Transgenic mice possessing increased numbers of nociceptors do not exhibit increased behavioral sensitivity in models of inflammatory and neuropathic pain

Glial cell line-derived neurotrophic factors (GFLs) and small molecules targeting RET receptor for the treatment of pain and Parkinson's disease

Rearranged during transfection (RET), in complex with glial cell line-derived (GDNF) family receptor alpha (GFRα), is the canonical signaling receptor for GDNF family ligands (GFLs) expressed in both central and peripheral parts of the nervous system and also in non-neuronal tissues. RET-dependent signaling elicited by GFLs has an important role in the development, maintenance and survival of dopamine and sensory neurons. Both Parkinson's disease and neuropathic pain are devastating disorders without an available cure, and at the moment are only treated symptomatically. GFLs have been studied extensively in animal models of Parkinson's disease and neuropathic pain with remarkable outcomes. However, clinical trials with recombinant or viral vector-encoded GFL proteins have produced inconclusive results. GFL proteins are not drug-like; they have poor pharmacokinetic properties and activate multiple receptors. Targeting RET and/or GFRα with small molecules may resolve the problems associated with using GFLs as drugs and can result in the development of therapeutics for disease-modifying treatments against Parkinson's disease and neuropathic pain.

Effect of naltrexone on neuropathic pain in mice locally transfected with the mutant μ-opioid receptor gene in spinal cord

BACKGROUND AND PURPOSE

Opioid antagonists, such as naloxone and naltrexone, exhibit agonistic properties at the mutated μ receptor, MOR-S196ACSTA. In our previous study, systemic naloxone (10 mg·kg(-1) , s.c.) elicited antinociceptive effect without the induction of tolerance, dependence or rewarding effect in mice 2 weeks after intrathecal administration of double-stranded adeno-associated virus-MOR-S196ACSTA-eGFP. Here, we have investigated if this antinociceptive paradigm would be effective in a mouse model of neuropathic pain.

EXPERIMENTAL APPROACH

Spinal nerves were ligated in male C57BL/6 mice 3 or 4 weeks after intrathecal injection of the lentivirus encoding the construct of MOR-S196ACSTA-eGFP (LV-MOR-S196ACSTA). Anti-allodynic effects of daily s.c.injections of saline, naltrexone (10 mg·kg(-1) ) or morphine (10 mg·kg(-1) ) were assessed by the von Frey test. After 14 days of treatment with saline, naltrexone or morphine, signs of natural withdrawal were measured at 22 and 46 h after the last injection. To determine the rewarding effects induced by morphine or naltrexone, the conditioned place preference test was carried out.

KEY RESULTS

Anti-allodynic effects, as measured by von Frey test, increased after naltrexone or morphine treatment in mice transfected with LV-MOR-S196ACSTA in the spinal cord. Cessation of treatment with morphine, but not naltrexone, induced natural withdrawal and rewarding effects.

CONCLUSIONS AND IMPLICATIONS

Systemic injection of naltrexone after the expression of a mutant μ opioid receptor, MOR-S196ACSTA, in the spinal cord may have therapeutic potential for chronic neuropathic pain, without the development of dependence or addiction.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

New insights into mechanisms of opioid inhibitory effects on capsaicin-induced TRPV1 activity during painful diabetic neuropathy

Painful diabetic neuropathy is a disease of the peripheral sensory neuron with impaired opioid responsiveness. Since μ-opioid receptor (MOR) activation can inhibit the transient receptor potential vanilloid 1 (TRPV1) activity in peripherally sensory neurons, this study investigated the mechanisms of impaired opioid inhibitory effects on capsaicin-induced TRPV1 activity in painful diabetic neuropathy. Intravenous injection of streptozotocin (STZ, 45 mg/kg) in Wistar rats led to a degeneration of insulin producing pancreatic β-cells, elevated blood glucose, and mechanical hypersensitivity (allodynia). In these animals, local morphine's inhibitory effects on capsaicin-induced nocifensive behavior as well as on capsaicin-induced TRPV1 current in dorsal root ganglion cells were significantly impaired. These changes were associated with a loss in MOR but not TRPV1 in peripheral sensory neurons. Intrathecal delivery of nerve growth factor in diabetic animals normalized sensory neuron MOR and subsequently rescued morphine's inhibitory effects on capsaicin-induced TRPV1 activity in vivo and in vitro. These findings identify a loss in functional MOR on sensory neurons as a contributing factor for the impaired opioid inhibitory effects on capsaicin-induced TRPV1 activity during advanced STZ-induced diabetes. Moreover, they support growing evidence of a distinct regulation of opioid responsiveness during various painful states of disease (e.g. arthritis, cancer, neuropathy) and may give novel therapeutic incentives.

