Key points• The peripheral terminals of sensory neurons encode physical and chemical signals into trains of action potentials (APs) and transmit these trains to the CNS.• Although modulation of this process is thought to predominantly reside at synapses, there are also indications that AP trains are incompletely propagated past points at which axons branch. One such site is the T-junction, where the single sensory neuron axon branches into peripheral and central processes.• In recordings from sensory neurons of dorsal root ganglia excised from adult rats, we identified use-dependent failure of AP propagation between the peripheral and central processes that results in filtering of rapid AP trains, especially in C-type neurons.• Propagation failure was regulated by membrane input resistance and Ca 2+ -sensitive K + and Cl − currents. Following peripheral nerve injury, T-junction filtering is reduced in C-type neurons, which may possibly contribute to pain generation.Abstract The T-junction of sensory neurons in the dorsal root ganglion (DRG) is a potential impediment to action potential (AP) propagation towards the CNS. Using intracellular recordings from rat DRG neuronal somata during stimulation of the dorsal root, we determined that the maximal rate at which all of 20 APs in a train could successfully transit the T-junction (following frequency) was lowest in C-type units, followed by A-type units with inflected descending limbs of the AP, and highest in A-type units without inflections. In C-type units, following frequency was slower than the rate at which AP trains could be produced in either dorsal root axonal segments or in the soma alone, indicating that the T-junction is a site that acts as a low-pass filter for AP propagation. Following frequency was slower for a train of 20 APs than for two, indicating that a cumulative process leads to propagation failure. Propagation failure was accompanied by diminished somatic membrane input resistance, and was enhanced when Ca 2+ -sensitive K + currents were augmented or when Ca 2+ -sensitive Cl − currents were blocked. After peripheral nerve injury, following frequencies were increased in axotomized C-type neurons and decreased in axotomized non-inflected A-type neurons. These findings reveal that the T-junction in sensory neurons is a regulator of afferent impulse traffic. Diminished filtering of AP trains at the T-junction of C-type neurons with axotomized peripheral processes could enhance the transmission of activity that is ectopically triggered in a neuroma or the neuronal soma, possibly contributing to pain generation.

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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 (1 μM) 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 (≤400 nM). 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.

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Mitochondria critically regulate cytoplasmic Ca2+ concentration ([Ca2+]c), but the effects of sensory neuron injury have not been examined. Using FCCP (1μM) to eliminate mitochondrial Ca2+ uptake combined with oligomycin (10μM) to prevent ATP depletion, we first identified features of depolarization-induced neuronal [Ca2+]c transients that are sensitive to blockade of mitochondrial Ca2+ buffering in order to assess mitochondrial contributions to [Ca2+]c regulation. This established the loss of a shoulder during the recovery of the depolarization (K+)-induced transient, increased transient peak and area, and elevated shoulder level as evidence of diminished mitochondrial Ca2+ buffering. We then examined transients in Control neurons and neurons from the 4th lumbar (L4) and L5 dorsal root ganglia after L5 spinal nerve ligation (SNL). The SNL L4 neurons showed decreased transient peak and area compared to control neurons, while the SNL L5 neurons showed increased shoulder level. Additionally, SNL L4 neurons developed shoulders following transients with lower peaks than Control neurons. Application of FCCP plus oligomycin elevated resting [Ca2+]c in SNL L4 neurons more than in Control neurons. Whereas application of FCCP plus oligomycin 2s after neuronal depolarization initiated mitochondrial Ca2+ release in most Control and SNL L4 neurons, this usually failed to release mitochondrial Ca2+ from SNL L5 neurons. For comparable cytoplasmic Ca2+ loads, the releasable mitochondrial Ca2+ in SNL L5 neurons was less than Control while it was increased in SNL L4 neurons. These findings show diminished mitochondrial Ca2+ buffering in axotomized SNL L5 neurons but enhanced Ca2+ buffering by neurons in adjacent SNL L4 neurons.