Inhibition of Inflammatory Pain by CRF at Peripheral, Spinal and Supraspinal Sites: Involvement of Areas Coexpressing CRF Receptors and Opioid Peptides

Abstract

There is conflicting evidence on the antinociceptive effects of corticotropin-releasing factor (CRF) along the neuraxis of pain transmission and the responsible anatomical sites of CRF's action at the level of the brain, spinal cord and periphery. In an animal model of tonic pain, that is, Freunds complete adjuvant (FCA) hindpaw inflammation, we systematically investigated CRF's ability to modulate inflammatory pain at those three levels of pain transmission by algesiometry following the intracerebroventricular, intrathecal, and intraplantar application of low, systemically inactive doses of CRF. At each level, CRF elicits potent antinociceptive effects, which are dose dependent and antagonized by local, but not systemic CRF receptor antagonist a-helical CRF indicating CRF receptor specificity. Consistently, we have identified by immunohistochemistry multiple brain areas, inhibitory interneurons within the dorsal horn of the spinal cord as well as immune cells within subcutaneous tissueFbut not peripheral sensory neuronsFthat coexpress both CRF receptors and opioid peptides. In line with these anatomical findings, local administration of CRF together with the opioid receptor antagonist naloxone dosedependently reversed CRF's antinociceptive effects at each of these three levels of pain transmission. Therefore, local application of low, systemically inactive doses of CRF at the level of the brain, spinal cord and periphery inhibits tonic inflammatory pain most likely through an activation of CRF receptors on cells that coexpress opioid peptides which results in opioid-mediated pain inhibition. Future studies have to delineate whether endogenous CRF at these three levels contributes to the body's response to cope with the stressful stimulus pain in an opioid-mediated manner.

Immune cell–derived opioids protect against neuropathic pain in mice

“…In accordance with the previously shown release of opioid peptides from leukocytes by corticotropin-releasing factor (CRF) (13)(14)(15), the majority of β-endorphin-, Met-enkephalin-, and dynorphin Apositive cells coexpressed CRF receptors at both stages of nerve injury ( Figure 1C). We did not find CRF receptors in the sciatic nerve fibers (data not shown), similar to previous studies (34). Administration of CRF (5-20 ng at 2 days or 20-100 ng at 14 days) at the site of nerve injury dose-dependently and fully reversed mechanical allodynia measured at 30 minutes after CRF application, at early and later stages of neuropathy (P < 0.001 and P = 0.003, respectively; Figure 2A).…”

“…At 2 and 14 days after CCI, mice (n = 3) were deeply anesthetized with isoflurane and perfused transcardially (10,16,34). Ligated parts of the sciatic nerves (approximately 1 cm long, including the ligation site and sites proximal and distal to it) were dissected, postfixed, cryoprotected, embedded in OCT compound (Miles Inc.), frozen (10,16,34), and cut into 10-μm-thick longitudinal sections that were then mounted on gelatin-coated slides. To examine the coexpression of opioid peptides with CRF receptors, we incubated the sections with goat anti-CRF receptor Ab recognizing both CRF receptor 1 and CRF receptor 2 (1:400; Santa Cruz Biotechnology Inc.) and with rabbit Abs against β-endorphin, Met-enkephalin (both at 1:500; Bachem), or dynorphin A (1:350; Abcam).…”

“…Staining was performed with a VECTA-STAIN avidin-biotin peroxidase complex according to the manufacturer's instructions using goat anti-rabbit biotinylated secondary Ab and avidinbiotin peroxidase (VECTASTAIN Elite Kit; Vector Laboratories) (16,34). For all experiments, specificity of the staining was confirmed by omission of the primary Abs or preabsorption (for 4 hours) of the primary Abs with the respective antigenic peptides (5-to 10-fold excess), which showed no staining (data not shown).…”

Corticotropin-releasing factor type 1 receptors mediate the visceral hyperalgesia induced by repeated psychological stress in rats

“…It is now well established that the brain CRF/CRF 1 signaling system modulates pain responses although the exact sites mediating this modulation remain unidentified (25,38). The first evidence that brain CRF system plays a role in the modulation of visceral pain in rats was shown by the elimination of visceral hyperalgesia to colorectal distention (CRD) induced by acute partial restraint stress by intracerebroventricular (ICV) injection of the nonselective CRF receptor antagonist, ␣-hCRF (19).…”

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

“…29 In brief, animals were anesthetized with isoflurane in oxygen via nose cone. A longitudinal skin incision was made in the lumbar region directly above the spinous processes of the L4 -L6 vertebrae.…”