Modulation of tight junction proteins in the perineurium to facilitate peripheral opioid analgesia.

BACKGROUND: Peripheral application of opioids reduces inflammatory pain but is less effective in noninflamed tissue of rats and human patients. Hypertonic solutions can facilitate the antinociceptive activity of hydrophilic opioids in noninflamed tissue in vivo. However, the underlying mechanisms are not well understood. We hypothesized that the enhanced efficacy of opioids may be because of opening of the perineurial barrier formed by tight junction-proteins like claudin-1. METHODS: Male Wistar rats were treated intraplantarly with 10% NaCl. Pain behavior (n = 6) and electrophysiological recordings (n = 9 or more) from skin-nerve preparations after local application of the opioid [d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO) were explored. Tight junction-proteins as well as permeability of the barrier were examined by immunohistochemistry and Western blot (n = 3 or more). RESULTS: Local administration of 10% NaCl facilitated increased mechanical nociceptive thresholds in response to DAMGO, penetration of horseradish peroxidase into the nerve, as well as a reduced response of C- but not Aδ-nociceptors to mechanical stimulation after application of DAMGO in the skin-nerve preparation. In noninflamed paw tissue, claudin-1 was expressed in the epidermis, blood vessels, and the perineurium, surrounding neurons immunoreactive for calcitonin gene-related peptide or protein gene product 9.5. Claudin-1 but not claudin-5 or occludin was significantly reduced after pretreatment with 10% NaCl. Intraplantar application of a metalloproteinase inhibitor (GM6001) completely reversed these effects. CONCLUSION: Hypertonic saline opens the perineurial barrier via metalloproteinase activation and claudin-1 regulation, thereby allowing access of hydrophilic drugs to peripheral opioid receptors. This principle may be used to specifically target hydrophilic drugs to peripheral neurons.

p38 Mitogen-activated protein kinase activation by nerve growth factor in primary sensory neurons upregulates µ-opioid receptors to enhance opioid responsiveness toward better pain control.

BACKGROUND: Sensory neuron opioid receptors are targets for spinal, epidural, and peripheral opioid application. Although local nerve growth factor (NGF) has been identified as a mediator of sensory neuron µ-opioid receptor (MOR) up-regulation, the signaling pathways involved have not been yet identified. METHODS: Wistar rats were treated with intraplantar vehicle, Freund's complete adjuvant, NGF, NGF plus intrathecal p38 mitogen-activated protein kinase (MAPK) inhibitors, or NGF plus extracellular signal-regulated kinase-1/2 MAPK inhibitors. After 4 days of treatment, paw pressure thresholds of an intraplantar full (fentanyl) or partial (buprenorphine) opioid agonist were determined by algesiometry. Tissue samples from rat dorsal root ganglia were subjected to radiolabeled ligand binding, Western blot analysis, and confocal immunofluorescence. RESULTS: Exogenous and endogenous NGF resulting from Freund's complete adjuvant inflammation produced significant potentiation and enhanced efficacy in fentanyl- and buprenorphine-induced dose-dependent antinociception, respectively. Furthermore, in the ipsilateral dorsal root ganglia, NGF produced a significant increase in MOR binding sites, proteins, and immunoreactive neurons. In parallel, phosphorylated p38-MAPK protein, the number of phosphorylated p38-MAPK immunoreactive neurons expressing MOR in dorsal root ganglia, and the peripherally directed axonal transport of MOR significantly increased. Finally, NGF-induced effects occurring in dorsal root ganglia, on axonal transport, and on the potentiation or enhanced efficacy of opioid antinociception were abrogated by inhibition of p38, but not extracellular signal-regulated kinase-1/2, MAPK. CONCLUSIONS: Local NGF through activation of the p38-MAPK pathway leads to adaptive changes in sensory neuron MOR toward enhanced susceptibility to local opioids. This effect may act as a counter-regulatory response to p38-MAPK-induced pain (e.g., inflammatory pain) to facilitate opioid-mediated antinociception.