Many approaches that use viral vectors to deliver transgenes have limited 
transduction efficiency yet require high levels of transgene expression. In 
particular, infection via axon terminals is relatively inefficient but is a 
powerful means of achieving infection of specific neuron types. Combining this 
with optogenetic approaches requires high gene expression levels that are not 
typically achieved with nontoxic retrogradely infecting vectors. We generated 
rabies glycoprotein-pseudotyped lentiviral vectors that use a positive feedback 
loop composed of a Tet promoter driving both its own tetracycline-dependent 
transcription activator (tTA) ("TLoop") and channelrhodopsin-2-YFP (ChR2YFP). We 
show that TLoop vectors strongly express proteins in a drug-controllable manner 
in neurons that project to injection sites within the mouse brain. After initial 
infection, the virus travels retrogradely, stably integrates into the host 
genome, and expresses gene products. The expression is robust and allows 
optogenetic studies of neurons projecting to the location of virus injection, as 
demonstrated by fluorescence-targeted intracellular recordings. ChR2YFP 
expression did not cause observable signs of toxicity and continued for up to 6 
mo after infection. Expression can be reversibly blocked by administration of 
doxycycline, if necessary, for expression of gene products that might be more 
toxic. Overall, we present a system that will allow researchers to achieve high 
levels of gene expression even in the face of inefficient viral transduction. 
The particular vectors that we demonstrate may enhance efforts to gain a precise 
understanding of the contributions of specific types of projection neurons to 
brain function.