Dopamine-dependent plasticity is heterogeneously expressed by presynaptic calcium activity across individual boutons of the Drosophila mushroom body

Abstract

The Drosophila mushroom body (MB) is an important model system for studying the synaptic mechanisms of associative learning. In this system, coincidence of odor-evoked calcium influx and dopaminergic input in the presynaptic terminals of Kenyon cells (KCs), the principal neurons of the MB, triggers long-term depression (LTD), which plays a critical role in olfactory learning. However, it is controversial whether such synaptic plasticity is accompanied by a corresponding decrease in odor-evoked calcium activity in the KC presynaptic terminals. Here, we address this question by inducing LTD by pairing odor presentation with optogenetic activation of dopaminergic neurons (DANs). This allows us to rigorously compare the changes at the pre- and postsynaptic sites in the same conditions. By imaging presynaptic acetylcholine release in the condition where LTD is reliably observed in the postsynaptic calcium signals, we show that neurotransmitter release from KCs is depressed selectively in the MB compartments innervated by activated DANs, demonstrating the presynaptic nature of LTD. However, total odor-evoked calcium activity of the KC axon bundles does not show concurrent depression. We further conduct calcium imaging in individual presynaptic boutons and uncover the highly heterogeneous nature of calcium plasticity. Namely, only a subset of boutons, which are strongly activated by associated odors, undergo calcium activity depression, while weakly responding boutons show potentiation. Thus, our results suggest an unexpected nonlinear relationship between presynaptic calcium influx and the results of plasticity, challenging the simple view of cooperative actions of presynaptic calcium and dopaminergic input.

Significance Statement

Dopamine-induced long-term synaptic depression in the mushroom body is a key mechanism of olfactory associative learning in Drosophila. Although multiple lines of evidence indicate the presynaptic origin of this plasticity, it has been controversial whether and how the plasticity affects odor-evoked presynaptic calcium influx. Here, we demonstrate that the plasticity of presynaptic calcium signals is highly heterogeneous across individual presynaptic boutons, even among those on the same, likely spiking axons. The mode of plasticity (i.e., potentiation or depression) depends on the original response size. These results challenge the simple view that coincidence of presynaptic calcium and dopaminergic input triggers depression and suggest the plasticity outcome is finely controlled by the activity of individual synapses and local neuromodulatory signals.