Background: Behavioral criteria that confirm neuropathic pain in animal injury models are undefined. Therefore, the authors sought clinically relevant measures that distinguish pain behavior of rats with peripheral nerve injury from those with sham injury. Methods: The authors examined mechanical and thermal sensory sensitivity, comparing responses at baseline to responses after spinal nerve ligation (SNL group), sham nerve injury (sham group), or skin incision alone (control group). Results: Substantial variance was evident in all sensory tests at baseline. After surgery, tests using brush, cold, or heat stimulation showed minimal distinctions between surgical groups. Postsurgical thresholds for flexion withdrawal from mechanical stimulation with von Frey fibers were decreased bilaterally in SNL and sham groups. In contrast, the probability of a complex hyperalgesia-type response with prolonged elevation, shaking, or licking of the paw was selectively increased on the ipsilateral side in the SNL group. Nonetheless, the effect of SNL on behavior was inconsistent, regardless of the sensory test. The behavioral measure that best distinguishes between SNL and sham groups and thereby best identifies animals with successful SNL-induced neuropathic pain is increased ipsilateral postsurgical probability of a hyperalgesia-type response to noxious mechanical stimulation. Using receiver operating characteristics analysis, mechanical hyperalgesia identifies a local SNL effect in approximately 60% of animals when specificity is required to be 90% or higher. Conclusions: Simple withdrawal from von Frey tactile stimulation, although frequently used, is not a valid measure of peripheral nerve injury pain in rats, whereas a complex hyperalgesic-type response is a specific neuropathy-induced behavior.

A regional approach can protect our patients from often unacceptable adverse effects produced by systematically applied drugs. Regional therapeutic approaches, as well as interventions at the level of the peripheral nervous system and particularly the dorsal root ganglion (DRG), represent an alternative to the systemic application of therapeutic agents. This article provides an overview of DRG anatomical peculiarities, explains why the DRG is an important therapeutic target, and how animal models of targeted drug delivery can help us in the translation of basic research into clinical practice.

Background: Painful peripheral nerve injury results in disordered sensory neuron function that contributes to the pathogenesis of neuropathic pain. However, the relative roles of neurons with transected axons versus intact adjacent neurons have not been resolved. An essential first step is identification of electrophysiologic changes in these two neuronal populations after partial nerve damage. Methods: Twenty days after spinal nerve ligation (SNL), intracellular recordings were obtained from axotomized fifth lumbar (L5) dorsal root ganglion neurons and adjacent, intact L4 neurons, as well as from control neurons and others subjected to sham-SNL surgery. Results: Pronounced electrophysiologic changes were seen only in L5 neurons after SNL. Both Aalpha/beta and Adelta neuron types showed increased action potential duration, decreased afterhyperpolarization amplitude and duration, and decreased current threshold for action potential initiation. Aalpha/beta neurons showed resting membrane potential depolarization, and increased repetitive firing during sustained depolarization developed in Adelta neurons. The afterhyperpolarization duration in neurons with C fibers shortened after axotomy. In contrast to the axotomized L5 neurons, neighboring L4 neurons showed no changes in action potential duration, afterhyperpolarization dimensions, or excitability after SNL. Depolarization rate (dV/dt) increased after SNL in L4 Aalpha/beta and Adelta neurons but decreased in L5 neurons. Time-dependent rectification during hyperpolarizing current injection (sag) was greater after SNL in Aalpha/beta L4 neurons compared with L5. Sham-SNL surgery produced only a decreased input resistance in Aalpha/beta neurons and a decreased conduction velocity in medium-sized cells. In the L5 ganglion after axotomy, a novel set of neurons, consisting of 24% of the myelinated population, exhibited long action potential durations despite myelinated neuron conduction velocities, particularly depolarized resting membrane potential, low depolarization rate, and absence of sag. Conclusions: These findings indicate that nerve injury-induced electrical instability is restricted to axotomized neurons and is absent in adjacent intact neurons.

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 Ca²⁺‐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 Ca²⁺‐sensitive K⁺ currents were augmented or when Ca²⁺‐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.