Gustatory receptor neurons (GRNs)—similar to OSNs—
are primary sensory neurons assembled in taste sensilla
that express gustatory receptors (GRs). These receptors
interact specifically with soluble chemicals, such as
sugars, amino acids, and repulsive (bitter-tasting) compounds.
All GRNs of the labial palps (LPs) and some of
the legs project axons to the subesophageal ganglion
(SOG) in the brain, the primary taste processing center,
but some axons of GRNs in the legs target the thoracic
ganglion [20,21]. At least two functionally distinct sets of
neurons in the LPs, one promoting feeding and the other
repulsion, express distinct subsets of Gr genes that
encode receptors for sugars and bitter-tasting compounds,
respectively. The axons of these two sets of neurons
converge onto different fields within the SOG [22]. No second-order projection neurons have been identified in
Drosophila, but studies in honeybees have shown that
interneurons connect the SOG with the calices of the
mushroom bodies [23], a structure that also receives
olfactory input through the ALs.