Background Microglial cells are the resident macrophages of the central nervous system (CNS). Their activation (termed “microgliosis”) is thought to modulate synaptic transmission in the dorsal horn, thereby promoting chronic pain. Fractalkine derived from the central termini of primary sensory afferents has been shown to contribute to microgliosis.

Objectives The primary aim of this study was to document the temporal profile of dorsal horn microgliosis after destabilization of the medial meniscus (DMM) in the mouse. Further, we assessed fractalkine release from dorsal root ganglia (DRG) after DMM surgery. We evaluated these pathological events in the pain pathway in three strains of mice: wildtypes (WT), which develop slowly progressive osteoarthritis following DMM surgery, associated with sustained pain-related behaviours during the 16-week follow-up period¹; Ccr2 null mice, which are protected from persistent pain in spite of joint damage¹; and Adamts5 null mice, which are protected from joint damage and associated pain-related behaviours after DMM surgery².

Methods DMM or sham surgery was performed in the right knee of 10-week old male wild type (WT), Ccr2 null or Adamts5 null C57BL/6 mice. Mechanical allodynia in the hindpaw was monitored using von Frey fibers. L4 dorsal horn microgliosis was assessed 4, 8 and 16 weeks after surgery, based on the morphology of Iba1-immunoreactive (ir) microglia: Iba1-ir cells were classified as “resting” if their process length was more than double the soma diameter, and as “activated” if their process length was less than double the soma diameter³. The observer was blinded to the treatment groups. DRG cells (L3-L5) from DMM, sham or naïve control mice were cultured and supernatants collected for fractalkine ELISA.

Results Four weeks after surgery in WT mice, the number of activated microglia in the L4 dorsal horn was not different between naïve and DMM mice. In contrast, 8 weeks after DMM, there was a significant increase in the number of activated microglia compared to shams. By 16 weeks after DMM, activated microglia were increased compared to age-matched naïve controls. DRG cultures from WT mice showed increased fractalkine release 8 and 16 weeks, but not 4 weeks, after DMM. In contrast, DRG cultures from Ccr2 null and Adamts5 null mice did not release increased fractalkine and L4 dorsal horn microgliosis was absent in these mice.

Conclusions DMM surgery leads to late stage dorsal horn microgliosis, 8–16 weeks after surgery. This suggests that involvement of the CNS is a late event in this experimental model. Further, the temporal correlation with fractalkine release by DRG cells suggests that fractalkine may contribute to microglial activation. Finally, reduced microgliosis in Ccr2 null and Adamts5 null mice, both of which fail to display persistent pain behaviors after DMM, suggests that microgliosis is associated with the persistence of OA pain. Joint protection and prevention of peripheral sensitization may attenuate one aspect of central sensitization.

1. Miller RE et al, PNAS 2012;

2. Malfait AM et al, Osteoarthritis Cart 2008;

3. Thakur M et al. PLoS One 2012.

Acknowledgement NIH, NIAMS R01-AR-064251 and R01-AR-060364

Disclosure of Interest None declared