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...

. Dopaminergic neurons are subject to a significant background GABAergic input in vivo. The presence of this GABAergic background might be expected to inhibit dopaminergic neuron firing. However, dopaminergic neurons are not all silent but instead fire in single-spiking and burst firing modes. Here we present evidence that phasic changes in the tonic activity of GABAergic afferents are a potential extrinsic mechanism that triggers bursts and pauses in dopaminergic neurons. We find that spontaneous singlespiking is more sensitive to activation of GABA receptors than phasic N-methyl-D-aspartate (NMDA)-mediated burst firing in rat slices (P15-P31). Because tonic activation of GABA A receptors has previously been shown to suppress burst firing in vivo, our results suggest that the activity patterns seen in vivo are the result of a balance between excitatory and inhibitory conductances that interact with the intrinsic pacemaking currents observed in slices. Using the dynamic clamp technique, we applied balanced, constant NMDA and GABA A receptor conductances into dopaminergic neurons in slices. Bursts could be produced by disinhibition (phasic removal of the GABA A receptor conductance), and these bursts had a higher frequency than bursts produced by the same NMDA receptor conductance alone. Phasic increases in the GABA A receptor conductance evoked pauses in firing. In contrast to NMDA receptor, application of constant AMPA and GABA A receptor conductances caused the cell to go into depolarization block. These results support a bidirectional mechanism by which GABAergic inputs, in balance with NMDA receptormediated excitatory inputs, control the firing pattern of dopaminergic neurons.

During reinforcement and sequence learning, dopaminergic neurons fire bursts of action potentials. Dopaminergic neurons in vivo receive strong background excitatory and inhibitory inputs, suggesting that one mechanism by which bursts may be produced is disinhibition. Unfortunately, these inputs are lost during slice preparation and are not precisely controlled during in vivo experiments. In the present study we show that dopaminergic neurons can be shifted into a balanced state in which constant synaptic N-methyl-d-aspartate (NMDA) and GABA(A) conductances are mimicked either pharmacologically or using dynamic clamp. From this state, a disinhibition burst can be evoked by removing the background inhibitory conductance. We demonstrate three functional characteristics of network-based disinhibition that promote high-frequency, short-latency bursting in dopaminergic neurons. First, we found that increasing the total background NMDA and GABA(A) synaptic conductances increased the intraburst firing frequency and reduced its latency. Second, we found that the disinhibition burst is sensitive to the proportion of background inhibitory input that is removed. In particular, we found that high-frequency, short-latency bursts were enhanced by increasing the degree of disinhibition. Third, the time course over which inhibition is removed had a large effect on the burst, namely, that synchronous removal of weak inhibitory inputs produces bursts of high intraburst frequency and shorter latency. Our results suggest that fast, more precisely timed bursts can be evoked by complete and synchronous disinhibition of dopaminergic neurons in a high-conductance state.