accumbens (NAc), a brain region also implicated in positive reinforcement. We find that optogenetic activation of dopaminergic terminals innervating the NAc is sufficient to drive ICSS, and that ICSS driven by optical activation of dopamine neuron somata in the VTA is significantly attenuated by intra-NAc injections of D1 or D2 receptor antagonists. These data demonstrate that the NAc is a critical efferent target sustaining dopamine neuron-specific ICSS, identify receptor subtypes through which dopamine acts to promote this behavior, and ultimately help to refine our understanding of the neural circuitry mediating positive reinforcement.     

Neurons in the amygdala with response-selectivity for anxiety in two ethologically based tests

The amygdala is a key area in the brain for detecting potential threats or dangers, and further mediating anxiety. However, the neuronal mechanisms of anxiety in the amygdala have not been well characterized. Here we report that in freely-behaving mice, a group of neurons in the basolateral amygdala (BLA) fires tonically under anxiety conditions in both open-field and elevated plus-maze tests. The firing patterns of these neurons displayed a characteristic slow onset and progressively increased firing rates. Specifically, these firing patterns were correlated to a gradual development of anxiety-like behaviors in the open-field test. Moreover, these neurons could be activated by any impoverished environment similar to an open-field; and introduction of both comfortable and uncomfortable stimuli temporarily suppressed the activity of these BLA neurons. Importantly, the excitability of these BLA neurons correlated well with levels of anxiety. These results demonstrate that this type of BLA neuron is likely to represent anxiety and/or emotional values of anxiety elicited by anxiogenic environmental stressors.     

Context-dependent coding in single neurons

The linear-nonlinear cascade model (LN model) has proven very useful in representing a neural system’s encoding properties, but has proven less successful in reproducing the firing patterns of individual neurons whose behavior is strongly dependent on prior firing history. While the cell’s behavior can still usefully be considered as feature detection acting on a fluctuating input, some of the coding capacity of the cell is taken up by the increased firing rate due to a constant “driving” direct current (DC) stimulus. Furthermore, both the DC input and the post-spike refractory period generate regular firing, reducing the spike-timing entropy available for encoding time-varying fluctuations. In this paper, we address these issues, focusing on the example of motoneurons in which an afterhyperpolarization (AHP) current plays a dominant role regularizing firing behavior. We explore the accuracy and generalizability of several alternative models for single neurons under changes in DC and variance of the stimulus input. We use a motoneuron simulation to compare coding models in neurons with and without the AHP current. Finally, we quantify the tradeoff between instantaneously encoding information about fluctuations and about the DC.     

Excitability of jcBNST Neurons Is Reduced in Alcohol-Dependent Animals during Protracted Alcohol Withdrawal

Alcohol dependence and withdrawal has been shown to cause neuroadaptive changes at multiple levels of the nervous system. At the neuron level, adaptations of synaptic connections have been extensively studied in a number of brain areas and accumulating evidence also shows the importance of alcohol dependence-related changes in the intrinsic cellular properties of neurons. At the same time, it is still largely unknown how such neural adaptations impact the firing and integrative properties of neurons. To address these problems, here, we analyze physiological properties of neurons in the bed nucleus of stria terminalis (jcBNST) in animals with a history of alcohol dependence. As a comprehensive approach, first we measure passive and active membrane properties of neurons using conventional current clamp protocols and then analyze their firing responses under the action of simulated synaptic bombardment via dynamic clamp. We find that most physiological properties as measured by DC current injection are barely affected during protracted withdrawal. However, neuronal excitability as measured from firing responses under simulated synaptic inputs with the dynamic clamp is markedly reduced in all 3 types of jcBNST neurons. These results support the importance of studying the effects of alcohol and drugs of abuse on the firing properties of neurons with dynamic clamp protocols designed to bring the neurons into a high conductance state. Since the jcBNST integrates excitatory inputs from the basolateral amygdala (BLA) and cortical inputs from the infralimbic and the insular cortices and in turn is believed to contribute to the inhibitory input to the central nucleus of the amygdala (CeA) the reduced excitability of the jcBNST during protracted withdrawal in alcohol-dependent animals will likely affect ability of the jcBNST to shape the activity and output of the CeA.     

Behavioral and neurophysiological evidence for a facilatory interaction between co-existing transmitters: cholecystokinin and dopamine

Cholecystokinin (CCK) and dopamine (DA) co-exist in ventral tegmental neurons which project via the mesencephalic pathway to the nucleus accumbens of the rat. CCK and DA are located in separate neurons in the substantia nigra which projects via the nigrostriatal pathway to the caudate nucleus in the rat. The functional significance of this peptide-amine co-localization was investigated using behavioral and neurophysiological techniques. CCK injected directly into the nucleus accumbens potentiated apomorphine-induced stereotypy and dopamine-induced hyperlocomotion. CCK injected directly into the caudate nucleus had no effect on apomorphine-induced stereotypy or dopamine-induced hyperlocomotion CCK injected alone into either site did not induce stereotypy or hyperlocomotion. The dose-response curve to apomorphine induction of stereotypy was shifted to the left by CCK, indicating increased sensitivity to the dopaminergic agonist. Neurophysiological analysis of the firing rate of ventral tegmental neurons demonstrated that CCK produced a left-shift in the dose-response curve of apomorphine on inhibition of neuronal firing. These data suggest that CCK acts as a modulator of dopamine, increasing neuronal responses to dopaminergic agonists. The potentiation of dopamine by CCK may be specific to the mesolimbic neurons, where CCK and DA co-exist in the rat.     

