Addiction is a maladaptive pattern of behavior following repeated use of reinforcing drugs in predisposed individuals, leading to lifelong changes. Common among these changes are alterations of neurons releasing dopamine in the ventral and dorsal territories of the striatum. The serotonin 5-HT_2Breceptor has been involved in various behaviors, including impulsivity, response to antidepressants, and response to psychostimulants, pointing toward putative interactions with the dopamine system. Despite these findings, it remains unknown whether 5-HT_2Breceptors directly modulate dopaminergic activity and the possible mechanisms involved. To answer these questions, we investigated the contribution of 5-HT_2Breceptors to cocaine-dependent behavioral responses. Male mice permanently lacking 5-HT_2Breceptors, even restricted to dopamine neurons, developed heightened cocaine-induced locomotor responses. Retrograde tracing combined with single-cell mRNA amplification indicated that 5-HT_2Breceptors are expressed by mesolimbic dopamine neurons.In vivoandex vivoelectrophysiological recordings showed that 5-HT_2B-receptor inactivation in dopamine neurons affects their neuronal activity and increases AMPA-mediated over NMDA-mediated excitatory synaptic currents. These changes are associated with lower ventral striatum dopamine activity and blunted cocaine self-administration. These data identify the 5-HT_2Breceptor as a pharmacological intermediate and provide mechanistic insight into attenuated dopamine tone following exposure to drugs of abuse.SIGNIFICANCE STATEMENTHere we report that mice lacking 5-HT_2Breceptors totally or exclusively in dopamine neurons exhibit heightened cocaine-induced locomotor responses. Despite the sensitized state of these mice, we found that associated changes include lower ventral striatum dopamine activity and lower cocaine operant self-administration. We described the selective expression of 5-HT_2Breceptors in a subpopulation of dopamine neurons sending axons to the ventral striatum. Increased burstingin vivoproperties of these dopamine neurons and a concomitant increase in AMPA synaptic transmission toex vivodopamine neurons were found in mice lacking 5-HT_2Breceptors. These data support the idea that the chronic 5-HT_2B-receptor inhibition makes mice behave like animals already exposed to cocaine with higher cocaine-induced locomotion associated with changes in dopamine neuron reactivity.