Neural correlates for neonicotinoid-induced impairment of olfactory long-term memory
Reports of olfactory learning and memory deficits in honey bees exposed to neonicotinoid pesticides have been accumulating over the past decades. As agonists of the nicotinic receptors to acetylcholine, neonicotinoids target most of the projection neurons conveying olfactory information to the memory brain centres, the mushroom bodies (MBs). However, the neural mechanisms by which neonicotinoids interfere with memory formation are poorly understood. Here, we investigated the consequences of chronic exposure to the neonicotinoid imidacloprid on the number of projection neuron terminal boutons and on their plasticity in the context of long-term memory formation. Using super-resolution STED microscopy, we also measured the density of synapsin-positive units (SPUs) within boutons, as synapsin is known to be enriched in new boutons following neuronal activation. We show that imidacloprid suppresses the synaptic pruning of projection neurons naturally occurring with age and experience. The resulting excessive number of boutons in the MBs of treated bees was associated with long-term memory deficits. The subset of treated bees that showed successful memory formation had a similar number of boutons as untreated bees, suggesting that synaptic pruning might have been involved in the memorization process. As the SPU density was not affected by the imidacloprid treatment, the high bouton number in the lip of treated bees was not due to synaptogenesis. Altogether, our experiments show that, by altering synaptic pruning, imidacloprid interferes with long-term memory formation.