When the Efferent Becomes Afferent ‐ the Cellular Response of Sympathetic Neurons to Sensory Stimuli

Abstract

Responding to the so called “inflammatory soup” has often been regarded as the sole domain of sensory afferent neurons. We tested the hypothesis that efferent sympathetic neurons can also directly respond to traditionally afferent stimuli. We used single cell RNA‐sequencing (scRNAseq) to explore transcript expression of membrane G protein and ion channel coupled receptors in the sympathetic neuronal population within the stellate ganglia of 5–6‐week‐old Wistar rats. Agonist responses were then evaluated using Fura‐2 AM calcium imaging and perforated patch clamp recordings in isolated cultured neurons.ScRNAseq identified transcript expression of transient receptor potential (TRP) channels (TRPV2>TRM7>TRPC1>>TRM4). Certain TRP channel activators (10 mM camphor n=18, 10 mM cannabidiol n=16, and 100 mM 2‐APB n=15) caused large baseline calcium influxes, which were blocked by the TRP channel inhibitor ruthenium red (RuR, 10 mM; Camphor n=7; Cannabidiol n=24; 2‐APB n=11). An accompanying depolarization of the resting membrane potential and spontaneous firing was also observed, which was reduced by RuR. Other TRP channel activators (100 mM menthol n=15, 1 mM capsaicin n=16, 500 mM AITC n=9, 1 mM vanillin n=15, 300 nM JT010 n=15, 100 mM cavracol n=13, 10 mM icillin n=10, 50 mM naltriben n=15, pH=6.5 n=16 and 100 ng/ml IGF‐1 n=12) failed to produce significant responses.ScRNAseq also identified transcript expression of receptors for inflammatory mediators, for example histamine (HRH3) and prostaglandin E2 (PTGER2 and PTGER3). Stimulation of these receptors (100 mM histamine n=11 and 1 mM prostaglandin E2 n=8) had no effect on baseline [Ca2+]I, but were able to reduce Ca2+ influx in response to KCl depolarization (50 mM) or nicotine (100 mM).Overall, these data are important when interpreting stellate ganglia sympathetic neurons as purely efferent, with evidence that neuronal excitability can be directly influenced by sensory signaling pathways.Support or Funding InformationThis project was supported by the Wellcome trust and the British Heart Foundation