Stimulation of muscarinic receptors mimics experience-dependent plasticity in the honey bee brain

Abstract

Honey bees begin life working in the hive. At approximately 3 weeks of age, they shift to visiting flowers to forage for pollen and nectar. Foraging is a complex task associated with enlargement of the mushroom bodies, a brain region important in insects for certain forms of learning and memory. We report here that foraging bees had a larger volume of mushroom body neuropil than did age-matched bees confined to the hive. This result indicates that direct experience of the world outside the hive causes mushroom body neuropil growth in bees. We also show that oral treatment of caged bees with pilocarpine, a muscarinic agonist, induced an increase in the volume of the neuropil similar to that seen after a week of foraging experience. Effects of pilocarpine were blocked by scopolamine, a muscarinic antagonist. Our results suggest that signaling in cholinergic pathways couples experience to structural brain plasticity.

Fig. 1.

Caged bees in the laboratory. Caged bees were fed either a 50% sucrose solution (in water) or cholinergic agents dissolved in the sucrose solution. The volume of neuropil of the mushroom bodies was estimated in age-matched bees as a function of foraging experience and drug treatment.

Fig. 2.

Control of foraging experience in honey bees. Extensive daily behavioral observations made at the entrance of hives in the field permitted identification of bees of known age on the day that they shifted from working in the hive to foraging. These one-day foragers were marked with a second paint dot and allowed to continue foraging (Week 1). Bees with 1 week of foraging experience were allowed to continue foraging in the field, confined to the hive, or confined in cages in the laboratory for 1 week and fed either sugar water or cholinergic agents dissolved in sugar water (Week 2). Dashed lines indicate bees prevented from foraging.

Fig. 3.

Effects of foraging experience or muscarinic agonist on the mushroom bodies. Estimates of the volume of the mushroom body neuropil were made for a total of 309 individuals, across nine experiments, conducted over three field seasons. To facilitate comparisons across all treatments, we calculated the % difference in neuropil volume relative to the caged control group in each experiment. Statistical analysis of these data used a mixed ANOVA model (F = 39.00; P = 0.0001; Table 1) and Dunnett post hoc tests (groups showing significant differences from the caged control group are indicated with the % difference). Key experimental groups are shown with the % difference indicated. Sample sizes are given in each bar (number of brains/number of trials). Complete data for these and all other groups are presented in Table 1.