The NeuroD6 subtype of VTA neurons contributes to psychostimulant sensitization and behavioral reinforcement

Abstract

Reward-related behavior is complex and its dysfunction correlated with neuropsychiatric illness. Dopamine neurons of the ventral tegmental area (VTA) have long been associated with different aspects of reward function, but it remains to be disentangled how distinct VTA dopamine neurons contribute to the full range of behaviors ascribed to the VTA. Here, a recently identified subtype of VTA neurons molecularly defined by NeuroD6 (NEX1M) was addressed. Among all VTA dopamine neurons, less than 15% were identified as positive for NeuroD6. In addition to dopaminergic markers, sparse NeuroD6 neurons expressed the Vesicular glutamate transporter 2 gene. To achieve manipulation of NeuroD6 VTA neurons, NeuroD6(NEX)-Cre-driven mouse genetics and optogenetics were implemented. First, expression of Vesicular monoamine transporter 2 was ablated to disrupt dopaminergic function in NeuroD6 VTA neurons. Comparing Vmat2^{lox/lox;NEX-Cre} conditional knockout (cKO) mice with littermate controls, it was evident that baseline locomotion, preference for sugar and ethanol, and place preference upon amphetamine- and cocaine-induced conditioning were similar between genotypes. However, locomotion upon repeated psychostimulant administration was significantly elevated above control level in cKO mice. Second, optogenetic activation of NEX-Cre VTA neurons was shown to induce dopamine release and glutamatergic post-synaptic currents within the nucleus accumbens. Third, optogenetic stimulation of NEX-Cre VTA neurons in vivo induced significant place preference behavior, while stimulation of VTA neurons defined by Calretinin failed to cause a similar response. The results show that NeuroD6 VTA neurons exert distinct regulation over specific aspects of reward-related behavior, findings that contribute to the current understanding of VTA neurocircuitry.

Significance statement Reward-related behavior is complex and its dysfunction is implicated in many neuropsychiatric disorders, including drug addiction. Midbrain dopamine neurons of the ventral tegmental area (VTA) are crucial for reward behavior, but due to recently uncovered heterogeneity, it remains to be fully resolved how they regulate reward responsiveness and how their dysfunction might contribute to disease. Here we show that the recently described NeuroD6 (NEX) subtype of VTA dopamine neurons is involved in psychostimulant sensitization and that optogenetic stimulation of NEX-Cre VTA neurons induces dopamine release, glutamatergic post-synaptic currents and real time place preference behavior. NeuroD6 VTA neurons thus exert distinct regulation over specific aspects of reward-related behavior, findings that contribute to the current understanding of VTA neurocircuitry.