Morphology of feedback neurons in the mushroom body of the honeybee, Apis mellifera.

The anatomy of gamma-aminobutyric acid (GABA)-immunoreactive, recurrent feedback neurons in the mushroom body (MB) of the honeybee, Apis mellifera, was investigated by using intraneuropilar injections of cobalt ions and light microscopic techniques. Each MB contains approximately 110 GABA-immunoreactive neurons, and approximately 50% of them are feedback neurons, i.e., they connect the MB output regions--the alpha-lobe, beta-lobe, and pedunculus--with its input regions--the calyces. Their somata are located in the lateral protocerebral lobe, and their primary neurites project medially and bifurcate near the alpha-lobe. In the alpha-lobe feedback neurons form narrow banded, horizontal arborizations in the dorsal and median alpha-lobe; each cell innervates a certain alpha-lobe layer. The neurons form additional branches in the pedunculus and the beta-lobe. All calycal subcompartments--the lip, collar, and basal ring--are innervated by feedback neurons. However, individual feedback neurons innervate exclusively a certain subcompartment in both the median and lateral calyx. Due to the arrangement of intrinsic Kenyon cells, each calycal subcompartment is connected to its specific, corresponding layer in the alpha-lobe. Feedback neurons interconnect the alpha-lobe and the calyces in either a corresponding or a noncorresponding fashion. With respect to their branching pattern in the alpha-lobe, the basal ring and the collar neuropil receive input from feedback neurons innervating the corresponding dorsal and median alpha-lobe layers. By contrast, the lip region, which receives olfactory antennal input, is innervated by feedback neurons with arborizations in a noncorresponding dorsal alpha-lobe layer.