Nicotine stimulates expression of proteins implicated in peripheral and central sensitization.

Pain patients who are nicotine dependent report a significantly increased incidence and severity of pain intensity. The goal of this study was to determine the effects of prolonged nicotine administration on inflammatory proteins implicated in the development of peripheral and central sensitization of the trigeminal system. Behavioral, immunohistochemical, and microarray studies were utilized to investigate the effects of nicotine administered daily for 14 days via an Alzet® osmotic pump in Sprague Dawley rats. Systemic nicotine administration caused a significant increase in nocifensive withdrawals to mechanical stimulation of trigeminal neurons. Nicotine stimulated expression of the pro-inflammatory signal transduction proteins phosphorylated-extracellular signal-regulated kinase (p-ERK), phosphorylated-c-Jun N-terminal kinase (p-JNK), and protein kinase A (PKA) in the spinal trigeminal nucleus. Nicotine also promoted elevations in the expression of glial fibrillary acidic protein (GFAP), a biomarker of activated astrocytes, and the microglia biomarker ionized calcium-binding adapter molecule 1 (Iba1). Similarly, levels of eleven cytokines were significantly elevated with the largest increase in expression of TNF-α. Levels of PKA, p-ERK, and p-JNK in trigeminal ganglion neurons were increased by nicotine. Our findings demonstrate that prolonged systemic administration of nicotine promotes sustained behavioral and cellular changes in the expression of key proteins in the spinal trigeminal nucleus and trigeminal ganglion implicated in the development and maintenance of peripheral and central sensitization.

Dual orexin receptor antagonist 12 inhibits expression of proteins in neurons and glia implicated in peripheral and central sensitization.

Sensitization and activation of trigeminal nociceptors is implicated in prevalent and debilitating orofacial pain conditions including temporomandibular joint (TMJ) disorders. Orexins are excitatory neuropeptides that function to regulate many physiological processes and are reported to modulate nociception. To determine the role of orexins in an inflammatory model of trigeminal activation, the effects of a dual orexin receptor antagonist (DORA-12) on levels of proteins that promote peripheral and central sensitization and changes in nocifensive responses were investigated. In adult male Sprague-Dawley rats, mRNA for orexin receptor 1 (OX1R) and receptor 2 (OX2R) were detected in trigeminal ganglia and spinal trigeminal nucleus (STN). OX1R immunoreactivity was localized primarily in neuronal cell bodies in the V3 region of the ganglion and in laminas I-II of the STN. Animals injected bilaterally with complete Freund's adjuvant (CFA) in the TMJ capsule exhibited increased expression of P-p38, P-ERK, and lba1 in trigeminal ganglia and P-ERK and lba1 in the STN at 2 days post injection. However, levels of each of these proteins in rats receiving daily oral DORA-12 were inhibited to near basal levels. Similarly, administration of DORA-12 on days 3 and 4 post CFA injection in the TMJ effectively inhibited the prolonged stimulated expression of protein kinase A, NFkB, and Iba1 in the STN on day 5 post injection. While injection of CFA mediated a nocifensive response to mechanical stimulation of the orofacial region at 2h and 3 and 5 days post injection, treatment with DORA-12 suppressed the nocifensive response on day 5. Somewhat surprisingly, nocifensive responses were again observed on day 10 post CFA stimulation in the absence of daily DORA-12 administration. Our results provide evidence that DORA-12 can inhibit CFA-induced stimulation of trigeminal sensory neurons by inhibiting expression of proteins associated with sensitization of peripheral and central neurons and nociception.