Immunohistochemical localization of a metabotropic glutamate receptor, mGluR7, in ganglion neurons of the rat; with special reference to the presence in glutamatergic ganglion neurons

GRM7 gene mutations and consequences for neurodevelopment

The metabotropic glutamate receptor 7 (mGlu7), encoded by the GRM7 gene in humans, is a presynaptic, G protein-coupled glutamate receptor that is essential for modulating neurotransmission. Mutations in or reduced expression of GRM7 have been identified in different genetic neurodevelopmental disorders (NDDs), and rare biallelic missense variants have been proposed to underlie a subset of NDDs. Clinical GRM7 variants have been associated with a range of symptoms consistent with neurodevelopmental molecular features, including hypomyelination, brain atrophy and defects in axon outgrowth. Here, we review the newest findings regarding the cellular and molecular defects caused by GRM7 variants in NDD patients.

Emerging Trends in Pain Modulation by Metabotropic Glutamate Receptors

Pain is an essential protective mechanism meant to prevent tissue damages in organisms. On the other hand, chronic or persistent pain caused, for example, by inflammation or nerve injury is long lasting and responsible for long-term disability in patients. Therefore, chronic pain and its management represents a major public health problem. Hence, it is critical to better understand chronic pain molecular mechanisms to develop innovative and efficient drugs. Over the past decades, accumulating evidence has demonstrated a pivotal role of glutamate in pain sensation and transmission, supporting glutamate receptors as promising potential targets for pain relieving drug development. Glutamate is the most abundant excitatory neurotransmitter in the brain. Once released into the synapse, glutamate acts through ionotropic glutamate receptors (iGluRs), which are ligand-gated ion channels triggering fast excitatory neurotransmission, and metabotropic glutamate receptors (mGluRs), which are G protein-coupled receptors modulating synaptic transmission. Eight mGluRs subtypes have been identified and are divided into three classes based on their sequence similarities and their pharmacological and biochemical properties. Of note, all mGluR subtypes (except mGlu6 receptor) are expressed within the nociceptive pathways where they modulate pain transmission. This review will address the role of mGluRs in acute and persistent pain processing and emerging pharmacotherapies for pain management.

Glutamate and Its Receptors as Therapeutic Targets for Migraine

There is substantial evidence indicating a role for glutamate in migraine. Levels of glutamate are higher in the brain and possibly also in the peripheral circulation in migraine patients, particularly during attacks. Altered blood levels of kynurenines, endogenous modulators of glutamate receptors, have been reported in migraine patients. Population genetic studies implicate genes that are involved with glutamate signaling in migraine, and gene mutations responsible for familial hemiplegic migraine and other familial migraine syndromes may influence glutamate signaling. Animal studies indicate that glutamate plays a key role in pain transmission, central sensitization, and cortical spreading depression. Multiple therapies that target glutamate receptors including magnesium, topiramate, memantine, and ketamine have been reported to have efficacy in the treatment of migraine, although with the exception of topiramate, the evidence for the efficacy of these therapies is not strong. Also, because all of these therapies have other mechanisms of action, it is not possible to conclude that the efficacy of these drugs is entirely due to their effects on glutamate receptors. Further studies are needed to more clearly delineate the possible roles of glutamate and its specific receptor subtypes in migraine and to identify new ways of targeting glutamate for migraine therapy.

The Glutamatergic System in Primary Somatosensory Neurons and Its Involvement in Sensory Input-Dependent Plasticity

Glutamate is the most common neurotransmitter in both the central and the peripheral nervous system. Glutamate is present in all types of neurons in sensory ganglia, and is released not only from their peripheral and central axon terminals but also from their cell bodies. Consistently, these neurons express ionotropic and metabotropic receptors, as well as other molecules involved in the synthesis, transport and release of the neurotransmitter. Primary sensory neurons are the first neurons in the sensory channels, which receive information from the periphery, and are thus key players in the sensory transduction and in the transmission of this information to higher centers in the pathway. These neurons are tightly enclosed by satellite glial cells, which also express several ionotropic and metabotropic glutamate receptors, and display increases in intracellular calcium accompanying the release of glutamate. One of the main interests in our group has been the study of the implication of the peripheral nervous system in sensory-dependent plasticity. Recently, we have provided novel evidence in favor of morphological changes in first- and second-order neurons of the trigeminal system after sustained alterations of the sensory input. Moreover, these anatomical changes are paralleled by several molecular changes, among which those related to glutamatergic neurotransmission are particularly relevant. In this review, we will describe the state of the art of the glutamatergic system in sensory ganglia and its involvement in input-dependent plasticity, a fundamental ground for advancing our knowledge of the neural mechanisms of learning and adaptation, reaction to injury, and chronic pain.

Investigating the expression of metabotropic glutamate receptors in trigeminal ganglion neurons and satellite glial cells: implications for craniofacial pain

CONTEXT/OBJECTIVE

Previous studies have demonstrated that various subtypes of the metabotropic glutamate receptors (mGluRs) are expressed in the dorsal root ganglion (DRG) of the peripheral nervous system (PNS), implicating that glutamate potentially contributes to sensory transmission through these receptors. While mGluR expression has been investigated largely in the DRG, the present study focused on mGluR expression on neurons and satellite glial cells (SGCs) of the trigeminal ganglion (TG).

MATERIALS AND METHODS

To address the presence of mGluRs in rat TG neurons and their corresponding SGCs, the trigeminal ganglia from six adult male Wistar rats were isolated and immunohistochemistry and immunocytochemistry were performed. The expression of mGluR1α-, mGluR2/3- and mGluR8 on TG neurons and SGCs was investigated in tissue slices and isolated cells.

RESULTS

35.1 ± 6.0% of the TG neurons were positive for mGluR1α, whereas 39.9 ± 7.7% and 55.5 ± 6.3% were positive for mGluR2/3 and mGluR8, respectively. Immunoreactive neurons expressing mGluRs were mainly medium- to large sized, with a smaller population of small-sized neurons showing immunoreactivity. The SGCs showed immunoreactivity toward mGluR1α and mGluR8, but not mGluR2/3, both in the tissue and in isolated cells.

CONCLUSIONS

Findings from the present study showed that trigeminal neurons express mGluR1α, mGluR2/3 and mGluR8, while SGCs only express mGluR1α and mGluR8. This novel evidence may advance investigations on a possible role of mGluRs in relation to trigeminal pain transmission within the craniofacial region.

Peripheral nerve injury decreases the expression of metabolic glutamate receptor 7 in dorsal root ganglion neurons

Group II and III metabolic glutamate receptors (mGluRs) are responsible for the glutamate-mediated postsynaptic excitation of neurons. Previous pharmacological evidences show that activation of mGluR7 could inhibit nociceptive reception. However, the distribution and expression patterns of mGluR7 after peripheral injury remain unclear. Herein we found that mGluR7 was expressed in the rat peptidergic dorsal root ganglion (DRG) neurons and large neurons, but rarely in isolectin B4 positive neurons. Sciatic nerve ligation experiment showed that mGluR7 was anterogradely transported from cell body to the peripheral site. Furthermore, after peripheral nerve injury, mGluR7 expression was down-regulated in both peptidergic and large DRG neurons. Our work suggests that mGluR7 might be involved in the regulation of pathological pain after peripheral nerve injury.

