Functional reorganization of sensory pathways in the rat spinal dorsal horn following peripheral nerve injury

Abstract

Functional reorganization of sensory pathways in the rat spinal dorsal horn following sciatic nerve transection was examined using spinal cord slices with an attached dorsal root. Slices were obtained from animals whose sciatic nerve had been transected 2-4 weeks previously and compared to sham-operated controls. Whole-cell recordings from substantia gelatinosa neurones in sham-operated rats, to which nociceptive information was preferentially transmitted, revealed that dorsal root stimulation sufficient to activate A afferent fibres evoked a mono- and/or polysynaptic EPSC in 111 of 131 (approximately 85%) neurones. This is in contrast to the response following A fibre stimulation, where monosynaptic EPSCs were observed in 2 of 131 (approximately 2%) neurones and polysynaptic EPSCs were observed in 18 of 131 (approximately 14%) neurones. In sciatic nerve-transected rats, however, a polysynaptic EPSC following stimulation of A afferents was elicited in 30 of 37 (81%) neurones and a monosynaptic EPSC evoked by A afferent stimulation was detected in a subset of neurones (4 of 37, approximately 11%). These observations suggest that, following sciatic nerve transection, large myelinated A afferent fibres establish synaptic contact with interneurones and transmit innocuous information to substantia gelatinosa. This functional reorganization of the sensory circuitry may constitute an underlying mechanism, at least in part, for sensory abnormalities following peripheral nerve injuries.

Figure 1. Patch-clamp and extracellular recordings from spinal cord slices with attached dorsal root

A, a schematic diagram of patch and extracellular recordings. Whole-cell patch-clamp recordings were made from neurones in the SG in a transverse slice that retained an attached dorsal root. The dorsal root was stimulated by either a suction electrode (same as that used for intracellular recordings from DRG neurones) or monopolar stimulating electrodes positioned at proximal and distal portions of the dorsal root (see Methods). B and C, representative extracellular recordings of compound action potentials from sham-operated (B) and SNT (C) rats, evoked at graded stimulus intensities (shown on the right of each trace). Calculated conduction velocities of the first and second (arrow) peaks, respectively, were 22 and 5 m s⁻¹ in sham-operated (B) and 18 and 4.3 m s⁻¹ in SNT (C) rats. These values are consistent with Aδ and Aβ fibre activation, respectively.

Figure 2. Identification of SG neurones in the transverse spinal cord slices

A1 and B 1, two representative SG neurones filled with neurobiotin from patch electrodes. A2 and B 2, the neurones in A1 and B 1 at a higher magnification. Scale bars: A1 and B 1, 100 μm; A2 and B 2, 50 μm.

Figure 3. Mono- and polysynaptic EPSCs in SG neurones evoked by stimulation of the dorsal root

A and B, representative mono- (A) and polysynaptic (B) EPSCs in response to Aδ afferent stimulation (38 and 41 μA, respectively, 0.1 ms duration) in sham-operated rats. EPSCs shown in A were obtained at low (0.2 Hz, A1) and high frequency (20 Hz, A2) stimulation. Note the short and constant latency, and absence of failures even though the amplitude of the EPSCs was markedly attenuated (A2). The first EPSC is indicated by an arrow. The latencies of EPSCs in a different neurone (B) were longer than those in A and were not variable at low frequency stimulation (B 1). At high frequency stimulation (20 Hz), however, the latency was prolonged and variable (B 2). In addition, some stimuli failed to evoke a response (double arrows).

Figure 5. Differences in the threshold stimulus intensity for initiating synaptic responses in sham-operated and SNT rats

A, the stimulus threshold intensity required to evoke either a mono- or a polysynaptic EPSC in sham-operated (Normal) and SNT (Transected) rats. For intact controls, the mean intensity for all Aδ-mediated events was 46 ± 11 μA (n = 111) and for all Aβ-mediated events in SNT rats it was 29 ± 5 μA (n = 34); these values were significantly different (P < 0.01, Student's unpaired t test). B, the distribution of threshold intensities for each neurone in normal (□) and SNT rats (). Most neurones in SNT rats responded at a stimulus intensity less than the Aδ afferent threshold (< 33 μA).

Figure 4. Poly- and monosynaptic EPSCs recorded from three SG neurones in SNT rats in response to Aβ afferent stimulation

A, long and variable latency polysynaptic EPSCs evoked by stimulation of Aβ afferents (30 μA at 0.2 Hz) are superimposed. B, in a different neurone, monosynaptic EPSCs, which have a short and constant latency elicited by Aβ stimulation (30 μA at 0.1 Hz), are superimposed. C and D, in a third neurone, neither failures nor changes in latency of the EPSCs (shown on different time scales) were observed following high frequency repetitive stimulation (30 μA, 0.1 ms, 20 Hz). Traces indicated by arrows are the first EPSCs, and subsequent EPSCs were markedly decreased in amplitude. Triangle in D indicates the onset of the EPSCs and highlights the constant latency of the evoked events. The conduction velocity of the fibre responsible for the EPSC shown in C and D was 21.0 m s⁻¹.

Figure 6. Schematic diagram of the possible reorganization of the sensory circuitry in the spinal dorsal horn following sciatic nerve transection

A, simplified sensory afferent termination of Aβ and Aδ fibres in the spinal dorsal horn (C fibre inputs are excluded). Aβ afferents, which convey tactile information, terminate at laminae III-VI, while Aδ afferents conveying noxious sensation terminate preferentially at the SG. B, following sciatic nerve transection, Aβ afferents not only directly sprout into the SG (c) but also promote either the establishment of synaptic connections between SG neurones and interneurones in other laminae previously innervated by Aβ afferents (a) or synaptic contacts between Aβ afferents and interneurones with pre-existing synapses onto SG neurones (b).