Sarah A. Woller

The search for novel analgesics: targets and mechanisms

The management of the pain state is of great therapeutic relevance to virtually every medical specialty. Failure to manage its expression has deleterious consequence to the well-being of the organism. An understanding of the complex biology of the mechanisms underlying the processing of nociceptive information provides an important pathway towards development of novel and robust therapeutics. Importantly, preclinical models have been of considerable use in determining the linkage between mechanism and the associated behaviorally defined pain state. This review seeks to provide an overview of current thinking targeting pain biology, the use of preclinical models and the development of novel pain therapeutics. Issues pertinent to the strengths and weaknesses of current development strategies for analgesics are considered.

Inflammation is increased with anxiety- and depression-like signs in a rat model of spinal cord injury

Spinal cord injury (SCI) leads to increased anxiety and depression in as many as 60% of patients. Yet, despite extensive clinical research focused on understanding the variables influencing psychological well-being following SCI, risk factors that decrease it remain unclear. We hypothesized that excitation of the immune system, inherent to SCI, may contribute to the decrease in psychological well-being. To test this hypothesis, we used a battery of established behavioral tests to assess depression and anxiety in spinally contused rats. The behavioral tests, and subsequent statistical analyses, revealed three cohorts of subjects that displayed behavioral characteristics of (1) depression, (2) depression and anxiety, or (3) no signs of decreased psychological well-being. Subsequent molecular analyses demonstrated that the psychological cohorts differed not only in behavioral symptoms, but also in peripheral (serum) and central (hippocampi and spinal cord) levels of pro-inflammatory cytokines. Subjects exhibiting a purely depression-like profile showed higher levels of pro-inflammatory cytokines peripherally, whereas subjects exhibiting a depression- and anxiety-like profile showed higher levels of pro-inflammatory cytokines centrally (hippocampi and spinal cord). These changes in inflammation were not associated with injury severity; suggesting that the association between inflammation and the expression of behaviors characteristic of decreased psychological well-being was not confounded by differential impairments in motor ability. These data support the hypothesis that inflammatory changes are associated with decreased psychological well-being following SCI.

Systemic TAK-242 prevents intrathecal LPS evoked hyperalgesia in male, but not female mice and prevents delayed allodynia following intraplantar formalin in both male and female mice: The role of TLR4 in the evolution of a persistent pain state

OBJECTIVEnPain resulting from local tissue injury or inflammation typically resolves with time. Frequently, however, this pain may unexpectedly persist, becoming a pathological chronic state. Increasingly, the innate and adaptive immune systems are being implicated in the initiation and maintenance of these persistent conditions. In particular, Toll-like receptor 4 (TLR4) signaling has been shown to mediate the transition to a persistent pain state in a sex-dependent manner. In the present work, we explored this contribution using the TLR4 antagonist, TAK-242.nnnMETHODSnMale and female C57Bl/6 mice were given intravenous (IV), intrathecal (IT), or intraperitoneal (IP) TAK-242 prior to IT delivery of lipopolysaccharide (LPS), and tactile reactivity was assessed at regular intervals over 72-h. Additional groups of mice were treated with IP TAK-242 prior to intraplantar formalin, and flinching was monitored for 1-h. Tactile reactivity was assessed at 7-days after formalin delivery.nnnRESULTSnLPS evoked TNF release from male and female macrophages and RAW267.4 cells, which was blocked in a concentration dependent fashion by TAK-242. In vivo, IT LPS evoked tactile allodynia to a greater degree in male than female mice. TAK-242, given by all routes, prevented development of IT LPS-induced tactile allodynia in male animals, but did not reverse their established allodynia. TLR4 deficiency and TAK-242 treatment attenuated IT LPS-induced allodynia in male, but not female mice. In the formalin model, pre-treatment with TAK-242 did not affect Phase 1 or Phase 2 flinching, but prevented the delayed tactile allodynia in both male and unexpectedly in female mice (Phase 3).nnnCONCLUSIONSnTogether, these results suggest that TAK-242 is a TLR4 antagonist that has efficacy after systemic and intrathecal delivery and confirms the role of endogenous TLR4 signaling in triggering the development of a delayed allodynia in both male and female mice.

Differences in cisplatin-induced mechanical allodynia in male and female mice.

Chemotherapeutic agents, such as cisplatin, are known to induce a persistent polyneuropathy. The mechanisms underlying the development of this pain are complex, and have only been investigated rodent models using male animals, despite an equivalent presentation of neuropathy between the sexes, clinically.