Potential mechanisms for hypoalgesia induced by anti-nerve growth factor immunoglobulin are identified using autoimmune nerve growth factor deprivation

Nerve growth factor (NGF) antagonism has long been proposed as a chronic pain treatment. In 2010, the FDA suspended clinical trials using tanezumab, a humanized monoclonal anti-NGF antibody, to treat osteoarthritis due to worsening joint damage in 16 patients. Increased physical activity in the absence of acute pain which normally prevents self-harm was purported as a potential cause. Such an adverse effect is consistent with an extension of tanezumab's primary mechanism of action by decreasing pain sensitivity below baseline levels. In animal inflammatory pain models, NGF antagonism decreases intraepidermal nerve fiber (IENF) density and attenuates increases in expression of nociception-related proteins, such as calcitonin gene-related peptide (CGRP) and substance P (SP). Little is known of the effects of NGF antagonism in noninflamed animals and the hypoalgesia that ensues. In the current study, we immunized rats with NGF or cytochrome C (cytC) and examined (1) nocifensive behaviors with thermal latencies, mechanical thresholds, the hot plate test, and the tail flick test, (2) IENF density, and (3) expression of CGRP, SP, voltage-gated sodium channel 1.8 (Nav1.8), and glutaminase in subpopulations of dorsal root ganglion (DRG) neurons separated by size and isolectin B4 (IB4) labeling. Rats with high anti-NGF titers had delayed responses on the hot plate test but no other behavioral abnormalities. Delayed hot plate responses correlated with lower IENF density. CGRP and SP expression was decreased principally in medium (400-800 μm(2)) and small neurons (<400 μm(2)), respectively, regardless of IB4 labeling. Expression of Nav1.8 was only decreased in small and medium IB4 negative neurons. NGF immunization appears to result in a more profound antagonism of NGF than tanezumab therapy, but we hypothesize that decreases in IENF density and nociception-related protein expression are potential mechanisms for tanezumab-induced hypoalgesia.

Preserved acute pain and impaired neuropathic pain in mice lacking protein interacting with C Kinase 1

Protein interacting with C Kinase 1 (PICK1), a PDZ domain-containing scaffolding protein, interacts with multiple different proteins in the mammalian nervous system and is believed to play important roles in diverse physiological and pathological conditions. In this study, we report that PICK1 is expressed in neurons of the dorsal root ganglion (DRG) and spinal cord dorsal horn, two major pain-related regions. PICK1 was present in approximately 29.7% of DRG neurons, most of which were small-less than 750 μm² in cross-sectional area. Some of these PICK1-positive cells co-labeled with isolectin B4 or calcitonin-gene-related peptide. In the dorsal horn, PICK1 immunoreactivity was concentrated in the superficial dorsal horn, where it was prominent in the postsynaptic density, axons, and dendrites. Targeted disruption of PICK1 gene did not affect basal paw withdrawal responses to acute noxious thermal and mechanical stimuli or locomotor reflex activity, but it completely blocked the induction of peripheral nerve injury-induced mechanical and thermal pain hypersensitivities. PICK1 appears to be required for peripheral nerve injury-induced neuropathic pain development and to be a potential biochemical target for treating this disorder.

Glutamate pharmacology and metabolism in peripheral primary afferents: physiological and pathophysiological mechanisms

In addition to using glutamate as a neurotransmitter at central synapses, many primary sensory neurons release glutamate from peripheral terminals. Primary sensory neurons with cell bodies in dorsal root or trigeminal ganglia produce glutaminase, the synthetic enzyme for glutamate, and transport the enzyme in mitochondria to peripheral terminals. Vesicular glutamate transporters fill neurotransmitter vesicles with glutamate and they are shipped to peripheral terminals. Intense noxious stimuli or tissue damage causes glutamate to be released from peripheral afferent nerve terminals and augmented release occurs during acute and chronic inflammation. The site of action for glutamate can be at the autologous or nearby nerve terminals. Peripheral nerve terminals contain both ionotropic and metabotropic excitatory amino acid receptors (EAARs) and activation of these receptors can lower the activation threshold and increase the excitability of primary afferents. Antagonism of EAARs can reduce excitability of activated afferents and produce antinociception in many animal models of acute and chronic pain. Glutamate injected into human skin and muscle causes acute pain. Trauma in humans, such as arthritis, myalgia, and tendonitis, elevates glutamate levels in affected tissues. There is evidence that EAAR antagonism at peripheral sites can provide relief in some chronic pain sufferers.

Fixative composition alters distributions of immunoreactivity for glutaminase and two markers of nociceptive neurons, Nav1.8 and TRPV1, in the rat dorsal root ganglion

Most, if not all, dorsal root ganglion (DRG) neurons use the neurotransmitter glutamate. There are, however, conflicting reports of the percentages of DRG neurons that express glutaminase (GLS), the enzyme that synthesizes glutamate, ranging from 30% to 100% of DRG neurons. Defining DRG neuron populations by the expression of proteins like GLS, which indicates function, is routinely accomplished with immunolabeling techniques. Proper characterization of DRG neuron populations relies on accurate detection of such antigens. It is known intuitively that fixation can alter immunoreactivity (IR). In this study, we compared the effects of five formaldehyde concentrations between 0.25% and 4.0% (w/v) and five picric acid concentrations between 0.0% and 0.8% (w/v) on the IR of GLS, the voltage-gated sodium channel 1.8 (Na(v)1.8), and the capsaicin receptor TRPV1. We also compared the effects of five incubation time lengths from 2 to 192 hr, in primary antiserum on IR. Lowering formaldehyde concentration elevated IR for all three antigens, while raising picric acid concentration increased Na(v)1.8 and TRPV1 IR. Increasing IR improved detection sensitivity, which led to higher percentages of labeled DRG neurons. By selecting fixation conditions that optimized IR, we found that all DRG neurons express GLS, 69% of neurons express Na(v)1.8, and 77% of neurons express TRPV1, indicating that some previous studies may have underestimated the percentages of DRG neurons expressing these proteins. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

Anatomy and function of group III metabotropic glutamate receptors in gastric vagal pathways

Metabotropic glutamate receptors (mGluR) are classified into groups I (excitatory), II and III (inhibitory) mGluR. Activation of peripheral group III mGluR (mGluR4, mGluR6, mGluR7, mGluR8), particularly mGluR8, inhibits vagal afferent mechanosensitivity in vitro which translates into reduced triggering of transient lower oesophageal sphincter relaxations and gastroesophageal reflux in vivo. However, the expression and function of group III mGluR in central gastrointestinal vagal reflex pathways is not known. Here we assessed the expression of group III mGluR in identified gastric vagal afferents in the nodose ganglion (NG) and in the dorsal medulla. We also determined the central action of the mGluR8a agonist S-3,4-DCPG (DCPG) on nucleus tractus solitarius (NTS) neurons with gastric mechanosensory input in vivo. Labelling for mGluR4 and mGluR8 was abundant in gastric vagal afferents in the NG, at their termination site in the NTS (subnucleus gelatinosus) and in gastric vagal motorneurons, while labelling for mGluR6 and mGluR7 was weaker in these regions. DCPG (0.1 nmol or 0.001-10 nmol i.c.v.) inhibited or markedly attenuated responses of 8/10 NTS neurons excited by isobaric gastric distension with no effect on blood pressure or respiration; 2 NTS neurons were unaffected. The effects of DCPG were significantly reversed by the group III mGluR antagonist MAP4 (10 nmol, i.c.v.). In contrast, 4/4 NTS neurons inhibited by gastric distension were unaffected by DCPG. We conclude that group III mGluR are expressed in peripheral and central vagal pathways, and that mGluR8 within the NTS selectively reduce excitatory transmission along gastric vagal pathways.

