Painful nerve injury decreases sarco-endoplasmic reticulum Ca2+-ATPase activity in axotomized sensory neurons

Abstract

The sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) is a critical pathway by which sensory neurons sequester cytosolic Ca2+ and thereby maintain intracellular Ca2+ homeostasis. We have previously demonstrated decreased intraluminal endoplasmic reticulum Ca2+ concentration in traumatized sensory neurons. Here we examine SERCA function in dissociated sensory neurons using Fura-2 fluorometry. Blocking SERCA with thapsigargin (1μM) increased resting [Ca2+]c and prolonged recovery (τ) from transients induced by neuronal activation (elevated bath K+), demonstrating SERCA contributes to control of resting [Ca2+]c and recovery from transient [Ca2+]c elevation. To evaluate SERCA in isolation, plasma membrane Ca2+ ATPase was blocked with pH 8.8 bath solution and mitochondrial buffering was avoided by keeping transients small (⩽400nM). Neurons axotomized by spinal nerve ligation (SNL) showed a slowed rate of transient recovery compared to control neurons, representing diminished SERCA function, whereas neighboring non-axotomized neurons from SNL animals were unaffected. Injury did not affect SERCA function in large neurons. Repeated depolarization prolonged transient recovery, showing that neuronal activation inhibits SERCA function. These findings suggest that injury-induced loss of SERCA function in small sensory neurons may contribute to the generation of pain following peripheral nerve injury.