Intrinsically photosensitive ganglion cells of the tiger salamander retina

The purpose of my thesis is to investigate and characterize intrinsic photosensitivity among ganglion cells in the salamander retina. Intrinsically photosensitive ganglion cells (ipRGCs) have been well characterized in mammalian systems, both in terms of anatomy and electrophysiological properties. They are able to respond to light in the absence of photoreceptor-based input, and their intrinsic responses to light are, relative to photoreceptor based responses, slow in onset, low in sensitivity, sustained for the duration of onset, and slow in offset. Many aspects of melanopsin, the pigment responsible for intrinsic activity in mice, and its associated signaling pathways bear a stronger resemblance to invertebrate photoreceptive systems than vertebrate systems. The genetic diversity and gene expression of melanopsin and other photopigments is also greater in non-mammalian vertebrates. In this context I was interested in studying ipRGC electrophysiological responses in lower vertebrates. In the course of my studies I examined the typical responses of salamander ipRGCs to prolonged stimuli of different intensities. We characterized the nature of synaptic input and found these cells received ON inputs as well as GABAergic steady inhibition from amacrine cells. We also characterized the relative sizes of the receptive fields based on synaptic input versus intrinsic photosensitivity, and found them to be comparable. We tested on our ipRGCs the effects of various drugs thought to block the intrinsic photosensitivity in mammalian ipRGCs. Finally, we characterized the spectral sensitivity of the intrinsic responses to different wavelengths of light.