VGLUTs in Peripheral Neurons and the Spinal Cord: Time for a Review

Vesicular glutamate transporters (VGLUTs) are key molecules for the incorporation of glutamate in synaptic vesicles across the nervous system, and since their discovery in the early 1990s, research on these transporters has been intense and productive. This review will focus on several aspects of VGLUTs research on neurons in the periphery and the spinal cord. Firstly, it will begin with a historical account on the evolution of the morphological analysis of glutamatergic systems and the pivotal role played by the discovery of VGLUTs. Secondly, and in order to provide an appropriate framework, there will be a synthetic description of the neuroanatomy and neurochemistry of peripheral neurons and the spinal cord. This will be followed by a succinct description of the current knowledge on the expression of VGLUTs in peripheral sensory and autonomic neurons and neurons in the spinal cord. Finally, this review will address the modulation of VGLUTs expression after nerve and tissue insult, their physiological relevance in relation to sensation, pain, and neuroprotection, and their potential pharmacological usefulness.

Repeated variate stress in male rats induces increased voiding frequency, somatic sensitivity, and urinary bladder nerve growth factor expression

Stress exacerbates symptoms of functional lower urinary tract disorders including interstitial cystitis (IC)/bladder pain syndrome (BPS) and overactive bladder (OAB) in humans, but mechanisms contributing to symptom worsening are unknown. These studies address stress-induced changes in the structure and function of the micturition reflex using an animal model of stress in male rats. Rats were exposed to 7 days of repeated variate stress (RVS). Target organ (urinary bladder, thymus, adrenal gland) tissues were collected and weighed following RVS. Evans blue (EB) concentration and histamine, myeloperoxidase (MPO), nerve growth factor (NGF), brain-derived neurotropic factor (BDNF), and CXCL12 protein content (ELISA) were measured in the urinary bladder, and somatic sensitivity of the hindpaw and pelvic regions was determined following RVS. Bladder function was evaluated using continuous, open outlet intravesical infusion of saline in conscious rats. Increases in body weight gain were significantly (P ≤ 0.01) attenuated by day 5 of RVS, and adrenal weight was significantly (P ≤ 0.05) increased. Histamine, MPO, NGF, and CXCL12 protein expression was significantly (P ≤ 0.01) increased in the urinary bladder after RVS. Somatic sensitivity of the hindpaw and pelvic regions was significantly (P ≤ 0.01) increased at all monofilament forces tested (0.1-4 g) after RVS. Intercontraction interval, infused volume, and void volume were significantly (P ≤ 0.01) decreased after RVS. These studies demonstrate increased voiding frequency, histamine, MPO, NGF, and CXCL12 bladder content and somatic sensitivity after RVS suggesting an inflammatory component to stress-induced changes in bladder function and somatic sensitivity.

