Metabotropic glutamate mGlu1 receptor mRNA expression in dorsal root ganglia of rats after peripheral nerve injury

Laminin-γ1 chain and stress inducible protein 1 synergistically mediate PrPC-dependent axonal growth via Ca2+ mobilization in dorsal root ganglia neurons

Prion protein (PrP(C)) is a cell surface glycoprotein that is abundantly expressed in nervous system. The elucidation of the PrP(C) interactome network and its significance on neural physiology is crucial to understanding neurodegenerative events associated with prion and Alzheimer's diseases. PrP(C) co-opts stress inducible protein 1/alpha7 nicotinic acetylcholine receptor (STI1/α7nAChR) or laminin/Type I metabotropic glutamate receptors (mGluR1/5) to modulate hippocampal neuronal survival and differentiation. However, potential cross-talk between these protein complexes and their role in peripheral neurons has never been addressed. To explore this issue, we investigated PrP(C)-mediated axonogenesis in peripheral neurons in response to STI1 and laminin-γ1 chain-derived peptide (Ln-γ1). STI1 and Ln-γ1 promoted robust axonogenesis in wild-type neurons, whereas no effect was observed in neurons from PrP(C) -null mice. PrP(C) binding to Ln-γ1 or STI1 led to an increase in intracellular Ca(2+) levels via distinct mechanisms: STI1 promoted extracellular Ca(2+) influx, and Ln-γ1 released calcium from intracellular stores. Both effects depend on phospholipase C activation, which is modulated by mGluR1/5 for Ln-γ1, but depends on, C-type transient receptor potential (TRPC) channels rather than α7nAChR for STI1. Treatment of neurons with suboptimal concentrations of both ligands led to synergistic actions on PrP(C)-mediated calcium response and axonogenesis. This effect was likely mediated by simultaneous binding of the two ligands to PrP(C). These results suggest a role for PrP(C) as an organizer of diverse multiprotein complexes, triggering specific signaling pathways and promoting axonogenesis in the peripheral nervous system.

Differential regulation of glutamate receptors in trigeminal ganglia following masseter inflammation

The present study examined whether N-methyl-D-aspartate receptor (NMDAR), 5-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) subunits and group I metabotropic glutamate receptors (mGluRs) are constitutively expressed in trigeminal ganglia (TG) using Western blot analysis in male Sprague-Dawley rats. We then investigated whether experimental induction of masseter inflammation influences glutamate receptor expressions by comparing the protein levels from naïve rats to those from complete Freund's adjuvant (CFA) inflamed rats. Our results showed that NMDAR1 (NR1), NMDAR2A (NR2A), NMDAR2B (NR2B), AMPAR1 (GluR1) and AMPAR2 (GluR2) subunits, and group I metabotropic glutamate receptor, mGluR5, are constitutively expressed in TG. Masseter inflammation significantly down-regulated NR1 subunit expression that persisted to 7 days post-CFA inflammation. NR2A and NR2B expressions were not significantly changed. GluR1 receptor subunit expression was slightly increased in TG 3 days after CFA-induced inflammation, but the change was not statistically significant. GluR2 protein level was not affected by CFA inflammation. The level of mGluR5 protein was significantly up-regulated in TG 3 days after CFA-induced masseter inflammation. There were no inflammation-induced changes in any of the proteins we analyzed in the contralateral, non-inflamed TG. These results suggested that muscle inflammation differentially modulates glutamate receptor subunits at the primary afferent level in male rats and that these inflammation-induced transcriptional changes may contribute to functionally different aspects of craniofacial muscle pain.

Colocalization of metabotropic glutamate receptors in rat dorsal root ganglion cells

Glutamate is the main excitatory transmitter in both central and peripheral nervous systems. Discovery of metabotropic glutamate receptors (mGluRs) made it clear that glutamate can have excitatory or inhibitory effects on neuronal function, with group I mGluRs enhancing cell excitability but group II and III mGluRs decreasing excitability. The present study investigated the colocalization of mGluR subtypes representing groups I, II, or III in rat L5 dorsal root ganglion (DRG) cells. The analyses show that group III has the highest expression, with 75.0% of DRG cells expressing mGluR8, followed by group II, with 51.6% expressing mGluR2/3, followed by group I, with only 6.8% expressing mGluR1alpha. mGluR8 is expressed by small, medium, and large diameter cells. In contrast, mGluR1alpha and mGluR2/3 are expressed by mainly small and medium cells. Approximately half of cells expressing group I mGluR1alpha also express either group II mGluR2/3 or group III mGluR8. These mGluR1alpha double-labeled populations are not likely to overlap since >1.0% of mGluR1alpha are triple-labeled. As expected from the high percentage of single-labeled mGluR2/3 and mGluR8 cells, there is a considerable population of double-labeled cells with approximately 30% of each population expressing both receptors. Due to the fact that the number of mGluR1alpha-expressing cells in the DRG is low, the percentage of triple-labeled cells is also low ( approximately 1-2%). The prevalence of groups II and III indicate that glutamate could have a substantial inhibitory effect of primary afferent function, reducing and/or fine-tuning sensory input before transmission to the spinal cord. These anatomical data highlight the potential inhibitory role glutamate may play in peripheral sensory transmission.

