Mika Ohmichi

Two-week cast immobilization induced chronic widespread hyperalgesia in rats.

It has been postulated that physical immobilization is an essential factor in developing chronic pain after trauma or surgery in an extremity. However, the mechanisms of sustained immobilization‐induced chronic pain remain poorly understood. The present study, therefore, aimed to develop a rat model for chronic post‐cast pain (CPCP) and to clarify the mechanism(s) underlying CPCP. To investigate the effects of cast immobilization on pain behaviours in rats, one hindlimb was immobilized for 2 weeks with a cast and remobilization was conducted for 10 weeks. Cast immobilization induced muscle atrophy and inflammatory changes in the immobilized hindlimb that began 2 h after cast removal and continued for 1 week. Spontaneous pain‐related behaviours (licking and reduction in weight bearing) in the immobilized hindlimb were observed for 2 weeks, and widespread mechanical hyperalgesia in bilateral calves, hindpaws and tail all continued for 5–10 weeks after cast removal. A sciatic nerve block with lidocaine 24 h after cast removal transitorily abolished bilateral mechanical hyperalgesia in CPCP rats, suggesting that sensory inputs originating in the immobilized hindlimb contribute to the mechanism of both ipsilateral and contralateral hyperalgesia. Intraperitoneal injection of the free radical scavengers 4‐hydroxy‐2,2,6,6‐tetramethylpiperydine‐1‐oxy1 or N‐acetylcysteine 24 h after cast removal clearly inhibited mechanical hyperalgesia in bilateral calves and hindpaws in CPCP rats. These results suggest that cast immobilization induces ischaemia/reperfusion injury in the hindlimb and consequent production of oxygen free radicals, which may be involved in the mechanism of widespread hyperalgesia in CPCP rats.

Treadmill running and static stretching improve long-lasting hyperalgesia, joint limitation, and muscle atrophy induced by cast immobilization in rats

The effects of exercise on chronic pain induced by immobilization are incompletely understood. The purpose of this study was to investigate whether 30min of treadmill running (TR; active exercise) and 10min of static stretching (SS; passive exercise) of the immobilized hindlimb reduce widespread chronic pain, joint limitation, and hindlimb muscle atrophy induced by cast immobilization in rats. One hindlimb of Sprague Dawley (SD) rats was immobilized for 2 weeks with a cast, and remobilization was conducted for 7 weeks. MRI study showed that cast immobilization had induced inflammatory changes in the immobilized hindlimb, beginning as early as 2h after cast removal; these changes continued for 2-3 days. Mechanical hyperalgesia in the calf and hindpaw developed as early as 2h after cast removal and continued for 7 weeks. TR and SS were initiated 3 days after cast removal and were continued 3 times per week for 2 weeks. Both forms of exercise significantly inhibited mechanical hyperalgesia in the calf and hindpaw in immobilized rats. Range-of-motion limitations in the knee and ankle joints and calf muscle atrophy after cast removal were also decreased by both TR and SS. This study is the first to demonstrate the beneficial effect of TR and SS on widespread chronic pain, joint limitation, and muscle atrophy in a cast-immobilized rat model.

Activated spinal astrocytes are involved in the maintenance of chronic widespread mechanical hyperalgesia after cast immobilization

BackgroundIn the present study, we examined spinal glial cell activation as a central nervous system mechanism of widespread mechanical hyperalgesia in rats that experienced chronic post-cast pain (CPCP) 2 weeks after cast immobilization. Activated spinal microglia and astrocytes were investigated immunohistologically in lumbar and coccygeal spinal cord segments 1 day, 5 weeks, and 13 weeks following cast removal.ResultsIn the lumbar cord, astrocytes were activated after microglia. Astrocytes also were activated after microglia in the coccygeal cord, but with a delay that was longer than that observed in the lumbar cord. This activation pattern paralleled the observation that mechanical hyperalgesia occurred in the hindleg or the hindpaw before the tail. The activating transcription factor 3 (ATF3) immune response in dorsal root ganglia (DRG) on the last day of cast immobilization suggested that nerve damage might not occur in CPCP rats. The neural activation assessed by the phosphorylated extracellular signal-regulated kinase (pERK) immune response in DRG arose 1 day after cast removal. In addition, L-α-aminoadipate (L-α-AA), an inhibitor of astrocyte activation administered intrathecally 5 weeks after cast removal, inhibited mechanical hyperalgesia in several body parts including the lower leg skin and muscles bilaterally, hindpaws, and tail.ConclusionsThese findings suggest that activation of lumbar cord astrocytes is an important factor in widespread mechanical hyperalgesia in CPCP.

Low rather than high dose lipopolysaccharide ‘priming’ of muscle provides an animal model of persistent elevated mechanical sensitivity for the study of chronic pain

Experimental animal pain models involving peripheral nerve lesions have expanded the understanding of the pathological changes caused by nerve damage. However models for the pathogenesis of chronic pain patients lacking obvious nerve injuries have not been developed to the same extent. Guided by clinical observations, we focused on the initiating noxious event, the context when applying nociceptive stimulation targeting long‐lasting pain elicited by muscle insult. The administration of a nociceptive agent (6% hypertonic saline: HS; 5‐time repeated‐injection: HS5) after pretreatment with an immuno‐inflammatory agent (lipopolysaccharide: LPS, 2μg/kg) into one gastrocnemius muscle produced markedly long‐persisting biphasic sustained mechanical hypersensitivity on the plantar surface of both hindpaws. In the acute phase, the blockade of afferent inputs from the injected‐site was effective in returning the contralateral enhanced‐responses to baseline levels. In contrast, similar blockade during the chronic phase did not affect the contralateral enhanced‐responses, indicating that the hypersensitivity in the two phases was probably induced by different mechanisms. However, increasing the dose of LPS (20μg/kg) before applying HS5 eliminated the development of mechanical hypersensitivity in the chronic phase, while the hypersensitivity in the acute phase was significantly more severe than with low‐dose LPS‐pretreatment. In this model, the development of hypersensitivity could be modulated by manipulating LPS‐doses prior to noxious stimulation. This novel chronic pain model based on a preceding ‘priming’ myalgic stimulus provides an intriguing means for studying the pathogenesis of chronic pain.

