Abstract:Craniofacial muscle pain including muscular temporomandibular disorders accounts for a substantial portion of all pain perceived in the head and neck region. In spite of its high clinical prevalence, the mechanisms of chronic craniofacial muscle pain are not well understood. Injection of acidic saline into rodent hindlimb muscles produces pathologies which resemble muscular pathologies in chronic pain patients. Here we investigated whether analogous transformations occur following repeated injections of acidic… Show more
“…As the use of intramuscular acidic saline injections increases, reports are emerging that pH 4.0 saline injection does not always generate demonstrable hypersensitivity. For example, a recent study by Ambalavanar et al3 reported that pH 4.0 saline injection into the masseter muscle failed to induce nociceptive behavioral responses. This has led some to question the reliability of acidic saline to induce hyperalgesia, but this result could also be explained simply by the inherent differences in various muscles to acidic saline injection (masseter versus gastrocnemius).…”
Most of our knowledge about chronic musculoskeletal pain is based on cutaneous pain models. To test the hypothesis that animals develop chronic muscular hyperalgesia following intramuscular acidic saline injections, primary hyperalgesia within the gastrocnemius muscle was analyzed and compared to secondary cutaneous hyperalgesia in the hind paw that develops following intramuscular acid saline injection. Two acidic saline (pH 4.0) injections were administrated into the gastrocnemius of female CF-1 mice. The results indicate that mice developed a robust hypersensitivity bilaterally in primary (gastrocnemius muscle) and secondary (cutaneous hind paw) sites that lasted up to 2 weeks. In addition, primary hyperalgesia correlated well with levels of Fos expression. Fos expression patterns in the spinal cord were different for primary and secondary site stimulation. Hind paw palpation stimulated ipsilateral Fos expression in the superficial spinal laminae at L4/L5 levels, and bilaterally in deep laminae at L2-L5 spinal levels. In contrast, gastrocnemius compression stimulated widespread Fos expression in all regions of the ipsilateral dorsal horn within L2-L6 spinal segments. These findings indicate that acidic saline injection induces primary hyperalgesia in muscle and that the patterns of Fos expression in response to primary versus secondary stimulation are strikingly different.
KeywordsFos; spinal cord; muscle pain; acidic saline; mice; cutaneous pain PERSPECTIVE This study assesses primary site muscular pain, which is the main complaint of people with musculoskeletal conditions, and identifies spinal patterns activated by noxious mechanical stimuli to the gastrocnemius. This study demonstrates approaches to test nociception arising
“…As the use of intramuscular acidic saline injections increases, reports are emerging that pH 4.0 saline injection does not always generate demonstrable hypersensitivity. For example, a recent study by Ambalavanar et al3 reported that pH 4.0 saline injection into the masseter muscle failed to induce nociceptive behavioral responses. This has led some to question the reliability of acidic saline to induce hyperalgesia, but this result could also be explained simply by the inherent differences in various muscles to acidic saline injection (masseter versus gastrocnemius).…”
Most of our knowledge about chronic musculoskeletal pain is based on cutaneous pain models. To test the hypothesis that animals develop chronic muscular hyperalgesia following intramuscular acidic saline injections, primary hyperalgesia within the gastrocnemius muscle was analyzed and compared to secondary cutaneous hyperalgesia in the hind paw that develops following intramuscular acid saline injection. Two acidic saline (pH 4.0) injections were administrated into the gastrocnemius of female CF-1 mice. The results indicate that mice developed a robust hypersensitivity bilaterally in primary (gastrocnemius muscle) and secondary (cutaneous hind paw) sites that lasted up to 2 weeks. In addition, primary hyperalgesia correlated well with levels of Fos expression. Fos expression patterns in the spinal cord were different for primary and secondary site stimulation. Hind paw palpation stimulated ipsilateral Fos expression in the superficial spinal laminae at L4/L5 levels, and bilaterally in deep laminae at L2-L5 spinal levels. In contrast, gastrocnemius compression stimulated widespread Fos expression in all regions of the ipsilateral dorsal horn within L2-L6 spinal segments. These findings indicate that acidic saline injection induces primary hyperalgesia in muscle and that the patterns of Fos expression in response to primary versus secondary stimulation are strikingly different.
KeywordsFos; spinal cord; muscle pain; acidic saline; mice; cutaneous pain PERSPECTIVE This study assesses primary site muscular pain, which is the main complaint of people with musculoskeletal conditions, and identifies spinal patterns activated by noxious mechanical stimuli to the gastrocnemius. This study demonstrates approaches to test nociception arising
“…Furthermore, Gerwin et al (2004) hypothesized that CGRP intensifies the response to excess ACh at the nerve terminal by enhancing ACh receptor activity and synthesis, supporting the role of neuropeptides in the MTrP pathophysiology. On the other hand, a study by Ambalavanar et al (2007) found that CGRP expression in the rat is muscle-specific; e.g. craniofacial muscles react differently to noxious stimuli than hindlimb muscles.…”
This article discusses muscle pain concepts in the context of myofascial pain syndrome (MPS) and summarizes microdialysis studies that have surveyed the biochemical basis of this musculoskeletal pain condition. Though MPS is a common type of non-articular pain, its pathophysiology is only beginning to be understood due to its enormous complexity. MPS is characterized by the presence of myofascial trigger points (MTrPs), which are defined as hyperirritable nodules located within a taut band of skeletal muscle. MTrPs may be active (spontaneously painful and symptomatic) or latent (non-spontaneously painful). Painful MTrPs activate muscle nociceptors that, upon sustained noxious stimulation, initiate motor and sensory changes in the peripheral and central nervous systems. This process is called sensitization. In order to investigate the peripheral factors that influence the sensitization process, a microdialysis technique was developed to quantitatively measure the biochemical milieu of skeletal muscle. Biochemical differences were found between active and latent MTrPs, as well as in comparison with healthy muscle tissue. In this paper we relate the findings of elevated levels of sensitizing substances within painful muscle to the current theoretical framework of muscle pain and MTrP development.
“…Inflammatory models of muscle pain involve injecting an irritating compound (mustard oil, carrageenan, complete freund's adjuvant, formalin, etc.) into the muscle, which triggers a robust inflammatory response [4;30;52;66;72;152] designed to mimic myositis and muscle strains in humans [115]. These compounds result in decreased mechanical withdrawal thresholds of muscle and paw, enhanced avoidance to noxious stimuli (PEAP), and decreased voluntary activity [16;79;80;149;152;175;196].…”
Pain is ultimately a perceptual phenomenon. It is built from information gathered by specialized pain receptors in tissue, modified by spinal and supraspinal mechanisms, and integrated into a discrete sensory experience with an emotional valence in the brain. Because of this, studying intact animals allows the multidimensional nature of pain to be examined. A number of animal models have been developed, reflecting observations that pain phenotypes are mediated by distinct mechanisms. Animal models of pain are designed to mimic distinct clinical diseases to better evaluate underlying mechanisms and potential treatments. Outcome measures are designed to measure multiple parts of the pain experience including reflexive hyperalgesia measures, sensory and affective dimensions of pain and impact of pain on function and quality of life. In this review we discuss the common methods used for inducing each of the pain phenotypes related to clinical pain syndromes, as well as the main behavioral tests for assessing pain in each model.
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