Online Issues
<< All Back-issues
<< This Issue's Table of Contents
ILAR Journal V40(3) 1999
Animal Models of Pain
Ke Ren and Ronald Dubner
| Ke Ren, M.D., Ph.D., is Assistant Professor, and Ronald Dubner, D.D.S., Ph.D., is Professor and Chairman in the Department of Oral and Craniofacial Biological Sciences, University of Maryland Dental School, Baltimore, Maryland. |
Introduction
During the 1990s, there has been a proliferation of animal models to study the effects of tissue injury on the development of persistent or chronic pain. In most of these studies, the animals are awake and exposed to pain. These models attempt to mimic human clinical conditions. A major purpose of such studies is to further knowledge that can ultimately be applied to the management of acute and chronic pain in humans and animals. Although scientists engaged in these studies feel morally justified in conducting such experiments, there is a need to demonstrate a continuing responsibility in the proper treatment of the animals that participate in these experiments. The animals should be exposed to the minimal pain necessary to carry out the experiment (Dubner 1983). Thus, both scientific and ethical considerations require that methods of assessing pain in animals be developed.
Assessing Pain in Inflammatory Models
To study the mechanisms of persistent pain, animal models of inflammatory hyperalgesia that mimic human clinical pain conditions have been developed by the injection of inflammatory agents into the rat or mouse hind paw (Hargreaves and others 1988; Iadarola and others 1988a; Larson and others 1986; Millan and others 1988; Qiu and others 1998). A paw withdrawal latency measure and the withdrawal duration (how long the limb remains off the glass plate) can be used to infer pain and hyperalgesia in inflamed animals. The animals withdraw their limb reflexively to noxious stimuli but also exhibit more complex organized behaviors such as paw licking and limb guarding (Hargreaves and others 1988). The effect of the radiant heat stimulus used in this method on the cutaneous temperature of inflamed carrageenan-treated and saline-treated rat paws is shown in Figure 1. The inflamed paws have greater initial resting paw temperatures than the paws receiving injections of saline. In addition, stimulation of the inflamed paws results in shorter paw withdrawal latencies than stimulation of saline-treated paws, and this shorter latency corresponds to a lower threshold temperature. Withdrawal latencies using this model parallel more complex organized behaviors such as withdrawal duration and paw licking. Shown in Figure 2 are the magnitude and time course of these changes in paw withdrawal latency after injection of carrageenan or complete Freund's adjuvant (CFA1) to the hind paw of the rat. There is a reduction of withdrawal latency from approximately 10 to 4 sec limited to the carrageenan-injected hind paw (Figure 2A). The paw withdrawal latency of the contralateral untreated hind paw is unchanged, suggesting that the noninjected side may be used as a control. The lack of any change on the contralateral side also suggests that the withdrawal is not an avoidance response. In Figure 2B, the time course of the change reveals that there is a rapid reduction in paw withdrawal latency that peaks in 2 to 6 hr depending on the agent used, and it persists 1 to 2 wk in CFA-treated rats (Iadarola and others 1988a).
Paw withdrawal responses can also be used to quantify mechanical sensitivity after the induction of inflammation (Ren and Dubner 1993; also see Chaplan and others 1994). Rats are placed on a meshed metal surface and a series of von Frey filaments are applied to the ventral or dorsal surface of the hind paw. Response threshold is defined as the lowest force of two or more consecutive von Frey filaments that produces a response. Figure 3A reveals that before inflammation, the median von Frey thresholds were 8.5 g for both sides. However, with this response measure, it is not clear whether the rat is withdrawing from a noxious or an innocuous stimulus. The response duration, a measure of the total time the rats withdraw and hold their hind paw away from the test surface, can be quantified and used as a measure of nocifensive behavior. The response duration is defined as the time from the start of a response to the return of the paw to the original position. A 0.2-sec duration is considered a quick withdrawal, characteristic of the response to an innocuous stimulus. As shown in Figure 3B, 8.5-g forces produced average response durations that were greater than 0.2 sec and likely represent a nocifensive behavior. The response frequency to a fixed intensity of von Frey filament stimulation is also often used as a measure of mechanical responsiveness (Kim and Chung 1992).
