Pheromone signalling
A picture is beginning to emerge of the molecular mechanisms by which odorants are detected by mammals and pheromones by insects. In Lepidoptera olfaction and pheromone detection are performed by different sensilla (hair like projections) on the antenna, whereas with mammals the same functions are performed by the olfactory tract and the vomeronasal organ respectively, although the exact role of the latter is still a cause of conjecture.
There are at least two distinct odorant detection pathways in the olfactory tract of mammals 102. In the first pathway, guanine nucleotide binding protein (G-protein) -coupled receptors, modulate adenylate cyclase to increase cAMP levels, this opens cyclic nucleotide gated ion channels which produce the action potential in the neuron. In the second pathway a similar sequence of events occurs except that inositol 1,4,5-triphosphate (IP3) is used as the "second messenger". The two pathways utilise different receptors and ion channels. In both cases sensitivity after stimulus is reduced by phosphorylation of the G-proteins by protein kinase A for the cAMP system and proteins kinase C for the IP3 system 103. In addition increased Ca2+ levels elicited by odorants reduce sensitivity by inhibiting adenylate cyclase 104 and strong odorants initiate NO synthase and cGMP production 105, which also reduces sensitivity 106 107. Any given odorant only activates one of these two systems, but there is no relationship between the class of odorant and the system activated 108 109.
Very little is known about insect olfaction, but more is known about how insect pheromones are detected. They are initially bound and transported by specific pheromone binding proteins (SPBP), which have molecular weights of 14Kda and are almost half a-helix 110 111. The binding of the pheromone or the SPBP-pheromone complex 112 to a G-protein coupled odorant receptor 113 activates a specific phospholipase C 114; IP3 is released and this gates the opening of an ion channel, which produces the action potential in the ion channel 115. As with mammalian olfaction the signal is "turned off" by phosphorylation of the receptor 116 and high cGMP levels 117.