Midbrain dopamine (DA) neurons fire in 2 characteristic modes, tonic and phasic, which are thought to modulate distinct aspects of behavior. However, the inability to selectively disrupt these patterns of activity has hampered the precise definition of the function of these modes of signaling. Here, we addressed the role of phasic DA in learning and other DA-dependent behaviors by attenuating DA neuron burst firing and subsequent DA release, without altering tonic neural activity. Disruption of phasic DA was achieved by selective genetic inactivation of NMDA-type, ionotropic glutamate receptors in DA neurons. Disruption of phasic DA neuron activity impaired the acquisition of numerous conditioned behavioral responses, and dramatically attenuated learning about cues that predicted rewarding and aversive events while leaving many other DA-dependent behaviors unaffected.cue-dependent learning ͉ mouse behavior ͉ electrophysiology ͉ cyclic voltammetry D opamine (DA) neurons of the ventral midbrain project to the dorsal and ventral striatum, as well as to other corticolimbic structures such as the hippocampus, amygdala, and prefrontal cortex. Differential DA release (tonic or phasic) is thought to activate distinct signal transduction cascades through the activation of postsynaptic inhibitory and excitatory G protein coupled receptors. Phasic DA is proposed to activate excitatory, low-affinity DA D1-like receptors (Rs) (1, 2) to facilitate long-term potentiation of excitatory synaptic transmission and enhance activity of the basal ganglia direct pathway facilitating appropriate action selection during goal-directed behavior. Conversely, tonic DA release is proposed to act on inhibitory, high-affinity DA D2Rs to facilitate long-term depression of cortico-striatal synapses and suppress activity of medium spiny neurons (MSNs) of the basal ganglia indirect pathway (1, 3-5). Thus, coordinate D1R and D2R activation modulates motor and cognitive function, and facilitates behavioral flexibility by a dichotomous control of striatal plasticity (5).During reinforcement learning shifts in phasic DA neuron responses from primary rewards, to reward predicting, stimuli are thought to reflect the acquisition of incentive salience for the predictive conditioned stimuli (6-10). Coincident DA and glutamate release onto MSNs during conditioned-stimulus response learning facilitates long-term potentiation of excitatory synapses that is thought to underlie reinforcement learning (1, 2, 11). Pharmacological or genetic disruption of D1R signaling impairs learning in numerous behavioral paradigms (2, 11); thus, phasic DA acting through D1R is thought to facilitate memory acquisition by ''stamping-in'' stimulus-response associations.Although considerable correlative electrophysiological evidence, as well as pharmacological and genetic evidence, supports an important role of phasic DA in stamping-in cue-reward associations, other evidence suggests that DA is not necessary for learning conditioned-stimulus responses. Mice genetically modified to...

Neurotransmission operates on a millisecond timescale, but is changed by normal experience or neuropathology over days, weeks or even months. Despite the great importance of long-term neurotransmitter dynamics, no technique exists to track these changes within a subject from day to day over extended periods of time. Here we describe and characterize a microsensor that can detect the neurotransmitter dopamine with subsecond temporal resolution over months in vivo in rats and mice.

Loss of dopamine in Parkinson's disease is hypothesized to impede movement by inducing hypo- and hyperactivity in striatal spiny projection neurons (SPNs) of the direct (dSPNs) and indirect (iSPNs) pathways in the basal ganglia, respectively. The opposite imbalance might underlie hyperkinetic abnormalities, such as dyskinesia caused by treatment of Parkinson's disease with the dopamine precursor L-DOPA. Here we monitored thousands of SPNs in behaving mice, before and after dopamine depletion and during L-DOPA-induced dyskinesia. Normally, intermingled clusters of dSPNs and iSPNs coactivated before movement. Dopamine depletion unbalanced SPN activity rates and disrupted the movement-encoding iSPN clusters. Matching their clinical efficacy, L-DOPA or agonism of the D dopamine receptor reversed these abnormalities more effectively than agonism of the D dopamine receptor. The opposite pathophysiology arose in L-DOPA-induced dyskinesia, during which iSPNs showed hypoactivity and dSPNs showed unclustered hyperactivity. Therefore, both the spatiotemporal profiles and rates of SPN activity appear crucial to striatal function, and next-generation treatments for basal ganglia disorders should target both facets of striatal activity.

The brain’s ability to associate different stimuli is vital to long-term memory, but how neural ensembles encode associative memories is unknown. Here we studied how cell ensembles in the basal and lateral amygdala (BLA) encode associations between conditioned and unconditioned stimuli (CS, US). Using a miniature fluorescence microscope, we tracked BLA ensemble neural Ca2+ dynamics during fear learning and extinction over six days in behaving mice. Fear conditioning induced both up- and down-regulation of individual cells’ CS-evoked responses. This bi-directional plasticity mainly occurred after conditioning and reshaped the CS ensemble neural representation to gain similarity to the US-representation. During extinction training with repetitive CS presentations, the CS-representation became more distinctive without reverting to its original form. Throughout, the strength of the ensemble-encoded CS-US association predicted each mouse’s level of behavioral conditioning. These findings support a supervised learning model in which activation of the US-representation guides the transformation of the CS-representation.

