Activation of corticostriatal circuitry relieves chronic neuropathic pain

Abstract

Neural circuits that determine the perception and modulation of pain remain poorly understood. The prefrontal cortex (PFC) provides top-down control of sensory and affective processes. While animal and human imaging studies have shown that the PFC is involved in pain regulation, its exact role in pain states remains incompletely understood. A key output target for the PFC is the nucleus accumbens (NAc), an important component of the reward circuitry. Interestingly, recent human imaging studies suggest that the projection from the PFC to the NAc is altered in chronic pain. The function of this corticostriatal projection in pain states, however, is not known. Here we show that optogenetic activation of the PFC produces strong antinociceptive effects in a rat model (spared nerve injury model) of persistent neuropathic pain. PFC activation also reduces the affective symptoms of pain. Furthermore, we show that this pain-relieving function of the PFC is likely mediated by projections to the NAc. Thus, our results support a novel role for corticostriatal circuitry in pain regulation.

Keywords: affective; nucleus accumbens; optogenetics; pain; prefrontal cortex.

Copyright © 2015 the authors 0270-6474/15/355247-13$15.00/0.

Figures

Figure 1.

SNI evokes sensory and depressive symptoms of chronic pain. A, B, SNI-treated rats developed mechanical and cold allodynia, compared with sham-treated rats. Two-way ANOVA with repeated measures and Bonferroni's post-test, n = 6, p < 0.0001. C, SNI-treated rats developed decreased preference for sucrose in the SPT, a test of anhedonia. Student's t test, n = 8, p < 0.001. D, SNI-treated rats developed decreased motivation or behavioral despair, as indicated by prolonged immobility on the FST. Student's t test, n = 4, p < 0.05. Error bars show mean and SEM.

Figure 2.

Functional expression of ChR2 in PL-PFC neurons. A, Histologic expression of ChR2-eYFP in the PL-PFC 14 d after viral injection. ChR2-eYFP was found to be restricted to neurons in the PL-PFC region. NeuN, Neuronal marker. B, Schematic showing electrophysiological recordings were conducted at the site of light delivery. Data were collected from prefrontal neurons 14 d after viral injections in the PL-PFC. C, Whole-cell patch-clamp recording showed that direct illumination with an LED in brain slices containing the PL-PFC elicited spikes with high fidelity in prefrontal neurons. LED pulses are represented below in blue. D, In vivo recording at the virus injection site with simultaneous photoactivation demonstrated that laser pulses reliably triggered spikes in PL-PFC neurons. Laser pulses are represented below in green.

Figure 3.

Activation of PL-PFC neurons relieves sensory allodynia in a rat chronic neuropathic pain (SNI) model. A, Optical fibers were positioned directly above viral injection sites in the PL-PFC. Photoactivation occurred during behavior tests. B, Illumination of ChR2-expressing neurons in the PL-PFC decreased mechanical allodynia in SNI-treated rats, but not in sham-treated rats. Light treatment did not have any effect on animals that received vectors containing eYFP only. Two-way ANOVA with Bonferroni's post-test, n = 8–9, p < 0.0001. Light treatment was performed ≥2 weeks after SNI/sham procedures. C, Illumination of ChR2-expressing neurons in the PL-PFC decreased cold allodynia in SNI-treated rats. Two-way ANOVA with Bonferroni's post-test, n = 8–9, p < 0.0001. Light treatment was performed ≥2 weeks after SNI/sham procedures. D, Schematic showing the locations of optic fibers. E, To test the contribution of local circuits within the PL-PFC to the antiallodynic effects of prefrontal activation, NBQX was injected into the PL-PFC before photoactivation and behavior assays. Tests were performed on SNI-treated rats that expressed ChR2. F, Compared with saline, NBQX infusion in the PFC did not alter mechanical allodynia in SNI-treated rats, with or without light activation of the PFC. Student's t test, n = 5 (light), n = 6–7 (no light), p > 0.05. G, NBQX infusion in the PFC did not alter cold allodynia in SNI-treated rats, with or without light activation of the PFC. Student's t test, n = 5 (light), n = 6–7 (no light), p > 0.05. H, Schematic showing the locations of NBQX infusion in the PL-PFC. Error bars show mean and SEM.

Figure 4.

Activation of PL-PFC neurons elevates acute nociceptive threshold and relieves chronic neuropathic pain. A, Photoactivation of the PL-PFC relieved mechanical allodynia of the injured paws 4 weeks after SNI. Student's t test, n = 5, p < 0.001. B, Activation of the PL-PFC relieved cold allodynia of the injured paws 4 weeks after SNI. Student's t test, n = 4, p < 0.01. C, Activation of the PL-PFC increased the latency to withdrawal of the uninjured (contralateral) paws of SNI-treated rats on the Hargreaves test. Student's t test, n = 5, p < 0.05. D, Activation of the PL-PFC increased the latency to withdrawal of the injured paws of SNI-treated rats. Student's t test, n = 5, p < 0.01. Error bars show mean and SEM.