Glutamate receptors and pain

Pain is an important survival and protection mechanism for animals. However, chronic/persistent pain may be differentiated from normal physiological pain in that it confers no obvious advantage. An accumulating body of pharmacological, electrophysiological, and behavioral evidence is emerging in support of the notion that glutamate receptors play a crucial role in pain pathways and that modulation of glutamate receptors may have potential for therapeutic utility in several categories of persistent pain, including neuropathic pain resulting from injury and/or disease of central (e.g., spinal cord injury) or peripheral nerves (e.g., diabetic neuropathy, radiculopathy) and inflammatory or joint-related pain (e.g., rheumatoid arthritis, osteoarthritis). This review focuses on the role of glutamate receptors, including both ionotropic (AMPA, NMDA and kainate) and metabotropic (mGlu1-8) receptors in persistent pain states with particular emphasis on their expression patterns in nociceptive pathways and their potential as targets for pharmacological intervention strategies.

Vagal afferent neurons projecting to the stomach and small intestine exhibit multiple N-methyl-D-aspartate receptor subunit phenotypes

Previous reports suggest that NMDA receptors participate in control of food intake via vagal afferent neurons that innervate the upper gastrointestinal (GI) tract. While messenger RNA coding for the NR1 NMDA receptor subunit is present in a majority of vagal afferent neurons of nodose ganglia (NG), immunoreactivity for other NMDA receptor subunits (NR2B, NR2C and NR2D) are expressed in more limited subpopulations of vagal afferents. To determine whether vagal afferent neurons that project to the stomach or duodenum exhibit distinct NMDA receptor subunit phenotypes, we examined immunoreactivity (IR) for NMDA receptor NR1, NR2B, NR2C and NR2D subunits in NG neurons that were labeled by injections of the retrograde tracer Fast Blue (FB) into the wall of the stomach or duodenum. FB injections into the fundus or corpus of the stomach labeled comparable numbers of neurons in both the left and right NG, while proximal duodenal injections labeled only neurons of left NG. NR1-IR expression was observed in most neurons innervating the upper GI tract (fundus, 97%; corpus, 95%; duodenum, 98%). Likewise, most neurons that innervated the upper GI tract expressed NR2B-IR (fundus, 98%; corpus, 85%; duodenum, 81%). NR2C-IR was observed in only 52%, 46% and 32% of FB-positive neurons projecting to the fundus, corpus or duodenum respectively, while NR2D-IR occurred in an even more restricted FB-labeled subpopulation (fundus, 13%; corpus, 26%; and duodenum, 18%). Our observations indicate that different subpopulations of vagal afferents express distinct NMDA receptor subunit phenotypes. However, the neuronal distribution of NMDA receptor subunits is not correlated with innervation of either the stomach or duodenum.

N-methyl-D-aspartate receptor subunit phenotypes of vagal afferent neurons in nodose ganglia of the rat

Most vagal afferent neurons in rat nodose ganglia express mRNA coding for the NR1 subunit of the heteromeric N-methyl-D-aspartate (NMDA) receptor ion channel. NMDA receptor subunit immunoreactivity has been detected on axon terminals of vagal afferents in the dorsal hindbrain, suggesting a role for presynaptic NMDA receptors in viscerosensory function. Although NMDA receptor subunits (NR1, NR2B, NR2C, and NR2D) have been linked to distinct neuronal populations in the brain, the NMDA receptor subunit phenotype of vagal afferent neurons has not been determined. Therefore, we examined NMDA receptor subunit (NR1, NR2B, NR2C, and NR2D) immunoreactivity in vagal afferent neurons. We found that, although the left nodose contained significantly more neurons (7,603), than the right (5,978), the proportions of NMDA subunits expressed in the left and right nodose ganglia were not significantly different. Immunoreactivity for NMDA NR1 subunit was present in 92.3% of all nodose neurons. NR2B immunoreactivity was present in 56.7% of neurons; NR2C-expressing nodose neurons made up 49.4% of the total population; NR2D subunit immunoreactivity was observed in just 13.5% of all nodose neurons. Double labeling revealed that 30.2% of nodose neurons expressed immunoreactivity to both NR2B and NR2C, whereas NR2B and NR2D immunoreactivities were colocalized in 11.5% of nodose neurons. NR2C immunoreactivity colocalized with NR2D in 13.1% of nodose neurons. Our results indicate that most vagal afferent neurons express NMDA receptor ion channels composed of NR1, NR2B, and NR2C subunits and that a minority phenotype that expresses NR2D also expresses NR1, NR2B, and NR2C.

A part of cholinergic fibers in mouse superior cervical ganglia contain GABA or glutamate

The localizations and functions of glutamate and GABA, the major amino acid neurotransmitters in the central nervous system, are still unclear in the peripheral nervous system. We immunohistochemically double-stained mouse superior cervical ganglia with antibodies for the vesicular acetylcholine transporter (VAchT), GAD65, the vesicular glutamate transporters 1-3 (VGluTs1-3), the marker of the sympathetic preganglionic neuron (SPN), GABAergic, and glutamatergic terminals, respectively. All GAD65-positive terminals showed VAchT immunoreactivity, indicating that GABAergic fibers originate from SPNs. VGluT2-immunoreactive terminals showing colocalization with VAchT were observed, but VGluT1 and 3 immunoreactive terminals were not. Colocalization of GAD65 and VGluT2 was rarely found. All VGluT2-immunopositive terminals were also immunopositive for neuronal nitric oxide synthase (nNOS), a marker for the subpopulation of the SPNs, while about half of the GABA-immunopositive fibers were immunopositive for nNOS. The origin of these fibers was discussed.

Physiological and behavioral evidence for focal nociception induced by epidural glutamate infusion in rats

STUDY DESIGN

Blinded animal study.

OBJECTIVES

To determine if an increased concentration of epidural glutamate can cause a focal nociceptive response in the lower extremities that is consistent with sciatica.

