| In general, olfactory receptor neurons (ORNs) are bipolar-shaped cells with an apical dendrite containing cilia or microvilli and a basal unmyelinated axon. In the squid, Lolliguncula brevis, five morphological variations on the general theme have been described. Each ORN subtype can be identified by its unique shape, relative size and location within the epithelium. In this dissertation, I hypothesized that these morphological differences reflect functional differences in odorant sensitivity and transduction. My studies found that although there were only five distinct morphological subtypes, there were many more functional subtypes. Using metabolite phenotyping I found that within a single morphological subtype there could be as many as 12 metabolite profiles. Compared to the metabolite profiles from chemosensory neurons of other species, the multiple metabolite profiles suggested functional heterogeneity. Functional diversity for each ORN subtype was supported by protein expression studies showing that three of the five subtypes contain proteins for more than one signal transduction pathway. As with the metabolite profiling, there was heterogeneity in transduction proteins within a morphological subtype. Finally, using an activity marker, we showed that each ORN subtype could respond to more than one odorant and had differential sensitivity to the odorants tested. These data clarify previous work showing that a single squid ORN can respond to multiple odorants and provides a molecular basis for the observed physiological response. The results from these studies support my hypothesis that morphologically distinct ORNs reflect functional differences, which we found in signal transduction protein expression and odorant sensitivity. |
