Pharmacological restoration of anti-nociceptive functions in the prefrontal cortex relieves chronic pain

Abstract

Chronic pain affects one in four adults, and effective non-sedating and non-addictive treatments are urgently needed. Chronic pain causes maladaptive changes in the cerebral cortex, which can lead to impaired endogenous nociceptive processing. However, it is not yet clear if drugs that restore endogenous cortical regulation could provide an effective therapeutic strategy for chronic pain. Here, we studied the nociceptive response of neurons in the prelimbic region of the prefrontal cortex (PL-PFC) in freely behaving rats using a spared nerve injury (SNI) model of chronic pain, and the impact of AMPAkines, a class of drugs that increase central glutamate signaling, on such response. We found that neurons in the PL-PFC increase their firing rates in response to noxious stimulations; chronic neuropathic pain, however, suppressed this important cortical pain response. Meanwhile, CX546, a well-known AMPAkine, restored the anti-nociceptive response of PL-PFC neurons in the chronic pain condition. In addition, both systemic administration and direct delivery of CX546 into the PL-PFC inhibited symptoms of chronic pain, whereas optogenetic inactivation of the PFC neurons or administration of AMPA receptor antagonists in the PL-PFC blocked the anti-nociceptive effects of CX546. These results indicate that restoration of the endogenous anti-nociceptive functions in the PL-PFC by pharmacological agents such as AMPAkines constitutes a successful strategy to treat chronic neuropathic pain.

Keywords: AMPAkine; CX546; Neuropathic pain; PFC; Prefrontal cortex; Prelimbic.

Copyright © 2021. Published by Elsevier Ltd.

Figures

Fig. 1.. Neurons in the prelimbic region of the prefrontal cortex (PL-PFC) increase their firing rates in response to noxious stimuli.

A. Schematic for electrophysiological recordings of the PL-PFC response to noxious (pinprick (PP) 28-gauge needle) or non-noxious (0.4g vF filament) stimulus in freely moving rats. B. Histology showing the location of the silicon probes in the PL-PFC contralateral to peripheral stimulations to the hind paw. Scale bar, 1000 μm. C. Raster plot and peri-stimulus time histogram (PSTH) of a representative PL-PFC neuron in a naïve rat. Time 0 indicates the onset of vF filament stimulation. D. Raster plot and PSTH of a representative PL-PFC neuron in a naïve rat. Time 0 indicates the onset of PP stimulation. E. In total, 8.55 % of PL-PFC neurons (n = 117 total neurons recorded) responded to a vF filament stimulus by increasing firing rates (FR). F. In total, 29.9 % of PL-PFC neurons (n = 117 total neurons recorded) responded to a PP stimulus by increasing FR. G. The difference in the percentage of neurons that increased their firing rates in response to PP stimulations compared to vF filament stimulations is statistically significant. p

Fig. 2.. Spared nerve injury (SNI) suppresses neuronal activity in the PL-PFC in response to noxious stimuli.

A. Schematic of the mechanical allodynia test of SNI-treated rats. B. Animals that underwent SNI surgery developed mechanical allodynia lasting at least 14 days. In comparison, sham-treated rats (control) did not demonstrate mechanical allodynia. A 50% withdrawal threshold was calculated (see Methods for details). n = 15; p

Fig. 3.. AMPAkines increase the nociceptive response of neurons in the PL-PFC in SNI-treated rats.

A. Raster plots and PSTHs of a representative PL-PFC neuron in a SNI-treated rat before (left) and after (right) systemic injection of CX546 (10 mg/kg). Time 0 indicates the onset of PP stimulation. B. CX546 (10 mg/kg) induced a higher peak FR in SNI-treated rats in response to noxious stimuli. n = 174; p = 0.0151, Wilcoxon paired signed-rank test. C. Raster plot and PSTH of a representative PL-PFC neuron in response to vF filament stimulation. Time 0 indicates the onset of vF filament stimulation. D. CX546 induced no changes in peak FR in SNI-treated rats in response to non-noxious stimuli. n = 174; p = 0.3219, Wilcoxon paired signed-rank test. E. Raster plots and PSTHs of a representative PL-PFC neuron in a sham treated rat before (left) and after (right) the injection of CX546 (10 mg/kg). F. CX546 induced no changes in peak FR in SHAM rats in response to PP stimuli. n= 145; p = 0.2819, Wilcoxon paired signed-rank test.

Fig. 4.. Local infusion of CX546 into the PL-PFC had similar analgesic effects as systemic infusion of CX546.

A. Schematics showing (left) systemic injection of CX546 (10 mg/kg) and (right) intra-PL-PFC infusion of CX546 (400 uM). B. Systemic injection and intracranial injection both relieved mechanical allodynia in SNI-treated rats. Baseline vs Systemic: n = 6; p = 0.0003, Baseline vs Intracranial: n = 6; p = 0.0004, Systemic vs Intracranial: n = 6; p = 0.4701, one-way ANOVA, Tukey’s multiple comparisons test with repeated measures. C. Systemic injection and intracranial injection of CX546 both relieved cold allodynia in SNI-treated rats. Baseline vs Systemic: n = 6; p = 0.0021, Baseline vs Intracranial: n = 6; p = 0.0017, Systemic vs Intracranial: n = 6; p = 0.6083, one-way ANOVA, Tukey’s multiple comparisons test with repeated measures.

Fig. 5.. Inactivation of the PL-PFC inhibits analgesic effects of CX546.

A. Histologic expression of NpHR-eYFP in the PL-PFC. Scale bar, 1000 μm. B. Schematic showing the systemic injection of CX546, with optogenetic inactivation of the PL-PFC. C. Optogenetic inhibition of the PL-PFC blocked the analgesic effects of systemic administration of CX546 in SNI-treated rats on the mechanical allodynia test. NpHR group n = 6, YFP group n = 6, p = 0.002, Mann-Whitney U test. D. Optogenetic inhibition of PL-PFC blocked the analgesic effects of systemic administration of CX546 in SNI-treated rats on the cold allodynia test. NpHR group n = 6, YFP group; n = 6, p = 0.015, Mann-Whitney U test. E. Schematic showing the experimental set up of systemic injection of CX546 and intra-PL-PFC infusion of NBQX. F. NBQX blocked the analgesic effects of systemic administration of CX546 in SNI-treated rats on the mechanical allodynia test. NBQX group n = 6, saline group, n = 6, p = 0.002, Mann-Whitney U test. G. NBQX blocked the analgesic effects of systemic administration of CX546 (10mg/kg) in SNI-treated rats on the cold allodynia test. NBQX group n = 6, saline group n = 6, p = 0.020, Mann-Whitney U test.