Synaptic homeostasis at the Drosophila neuromuscular junction is a reversible signaling process that is sensitive to high temperature

Homeostasis is a vital mode of biological self-regulation. The hallmarks of homeostasis for any biological system are a baseline set point of physiological activity, detection of unacceptable deviations from the set point, and effective corrective measures to counteract deviations. Homeostatic synaptic plasticity (HSP) is a form of neuroplasticity in which neurons and circuits resist environmental perturbations in order to maintain appropriate levels of activity. One assumption is that if an environmental perturbation triggers homeostatic corrective changes in neuronal properties, those corrective measures should be reversed upon removal of the perturbation. We test the reversibility and limits of HSP at a well-studied model synapse, the Drosophila melanogaster neuromuscular junction (NMJ). At the Drosophila NMJ, impairment of glutamate receptors causes a decrease in quantal size, which is offset by a corrective, homeostatic increase in the number of vesicles released per evoked presynaptic stimulus, or quantal content. This process has been termed presynaptic homeostatic potentiation (PHP). Taking advantage of a GAL4/GAL80TS/UAS expression system, we triggered PHP by expressing a dominant-negative glutamate receptor subunit at the NMJ. We then reversed PHP by halting expression of the dominant-negative receptor. Our data show that PHP is fully reversible over a time course of 48-72 hours after the dominant-negative glutamate receptor stops being genetically expressed. Additionally, we found that the PHP response triggered by the dominant-negative subunit was ablated at high temperatures. Our data show that the long-term maintenance of PHP at the Drosophila NMJ is a reversible regulatory process that is sensitive to temperature.\n\nSIGNIFICANCE STATEMENTBiological homeostatic systems must upregulate or downregulate cellular parameters in order to maintain appropriate set points of physiological activity. Homeostasis is a well-documented mode of regulation in metazoan nervous systems. True homeostatic control should be a reversible process - but due to technical difficulties of presenting and removing functional challenges to living synapses, the reversibility of homeostatic forms of synapse regulation has not been rigorously examined in vivo over extended periods of developmental time. Here we formally demonstrate that homeostatic regulation of Drosophila melanogaster neuromuscular synapse function is reversible and temperature-labile. This is significant because developing methods to study how homeostatic regulatory systems are turned on and off could lead to fundamental new insights about control of synaptic output.