The method of measuring nocifensive behavior has been applied to the orofacial region recently (Imamura and others. 1997). Rats are habituated in a specially designed box with an opening at one end, thus allowing a portion of the orofacial area to be exposed. The radiant heat source is directed to the targeted facial spot, and the head withdrawal latency is determined automatically. Using comparable intensities of heat stimuli, the head withdrawal latency to a noxious heat stimulus is within the range of the paw withdrawal latency. Compared with hind paw testing, this method requires moderate training of the animals so that they become acclimated to the testing environment. The training process typically involves a few daily sessions of handling and exposure to the box followed by a few sessions of heat stimulation. A low dose (20 mg/kg) of pentobarbital sodium may be introduced to help calm the animals (Ren and Dubner 1996). Using this system, the mechanical sensitivity of the orofacial region can also be tested with von Frey filaments (Ren and Dubner 1996). In general, the mechanical threshold for the perioral skin was significantly lower than that of the hind paw skin. The other method of assessing the mechanical sensitivity of the orofacial region is to deliver mechanical stimulation to freely moving rats that are placed in a small plastic cage (Vos and others. 1994).
All of the behaviors described above provide the animal with control of the intensity or duration of the stimulus because the behavior results in escape from the aversive stimulus. In contrast, there are assessment methods in which the animal does not have control of stimulus intensity or duration. For example, the writhing response, which is produced in rodents by injecting various chemicals, is considered a model of visceral pain (Ness 1999; Vyklicky 1979). In addition to the lack of stimulus control offered the animal with this method, the experimenter cannot control the duration of the stimulus (Ness 1999). Vocalization is another commonly used unlearned reaction to painful stimuli (Kayser and Guilbaud 1987). The stimulus intensity necessary to elicit a vocal response from the animal is determined. The stimulus can be applied to any part of the body, and the animal cannot control its intensity or duration.
Inflammatory Models of Persistent Pain
Animal models of tissue injury and inflammation can be subdivided into those that produce inflammation of cutaneous and subcutaneous tissues, joint inflammation, inflammation of muscle, and others. Each type of model is discussed below.
Cutaneous and Subcutaneous Inflammation
In one model, formalin is given by injection beneath the footpad of a rat or cat (Dubuisson and Dennis 1977; Kaneko and Hammond 1997; see Abbott and others 1995 for review). Formalin produces complex response patterns that last for approximately 1 hr. Initially, the animals elevate the injected limb and do not place it on the cage floor; but within 15 to 30 min, they begin to use it as a weight-bearing limb. Two phases of nocifensive behavior are typically described: The first or acute phase lasts for about 5 min and is followed by a longer-lasting, more persistent phase (about 40 min) that is characterized by shaking or licking of the paw. It is generally agreed that the first phase results at least in part from direct activation of primary afferent fibers, both low-threshold mechanoreceptive and nociceptive types (Puig and Sorkin 1996). There has been disagreement about the underlying mechanisms of the second phase. Early studies suggested that the second phase resulted from an increase in the excitability of dorsal horn neurons, whereas more recently it has been demonstrated that ongoing activity of primary afferent fibers is necessary for the development of the second phase (Abbadie and others 1997; Taylor and others 1995). Many different response measures are used for assessing pain after formalin. These measures include single parameters, such as flinching, shaking, or jerking of the injected foot, and complex scores that are derived from several nocifensive behaviors, such as licking and guarding (Clavelou and others 1995).
The injection of carrageenan, zymosan, or CFA into the footpad produces more persistent pain and hyperalgesia (Hargreaves and others 1988; Iadarola and others 1988a; Meller and Gebhart 1997). These models mimic more closely the time course of postoperative pain or other types of persistent injury. After injection of CFA into the footpad, the cutaneous inflammation appears within 2 hr and peaks within 6 to 8 hr (Figure 2B). Hyperalgesia and edema are present for approximately 1 to 2 wk. Although carrageenan produces a hyperalgesia of similar magnitude, its duration is usually less than 1 wk unless repeated injections are used. The physiological and biochemical effects of these inflammatory agents are limited to the affected limb (Iadarola and others 1988b), and there are no signs of an immune response or systemic disease. The animals cannot control the pain associated with these inflammation models, therefore, it is important to determine that the levels of pain are below the tolerance level of the animals. It has been shown that rats with CFA- or carrageenan-induced inflammation exhibit minimal reductions in weight and show normal grooming behavior (Iadarola and others 1988a). Exploratory motor behavior is normal, and no significant alterations occur in an open field locomotion test (Iadarola and others 1988a). The rats will use the affected limb for support, if necessary. Thus, the impact of the inflamed limb on the rat's behavior is minimal.
