Abstract
Purpose :
Transient receptor potential vanilloid (TRPV1) cation channel activation is associated with stressors such as heat, pH and pressure. Previously, we showed sensitivity to increased intraocular pressure (IOP), the only modifiable symptom of glaucomatous optic neuropathy, causes a transient increase in TRPV1 expression and calcium conductance in retinal ganglion cells (RGC). Here, we assess how genetic deletion of TRPV1 effects RGC morphology and function in a rodent model of glaucoma.
Methods :
To model glaucoma, IOP was raised in C57/B6 (WT) and TRPV1 knockout (TRPV1KO) mice by microbead occlusion of aqueous outflow. An equal volume of PBS was injected into fellow eyes. IOP was monitored over two weeks. To examine RGC function and morphology we performed whole-cell recordings, using dye-filled (Alexa-555) pipettes, in RGCs from acutely-dissected retinas. We measured RGC dendritic input and axonal output to light using voltage- and current-clamp modes under bath conditions to block specific receptors. Here, we limit our analysis to alpha-ON sustained (αONs) RGCs. Optic nerve (ON) compound action potentials (CAP) were obtained by injecting a series of voltages through the proximal section of the ON while recording from the distal. After the stimulating voltage producing a supramaximal CAP was reached, the recording pipette was adjusted several times to assess the change in response to the stimulating voltage as a function of pipette resistance. We analyzed the slope of the regression line obtained from plotting CAP area against the resistance.
Results :
TRPV1KO RGCs showed significant reduction in dendritic complexity and total length compared to WT controls (n≥12, p<0.001). TRPV1KO has no further effect on RGC dendritic morphology at two weeks of IOP elevation (n≥12). Next, we assessed AMPA-mediated light-evoked responses. Increased IOP was associated with increased AMPA-mediated light response in WT αONs RGCs but absent in TRPV1KO αONs RGCs (n≥8, p<0.01). We then measured light-driven spike output of these cells. Increased IOP was correlated with increased spike output in WT but not TRPV1KO αONs RGCs (n≥13, p<0.001). Finally, we examined the CAP of the optic nerve proper and found IOP elevation was associated with increased evoked potentials in TRPV1KO but not in WT nerves (n≥5, p<0.05).
Conclusions :
In conclusion, we find TRPV1 channels contribute to IOP-induced enhanced excitation and excitability.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.