AAV-only targeting of ventral tegmental area dopamine neurons for optical self-stimulation studies in mice

Optogenetic studies in rodents have highlighted the importance of ventral tegmental area (VTA) dopamine (DA) neurons to reward, learning, and motivation. Selective manipulation of VTA DA neurons is generally achieved with transgenic rodent lines that use a DA neuron-specific promoter to express Cre recombinase, combined with intra-VTA infusion of adeno-associated virus (AAV) vectors harboring Cre-dependent expression cassettes. These studies incur expenses related to line acquisition and husbandry and are typically rate-limited by in-house breeding capacity. Here, we describe a dual AAV approach that permits genetic access to VTA DA neurons and supports robust optical self-stimulation in mice. One vector employs a mouse tyrosine hydroxylase (TH) promoter fragment to drive Cre expression (AAV8-mTH-Cre). Intra-VTA co-infusion of AAV8-mTH-Cre with another vector harboring a Cre-dependent reporter labeled TH-positive VTA neurons in C57BL/6J mice with an efficiency (82%) and selectivity/fidelity (73%) comparable to studies involving TH-Cre knock-in mice. Co-infusion of AAV8-mTH-Cre with a Cre-dependent channelrhodopsin (ChR2) expression vector permitted optical regulation of VTA neurons exhibiting electrophysiological features consistent with conventional DA neurons and supported robust optical self-stimulation. Thus, this AAV-only approach allows investigation of reward-related behavior when transgenic Cre lines are not available, and in studies involving mutant mouse lines or inbred mouse strains.