Reduced number, G protein coupling, and antinociceptive efficacy of spinal mu-opioid receptors in diabetic rats are reversed by nerve growth factor

This study investigated putative mechanisms of impaired spinal opioid antinociception such as a downregulation of mu-opioid receptor (MOR) number, coupling, and efficacy in rats with advanced (12 weeks) streptozotocin (STZ)-induced diabetes. Intravenous injection of STZ (45 mg/kg) in Wistar rats led to selective degeneration of insulin-producing pancreatic ß-cells, elevated blood glucose, and mechanical hyperalgesia. In these animals, dose-dependent and naloxone-reversible intrathecal fentanyl antinociception was significantly impaired and associated with a loss in MOR immunoreactivity of calcitonin gene-related peptide-immunoreactive (CGRP-IR) sensory nerve terminals, membrane-bound MOR binding sites, and MOR-stimulated G protein coupling within the dorsal horn of the spinal cord. Intrathecal delivery of nerve growth factor (NGF) in diabetic animals normalized spinal MOR number and G protein coupling and rescued spinal fentanyl-induced antinociception. These findings identify for the first time a loss in functional MOR on central terminals of sensory neurons as a contributing factor for the impaired spinal opioid responsiveness during advanced STZ-induced diabetes that can be reversed by NGF. Moreover, they support growing evidence of a distinct regulation of opioid responsiveness during various painful states of disease (eg, arthritis, cancer, neuropathy) and may give novel therapeutic incentives.

PERSPECTIVE

In diabetic neuropathy a loss in sensory neuron mu-opioid receptor number and coupling contributes to impaired spinal opioid antinociception that can be reversed by NGF. These findings support growing evidence of a distinct regulation of opioid responsiveness during various painful diseases and may give novel therapeutic incentives.

Glial cell line-derived neurotrophic factor modulates the excitability of nociceptive trigeminal ganglion neurons via a paracrine mechanism following inflammation

Previous our report indicated that acute application of glial cell line-derived neurotrophic factor (GDNF) enhances the neuronal excitability of adult rat small-diameter trigeminal ganglion (TRG) neurons, which innervate the facial skin in the absence of neuropathic and inflammatory conditions. This study investigated whether under in vivo conditions, GDNF modulates the excitability of nociceptive Aδ-TRG neurons innervating the facial skin via a paracrine mechanism following inflammation. We used extracellular electrophysiological recording with multibarrel-electrodes in this study. Spontaneous Aδ-TRG neuronal activity was induced in control rats after iontophoretic application of GDNF into the trigeminal ganglia (TRGs). Noxious and non-noxious mechanical stimuli evoked Aδ-TRG neuronal firing rate were significantly increased by iontophoretic application of GDNF. The mean mechanical threshold of nociceptive TRG neurons was significantly decreased by GDNF application. The increased discharge frequency and decreased mechanical threshold induced by GDNF were antagonized by application of the protein tyrosine kinase inhibitor, K252b. The number of Aδ-TRG neurons with spontaneous firings and their firing rates in rats with inflammation induced by Complete Freund's Adjuvant were significantly higher than control rats. The firing rates of Aδ-TRG spontaneous neuronal activity were significantly decreased by iontophoretic application of K252b in inflamed rats. K252b also inhibited Aδ-TRG neuron activity evoked by mechanical stimulation in inflamed rats. These results suggest that in vivo GDNF enhances the excitability of nociceptive Aδ-TRG neurons via a paracrine mechanism within TRGs following inflammation. GDNF paracrine mechanism could be important as a therapeutic target for trigeminal inflammatory hyperalgesia.

Cutaneous C-polymodal fibers lacking TRPV1 are sensitized to heat following inflammation, but fail to drive heat hyperalgesia in the absence of TPV1 containing C-heat fibers

BACKGROUND

Previous studies have shown that the TRPV1 ion channel plays a critical role in the development of heat hyperalgesia after inflammation, as inflamed TRPV1-/- mice develop mechanical allodynia but fail to develop thermal hyperalgesia. In order to further investigate the role of TRPV1, we have used an ex vivo skin/nerve/DRG preparation to examine the effects of CFA-induced-inflammation on the response properties of TRPV1-positive and TRPV1-negative cutaneous nociceptors.