The presence of pain in the inflammation models is inferred by an increased response to a noxious stimulus (hyperalgesia) or a nocifensive behavior in response to an innocuous stimulus normally not perceived as painful (allodynia). The literature on these animal models often produces the same confused use of these terms, hyperalgesia and allodynia, as in the clinical literature. The terms are defined based only on behavior (Mersky 1979), although many authors attribute mechanisms to them. For example, allodynia is thought to result from sensitization of peripheral nociceptive afferent fibers, or to be mediated by low-threshold mechanoreceptive afferent fibers after increases in central nervous system (CNS1) excitability (central sensitization), or both. Hyperalgesia is thought to be due to central sensitization, to sensitization of nociceptive afferent fibers, or to both. Adding to the confusion, there are thermal or mechanical forms of allodynia and thermal or mechanical forms of hyperalgesia. Often the term hyperalgesia is used for both hyperalgesia and allodynia. For example, the paw withdrawal to a thermal stimulus after inflammation (see Figure 1) is thermal allodynia because withdrawal occurs at a normally innocuous temperature (38.5°C), yet it is typically described as thermal hyperalgesia (Hargreaves and others 1988). Nevertheless, a distinction can be made between these two types of behaviors in animals and the underlying mechanisms investigated.
As shown in Figure 3A, the injection of CFA into the rat's hind paw reduced the yon Frey threshold from 8.5 to 1.2 g. This result suggests that innocuous mechanical stimuli that are ordinarily barely perceptible now produce a paw withdrawal response. However, is this hyperalgesia or allodynia? The use of response duration as a measure (Figure 3B) suggests that this is mechanical allodynia. After CFA, rather than rapidly returning the stimulated paws to the test surface, the rats hold it off the floor for longer durations, sometimes shake it, and sometimes lick it. More intense mechanical stimuli that normally result in an increase in response duration result in additional increases, suggesting the presence of mechanical hyperalgesia (Figure 3B).
Intradermal capsaicin produces a model of neurogenic inflammation and hyperalgesia (LaMotte and others 1991) similar to one utilized in human subjects. The intradermal injection of capsaicin results in primary hyperalgesia at the site of injection and a surrounding area of secondary hyperalgesia to light touch. A flare reaction extends into the zone of secondary hyperalgesia. This neurogenic inflammation model has been used in monkeys to study changes in nociceptor activity and changes in the responses of spinal dorsal horn neurons (LaMotte and others 1991; Simone and others 1991). The model has recently been adapted to behavioral studies in the rat (Gilchrist and others 1996). Withdrawal responses to heat and mechanical stimuli were assessed using the paw withdrawal latency method described above (Hargreaves and others 1988; Ren and Dubner 1993). Intraplantar injection of capsaicin evokes nocifensive behavior characterized by lifting and guarding of the injected paw that lasts about 3 min. Capsaicin produces changes in paw withdrawal latencies and their duration to heat and mechanical stimuli in a dose-dependent manner. Reduced withdrawal latencies to heat last up to 45 min, whereas the effects of mechanical stimuli persist up to 4 h.
Other inflammatory agents such as mustard oil, a small fiber irritant, and zymosan have been used to produce behavioral and physiological changes. The effects of mustard oil are relatively short (a few minutes) when applied topically to cutaneous tissues (Ma and Woolf 1996a; Neumann and others 1996). Mustard oil can also be given by injecting subcutaneously or into muscle, in which case the changes last up to 20 min (see below) (Yu and others 1994). The intraplantar injection of zymosan produces a persistent dose- and time-dependent thermal and mechanical hyperalgesia associated with an intense inflammation (Meller and Gebhart 1997). A comparison of the onset and duration of inflammatory hyperalgesia produced by these inflammatory agents appears in Table 1.
Orofacial Inflammation. The formalin test is also applicable to the study of orofacial pain mechanisms. Clavelou and others (1989) gave injections of formalin into the upper lip of the rat. They observed that the rat's initial reaction to the formalin injection involved immediate head movement often accompanied by vocalization. After 15 to 30 sec, rats start to rub vigorously the injection site with forelimbs or hindlimbs; the duration of rubbing appears to correlate well with the pain intensity. The first phase of the perioral formalin test lasts only about 1 min, which is shorter than that seen in the paw formalin test. The timing of the second phase of the formalin test is similar after perioral or paw injections. This method has been shown to be a reliable method for assessing pain in the trigeminal region.
Mustard oil is also used to produce acute orofacial inflammation. After injection of mustard oil into the temporomandibular joint of the rat, inflammation develops within 30 min and reaches maximum in 2 hr (Haas and others 1992). The electromyographic (EMG1) activity is significantly increased in digastric and masseter muscles after mustard oil injection into the temporomandibular region (Hu and others 1994). The increase in EMG activity lasts for several minutes, which suggests the involvement of a central sensitization process. Because the effects of mustard oil last a relatively short time, this model has not been used to assess behavioral hyperalgesia and allodynia in the orofacial region.
