Brain plasticity in Diptera and Hymenoptera

Abstract

To mediate different types of behaviour, nervous systems need to coordinate the proper operation of their neural circuits as well as short- and long-term alterations that occur within those circuits. The latter ultimately devolve upon specific changes in neuronal structures, membrane properties and synaptic connections that are all examples of plasticity. This reorganization of the adult nervous system is shaped by internal and external influences both during development and adult maturation. In adults, behavioural experience is a major driving force of neuronal plasticity studied particularly in sensory systems. The range of adaptation depends on features that are important to a particular species, and is therefore specific, so that learning is essential for foraging in honeybees, while regenerative capacities are important in hemimetabolous insects with long appendages. Experience is usually effective during a critical period in early adult life, when neural function becomes tuned to future conditions in an insect's life. Tuning occur at all levels, in synaptic circuits, neuropile volumes, and behaviour. There are many examples, and this review incorporates only a select few, mainly those from Diptera and Hymenoptera.

Figure 1

Neuropile regions of the worker honeybee brain and of the fruit fly. Frontal vibratome slices from the brains of both species. A: Mid-depth of the honeybee brain. Antennal lobe (AL) glomeruli are the first relay of projection neurons in the olfactory system that project to the mushroom bodies. Each of these has a lateral and medial calyx (CA), two input neuropiles, and corresponding output regions, the peduncles (PED), which straddle the midline neuropiles of the central body complex (CB). B: Pre- and postsynaptic labelling of microglomeruli (MG) of the lip region of the bee calyx. Each microglomerulus (inset) comprises a presynaptic bouton from a projection neuron at its centre (red), surrounded by postsynaptic Kenyon cell spines (green). LP: lip. C: Central plane of the fly's brain, anterior relative to that in A, showing the antennal lobes (AL), the calyces (CA), and the peduncle (PED). Expression of green fluorescent protein (GFP) in the mushroom body is driven by means of the Gal4/UAS system, using OK107-Gal4 and UAS-LimK to target GFP to the membranes of most Kenyon cells. D: Pre- and postsynaptic labelling of microglomeruli of the fly's calyx, as in B, with individual microglomerulus (inset). Scale bars: A, (200μm); B, (15 μm; inset: 3μm); C, (200μm); D (10 μm; inset: 2 μm).