title: Role of C-fibers in pain and morphine induced analgesia/hyperalgesia in rats.
abstract: BACKGROUND
  Usual dosage of morphine (10 mg/kg) induces analgesia and ultra-low dose (ULD) of morphine (1 µg/kg); hyperalgesia, and C-fibers are also bearing µ-opioid receptors; here the importance of C-fibers on pain and morphine induced analgesia/hyperalgesia is questioned and investigated using pain evaluation methods and infant capsaicin treating for C-fibers lesioning.
  
  
  METHODS
  Wistar male rats (200-250 grams) were assigned to three categories i.e. control, sham (receiving neonatal capsaicin vehicle) and c-lesion (receiving neonatal capsaicin), each one with three groups (n=7). They were injected intraperitoneally with single dosage of saline, 10 mg/kg or 1 µg/kg morphine, respectively. Thermal pain threshold was evaluated using the tail flick test before and 30 minutes after the injections. Chemical pain was assessed using the formalin test (FT) 30 minutes after the administrations.
  
  
  RESULTS
  Results indicated that thermal (P < 0.001) and chemical pains in both neurogenic and inflammatory phases of FT (P < 0.05) were reduced in C-lesion animals. In the C-normal and C-lesion animals, 10 mg/kg morphine exerted analgesia both in thermal (P < 0.001) and two phases of FT (P < 0.01), but it was more potent in C-lesion animals (P < 0.05). Although ULD of morphine in C-normal animals produced hyperalgesic effect in thermal and chemical pains (P < 0.001), in C-lesion animals, it produced analgesia (P < 0.05) at the neurogenic phase of FT.
  
  
  CONCLUSIONS
  Results can raise the C-fibers involvement for a significant portion of nociceptive transmission, because C-lesioning potentiated morphine induced analgesia and eliminated ULD of morphine induced hyperalgesia. Therefore C and Aδ fibers can be involved in morphine analgesia; while, just C-fibers are possibly responsible for only presynaptically hyperalgesic/excitatory action of ULD in morphine.
authors.affiliation: Department of Biology, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran.
authors.forename: Zahra
authors.initials: Z
authors.lastname: Alizadeh
authors.affiliation: Department of Biology, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran.
authors.forename: Masoud
authors.initials: M
authors.lastname: Fereidoni
authors.affiliation: Department of Biology, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran.
authors.forename: Morteza
authors.initials: M
authors.lastname: Behnam-Rassouli
authors.affiliation: Department of Biology, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad, Iran.
authors.forename: Shirin
authors.initials: S
authors.lastname: Hosseini
keywords: Analgesia; C-Fibers; Hyperalgesia; Morphine; Pain