title: Corticofugal outputs facilitate acute, but inhibit chronic pain in rats.
abstract: It has been widely accepted that the primary somatosensory cortex (SI) plays an essential role in the sensory-discriminative aspect of pain perception. However, it remains unclear whether the SI has a role in the descending modulation of pain. Although there are abundant fibers projecting back from sensory cortex to thalamic nuclei, and the influence of cortical modulation from SI on the thalamic nociceptive relay neurons has been addressed, little is known about how the cortical outputs modulate the nociceptive behaviors resulting from tissue injury or evoked by painful stimulation. The present study was designed to test whether the cortical outputs influenced the nociceptive behaviors using rat models of noxious thermal-induced acute pain, formalin-induced acute and CFA-evoked chronic inflammatory pain. The results showed that intracortical microinjection of GABAA agonist muscimol significantly reduced the first and second phase behaviors in formalin tests and elevated the nociceptive thresholds in the thermal stimulus-elicited acute pain, suggesting a facilitatory influence of SI on the acute pain sensation. By contrast, microinjection of GABAA antagonist bicuculline remarkably reduced the thermal hyperalgesia of the CFA-inflamed hindpaws, indicating an inhibitory effect of SI output in the chronic pain state. The opposite modulatory effects in acute and chronic pain states suggest that there exists a functional switch for the SI cortex at different stages of pain disease, which is of great significance for the biological adaptation.
authors.affiliation: Neuroscience Research Institute and Department of Neurobiology, Peking University, Beijing, PR China.
authors.forename: Ning
authors.initials: N
authors.lastname: Wang
authors.forename: Jin-Yan
authors.initials: JY
authors.lastname: Wang
authors.forename: Fei
authors.initials: F
authors.lastname: Luo
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D001640:Bicuculline / Q000494:pharmacology; D002908:Chronic Disease; D004195:Disease Models, Animal; D005620:Freund's Adjuvant / Q000009:adverse effects; D007839:Functional Laterality; D018755:GABA Agonists / Q000494:pharmacology; D018756:GABA Antagonists / Q000494:pharmacology; D006358:Hot Temperature / Q000009:adverse effects; D008297:Male; D009118:Muscimol / Q000494:pharmacology; D009434:Neural Pathways / Q000503:physiopathology; D010146:Pain / Q000209:etiology / Q000473:pathology* / Q000503:physiopathology; D010147:Pain Measurement; D051381:Rats; D017207:Rats, Sprague-Dawley; D013003:Somatosensory Cortex / Q000187:drug effects / Q000503:physiopathology*; D013133:Spinothalamic Tracts / Q000503:physiopathology*; D013787:Thalamic Nuclei / Q000503:physiopathology*; D013997:Time Factors
chemical_list: D018755:GABA Agonists; D018756:GABA Antagonists; D009118:Muscimol; D005620:Freund's Adjuvant; D001640:Bicuculline