  Precise spatial and temporal manipulation of neural activity in specific
genetically defined cell populations is now possible with the advent of
optogenetics. The emerging field of optogenetics consists of a set of
naturally-occurring and engineered light-sensitive membrane proteins that are
able to activate (e.g., channelrhodopsin-2, ChR2) or silence (e.g.,
halorhodopsin, NpHR) neural activity. Here we demonstrate the technique and the
feasibility of using novel adeno-associated viral (AAV) tools to activate
(AAV-CaMKll{\alpha}-ChR2-eYFP) or silence (AAV-CaMKll{\alpha}-eNpHR3.0-eYFP)
neural activity of rat prefrontal cortical prelimbic (PL) pyramidal neurons in
vivo. In vivo single unit extracellular recording of ChR2-transduced pyramidal
neurons showed that delivery of brief (10 ms) blue (473 nm) light-pulse trains
up to 20 Hz via a custom fiber optic-coupled recording electrode (optrode)
induced spiking with high fidelity at 20 Hz for the duration of recording (up
to two hours in some cases). To silence spontaneously active neurons we
transduced them with the NpHR construct and administered continuous green (532
nm) light to completely inhibit action potential activity for up to 10 seconds
with 100% fidelity in most cases. These versatile photosensitive tools combined
with optrode recording methods provide experimental control over activity of
genetically defined neurons and can be used to investigate the functional
relationship between neural activity and complex cognitive behavior.
