Experience-dependent strengthening of Drosophila neuromuscular junctions

The genetic analysis of larval neuromuscular junctions (NMJs) of Drosophila has provided detailed insights into molecular mechanisms that control the morphological and physiological development of these glutamatergic synapses. However, because of the chronic defects caused by mutations, a time-resolved analysis of these mechanisms and their functional relationships has been difficult so far. In this study we provide a first temporal map of some of the molecular and cellular key processes, which are triggered in wild-type animals by natural larval locomotor activity and then mediate experience-dependent strengthening of larval NMJs. Larval locomotor activity was increased either by chronically rearing a larval culture at 29 degrees C instead of 18 or 25 degrees C or by acutely transferring larvae from a culture vial onto agar plates. Within 2 hr of enhanced locomotor activity, NMJs showed a significant potentiation of signal transmission that was rapidly reversed by an induced paralysis of the temperature-sensitive mutant parats1. Enhanced locomotor activity was also associated with a significant increase in the number of large subsynaptic translation aggregates. After 4 hr, postsynaptic DGluR-IIA glutamate receptor subunits started to transiently accumulate in ring-shaped areas around synapses, and they condensed later on, after chronic locomotor stimulation at 29 degrees C, into typical postsynaptic patches. These NMJs showed a reduced perisynaptic expression of the cell adhesion molecule Fasciclin II, an increased number of junctional boutons, and significantly more active zones. Such temporal mapping of experience-dependent adaptations at developing wild-type and mutant NMJs will provide detailed insights into the dynamic control of glutamatergic signal transmission.