Inflammation-induced hyperalgesia and spinal microglia reactivity in neonatal rats.

BACKGROUND: Peripheral inflammation and nerve injury evoke pain behaviours in adult rodents mediated by sensitization, a process that involves the activation of microglia in the spinal cord. In neonates, however, peripheral inflammation, but not nerve injury, induces a lasting hyperalgesia. It is known that microglia does not activate after nerve injury in young pups; however, changes in microglia associated with inflammation in neonatal animals have not been studied. METHODS: Inflammation was induced by unilateral intraplantar injection of carrageenan, complete Freund's adjuvant or zymosan in 10-day-old rats. Rats were tested for mechanical sensitivity in response to punctuate stimulation of the dorsal surface of the hind paw using calibrated von Frey filaments. Immunohistochemistry was used to detect changes in size and density of microglial cells using the specific marker Iba-1. The effects of minocycline applications (120 µg, i.t.) on spinal microglia and behaviour induced by zymosan inflammation were studied. RESULTS: Hind paw inflammation in young P10 rats, with either of the agents used, produced an immediate hyperalgesia, which lasted more than 7 days. A concomitant and significant increase in cell size and density in Iba-1-positive cells was observed in the spinal dorsal horn. These morphological changes in spinal microglia were observed as early as 1-h post-inflammation. Intrathecal and systemic administration of minocycline blocked the hyperalgesia and the changes in spinal microglia produced by zymosan. CONCLUSIONS: Results suggest a key role for spinal microglia activation in the development of hyperalgesia following inflammation in neonatal animals.

Expression of Ankrd2 in fast and slow muscles and its response to stretch are consistent with a role in slow muscle function.

In striated muscle, the structural genes associated with muscle fiber phenotype determination as well as muscle mass accretion are regulated largely by mechanical stimuli. Passive stretch of skeletal muscle stimulates muscle growth/hypertrophy and an increased expression of slow muscle genes. We previously identified Ankyrin repeat-domain protein (Ankrd2) as a novel transcript expressed in fast tibialis anterior muscles after 7 days of passive stretch immobilization in vivo. Here, we test the hypothesis that the expression of Ankrd2 in stretched fast muscle is associated with the stretch-induced expression of slow muscle phenotype rather than the hypertrophic response. Our results show that, in 4- and 7-day stretched tibialis anterior muscle, the expression of Ankrd2 mRNA and protein was significantly upregulated (P > 0.001). However, in fast muscles of kyphoscoliotic mutant mice, which lack the hypertrophic response to overload but have a slower muscle phenotype than wild-type, Ankrd2 expression was significantly upregulated. The distribution pattern of Ankrd2 in fast and slow muscle is also in accord with their slow fiber composition. Furthermore, it was markedly downregulated in denervated rat soleus muscle, which produces a pronounced shift toward the fast muscle phenotype. Using a sensitive proteomics approach (Ciphergen Technology), we observed that Ankrd2 protein was undetectable in soleus after 4 wk of denervation. We suggest that Ankrd2, which is also a titin binding protein, is a stretch-response gene associated with slow muscle function and that it is part of a separate mechanotransduction system to the one that regulates muscle mass.