A Hitchhiker’s Guide to the Selection of Viral Vectors for Optogenetic Studies

The very first article to describe optogenetics in neural systems used viruses as delivery vectors (Boyden et al., Nat Neurosci 8(9):1263–1268, 2005). Since then, viral-mediated gene delivery has become the method of choice for opsin expression in the field. There are many classes of viruses, each with unique attributes that can be taken advantage of to serve specific experimental needs. For example, precise cellular targeting can be achieved by exploiting the propensity of different vectors to transduce specific cell types. Distinct anatomical inputs or outputs to defined regions can be identified and manipulated by choosing vectors for opsin expression with retrograde or anterograde trafficking abilities. Some vectors also have the capability to spread between synaptically connected neurons, and this holds great potential for the determination of structure–function relationships across complex networks. Here we review the major viral vector types used in optogenetic studies and offer a detailed protocol for the production of adeno-associated virus, which has become the most popular vector for optogenetic applications. This chapter is intended to provide an understanding of basic principles in vectorology and to serve as a user's guide to aid in the selection of appropriate vector. The engineering of recombinant viruses promises to expand the level of experimental precision and control, and may one day even lead to effective optogenetic therapies.