The Role of Glial Activation in Descending Facilitation from the Rostroventromedial Medulla (RVM) in Models of Persistent Pain

Abstract

Substantial evidence shows that activation of glial cells in the spinal cord may promote central sensitization and enhancement of pain. Descending facilitation from the rostroventromedial medulla (RVM) is also recognized as a critical component in the maintenance of chronic pain states, although the precise mechanisms driving this activity are unclear. Here, we investigated the possibility that glial activation in the RVM could promote descending facilitation from the RVM in states of enhanced pain. Peripheral inflammation was induced with carrageenan injected into the plantar aspect of the hindpaw of male Sprague-Dawley rats and behavioral responses to noxious thermal and light tactile stimuli were determined. Microinjection of the glial inhibitors minocycline or fluorocitrate, or of SB 203580, a p38 MAPK inhibitor, produced a significant and time-related reversal of behavioral hypersensitivity resulting from hindpaw inflammation. Moreover, carrageenan-induced inflammation appeared to produce an increase in immunolabeling for activated microglia and astrocytes in the RVM, as well as for phosphorylated p38 MAPK; the latter was localized to both microglia and neurons of the RVM. Microinjection of the glial inhibitors into the RVM appeared to diminish immunofluorescent labeling for activated RVM microglia and astrocytes. Carrageenan-induced inflammation also increased RVM protein levels of Iba1 and GFAP and administration of minocycline or fluorocitrate into the RVM attenuated this effect. To examine a possible mechanism of glial activation, α, β-methylene-ATP was microinjected into the RVM, inducing thermal hyperalgesia, and pre-treatment with the P2X antagonists, PPADS and TNP-ATP, delayed the initiation of ATP-induced hyperalgesia. Post-treatment with the antagonists had no effect on established ATP-induced or carrageenan-induced hypersensitivity. The activation of P2X receptors initiates a signaling cascade leading to the production and release of nociceptive mediators, including BDNF. The RVM microinjection of an anti- BDNF antibody reversed carrageenan-induced thermal hyperalgesia. A model of morphine-induced paradoxical pain was also used to examine the role of glial activation in the RVM. Sustained morphine administration induced tactile allodynia and RVM microinjection of minocycline, but not fluorocitrate, attenuated the behavioral hypersensitivity. Sustained morphine also induced morphological changes in microglia of the RVM, suggesting microglial activation. A third model of enhanced pain used to study medullary glial activation was the spinal nerve ligation (SNL) model of neuropathic pain. The SNL injury induced astrocyte activation within the RVM and microinjection of the astrocyte inhibitor fluorocitrate attenuated the nerve injury-induced tactile allodynia. Minocycline administered to the RVM did not attenuate the behavioral hypersensitivity, suggesting a role for astrocytes, not microglia, in nerve injury-induced enhanced pain. The data show that inflammatory, opioid-induced and neuropathic pain is associated with glial activation in the RVM which likely participates in driving descending pain facilitation via glial-neuronal communication. These findings reveal a novel site of glial modulation of pain.