Anyone who has swatted flies buzzing over their food knows how deftly they manoeuvre their bodies in the air, and how quickly they learn, forcing you to readjust your striking strategy. Their intelligence is almost humbling for humans. Flies display their learning abilities in various contexts, and a well-studied example is their memory linking odours to noxious stimuli.
The fruit fly Drosophila can associate conditioned stimuli (such as odour) with unconditioned stimuli (a foot shock) and learn to increase avoidance with the presentation of odour. The association between odour and shock can be retained over a long period. In order to understand how such memories in a classical conditioning paradigm are formed at the cellular level, delineating the neural networks responsible is critical. Over the years, the anatomical basis of such learning processes has been studied, and the critical role of the brain structure known as the mushroom body has been established (de Belle & Heisenberg, 1994). In this issue of The Journal of Physiology, an article by Ueno et al. (2012), using an innovative, state of the art technique, examined how information is integrated at synapses in the mushroom body. The authors isolated fly brains and cultured them in vitro, even over a week in some experiments. Using genetically encoded Ca2+ indicators in the cultured brain preparation, the authors showed that information encoding odour and shock, transmitted through distinct pathways, is integrated in specific areas of the mushroom body. The pathways for odour and shock are antennal lobes (AL) and ascending fibres of the ventral nerve cord (AFV), respectively. The use of cultured brains enabled the direct stimulation of AL and AFV in a mechanically stable condition, emulating odour and shock input. Simultaneous stimulation of both pathways led to long-term enhancement of the odour-induced response, persisting over 2 h. The authors went further and elucidated the neurotransmitters and their receptors playing roles in the memory formation. The input of odour information to the mushroom body is mediated by nicotinic acetylcholine receptors, while that of shock information is mediated by NMDA receptors. Moreover, D1 dopamine receptors, which are not required for individual AL- or AFV-mediated Ca2+ responses, are necessary during the simultaneous stimulation for establishing long-term enhancement. Importantly, the properties of long-term enhancement, both in terms of physiology and genetics, resembled those of memory formation at the behavioural level, suggesting that the revealed network underlies the behaviour.
An intriguing aspect of Drosophila neurons, highlighted in the cultured brain preparation, is the surprisingly high concentration of Mg2+, above 20 mm, in the haemolymph. In a paper recently published in Neuron, the same group reported that the Mg2+ block of NMDA receptors, which occurs only in high Mg2+ concentrations, plays a critical role in the formation of long-term memory (Miyashita et al. 2012). Twenty millimolar Mg2+ in the semi-intact cultured brain system revealed physiologically relevant areas in the mushroom body for signal transduction.
The synaptic basis of olfactory aversive conditioning and the receptors involved in the process pose interesting questions for synapse physiology. In particular, interactions of the dopaminergic and cholinergic systems have been implicated in the mushroom body (Tsydzik & Wright, 2009), and the role of D1 dopamine receptors in the memory circuit is highly intriguing. The region-specific rescue experiment suggests that dopamine receptors expressed in the mushroom body are critical. Do they modify postsynaptic nicotinic acetylcholine receptors, or do they regulate release of acetylcholine vesicles in a retrograde fashion? How are they activated and modified in the learning process?
The mechanism of memory formation is a hotly studied area of neuroscience, and researchers use different models. Thanks to a plethora of available genetic tools, Drosophila provides valuable information to advance the field. With newly developed experimental techniques, Drosophila neuroscientists will continue to explore the physiological basis of memory. In the meantime, based on their efforts, we can envision a neural circuit taking shape in the brains of tiny insects the next time we narrowly miss a fly hovering over our meal!