Figure 5.

Prefrontal activation relieves the aversive quality and depressive symptoms associated with chronic pain. A, Schematic of the light treatment. B, Light activation in the PL-PFC induced chamber preference in SNI-treated, but not sham-treated, rats. Two-way ANOVA with repeated measures and Bonferroni's post-test was used to compare preconditioning with test values, n = 7, p < 0.05 for the SNI group, n = 5, p > 0.05 for the sham group. C, Photoactivation of the PL-PFC improved sucrose preference in animals with chronic pain. Two-way ANOVA with Bonferroni's post-test, n = 8–10, p < 0.0001. D, Photoactivation of the PL-PFC had no effect on total fluid consumption during the SPT in SNI-treated rats. Student's t test, p > 0.05. E, Activation of the PL-PFC caused a trend of increase in sucrose consumption during the SPT in SNI-treated rats. Student's t test, p > 0.05. F, Activation of the PL-PFC caused a trend of decrease in water consumption during the SPT in SNI-treated rats. Student's t test, p > 0.05. G, Photoactivation of the PL-PFC decreased immobility on the FST in SNI-treated rats. Student's t test, n = 5–9, p < 0.01. H, Locomotor activity was not altered by PL-PFC photoactivation. Two-way ANOVA with repeated measures, n = 4–8, p > 0.05. Error bars show mean and SEM.

Figure 6.

Light treatment in the NAc core selectively activates axon terminals of prefrontal neurons that project to the NAc core. A, ChR2-eYFP expression was found in the axon terminals of PL-PFC neurons in the NAc core 4 weeks after viral injections in the PL-PFC. B, In vivo recordings from the NAc core showed that light treatment in the NAc produced spikes. C, Whole-cell patch-clamp recordings from slices containing the NAc core showed that LED pulses produce EPSPs and spikes in MSNs of the NAc core. D, Light-induced EPSPs in the NAc were blocked by bath application of NBQX.

Figure 7.

Activation of prefrontal projections to the NAc core has antinociceptive effects in the chronic neuropathic pain state. A, Schematic showing the experimental paradigm. Four weeks after viral injections in the PL-PFC, light treatment was performed in the NAc core to activate prefrontal neurons that project to the NAc during behavior tests. B, Photoactivation of PL-PFC axons in the NAc core reduced mechanical allodynia in SNI-treated rats expressing ChR2-eYFP. Two-way ANOVA, n = 4–7, p < 0.001. C, Activation of PL-PFC axons in the NAc core reduced cold allodynia in SNI-treated rats expressing ChR2-eYFP. Two-way ANOVA, n = 4–8, p < 0.01. D, Schematic showing the locations of optic fibers in the NAc core. Error bars show mean and SEM.

Figure 8.

Activation of prefrontal projections to the NAc core relieves the affective symptoms associated with chronic pain. A, Schematic of the light treatment. B, Light activation of the corticostriatal circuit induced chamber preference in SNI-treated, but not sham-treated, rats. Two-way ANOVA with repeated measures and Bonferroni's post-test, n = 8, p < 0.01 for the SNI group; n = 7, p > 0.05 for the sham group. C, Photoactivation of the prefrontal projection to the NAc core improved sucrose preference in SNI-treated rats. Two-way ANOVA, n = 6–7, p < 0.001. D, Photoactivation of the PL–NAc circuit had no effect on the total fluid consumption of SNI-treated rats during the SPT. Student's t test, p > 0.05. E, Activation of the PL–NAc circuit increased sucrose consumption in SNI-treated rats during the SPT. Student's t test, p < 0.01. F, Activation of the PL–NAc circuit decreased water consumption in SNI-treated rats during the SPT. Student's t test, p < 0.05. G, Activation of the prefrontal projection to the NAc core decreased immobility on the FST in SNI-treated rats. Student's t test, n = 5–6, p < 0.05. Error bars show mean and SEM.

Figure 9.

NBQX in the NAc blocks the pain-relieving effects of photoactivation of the PL-PFC. A, To test the role of the corticostriatal circuit in pain regulation, NBQX was injected into the NAc core before photoactivation of the PL-PFC and behavior tests. Tests were performed on SNI-treated rats that expressed ChR2. B, NBQX infusion in the NAc blocked the effect of PFC activation on mechanical allodynia in SNI-treated rats. Student's t test, n = 9–11, p < 0.01. C, NBQX infusion in the NAc blocked the effect of PL-PFC activation on cold allodynia in SNI-treated rats. Student's t test, n = 8–10, p < 0.05. D, NBQX infusion in the NAc blocked the effect of light treatment in the PL-PFC on sucrose preference in SNI-treated rats. Student's t test, n = 4–5, p < 0.05. E, Schematic showing the locations of NBQX infusion in the NAc core. Error bars show mean and SEM.