Perception of chemical stimuli from the environment is essential to most animals; accordingly, they are equipped with a complex olfactory system capable of receiving a nearly unlimited number of odorous substances and pheromones. This enormous task is accomplished by olfactory sensory neurons (OSNs) arranged in several chemosensory compartments in the nose. The sensitive and selective responsiveness of OSNs to odorous molecules and pheromones is based on distinct receptors in their chemosensory membrane; consequently, olfactory receptors play a key role for a reliable recognition and an accurate processing of chemosensory information. They are therefore considered as key elements for an understanding of the principles and mechanisms underlying the sense of smell. The repertoire of olfactory receptors in mammals encompasses hundreds of different receptor types which are highly diverse and expressed in distinct subcompartments of the nose. Accordingly, they are categorized into several receptor families, including odorant receptors (ORs), vomeronasal receptors (V1Rs and V2Rs), trace amine-associated receptors (TAARs), formyl peptide receptors (FPRs), and the membrane guanylyl cyclase GC-D. This large and complex receptor repertoire is the basis for the enormous chemosensory capacity of the olfactory system.

Olfactory receptors are supposed to act not only as molecular sensors for odorants but also as cell recognition molecules guiding the axons of olfactory neurons to their appropriate glomerulus in the olfactory bulb. This concept implies that olfactory receptor proteins are located in sensory cilia and in the axons. To approach this critical issue, antibodies were generated against two peptides, one derived from olfactory receptor mOR256-17, one derived from the “mOR37” subfamily. By means of immunohistochemistry and double-labeling studies using transgenic mouse lines as well as Western blot analyses, it was demonstrated that the newly generated antibodies specifically recognized the receptor proteins. To scrutinize the hypothesis that olfactory receptor proteins may also be present in the axonal processes and the nerve terminals, serial sections through the olfactory bulb were probed with the antibodies. Two glomeruli in each bulb were stained by anti-mOR256-17, one positioned in the medial, one in the lateral hemisphere. Fiber bundles approaching the glomeruli through the outer nerve layer also displayed intense immunofluorescence. A similar picture emerged for the antibody anti-mOR37, a small number of glomeruli in the ventral domain of the bulb was stained. On serial sections through the olfactory bulb of mOR37-transgenic mouse lines, double-labeling experiments demonstrated that distinct immunoreactive glomeruli corresponded to glomeruli that were targeted by neurons expressing a particular member of the mOR37 receptor subfamily. These data indicate that olfactory receptor (OR) proteins are indeed present in the axonal processes and nerve terminals of olfactory sensory neurons, thus supporting the notion that ORs may participate in the molecular processes underlying the fasciculation and targeting of olfactory axons.

Pheromones are chemicals that serve intraspecific communication. In animals, the ability to detect and discriminate pheromones in a complex chemical environment substantially contributes to the survival of the species. Insects widely use pheromones to attract mating partners, to alarm conspecifics or to mark paths to rich food sources. The various functional roles of pheromones for insects are reflected by the chemical diversity of pheromonal compounds. The precise detection of the relevant intraspecific signals is accomplished by specialized chemosensory neurons housed in hair-like sensilla located on the surface of body appendages. Current data indicate that the extraordinary sensitivity and selectivity of the pheromone-responsive neurons (PRNs) is largely based on specific pheromone receptors (PRs) residing in their ciliary membrane. Besides these key elements, proper ligand-induced responses of PR-expressing neurons appear to generally require a putative co-receptor, the so-called “sensory neuron membrane protein 1” (SNMP1). Regarding the PR-mediated chemo-electrical signal transduction processes in insect PRNs, ionotropic as well as metabotropic mechanisms may be involved. In this review, we summarize and discuss current knowledge on the peripheral detection of pheromones in the olfactory system of insects with a focus on PRs and their specific role in the recognition and transduction of volatile intraspecific chemical signals.

The Grueneberg ganglion (GG) in the vestibule of the anterior nasal cavity is considered as an olfactory subcompartment based on expression of the olfactory marker protein (OMP) and axonal projection to the olfactory bulb. Searching for olfactory receptors present in the GG, it has been observed recently that V2r83, a member of the V2R class of olfactory receptors, is expressed in numerous cells in the GG of mice. However, no other olfactory receptors have been found to be present in a considerable number of GG neurons so far. Here, we report that GG neurons express trace amine–associated receptors (TAARs) that have most recently been described as a novel class of olfactory receptors. It was observed that several TAAR subtypes are expressed by defined subpopulations of GG neurons distinct from the V2r83-positive cells. Analyzing the time course of TAAR expression during pre- and postnatal development revealed that TAARs are expressed by a substantial portion of GG neurons in late embryonic and neonatal stages, whereas in juveniles and adults, the number of TAAR-positive cells in the GG was significantly decreased.