These neurons, which function in information processing and output pathways of the vertebrate brain, are commonly thought to be shielded from environmental impact. The results question the common perception that only sensory neurons react to sensory stimuli, and suggests that vertebrates could process information differently, dependent on if they are in the sun or in the shade.
Ruth Fischer, a PhD student in the lab, initially wanted to better understand the function of an enigmatic set of putative light receptive molecules, Encephalopsin and TMT-opsins to test if they might play a role in the regulation of known light-dependent brain functions such as seasonal reproduction. By analyzing the localization of the mRNA and protein in the brains of two teleost fishes, medaka and zebrafish, she confirmed that TMT-opsins are indeed present in cells of the diencephalon, which was already established to possess photoreceptive cells. But surprisingly, she also found these photoreceptors in inter- and motorneurons of the mid- and hindbrain. Interneurons are conventionally thought of as exclusively processing information coming from other neurons, while motorneurons control muscles, and neither type of neuron was thought to be responsive to light. In collaboration with the lab of Satchin Panda at the Salk Insitute, Ruth also confirmed that Encephalopsin, as well as TMT-opsins are indeed able to act as light sensors- at least in cells in a dish. But would this also be true for cells inside the brain?
“The findings led us wonder if these photoreceptor molecules will indeed transmit light information to the brain’s inter- and motorneurons.”, Ruth says. In collaboration with Bruno Fontinha and Simon Rumpel at IMP, the team tested if the way in which neurons talk to each other can be influenced by directly shining light on them. By measurements on slices of adult fish brains they showed that this is indeed the case.
But what could this mean? Kristin Teßmar-Raible explains: “These findings support two hypotheses: First, that sensory neurons, interneurons and motorneurons in the modern vertebrate brain evolved from a multifunctional „sensory-inter-motorneuron“ that combined all three tasks in one cell”, similar to what can be found in today’s jellyfish.. And second, that environmental light may modulate information transmission and processing in certain non-sensory neurons in the vertebrate brain”. If and how this may influence the way animals act and think remains to be elucidated…
Original publication in PLOS Biology
Fischer RM, Fontinha BM, Kirchmaier S, Steger J, Bloch S, Inoue D, Panda S, Rumpel S, Tessmar-Raible K. Co-Expression of VAL- and TMT-Opsins Uncovers Ancient Photosensory Interneurons and Motorneurons in the Vertebrate Brain. PLoS Biol. 2013 Jun.
DOI: 10.1371/journal.pbio.1001585.