Analysis of Synaptic Microcircuits in the Mushroom Bodies of the Honeybee.

Claudia Groh,Wolfgang Rössler

doi:10.3390/insects11010043

Abstract

Mushroom bodies (MBs) are multisensory integration centers in the insect brain involved in learning and memory formation. In the honeybee, the main sensory input region (calyx) of MBs is comparatively large and receives input from mainly olfactory and visual senses, but also from gustatory/tactile modalities. Behavioral plasticity following differential brood care, changes in sensory exposure or the formation of associative long-term memory (LTM) was shown to be associated with structural plasticity in synaptic microcircuits (microglomeruli) within olfactory and visual compartments of the MB calyx. In the same line, physiological studies have demonstrated that MB-calyx microcircuits change response properties after associative learning. The aim of this review is to provide an update and synthesis of recent research on the plasticity of microcircuits in the MB calyx of the honeybee, specifically looking at the synaptic connectivity between sensory projection neurons (PNs) and MB intrinsic neurons (Kenyon cells). We focus on the honeybee as a favorable experimental insect for studying neuronal mechanisms underlying complex social behavior, but also compare it with other insect species for certain aspects. This review concludes by highlighting open questions and promising routes for future research aimed at understanding the causal relationships between neuronal and behavioral plasticity in this charismatic social insect.

Highlights

The honeybee represents a powerful experimental model for investigating the changes in synaptic circuits that occur in the brain during adult behavioral development and learning in an insect that has a complex social life
Schematic of a single innervated by an synaptic contacts with multiple active zones of the Further abbreviations: AL: antennal individual claw of a class II Kenyoncells cells (KCs) dendrite with multiple dendritic protrusions forming synaptic contacts lobe, CB: central body, LA:oflamina, lateralFurther horn, PED: peduncle, projection neurons (PNs): RET: with multiple active zones the PNLH: bouton
We largely focus on plastic changes in the PN–KC connectivity within the MG of the Mushroom bodies (MBs) calyx

Summary

Introduction

The honeybee represents a powerful experimental model for investigating the changes in synaptic circuits that occur in the brain during adult behavioral development and learning in an insect that has a complex social life. Mushroom bodies (MBs), prominent centers in the honeybee brain, are neuronal substrates for multisensory integration, learning and memory formation [7,8,9] They are viewed as an experience-dependent re-coding device transforming. Structural analyses of microcircuits at the main MB input (MB-calyx microglomeruli, MG) have been performed along the transition from inside the dark hive to outdoor foraging (interior–exterior transition) and correlated with external and internal variables such as conditions that occur before adult emergence during brood care and during age- and task-related adult behavioral maturation [11,12,13,14,15,16,17]. Learning experiments on foraging adult bees have revealed that the formation of stable olfactory long-term memory (LTM) leads to structural neuronal plasticity of microcircuits in the MB calyces [18]. The goal is to stimulate future multidisciplinary approaches aimed at understanding causal relationships between neuronal and behavioral plasticity in a social insect

Honeybee Mushroom Bodies

KC dendrite with multiple dendritic protrusions forming innervated

Open Questions

Findings

New Approaches and Methodological Advances