Postsynaptic sensitivity to glutamate was genetically manipulated at the Drosophila neuromuscular junction (NMJ) to test whether postsynaptic activity can regulate presynaptic function during development. We cloned the gene encoding a second muscle-specific glutamate receptor, DGluRIIB, which is closely related to the previously identified DGluRIIA and located adjacent to it in the genome. Mutations that eliminate DGluRIIA (but not DGluRIIB) or transgenic constructs that increase DGluRIIA expression were generated. When DGluRIIA is missing, the response of the muscle to a single vesicle of transmitter is substantially decreased. However, the response of the muscle to nerve stimulation is normal because quantal content is significantly increased. Thus, a decrease in postsynaptic receptors leads to an increase in presynaptic transmitter release, indicating that postsynaptic activity controls a retrograde signal that regulates presynaptic function.

At the Drosophila glutamatergic neuromuscular junction, the postsynaptic cell can regulate synaptic strength by both changing its sensitivity to neurotransmitter and generating a retrograde signal that regulates presynaptic transmitter release. To investigate the molecular mechanisms underlying these forms of plasticity, we have undertaken a genetic analysis of two postsynaptic glutamate receptors that are expressed at this synapse. Deletion of both genes results in embryonic lethality that can be rescued by transgenic expression of either receptor. Although these receptors are redundant for viability, they have important differences. By transgenically rescuing the double mutant, we have investigated the relationship of receptor gene dosage and composition to synaptic function. We find that the receptor subunit composition regulates quantal size, Argiotoxin sensitivity, and receptor desensitization kinetics. Finally, we show that the activity of the receptor can regulate the retrograde signal functioning at this synapse. Thus, the diversity of receptors expressed at this synapse provides the cell with mechanisms for generating synaptic plasticity.

Two distinct mechanisms regulate synaptic efficacy at the Drosophila neuromuscular junction (NMJ): a PKA-dependent modulation of quantal size and a retrograde regulation of presynaptic release. Postsynaptic expression of a constitutively active PKA catalytic subunit decreases quantal size, whereas overexpression of a mutant PKA regulatory subunit (inhibiting PKA activity) increases quantal size. Increased PKA activity also decreases the response to direct iontophoresis of glutamate onto postsynaptic receptors. The PKA-dependent modulation of quantal size requires the presence of the muscle-specific glutamate receptor DGluRIIA, since PKA-dependent modulation of quantal size is lost in homozygous viable DGluRIIA- mutants. Furthermore, elevated postsynaptic PKA reduces the quantal amplitude and the time constant of miniature excitatory junctional potential (mEJP) decay to values that are nearly identical to those observed in DGluRIIA mutants. The PKA-dependent reduction in quantal size is accompanied developmentally by an increase in presynaptic quantal content, indicating the presence of a retrograde signal that regulates presynaptic release.