Regulation of galanin and galanin receptor 2 expression by capsaicin in primary cultured dorsal root ganglion neurons

TRPV1-Targeted Drugs in Development for Human Pain Conditions

The transient receptor potential vanilloid-1 (TRPV1) is a non-specific cation channel known for its sensitivity to pungent vanilloid compound (i.e. capsaicin) and noxious stimuli, including heat, low pH or inflammatory mediators. TRPV1 is found in the somatosensory system, particularly primary afferent neurons that respond to damaging or potentially damaging stimuli (nociceptors). Stimulation of TRPV1 evokes a burning sensation, reflecting a central role of the channel in pain. Pharmacological and genetic studies have validated TRPV1 as a therapeutic target in several preclinical models of chronic pain, including cancer, neuropathic, postoperative and musculoskeletal pain. While antagonists of TRPV1 were found to be a valuable addition to the pain therapeutic toolbox, their clinical use has been limited by detrimental side effects, such as hyperthermia. In contrast, capsaicin induces a prolonged defunctionalisation of nociceptors and thus opened the door to the development of a new class of therapeutics with long-lasting pain-relieving effects. Here we review the list of TRPV1 agonists undergoing clinical trials for chronic pain management, and discuss new indications, formulations or combination therapies being explored for capsaicin. While the analgesic pharmacopeia for chronic pain patients is ancient and poorly effective, modern TRPV1-targeted drugs could rapidly become available as the next generation of analgesics for a broad spectrum of pain conditions.

Elevated galanin receptor type 2 primarily contributes to mechanical hypersensitivity after median nerve injury

In this study, we investigated temporal changes in galanin receptor type 2 (GalR2) expression in NF200-, galanin-, neuropeptide Y (NPY)-, and neuronal nitric oxide synthase (nNOS)-like immunoreactive (LI) dorsal root ganglion (DRG) neurons after median nerve chronic constriction injury (CCI), and the effects of GalR2 on c-Fos expression in the cuneate nucleus (CN). Double immunofluorescence labeling methods were used to appraise changes in GalR2 expression in NF200-LI, galanin-LI, NPY-LI, and nNOS-LI DRG neurons after CCI. The von Frey assay was used to assess the efficiency of intraplantar administration of saline, M871 (a GalR2 antagonist), or AR-M1896 (a GalR2 agonist) on neuropathic signs of rats with CCI. The effects of alterations in c-Fos expression were assessed in all treatments. The percentage of GalR2-LI neurons in lesioned DRGs increased and peaked at 1 week after CCI. We further detected that percentages of GalR2-LI neurons labeled for NF200, galanin, NPY, and nNOS significantly increased following CCI. Furthermore, M871 remarkably attenuated tactile allodynia, but the sensation was slightly aggravated by AR-M1896 after CCI. Consequentially, after electrical stimulation of the CCI-treated median nerve, the number of c-Fos-LI neurons in the cuneate nucleus (CN) was significantly reduced in the M871 group, whereas it increased in the AR-M1896 group. These results suggest that activation of GalR2, probably through NPY or nitric oxide, induces c-Fos expression in the CN and transmits mechanical allodynia sensations to the thalamus.

Different responses of galanin and calcitonin gene-related peptide to capsaicin stimulation on dorsal root ganglion neurons in vitro

Both galanin (Gal) and calcitonin gene-related peptide (CGRP) are sensory neuropeptides which expressed in dorsal root ganglion (DRG) neurons and are involved in nociceptive processing. Capsaicin (CAP) influences nociceptive processing via influencing the expression of sensory neuropeptides in primary sensory neurons. However, little is known about the alterations of Gal and CGRP expression at the same condition stimulated by CAP. In the present study, primary cultured DRG neurons were used to determine the different responses of Gal and CGRP to CAP stimulation. DRG neurons were cultured for 48 hours and then exposed to CAP (2 μmol/L), capsazepine (CPZ) (2 μmol/L) plus CAP (2 μmol/L), or extracellular signal-regulated kinase 1/2 (ERK1/2) inhibitor PD98059 (10 μmol/L) plus CAP (2 μmol/L) for an additional 24hours. The DRG neurons were continuously exposed to culture media as a control. After that, the levels of Gal mRNA and CGRP mRNA of DRG neurons were determined using real time-PCR analysis. Gal and CGRP expression in situ was detected by an immunofluorescent labeling technique. The levels of phosphorylated-ERK1/2 (pERK1/2) protein were detected using a Western blot assay. The results showed that CAP evoked increases of Gal and its mRNA and decreases of CGRP and its mRNA in DRG neurons. Administration of either CPZ or PD98059 blocked the effects of CAP. These data indicate that Gal and CGRP shared different responses to CAP stimulation. Gal and CGRP may have different effects in nociceptive processing during neurogenic inflammation.

