Meeting Abstract
Neuronatomy and electrophysiology indicate that trigeminal sensory neurons innervating python pit organs project to the ipsilateral brainstem nucleus of the lateral descending trigeminal tract (nLTTD), from which information is routed to the contralateral optic tectum, where visual and thermal maps of space are integrated. Previous functional analyses of sensory function in the snake brain involved technically difficult, invasive procedures in heavily sedated snakes. Here we set out to develop functional magnetic resonance imaging for large snakes, and to use it to measure brain response when pit organs were presented with physiologically relevant thermal stimuli and agents designed to elucidate the mechanisms of thermosensory signal transduction. Sedated snakes exposed unilaterally to thermal stimulation of pit organs exhibited differential blood-oxygenation-level-dependent (BOLD) response in the optic tecta and nLTTD. Heat applied to the left labial pits elicited an average intensity increase of 14% and 13.75% in ipsilateral nLTTD and contralateral optic tectum, when compared to corresponding control nLTTD and optic tectum (contralateral and ipsilateral, respectively) in the same snakes. Topical application of the TRPA1-channel antagonist A-967079 negated responses to heat (average differences in heat-induced voxel intensity in nLTTD and tectum were 3.5% and 0.5%, respectively). These results provide the first in vivo physiological demonstration of TRPA1 channels as molecular thermosensors in any infrared-imaging snake species, validate fMRI for physiological assessment of brain function in live snakes, and provide the first imaging-based confirmation of the infrared pathway in the snake brain.
