Carbachol increases locus coeruleus activation by targeting noradrenergic neurons, inhibitory interneurons and inhibitory synaptic transmission

The locus coeruleus (LC) consists of noradrenergic (NA) neurons and plays an important role in controlling behaviours. Although much of the knowledge regarding LC functions comes from studying behavioural outcomes upon administration of muscarinic acetylcholine receptor (mAChR) agonists into the nucleus, the exact mechanisms remain unclear. Here, we report that the application of carbachol (CCh), an mAChR agonist, increased the spontaneous action potentials (sAPs) of both LC‐NA neurons and local inhibitory interneurons (LC I‐INs) in acute brain slices by activating M1/M3 mAChRs (m1/3AChRs). Optogenetic activation of LC I‐INs evoked inhibitory postsynaptic currents (IPSCs) in LC‐NA neurons that were mediated by γ‐aminobutyric acid type A (GABAA) and glycine receptors, and CCh application decreased the IPSC amplitude through a presynaptic mechanism by activating M4 mAChRs (m4AChRs). LC‐NA neurons also exhibited spontaneous phasic‐like activity (sPLA); CCh application increased the incidence of this activity. This effect of CCh application was not observed with blockade of GABAA and glycine receptors, suggesting that the sPLA enhancement occurred likely because of the decreased synaptic transmission of LC I‐INs onto LC‐NA neurons by the m4AChR activation and/or increased spiking rate of LC I‐INs by the m1/3AChR activation, which could lead to fatigue of the synaptic transmission. In conclusion, we report that CCh application, while inhibiting their synaptic transmission, increases sAP rates of LC‐NA neurons and LC I‐INs. Collectively, these effects provide insight into the cellular mechanisms underlying the behaviour modulations following the administration of muscarinic receptor agonists into the LC reported by the previous studies. CCh application increases firing rates of LC‐NA neuron and LC I‐INs by acting on m1/3AChRs; it also exerts a presynaptic inhibition of the synaptic transmission from LC I‐INs onto LC‐NA neurons by activating on m4AChRs. Collectively, these effects by CCh application enhance the tonic firing rate and the spontaneous phasic‐like activity of LC‐NA neurons ex vivo.