Molecular principles of olfaction
The olfactory sensory cells of insects are bipolar neurons, as it is for vertebrate olfactory neurones. Their dendrites are exposed to the volatile stimuli coming from the air and their axons innervate into the antennal lobes, which are analogue to the olfactory bulbs of vertebrate. Olfactory neurons project their dendrites into the olfactory sensilla (sg.= sensillum), which are distinct morphological structures often made by protruding cuticle and present on several parts of the insect body.
Based on their external morphology, ultrastructure and physiological properties they are classified in several groups, as for instance sensilla trichodea, sensilla basiconica, wall-pore sensilla, terminal-pore sensilla, single-wall sensilla, and double-wall sensilla.
The distinction between olfaction (=sense of smell) and gustation (=taste of sense) is not always easy as it is in vertebrate where the two processes take place in specific and anatomical distinct organs. Therefore in insect science these two processes are often grouped in the term chemoperception and sensilla are then named chemosensory sensilla.
These sensilla are not present only on insect antennae, but also on mouthparts, wings, legs and genital organs. However, a sharp distinction between olfaction and taste is also possible in insects by assuming that all signals reaching the antennal lobes are olfactory stimuli and the others are taste stimuli, as a parallel model of the vertebrate classification.
In insects chemosensory transduction takes place in the sensilla by the adsorption of molecules into pore channels (width ca. 10 nm) within the cuticular surface. The internal cavity of the chemosensory sensilla is filled with an extracellular solution called sensillar lymph, where two or more sensory neurones project their dendrites.
Volatile molecules that elicit neuronal signals are called odorants and have to be transported through the hydrophilic medium of the sensillar lymph. After they reach the cell membrane of the olfactory neurons they activate a group of transmembrane proteins, called olfactory receptors. Olfactory receptors were found to be G-protein coupled receptors with 7 transmembrane domains. They activate internal signal cascades, leading to the formation of an action potential and, therefore, an electrical signal.
Since the majority of the odorants are hydrophobic, it has been proposed that specific proteins, called Odorant Binding Proteins (OBPs) are responsible for carrying the volatile compounds to the membrane of the olfactory neurons. These proteins have been discovered in a number of insect species and are secreted only in the sensillar lymph of olfactory sensilla, with some exceptions.
In Lepidoptera a distinction is made between the Pheromone Binding Proteins (PBP) and the General Odorant Binding Protein (GOBP). Binding of the odorants by these small (12-15 kDa) soluble proteins seems to enhance the efficiency of stimulus perception. Classical OBPs are characterised by having 6 cysteines in conserved positions. These cysteines are responsible for the formation of three disulphide bridges inside each protein monomer. Recently a new family of proteins, called ChemoSensory Proteins (CSPs), showed similar rules in insect chemosensation, although these proteins have only 4 cysteines and some members are expressed in non-chemosensory organs. The exact rule of OBPs and CSPs is not jet clear.
Questions like the following ones are still unsolved: Are they really necessary in insect to code olfactory stimuli? Do they participate in selectivity of the chemical recognition? Do they transport the volatiles to the membrane receptors or do they actually remove the volatiles once the receptors are activated?
Thank to the genomics information that became available for several insect genomes during the last years, a heterogeneity of OBPs and CSPs was observed. For instance in Drosophila melanogaster (Diptera), Apis mellifera, (Hymenoptera) and Tribolium castaneum (Coleoptera) were fund 51 OBPs and 4 CSPs, 21 OBPs and 6 CSPs, and 47 OBPs and 21 CSPs, respectively. The early processes of the chemosensory transduction are still far to be fully understood, although experimental data combined with theoretical models are now proposing possible pathways.