The Python pit organ: imaging and immunocytochemical analysis of an extremely sensitive natural infrared detector

The Python infrared-sensitive pit organ is a natural infrared imager that c ombines high sensitivity, ambient temperature function, microscopic dimensi ons, and self-repair. We are investigating the spectral sensitivity and sig nal transduction process in snake infrared-sensitive neurons, neither of wh ich is understood. For example, it is unknown whether infrared receptor neu rons function on a thermal or a photic mechanism. We imaged pit organs in l iving Python molurus and Python regius using infrared-sensitive digital vid eo cameras. Pit organs were significantly more absorptive and/or emissive t han surrounding tissues in both 3-5 mu m and 8-12 mu m wavelength ranges, P it organs exhibited greater absorption/emissivity in the 8-12 mu m range th an in the 3-5 mu m range. To directly test the relationship between photore ceptors and pit organ infrared-sensitive neurons, we performed immunocytoch emistry using antisera directed against retinal photoreceptor opsins. Retin al photoreceptors were labeled with antisera specific for retinal opsins, b ut these antisera failed to label terminals of infrared-sensitive neurons i n the pit organ. Infrared-receptive neurons were also distinguished from re tinal photoreceptors on the basis of their calcium-binding protein content. These results indicate that the pit organ absorbs infrared radiation in tw o major atmospheric transmission windows, one of which (8-12 mu m) matches emission of targeted prey, and that infrared receptors are biochemically di stinct from retinal photoreceptors. These results also provide the first id entification of prospective biochemical components of infrared signal trans duction in pit organ receptor neurons. (C) 1999 Elsevier Science S.A. All r ights reserved.