Effects of exogenous galanin on neuropathic pain state and change of galanin and its receptors in DRG and SDH after sciatic nerve-pinch injury in rat

A large number of neuroanatomical, neurophysiologic, and neurochemical mechanisms are thought to contribute to the development and maintenance of neuropathic pain. However, mechanisms responsible for neuropathic pain have not been completely delineated. It has been demonstrated that neuropeptide galanin (Gal) is upregulated after injury in the dorsal root ganglion (DRG) and spinal dorsal horn (SDH) where it plays a predominantly antinociceptive role. In the present study, sciatic nerve-pinch injury rat model was used to determine the effects of exogenous Gal on the expression of the Gal and its receptors (GalR1, GalR2) in DRG and SDH, the alterations of pain behavior, nerve conduction velocity (NCV) and morphology of sciatic nerve. The results showed that exogenous Gal had antinociceptive effects in this nerve-pinch injury induced neuropathic pain animal model. It is very interesting that Gal, GalR1 and GalR2 change their expression greatly in DRG and SDH after nerve injury and intrathecal injection of exougenous Gal. Morphological investigation displays a serious damage after nerve-pinch injury and an amendatory regeneration after exogenous Gal treatment. These findings imply that Gal, via activation of GalR1 and/or GalR2, may have neuroprotective effects in reducing neuropathic pain behaviors and improving nerve regeneration after nerve injury.

Effects of norepinephrine on galanin expression in dorsal root ganglion neurons in vitro

BACKGROUND

Norepinephrine (NE) is a key neurotransmitter that functionally activates adrenoreceptors expressed in sympathetic neurons. Functional α1-adrenoreceptors are also expressed in dorsal root ganglion (DRG) primary sensory neurons and regulate neurogenic inflammation and nociceptive responses. Galanin is involved in inflammation and nociception. It has been suggested that galanin receptor (GalR) 1 and GalR3 activation induces analgesia at the level of the spinal cord, while activation of GalR2 has a pronociceptive role in the periphery. Whether activation or inhibition of α-adrenoreceptors influences galanin expression remains unknown.

OBJECTIVE

The aim of the present study was to investigate whether the α-adrenoreceptor agonist NE, the α1-adrenoreceptor antagonist prazosin, and the α2-adrenoreceptor antagonist yohimbine affect galanin expression in primary cultured DRG neurons.

METHODS

DRG was dissected from 240 embryonic 15-day-old Wistar rats, cultured as dissociated cells for 2 days, and then exposed to NE (10(-4) mol/L) for another 4 days. In the NE + prazosin group and the NE + yohimbine group, DRG neurons were pretreated with prazosin (10(-6) mol/L) and yohimbine (10(-5) mol/L), respectively, 10 minutes prior to the NE challenge. The neurons cultured continuously in media served as the controls. All of the cultured samples were processed to detect galanin mRNA and galanin peptide expression by reverse transcriptase-polymerase chain reaction and Western blot, respectively. Five samples were tested for each procedure.

RESULTS

Forty samples were prepared for this study and included in the analysis. After 4 days of incubation, mean (SD) galanin mRNA/β-actin mRNA concentration ratio was significantly increased with NE compared with controls (0.3349 [0.0413] vs 0.2411 [0.0519]; P < 0.05). Pretreatment with prazosin seemed to block the effects of NE (0.2522 [0.0496]; P < 0.05 vs NE), while yohimbine did not appear to significantly alter the effects of NE on elevation of galanin mRNA/β-actin mRNA concentration (0.3154 [0.0239]; P < 0.05 vs controls). After 4 days of incubation, galanin/β-actin concentration ratio was significantly higher with NE compared with controls (0.4406 [0.0655] vs 0.2295 [0.0794]; P < 0.01). Pretreatment with prazosin appeared to inhibit NE-induced galanin peptide expression (0.3156 [0.0942]; P < 0.05 vs NE), while yohimbine did not appear to alter the effects of NE on elevation of galanin peptide concentration (0.3700 [0.0533]; P < 0.05 vs controls). Coclusions: In this small in vitro study, NE, likely due to action on α1-adrenoreceptors but not α2-adrenoreceptors, was associated with an increase in galanin mRNA concentration and galanin peptide expression in these DRG neurons. These findings might be relevant to noradrenergic pain modulation.