To produce more persistent orofacial inflammation, CFA is given by injection into the orofacial region (Ren and Dubner 1996). The orofacial inflammation produced by CFA lasts more than 1 wk as indicated by plasma extravasation in the injected site (Zhou and others 1997). Interestingly, the same amount of the inflammatory agent produces significantly more intense inflammation after injection into the temporomandibular joint versus perioral skin. The behavioral hyperalgesia and allodynia have been evaluated after CFA-induced orofacial inflammation. By employing the method described above, it was found that thermal hyperalgesia developed at 5 hr, peaked at 24 hr, and lasted at least 1 wk after CFA injection into the temporomandibular joint or perioral skin (Ren and Dubner 1996). This time course of hyperalgesia is very comparable with that seen in the hind paw inflammation models (Hargreaves and others 1988; Hylden and others 1989). The mechanical sensitivity of the facial skin at the perioral injection site also was significantly increased, suggesting the development of mechanical allodynia.
Hyperalgesia and Allodynia. Behavioral models of hyperalgesia and allodynia have been useful in the study of peripheral and central mechanisms of hyperalgesia and allodynia. They have been correlated with neural events in primary afferent neurons and CNS neurons, particularly at the level of the medullary and spinal dorsal horns. Although correlative electrophysiological and behavioral studies have been informative (LaMotte and others 1991; Ren and Dubner 1993; Ren and others 1992; Simone and others 1991; Woolf and Thompson 1991 ;Yu and others 1993), only a few neurons can be studied. Recently, Fos, the protein product of the c-fos immediate early gene, has been used as a measure of neuronal activity (Bullitt 1990; Hunt and others 1987). Fos protein is induced by neuronal activity and appears to play a role in long-term changes in the CNS after neural activity (Goelet and others 1986). Fos expression increases in many nociceptive neurons in the dorsal horn after inflammation, and these changes can be localized to specific populations of neurons using immunocytochemical methods. The findings can be correlated with behavioral hyperalgesia and allodynia after injection of inflammatory agents such as CFA, carrageenan, mustard oil, or formalin (Draisci and Iadarola 1989; Gogas and others 1991; Ma and Woolf 1996b; Menétrey and others 1989; Presley and others 1990; Ren and Ruda 1996; Wei and others 1998, 1999).
Joint Inflammation
Acute arthritis can also be induced by the injection of carrageenan and kaolin into the cat or monkey knee joint just below the patella (Dougherty and others 1992; Schaible and others 1987). Changes in joint receptor and spinal dorsal horn neuronal activity begin as soon as 1 to 2 hr after injection and build up for several hours. After injection of kaolin and carrageenan into the knee joint of the rat, the thermal hyperalgesia, as indicated by a reduction in paw withdrawal latency, starts to develop at 4 hr and is maintained for about 24 hr (Sluka and Westlund 1993). The hyperalgesia in this model resolves completely within 2 to 3 days, which is of shorter duration than that in the CFA-induced hind paw inflammation model. The magnitude of hyperalgesia is also relatively low compared with the hind paw inflammation model. It should be kept in mind that the test site (paw) in the joint inflammation model is remote from the injury site. What is measured by paw withdrawal latency in the model of knee joint inflammation is likely only secondary hyperalgesia.
Other methods that have been developed in an attempt to mimic human conditions of persistent or chronic pain include models of polyarthritis in which CFA is injected into the rat's tail (De Castro Costa and others 1981). The CFA results in a delayed hypersensitivity reaction with inflammation and hyperalgesia of multiple joints occurring after 10 days to 3 wk. Pain is inferred from scratching behaviors, reduced motor activity of the animal, weight loss, vocalization when the affected limbs are pinched, and a reduction in these behaviors after the administration of opioids. It should be noted that this is a systemic disease that includes skin lesions, destruction of bone and cartilage, impairment of liver function, and lymphadenopathy (Coderre and Wall 1987). These systemic lesions make it more difficult to associate the animal's behavior with pain as opposed to generalized malaise and debilitation. The likely presence of CNS changes associated with the alterations in immune function also bring into question the use of this model to correlate neural activity and neurochemical alterations with behavior presumably related to pain. Other models of arthritis have been developed in which sodium urate crystals are injected into the ankle joint of rat or cat (Coderre and Wall 1987; Okuda and others 1984). The arthritis is fully developed within 24 hr. These animals reduce the weight placed on the treated hind limb and exhibit guarded movement of the limb. In the rat, touch, pressure, and thermal stimuli applied to the affected paw result in decreased responsiveness, presumably due to the pain associated with the movement. No signs of systemic disease have been observed in the urate arthritis model other than joint pathology secondary to tissue edema and the infiltration of polymorphonuclear leucocytes (Coderre and Wall 1987).
