TUMOR NECROSIS FACTOR-α INCREASES BRAIN-DERIVED NEUROTROPHIC FACTOR EXPRESSION IN TRIGEMINAL GANGLION NEURONS IN AN ACTIVITY-DEPENDENT MANNER

Many chronic trigeminal pain conditions, such as migraine or temporo-mandibular disorders, are associated with inflammation within peripheral endings of trigeminal ganglion (TG) sensory neurons. A critical role in mechanisms of neuroinflammation is attributed to proinflammatory cytokines, such as interleukin-1 beta and tumor necrosis factor-alpha (TNF alpha) that also contribute to mechanisms of persistent neuropathic pain resulting from nerve injury. However, the mechanisms of cytokine-mediated synaptic plasticity and nociceptor sensitization are not completely understood. In the present study, we examined the effects of TNF alpha on neuronal expression of brain-derived neurotrophic factor (BDNF), whose role in synaptic plasticity and sensitization of nociceptive pathways is well documented. We show that 4- and 24-h treatment with TNF alpha increases BDNF mRNA and protein, respectively, in neuron-enriched dissociated cultures of rat TG. TNF alpha increases the phosphorylated form of the cyclic AMP-responsive element binding protein (CREB), a transcription factor involved in regulation of BDNF expression in neurons, and activates transcription of BDNF exon IV (former exon III) and, to a lesser extent, exon VI (former exon IV), but not exon I. TNF alpha-mediated increase in BDNF expression is accompanied by increase in calcitonin gene-related peptide (CGRP), which is consistent with previously published studies, and indicates that both peptides are similarly regulated in TG neurons by inflammatory mediators. The effect of TNF alpha on BDNF expression is dependent on sodium influx through TTX-sensitive channels and on p38-mitogen-activated protein kinase. Moreover, electrical stimulation and forskolin, known to increase intracellular cAMP, potentiate the TNF alpha-mediated upregulation of BDNF expression. This study provides new evidence for a direct action of proinflammatory cytokines on TG primary sensory neurons, and reveals a mechanism through which TNF alpha stimulates de novo synthesis of BDNF in these neurons. Thus, TNF alpha should be considered in mechanisms of BDNF-dependent neuronal plasticity. (c) 2011 IBRO. Published by Elsevier Ltd. All rights reserved.