Carbon Dioxide Euthanasia Selectively Affects Physiology of Murine Retinal Cells, Implicating Carbonic Anhydrase-Expressing Cell Populations

Purpose: Laboratory rodents are commonly euthanized by exposure to gradually increasing concentrations of carbon dioxide (CO2). CO2 exposure induces respiratory acidosis, reduces dopamine levels, and causes hypoxia in central nervous system tissues, potentially affecting their physiology. These effects may be critical for brain and retinal tissues, yet the impact of CO2 euthanasia remains largely unclear. Methods: Using dark-adapted transretinal electroretinography (tERG), we tested the hypothesis that terminal CO2 overdose alters mouse retinal physiology. Two CO2 displacement rates were used, 30% and 60% of the chamber volume/min, with cervical dislocation as a reference method. Results: Neither slow nor fast CO2 overdose euthanasia affects rod photoreceptor and ON-bipolar cell flash responses. Activation and deactivation of rod phototransduction were not affected by CO2 overdose. However, both flow rates of CO2 exposure led to decreased cone photoreceptor response amplitudes and increased power spectral density integrals of oscillatory potentials (OPs). Moreover, Müller glia flash response amplitudes were reduced, and OPs were faster and more synchronized with the slower CO2 displacement rate compared to the two other euthanasia methods. In the mammalian retina, carbonic anhydrase is expressed in Müller glia, retinal pigment epithelium, most cone photoreceptors and a subset of amacrine cells. Conclusions: Our findings indicate that CO2 euthanasia can generally be considered a safe termination method for retinal research, but caution should be taken when studying the physiology of carbonic anhydrase-expressing cells.