SUMMARY OF BACKGROUND DATA

It is believed that the origin of sciatic pain is related to more than physical pressure on the nerve roots. Recently, it was determined that disc material may be a significant source of free glutamate, resulting from the enzymatic degradation of matrix aggrecan proteins. We believe that this free glutamate acts as a neurotransmitter at glutamate receptors on the dorsal root ganglion (DRG) cell bodies, thereby initiating a nociceptive response.

METHODS

Rats were subject to a 72-hour epidural glutamate infusion via a mini osmotic pump. Von Frey behavioral testing was performed 24 hours before, and 24 and 72 hours after the onset of the infusion. DRG and dorsal horn tissues were analyzed for changes in receptor expression, which have been previously shown to correlate with a nociceptive state.

RESULTS

Von Frey behavioral tests showed focal hyperalgesia that was maximal at the 0.02 mmol/L glutamate concentration. Significant changes in DRG glutamate receptor expression were seen for alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid, kainite, and N-methyl-D aspartate receptors. Analysis of dorsal horn glutamate receptors also showed patterns in alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid and kainate receptor expression that were consistent with a nociceptive state.

CONCLUSIONS

Epidural glutamate elicits a focal nociceptive response. Free glutamate that has been liberated from the disc material may be an important factor in the development of sciatic pain.

Intracerebroventricular administration of MK-801 increases food intake through mechanisms independent of gastric emptying

Systemic or hindbrain administration of MK-801, a noncompetitive N-methyl-D-aspartate receptor antagonist, increases meal size. To examine whether MK-801 enhances intake by increasing gastric emptying, we administered MK-801 (2.0 microg/3.0 microl) into the fourth ventricle [intracerebroventricular (ICV)] and measured feeding and gastric emptying of 5-ml NaCl or 15% sucrose loads. In a parallel experiment, we examined food intake and gastric emptying following intraperitoneal (IP) injection of MK-801 (100 microg/kg). MK-801, either IP or ICV, increased 30-min sucrose intake compared with control (12.3 +/- 0.7 vs. 9.8 +/- 0.5 and 16.6 +/- 2.0 vs. 10.7 +/- 0.7 ml, for IP and ICV administration, respectively). Also, IP MK-801 increased 5-min gastric emptying of NaCl (4.13 +/- 0.1 ml emptied) and sucrose (3.11 +/- 0.1 ml emptied) compared with control (3.75 +/- 0.2 and 2.28 +/- 0.1 ml emptied for NaCl and sucrose loads, respectively). In contrast, ICV MK-801 did not alter NaCl emptying (3.82 +/- 0.1 ml emptied) compared with control (3.82 +/- 0.3 ml emptied) and actually reduced gastric emptying of sucrose (2.1 +/- 0.2 and 2.94 +/- 0.1 ml emptied, for MK and vehicle, respectively). These data confirm previous results that systemic as well as hindbrain injection of MK-801 increases food intake. However, because ICV MK-801 failed to increase gastric emptying, these results indicate that MK-801 increases food intake through mechanisms independent of altered gastric emptying.

Mechanotransduction by intraganglionic laminar endings of vagal tension receptors in the guinea-pig oesophagus

Vagal mechanoreceptors to the guinea-pig oesophagus, recorded extracellularly, in vitro, fired spontaneously at 3.3 +/- 0.2 Hz, (n = 75, from 57 animals), and had low thresholds to circumferential stretch. In this study, we have investigated whether mechanotransduction by intraganglionic laminar endings (IGLEs) directly relies on mechano-gated ion channels, or whether it is due to chemical activation by neurotransmitters (glutamate or ATP) released from other cells during mechanical distortion. Rapid distortion of focal transduction sites (IGLEs) evoked action potentials with a latency of < 10 ms. Antagonists to ionotropic (AP5, memantine and 6,7-dinitroquinoxaline-2,3-dione (DNQX)) and metabotropic glutamate receptors (N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide (PHCCC) and (RS)-a-methyl-4-phosphono-phenylglycine (MPPG)) did not affect mechano-transduction. Glutamate, NMDA and the selective mGluR group II and III agonists, (2R, 4R)-APDC and L-AP4, had no effect on spontaneous or stretch-induced firing. The P2X purinoreceptor agonist, alpha,beta-methylene ATP, caused concentration-dependent excitation of vagal mechanoreceptors (EC50 = 22.2 microM) which was blocked by the non-selective P2 antagonist PPADS (30 microM). On its own, PPADS affected neither stretch-induced firing nor spontaneous firing. Neither Ca(2+)-free solution (1 mM EDTA, 3.6 mM Mg(2+)) solution nor Cd(2+) (100 microM) blocked stretch-induced firing. Thus chemical transmission is not involved in activation of vagal mechanoreceptors. The blocker of stretch-activated channels, Gd(3+) (300 microM), did not inhibit stretch-induced firing. However, benzamil (100 microM) significantly inhibited spontaneous and distension-evoked firing in a stretch-dependent manner; proportionally greater inhibition was seen with larger stretches. The results suggest that IGLEs of vagal tension receptors directly transduce mechanical stimuli probably via benzamil-sensitive, Gd3+-insensitive, stretch-activated ion channels, and that chemical transmission is not involved in transduction.

Metabotropic glutamate receptor expression in the rat superior cervical ganglion

To begin to determine what role metabotropic glutamate receptors (mGluRs) play in the peripheral nervous system, reverse transcriptase-polymerase chain reaction (RT-PCR) was used to probe RNA isolated from sympathetic neurons of the rat superior cervical ganglion (SCG). RT-PCR primers were designed to detect each of the eight rat mGluR transcripts. Only one, mGluR7, was detected in RNA from rat SCG, though each appeared to be present in RNA from whole rat brain. Although mGluR7 messenger RNA is apparently present in rat SCG, functional mGluR7 was not observed, as application of neither glutamate nor the group III mGluR agonist L-2-amino-4-phosphonobutyrate (L-AP4) produced calcium current modulation in isolated SCG neurons, as would be expected. However, following mGluR7 heterologous expression, application of L-AP4 did produce moderate calcium current modulation in isolated neurons, indicating that mGluR7 can express on the surface of SCG soma, and that its activation can modulate calcium currents.

Qualitative light and electron microscope study of glutamate receptors in the caudal spinal trigeminal nucleus of the rat

Though ionotropic and metabotropic glutamate receptors have recently been recognized to play important roles in the transmission of orofacial nociceptive impulses, their detailed distribution in the spinal trigeminal nucleus has not been systematically investigated. There is also controversy regarding the electron microscope localization of metabotropic receptors. We therefore undertook this investigation to address the above-mentioned issues in the caudal part of the spinal trigeminal nucleus, using light and electron microscope immunocytochemistry, to provide baseline information for the development of agonists and antagonists of these receptors in the clinical treatment of orofacial pain. The results showed some moderately to strongly stained glutamate receptor 1 neurons, and many strongly stained glutamate receptor 2/3 neurons in lamina II of the nucleus, suggesting that the latter may play an important role in orofacial pain processing, with the former playing a minor role. The metabotropic glutamate receptor 1 immunoreactive product was localized mostly in dendrites, while most of the metabotropic glutamate receptors 2/3 immunoreactive product was deposited in axon terminals containing synaptic vesicles of different shapes, suggesting that glutamate receptors 2/3 may control the release of both excitatory and inhibitory neurotransmitters.