Fluorescence imaging offers expanding capabilities for recording neural dynamics in behaving mammals, including the means to monitor hundreds of cells targeted by genetic type or connectivity, track cells over weeks, densely sample neurons within local microcircuits, study cells too inactive to isolate in extracellular electrical recordings, and visualize activity in dendrites, axons, or dendritic spines. We discuss recent progress and future directions for imaging in behaving mammals from a systems engineering perspective, which seeks holistic consideration of fluorescent indicators, optical instrumentation, and computational analyses. Today, genetically encoded indicators of neural Ca2+ dynamics are widely used, and those of trans-membrane voltage are rapidly improving. Two complementary imaging paradigms involve conventional microscopes for studying head-restrained animals and head-mounted miniature microscopes for imaging in freely behaving animals. Overall, the field has attained sufficient sophistication that increased cooperation between those designing new indicators, light sources, microscopes, and computational analyses would greatly benefit future progress.

During Pavlovian conditioning, phasic dopamine (DA) responses emerge to reward-predictive stimuli as the subject learns to anticipate reward delivery. This observation has led to the hypothesis that phasic dopamine signaling is important for learning. To assess the ability of mice to develop anticipatory behavior and to characterize the contribution of dopamine, we used a food-reinforced Pavlovian conditioning paradigm. As mice learned the cuereward association, they increased their head entries to the food receptacle in a pattern that was consistent with conditioned anticipatory behavior. D1-receptor knockout (D1R-KO) mice had impaired acquisition, and systemic administration of a D1R antagonist blocked both the acquisition and expression of conditioned approach in wild-type mice. To assess the specific contribution of phasic dopamine transmission, we tested mice lacking NMDA-type glutamate receptors (NMDARs) exclusively in dopamine neurons (NR1-KO mice). Surprisingly, NR1-KO mice learned at the same rate as their littermate controls. To evaluate the contribution of NMDARs to phasic dopamine release in this paradigm, we performed fast-scan cyclic voltammetry in the nucleus accumbens of awake mice. Despite having significantly attenuated phasic dopamine release following reward delivery, KO mice developed cueevoked dopamine release at the same rate as controls. We conclude that NMDARs in dopamine neurons enhance but are not critical for phasic dopamine release to behaviorally relevant stimuli; furthermore, their contribution to phasic dopamine signaling is not necessary for the development of cue-evoked dopamine or anticipatory activity in a D1R-dependent Pavlovian conditioning paradigm.D1 receptors | fast-scan cyclic voltammetry | nucleus accumbens | reinforcement learning | reward-prediction D uring an appetitive Pavlovian conditioning paradigm, a discrete cue (CS) is repeatedly paired with the delivery of a reward (US). As an animal learns to associate CS presentation with US delivery, a number of conditioned responses (CR) may occur in anticipation of reward delivery (1). The emergence of these anticipatory responses has served as a behavioral proxy by which the neural processes that underlie stimulus-reward learning may be studied. One commonly observed CR in rodents is the development of conditioned approach (CA) behavior; as the CS becomes predictive of US delivery, an animal will physically approach the location of the CS or US with increased frequency in the presence of the CS (2-5). Recent studies using electrophysiological and pharmacological manipulations have specifically implicated the dopamine system in the acquisition of CA behavior.The presentation of behaviorally relevant stimuli elicits burstfiring by the dopamine neurons of the ventral midbrain (6) and transient dopamine release in terminal regions including the nucleus accumbens core (AcbC) (7). During Pavlovian conditioning, a previously neutral stimulus (CS) elicits a phasic dopamine response as it becomes a predictor of the reward (US) (2...

The ability to control the electrical activity of a neuronal subtype is a valuable tool in deciphering the role of discreet cell populations in complex neural circuits. Recent techniques that allow remote control of neurons are either labor intensive and invasive or indirectly coupled to neural electrical potential with low temporal resolution. Here we show the rapid, reversible and direct activation of genetically identified neuronal subpopulations by generating two inducible transgenic mouse models. Confined expression of the capsaicin receptor, TRPV1, allows cell-specific activation after peripheral or oral delivery of ligand in freely moving mice. Capsaicin-induced activation of dopaminergic or serotonergic neurons reversibly alters both physiological and behavioural responses within minutes, and lasts ~10 min. These models showcase a robust and remotely controllable genetic tool that modulates a distinct cell population without the need for invasive and labour-intensive approaches.

Dopamine-producing neurons fire with both basal level tonic patterns and phasic bursts. Varying affinities of the five dopamine receptors have led to a hypothesis that higher affinity receptors are primarily activated by basal level tonic dopamine, while lower affinity receptors may be tuned to be sensitive to higher levels caused by phasic bursts. Genetically modified mice provide a method to begin to probe this hypothesis. Here we discuss three mouse models. Dopamine-deficient mice were used to determine which behaviors require dopamine. These behaviors were then analyzed in mice lacking D1 receptors and in mice with reduced phasic dopamine release. Comparison of the latter two mouse models revealed a similar failure to learn about and respond normally to cues that indicate either a positive or negative outcome, giving support to the hypothesis that phasic dopamine release and the D1 receptor act in the same pathway. However, the D1 receptor likely has additional roles beyond those of phasic dopamine detection, because D1 receptor knockout mice have deficits in addition to what has been observed in mice with reduced phasic dopamine release.