Injury of Muscular but not Cutaneous Nerve Drives Acute Neuropathic Pain in Rats

Abstract

Acute pain is a common complication after injury of a peripheral nerve but the underlying mechanism is obscure. We established a model of acute neuropathic pain via pulling a pre-implanted suture loop to transect a peripheral nerve in awake rats. The tibial (both muscular and cutaneous), gastrocnemius-soleus (muscular only), and sural nerves (cutaneous only) were each transected. Transection of the tibial and gastrocnemius-soleus nerves, but not the sural nerve immediately evoked spontaneous pain and mechanical allodynia in the skin territories innervated by the adjacent intact nerves. Evans blue extravasation and cutaneous temperature of the intact skin territory were also significantly increased. In vivo electrophysiological recordings revealed that injury of a muscular nerve induced mechanical hypersensitivity and spontaneous activity in the nociceptive C-neurons in adjacent intact nerves. Our results indicate that injury of a muscular nerve, but not a cutaneous nerve, drives acute neuropathic pain.

Keywords: Acute neuropathic pain; Cutaneous nerve; Muscular nerve.

Fig. 1

Nerve branches and plantar territories. A Diagram of the sciatic nerve and its branches: tibial (both muscular and cutaneous), gastrocnemius-soleus (GS; muscular only) and sural (cutaneous only). In nerve transection, the transparent tube was held steady and the suture loop was pulled rapidly. B The plantar hind paw is divided into three territories: lateral, middle, and medial, innervated by the sural, tibial, and saphenous nerves, respectively. Paw withdrawal thresholds were measured at sites 1, 2, and 3 (dashed circles) in the corresponding territories.

Fig. 2

Paw withdrawal thresholds (PWTs) in each territory and spontaneous pain duration. A PWTs of the lateral territory. In the tibial and GS transection groups, PWTs significantly decreased immediately after transection and lasted for 14 days. In the sural and sham groups, PWTs showed no significant change at most time points. B PWTs of the medial territory. PWTs decreased significantly after GS nerve transection. In the tibial, sural, and sham groups, the PWTs at most time points showed no significant change. C PWTs of the middle territory. PWTs increased significantly after tibial nerve transection and dropped after GS nerve transection with a potential recovery tendency after 1 day. D Spontaneous pain duration. After tibial nerve transection and GS nerve transection, the spontaneous pain duration extended significantly, and a recovery tendency occurred over time. In the sural and sham groups, spontaneous pain duration did not significantly change at most time points [time points vs baseline, *P < 0.05, **P < 0.01 (tibial group); ^#P < 0.05, ^##P < 0.01 (GS group); ^&P < 0.05, ^&&P < 0.01 (sural group); ^{^}P < 0.05, ^{^^}P < 0.01 (sham group); arrowheads, implantation of suture loop and tube without nerve transection on day − 2; arrows, nerve transection].

Fig. 3

In vivo responses of nociceptive C-neurons to mechanical stimuli in the tibial, GS, and sural nerve transection groups. A Bright-field image of the L4 DRG surface showing a neuron cell body (arrow) and an extracellular recording electrode (dashed lines) (scale bar, 50 μm). B Conduction velocity (CV) was measured by electrically stimulating the peripheral receptive field on the dorsal surface of the paw (black dot) (bar, 100 ms). C Typical response of the C-neuron to 50 mN mechanical stimulation. Action potentials (APs) in the original recording trace (Ie) are presented as corresponding tic marks below. D Representative response of the C-neuron to nociceptive thermal stimulation (51 °C for 5 s). E Representative response of the C-neuron to cold stimulation (0 °C for 10 s). F Receptive fields of nociceptive C-neurons after tibial (△), GS (×), sural (□), and sham (○) transection. G Action potentials (vertical lines) evoked by different mechanical stimuli—Q-tip (Q), light brush (Br), and von-Frey filaments with 5 mN, 10 mN, 30 mN, and 50 mN bending forces—immediately after nerve transection. H Mechanical thresholds to evoke the discharge of nociceptive C-neurons (*P < 0.05, **P < 0.01, post-transection vs pre-transection).

Fig. 4

Rapid-onset of spontaneous C-neuron discharges in the L4 DRG following muscular nerve transection. A Schematic of the recording setup. B, C An initially quiescent C-nociceptive neuron showed spontaneous activity within 3 min after tibial and GS nerve transections, with inhibitory modulation by A-fiber strength stimuli (horizontal bar, electrical stimulation at 10 Hz, 0.5 mA for 3 min; arrowheads, mechanical stimulation in the receptive field). D Percentage of C-neurons exhibiting spontaneous discharges after nerve transection. The number of neurons exhibiting spontaneous discharge/total neurons recorded is indicated above each column (*P < 0.05, **P < 0.01, tibial, GS, and sural groups vs sham).

Fig. 5

Evans blue extravasation and cutaneous temperature after nerve transection. A Evans blue clearly extravasated in the affected skin 30 min after tibial nerve transection (left), and the temperature of the lateral territory of the plantar hind paw increased significantly after the transection (right) (*P < 0.05, **P < 0.01, lateral vs middle territory). B Evans blue extravasated significantly after GS nerve transection (left), but with no significant change in the temperature of the ipsilateral paw (right) (*P < 0.05, **P < 0.01, ipsilateral middle vs contralateral middle territory). C No significant Evans blue extravasation (left) and cutaneous temperature change (right) were found following sural nerve transection (*P < 0.05, **P < 0.01, lateral vs middle territory). D Concentration of Evans blue dye in the dorsal skin after tibial, GS, and sural nerve transections (*P < 0.05, **P < 0.01, ipsilateral vs contralateral skin).