The characterization of retinal electrical signals of the in-vitro and in-vivo porcine eye : a novel large mammalian eye model

Abstract

Introduction: A convenient and easily available experimental eye model would facilitate studies to understand the physiology and pathophysiology of the complex retina. In the animal kingdom, primate eyes are most similar to human eyes. However, there are ethical and legal issues in using primates for research studies. Thus, it is essential to develop an alternative animal model for eye research. In terms of structure and physiology, the porcine eye is a good alternative to the non-human primate as a model for eye research. Although the photoreceptor distribution of the porcine retina is different from that of the primate, many retinal characteristics of the pig are similar to that of human. In this study, we investigate the characteristics of retinal electrical activity in the porcine eye using multifocal electroretmography (mfERG) to determine the suitability of this model for future eye research in different aspects, including in-vivo and in-vitro conditions as well as normal and abnormal conditions. Objectives: - To study the cellular origins of the porcine mfERG in-vivo by use of pharmacological agents blocking specific retinal pathways. - To identify alterations of the porcine mfERG in-vivo by use of a transgenic pig model with known retinal re-wiring. - To study the cellular origins of the porcine mfERG in-vitro by use of pharmacological agents blocking specific retinal pathways. - To identify the alterations of the porcine mfERG in-vitro by use of anesthetics and variations of the culture environment. The findings from this study will enhance our understanding of the characteristics of retinal electrical signals of the in-vivo and in-vitro porcine eyes. Methods: There were three experiments in this study. In Experiment 1, the mfERG was recorded from normal (wild type) porcine eye to characterize the cellular origins of the porcine mfERG by injection of pharmacological agents known to block specific retinal pathways. In Experiment 2, the mfERG was recorded from the transgenic pig (Tg) with retinitis pigmentosa (RP). The mfERG from the transgenic pig was compared with the mfERG obtained from Experiment 1 to characterize the alteration of retinal signal pathway at an early stage of RP. In Experiment 3, the mfERG was recorded from isolated perfused pig eye to assess the properties of retinal components in-vitro. The cellular origins of mfERG in-vitro were characterized by injection of the same pharmacological agents used in Experiment 1. The effect of anesthesia on the mfERG was also assessed. Results: From Experiment 1, the cellular origins of the porcine mfERG were found to be similar to that of the primate. The cellular origins of the first order kernel (Kl) and the first slice of the second order kernel (K2.1) porcine mfERG appear to be both the inner and outer retina. In Kl mfERG, the nl involve responses of cone photoreceptors and OFF-bipolar cells. The leading edge of P1 was dominated by ON-bipolar cell depolarization. The rear edge of P1, n2 and p2 was dominated by ON-bipolar activities and shaped by the activities of OFF-bipolar cells and retinal cells with NMDAr and voltage gated sodium channels other than ganglion cells. The p3 was mainly inner retinal activities. In Experiment 2, the alteration in porcine mfERG associated with neural re-wiring in the degenerating porcine RP model (six-week P347L transgenic pig (Tg)) was demonstrated. The signal alterations in the Tg pigs involved both outer and inner retinal responses. In particular, the early components of the inner retinal contribution were obviously altered in Tg mfERG; the inner retinal components at about 24 and 40ms appeared to be inverted. The differences in the estimates of OFF-bipolar cell pathway contributions were minimal. There was no change of cone cell responses in Tg mfERG In Experiment 3, the functional cellular components of porcine mfERG in-vitro were demonstrated by use of pharmacological agents as in Experiment 1. A close relationship between inner retinal activity and effect of commonly used anesthesia was found. Isoflurane and propofol caused influence on mfERG resembled the inner retinal activities sensitive to TTX+NMDA application. Conclusions: The porcine eye could be a good alternative to primate eye for studies of retinal physiology. Experiments 1 and 3 showed the possibilities of obtaining useful mfERG information from the porcine model, and the similarity of cellular origins of mfERG between pig and primate. Experiment 2 and 3 illustrated a good example of using porcine mfERG in-vivo and in-vitro for experiments that may not be easily performed in primates.