title: Effects of simulated weightlessness on intramuscular hypertonic saline induced muscle nociception and spinal Fos expression in rats.
abstract: We assessed the effects of simulated weightlessness, hindlimb unloading (HU) by 7 days of tail suspension, on noxious mechanically and heat evoked spinal withdrawal reflexes and spinal Fos expression during muscle nociception elicited by intramuscular (i.m.) injection of hypertonic (HT; 5.8%) saline into gastrocnemius muscle in rats. In HU rats, i.m. HT saline-induced secondary mechanical hyperalgesia was enhanced, and secondary heat hypoalgesia was significantly delayed. After 7 days of HU, basal Fos expression in spinal L4-6 segments was bilaterally enhanced only in superficial (I-II) but not middle and deep laminae (III-VI) of the spinal dorsal horn, which finding was not influenced by tail denervation. Unilateral i.m. HT saline injection increased spinal Fos expression bilaterally in both the control rats and 7 days of HU rats. The HT saline-induced bilateral increase of spinal Fos occurred within 0.5h and reached its peak within 1h, after which it gradually returned to the control levels within 8h. Spatial patterns of spinal Fos expression differed between the control group and 7 days of HU group. In superficial laminae, the HT saline-induced increases in Fos expression were higher and in the middle and deep laminae V-VI lower in the 7 days of HU than control rats. It is suggested that supraspinal mechanisms presumably underlie the effects of HU on spinally-organized nociception. Simulated weightlessness may enhance descending facilitation and weaken descending inhibition of nociception.
authors.affiliation: Center for Biomedical Research on Pain (CBRP), College of Medicine, Xi׳an Jiaotong University, Xi׳an 710061, PR China.
authors.forename: Jing
authors.initials: J
authors.lastname: Lei
authors.affiliation: Institute of Biomedicine/Physiology, University of Helsinki, POB 63, Helsinki 00014, Finland.
authors.forename: Antti
authors.initials: A
authors.lastname: Pertovaara
authors.affiliation: Center for Biomedical Research on Pain (CBRP), College of Medicine, Xi׳an Jiaotong University, Xi׳an 710061, PR China. Electronic address: yhj@mail.xjtu.edu.cn.
authors.forename: Hao-Jun
authors.initials: HJ
authors.lastname: You
mesh_terms: D000818:Animals; D019417:Hindlimb Suspension; D006930:Hyperalgesia / Q000378:metabolism; D007150:Immunohistochemistry; D007273:Injections, Intramuscular; D008297:Male; D018482:Muscle, Skeletal / Q000378:metabolism; D063806:Myalgia / Q000209:etiology / Q000378:metabolism*; D059225:Nociception / Q000502:physiology*; D016760:Proto-Oncogene Proteins c-fos / Q000096:biosynthesis*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012462:Saline Solution, Hypertonic / Q000633:toxicity; D013116:Spinal Cord / Q000378:metabolism*; D014893:Weightlessness / Q000009:adverse effects*
chemical_list: D016760:Proto-Oncogene Proteins c-fos; D012462:Saline Solution, Hypertonic
keywords: Endogenous descending control; Fos expression; Muscle pain; Simulated weightlessness; Spinal cord