Neurotrophic factors and target-specific retrograde signaling interactions define the specificity of classical and neuropeptide cotransmitter release at identified Lymnaea synapses

Frank Visser,Tara A Janes,Naweed I Syed,Wali Zaidi,Angela M Getz

doi:10.1038/s41598-020-70322-5

Abstract

Many neurons concurrently and/or differentially release multiple neurotransmitter substances to selectively modulate the activity of distinct postsynaptic targets within a network. However, the molecular mechanisms that produce synaptic heterogeneity by regulating the cotransmitter release characteristics of individual presynaptic terminals remain poorly defined. In particular, we know little about the regulation of neuropeptide corelease, despite the fact that they mediate synaptic transmission, plasticity and neuromodulation. Here, we report that an identified Lymnaea neuron selectively releases its classical small molecule and peptide neurotransmitters, acetylcholine and FMRFamide-derived neuropeptides, to differentially influence the activity of distinct postsynaptic targets that coordinate cardiorespiratory behaviour. Using a combination of electrophysiological, molecular, and pharmacological approaches, we found that neuropeptide cotransmitter release was regulated by cross-talk between extrinsic neurotrophic factor signaling and target-specific retrograde arachidonic acid signaling, which converged on modulation of glycogen synthase kinase 3. In this context, we identified a novel role for the Lymnaea synaptophysin homologue as a specific and synapse-delimited inhibitory regulator of peptide neurotransmitter release. This study is among the first to define the cellular and molecular mechanisms underlying the differential release of cotransmitter substances from individual presynaptic terminals, which allow for context-dependent tuning and plasticity of the synaptic networks underlying patterned motor behaviour.

Highlights

Many neurons concurrently and/or differentially release multiple neurotransmitter substances to selectively modulate the activity of distinct postsynaptic targets within a network
The Lymnaea cardiorespiratory interneuron visceral dorsal 4 (VD4) was of interest for this study as it is well known to use the classical small molecule neurotransmitter acetylcholine (ACh) alongside a mixture of neuropeptides derived from the heptapeptide transcript of the FMRFamide gene[25,26,27,28]
VD4 is integrated into a three-neuron network via reciprocal inhibitory synapses with the FMRFamidergic input 3 interneuron (IP3I) and the giant dopaminergic neuron right pedal dorsal 1 (RPeD1)

Summary

Introduction

Many neurons concurrently and/or differentially release multiple neurotransmitter substances to selectively modulate the activity of distinct postsynaptic targets within a network. This study is among the first to define the cellular and molecular mechanisms underlying the differential release of cotransmitter substances from individual presynaptic terminals, which allow for context-dependent tuning and plasticity of the synaptic networks underlying patterned motor behaviour. Several studies have indicated that postsynaptic target identity influences the selective use of classical and peptide neurotransmitters at the different presynaptic terminals of a given neuron[6,19,21,22] Taken together, these observations suggest that the peptidergic characteristics of cotransmitting terminals can be highly variable, the molecular mechanisms that give rise to such differences are yet to be defined. Invertebrate models are especially valuable for functional studies on peptidergic transmission because they are one of the few systems in which electrophysiologically measurable synaptic responses can be directly attributed to the actions of identified peptide neurotransmitters at individual synapses

Methods

Results

Conclusion