Receptors and transmission in the brain-gut axis. II. Excitatory amino acid receptors in the brain-gut axis

In the last decade, there has been a dramatic increase in academic and pharmaceutical interest in central integration of vago-vagal reflexes controlling the gastrointestinal tract. Associated with this, there have been substantial efforts to determine the receptor-mediated events in the dorsal vagal complex that underlie the physiological responses to distension or variations in the composition of the gut contents. Strong evidence supports the idea that glutamate is a transmitter in afferent vagal fibers conveying information from the gut to the brain, and the implications of this are discussed in this themes article. Furthermore, both ionotropic and metabotropic glutamate receptors mediate pre- and postsynaptic control of glutamate transmission related to several reflexes, including swallowing motor pattern generation, gastric accommodation, and emesis. The emphasis of this themes article is on the potential therapeutic benefits afforded by modulation of these receptors at the site of the dorsal vagal complex.

Cellular and subcellular distribution of the amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor subunit GluR2 in the rat dorsal vagal complex

Amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) type glutamate receptors are ligand gated ion channels made up of various combinations of four subunits termed GluR1-4. The GluR2 subunit controls several key features of the receptor including calcium permeability and inward rectification. In the present study, we analysed by immunocytochemistry the cellular and subcellular distribution of the GluR2 subunit in neurons of the dorsal vagal complex of the rat. GluR2 immunoreactivity was found both in the neuropile and in neuronal cell bodies. Perikaryal staining was strong in the dorsal motor nucleus of the vagus nerve and moderate in the medial part of the nucleus tractus solitarii as well as in the area postrema. The lateral part of the nucleus tractus solitarii was almost devoid of immunoreactivity except for the interstitial subnucleus which was filled with numerous strongly immunoreactive perikarya and large cell processes. Ultrastructural examination was carried out in the interstitial subnucleus. Peroxidase staining indicative of GluR2 immunoreactivity was observed in neuronal cell bodies and dendrites. No labeled axon terminal or glial cell body was found. Additional experiments performed using pre-embedding immunogold showed that most of the labeling in immunoreactive dendrites was intracytoplasmic. These results indicate that GluR2 immunoreactivity is differentially distributed among neurons in the dorsal vagal complex, thereby suggesting differences in the functional properties of AMPA receptors between neuronal populations. These results also suggest that AMPA receptors, at least those containing the GluR2 subunit, have no major role as presynaptic receptors within this region. Finally, they indicate the existence of large intracellular pools of GluR2 subunits within dendrites of immunoreactive neurons.

Group II and group III metabotropic glutamate receptor agonists depress synaptic transmission in the rat spinal cord dorsal horn

The effects of group II and group III metabotropic glutamate receptor agonists on synaptic responses evoked by primary afferent stimulation in the dorsal horn, but mostly substantia gelatinosa, neurons were studied in the spinal cord slice preparation using conventional intracellular recording technique. Bath application of a potent metabotropic glutamate receptor 2- and 3-selective agonist (2S,1'R,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl) glycine reversibly suppressed monosynaptic and polysynaptic excitatory postsynaptic potentials evoked by A primary afferent fibers stimulation, the effect likely mediated by mGlu3 receptor subtype. This suppressing effect of (2S,1'R,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl) glycine on primary afferent neurotransmission was dose dependent and reduced by (S)-alpha-ethylglutamate, a group II metabotropic glutamate receptor antagonist. (2S,1'R,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl) glycine suppressed excitatory postsynaptic potentials without inducing detectable changes of postsynaptic membrane potential and neuronal input resistance in dorsal horn neurons. The paired-pulse depression at excitatory synapses between primary afferent fibers and dorsal horn neurons was reduced by (2S,1'R,2'R,3'R)-2-(2', 3'-dicarboxycyclopropyl) glycine application, suggesting a presynaptic site of action. The selective group III metabotropic glutamate receptor agonist (S)-2-amino-4-phosphonobutanoate also depressed A afferent fibers-evoked monosynaptic and polysynaptic excitatory postsynaptic potentials in a dose-dependent and reversible manner. The concentration-dependence of (S)-2-amino-4-phosphonobutanoate-mediated depression was most consistent with activation of mGlu receptor subtypes 4 and 7. However, on the basis of anatomical distribution of mGlu 4 and 7 subtypes, it is also possible that the (S)-2-amino-4-phosphonobatanoate effect is due to interaction with mGlu 7 receptor alone. (RS)-alpha-cyclopropyl-4-phosphonophenylglycine a preferential antagonist at group III metabotropic glutamate receptors, completely reversed the depressant effects of (S)-2-amino-4-phosphonobutanoate on both monosynaptic and polysynaptic responses. (S)-2-amino-4-phosphonobutanoate reduced the paired-pulse depression at excitatory synapses between primary afferent fibers and dorsal horn neurons, but did not alter their postsynaptic membrane potential and input resistance. A clear facilitation of the (S)-2-amino-4-phosphonobutanoate-induced depression of monosynaptic and polysynaptic excitatory postsynaptic potentials in the absence of gamma-aminobutyric acid-subtype A receptor- and glycine-mediated synaptic inhibition was shown. Besides the depressant effect on excitatory synaptic transmission, inhibitory actions of group II and III metabotropic glutamate receptor agonists on the inhibitory postsynaptic potentials evoked by primary afferent stimulation in dorsal horn neurons were observed. These results suggest that group II and group III metabotropic glutamate receptors are expressed at primary afferent synapses in the dorsal horn region, and activation of the receptors suppresses synaptic transmission by an action on the presynaptic site.

Groups II and III metabotropic glutamate receptors differentially modulate brief and prolonged nociception in primate STT cells

