Title

 

Authors

A. De Corato

Doctorate School in Neuroscience
Institute of Pharmacology, Catholic University School of Medicine, Rome

 

Abstract

Migraine is a complex, chronic, painful, neurovascular disorder characterized by episodic activation of the trigeminal system. Increased calcitonin gene-related peptide (CGRP) production is found at different levels during migraine attacks. CGRP actions are mediated by the activation of specific G protein coupled receptors, which increases intracellular cAMP content. Interestingly, CGRP is also released within the trigeminal ganglia suggesting possible local effects on satellite cells, a specialized type of glia that ensheaths trigeminal neurons. CGRP was shown to enhance satellite-cell production of interleukin 1β (IL-1β), while trigeminal neurons express an activity-dependent production of nitric oxide (NO). Thus, in the present study we tested the hypothesis that IL-1β and NO induce trigeminal satellite cell activation, and that once activated these cells can influence neuronal responses. Moreover, we detected the expression of all the three components of the functional CGRP receptor in trigeminal glial cells and characterized the pro-inflammatory effects of CGRP. Primary cultures of rat trigeminal satellite cells isolated from neuronal cultures were characterized in vitro (1). Cyclooxygenase (COX) expression and activity were taken as a marker of glial pro-inflammatory activation. IL-1β and the NO donor diethylenetriamine/nitric oxide (DETA/ NO) elevated PGE2 release by satellite cells. The stimulatory effect of IL-1β was mediated mainly by upregulation of the inducible form of COX enzyme (COX2), while NO increased the constitutive COX activity. Short exposures of trigeminal satellite cells to both activators induced intracellular modifications which led to significant PGE2 production after the removal of the pro-inflammatory stimuli. This effect allowed us to harvest medium from activated satellite cells (so-called 'conditioned medium') that did not contain any stimulus, and thus test the effects of glial factors on neuronal activation. Conditioned medium from satellite cells activated by either IL-1β or NO increased neuronal CGRP release induced by different triggers. Since activated neurons release CGRP also within the trigeminal ganglion, the peptide may directly target satellite cells generating a pro-inflammatory loop that sustain trigeminal signaling and inflammatory pain transmission. In our experimental model, direct exposure of trigeminal glia to CGRP triggered only modest proinflammatory effects, while the peptide significantly enhanced glial activation induced by IL-1β. CGRP increased IL-1β dependent COX2 expression and activity, as well as the expression of the inducible nitric oxide synthase (NOS2) and IL-1β stimulated by the cytokine. These effects were, at least in part, mimicked by a cell-permeable analogue of  cAMP, and were significantly reduced by the specific rat CGRP receptor antagonist, MK8825. Moreover, considering the possible involvement of glial cells in the development of central sensitization leading to chronic pain, we further evaluated the effects of CGRP on primary cultures of rat cortical astrocytes, taken as an experimental model of central glial activation. In this paradigm, CGRP exerted mild proinflammatory actions, thus suggesting a possible role in the modulation of central glial responses in those states of inflammatory pain accompanied by increased blood brain barrier permeability. Taken together, these findings support the notion that satellite cells areactively involved in the modulation of trigeminal neuronal activity, can amplify and sustain inflammatory processes within the ganglia thus contributing to migraine-related neurochemical events. In conclusion, glial cells should be envisioned as an additional target for therapeutic intervention in migraine.
 
1.  Capuano A et al. (2009)- Mol Pain 5:43.