Input-specific deployment of the active zone scaffold RIM tunes synaptic release probability and plasticity.

RIM is essential for transmission and acute homeostatic potentiation at the high-release probability phasic synapse at the Drosophila neuromuscular junction, but largely dispensable at the neighboring low-release probability tonic synapse.

• Why it matters: It was unclear whether the presynaptic active zone scaffold RIM is uniformly required across synapses that differ in release probability and plasticity rules, especially at convergent inputs.
• What they did: Using neurotoxin-based silencing to isolate transmission at the Drosophila neuromuscular junction, the authors compared RIM dependence at phasic versus tonic synapses and used super-resolution imaging to map RIM relative to CaV2 channels during acute plasticity.
• The result: RIM’s proximity to CaV2 channels is greater at phasic synapses, and during acute plasticity it coordinates their nanoscale reorganization to expand the readily releasable pool, enabling rapid homeostatic potentiation.

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