RESULTS

In wildtype mice we found that polymodal C-fibers (CPMs) lacking TRPV1 were sensitized to heat within a day after CFA injection. This sensitization included both a drop in average heat threshold and an increase in firing rate to a heat ramp applied to the skin. No changes were observed in the mechanical response properties of these cells. Conversely, TRPV1-positive mechanically insensitive, heat sensitive fibers (CHs) were not sensitized following inflammation. However, results suggested that some of these fibers may have gained mechanical sensitivity and that some previous silent fibers gained heat sensitivity. In mice lacking TRPV1, inflammation only decreased heat threshold of CPMs but did not sensitize their responses to the heat ramp. No CH-fibers could be identified in naïve nor inflamed TRPV1-/- mice.

CONCLUSIONS

Results obtained here suggest that increased heat sensitivity in TRPV1-negative CPM fibers alone following inflammation is insufficient for the induction of heat hyperalgesia. On the other hand, TRPV1-positive CH fibers appear to play an essential role in this process that may include both afferent and efferent functions.

Thermal nociception and TRPV1 function are attenuated in mice lacking the nucleotide receptor P2Y2

Recent studies indicate that ATP and UTP act at G protein-coupled (P2Y) nucleotide receptors to excite nociceptive sensory neurons; nucleotides also potentiate signaling through the pro-nociceptive capsaicin receptor, TRPV1. We demonstrate here that P2Y(2) is the principal UTP receptor in somatosensory neurons: P2Y(2) is highly expressed in dorsal root ganglia and P2Y(2)-/- mice showed profound deficits in UTP-evoked calcium transients and potentiation of capsaicin responses. P2Y(2)-/- mice were also deficient in the detection of painful heat: baseline thermal response latencies were increased and mutant mice failed to develop thermal hypersensitivity in response to inflammatory injury (injection of complete Freund's adjuvant into the hindpaw). P2Y(2) was the only Gq-coupled P2Y receptor examined that showed an increase in DRG mRNA levels in response to inflammation. Surprisingly, TRPV1 function was also attenuated in P2Y(2)-/- mice, as measured by the frequency and magnitude of capsaicin responses in vitro and behavioral responses to capsaicin administration in vivo. However, TRPV1 mRNA levels and immunoreactivity were not reduced, and behavioral sensitivity to capsaicin could be largely restored in P2Y(2)-/- mice by pretreatment with bradykinin, suggesting that normal function of TRPV1 requires ongoing modulation by G protein-coupled receptors. These results indicate that nucleotide signaling through P2Y(2) plays a key role in thermal nociception.

Species and strain differences in rodent sciatic nerve anatomy: implications for studies of neuropathic pain

Hindlimb pain models developed in rats have been transposed to mice, but assumed sciatic nerve neuroanatomic similarities have not been examined. We compared sciatic nerve structural organization in mouse strains (C57BL/6J, DBA/2J, and B6129PF2/J) and rat strains (Wistar, Brown Norway, and Sprague-Dawley). Dissection and retrograde labeling showed mouse sciatic nerve origins predominantly from the third lumbar (L3) and L4 spinal nerves, unlike the L4 and L5 in rats. Proportionate contributions by each level differed significantly between strains in both mice and rats. Whereas all rats had six lumbar vertebrae, variable patterns in mice included mostly five vertebrae in DBA/2J, mostly six vertebrae in C57BL/6J, and a mix in B6129PF2/J. Mice with a short lumbar vertebral column showed a rostral shift in relative contributions to the sciatic nerve by L3 and L4. Ligation of the mouse L4 nerve created hyperalgesia similar to that in rats after L5 ligation, and motor changes were similar after mouse L4 and rat L5 ligation (foot cupping) and after mouse L3 and rat L4 ligation (flexion weakness). Thus, mouse L3 and L4 neural segments are anatomically and functionally homologous with rat L4 and L5 segments. Neuronal changes after distal injury or inflammation should be sought in the mouse L3 and L4 ganglia, and the spinal nerve ligation model in mice should involve ligation of the L4 nerve while L3 remains intact. Strain-dependent variability in segmental contributions to the sciatic nerve may account in part for genetic differences in pain behavior after spinal nerve ligation.