Muscle Inflammation
There have been relatively fewer studies devoted to the pain associated with muscle inflammation compared with other inflammation models. Some algesic compounds such as bradykinin and prostaglandin E2 have been given by injection to excite nociceptors in muscle and to elicit muscle pain (reviewed in Mense 1991). An experimental myositis is induced by injection of carrageenan into the gastrocnemius-soleus muscle (Hoheisel and others. 1994, 1997). Mustard oil has been given by injection into the deep masseter or tongue muscle of the rat to produce facilitation of trigeminal brain-stem nociceptive neurons and EMG (Hu and others 1992; Yu and others 1993). The inflammatory agent, CFA, is also injected into the masseter to produce persistent inflammation and central Fos protein expression (Imbe and others 1998). Although these studies have revealed neurochemical and physiological changes in the CNS, behavioral studies related to these models have not been performed. It is worth mentioning that the intramuscular injection of hyper-tonic saline has been used to produce jaw muscle pain in humans (Stohler and others 1991). The resulting pain is described as "aching," "throbbing," "cramping," "hot-burning," and "stabbing." This model has proven to be reliable in inducing jaw muscle pain and has recently been utilized in studies of awake monkeys trained in a jaw position task (Toms and others 1996). In rats, the continuous intraarterial infusion of one hind limb of the rat with a free-radical donor produces an inflammatory response in the per-fused hind limb, including the gastrocnemius muscle, and behavioral signs of hyperalgesia (van der Laan and others 1997).
Other Models
The success of many of the models described above has resulted in a plethora of new inflammation models in recent years. Some examples follow.
Bone lesions in rats produced by drilling a hole through the tibia result in secondary mechanical hyperalgesia and allodynia as well as cold allodynia (Houghton and others 1997). Nocifensive behavior is characterized by a lifting and guarding of the damaged limb. The model was developed to study mechanisms underlying bone pain.
An animal model of pain has been produced by systemic administration of an immunotherapeutic antiganglioside antibody (Slart and others 1997). The antibody in rat produces mechanical allodynia and provides a model for studying the pharmacology of this allodynia, which occurs in children exposed to the antibody in the treatment of neuroblastomas.
All of the models described above, which attempt to mimic human conditions of chronic or persistent pain, produce pain that the animal cannot control. Therefore, it is important that investigators assess the level of pain in these animals and provide analgesic agents when it does not interfere with the purpose of the experiment. Pain in these studies can be inferred from ongoing behavioral variables such as feeding and drinking, sleep-waking cycle, grooming, and social behavior (Sternbach 1976). Significant deviation from normal behavior suggests that the animal is in pain and possibly distressed.
1Abbreviations used in this article: CFA, complete Freund's adjuvant; CNS, central nervous system; EMG, electromyographic.
References
Abbadie C, Taylor KT, Peterson MA, Basbaum AI. 1997. Differential contribution of the two phases of the formalin test to the pattern of c-fos expression in the rat spinal cord: Studies with remifentanil and lidocaine. Pain 69:101-110.
Abbott FV, Franklin KB J, Westbrook RF. 1995. The formalin test: Scoring properties of the first and second phases of the pain response in rats. Pain 60:91-102.
Bullitt E. 1990. Expression of c-fos-like protein as a marker for neuronal activity following noxious stimulation in the rat. J Comp Neurol 296:517-530.
Chaplan SR, Bach FW, Pogrel JW, Chung JM, Yaksh TL. 1994. Quantitative assessment of tactile allodynia in the rat paw. J Neurosci Methods 53:55-63.
Clavelou P, Dallel R, Orliaguet T, Woda A, Raboisson P. 1995. The orofacial formalin test in rats: Effects of different formalin concentrations. Pain 62:295-301.
Clavelou P, Pajot J, Daliel R, Raboisson P. 1989. Application of the formalin test to the study of orofacial pain in the rat. Neurosci Lett 103:349-353.
Coderre TJ, Wall PD. 1987. Ankle joint urate arthritis (AJUA) in rats: An alternative animal model of arthritis to that produced by Freund's adjuvant. Pain 28:379-393.
De Castro Costa M, De Sutter P, Gybels J, Van Hees J. 1981. Adjuvant-induced arthritis in rats: A possible animal model of chronic pain. Pain 10:173-186.
Doughetty PM, Sluka KA, Sorkin LS, Westlund KN, Willis WD. 1992. Neural changes in acute arthritis in monkeys. 1. Parallel enhancement of responses of spinothalamic tract neurons to mechanical stimulation and excitatory amino acids. Brain Res Rev 17:1-13.
Draisci G, Iadarola MJ. 1989. Temporal analysis of increases in c-fos, preprodynorphin and preproenkephalin mRNAs in rat spinal cord. Mol Brain Res 6:31-37.
Dubner R. 1983. Pain research in animals. Ann NY Acad Sci 406:128-132. Dubuisson D, Dennis SG. 1977. The formalin test: A quantitative study of the analgesic effects of morphine, meperidine, and brain-stem stimulation in rats and cats. Pain 4:161174.
