Changes in the firing pattern of midbrain dopamine neurons are thought to encode information for certain types of reward-related learning. In particular, the burst pattern of firing is predicted to result in more efficient dopamine release at target loci, which could underlie changes in synaptic plasticity. In this study, the effects of dopamine on the firing patterns of dopaminergic neurons in vivo and their electrophysiological characteristics in vitro were examined by using a genetic dopamine-deficient (DD) mouse model. Extracellular recordings in vivo showed that, although the firing pattern of dopamine neurons in normal mice included bursting activity, DD mice recordings showed only a single-spike pattern of activity with no bursts. Bursting was restored in DD mice after systemic administration of the dopamine precursor, L-3,4-dihydroxyphenylalanine (L-dopa). Whole-cell recordings in vitro demonstrated that the basic electrophysiology and pharmacology of dopamine neurons were identical between DD and control mice, except that amphetamine did not elicit a hyperpolarizing current in slices from DD mice. These data suggest that endogenously released dopamine plays a critical role in the afferent control of dopamine neuron bursting activity and that this control is exerted via a network feedback mechanism.T he activity of dopamine neurons has been shown to correlate with behavioral adaptations during reward-related learning in primates and rodents (1-4). Dopamine neurons fire spontaneously in vivo in a spectrum of patterns ranging from pacemaker, to random, to bursting modes (5, 6). Clusters of two to eight spikes characterize the burst mode (7,8). The random mode is the most common pattern encountered in vivo and is characterized by bursts of spikes followed by single-spike activity (5, 9). The pacemaker pattern, encountered in Ï·20% of neurons recorded in vivo, is characterized by single spikes firing in a clock-like manner, interrupted by infrequent bursts (2,5,6). Determining the origins and mechanisms responsible for burst firing in vivo is of interest because this firing pattern is thought to be responsible for large increases in dopamine release in the striatum that may mediate synaptic plasticity and contribute to reward-related learning (4, 10-17).The only pattern recorded spontaneously in vitro is the single-spike, pacemaker pattern without bursts (18)(19)(20). This contrasts markedly with recordings in vivo where bursts can still be encountered even if a neuron is classified as firing in a pacemaker mode (2). This disparity between in vivo and in vitro recordings suggests that afferents play a critical role in the control of dopamine neuron firing pattern.Release of dopamine in the basal ganglia and other projection areas may influence the afferent regulation of dopamine neurons through reciprocal and other long distance, multisynaptic connections (e.g., see ref. 21). This study investigates the effects of removing dopamine on the activity of dopamine neurons by using mice that were rendered dopamine-...