Mammalian chemosensory system

The receptor cells are organized in three corresponding
molecular and cellular chemosensory
systems. Each neuron of the main olfactory
sensory system (1) expresses one of the different
olfactory receptor genes and sends axons to
specific glomeruli of the main olfactory bulb
(mitral cells). The odorant receptor (OR) gene
family comprises about 1000 members, each
encoding a seven-transmembrane cyclic nucleotide-
gated channel with distinct odorant
specificity (G-olfactory proteins, Golf). The bitter
taste sensory system (2) connects axonal projections
of receptor cells in the taste sensory
epithelium of the taste buds to gustatory nuclei
of the brain stem. Two families of taste receptors,
the TIRs (two genes) and T2Rs (50–80
genes of the gustducin class) have been described.
Two families of mammalian putative
pheromone receptors (V1Rs and V2Rs) are encoded
by 30–50 and over 100 genes, respectively
(3).