Freie Universität Berlin, Fachbereich Biologie, Institut für Tierphysiologie (WE 5), Neurobiologie, Königin-Luise-Str. 28-30, 14195 Berlin
A top-down approach as applied to learning and memory in honeybees provides the opportunity of relating different levels of complexity to each other, and of analyzing the rules and mechanisms from viewpoint of the respective next higher level. Olfactory conditioning of harnessed bees exemplifies essential elements of associative learning and, in general, forms a bridge between the systems and the cellular levels of analysis. Intracellular recordings of identified neurons during olfactory conditioning allow testing of the assumptions made by modern behavioral theories of associative learning and provide access to cellular and molecular studies. Analysis at this intermediate level of complexity is particularly profitable in the bee, because essential neural elements of the associative network are known and can be tested during ongoing learning behavior. In this respect, the honeybee offers unique properties for the building of bridges between the molecular, cellular, neuronal, neuropilar and behavioral levels of associative learning.
At the circuit level, the neural substrate of olfactory conditioning is characterized by two neuropiles, antennal lobe, mushroom bodies (mb), and three tracts (the m-, ml- and lAGTs), furthermore by and identified neuron (VUM[mxl]) whose excitation following the olfactory stimulus is sufficient to initiate associative learning. The VUM neuron is immunoreactive to an antibody against octopamine (OA). The antennal lobes and mb calyces, but not the lateral protocerebrum, appear to develop an associative memory trace. Since the mb calyces receive inputs from other sensory modalities, and the mbs are the substrates for intense multisensory integration at the highest level of the insect nervous system. The memory traces in the antennal lobes and mbs may store different aspects of the memory, the antennal lobes, the stimulus related non-associative and associative components, the mb the context dependencies.
At the cellular level the coupling between transmitters, modulators and second messengers turns out to be rather complex in the mbs. The modulation of currents as measured with the whole cell patch configuration of Kenyon cell somata indicates down regulation of a nAChR-current and upregulation of a shaker type A-current by several transmitters including OA. Nitric oxide is found to be implicated in the formation of a particular form of long-term memory. Proteins synthesis appears to be involved selectively in the formation of a form of long-term memory which develops after spaced learning trials, and thus is different from long-term memory after massed learning trials.
The data on bee olfactory learning shall be compared with corresponding data on Drosophila with the aim to elucidate common neural and cellular mechanisms.