Neuronal diversity of neuropeptide signaling, including galanin, in the mouse locus coeruleus
Purpose: The locus coeruleus (LC) is a small nucleus in the pons, with norepinephrine (NE) as its major transmitter. It plays key roles in wakefulness and is associated with disorders like depression. The study aims to analyze single-cell RNA-seq data to subdivide neurons in the LC and chart their neuropeptide, co-transmitter, and receptor profiles. It also seeks to build a precise map of galanin and its receptors in the mouse LC, and understand the role of neuropeptide expression and signaling. Results: The study identified 24 clusters in total, with 3 NE, 17 glutamate, and 5 GABA subtypes. NE neurons expressed at least 19 neuropeptide transcripts, mainly galanin (Gal) but not Npy, and >30 neuropeptide receptors. Among galanin receptors, Galr1 was expressed in ~19% of NE neurons, and highly expressed in GABA neurons surrounding the NE ensemble. Patch-clamp electrophysiology and cell-type-specific Ca2+-imaging using GCaMP6s revealed that a GalR1 agonist inhibits no more than ~35% of NE neurons, with the effect being direct and not relying on feed-forward GABA inhibition. Conclusions: The results define a role for the galanin system in NE functions and provide a conceptual framework for the action of many other peptides and their receptors. The study found that most NE neurons express mRNA transcripts for galanin, at least 19 neuropeptides (but not Npy), and >30 neuropeptide receptors, including Galr1 at low levels. Selective activation of GalR1 inhibited the spontaneous activity of a subset of NE neurons possibly through the activation of Kv currents. These findings offer new insights into the role of neuropeptide expression and signaling in mouse LC. Overall design: The scRNA-seq experiment involved collecting cells from 3-4-week-old C57Bl/N mice of both sexes, dissociating the LC tissue, and fixing single cells in ice-cold methanol. cDNA libraries were prepared, and sequencing was performed using the 10x Genomics Chromium Single Cell Kit. Two independent samples were prepared and sequenced, resulting in a total of 2,578 cells. Data was analyzed using the Seurat pipeline, including quality control, filtering, and clustering.