Transduction and odor mapping
Amino acids are known water-borne olfactory stimuli that elicit responses in olfactory receptor neurons of larval Xenopus laevis (see Hassenklöver et al., 2012). We were able to show that a large subgroup of amino acid-sensitive receptor neurons does not possess the canonical cAMP-mediated transduction pathway (see Manzini et al., 2003). We hypothesized that these amino acid responses might be mediated via a vomeronasal-like transduction machinery. We could show that amino acid-responsive receptor neurons in the lateral MOE employ a phospholipase C (PLC) and diacylglycerol-mediated (DAG) transduction cascade (see Sansone et al., 2014). On the other hand, a substantial number of ORNs possess the well described cAMP-mediated transduction pathway and are characterized by responses to alcohols, aldehydes and ketons. A subgroup of amino acid-sensitive receptor neurons does not possess the canonical cAMP-mediated transduction pathway, but signals via PLC and DAG.
The above mentioned subpopulations of receptor neurons project into different glomerular clusters of the main olfactory bulb. Glomeruli activated by amino acids are located laterally compared to those activated by alcohols, aldehydes and ketones. Responses to amines and bile acids, two additional groups of waterborne odorants, are more broadly distributed in the olfactory bulb. This unequal mapping of odorants in the main olfactory bulb is consistent with our finding that the glomerular layer in larval Xenopus laevis is organized in spatially distinct clusters, and that the presynaptic proteins synaptophysin and synaptotagmin are differentially expressed in glomeruli in the lateral and medial main olfactory bulb (see Manzini et al., 2007).
Taken together, the glomerular layer of the olfactory bulb of larval Xenopus laevis is organized in spatially distinct clusters that receive input from different subsets of receptor neurons with varying transduction mechanisms, odorant spectra and an unequal composition of presynaptic proteins. This shows that the main olfactory system of larval Xenopus laevis is made up of at least two diverse subsystems with different functional relevance possibly emerged at different points in the evolution of the olfactory system.