Sodium channel expression in the ventral posterolateral nucleus of the thalamus after peripheral nerve injury

Abstract

Peripheral nerve injury is known to up-regulate the expression of rapidly-repriming Nav1.3 sodium channel within first-order dorsal root ganglion neurons and second-order dorsal horn nociceptive neurons, but it is not known if pain-processing neurons higher along the neuraxis also undergo changes in sodium channel expression. In this study, we hypothesized that after peripheral nerve injury, third-order neurons in the ventral posterolateral (VPL) nucleus of the thalamus undergo changes in expression of sodium channels. To test this hypothesis, adult male Sprague-Dawley rats underwent chronic constriction injury (CCI) of the sciatic nerve. Ten days after CCI, when allodynia and hyperalgesia were evident, in situ hybridization and immunocytochemical analysis revealed up-regulation of Nav1.3 mRNA, but no changes in expression of Nav1.1, Nav1.2, or Nav1.6 in VPL neurons, and unit recordings demonstrated increased background firing, which persisted after spinal cord transection, and evoked hyperresponsiveness to peripheral stimuli. These results demonstrate that injury to the peripheral nervous system induces alterations in sodium channel expression within higher-order VPL neurons, and suggest that misexpression of the Nav1.3 sodium channel increases the excitability of VPL neurons injury, contributing to neuropathic pain.

Figure 1

Two-dimensional distribution of 10 histologically identified recording sites plotted on a schematic diagram [16] of the ventrobasal complex of the thalamus corresponding to bregma -3.14 mm, which delineates the posterior nucleus group (Po), ventral posteromedial nucleus (VPM), and VPL. Units from intact control (open circles), 10 days after chronic constriction injury (CCI) ipsilateral side (open squares), and CCI contralateral side (filled triangles) groups are shown (A). All units used in this analysis were confined to the VPL. Representative extracellular multireceptive unit recordings plotted as peristimulus time histograms as well as unit activity are shown for intact (B), and ipsilateral (C) and contralateral (D) sides after CCI in response to phasic brush, 144 g/mm² press, and 583 g/mm² pinch, stimulation (10 sec) of peripheral receptive fields located on the corresponding hindpaw. Quantification (E) of spikes/second show that 10 days after CCI, on the contralateral side, evoked discharge rates were significantly (*p < 0.05) elevated in response to all peripheral stimuli compared to intact animals and the ipsilateral side of CCI animals.

Figure 2

Representative recording of spontaneous and evoked activity of a contralateral VPL neuron with a hindlimb receptive field demonstrated spontaneous discharge 10 days after CCI (A). The VPL unit was continuously recorded, and the spinal cord was acutely transected at T6 following application of 2% lidocaine (lido+tx, at t = 120 s). The corresponding unit waveform is shown. Spontaneous background (BK) activity and evoked responses to brush and press (PR, bar) stimuli are shown on an expanded time scale before (a, t = 50–59 sec) and after (b, t = 300–309 sec) cord transection. In CCI animals, spontaneous firing of VPL neurons was unaffected and occurred at a frequency of 5–12 spikes/s following cord transection, but no evoked responses to PR could be elicited (b). Quantification (B) revealed that evoked responses could no longer be elicited after cord transection in intact and CCI (contralateral) groups, and that background activity remained significantly (*p < 0.05) elevated in CCI animals before (pre) and after interruption (tx) of ascending afferent barrage compared with intact animals.

Figure 3

Schematic representation of a coronal brain section corresponding to bregma -3.14 [16] showing the location of the VPL and image locations. Ten days after CCI, representative images from in situ hybridization for detection of Nav1.3 mRNA are shown for the ipsilateral (B) and contralateral (C) sides. On the ipsilateral side, only very light Nav1.3 signal was detectable. On the contralateral side, punctuate Nav1.3 signal was present in CCI animals. Higher magnification insets are shown for the ipsilateral (B') and contralateral (C') sides. Quantification of the number of Nav1.3-positive cells exhibiting a neuronal morphology from each group is shown in (D). Compared to intact animals, and to the ipsilateral side after CCI, the contralateral VPL exhibited a significantly (*p < 0.05) increased number of Nav1.3-positive profiles after CCI. Nav1.3 signal intensity was very low in intact animals, and on the ipsilateral side after CCI, whereas on the contralateral side signal intensity was significantly increased after CCI.

Figure 4

Representative images showing Nav1.1 (A, B), Nav1.2 (C, D), and Nav1.6 (E, F) mRNA transcripts within regions corresponding to the ipsilateral (A, C, E) and contralateral (B, D, F) VPL in coronal brain sections collected at bregma -3.14 10 days following CCI. Signal was detectable in cells exhibiting a neuronal morphology. Quantification of the number of neurons per section expressing each sodium channel isoforms (G) did not reveal any significant differences in ipsilateral or contralateral expression after CCI when compared to intact animals. Quantification of sodium channel in situ signal intensity (H) showed moderate expression of all channel isoforms in intact animals, and that 10 days after CCI no isoforms exhibited changes in signal intensity.