Journal Club: Demonstration of gene delivery + optogenetic stimulation of auditory neurons in the guinea pig model

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Today's journal article

Liu C, Yu N. Optogenetic Infection and Optical Stimulation: A Study on Auditory Responses in Guinea Pig Cochlear Neurons. 

Why I picked this article

"Optogenetics" is a method that combines genetic modification to place light‑sensitive proteins (such as channelrhodopsins) into selected neurons, so brief flashes of light can artificially turn these neurons on or off. Because light can be focused very precisely, it is a useful technique to activate neurons in a very specific and regulated manner. Optogenetics has been used as a useful tool in neuroscience research to understand neural functions; however, it has also been looked at as a potential therapeutic methodology to regain control of the neural network. 

For inner ear research, optogenetics has been investigated as a potential new methodology for future cochlear implants by different research groups. The current electrical cochlear implants restore hearing but have limited frequency resolution because the electric current spreads in fluid. Optogenetics may have an advantage to provide more precise, light‑based stimulation of auditory neurons if expressing light‑sensitive proteins in the inner ear can be achieved. 

This research shows the use of the adult guinea pig animal model and demonstrates that optical stimulation can drive compound action potentials comparable to sound.

Some of the research findings

Animal model & methodology: 
  • Species and age: adult guinea pigs, 25–30 weeks, either sex. 
  • Vector for light-sensitive protein: AAV2/8‑ChR2(H134R)‑hSyn‑eYFP
  • Vector was used at the dose 1.1×10^13 vg/mL. 
  • Vector administration: 5 µL injected via the round window membrane using a microinjection pump (retroauricular approach). 
  • Treatment: Right ear injected; left ear control. 
  • Timepoints: 4–6 weeks post‑injection before assays. 
  • Activation of Optogenetics: Stimulation: 470 nm blue laser
  • How auditory neuron activity is measured: researchers recorded optical compound action potentials (oCAPs). Acoustic clicks for aCAPs. 
Outcome: 
  • ChR2(H134R)‑eYFP expression confirmed in spiral ganglion neurons (SGNs) by immunohistochemistry and RT‑PCR. 
  • The light-sensitive protein was detected in 90% of tested animals (18/20 injected cochleae). 
  • oCAP amplitudes increased with laser intensity, suggesting that optogenetic stimulation of the auditory neurons is working. 
  • oCAP at 5.80 mW had similar amplitude and latency to an aCAP evoked by a 40 dB SPL click sound. 
  • Auditory thresholds as measured by auditory brainstem response were increased. It's likely caused by the surgery and injection. The viral solution itself did not further elevate thresholds relative to saline control. 
  • Concluding remark is that AAV‑round window delivery produced robust SGN expression without clear additional auditory pathway damage beyond surgical handling. The model to combine round-window gene transfection and optogenetics is feasible and cost-effective for animal models.
Figure 8. Laser-intensity increase and the response from auditory neurons as measured by oCAP. Liu and Yu (2025)

Haruna's takeaway

This is like back to cool topic of optogenetic research. It is proof-of-concept that the combination of delivering genes via the round window and then using the gene to use optogenetic techniques could at least work in healthy guinea pig cochlea. There are a lot of challenges ahead, like how to ensure delivery in humans, across the entire frequency range, consistently. The technology to stimulate neurons with precision will also be needed. I wonder if the biggest challenge is the pathological status of the cochlea, where adult and very unwell spiral ganglion neurons may be hard to work with in the same way. The fact that the gene delivered to auditory neurons seems very high is also interesting; I thought it was really hard to get them in there, but maybe it's not as hard as I initially thought. 

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This is Haruna's 90/100 of the 100-day challenge to post a science blog article every day! I love inner ear biology & cochlear physiology.