The effect of gabapentin and ketorolac on allodynia and conditioned place preference in antibody-induced inflammation.

Glucose‐6‐phosphate isomerase and collagen type II antibody‐induced arthritis models (K/BxN and CAIA, respectively) have an inflammatory and a post‐inflammatory phase. Both phases display robust tactile allodynia. In previous work, inflammatory phase allodynia was reversed by gabapentin and ketorolac, whereas in late phase only gabapentin was effective. Here, we sought to determine if the effects of these two drugs during the early and late phases of the two arthritis models were observed in the conditioned place preference (CPP) paradigm, indicating a differential drug effect on the aversive state.

Origins of antidromic activity in sensory afferent fibers and neurogenic inflammation

Neurogenic inflammation results from the release of biologically active agents from the peripheral primary afferent terminal. This release reflects the presence of releasable pools of active product and depolarization-exocytotic coupling mechanisms in the distal afferent terminal and serves to alter the physiologic function of innervated organ systems ranging from the skin and meninges to muscle, bone, and viscera. Aside from direct stimulation, this biologically important release from the peripheral afferent terminal can be initiated by antidromic activity arising from five anatomically distinct points of origin: (i) afferent collaterals at the peripheral-target organ level, (ii) afferent collaterals arising proximal to the target organ, (iii) from mid-axon where afferents lacking myelin sheaths (C fibers and others following demyelinating injuries) may display crosstalk and respond to local irritation, (iv) the dorsal root ganglion itself, and (v) the central terminals of the afferent in the dorsal horn where local circuits and bulbospinal projections can initiate the so-called dorsal root reflexes, i.e., antidromic traffic in the sensory afferent.

Inhibition of Neuroinflammation by AIBP: Spinal Effects upon Facilitated Pain States

Apolipoprotein A-I binding protein (AIBP) reduces lipid raft abundance by augmenting the removal of excess cholesterol from the plasma membrane. Here, we report that AIBP prevents and reverses processes associated with neuroinflammatory-mediated spinal nociceptive processing. The mechanism involves AIBP binding to Toll-like receptor-4 (TLR4) and increased binding of AIBP to activated microglia, which mediates selective regulation of lipid rafts in inflammatory cells. AIBP-mediated lipid raft reductions downregulate LPS-induced TLR4 dimerization, inflammatory signaling, and expression of cytokines in microglia. In mice, intrathecal injections of AIBP reduce spinal myeloid cell lipid rafts, TLR4 dimerization, neuroinflammation, and glial activation. Intrathecal AIBP reverses established allodynia in mice in which pain states were induced by the chemotherapeutic cisplatin, intraplantar formalin, or intrathecal LPS, all of which are pro-nociceptive interventions known to be regulated by TLR4 signaling. These findings demonstrate a mechanism by which AIBP regulates neuroinflammation and suggest the therapeutic potential of AIBP in treating preexisting pain states.

118. The interplay between psychological well-being, pain and inflammation in a rat model of spinal cord injury

We hypothesize that inflammation, inherent to spinal cord injury (SCI), contributes to decreased psychological well-being. To test this, male Sprague–Dawley rats (Nxa0=xa047) received a mild, moderate, or severe contusion or were intact controls. Behavioral measures of anxiety, depression and pain were collected prior to injury, and on days 10 and 21 post injury. Behavioral tests assessed anhedonia (sucrose preference), anxiety (center activity and shock probe burying), psychomotor activity (open field activity) and social interest (social exploration). Pain tests assessed mechanical allodynia (tactile reactivity) and thermal hyperalgesia (tail-flick). Serum alpha-2 macroglobulin levels were also measured on Days 1, 10 and 21, and thymus weight was recorded on Day 25. First, subjects’ average scores (Days 10 and 21) on behavioral measures of depression and anxiety were subjected to principal components and hierarchical cluster analyses. The analyses derived three subject clusters, identified via ANOVAs as: “Depression/Anxiety” (nxa0=xa015), “Depression” (nxa0=xa012) and “Healthy” (nxa0=xa020). The “Depression/Anxiety” cluster showed increased mechanical allodynia, anxiety, and peripheral inflammation relative to the “Healthy” cluster. The “Depression” cluster also showed mechanical allodynia, increased alpha-2 macroglobulin levels and decreased thymus weight relative to the ”Healthy” cluster. These data provide preliminary support for a role of inflammation in decreasing psychological well-being after SCI. Overall, we demonstrate, post SCI, an interplay between decreased psychological well-being, increased peripheral inflammation and increased pain reactivity.