Gilchrist HD, Allard BL, Simone DA. 1996. Enhanced withdrawal responses to heat and mechanical stimuli following intraplantar injection of capsaicin in rats. Pain 67:179-188.
Goelet P, Castellucci VF, Schacher S, Kandel ER. 1986. The long and short of long-term memory--A molecular framework. Nature 322:419-422.
Gogas KR, Presley RW, Levine JD, Basbaum AI. 199 I. The antinociceptive action of supraspinal opioids results from an increase in descending inhibitory control: Correlation of nociceptive behavior and c-fos expression. Neuroscience 42:617-628.
Haas DA, Nakanishi O, MacMillan RE, Jordan RC, Hu JW. 1992. Development of an orofacial model of acute inflammation in the rat. Arch Oral Biol 37:417-422.
Hargreaves K, Dubner R, Brown F, Flores C, Joris J. 1988. A new and sensitive method for measuring thermal nociception in cutaneous hyperalgesia. Pain 32:77-88.
Hoheisel U, Koch K, Mense S. 1994. Functional reorganization in the rat dorsal horn during an experimental myositis. Pain 59:111-118.
Hoheisel U, Sander B, Mense S. 1997. Myositis-induced functional reorganisation of the rat dorsal horn: Effects of spinal superfusion with antagonists to neurokinin and glutamate receptors. Pain 69:219-230.
Houghton AK, Hewitt E, Westlund KN. 1997. Enhanced withdrawal responses to mechanical and thermal stimuli after bone injury. Pain 73:325-337.
Hu JW, Sessle BJ, Raboisson P, Daliel R, Woda A. 1992. Stimulation of craniofacial muscle afferents induces prolonged facilitatory effects in tfigeminal nociceptive brain-stem neurones. Pain 48:53-60.
Hu JW, Yu X-M, Sunakawa M, Chiang CY, Haas DA, Kwan CL, Tsai C-M, Vernon H, Sessle BJ. 1994. Electromyographic and trigeminal brainstem neuronal changes associated with inflammatory irritation of superficial and deep craniofacial tissues in rats. In: Gebhart GF, Hammond DL, Jensen TS, editors. Proc 7th World Congress on Pain~ Elsevier, Amsterdam, p 325-336.
Hunt SP, Pini A, Even G. 1987. Induction of c-fos-like protein in spinal cord neurons following sensory stimulation. Nature 328:632-634.
Hylden JLK, Nahin RL, Traub RJ, Dubner R. 1989. Expansion of receptive fields of spinal lamina I projection neurons in rats with unilateral adjuvant-induced inflammation: The contribution of dorsal horn mechanisms. Pain 37:229-243.
Iadarola MJ, Brady LS, Draisci G, Dubnet R. 1988a. Enhancement of dynorphin gene expression in spinal cord following experimental inflammation: Stimulus specificity, behavioral parameters and opioid receptor binding. Pain 45:313-326.
Iadarola M J, Douglass J, Civelli O, Naranjo JR. 1988b. Differential activation of spinal cord dynorphin and enkephalin neurons during hyperalgesia: Evidence using cDNA hybridization. Brain Res 455:202-212.
Imamura Y, Kawamoto H, Nakanishi O. 1997. Characterization of heat-hyperalgesia in an experimental trigeminal neuropathy in rats. Exp Brain Res 116:97-103.
Imbe H, Dubner R, Ren K. 1998. Masseter inflammation-induced Fos protein expression in brainstem neurons depends on somatosensory-vagal-adrenal integration. (Abstract). Soc Neurosci Abs 24:388.
Kaneko M, Hammond DL. 1997. Role of spinal gamma-aminobutyric acid A receptors in formalin-induced nociception in the rat. J Pharmacol Exp Ther 282:928-38.
Kayser V, Guilbaud G. 1987. Local and remote modifications of nociceptive sensitivity during carrageenin-induced inflammation in the rat. Pain 28:99-107.
Kim SH, Chung JM. 1992. An experimental model for peripheral neuropathy produced by segmental spinal nerve ligation in the rat. Pain 50:355-363.
LaMotte RH, Shain CN, Simone DA, Tsai E-FP. 199 I. Neurogenic hyperalgesia: Psychophysical studies of underlying mechanisms. J Neurophysiol 66:190-211.
Larson AA, Brown DR, E1-Atrash S, Walser MM. 1986. Pain threshold changes in adjuvant-induced inflammation: A possible model of chronic pain in the mouse. Pharmacol Biochem Behav 24:49-53.
