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-CaMKllα-ChR2-eYFP) or silence (AAV-CaMKllα-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.