Spike-timing precision and neuronal synchrony are enhanced by an interaction between synaptic inhibition and membrane oscillations in the amygdala

The basolateral complex of the amygdala (BLA) is a critical component of the neural circuit regulating fear learning. During fear learning and recall, the amygdala and other brain regions, including the hippocampus and prefrontal cortex, exhibit phase-locked oscillations in the high delta/low theta frequency band (～2-6 Hz) that have been shown to contribute to the learning process. Network oscillations are commonly generated by inhibitory synaptic input that coordinates action potentials in groups of neurons. In the rat BLA, principal neurons spontaneously receive synchronized, inhibitory input in the form of compound, rhythmic, inhibitory postsynaptic potentials (IPSPs), likely originating from burst-firing parvalbumin interneurons. Here we investigated the role of compound IPSPs in the rat and rhesus macaque BLA in regulating action potential synchrony and spike-timing precision. Furthermore, because principal neurons exhibit intrinsic oscillatory properties and resonance between 4 and 5 Hz, in the same frequency band observed during fear, we investigated whether compound IPSPs and intrinsic oscillations interact to promote rhythmic activity in the BLA at this frequency. Using whole-cell patch clamp in brain slices, we demonstrate that compound IPSPs, which occur spontaneously and are synchronized across principal neurons in both the rat and primate BLA, significantly improve spike-timing precision in BLA principal neurons for a window of ～300 ms following each IPSP. We also show that compound IPSPs coordinate the firing of pairs of BLA principal neurons, and significantly improve spike synchrony for a window of ～130 ms. Compound IPSPs enhance a 5 Hz calcium-dependent membrane potential oscillation (MPO) in these neurons, likely contributing to the improvement in spike-timing precision and synchronization of spiking. Activation of the cAMP-PKA signaling cascade enhanced the MPO, and inhibition of this cascade blocked the MPO. We discuss these results in the context of spike-timing dependent plasticity and modulation by neurotransmitters important for fear learning, such as dopamine.     

Selective activation of cholinergic interneurons enhances accumbal phasic dopamine release: setting the tone for reward processing

Dopamine plays a critical role in motor control, addiction, and reward-seeking behaviors, and its release dynamics have traditionally been linked to changes in midbrain dopamine neuron activity. Here, we report that selective endogenous cholinergic activation achieved via in vitro optogenetic stimulation of nucleus accumbens, a terminal field of dopaminergic neurons, elicits real-time dopamine release. This mechanism occurs via direct actions on dopamine terminals, does not require changes in neuron firing within the midbrain, and is dependent on glutamatergic receptor activity. More importantly, we demonstrate that in vivo selective activation of cholinergic interneurons is sufficient to elicit dopamine release in the nucleus accumbens. Therefore, the control of accumbal extracellular dopamine levels by endogenous cholinergic activity results from a complex convergence of neurotransmitter/neuromodulator systems that may ultimately synergize to drive motivated behavior.     

Stimulation of midbrain dopaminergic structures modifies firing rates of rat lateral habenula neurons

Ventral tegmental area (VTA) and substantia nigra pars compacta (SNpc) are midbrain structures known to be involved in mediating reward in rodents. Lateral habenula (LHb) is considered as a negative reward source and it is reported that stimulation of the LHb rapidly induces inhibition of firing in midbrain dopamine neurons. Interestingly, the phasic fall in LHb neuronal activity may follow the excitation of dopamine neurons in response to reward-predicting stimuli. The VTA and SNpc give rise to dopaminergic projections that innervate the LHb, which is also known to be involved in processing painful stimuli. But it's unclear what physiological effects these inputs have on habenular function. In this study we distinguished the LHb pain-activated neurons of the Wistar rats and assessed their electrophysiological responsiveness to the stimulation of the VTA and SNpc with either single-pulse stimulation (300 μA, 0.5 Hz) or tetanic stimulation (80 μA, 25 Hz). Single-pulse stimulation that was delivered to either midbrain structure triggered transient inhibition of firing of ～90% of the LHb pain-activated neurons. However, tetanic stimulation of the VTA tended to evoke an elevation in neuronal firing rate. We conclude that LHb pain-activated neurons can receive diverse reward-related signals originating from midbrain dopaminergic structures, and thus participate in the regulation of the brain reward system via both positive and negative feedback mechanisms.