Therapeutic effects of diclofenac, pregabalin, and duloxetine on disuse-induced chronic musculoskeletal pain in rats

The aim of this study was to clarify the mechanism of disuse-induced muscle hyperalgesia through the evaluation of the pharmacological behaviour of muscle hyperalgesia profiles in chronic post-cast pain (CPCP) rats with acute and chronic-phase mirror-image muscle hyperalgesia treated with diclofenac (NSAID), pregabalin (an inhibitor of Ca2+ channel α2δ), and duloxetine (SNRI). After 2 weeks of cast immobilization, the peak cross-sectional area and muscle wet weight of the ipsilateral soleus and gastrocnemius muscles decreased more significantly in CPCP rats than in untreated rats. Histological findings revealed disuse-induced muscle atrophy in CPCP rats. The blood biochemical parameters of CPCP rats in acute and chronic phases did not differ significantly from those of untreated rats. The diclofenac and pregabalin-treated groups exhibited no improvement in acute or chronic muscle hyperalgesia. In contrast, the duloxetine-treated group exhibited an improvement in acute muscle hyperalgesia, but showed no apparent effect on chronic muscle hyperalgesia on ipsilateral or contralateral sides. However, the chronic muscle hyperalgesia was reversed by intrathecal administration of DAMGO (a μ-opioid receptor agonist). The results suggest that chronic muscle hyperalgesia in CPCP rats did not result from an inflammatory mechanism, and there is only a low probability that it’s caused by a neuropathic mechanism.

Vascular branches from cutaneous nerve of the forearm and hand: Application to better understanding Raynaud's disease

Cutaneous nerves have branches called vascular branches (VBs) that reach arteries. VBs are thought to be involved in arterial constriction, and this is the rationale for periarterial sympathectomy as a treatment option for Raynauds disease. However, the branching patterns and distribution areas of the VBs remain largely unclear. The aim of the present study was to investigate the anatomical structures of the VBs of the cutaneous nerves. Forty hands and forearms were examined to assess the branching patterns and distribution areas of the VBs of the superficial branch of the radial nerve (SBRN), the lateral antebrachial cutaneous nerve (LACN), the medial antebrachial cutaneous nerve (MACN), and the palmar cutaneous branch of the ulnar nerve (PCUN). VBs reaching the radial and ulnar arteries were observed in all specimens. The branching patterns were classified into six types. The mean distance between the radial styloid process and the point where the VBs reached the radial artery was 34.3 ± 4.8 mm in the SBRN and 38.5 ± 15.8 mm in the LACN. The mean distance between the ulnar styloid process and the point where the VBs reached the ulnar artery was 60.3 ± 25.9 mm in the MACN and 43.8 ± 26.0 mm in the PCUN. This study showed that the VBs of the cutaneous nerves have diverse branching patterns. The VBs of the SBRN had a more limited distribution areas than those of the other nerves. Clin. Anat. 31:734–741, 2018.

Visual recognition of mirror, video-recorded, and still images in rats

Several recent studies have claimed that rodents have good visual recognition abilities. However, the extent to which rats can recognize other rats and distinguish between males and females using visual information alone remains unclear. In the present study, we investigated the ability of rats to visually recognize mirror, video-recorded, and still images and to discriminate between images of males and females. Rats were tested in a place preference apparatus with a mirror, a video-recorded image of a rat, or a still image of a rat at one end. The data were assessed using t-test with Bonferroni correction. Male and female rats spent significantly more time in the mirror chamber and the video-recorded image chamber than in their respective blank chambers (P < 0.05), and male rats also spent more time in the chamber containing a still image. Furthermore, it was found that male rats exhibited significantly more sniffing behavior around the mirror than in the blank chamber (P < 0.05), whereas female rats were no significant differences in the sniffing behaviors in the mirror, moving or still image experiments. Identical results were obtained regardless of whether the rat in the image was the same or opposite sex. These results indicate that rats can process the differences in mirror, video-recorded, and still images as visual information, but are unable to use this information to distinguish between the sexes.

A case of pancake kidney with a single ureter in the retroperitoneal space

The pancake kidney (PK) is a rare type of renal anomaly in which both kidneys completely fuse without an isthmus. In the previous reports, PKs have double ureters and are located in the pelvic cavity. We encountered a rare case of PK with a single ureter, which is located in the left retroperitoneal space, in a 95-year-old female cadaver, which was detected during a dissection course. In our case, the major calyces joined to form a single renal pelvis, which continued as a single ureter. To the best of our knowledge, this is the first report on PK with a single ureter that is located not in the pelvic cavity but in the retroperitoneal space. The knowledge of such anomalous presentation is important to avoid any complications during retroperitoneal surgery.