The physiology and circuitry associated with dorsal cochlear nucleus neurons 
(DCN) have been well described. The ability to remotely manipulate neuronal 
activity in these neurons would represent a step forward in the ability to 
understand the specific function of DCN neurons in hearing. Although, 
optogenetics has been used to study the function of pathways in other systems 
for several years, in the auditory system only neurons in the auditory cortex 
have been studied using this technique. Adeno-associated viral vectors with 
either channelrhodopsin-2 fused with GFP (ChR2-GFP) or halorhodopsin fused with 
mCherry (HaloR-mCherry), capable of expressing light sensitive cation channels 
or chloride pumps, respectively, were delivered into the dorsal cochlear nucleus 
(DCN). One to 18 months later, expression of ChR2 and HaloR was observed 
throughout the DCN. Rhodopsin distribution within the DCN was determined to be 
within several cell types identified based on morphology and location within the 
DCN. Expression of ChR2-GFP and HaloR-mCherry was found at both the injection 
site as well as in regions receiving projections from the site. Wavelength 
appropriate optical stimulation in vivo resulted in neuronal activity that was 
significantly increased over pre-stimulation levels with no return to baseline 
levels during the time of the light exposure. We also examined the effects of 
optically driven neuronal activity on subsequent tone driven responses in the 
DCN. In the DCN 75% of the 16 electrode sites showed decreased neuronal activity 
in response to a tone immediately following light stimulation while six percent 
were decreased following tone stimulation and 19% of the electrode sites showed 
no change. This is in contrast to tone driven neuronal activity prior to the 
light exposure in which the majority of electrode sites showed increased 
neuronal activity. Our results indicate that expression and activation of 
rhodopsin within neurons involved in auditory processing does not appear to have 
deleterious effects on hearing even 18 months following expression. In addition, 
virally targeted rhodopsins may be useful as tract tracers to delineate as well 
as modulate the activity of pathways and specific neurons. In the future 
rhodopsins can be targeted to specific subpopulations of auditory neurons. 
Ultimately, photostimulation may provide a physiologically relevant method for 
modulating the function of auditory neurons and affecting hearing outcomes. This 
article is part of a Special Issue entitled Optogenetics (7th BRES).