The skin as a neurotrophic organ

The ability of the skin to serve as a protective shield against environmental challenges and as a sensitive detector and responder to thermal, chemical, and mechanical stimuli speaks to its exquisite design. A central feature of this design is the diverse array of neuronal afferents that convey and respond to sensory stimuli that the skin encounters. Cutaneous neuron development, form, and function are highly dependent on communication with the skin through its production of multiple growth factor proteins that modulate afferent development, maturation, and function. Production by the skin of neurotrophin growth factors and members of the glial cell line-derived neurotrophic factor family are particularly important for support of specific subsets of sensory neurons with unique phenotypic and functional properties. Although these proteins have central roles in afferent development and function, challenges remain in identifying specific molecular mechanisms of growth factor communication and understanding how activation of signaling pathways direct neuron differentiation and function under normal and pathological conditions.

Artemin overexpression in skin enhances expression of TRPV1 and TRPA1 in cutaneous sensory neurons and leads to behavioral sensitivity to heat and cold

Artemin, a neuronal survival factor in the glial cell line-derived neurotrophic factor family, binds the glycosylphosphatidylinositol-anchored protein GFRalpha3 and the receptor tyrosine kinase Ret. Expression of the GFRalpha3 receptor is primarily restricted to the peripheral nervous system and is found in a subpopulation of nociceptive sensory neurons of the dorsal root ganglia (DRGs) that coexpress the Ret and TrkA receptor tyrosine kinases and the thermosensitive channel TRPV1. To determine how artemin affects sensory neuron properties, transgenic mice that overexpress artemin in skin keratinocytes (ART-OE mice) were analyzed. Expression of artemin caused a 20.5% increase in DRG neuron number and increased the level of mRNA encoding GFRalpha3, TrkA, TRPV1, and the putative noxious cold-detecting channel TRPA1. Nearly all GFRalpha3-positive neurons expressed TRPV1 immunoreactivity, and most of these neurons were also positive for TRPA1. Interestingly, acid-sensing ion channel (ASIC) 1, 2a, 2b, and 3 mRNAs were decreased in the DRG, and this reduction was strongest in females. Analysis of sensory neuron physiological properties using an ex vivo preparation showed that cutaneous C-fiber nociceptors of ART-OE mice had reduced heat thresholds and increased firing rates in response to a heat ramp. No change in mechanical threshold was detected. Behavioral testing of ART-OE mice showed that they had increased sensitivity to both heat and noxious cold. These results indicate that the level of artemin in the skin modulates gene expression and response properties of afferents that project to the skin and that these changes lead to behavioral sensitivity to both hot and cold stimuli.

Glial cell line-derived neurotrophic factor family members sensitize nociceptors in vitro and produce thermal hyperalgesia in vivo

Nerve growth factor (NGF) has been implicated as an effector of inflammatory pain because it sensitizes primary afferents to noxious thermal, mechanical, and chemical [e.g., capsaicin, a transient receptor potential vanilloid receptor 1 (TRPV1) agonist] stimuli and because NGF levels increase during inflammation. Here, we report the ability of glial cell line-derived neurotrophic factor (GDNF) family members artemin, neurturin and GDNF to potentiate TRPV1 signaling and to induce behavioral hyperalgesia. Analysis of capsaicin-evoked Ca2+ transients in dissociated mouse dorsal root ganglion (DRG) neurons revealed that a 7 min exposure to GDNF, neurturin, or artemin potentiated TRPV1 function at doses 10-100 times lower than NGF. Moreover, GDNF family members induced capsaicin responses in a subset of neurons that were previously insensitive to capsaicin. Using reverse transcriptase-PCR, we found that artemin mRNA was profoundly upregulated in response to inflammation induced by hindpaw injection of complete Freund's adjuvant (CFA): artemin expression increased 10-fold 1 d after CFA injection, whereas NGF expression doubled by day 7. No increase was seen in neurturin or GDNF. A corresponding increase in mRNA for the artemin coreceptor GFRalpha3 (for GDNF family receptor alpha) was seen in DRG, and GFRalpha3 immunoreactivity was widely colocalized with TRPV1 in epidermal afferents. Finally, hindpaw injection of artemin, neurturin, GDNF, or NGF produced acute thermal hyperalgesia that lasted up to 4 h; combined injection of artemin and NGF produced hyperalgesia that lasted for 6 d. These results indicate that GDNF family members regulate the sensitivity of thermal nociceptors and implicate artemin in particular as an important effector in inflammatory hyperalgesia.

