Sublethal Dosage of Imidacloprid Reduces the Microglomerular Density of Honey Bee Mushroom Bodies

This study aimed to identify the regions of the brain of adult bees that become impaired by the neurotoxin imidacloprid and provides evidence that the binding of imidacloprid to the nicotinic acetylcholine receptor leads to irreversible effects, explaining the dose-response relationship of imidacloprid in arthropods, first described by Francisco Sánchez-Bayo in 2009, and confirming the mechanism of action described by Henk Tennekes in 2010.
The nAChRs are mainly located in the bilaterally symmetrical mushroom bodies (MBs), which are responsible for multisensory integration. MBs are thought to be related to learning, memory, cognitive processes, and the management of complex behaviours, as well as the formation, consolidation, and recall of olfactory memories. Among the functions in which MBs are involved, synaptic plasticity is a powerful mechanism that is the basis for coping with any task encountered and for making internal adjustment. The cup-shaped calyces in the MBs, which consist of many Kenyon cell dendrites, are important regions for sensory input. Different areas in the calyces receive different types of sensory input: the lip regions mainly receive information from the olfactory system, the collar regions are stimulated by neurons from the optic ganglia, and the basal rings receive both visual and olfactory information. In the calyces, micro-glomerulus (MG), which consists of many synaptic units, have been chosen as the indicator of synaptic connectivity for distinct synaptic units.
Long-term memory (LTM) is important for honey bees during foraging activity. When foraging, foragers are able to efficiently learn cues from different flowers because of the reward they provide. Foraging activity is guided by the benefits the foragers obtain through specific foraging behaviour. Flowers appear in patches distributed at various distances, which can be categorized by the various travel times between them. These time intervals may vary from minutes to months. In addition, flower fidelity is guided by the bees’ LTM. When their LTM is damaged by imidacloprid, honey bees are likely to become lost while foraging. The decrease in micro-glomerulus (MG) density in the calyces of a honey bee after having been exposed to imidacloprid during the larval stage might affect the adult in various ways, leading to diminished olfactory learning ability, reduced LTM, and consequently limited foraging efficiency. From both a behavioural and a neurophysiological perspective, the evidence supports the conclusion that a sublethal dose of imidacloprid negatively affects the development of the honey bee nervous system. It is thus reasonable to speculate that imidacloprid residue in the environment would impair bees’ neural connectivity, eventually resulting in the decline and collapse of colonies. The decrease in MG density in the calyces of honey bees has a profound impact on their nervous system and behaviour.
Sources:
Sánchez-Bayo, F. (2009) From simple toxicological models to prediction of toxic effects in time. Ecotoxicology 18:343-354
Peng, Y.-C. and Yang, E.-C. Sublethal Dosage of Imidacloprid Reduces the Microglomerular Density of Honey Bee Mushroom Bodies. Sci. Rep. 6, 19298; doi: 10.1038/srep19298 (2016)
http://www.nature.com/articles/srep19298
Tennekes, H.A. (2010) The significance of the Druckrey–Küpfmüller equation for risk assessment—The toxicity of neonicotinoid insecticides to arthropods is reinforced by exposure time. Toxicology 276, 1-4