The heterogeneous family of G-protein-coupled metabotropic glutamate receptors (mGluRs) provides excitatory and inhibitory controls of synaptic transmission and neuronal excitability in the nervous system. Eight mGluR subtypes have been cloned and are classified in three subgroups. Group I mGluRs can stimulate phosphoinositide hydrolysis and activate protein kinase C whereas group II (mGluR2 and 3) and group III (mGluR4, 6, 7, and 8) mGluRs share the ability to inhibit cAMP formation. The present study examined the roles of groups II and III mGluRs in the processing of brief nociceptive information and capsaicin-induced central sensitization of primate spinothalamic tract (STT) cells in vivo. In 11 anesthetized male monkeys (Macaca fascicularis), extracellular recordings were made from 21 STT cells in the lumbar dorsal horn. Responses to brief (15 s) cutaneous stimuli of innocuous (brush), marginally and distinctly noxious (press and pinch, respectively) intensity were recorded before, during, and after the infusion of group II and group III mGluR agonists into the dorsal horn by microdialysis. Different concentrations were applied for at least 20 min each (at 5 microliter/min) to obtain cumulative concentration-response relationships. Values in this paper refer to the drug concentrations in the microdialysis fibers; actual concentrations in the tissue are about three orders of magnitude lower. The agonists were also applied at 10-25 min after intradermal capsaicin injection. The group II agonists (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (LCCG1, 1 microM-10 mM, n = 6) and (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4, 6-dicarboxylate (LY379268; 1 microM-10 mM, n = 6) had no significant effects on the responses to brief cutaneous mechanical stimuli (brush, press, pinch) or on ongoing background activity. In contrast, the group III agonist L(+)-2-amino-4-phosphonobutyric acid (LAP4, 0. 1 microM-10 mM, n = 6) inhibited the responses to cutaneous mechanical stimuli in a concentration-dependent manner, having a stronger effect on brush responses than on responses to press and pinch. LAP4 did not change background discharges significantly. Intradermal injections of capsaicin increased ongoing background activity and sensitized the STT cells to cutaneous mechanical stimuli (ongoing activity > brush > press > pinch). When given as posttreatment, the group II agonists LCCG1 (100 microM, n = 5) and LY379268 (100 microM, n = 6) and the group III agonist LAP4 (100 microM, n = 6) reversed the capsaicin-induced sensitization. After washout of the agonists, the central sensitization resumed. Our data suggest that, while activation of both group II and group III mGluRs can reverse capsaicin-induced central sensitization, it is the actions of group II mGluRs in particular that undergo significant functional changes during central sensitization because they modulate responses of sensitized STT cells but have no effect under control conditions.

Expression of metabotropic glutamate receptors mRNA in the thalamus and brainstem of monoarthritic rats

Evidence for the involvement of metabotropic glutamate receptors (mGluR) in sensory processing has been emerging. Additionally, the differential distribution of distinct mGluR subtypes mRNA in particular thalamic nuclei of normal rats suggests that they could be involved in the processing of somatosensory information. In the present study, mGluR1, 3, 4 and 7 mRNAs expression was investigated by in situ hybridisation in selected brainstem and thalamic nuclei of adult monoarthritic rats at different time points of the disease (2, 4 and 14 days). Monoarthritic rats displayed behavioural and physical signs of painful arthritis at all time points. At 2 days of monoarthritis the mGluR1 mRNA expression was decreased mainly in the ventrobasal complex (VB) and in the posterior thalamic nuclei (Po) contralateral to the inflamed joint. The mGluR4 mRNA expression was also reduced, but minimum values were found at 4 days of monoarthritis, when no changes could be found in mGluR1 mRNA expression. At 14 days, mGluR4 mRNA expression was similar to controls, while mGluR1 mRNA was again reduced. Similar decreases of mGluR7 mRNA expression in the VB and Po were found at all time points, while mGluR3 mRNA expression was bilaterally increased in the reticular thalamic nucleus (Rt). In the brainstem no changes could be found in the expression of any mGluR subtype mRNA. The reduced expression of mGluR1, 4 and 7 transcripts in VB and Po, and the increases of mGluR3 mRNA in the Rt may contribute to counteract the increased noxious input arising from the periphery.

Herniated lumbar disc material as a source of free glutamate available to affect pain signals through the dorsal root ganglion

STUDY DESIGN

Combined prospective human cohort and prospective controlled animal model.

OBJECTIVES

To determine whether free glutamate is available in herniated disc material in concentrations sufficient to diffuse to glutamate receptors and affect the activity of neurons in the dorsal root ganglion that may transmit pain information.

SUMMARY OF BACKGROUND DATA

The severity of lumbar radicular pain cannot be fully explained by physical pressure on nerve roots or ganglions. In experimental models, inflammatory processes are relatively modest under conditions of disc herniation. The hypothesis for the current study was that the proteoglycan link and core proteins, which contain high fractions of acidic amino acids, may be a source of glutamate when enzymatically degraded in an environment without glutamate reuptake systems. Glutamate would be free to diffuse to the dorsal root ganglion to affect glutamate receptors.

METHODS

Disc material was harvested during surgery from herniated and nonherniated portions in patients undergoing elective lumbar disc surgery and subjected to immunohistochemistry and high-performance liquid chromatography for assessment of the presence of extracellular disc matrix glutamate. Miniosmotic pumps with differing concentrations of radiolabeled glutamate based on human data were implanted in the rat epidural space for 72 hours and dorsal root ganglion (DRG) in the region were harvested.

RESULTS

Densitometry of disc matrix demonstrated immunohistochemical evidence for significant extracellular glutamate (P < 0.002). High performance liquid chromatography showed significant concentrations of glutamate in disc material and significantly more in herniated than in nonherniated disc material (P < 0.05). Significant radiolabeling of the dorsal root ganglion after epidural glutamate infusion was found at concentrations two orders of magnitude below measured disc glutamate levels. Autoradiography demonstrated radiolabeling of adjacent DRG.

CONCLUSIONS

Glutamate originating from degenerated disc proteoglycan may diffuse to the dorsal root ganglion and effect glutamate receptors. Consideration may be given to treating disc radiculopathy with epidural glutamate receptor antagonists.

Non-NMDA glutamate receptors modulate capsaicin induced c-fos expression within trigeminal nucleus caudalis

1. We examined the effects of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzol[f]quinoxaline-7-sulpho namide (NBQX), the kainate receptor antagonists gamma-(R-)-glutamylaminomethanesulphonic acid (GAMS) and 6,7,8,9-tetrahydro-5-nitro-1H-benz[g]indole-2,3-dione-3-oxime (NS-102), and the group III metabotropic glutamate receptor (mGluR) agonist 2-amino-4-phosphono-S-butanoic acid (L-AP4) on c-fos-like immunoreactivity (c-fos LI) in trigeminal caudalis (Sp5C), lateral reticular (LRt), medullary reticular (Md) and solitary tract (Sol) nuclei, after intracisternal injection of capsaicin in urethane anaesthetized Sprague-Dawley rats. 2. Few c-fos labelled cells were observed within Sp5C in capsaicin-vehicle treated animals. The number of positive c-fos cells increased by 17 fold after intracisternal capsaicin (5 nmol) administration. 3. Pretreatment with CNQX (0.02, 0.1, 0.6, 3 and 15 mg kg-1) or NBQX (0.01, 0.1 and 1 mg kg-1), administered intraperitoneally 15 min before capsaicin, significantly reduced labelled cells within Sp5C by a maximum of 45 and 34%, respectively. The number of c-fox LI cells within LRt, Md and Sol was not affected. Pretreatment with L-AP4 (1, 3 and 10 mg kg-1) decreased the number of Sp5C c-fos LI cells by a maximum of 30%, whereas GAMS (1 and 10 mg kg-1) and NS-102 (1 and 5 mg kg-1) did not show any significant effect. 4. These results suggest that blockade of AMPA receptors, but not kainate receptors, or the activation of group III mGluRs, decrease the response of Sp5C neurons to trigeminovascular activation. Thus, in addition to NMDA receptors, mGluRs and AMPA receptors may modulate cephalic pain and may provide a potential therapeutic target for antimigraine drugs.

