# Genetic Activation of Locus Coeruleus Noradrenergic Neurons Modulates Cerebellar MF-GrC Synaptic Plasticity via Presynaptic α2-AR/PKA Signaling in Mice

## TL;DR

This study shows that activating brain cells in mice that release a chemical called norepinephrine can change how connections in the cerebellum strengthen or weaken, using a specific signaling pathway.

## Contribution

The study reveals a novel presynaptic α2A-AR/PKA signaling mechanism by which LC noradrenergic neurons modulate cerebellar synaptic plasticity.

## Key findings

- Optogenetic activation of LC noradrenergic neurons impairs MF-GrC LTP via α2-AR signaling.
- Facial stimulation-induced glutamate sensor fluorescence LTP is abolished by chemogenetic LC activation.
- Optogenetically induced LTD is blocked by PKA inhibition but not by PKC inhibition.

## Abstract

The aim of this study was to investigate how optogenetic or chemogenetic activation of locus coeruleus (LC) noradrenergic neurons modulates cerebellar mossy fiber-granule cell (MF-GrC) long-term synaptic plasticity in DBH-Cre mice. The results showed that facial stimulation induced MF-GrC long-term potentiation (LTP) under control conditions and that this LTP was impaired by optogenetic activation of LC noradrenergic neurons via α2-ARs. Meanwhile, facial stimulation induced glutamate sensor fluorescence LTP in the granular layer, which was abolished by chemogenetic activation of LC noradrenergic neurons. In the absence of N-methyl-D-aspartate receptor activity, optogenetic activation of LC noradrenergic neurons triggered facial stimulation-induced MF-GrC long-term depression (LTD) via α2A-ARs. Optogenetically induced MF-GrC LTD by LC noradrenergic neuron activation was abolished by protein kinase A (PKA) inhibition but not by protein kinase C inhibition. Immunofluorescence results revealed abundant α2A-AR expression in the granular layer, with particularly high levels in glomeruli. These results indicate that optogenetic activation of LC noradrenergic neurons impairs MF-GrC LTP by triggering presynaptic LTD via the α2A-AR/PKA signaling pathway. Collectively, these findings provide novel evidence that LC-derived noradrenergic afferents modulate synaptic plasticity within the cerebellar cortical circuitry in intact animals.

Locus coeruleus (LC) noradrenergic neurons project their axons to the cerebellar cortex and modulate cerebellar circuit function via distinct adrenergic receptor (AR) subtypes. The present study investigated the mechanism by which optogenetic activation of LC noradrenergic neurons modulates facial stimulation-evoked long-term synaptic plasticity at cerebellar mossy fiber-granule cell (MF-GrC) synapses in urethane-anesthetized DBH-Cre mice. Blockade of GABAA receptors, 20 Hz facial stimulation induced MF-GrC long-term potentiation (LTP) in the control group, and this LTP was impaired by optogenetic activation of LC noradrenergic neurons via α2-ARs. Meanwhile, facial stimulation induced LTP of glutamate sensor fluorescence in the granular layer, which was abolished by chemogenetic activation of LC noradrenergic neurons. Following NMDA receptor blockade, optogenetic activation of LC noradrenergic neurons triggered facial stimulation-induced MF-GrC long-term depression (LTD) via α2A-ARs. Optogenetically activated LC noradrenergic neuron-induced MF-GrC LTD was abolished by protein kinase A (PKA) inhibition but not by protein kinase C inhibition. Immunofluorescence results revealed abundant α2A-AR expression in the granular layer, with particularly high levels in glomeruli, and no colocalization with the glutamate sensor. These results indicate that optogenetic activation of LC noradrenergic neurons impairs facial stimulation-induced MF-GrC LTP by triggering presynaptic LTD via the α2A-AR/PKA signaling cascade.