The peripheral role of group I metabotropic glutamate receptors on nociceptive behaviors in rats with knee joint inflammation

We examined whether the mGluR1 and mGluR5 were involved in development and maintenance of behavioral signs of non-evoked pain and secondary mechanical hyperalgesia induced by knee joint inflammation. Selective mGluR1 antagonist, (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA: 50, 100, 200 microM/25 microl, n=10 per group) and selective mGluR5 antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP: 50, 100, 200 nM/25 microl, n=10 per group) was intra-articularly (i.a.) injected 30 min before and 4h after carrageenan injection and behavioral tests were conducted. In the pre-treatment, only a higher dose (200 nM) of MPEP significantly prevented the magnitude of weight load reduction, whereas AIDA (200 microM) and MEPE (50, 100 and 200 nM) significantly reduced the development of mechanical hyperalgesia compared to saline treated group. In the post-treatment, AIDA (200 microM) and MPEP at 100 and 200 nM partially reversed the reduction of weight load induced by carrageenan. MPEP significantly increased the withdrawal threshold to mechanical stimulation in a dose-dependent manner, whereas AIDA had significantly reversed the decreased the paw withdrawal threshold only at 200 microM. The present study demonstrated that i.a. MPEP, selective mGluR5 antagonist is more effective than selective mGluR1 antagonist, AIDA on non-evoked pain as well as mechanical hyperalgesia in both induction and maintenance phase in knee joint inflammation. It is suggested that peripheral mGlu5 receptors play a more prominent role in inflammatory pain including evoked and spontaneous pain. Thus, selective mGluR5 antagonist could be effective therapeutic tools in clinical setting.

The antinociceptive and anxiolytic-like effects of the metabotropic glutamate receptor 5 (mGluR5) antagonists, MPEP and MTEP, and the mGluR1 antagonist, LY456236, in rodents: a comparison of efficacy and side-effect profiles

RATIONALE

Modulation of metabotropic glutamate receptor (mGluR) subtypes represents a novel approach for the treatment of neurological and psychiatric disorders.

OBJECTIVES

This study was conducted to investigate the role of the mGluR5 and mGluR1 subtypes in the modulation of pain and anxiety.

METHODS

The mGluR5 antagonists, 2-methyl-6-(phenylethynyl)pyridine (MPEP) and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP), and the mGluR1 antagonist, (4-methoxy-phenyl)-(6-methoxy-quinazolin-4-yl)-amine HCl (LY456236), were tested in models of pain [mouse formalin test, rat spinal nerve ligation (SNL)] and anxiety [Vogel conflict, conditioned lick suppression (CLS)], and their efficacious effects were compared to any associated side effects.

RESULTS

The systemic administration of MPEP, MTEP, and LY456236 reduced hyperalgesia induced by formalin and mechanical allodynia following SNL. However, only LY456236 completely reversed the allodynia. In the anxiety models, MPEP (3--30 mg/kg), MTEP (3--10 mg/kg), and LY456236 (10--30 mg/kg) produced anxiolytic-like effects similar to the benzodiazepine, chlordiazepoxide (CDP, 6 mg/kg). However, only MPEP and MTEP were able to produce a level of anxiolysis comparable to CDP. In a series of tests examining potential side effects, MPEP and MTEP reduced body temperature and locomotor activity and impaired operant responding for food and rotarod performance at doses of 3--30 and 1--30 mg/kg, respectively. LY456236 reduced operant responding at 30 mg/kg.

CONCLUSION

Both mGluR5 and mGluR1 antagonists are effective in models of pain and anxiety. However, an mGluR1 antagonist was more efficacious than the two mGluR5 antagonists in the pain models, which, conversely, appeared more efficacious in the anxiety models. These findings support the potential utility of mGluR5 and mGluR1 antagonists for both the treatment of chronic pain and as novel anxiolytics.

Pharmacological sensitivity and gene expression analysis of the tibial nerve injury model of neuropathic pain

The tibial nerve injury model is a novel, surgically uncomplicated, rat model of neuropathic pain based on a unilateral transection (neurotomy) of the tibial branch of the sciatic nerve. The aim of the present study was to describe some behavioral and molecular features of the model, and to test its sensitivity to a number of drugs which are currently used for the treatment of neuropathic pain. The model was characterized by a pronounced mechanical allodynia which was present in all subjects and a less robust thermal hyperalgesia. Mechanical allodynia developed within 2 weeks post-surgery and was reliably present for at least 9 weeks. Neurotomized rats showed no autotomy and their body weight developed normally. Gene expression in ipsilateral L5 dorsal root ganglia, analyzed by quantitative polymerase chain reaction (PCR), showed a pronounced up-regulation of galanin and vasointestinal peptide (VIP). This up-regulation developed rapidly (within 1 to 2 days following neurotomy) and remained present for at least 12 days. On the other hand, expression of calcitonin gene-related peptide (CGRP) and substance P mRNA was down-regulated 12 days following neurotomy. Mechanical allodynia was completely reversed by morphine [minimal effective dose (MED): 8 mg/kg, i.p.] and partially reversed by carbamazepine (MED: 64 mg/kg, i.p.), baclofen (MED: 3 mg/kg, i.p.) and amitriptyline (trend for efficacy at 32 mg/kg, i.p.), but not by gabapentin (50-100 mg/kg, i.p.). The finding that the tibial nerve injury model shows a robust and persistent mechanical allodynia which is sensitive to a number of established analgesics, as well as a gene expression profile which is compatible with that obtained in other models of neuropathic pain, further supports its validity as a reliable and surgically uncomplicated model for the study of neuropathic pain.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.