Distribution and developmental changes in metabotropic glutamate receptor messenger RNA expression in the rat lumbar spinal cord

Using in situ hybridisation, the regional distribution of primary transcripts and splice variants of all metabotropic glutamate receptor subtypes (mGluR) currently known to be expressed in the spinal cord have been studied in the lumbar enlargement of the rat spinal cord. In adult animals, the messenger RNA of the mGluR subtypes 1, 5, 3, 4 and 7 were differentially expressed. The transcripts of mGluR1 and 5 were most abundant with mGluR5 messenger RNA being concentrated in the superficial dorsal horn. In contrast, the mGluR2 transcript was not detectable with the sensitivity of the method. Secondly, age related changes (postnatal days 1, 7, 12, 21) in the postnatal expression of mGluR1-5 and 7 transcripts have been investigated. mGluR1 and 7 messenger RNA showed a general decrease in spinal expression from postnatal day 1 to day 21. Quantitative densitometry showed high mGluR3 and 5 messenger RNA levels especially in the superficial dorsal horn at birth, however these levels decreased with age. In addition to changes in density, the regional distribution of mGluR3 messenger RNA was altered with postnatal development. Up to postnatal day 12, mGluR3 messenger RNA expression was almost exclusively restricted to the spinal grey matter, but with postnatal day 21 a strong additional expression in the white matter occurred. Distribution of mGluR4 messenger RNA showed little change in the dorsal horn, however motoneuronal expression emerged during development. These changes may suggest different roles for mGluRs in the maturation of spinal transmission of the rat nervous system.

Visceral afferent participation in delayed satiation following NMDA receptor blockade

We previously reported that rats increase their food intake, but not water intake, following intraperitoneal (i.p.) injection of MK-801, a noncompetitive antagonist of N-methyl-D-aspartate (NMDA)-activated ion channels. The drug appears to specifically interfere with signals that participate in satiation, thereby prolonging the meal and increasing its size. The mechanism by which delayed satiation occurs is not known. However, some well-studied satiety signals are carried by visceral sensory fibers that innervate the abdominal viscera. We hypothesized that MK-801 might increase food intake by interfering with satiety signals transmitted by visceral afferent neurons. To test this hypothesis, we examined MK-801's effect on food intake in rats systemically treated with capsaicin, a neurotoxin that destroys small unmyelinated visceral afferent neurons. Capsaicin treatment significantly attenuated increased sucrose intake following MK-801. We also investigated whether the effects of MK-801 on food intake would persist in rats treated with total subdiaphragmatic vagotomies. MK-801 increased the intake of 15% sucrose by sham-vagotomized rats, while vagotomized rats did not increase their intake following MK-801. Taken together, these results support the hypothesis that capsaicin-sensitive visceral sensory neurons are involved in increased food intake following systemic NMDA receptor blockade. This, in turn, suggests that NMDA receptor activation may be an important component of the neural circuitry involved in satiation.

Delay in meal termination follows blockade of N-methyl-D-aspartate receptors in the dorsal hindbrain

We have reported that rats increased their intake of food, but not water, following an intraperitoneal injection of MK-801, a non-competitive antagonist of N-methyl-d-aspartate (NMDA)-activated ion channels. The antagonist appears to specifically interfere with signals that participate in meal termination (satiety), thereby prolonging the meal and increasing its size. The anatomical site at which MK-801 acts to increase food intake is not known. However, vagal sensory neurons are known to participate in satiation for food. Furthermore, NMDA receptor immunoreactivity is present in the caudal nucleus of the solitary tract (NTS) where vagal sensory fibers terminate. Therefore, we hypothesized that MK-801 might increase food intake by blocking NMDA receptors in the NTS. To test this hypothesis, we microinjected MK-801 directly into the hindbrain, immediately prior to a deprivation-induced meal of 15% sucrose. We found that sucrose intake was significantly increased following injection of MK-801 (2 microgram/3 microliter) into the fourth ventricle. When MK-801 was injected directly into the caudomedial NTS, intake was increased significantly by doses as small as 198 ng/30 nl, while equivalent injections into other hindbrain areas or the fourth ventricle did not increase food intake. These data are consistent with control of food intake by endogenous glutamate and NMDA-type glutamate receptors located in the caudomedial NTS.

The origin and development of the vagal and spinal innervation of the external muscle of the mouse esophagus

Retrograde and anterograde tracing and immunohistochemical techniques were used to examine the origin of the extrinsic innervation, and the development of the vagal innervation to the mouse esophagus. Cholinergic nerve terminals were localised using an antiserum to the vesicular acetylcholine transporter and cholinergic cell bodies were localised using an antiserum to choline acetyltransferase. Cholinergic nerve terminals, which also contained calcitonin gene-related peptide, were present at the motor end plates in the external (striated) muscle of the esophagus. Following injection of Fast Blue into subdiaphragmatic or cervical levels of the esophagus, the only retrogradely-labelled cholinergic nerve cell bodies that also contained calcitonin gene-related peptide were found in the nucleus ambiguus. Neurons in the dorsal motor nucleus of the vagus, the nodose ganglia and dorsal root ganglia gave rise to a number of different types of nerve terminals within the myenteric plexus. Retrogradely-labelled neurons in the dorsal motor nucleus of vagus contained cholinergic markers only, nitric oxide synthase only or cholinergic markers plus nitric oxide synthase, retrogradely-labelled neurons in the dorsal root ganglia contained calcitonin gene-related peptide only, and a small number of retrogradely-labelled neurons in the nodose ganglia contained tyrosine hydroxylase. The development of the vagal innervation to the esophagus was examined following application of DiI to the vagus nerve of fixed mouse embryos. Anterogradely-labelled nerve fibres, which arose from both nodose ganglia and the medulla, were already present in the esophagus of embryonic day 12 (E12) mice. Some of the DiI-labelled vagal nerve fibres were present in among the smooth muscle cells of the external muscle layer prior to their transdifferentiation to striated muscle. We conclude that the neurons in the nucleus ambiguus that project to the esophagus differ from other extrinsic neurons in their chemistry as well as their targets within the esophagus. The development of the extrinsic innervation precedes the transdifferentiation of the external muscle to striated muscle, raising the possibility that, during development, smooth muscle of the esophagus is innervated transiently by vagal neurons.

Co-localization of mu-opioid receptor-like immunoreactivity with substance P-LI, calcitonin gene-related peptide-LI and nitric oxide synthase-LI in vagal and glossopharyngeal afferent neurons of the rat

Co-localization of mu-opioid receptor (MOR)-like immunoreactivity (-LI) with substance P (SP)-LI, calcitonin gene-related peptide (CGRP)-LI and nitric oxide synthase (NOS)-LI in the nodose, petrosal and jugular ganglia was examined in the rat by a double immunofluorescence histochemical method. About 0.6%, 41% and 95% of neurons with MOR-LI, respectively, in the nodose, petrosal and jugular ganglia showed SP-LI; about 2%, 51% and 66% of MOR-like immunoreactive neurons displayed CGRP-LI in the nodose, petrosal and jugular ganglia, respectively. In addition, about 59% of MOR-like immunoreactive neurons in the nodose ganglia displayed NOS-LI, whereas no NOS-LI was detected in the petrosal or jugular ganglion. These data provide evidence for co-localization of MOR-LI with SP-LI, CGRP-LI and NOS-LI in the vagal and glossopharyngeal afferent neurons, and suggest that MOR may regulate the release of SP, CGRP and nitric oxide from the visceral primary afferent terminals in the nucleus of the solitary tract of the rat.

