Cyclic nucleotide signaling mediates an odorant-suppressible chloride conductance in lobster olfactory receptor neurons

Amino acid- vs. peptide-odorants: responses of individual olfactory receptor neurons in an aquatic species

Amino acids are widely used waterborne olfactory stimuli proposed to serve as cues in the search for food. In natural waters the main source of amino acids is the decomposition of proteins. But this process also produces a variety of small peptides as intermediate cleavage products. In the present study we tested whether amino acids actually are the natural and adequate stimuli for the olfactory receptors they bind to. Alternatively, these olfactory receptors could be peptide receptors which also bind amino acids though at lower affinity. Employing calcium imaging in acute slices of the main olfactory epithelium of the fully aquatic larvae of Xenopus laevis we show that amino acids, and not peptides, are more effective waterborne odorants.

Dopamine modulates Ih in a motor axon

We studied the axons of the pyloric dilator neurons in the stomatogastric nervous system of the lobster. The several-centimeters-long portions of these axons in the motor nerves depolarize in response to low concentrations of dopamine (DA) and exhibit peripheral spike initiation in the absence of centrally generated activity. This effect is inhibited by blockers of hyperpolarization-activated inward current (I(h)). We show here that peripheral spike initiation was also elicited by D(1)-type receptor agonists and drugs that increase cAMP. This suggests that DA acts via a D(1)-type receptor mechanism to modulate hyperpolarization-activated cyclic nucleotide-gated channels. We used two-electrode voltage clamp of the axon to directly study the effect of DA on I(h). Surprisingly, DA decreased the maximal conductance. However, because of a shift of the activation curve to more depolarized potentials, and a change in the slope, conductance was increased at biologically relevant membrane potentials. These changes were solely caused by modulation of I(h), as DA had no discernible effect when I(h) was blocked. In addition, they were not induced by repeated activation and could be mimicked by application of drugs that increase cAMP concentration. DA modulation of I(h) persisted in the presence of a protein kinase A inhibitor and is therefore potentially mediated by a phosphorylation-independent direct effect of cAMP on the ion channel. A computer model of the axon showed that the changes in maximal conductance and voltage dependence were not qualitatively affected by space-clamp problems.

Perfusion with cAMP analogue affects pheromone-sensitive trichoid sensilla of the hawkmoth Manduca sexta in a time-dependent manner

Octopamine causes time-dependent disadaptation of pheromone-sensitive olfactory receptor neurons (ORNs) of Manduca sexta. Because the majority of insect octopamine receptors are positively coupled to adenylyl cyclases we examined whether cyclic adenosine monophosphate (cAMP) mimics octopamine-dependent modulation of pheromone transduction in a time-dependent manner. Long-term tip recordings of single trichoid sensilla of Manduca sexta were performed during three zeitgeber times (ZTs, ZT 0=lights on), while stimulating the sensilla with two doses of the main pheromone component bombykal in a non-adapting protocol. The membrane-permeable cAMP analogue 8bcAMP increased the normalized sensillar potential amplitude in a time- and bombykal dose-dependent way. At the higher bombykal dose only, the applied 8bcAMP antagonized an endogenous decrease in the mean sensillar potential amplitude at ZT 1-4 and ZT 8-11 when ORNs were adapted but not at ZT 22-1, when ORNs were sensitized. In contrast to octopamine, 8bcAMP did not consistently affect the initial pheromone-dependent action potential frequency, the phasic/tonic response pattern, or the time-dependent shift to lower mean action potential frequencies at ZT 8-11. Furthermore, 8bcAMP increased the spontaneous action potential frequency time dependently, but differently from octopamine. In conclusion, our results show that cAMP only partly mimics the octopamine-dependent disadaptation of olfactory receptor neurons during photophase, apparently due to another missing octopamine-dependent synergistic factor such as defined intracellular calcium levels.

Evidence for multiple signaling pathways in single squid olfactory receptor neurons

At least two different G-protein-mediated transduction cascades, the adenylate cyclase and phospholipase C (PLC) pathway, process chemosensory stimuli for various species. In squid olfactory receptor neurons (ORNs), physiological studies indicate that both pathways may be present; however, confirmation of the transduction molecules at the protein level is absent. Here we provide evidence that the G-proteins involved in both adenylate cyclase and PLC pathways are present in squid ORNs (Lolliguncula brevis). We used immunoblotting to show that Galpha(olf), Galpha(q), and a downstream effector, enzyme PLC140, are present in the squid olfactory epithelium (OE). To localize these proteins to one or more of the five morphological cell types described for squid OE, paraformaldehyde-fixed olfactory organs were cryosectioned (10 microm), double-labeled for Galpha(olf), Galpha(q), or PLC140, and imaged. Analysis of serial sections from entire olfactory organs for epithelial area and patterns of immunofluorescence revealed a region of highest immunoreactivity at the anterior half of the organ. At the cellular level, type 1 cells could not be distinguished morphologically and were not included in the analysis. The three labeling patterns observed in type 2 cells were Galpha(q) alone, PLC140 alone, and colocalization of Galpha(q) and PLC140. Subsets of cell types 3, 4, and 5 showed colocalization of Galpha(olf) with Galpha(q) but not with PLC140. These data suggest that the PLC pathway predominates in type 2 cells; however, coexpression of Galpha(olf) with Galpha(q) in cell types 3, 4, and 5 suggests that both pathways may participate in olfactory transduction in non-type 2 squid ORNs.

Gene expression and specificity in the mature zone of the lobster olfactory organ

The lobster olfactory organ is an important model for investigating many aspects of the olfactory system. To facilitate study of the molecular basis of olfaction in lobsters, we made a subtracted cDNA library from the mature zone of the olfactory organ of Homarus americanus, the American lobster. Sequencing of the 5′-end of 5,184 cDNA clones produced 2,389 distinct high-quality sequences consisting of 1,944 singlets and 445 contigs. Matches to known sequences corresponded with the types of cells present in the olfactory organ, including specific markers of olfactory sensory neurons, auxiliary cells, secretory cells of the aesthetasc tegumental gland, and epithelial cells. The wealth of neuronal mRNAs represented among the sequences reflected the preponderance of neurons in the tissue. The sequences identified candidate genes responsible for known functions and suggested new functions not previously recognized in the olfactory organ. A cDNA microarray was designed and tested by assessing mRNA abundance differences between two of the lobster's major chemosensory structures: the mature zone of the olfactory organ and the dactyl of the walking legs, a taste organ. The 115 differences detected again emphasized the abundance of neurons in the olfactory organ, especially a cluster of mRNAs encoding cytoskeletal-associated proteins and cell adhesion molecules such as 14-3-3ζ, actins, tubulins, trophinin, Fax, Yel077cp, suppressor of profilin 2, and gelsolin.