Impaired inflammatory pain and thermal hyperalgesia in mice expressing neuron-specific dominant negative mitogen activated protein kinase kinase (MEK)

BACKGROUND

Numerous studies have implicated spinal extracellular signal-regulated kinases (ERKs) as mediators of nociceptive plasticity. These studies have utilized pharmacological inhibition of MEK to demonstrate a role for ERK signaling in pain, but this approach cannot distinguish between effects of ERK in neuronal and non-neuronal cells. The present studies were undertaken to test the specific role of neuronal ERK in formalin-induced inflammatory pain. Dominant negative MEK (DN MEK) mutant mice in which MEK function is suppressed exclusively in neurons were tested in the formalin model of inflammatory pain.

RESULTS

Formalin-induced second phase spontaneous pain behaviors as well as thermal hyperalgesia measured 1 - 3 hours post-formalin were significantly reduced in the DN MEK mice when compared to their wild type littermate controls. In addition, spinal ERK phosphorylation following formalin injection was significantly reduced in the DN MEK mice. This was not due to a reduction of the number of unmyelinated fibers in the periphery, since these were almost double the number observed in wild type controls. Further examination of the effects of suppression of MEK function on a downstream target of ERK phosphorylation, the A-type potassium channel, showed that the ERK-dependent modulation of the A-type currents is significantly reduced in neurons from DN MEK mice compared to littermate wild type controls.

CONCLUSION

Our results demonstrate that the neuronal MEK-ERK pathway is indeed an important intracellular cascade that is associated with formalin-induced inflammatory pain and thermal hyperalgesia.

Nerve growth factor mediates peripheral mechanical hypersensitivity that accompanies experimental cystitis in mice

Increased sensitivity to somatic stimuli has been noted in the presence of visceral inflammation. Cystitis was induced by intraperitoneal injection of cyclophosphamide (CYP) in female mice. Sensitivity of hind paws to mechanical stimuli was determined prior to and 4, 9 and 24 h after CYP, and sensitivity of the tail to thermal stimuli was determined prior to, 4 and 24 h after CYP treatment. To investigate the role of nerve growth factor (NGF) in these processes, other groups of mice received NGF antiserum, normal serum, or K252a intravenously 30 min after CYP administration. CYP induced bladder inflammation that was not ablated by treatment with NGF antiserum or K252a. Sensitivity to mechanical stimuli was increased 4 and 9 h after CYP administration. This was reversed by NGF antiserum or K252a but not by normal serum. After 24 h, no differences were observed in withdrawal threshold among groups. None of the treatments had any effect on sensitivity to thermal stimuli. To further investigate the role of NGF in this process, NGF was instilled into the bladders of mice in the presence or absence of intravenous NGF antiserum. Four hours after intravesical instillation of NGF, the threshold of the hind paws to mechanical stimulation was significantly decreased, and this effect was reversed by prior treatment with NGF antiserum. This model of visceral pain causes increased sensitivity to peripheral application of mechanical stimuli. This effect is at least partially mediated by NGF, and the bladder may be the source of NGF in this process.

