PUTATIVE ODOR RECEPTORS LOCALIZE IN CILIA OF OLFACTORY RECEPTOR-CELLSIN RAT AND MOUSE - A FREEZE-SUBSTITUTION ULTRASTRUCTURAL-STUDY

Two different polyclonal antibodies were raised to synthetic peptides corresponding to distinct putative odour receptors of rat and mouse. B oth antibodies selectively labelled olfactory cilia as seen with cryof ixation and immunogold ultrastructural procedures. Regions of the olfa ctory organ where label was detected were consistent with those found at LM levels. Immunopositive cells were rare; only up to about 0.4% of these receptor cells were labelled. Despite chemical, species, and to pographic differences both antibodies behaved identically in their ult rastructural labelling patterns. For both antibodies, labelling was ve ry specific for olfactory cilia; both bound amply to the thick proxima l and the thinner and long distal parts of the cilia. Dendritic knobs showed little labelling if any. Dendritic receptor cell structures bel ow the knobs, supporting cell structures, and respiratory cilia did no t immunolabel. There were no obvious differences in morphology between labelled and unlabelled receptor cells and their cilia. Labelling cou ld be followed up to a distance of about 15 mu m from the knobs along the distal parts of the cilia. When labelled cells were observed, this signal was detectable in two, sometimes three, sections taken through these cells while being consistently absent in neighbouring cells. Th is pattern argues strongly for the specificity of the labelling. In co nclusion, very few receptor cells labelled with the antibodies to puta tive odour receptors. Additionally the olfactory cilia, the cellular r egions that first encounter odour molecules and that are thought to tr ansduce the odorous signal, displayed the most intense labelling with both antibodies. Consequently, the results showed these cilia as havin g many copies of the putative receptors. Finally, similar patterns of subcellular labelling were displayed in two different species, despite the use of different antibodies. Thus, this study provides compelling evidence that the heptahelical putative odour receptors localize in t he olfactory cilia.