Physiological and functional heterogeneity in the mouse locus coeruleus

Abstract

Neurons in the locus coeruleus (LC) have been traditionally viewed as a homogenous population. Recent studies are beginning to uncover heterogeneous molecular phenotypes and projection targets in this nucleus, but how such heterogeneity is ultimately manifested in neuronal physiology and function is largely unknown. We took an orthogonal approach to directly assess the physiological and functional heterogeneity in the LC. By identifying noradrenergic neurons using a genetic-based tagging approach, our study revealed a subset of neurons exhibiting a distinctive narrow spike waveform and lacking the prolonged after-hyperpolarization. We further provided evidence establishing a link between the proximity of LC neurons and their response latency, where putatively proximal neurons tended to have a long latency to optogenetic activation. Finally, we found that response latency of LC neurons was correlated with their burst firing and distinct relationship with pupil diameter. Together, our study presents novel evidence to reveal and link LC heterogeneity at the physiological level to that at the functional level.

Juxtacellular recordings from identified neurons in the mouse locus coeruleus

“…Spontaneous in vivo electrophysiological features alone were sufficient for classifying TH‐positive and TH‐negative cells with 76% accuracy (see Figure S5G–H). These observations are consistent with recent studies, which also describe neuronal subpopulations with relatively narrow spike waveforms in the mouse, rat and primate LC (Su & Cohen, 2022; Tortorelli et al, 2022; Totah et al, 2018). Morphologically, TH‐positive and TH‐negative neurons displayed largely overlapping features (Figure S3), which prevented cell type classification based solely on morphological criteria.…”

“…Previous studies differentiated heterogeneous LC neurons by their average spontaneous firing rate and spike width, which motivated us to compare these characteristics between TH‐positive and TH‐negative neurons (e.g., Tortorelli et al, 2022; Totah et al, 2018). On average, compared with TH‐positive neurons, TH‐negative cells displayed lower spontaneous mean firing rates (TH‐pos, 1.21 ± 0.78 Hz; TH‐neg, 0.77 ± 0.62 Hz; p = 0.036; Figure 2a) and narrower spike waveforms (spike half‐width durations; 0.83 ± 0.18 Hz; TH‐neg, 0.69 ± 0.14 Hz; p = 0.008; Figure 2b,c).…”

“…In the present study, these possibly diverse TH-negative neurons were grouped in order to test whether TH-positive neurons display unique electrophysiological features compared with the heterogenous surround of non-noradrenergic cell types. We also note that noradrenergic neurons have also been shown to be heterogeneous (Chandler et al, 2014;McKinney et al, 2022;Tortorelli et al, 2022;Totah et al, 2018), but our limited sample size prevented further within-class analysis. Future work will be required to resolve additional structure-function relationships within these heterogeneous TH-negative (and TH-positive) cell classes, which was beyond the scope of the present study.…”

“…Early anatomical studies divided LC neurons into several types based on their distinct morphological characteristics (Cintra et al, 1982;Loughlin et al, 1986). More recent work indicates that LC neurons are also heterogeneous in terms of their molecular expression profiles, intrinsic electrophysiological properties and projection targets (Chandler et al, 2014(Chandler et al, , 2019Kalwani et al, 2014;Li et al, 2016;McKinney et al, 2023;Plummer et al, 2017;Schwarz et al, 2015;Su & Cohen, 2022;Tortorelli et al, 2022;Totah et al, 2018;Uematsu et al, 2017). However, whether and how this structural heterogeneity of LC neurons relates to in vivo activity has remained largely unresolved.…”

“…Elegant pharmacology‐based approaches have been classically employed for inferring the putative noradrenergic identity of the recorded units (Aston‐Jones & Bloom, 1981a; Manohar et al, 2017; Sara & Segal, 1991; Shea et al, 2008; Totah et al, 2018). More recent approaches involving high‐density extracellular recordings, in combination with optogenetic tagging, have increased the efficiency of LC sampling and enabled the in vivo characterization of opto‐identified LC units (Breton‐Provencher et al, 2022; Carter et al, 2010; Hickey et al, 2014; Hirschberg et al, 2017; Ito et al, 2020; Martins & Froemke, 2015; McBurney‐Lin et al, 2022; Megemont et al, 2022; Swift et al, 2018; Tortorelli et al, 2022; Uematsu et al, 2017; Xiang et al, 2019; Yamasaki & Takeuchi, 2017). When cell‐type specific properties are very distinct and under low levels of recurrent connectivity, optotagging methods can achieve excellent separation of the extracellularly recorded units (as, e.g., for LC units from the medially located Barrigton's nucleus, Ito et al, 2020); however, if differences are more subtle and neural circuits more intermingled, extracellular optotagging becomes far from trivial and more critically dependent upon the performance of spike‐sorting algorithms.…”