title: Roles of the periaqueductal gray in descending facilitatory and inhibitory controls of intramuscular hypertonic saline induced muscle nociception.
abstract: Despite the importance of the periaqueductal gray (PAG) in the modulation of nociception and pain, many aspects of the roles of the different columns of the PAG in descending controls: facilitation and inhibition, are not understood. Employing a tonic muscle pain model established by i.m. injection of 5.8% saline into the gastrocnemius muscle, we now report the results of investigations designed to explore any differences in Fos expression in the different functional columns of the PAG in male Sprague-Dawley rats. In a second series of experiments, effects of the PAG on descending control of spinally-organized nociception were assessed by measuring hind paw withdrawal reflexes to noxious mechanical and heat stimulation before and after electrolytic lesion of specific columns of the PAG. Our results show that Fos expression within different columns of the PAG increases significantly and differentially following i.m. injection of 5.8% saline. The mean number of Fos positive neurons in the dorsolateral (dl), lateral (l), dorsomedial (dm) PAG elicited by i.m. injection of 5.8% saline reached a peak at 4h with a gradual decrease over time, whereas the maximum number of Fos-positive neurons in the ventrolateral (vl) PAG was observed 8h after i.m. injection. Contralateral lesion of the dl PAG significantly depressed ipsilateral secondary mechanical hyperalgesia in intramuscularly induced (5.8% saline) nociception (P<0.05), whereas heat hypoalgesia was not affected (P>0.05). By contrast, contralateral lesion of the vl PAG completely blocked the occurrence of ipsilateral heat hypoalgesia (P<0.05), while bilateral mechanical hyperalgesia was unaffected (P>0.05). In conclusion, functions of specific columns of the PAG in the control of spinal nociceptive activities are not homogeneous. It is suggested that, in this muscle pain model, the dl PAG and vl PAG participate in descending facilitation and inhibition of nociception, respectively.
authors.affiliation: Center for Biomedical Research on Pain (CBRP), College of Medicine, Xi'an Jiaotong University, Xi'an, Shaanxi 710061, PR China.
authors.forename: Jing
authors.initials: J
authors.lastname: Lei
authors.affiliation: Center for Biomedical Research on Pain (CBRP), College of Medicine, Xi'an Jiaotong University, Xi'an, Shaanxi 710061, PR China.
authors.forename: Tao
authors.initials: T
authors.lastname: Sun
authors.affiliation: Department of Physiology and Pharmacology, University of Bristol, Bristol, UK.
authors.forename: Bridget M
authors.initials: BM
authors.lastname: Lumb
authors.affiliation: Center for Biomedical Research on Pain (CBRP), College of Medicine, Xi'an Jiaotong University, Xi'an, Shaanxi 710061, PR China. Electronic address: yhj@mail.xjtu.edu.cn.
authors.forename: Hao-Jun
authors.initials: HJ
authors.lastname: You
mesh_terms: D000344:Afferent Pathways / Q000502:physiology*; D000818:Animals; D004573:Electrolytes / Q000009:adverse effects; D006930:Hyperalgesia / Q000503:physiopathology; D007273:Injections, Intramuscular; D008297:Male; D018482:Muscle, Skeletal / Q000294:innervation*; D009433:Neural Inhibition / Q000187:drug effects / Q000502:physiology*; D059225:Nociception / Q000187:drug effects*; D016761:Oncogene Proteins v-fos / Q000378:metabolism; D010147:Pain Measurement; D017288:Pain Threshold / Q000187:drug effects / Q000502:physiology; D010487:Periaqueductal Gray / Q000293:injuries / Q000378:metabolism / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012018:Reflex / Q000187:drug effects / Q000502:physiology; D012462:Saline Solution, Hypertonic / Q000494:pharmacology*; D013997:Time Factors
chemical_list: D004573:Electrolytes; D016761:Oncogene Proteins v-fos; D012462:Saline Solution, Hypertonic