F-actin at identified synapses in the mushroom body neuropil of the insect brain.

The distribution of f-actin stained by fluorescent phalloidin was investigated in the brain of several insect species, with a special focus on the mushroom body. For localizing f-actin in identified neurons and at synapses, additional staining with fluorescent dextrans and anti-synapsin I immunostaining was employed. Intense f-actin staining was consistently found in synaptic complexes of the mushroom body calyces (calycal microglomeruli [MG]). These MG contain a central core of presynaptic boutons, predominantly belonging to deutocerebral cholinergic excitatory projection neurons, which are surrounded by a shell of numerous Kenyon cell (KC) dendritic tips. In the cricket Gryllus bimaculatus, high-resolution confocal laser scanning imaging revealed colocalization of f-actin with KC dendritic spine parts within MG. Although presynaptic boutons appear to be mainly devoid of f-actin-phalloidin fluorescence, there appears to be an accumulation of f-actin in KC dendritic spines synaptically contacting the boutons. Electron microscopy of boutons and dextran-stained KC dendrites revealed their pre- and postsynaptic sites, with KCs being strictly postsynaptic elements. Their subsynaptic membrane appositions are considered to be associated with f-actin. Focal accumulation of f-actin in the dendritic tips of KCs was found to be a general feature of MG, with either spheroidal or indented boutons of different sizes, as encountered in the mushroom bodies of the cricket, honey bee, ant, and fruit fly. The structural similarities of calycal MG and f-actin accumulation in KC dendrites with cerebellar microglomeruli are considered comparatively. The accumulation of f-actin in KC dendrites is discussed in view of mushroom body plasticity and its potential role in learning and memory formation. Copyright 2004 Wiley-Liss, Inc.