Physiologic and Nanoscale Distinctions Define Glutamatergic Synapses in Tonic vs Phasic Neurons

Abstract

Neurons exhibit a striking degree of functional diversity, each one tuned to the needs of the circuitry in which it is embedded. A fundamental functional dichotomy occurs in activity patterns, with some neurons firing at a relatively constant “tonic” rate, while others fire in bursts - a “phasic” pattern. Synapses formed by tonic vs phasic neurons are also functionally differentiated, yet the bases of their distinctive properties remain enigmatic. A major challenge towards illuminating the synaptic differences between tonic and phasic neurons is the difficulty in isolating their physiological properties. At theDrosophilaneuromuscular junction (NMJ), most muscle fibers are co-innervated by two motor neurons, the tonic “MN-Ib” and phasic “MN-Is”. Here, we employed selective expression of a newly developed botulinum neurotoxin (BoNT-C) transgene to silence tonic or phasic motor neurons inDrosophilalarvae of either sex. This approach highlighted major differences in their neurotransmitter release properties, including probability, short-term plasticity, and vesicle pools. Furthermore, Ca2+imaging demonstrated ∼two-fold greater Ca2+influx at phasic neuron release sites relative to tonic, along with an enhanced synaptic vesicle coupling. Finally, confocal and super-resolution imaging revealed that phasic neuron release sites are organized in a more compact arrangement, with enhanced stoichiometry of voltage-gated Ca2+channels relative to other active zone scaffolds. These data suggest that distinctions in active zone nano-architecture and Ca2+influx collaborate to differentially tune glutamate release at tonic vs phasic synaptic subtypes.SIGNIFICANCE STATEMENT:“Tonic” and “phasic” neuronal subtypes, based on differential firing properties, are common across many nervous systems. Using a recently developed approach to selectively silence transmission from one of these two neurons, we reveal specialized synaptic functional and structural properties that distinguish these specialized neurons. This study provides important insights into how the input-specific synaptic diversity is achieved, which could have significant implications for the development of therapeutic interventions for neurological disorders that involve changes in synaptic function.

Excess glutamate release triggers subunit-specific homeostatic receptor scaling

“…First, most Drosophila muscle fibers are co-innervated by two distinct motor neuron subtypes, MN-Is and MN-Ib, that differ in morphology and function. 11 , 24 , 25 mEPSCs are therefore blended events of spontaneous activity emanating from both inputs. Because our electrophysiological recordings are a blend of events deriving from both MN-Ib and MN-Is inputs, it is possible that the average mEPSC amplitudes recorded do not accurately reflect GluR changes at both NMJs.…”

Neto proteins differentially modulate the gating properties ofDrosophilaNMJ glutamate receptors

“…By contrast, the mEPSC decay time constant in GluRIIA mutants that correspond to fast-desensitizing vertebrate GluA2 mutants resembles the fast kinetics of type-B receptors. This critical role for desensitization in the kinetics of synaptic transmission may constitute an adaptation of larval NMJ to the high concentration of glutamate (1.8 – 2.0 mM) in the hemolymph, which is actively maintained ( Augustin et al 2007 ; He et al 2023 ). Mutations that reduce the hemolymph glutamate concentration (to ~1 mM) or culturing larvae in glutamate-depleted conditions trigger a 250–500% increase in the levels of postsynaptic type-A and B receptors.…”

Mitochondrial phosphagen kinases support the volatile power demands of motor nerve terminals

“…2A) (Hoang & Chiba, 2001). Functional differentiation has been demonstrated between terminal types (He et al, 2023) and between Left: schematic of a single transmission electron micrograph through the NMJs of muscle six. 3D reconstructions were made from such series from multiple larvae allowing morphometric analyses of presynaptic mitochondria (Justs et al, 2022).…”