Foraging experience, glomerulus volume, and synapse number: A stereological study of the honey bee antennal lobe

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 the glomeruli can be identified according to their predictable size and location. This study examines T1-44, a prominent glomerulus on the dorsal surface of the antennal lobe. Previously, we have shown that the volume of T1-44 in 4-day-old workers performing tasks within the hive is significantly smaller than in foragers and that increases in volume are accompanied by an increase in total synapse number in this glomerulus. Here we examine whether foraging experience is essential for either changes in volume or for changes in synapse numbers in glomerulus T1-44. Five-day-old bees reared under normal colony conditions were compared with 5-day-old bees reared under isolated conditions, and also to 5-day-old bees that had been induced to forage precociously. A combination of light and electron microscopy was used to compare T1-44 volumes and synapse numbers in these three groups. Two groups of 11-day-old bees, precocious foragers and nonforagers, were also examined. 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. This study shows that while both volume and synapse numbers can be induced to increase prematurely in young (5-day-old) precocious foragers, foraging experience is not essential for these structural changes to occur in glomerulus T1-44.