TJC and AL co-wrote the paper with assistance from GJB. of cutaneous allodynia, and encourage the study of how these mechanisms contribute to chronic pain. We anticipate that focus on the pain mechanisms associated with microvascular dysfunction in muscle will provide new effective treatments for chronic pain patients with cutaneous tactile allodynia. == Background == Cutaneous tactile allodynia (referred to henceforth as allodynia) is often found in patients with neuropathic pain, and is generally assumed to depend on the sensitization of the central nervous system in response to aberrant activity in damaged peripheral nerves [1]. However, allodynia is also caused by other injuries, such as that produced by ultraviolet radiation, and occurs in association with migraine headache [2] and fibromyalgia [3]. Allodynia is also prominent in complex regional pain syndrome (CRPS) [4], which can be initiated by either soft tissue (CRPS type-I) or nerve (CRPS type-II) injuries. Importantly, what both CRPS subtypes share with UV injury, migraine and fibromyalgia, besides allodynia, are significant vascular abnormalities caused by microvascular dysfunction [5-9]. Also since CNS sensitization, which is critical for allodynia, is more pronounced following deep tissue injury than after cutaneous injury [10], it is possible that microvascular dysfunction AMAS in muscle may induce AMAS significant allodynia. However, few investigators have assessed vascular abnormalities in the etiology of chronic pain, and none have studied whether microvascular dysfunction in muscle contributes to allodynia. To address these questions, we investigated whether an ischemia-reperfusion (IR) injury produces allodynia in rats, and whether the allodynia is associated with microvascular dysfunction in muscle, and key mechanisms that underlie it. We show that microvascular dysfunction leads to persistent muscle ischemia, a reduction of intraepidermal nerve fibers, and allodynia correlated with muscle ischemia, but not with skin nerve loss. The affected hind paw muscle shows lipid peroxidation, an upregulation of nuclear factor kappa B, and enhanced pro-inflammatory cytokines, while allodynia is relieved by agents that inhibit oxidative stress, nuclear factor kappa B and Mouse monoclonal to CD48.COB48 reacts with blast-1, a 45 kDa GPI linked cell surface molecule. CD48 is expressed on peripheral blood lymphocytes, monocytes, or macrophages, but not on granulocytes and platelets nor on non-hematopoietic cells. CD48 binds to CD2 and plays a role as an accessory molecule in g/d T cell recognition and a/b T cell antigen recognition cytokine activity. Allodynia is increased, along with hind paw muscle lactate, when these rats exercise, and is reduced by an acid sensing ion channel antagonist. Allodynia is also significantly correlated with muscle lactate before and after exercise. == Results and discussion == We first tested whether allodynia is exhibited in rats with IR injury of the hind paw. A AMAS persistent significant reduction in mechanical paw-withdrawal threshold was observed following a 3 h IR injury induced using a tourniquet at the ankle (P = 0.0001) (Fig.1a). This procedure produces a complete occlusion of blood flow to the hind paw, followed by prolonged reactive hyperemia (Fig.2) and edema [11] on reperfusion. In addition to tactile allodynia, rats with what we have called chronic post-ischemia pain (CPIP) exhibit cold allodynia and mechanical hyperalgesia [11], as well as vascular abnormalities [12] that resemble symptoms in CRPS patients (Fig.3). == Figure 1. == Allodynia, endothelial cell injury and microvascular dysfunction in muscle induced by hind paw IR injury. a, 3 h tourniquet IR (CPIP) (n = 12), but not sham (n = 8) treatment, induces a significant reduction in paw-withdrawal threshold (PWT, g) for 4 weeks post-reperfusion (*P < 0.05 compared to baseline (Bas) or sham). b, 3 h clamping of the blood vessels supplying the hind paw (clamp), but not for 5 min (sham), also induces a significant reduction in PWT (g) 2 and 7 days post-reperfusion compared to rats that were only anesthetized (controls) (all groups n = 8) (*P < 0.05, compared to control). c, There are significantly greater dose-dependent norepinephrine (NE)-induced reductions from baseline in blood flow (peak % decrease in flux) in CPIP (n = 8), as compared to sham (n = 13) rats at 2 days post-reperfusion (*P < 0.05) (data from [12]). d, e, Electron micrographs of hind paw digital muscle (HPDM) capillaries from a sham-treated (d) and 7 day CPIP (e) rat (muscle fiber (F), endothelial cell nucleus (N), pericyte (P), lumen (*)). f, Capillary walls are thicker (m) in CPIP (n = 370 from 4 rats), as compared to sham-treated (n =.