Enhanced thermal avoidance in mice lacking the ATP receptor P2X3

P2X3 is an ATP-gated cation channel subtype expressed by a subpopulation of primary sensory neurons. In vivo spinal cord recordings in mice lacking P2X3 (P2X3-/-) have suggested that this protein may be important for the coding of peripheral warm stimuli. To explore this possibility more thoroughly, we examined behavioral and electrophysiological responses to thermal stimuli in P2X3-/- mice. As previously reported, recording from the spinal cord dorsal horn of anesthetized P2X3-/- mice revealed a blunted response of wide dynamic range neurons to hind paw heating. When placed in a thermal gradient, however, P2X3-/- mice exhibited an unexpectedly enhanced avoidance of both hot and cold temperatures, relative to controls. In the tail immersion test, mutant mice exhibited shorter withdrawal latencies at temperatures above and below thermoneutrality. Consistent with these changes, P2X3-/- mice exhibited enhanced induction of spinal cord c-FOS following hind paw heating to 45 degrees C. Thus, gain- and loss-of-function thermosensory phenotypes coexist in P2X3-/- mice. No changes in thermal preference were observed in wild-type mice injected subcutaneously with the P2X3 antagonist, A317491 or intrathecally with the P2X3 and P2X1 antagonist TNP-ATP. The reason for this apparent discrepancy is unclear, but we cannot exclude the possibility that compensatory events contribute, at least in part, to the P2X3-/- phenotype. Regardless, this study illustrates the utility of thermal preference assays as part of a comprehensive approach to the analysis of mouse thermosensation.

Overexpression of NGF or GDNF alters transcriptional plasticity evoked by inflammation

Transcriptional changes evoked in nociceptive sensory neurons by inflammatory injury play a substantial role in the generation of and recovery from painful hypersensitivity. Transgenic mice overexpressing nerve growth factor (NGF) or glial cell line-derived neurotrophic factor (GDNF) in the skin possess a greatly increased number of nociceptors. Surprisingly, NGF-overexpressers display reduced hypersensitivity and recovered more rapidly in response to inflammation, suggesting a compensatory suppression of nociceptive transmission in these mice. To determine whether these transgenic mice show changes in inflammation-evoked transcriptional plasticity, we examined the expression of a panel of genes implicated in nociceptive signaling in response to injection of complete Freund's adjuvant into the hindpaw. Relative mRNA levels were quantified 1, 4 and 15 days after injection using real-time PCR. In wild type mice CFA injection elicited a reproducible pattern of altered gene expression that returned to baseline over a 2-week period. In mice overexpressing NGF or GDNF the expression patterns for several genes were substantially altered; these changes in injury-evoked patterns of gene expression suggest the existence of endogenous regulatory mechanisms that can compensate for increased nociceptive input by modulating the expression of a limited subset of genes.

Differential Sensitivity of N- and P/Q-Type Ca2+ Channel Currents to a μ Opioid in Isolectin B -Positive and -Negative Dorsal Root Ganglion Neurons

Opioids have a selective effect on nociception with little effect on other sensory modalities. However, the cellular mechanisms for this preferential effect are not fully known. Two broad classes of nociceptors can be distinguished based on their growth factor requirements and binding to isolectin B4(IB4). In this study, we determined the difference in the modulation of voltage-gated Ca2+ currents by [D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin (DAMGO, a specific mu opioid agonist) between IB4-positive and -negative small dorsal root ganglion (DRG) neurons. Whole-cell voltage-clamp recordings were performed in acutely isolated DRG neurons in adult rats. Both 1-10 microM DAMGO and 1 to 10 microM morphine had a greater effect on high voltage-activated Ca2+ currents in IB4-negative than IB4-positive cells. However, DAMGO had no significant effect on T-type Ca2+ currents in both groups. The N-type Ca2+ current was the major subtype of Ca2+ currents inhibited by DAMGO in both IB4-positive and -negative neurons. Although DAMGO had no effect on L-type and R-type Ca2+ currents in both groups, it produced a larger inhibition on N-type and P/Q-type Ca2+ currents in IB4-negative than IB4-positive neurons. Furthermore, double labeling revealed that there was a significantly higher mu opioid receptor immunoreactivity in IB4-negative than IB4-positive cells. Thus, these data suggest that N-and P/Q-type Ca2+ currents are more sensitive to inhibition by the mu opioids in IB4-negative than IB4-positive DRG neurons. The differential sensitivity of voltage-gated Ca2+ channels to the mu opioids in subsets of DRG neurons may constitute an important analgesic mechanism of mu opioids.