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

The anatomy of gamma-aminobutyric acid ( GABA)-immunoreactive, recurrent fe edback neurons in the mushroom body (NIB) of the honeybee, Apis mellifera, was investigated by using intraneuropilar injections of cobalt ions and lig ht microscopic techniques. Each MB contains approximately 110 GABA-immunore active neurons, and approximately 50% of them are feedback neurons, i.e., t hey connect the MB output regions-the alpha-lobe, beta-lobe, and pedunculus -with its input regions-the calyces. Their somata are located in the latera l protocerebral lobe, and their primary neurites project medially and bifur cate near the alpha-lobe. In the alpha-lobe feedback neurons form narrow ba nded, horizontal arborizations in the dorsal and median alpha-lobe; each ce ll innervates a certain oc-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, individ ual feedback neurons innervate exclusively a certain subcompartment in both the median and lateral calyx. Due to the arrangement of intrinsic Kenyon c ells, each calycal subcompartment is connected to its specific, correspondi ng layer in the alpha-lobe. Feedback neurons interconnect the alpha-lobe an d the calyces in either a corresponding or a noncorresponding fashion. With respect to their branching pattern in the alpha-lobe, the basal ring and t he collar neuropil receive input from feedback neurons innervating the corr esponding 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. J. Comp. Neurol. 404:114-126, 1999. (C) 1999 Wiley-Liss, Inc.