Sensory neurons have an axon initial segment that initiates spontaneous activity in neuropathic pain

Abstract

The axon initial segment is a specialized compartment of the proximal axon of CNS neurons where action potentials are initiated. However, it remains unknown whether this domain is assembled in sensory dorsal root ganglion neurons, in which spikes are initiated in the peripheral terminals. Here we investigate whether sensory neurons have an axon initial segment and if it contributes to spontaneous activity in neuropathic pain. Our results demonstrate that myelinated dorsal root ganglion neurons assemble an axon initial segment in the proximal region of their stem axon, enriched in the voltage-gated sodium channels Nav1.1 and Nav1.7. Using correlative immunofluorescence and calcium imaging, we demonstrate that the Nav1.7 channels at the axon initial segment are associated with spontaneous activity. Computer simulations further indicate that the axon initial segment plays a key role in the initiation of spontaneous discharges by lowering their voltage threshold. Finally, using a Cre-based mouse model for time-controlled axon initial segment disassembly, we demonstrate that this compartment is a major source of spontaneous discharges causing mechanical allodynia in neuropathic pain. Thus, an axon initial segment domain is present in sensory neurons and facilitates their spontaneous activity. This study provides a new insight in the cellular mechanisms that cause pathological pain and identifies a new potential target for chronic pain management.

The axon initial segment as a source of neuropathic pain

“… 4 However, the subcellular source of this aberrant, pain-inducing neuronal activity remains unknown. Now, in this issue of Brain , Nascimento and colleagues 5 present an impressive series of experiments suggesting that one major player is a specialized neuronal compartment known as the axon initial segment (AIS).…”

“…Here, Nascimento and colleagues 5 began by providing convincing anatomical evidence that DRG neurons in the intact adult mouse can indeed have a bone fide AIS. Using immunohistochemistry in ex vivo DRG tissue, they identified ankyrin-G-positive elongated structures in the proximal axon of medium and large—but not small—DRG cells, cell types which also possess myelinated axons and nodes of Ranvier.…”

“…Finally, to provide causal evidence for the role of the DRG in producing pain-associated neuronal activity, Nascimento et al . 5 selectively removed AISs from DRG neurons before assessing the effects on spontaneous activity generation and behavioural measures of pain. They triggered AIS disassembly in adult DRG neurons by crossing the sensory ganglia-specific Advillin-Cre ERT2 inducible conditional line with mice floxed for the ankyrin-G gene Ank3 (Avil-Ank3 fl/fl ).…”

“…Nascimento et al 5 used this approach to ask two key questions. First, what happens to spontaneous activity when DRGs lack an AIS?…”

“…The second, crucial question was therefore: when DRGs lack an AIS, what happens to neuropathic pain itself? To address this, Nascimento and colleagues 5 carried out behavioural tests for mechanical allodynia, where pain is experienced after exposure to harmless tactile stimuli. 10 Before CCI induction, no differences were found between wild type and Avil-Ank3 fl/fl tamoxifen-injected mice in the allodynia measure of paw withdrawal threshold.…”