Background/Purpose:

The aim of this study was to investigate the effects of prolonged chemogenetic inhibition of Na_V1.8 nociceptors on pain-related behaviors and cellular changes in the dorsal root ganglia (DRG) in a murine model of OA.

After destabilization of the medial meniscus (DMM), mice develop pain-related behaviors in association with progressive joint damage, including knee hyperalgesia and mechanical allodynia in the ipsilateral hindpaw. In addition, cellular changes occur in the DRG, including increased numbers of macrophages 8 and 16 weeks after DMM, compared to sham and naïve age-matched controls.

Chemogenetic silencing of nociceptors involves the use of Designer Receptors Exclusively Activated by Designer Drugs (DREADD), also termed Pdi, genetically targeted to the Voltage-Gated Sodium Channel 1.8 (Na_V1.8). Na_V1.8 is expressed by the majority of nociceptors. Activation of Pdi with the synthetic chemical agonist clozapine N-oxide (CNO) inhibits the neuronal activity of these nociceptors (Miller RE et al, Arthr Rheum 2017).

Methods:

CNO- or saline-loaded Alzet minipumps were implanted into the peritoneal cavity of 10-week old male Na_V1.8-Pdi C57BL/6 mice, which allows for chronic release of CNO or saline for up to 6 weeks. Three days after implantation, DMM surgery was performed on the right knees of these mice.

A blinded observer performed the following behavioral tests after surgery: Knee hyperalgesia was measured using a Pressure Application Measurement (PAM) device (Ugo Basile), and mechanical allodynia in the ipsilateral hind paw was measured using von Frey fibers and the up-down staircase technique. To assess cellular changes in the peripheral pain pathway, L4 DRG sections were immunostained with the macrophage marker, F4/80. The total number of macrophages per DRG section was quantified using ImageJ and normalized to the area of each DRG.

Results:

Evaluation of knee hyperalgesia 2, 4 and 6 weeks after DMM surgery in saline (n=5) and CNO (n=11) implanted Na_V1.8-Pdi mice revealed that CNO-implanted mice had significantly less knee hyperalgesia at 4 (p= 0.0004) and 6 (p= 0.01) weeks after DMM, compared to saline-implanted mice. Mechanical allodynia was assessed at 6 weeks after DMM surgery, and CNO-implanted mice had significantly less mechanical allodynia compared to saline-implanted mice (p= 0.03). Macrophage infiltration in the ipsilateral L4 DRG was significantly attenuated in CNO-implanted mice compared to vehicle controls (p=0.03).

Conclusion:

The results showed that prolonged (i.e., 6 weeks) chemogenetic inhibition of nociceptors in Na_V1.8-Pdi mice significantly attenuated knee hyperalgesia and mechanical allodynia. In addition, prolonged chemogenetic inhibition also affected cellular changes in the DRGs, with fewer macrophages in CNO-implanted mice. This suggests that prolonged blockade of nociceptor activity can attenuate pain-related behaviors and influence cellular changes in the DRGs in murine OA. Histological assessment of the knee joints is ongoing.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/effects-of-prolonged-chemogenetic-inhibition-of-nociceptors-in-a-murine-surgical-model-of-osteoarthritis/