Ma Q-P, Woolf CJ. 1996a. Progressive tactile hypersensitivity: An inflammation-induced incremental increase in the excitability of the spinal cord. Pain 67:97-106.
Ma Q-P, Woolf CJ. 1996b. Basal and touch-evoked fos-like immunoreactivity during experimental inflammation in the rat. Pain 67:307-316.
Meller ST, Gebhart GF. 1997. Intraplantar zymosan as a reliable, quantifiable model of thermal and mechanical hyperalgesia in the rat. Eur J Pain 1:43-52.
Menétrey D, Gannon A, Levine JD, Basbaum Al. 1989. Expression of c-fos protein in interneurons and projection neurons of the rat spinal cord in response to noxious somatic, articular, and visceral stimulation. J Comp Neurol 285:177-195.
Mense S. 1991. Considerations concerning the neurobiological basis of muscle pain. Can J Physiol Pharmacol 69:610-616.
Mersky H. 1979. Pain terms: A list with definitions and notes on usage. Pain 6:249-252.
Millan M J, Czlonkowski A, Morris B, Stein C, Arendt R, Huber A, H611t V, Herz A. 1988. Inflammation of the hind limb as a model of unilateral, localized pain: Influence on multiple opioid systems in the spinal cord of the rat. Pain 35:299-312.
Ness TJ. 1999. Models of visceral nociception. ILAR J 40:119-128.
Neumann S, Doubell TP , Leslie T, Woolf CJ. 1996. Inflammatory pain hypersensitivity mediated by phenotypic switch in myelinated primary sensory neurons. Nature 384:360-364.
Okuda K, Nakahama H, Miyakawa H, Shima K. 1984. Arthritis induced in cats by sodium urate: A possible animal model for chronic pain. Pain 18:287-297.
Presley RW, Menétrey D, Levine JD, Basbaum Al. 1990. Systemic morphine suppresses noxious stimulus-evoked Fos protein-like immunoreactivity in the rat spinal cord. J Neurosci 10:323-335.
Puig S, Sorkin LS. 1996. Formalin-evoked activity in identified primary afferent fibers: Systemic lidocaine suppresses phase-2 activity. Pain 64:345-355.
Qiu C, Sofa I, Ren K, Uhl G, Dubner R. 1998. Opioid receptor plasticity in mu opioid receptor knockout (MOR KO) mice following persistent inflammation. (Abstract). Soc Neurosci Abs 24:892.
Ren K, Dubner R. 1993. NMDA receptor antagonists attenuate mechanical hyperalgesia in rats with unilateral inflammation of the hindpaw. Neurosci Lett 163:22-26.
Ren K, Dubner R. 1996. An inflammation/hyperalgesia model for the study of orofacial pain. (Abstract). J Dent Res 75:217.
Ren K, Hylden JLK, Williams GM, Ruda MA, Dubner R.1992. The effects of an NMDA receptor antagonist, MK-801, on behavioral hyperalgesia and dorsal horn neuronal activity in rats with adjuvant-induced inflammation. Pain 50:331-344.
Ren K, Ruda MA. 1996. Descending modulation of Fos expression after persistent peripheral inflammation. Neuroreport 7:2186-2190.
Schaible H-G, Schmidt RF, Willis WD. 1987. Enhancement of the responses of ascending tract cells in the cat spinal cord by acute inflammation of the knee joint. Exp Brain Res 66:489-499.
Simone DA, Sorkin LS, Oh U, Chung JM, Owens C, LaMotte RH, Willis WD. 1991. Neurogenic hyperalgesia: Central neural correlates in responses of spinothalamic tract neurons. J Neurophysiol 66:228-246.
Slart R, Yu AL, Yaksh TL, Sorkin LS. 1997. An animal model of pain produced by systemic administration of an immunotherapeutic anti-gan-glioside antibody. Pain 69:119-125.
Sluka KA, Westlund KN. 1993. Behavioral and immunohistochemical changes in an experimental arthritis model in rats. Pain 55:367-377.
Steinbach RA. 1976. The need for an animal model of chronic pain. Pain 2:2-4.
Stohler CS, Zhang X, Ashton-Miller JA. 1991. An experimental model of jaw muscle pain in man. In: Davidovitch Z, editor. The Biological Mechanisms of Tooth Movement and Craniofacial Adaptation. Birmingham: EBSCO Media. p 261-267.
Taylor BK, Peterson MA, Basbaum AI. 1995. Persistent chemical nociception in the rat requires ongoing peripheral nerve input. J Neurosci 15:7575-7584.
Toms S, Benevento LA, Capra NF. 1996. Jaw position task performance during acute muscle pain in monkeys. (Abstract). J Dent Res 75:133.
van der Laan L, Kapitein PJ, Verhofstad AA, Hendriks T, Goris RJ. 1997. A novel animal model to evaluate oxygen derived free radical damage in soft tissue. Free Radic Res 26:363-372.
