cAMP-independent responses of olfactory neurons in Xenopus laevis tadpoles and their projection onto olfactory bulb neurons.

We report on responses of olfactory receptor neurons (ORNs) upon application of amino acids and forskolin using a novel slice preparation of the olfactory epithelium of Xenopus laevis tadpoles. Responses were measured using the patch-clamp technique. Both amino acids and forskolin proved to be potent stimuli. Interestingly, a number of ORNs that responded to amino acids did not respond to forskolin. This suggests that some amino acids activate transduction pathways other than the well-known cAMP-mediated one. The differential processing of cAMP-mediated stimuli on the one hand and amino acid stimuli on the other was further elucidated by calcium-imaging of olfactory bulb neurons using a novel nose-olfactory bulb preparation of Xenopus laevis tadpoles. The projection pattern of amino acid-sensitive ORNs to olfactory bulb neurons differed markedly from the projection pattern of forskolin-sensitive ORNs. Olfactory bulb neurons activated by amino acids were located laterally compared to those activated by forskolin, and only a small proportion responded to both stimuli. The ensemble of neurons activated by forskolin was also activated by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) and the membrane-permeant cAMP analogue 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (pCPT-cAMP). We therefore conclude that sensory transduction of a number of amino acids is cAMP independent, and amino acid- and cAMP-mediated responses are processed differentially at the level of the olfactory bulb.