Enhanced expression of metabotropic glutamate receptor 3 messenger RNA in the rat spinal cord during ultraviolet irradiation induced peripheral inflammation

Metabotropic glutamate receptors are thought to play a role in the development and maintenance of spinal hyperexcitability resulting in hyperalgesia and pain. In this study we have used in situ hybridization to investigate the distribution of metabotropic glutamate receptors mGluR1-7 messenger RNA in the rat spinal cord in a model of inflammatory hyperalgesia. Hyperalgesia was induced in nine-day-old rats by exposure of the left hindpaw to an ultraviolet light source. Lumbar portions of spinal cords were removed from control and ultraviolet-treated animals. In situ hybridization with specific oligonucleotide probes was used to localize metabotropic glutamate receptor messenger RNAs. mGluR1, 3-5 and 7 subtype messenger RNA was detected in the gray matter of the spinal cord with distribution being specific for the different subtypes. A significant increase in the expression of mGluR3 messenger RNA was seen in cells of the dorsal laminae in both sides of the lumbar spinal cord. This increase was most pronounced in laminae II, III and IV but gradually decreased and disappeared by the third day of inflammation. In parallel with this, behavioural experiments revealed mechanical hyperalgesia in both hindlimbs after ultraviolet irradiation. There was no change in mGluR3 messenger RNA expression in the thoracic segments. No changes have been detected in the levels of expression of mGluR 1,2,4,5,7 subtype messenger RNA in spinal cords taken from hyperalgesic animals. These observations show that during ultraviolet irradiation induced inflammation, the synthesis of mGluR3 messenger RNA is altered suggesting that regulation of metabotropic glutamate receptor expression may be instrumental in plastic changes within the spinal cord during the development of hyperalgesia and pain.

A novel splice variant of a metabotropic glutamate receptor, human mGluR7b

Two splice variants of the human metabotropic glutamate receptor 7, named hmGluR7a and hmGluR7b, were isolated from a human brain cDNA library. The isoforms differ by an out-of-frame insertion of 92 nucleotides close to the C-terminus of the hmGluR7 coding region, hmGluR7a has a length of 915 amino acids and represents the human homolog of the recently cloned rat mGluR7. hmGluR7b is seven amino acids longer and exhibits a novel C-terminus of 23 amino acids in length. RT-PCR analysis demonstrated the existence of mGluR7b transcripts in wild-type mouse brain and its absence in mGluR7 knockout mice. Northern blot analysis indicate that mGluR7 expression is developmentally regulated. It is expressed at high levels in human fetal brain and at a lower level in many regions of adult human brain. Stimulation of hmGluR7b with L-2-amino-4-phosphonobutyrate (L-AP4), L-serine-O-phosphate (L-SOP) or L-glutamate in stably transfected Chinese hamster ovary (CHO) cells depressed forskolin-induced cAMP accumulation, whereas (1S,3R)-1-aminocyclopentane-1,3,-dicarboxylic acid ((1S,3R)-ACPD) and quisqualate (both at 1mM) had no significant effects. As described for rat mGluR7, the rank order of agonist potencies is: L-SOP, L-AP4 > L-glutamate > (1S,3R)-ACPD, quisqualate.

Localization of mu-opioid receptor-like immunoreactivity in the central components of the vagus nerve: a light and electron microscope study in the rat

mu-Opioid receptor, the opioid receptor that shows the highest affinity for morphine, appears to induce a variety of side-effects, at least partly, directly through the mu-opioid receptor on neurons constituting the autonomic part of the vagus nerve. Thus, in the present study, location of mu-opioid receptor-like immunoreactivity in the central components of the autonomic part of the vagus nerve was examined in the rat. The intense immunoreactivity was observed light microscopically in the neuropil of the commissural subnucleus and the dorsal part of the medial subnucleus of the nucleus of the solitary tract, and in the neuropil of the rostral half of the ambiguus nucleus. The immunoreactivity was moderate in the neuropil of the rostral and lateral subnuclei and ventral part of the medial subnucleus of the nucleus of the solitary tract, and weak in the neuropil of the dorsal motor nucleus of the vagus nerve. In the nodose ganglion, many neurons of various sizes (17-48 microns in soma diameter) showed moderate immunoreactivity. After unilateral vagotomy at a level proximal to the nodose ganglion, the immunoreactivity in the ipsilateral ambiguus nucleus was apparently reduced within 48 h of the operation, and completely disappeared by the seventh day after the operation. In the nucleus of the solitary tract and dorsal motor nucleus of the vagus nerve, the reduction of immunoreactivity after the ganglionectomy was detectable on the fourth day after the operation, and became readily apparent by the seventh day after the operation; the immunoreactivity, none the less, still remained on the 10th day after the operation. Electron microscopically, the immunoreactivity in the ambiguus nucleus was seen mainly on dendritic profiles and additionally on somatic ones; no immunoreactivity was detected in axonal profiles. The immunoreactivity in the dorsal motor nucleus of the vagus nerve was observed only on dendritic profiles. The immunoreactivity in the nucleus of the solitary tract was seen on axonal and dendritic profiles, but not on somatic profiles. The immunoreactive axon terminals in the nucleus of the solitary tract were filled with spherical synaptic vesicles and made asymmetric synapses with dendritic profiles. The results indicate that the mu-opioid receptor in the central components of the autonomic part of the vagus nerve is located on dendrites and cell bodies of efferent neurons in the ambiguus, on dendrites of efferent neurons in the dorsal motor nucleus, and on axons which arise from nodose ganglion neurons and terminate in the nucleus of the solitary tract. The receptors on these structures may constitute the targets of enkephalin-containing and beta-endorphin-containing afferent axons arising from brainstem neurons. The receptors on the axon terminals of nodose ganglion neurons may be involved in regulation of the release of neurotransmitters and/or neuromodulators.

Sensory afferent processing in multi-responsive DRG neurons

The recent advance in molecular and neurobiological techniques disclosed the multi-responsive nature of DRG neurons. The survival, phenotype expression and electrical properties of these neurons are under the control of a variety of substances through their specific receptors. In pathological conditions, such as tissue inflammation or nerve injury, DRG neurons change their responsiveness through the dynamic reconstruction of their receptor system. This reconstruction is initiated by environmental stimuli. Thus the properties of polymodal nociceptors can be altered according to the environmental conditions. The whole story of this mechanism is not disclosed yet. In order to understand this mechanism, it is basically important to identify various receptor mRNAs in DRG neurons, precise localization of receptor proteins, site of synthesis and route of supply of ligands for these receptors.