Neuronal Plasticity in the Mushroom Bodies of Winter Bees Is Retained Despite Substantially Advanced Age.

Abstract

Honeybee (Apis mellifera) workers exhibit remarkable behavioral plasticity throughout adult life. In spring and summer, they transition through diverse tasks over a short lifespan of 4-6 weeks. This involves dramatic changes in sensory environment and cognitive demands associated with pronounced structural neuronal plasticity in the mushroom bodies (MBs), high-order brain centers for sensory integration, learning, and memory. This plasticity manifests as age- and experience-related volume increase in sensory input regions of the MB calyces, accompanied by pruning of projection neuron (PN) boutons in synaptic microcircuits within visual and olfactory compartments. As winter approaches, honeybees suspend brood rearing and foraging activities to survive the cold months by forming a tight, thermoregulated cluster. Unique physiological adaptations enable winter bees to live up to 8 months until a new generation emerges in spring. This extended lifespan occurs during a period of reduced sensory input and high metabolic costs raising the question of how such conditions affect structural neuronal plasticity. Using synapsin immunolabeling and 3D confocal-microscopy image analyses of MB synaptic neuropils in whole-mount brains of age-controlled worker bees, we found that winter bees retain a high degree of neuronal plasticity throughout their lifespan. MB calyces exhibit an initial volume increase followed by a period of stagnation to then undergo another expansion at the onset of spring foraging. While olfactory PN boutons exhibit continuous pruning, visual bouton numbers remain stable during winter. We conclude that winter bees retain comparable neuronal capacities to summer bees, despite strong differences in lifespan, physiological, and environmental conditions.