Takao Kumazawa

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.

Molecular cloning of prostaglandin EP3 receptors from canine sensory ganglia and their facilitatory action on bradykinin-induced mobilization of intracellular calcium

We previously demonstrated that the activation of prostaglandin E‐prostanoid‐3 (EP3) receptor sensitized the canine nociceptor response to bradykinin (BK). To elucidate the molecular mechanism for this sensitization, we cloned two cDNAs encoding EP3s with different C‐terminals, from canine dorsal root ganglia, and established the transformed cell lines stably expressing them. In both transformants, EP3 agonist did not increase intracellular cAMP levels, but it attenuated forskolin‐dependent cAMP accumulation in a pertussis toxin (PTX)‐sensitive manner and increased intracellular calcium levels in a PTX‐resistant manner, indicating that both EP3s can couple with Gi and Gq, but not with Gs proteins. As the nociceptor response to BK is mediated by BK B2 receptor, it was transfected into the transformants and the effects of EP3 agonist on BK‐dependent calcium mobilization were investigated. When BK was applied twice with a 6‐min interval, the second response was markedly attenuated. Pre‐treatment with EP3 agonist had no effect on the initial response, but restored the second response in a PTX‐sensitive manner. A protein kinase A inhibitor mimicked the effect of EP3 agonist. These results demonstrate that the activation of EP3 restores the response to BK by attenuating the desensitization of BK B2 receptor activity via Gi protein.

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.