Revival of light-evoked neuronal signals in the post-mortem mammalian and human retina

The retina, a highly metabolic tissue in the central nervous system, consumes the most oxygen and energy stores in the body by tissue mass/volume. Consequently, it is not surprising that retinal ischemia leads to a rapid loss of retinal light responses and electrical transmission. In this study, we show that despite a swift decline of retinal light responses after circulatory death (decay time constant {tau} = [~]1-2 min), we were able to restore mouse rod and cone photoreceptor light signals from enucleated eyes up to 3 h postmortem with significantly better postmortem recovery of cone versus rod light responses. We also demonstrate that both rod and cone phototransduction are more resistant to postmortem enucleation delay than synaptic transmission to second order neurons (bipolar cells). Encouraged by these analyses and the lack of previously successful postmortem retinal light recordings from human foveal/macular photoreceptors, we attempted to restore light responses in the human macula using donor eyes harvested 0.5 - 5 hours postmortem. Here we show successful recordings of human macular cone photoresponses in samples obtained from eyes enucleated up to 5 hours postmortem. Comparing freshly enucleated non-human primate eyes with human eyes enucleated and reoxygenated at different times after death, we show the exponential decay of the light response amplitudes has a time constant of 74 min. We find that both cause of death and donor age are useful parameters for predicting the recovery of retinal light responses in postmortem human macular tissue. Moreover, we present evidence that hypoxia and secondary acidosis, two modifiable factors, are primary contributors to the rapid loss of retinal light signaling after death. Finally, we show postmortem hypoxia rather than acidosis is the major cause of irreversible decay of the light response amplitudes. The criteria and methodology that we have established here for reviving electrical photoresponses in the postmortem human eye will serve as a new starting point for studying neurophysiology and disease in the human retina.