Priming enhances endotoxin-induced thermal hyperalgesia and mechanical allodynia in rats

Abstract

Central inflammation is an integral component and contributor of the pathology of many debilitating diseases and has been shown to produce spontaneous pain and hyperalgesia. Recently, administration of lipopolysaccharide (LPS) into the lateral ventricle of rats was shown to elicit both thermal hyperalgesia and tactile allodynia [K. Walker, A. Dray, M. Perkins, Hyperalgesia in rats following intracerebroventricular administration of endotoxin: effect of bradykinin B₁ and B₂ receptor antagonist treatment, Pain 65 (1996) 211–219]. In this study, we have replicated the LPS model with some adaptations and correlated the nociceptive behaviors with an increased expression of activated macrophages in the central nervous system. We also examined the effects of priming on LPS-induced decreases in thermal nociceptive thresholds and mechanical response thresholds following either central or peripheral administration. Intracerebroventricular (i.c.v.) administration of LPS (0.2 μg/rat) did not alter either thermal (hot plate) or mechanical (von Frey filaments) thresholds compared to baseline values in the first few hours after injection. However, priming rats by pretreating with i.c.v. LPS (0.2 μg) 24 h prior to testing with i.c.v. LPS (0.2 μg) produced significant mechanical allodynia and thermal hyperalgesia. The mechanical allodynia had an onset of 80 min after injection and a duration of 5 h. A similar time course was observed for thermal hyperalgesia, although its expression was less pronounced. Immunohistochemical studies indicated an increased expression of activated macrophages in the brain parenchyma of primed rats but not in unprimed rats. Intraperitoneal (i.p., 2 mg/kg) administration of LPS had no significant effect on either thermal or mechanical thresholds in the first few hours after injection; however, priming rats via i.p. (0.2 mg/kg) or i.c.v. (0.2 μg) LPS produced a reduction in both thermal nociceptive thresholds and mechanical response thresholds in rats given a subsequent i.p. injection of LPS. This study demonstrates that priming is an effective protocol for the induction of central inflammation and increases the duration of these behaviors after i.c.v. administration.

Introduction

The mechanisms involved in nociception following peripheral inflammation have been extensively studied in animal models. While these studies have focused on the pathophysiology and identification of endogenous chemicals involved in nociceptive behaviors resulting from peripheral inflammation 12, 16, our knowledge of the chemical mediators and pathways involved in nociceptive transmission generated by central inflammation has yet to be elucidated. Inflammation of the central nervous system (CNS) remains an important component of the pathological states associated with many debilitating conditions including cerebrovascular and traumatic lesions, cerebral ischemia, multiple sclerosis, acquired immunodeficiency syndrome and cerebral tumors. Furthermore, inflammatory processes resulting from lesions of the brain and spinal cord can produce central pain syndromes 4, 6, 30. One obstacle in assessing pain related to central inflammation has been the absence of animal models which exhibit physiological and behavioral signs. Recently, Walker et al. [33]described an animal model of central inflammation which was characterized by hyperalgesia and allodynia in the rat hind paws following intracerebroventricular (i.c.v.) administration of bacterial lipopolysaccharide (LPS).

LPS has been utilized as an illness-inducing agent and as a means of initiating an acute phase (inflammatory) response. Consequently, LPS has been used to investigate the mediators involved in host defense mechanisms and has more recently been utilized to induce hyperalgesia associated with inflammation 19, 35, 36. Nociceptive responses in rodents has been reported following various routes of administration. Intraperitoneal (i.p.) administration of LPS produced hyperalgesia in the rat's hind paws as measured by thermal threshold and formalin nociceptive tests [39], and while the effect was produced by systemic administration, central pathways have been implicated in the behavioral hyperalgesic response 28, 36, 38. Central administration of LPS has also been shown to induce hyperalgesia. Intrathecal administration of LPS has been found to produce thermal hyperalgesia as measured by the plantar test [20], whereas both thermal hyperalgesia and tactile allodynia were observed in the rat hind paw following i.c.v. administration [33]. In these latter studies, the onset and duration of hyperalgesia were typically 4–8 h and 6–24 h, respectively.

In addition to initiating an inflammatory response, LPS has been found to act as a priming agent and thereby greatly enhance the inflammatory response to subsequent exposure of endotoxins, compared to the effects produced by a single exposure of the endotoxin. Priming has been shown to augment LPS-induced cytokine production, mast cell degranulation and arachidonic acid metabolite production, all of which are generated during inflammation and can produce pain-like behaviours 1, 14. Thus, priming may enhance hyperalgesia and allodynia induced by LPS.

Microglia, the resident macrophage within the neuroaxis, are immunocompetent cells that play a pivotal role in neuroinflammation [42]. While it is known that endotoxins, including LPS, are known to cause the activation of microglia 2, 21, 23, 29and produce behavioral hyperalgesia, there have been no experiments that have attempted to link LPS-induced activation of microglia with changes in nociceptive response thresholds. It has been postulated that cytokines are released from microglia during central inflammation [7], and thus may be an important cellular source of mediators involved in triggering nociceptive responses.

In the present study, we evaluated mechanical allodynia and thermal hyperalgesia following central administration of LPS in primed rats. Since priming is a phenomenon that has been observed both in vitro and in vivo, it is of interest to determine whether LPS priming of rats would shorten the onset of hyperalgesia and enhance the reduction in response thresholds. Accordingly, thermal hyperalgesia and mechanical allodynia following either peripheral or central administration of LPS were evaluated in primed and unprimed rats. The onset and time course of the resulting nociception were determined for both of these groups to evaluate the predictability and reproducibility of the model, and furthermore, the hyperalgesia and allodynia induced by LPS were correlated with the expression of activated macrophages in the central nervous system as a marker of central inflammation.