POSTSYNAPTIC ATP RELEASE MEDIATES HOMEOSTATIC SYNAPTIC DEPRESSION AT THE NEUROMUSCULAR JUNCTION

Excitability varies among muscle fibers, yet the neuromuscular synapse maintains a remarkable fidelity of execution owing to robust homeostatic control of synaptic efficacy. Classically, presynaptic neurotransmitter release is viewed as the primary adjustable parameter tuning synaptic strength to postsynaptic excitability, under retrograde control from the muscle fiber. In previous work, we showed in vertebrates that this retrograde control is governed by postsynaptic calcium signaling. Local postsynaptic Ca²⁺ accumulation, driven by the Ca²⁺ permeability of nicotinic synaptic receptors, triggers a positive feedback onto presynaptic release, whereas global “depolarization-induced Ca²⁺ release” (DICR) associated with the muscle action potential triggers a negative feedback. We proposed that the balance between these opposing feedbacks sets neurotransmitter release within a suprathreshold operational range of synaptic strength.
In the present study, we investigated the signaling pathway underlying the negative feedback onto presynaptic release. Extracellular ATP is known to negatively regulate evoked neurotransmitter release; however, although both nerve terminals and muscle fibers release ATP, postsynaptic ATP release is generally considered to mediate autocrine signaling, whereas synaptic effects are attributed exclusively to ATP co-released with the neurotransmitter. Using electrophysiological recordings combined with a pharmacological approach, we show that postsynaptic ATP release through DICR-activated Pannexin1 hemichannels alone mediates the negative feedback required for homeostatic control of synaptic efficacy. Furthermore, we demonstrate that this signaling pathway is impaired in an experimental model of a human disease caused by mutations in the replication factor Rif1, leading to excessive neurotransmitter release.