Assessment of the AAV-mediated expression of channelrhodopsin-2 and halorhodopsin in brainstem neurons mediating auditory signaling.

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).