Intrinsic braking role of descending locus coeruleus noradrenergic neurons in acute and chronic itch in mice

Abstract

Itch is defined as an unpleasant sensation that provokes a desire to scratch. Our understanding of neuronal circuits for itch information transmission and processing in the spinal dorsal horn (SDH) has progressively advanced following the identification of SDH neuron subsets that are crucial for scratching behavior in models of itch. However, little is known about the control of acute and chronic itch by descending signals from the brain to the SDH. In this study, using genetic approaches that enable cell-type and circuit-specific functional manipulation, we reveal an intrinsic potential of locus coeruleus (LC)-noradrenergic (NAergic) neurons that project to the SDH to control acute and chronic itch. Activation and silencing of SDH-projecting LC-NAergic neurons reduced and enhanced scratching behavior, respectively, in models of histamine-dependent and -independent acute itch. Furthermore, in a model of chronic itch associated with contact dermatitis, repetitive scratching behavior was suppressed by the activation of the descending LC-NAergic pathway and by knocking out NA transporters specific to descending LC-NAergic neurons using a CRISPR-Cas9 system. Moreover, patch-clamp recording using spinal slices showed that noradrenaline facilitated inhibitory synaptic inputs onto gastrin-releasing peptide receptor-expressing SDH neurons, a neuronal subset known to be essential for itch transmission. Our findings suggest that descending LC-NAergic signaling intrinsically controls acute and chronic itch and provide potential therapeutic strategies for the treatment of acute and chronic itch.

Intersectional targeting of defined neural circuits by adeno‐associated virus vectors

“…The impact of intersectional gene expression extends well beyond circuit tracing. Intersectional expression of optogenetic ion channels and chemogenetic designer receptors exclusively activated by designer drugs (DREADDs) permits functional interrogation of circuit activity (Table 1) (An et al., 2020; Jaramillo et al., 2020; Koga et al., 2020; Li et al., 2018; Zingg et al., 2017). Neural circuits can also be specifically ablated by intersectional expression of diphtheria toxin receptor (Table 1) (Azim et al., 2014).…”

“…Although serotypes other than AAV1 and AAV9 were not reported to drive trans‐synaptic Cre expression even at high titers (Zingg et al., 2017, 2020), we recommend using the lowest effective dose of AAV2‐retro to minimize the potential for off‐target labeling in bidirectional circuits. This approach has been used to study the role of lateral hypothalamus outputs to the periaqueductal gray in predation and evasion (Li et al., 2018), to study the role of locus coeruleus outputs to the spinal dorsal horn in acute and chronic itch (Koga et al., 2020), and to study the role of peri‐habenular nucleus outputs to the nucleus accumbens in depressive‐like behavior caused by light exposure at night (An et al., 2020).…”

α1A-adrenaline receptors in dorsal horn inhibitory neurons have an inhibitory role in the regulation of chloroquine-induced itch in mice

“…Specifically, gastrin-releasing peptide receptor (GRPR)-expressing (GRPR + ) neurons in the SDH have been identified as an essential component of spinal itch transmission [ 2 ]. Furthermore, it has also been shown that GRPR + neurons are controlled not only locally by subsets of excitatory and inhibitory interneurons in the SDH [ 3 ] but also remotely by descending monoaminergic neurons from the brainstem [ 4 , 5 ]. A recent study has shown that the activation of descending serotonergic (5-HTergic) neurons facilitates GRPR signaling and itch transmission in the SDH via activation of 5-HT 1A receptors [ 4 ].…”

“…As for noradrenaline (NA), we recently demonstrated that NAergic neurons descending from the locus coeruleus (LC) to the SDH negatively control itch-related scratching behaviors. Furthermore, application of either NA or an α 1A -adrenaline receptor (α 1A -AR) agonist to spinal cord slices facilitates the transmission of inhibitory synaptic inputs into GRPR + SDH neurons [ 5 ]. Thus, it is possible that α 1A -ARs in inhibitory SDH interneurons may play a role in controlling itch-related behavior, although this is entirely unknown.…”

“…In addition, nociceptive behavior (wiping responses to the capsaicin-injected cheek) was not different between the two genotypes ( Adra1a flox/flox mice, 46.0 ± 9.6, n = 5; Vgat-Cre ; Adra1a flox/flox mice, 40.2 ± 9.7, n = 6; P = 0.623, unpaired t-test). Our previous study showed that the application of α 1A -AR agonist to spinal cord slices facilitates the transmission of inhibitory synaptic inputs into GRPR + SDH neurons, raising the possibility that α 1A -ARs in inhibitory SDH interneurons have a role in the itch response [ 5 ]. We examined the expression of α 1A -ARs in the SDH using RNAscope in situ hybridization and found that Adra1a mRNA was clearly detected in inhibitory interneurons positive for Slc32a1 mRNA (also known as Vgat ) in the SDH of control Adra1a flox/flox mice (Fig.…”