Role of spinal microglia in myositis‐induced central sensitisation: An immunohistochemical and behavioural study in rats

Abstract

There is increasing evidence that spinal glial cells play an important role in chronic pain states. However, so far no data on the role of microglia in muscle pain are available. The aim of the present study was to investigate the involvement of spinal microglial cells in chronic muscle pain. In a rat model of chronic muscle inflammation (injection of complete Freund s adjuvant into the gastrocnemius-soleus muscle) alterations of microglia were visualized with quantitative OX-42 immunohistochemistry in the dorsal horn of the segments L4 and L5 12 days after induction of inflammation. In behavioural experiments the influence of chronic intrathecally applied minocycline - a specific microglia inhibitor - or an antibody against tumour necrosis factor-alpha (TNF-alpha; a cytokine released from microglia) on pain-related behaviour was investigated after 1, 3, 6, and 12 days. The immunhistochemical data show that in the deep laminae of the spinal dorsal horn microglial cells reacted with morphological changes to the muscle inflammation. Following inflammation, the mean boundary length surrounding the OX-42 immunostained area was significantly shorter. This indicates that microglial cells were activated by the myositis and withdrew their processes. Chronic intrathecal administration of minocycline or anti TNF-alpha with an osmotic mini-pump largely normalised the inflammation-induced changes in spontaneous exploratory behaviour and attenuated the hypersensitivity to mechanical stimulation. Both the immunohistochemical and behavioural data show that spinal microglial cells are involved in nociceptive processes in the cause of a chronic muscle inflammation.

Beneficial Effects of Minocycline on Cuprizone Induced Cortical Demyelination

“…In this recently published work no differences were found for locomotor activity and exploration in the open field test and the elevated plus maze test in 8-week old mice fed with cuprizone. Beneficial behavioural effects of minocycline have already been evaluated in various animal disease models [14,[42][43][44]. In mice challenged with LPS minocycline not only blocked the LPS-stimulated cytokine secretion but also facilitated the recovery from sickness behaviour and prevented anhedonia [43].…”

Anatomical and Physiological Factors Contributing to Chronic Muscle Pain

“…Noxious substances including, but not limited to, acidic saline (Sluka et al 2001), hypertonic saline (Ro et al 2007), carrageenan (Diehl et al 1988; Radhakrishnan et al 2003), mustard oil(Han et al 2008), complete freund’s adjuvant(CFA) (Ambalavanar et al 2007; Chacur et al 2009), and PGE2 combined with carrageenan (Dina et al 2008; Dina et al 2011) are commonly used to produce muscle hypersensitivity. Of these, repeated acidic saline (pH 4 in 0.9% saline) (Sluka et al 2001), CFA (Ambalavanar et al 2007; Chacur et al 2009), carrageenan (3% in 0.9% saline)(Diehl et al 1988; Radhakrishnan et al 2003), and PGE2 (Dina et al 2008; Dina et al 2011) injections produce long-lasting enhanced pain behavior in animals. Notably, in each of these paradigms, the enhanced pain behaviors last substantially longer than inflammatory or histological changes in the affected muscle.…”

Tetracyclines and pain

“…Local microinjection of minocycline inhibits thermal hyperalgesia in a model of orofacial pain induced by complete Freund's adjuvant (CFA) injection into the masseter muscle (Shimizu et al 2009). In a model of muscle inflammatory pain in which CFA is injected into the gastrocnemius-soleus muscle, chronic intrathecal minocycline inhibits mechanical allodynia (Chacur et al 2009). If CFA is injected directly into the ankle articular cavity of rats, a model of acute monoarthritis, there is development of mechanical allodynia and thermal hyperalgesia, and these phenomena are also inhibited by intrathecal minocycline (Shan et al 2007).…”

“…IL-6 concentrations in the spinal cord or cerebrospinal fluid were unaffected in this experimental setting. If the central sensitisation is induced by intrathecal LPS or injection of CFA into the gastrocnemius-soleus muscle, minocycline antiallodynic effect is also associated with reduced TNF-α concentration in the cerebrospinal fluid (Saito et al 2010) or in the L4-L5 dorsal horn segments (Chacur et al 2009). Raghavendra et al (2003) showed that the intrathecal preemptive treatment with minocycline induces antiallodynic and antihyperalgesic effects in rats submitted to spinal nerve transection, and these effects are associated with reduction of gene expression and concentrations of the inflammatory cytokines IL-1β, IL-6 and TNF-α in the L5 lumbar spinal cord segment.…”