Stereological Analysis Reveals Striking Differences in the Structural Plasticity of Two Readily Identifiable Glomeruli in the Antennal Lobes of the Adult Worker Honeybee

Abstract

The primary antennal sensory centers (antennal lobes) in the brain of the honeybee are highly compartmentalized into discrete spheres of synaptic neuropil called glomeruli, many of which can be identified according to their predictable size and location. Glomeruli undergo significant changes in volume during the lifetime of the adult worker bee, at least some of which are activity dependent. This study tests the commonly expressed assumption that increases in neuropil volume are accompanied by an underlying increase in the number of synapses present in the tissue. A combination of light and electron microscopy was used to determine total synapse number within two glomeruli, T1-44 and T4-2(1). The Cavalieri direct estimator of volume was applied to 1.5 microm sections of resin-embedded brains. Selected sections were then re-embedded and prepared for transmission electron microscopy. Synapse densities were determined using the physical disector method on electron micrographs. Synapse density and glomerulus volume were combined to give an unbiased estimate of the total number of synapses. In glomerulus T1-44, a significant increase in volume was accompanied by a significant increase in the total number of synapses. In contrast, synapse counts in T4-2(1) remained unchanged, despite a significant increase in the volume of this glomerulus. These results demonstrate that synapse proliferation in antennal lobes of the adult worker bee is highly site specific. Although volumetric changes and changes in synapse number both contribute to the structural plasticity of the antennal lobes, these two components of plasticity appear to be independent processes.

Experience-dependent modulation of antennal sensitivity and input to antennal lobes in male moths (Spodoptera littoralis) pre-exposed to sex pheromone

“…We hypothesise that the enlarged glomerular volume may originate from reorganization of the neural network (e.g. changes in synapse number), as discussed earlier (Hourcade et al, 2009), but other mechanisms, such as glial growth cannot be included (Brown et al, 2002). It should be noted that this effect of pre-exposure was measured on the normalised glomerular volume, i.e.…”

Multiple olfactory receptor neurons and their axonal projections in the antennal lobe of the honeybee Apis mellifera

“…In the template AL we did not find enlarged glomeruli in the T4 region. The difference between these two ALs probably reflects the interindividual variation, which might be, e.g., experience dependent (Winnington et al, 1996;Sigg et al, 1997;Brown et al, 2002Brown et al, , 2004. The arborization of ORNs in the glomeruli that we assigned to subpopulation T4 differed from the arborization of all other ORNs investigated.…”

Standard three-dimensional glomeruli of the Manduca sexta antennal lobe: a tool to study both developmental and adult neuronal plasticity

“…Work on adult neuronal plasticity in the honeybee has demonstrated that increases in adult glomerular volume are activity-dependent and correlated with better learning performance (Winnington et al 1996;Sigg et al 1997). Quantitative electron-microscopic studies have lead to the suggestion that volume increase and synapse number changes are independent processes that both contribute to structural plasticity in the AL, although synapse reorganization might only play a minor role on neuropilar volume (Brown et al 2002). Age effects on bee mushroom body volumes must be distinguished from experience effects; the first 5 days of adult brain development seem to be functionally separated from later maturation effects (Farris et al 2001); this is supported by synapse counts in certain glomeruli (Brown et al 2004).…”