Vos BP, Strassman AM, Maciewicz RJ. 1994. Behavioral evidence of trigeminal neuropathic pain following chronic constriction injury to the rat's infraorbital nerve. J Neurosci 14:2708-2723.
Vyklicky L. 1979. Techniques for the study of pain in animals. In: Bonica JJ, Liebeskind JC, Albe-Fessard DG, editors. Advances in Pain Research and Therapy. Vol 3. New York NY: Raven Press. p 727-745.
Wei F, Dubnet R, Ren K. 1999. Nucleus reticularis gigantocellularis and nucleus raphe magnus in the brain stem exert opposite effects on behavioral hyperalgesia and spinal Fos protein expression after peripheral inflammation. Pain 80:127-141.
Wei F, Ren K, Dubner R. 1998. Inflammation-induced Fos protein expression in the rat spinal cord is enhanced following dorsolateral or ventrolateral funiculus lesions. Brain Res 782:136-141.
Woolf CJ, Thompson SW. 1991. The induction and maintenance of central sensitization is dependent on N-methyl-D-aspartic acid receptor activation: Implications for the treatment of post-injury pain hypersensitivity states. Pain 44:293-299.
Yu X-M, Sessle BJ, Hu JW. 1993. Differential effects of cutaneous and deep application of inflammatory irritant on mechanoreceptive field properties of trigeminal brain stem nociceptive neurons. J Neurophysiol 70:1704-1707.
Yu X-M, Sessle BJ, Vernon H, Hu JW. 1994. Administration of opiate antagonist naloxone induces recurrence of increased jaw muscle activities related to inflammatory irritant application to rat temporomandibular joint region. J Neurophysiol 72:1430-1433.
Zhou Q-Q, Imbe H, Dubner R, Ren K. 1997. Persistent Fos protein expression after orofacial deep or cutaneous tissue inflammation. (Abstract). Soc Neurosci Abstr 23:1809.
Table 1 Comparison of inflammatory models of pain and hyperalgesia
| Chemical | Hyperalgesia | Allodynia | Time of onset | Duration |
| CFAa | Yes | Yes | 2-6 hr | 1-2 wk |
| Carrageenan | Yes | Yes | 1 hr | 24 hr |
| Mustard oil | Yes | Yes | 5 min | < 1 hr |
| Capsaicin | Yes | Yes | 1 min | < 1 hr |
| Formalin | ||||
| Phase I | NAa | NA | < 1 min | 5-10 min |
| Phase II | NA | NA | 10 min | 1 hr |
| Zymosan | Yes | Yes | 30 min | 24 hr |
Figure 1 Effect of radiant heat on cutaneous temperature of rat hind paws after saline administration and after carrageenan (CARRA)-induced inflammation. Animals are unrestrained in an enclosed plastic chamber, and the radiant heat stimulus is positioned under the glass floor directly beneath the hind paw. The inset illustrates the mean initial temperature and the mean temperature when the rats withdrew their hind paws (flick temperature) under both conditions. Mean withdrawal latencies are also shown. Squares and circles indicate results from CARRA- and saline-injected rats, respectively. From Hargreaves and others (1988), with permission.
Figure 2 (A) Histograms reveal the thermal hyperalgesia induced by the subcutaneous injection of carrageenan (4.5 to 6.0 mg). The peak effect at 3 to 7 hr after the injection is shown; baselines were measured immediately before the injection. **Significantly different from the noninjected paws, p < 0.001. (B) Time course of thermal hyperalgesia after the injection of complete Freund's adjuvant (CFA) into the hind paw. Control values were obtained immediately before the CFA injection. Adapted from Hylden and others (1989) and Ren and others (1992).
Figure 3 Mechanical hyperalgesia and mechanical allodynia after complete Freund's adjuvant (CFA)-induced inflammation. (A)Mechanical thresholds of all animals (n = 40) are shown as a scatter plot, with medians indicated by horizontal bars. Half-filled diamonds indicate nonresponders. There was a marked reduction of the mechanical threshold induced by the CFA. **Significantly different from noninjected contralateral paws, p < 0.001 (Mann-Whitney U). (B) Response duration measure of mechanical hyperalgesia and allodynia. Force is expressed in grams (g) of each von Frey filament. The averages and SEMs in each histogram are based on the responders only; the number of responders is shown above each bar (total n -- 40). Adapted from Ren and Dubner (1993).
Copyright © 2008. National Academy of Sciences.
All rights reserved.
500 Fifth St. N.W., Washington, D.C. 20001.
Terms of Use and Privacy Statement