Analysis of Dendritic Specializations in Two Classes of Kenyon Cells in the Mushroom Body of the Adult Honeybee, Apis mellifera

The mushroom bodies (MBs) in the insect brain serve sensory integration and memory formation. In the honeybee, they house two classes of intrinsic neurons: class I (spiny) and II (clawed) Kenyon cells (KCs). Both classes form postsynaptic elements in synaptic complexes (microglomeruli) comprising large axonal boutons from olfactory and visual projection neurons. To adapt their neuronal information processing systems, MB microglomeruli undergo age-, memory-, and environment-related structural plasticity. To analyze KC dendritic specializations and their connections with presynaptic boutons, we combined tracer injections in small groups of KCs from different age cohorts (freshly emerged bees to foragers) with presynaptic anti-synapsin immunolabeling. Using high-resolution confocal 3D reconstructions, we analyzed shape and contacts of class I and II KC dendrites in the olfactory (lip) and visual (collar) input sites of the MB calyx. In both KC classes, dendrites are always restricted to either the lip or the collar. We classified two types of class II KCs regarding the spatial distribution of dendritic branches: large-clustered and small-distributed. Individual claws of class II KCs largely vary regarding surface areas covered on individual axonal boutons (∼5%-70%). In class I KCs, we found four distinct morphological spine categories: stubby, thin, mushroom-shaped, and branched. Interestingly, the overall frequency of putative spine-bouton contacts in class I KCs remains largely constant throughout age cohorts. We discuss the results in the light of structural dynamics in MB microglomerular circuits and their role in multisensory information processing.