Deafferentation up-regulates the expression of the mGlu1a metabotropic glutamate receptor protein in the olfactory bulb

Deafferentation-induced alterations in mitral cell dendritic morphology in the adult zebrafish olfactory bulb

The removal of afferent input to the olfactory bulb by both cautery and chemical olfactory organ ablation in adult zebrafish results in a significant decrease in volume of the ipsilateral olfactory bulb. To examine the effects of deafferentation at a cellular level, primary output neurons of the olfactory bulb, the mitral cells, were investigated using retrograde tract tracing with fluorescent dextran using ex vivo brain cultures. Morphological characteristics including the number of major dendritic branches, total length of dendritic branches, area of the dendritic arbor, overall dendritic complexity, and optical density of the arbor were used to determine the effects of deafferentation on mitral cell dendrites. Following 8 weeks of permanent deafferentation there were significant reductions in the total length of dendritic branches, the area of the dendritic arbor, and the density of fine processes in the dendritic tuft. With 8 weeks of chronic, partial deafferentation there were significant reductions in all parameters examined, including a modified Sholl analysis that showed significant decreases in overall dendritic complexity. These results show the plasticity of mitral cell dendritic structures in the adult brain and provide information about the response of these output neurons following the loss of sensory input in this key model system.

Metabotropic glutamate receptors and dendrodendritic synapses in the main olfactory bulb

The main olfactory bulb (MOB) is the first site of synaptic processing in the central nervous system for odor information that is relayed from olfactory receptor neurons in the nasal cavity via the olfactory nerve (ON). Glutamate and ionotropic glutamate receptors (iGluRs) play a dominant role at ON synapses. Similarly, glutamate and iGluRs mediate dendrodendritic transmission between several populations of neurons within the MOB network. Neuroanatomical studies demonstrate that metabotropic glutamate receptors (mGluRs) are densely expressed through the MOB network, and they are particularly abundant at dendrodendritic synapses. Until recently, the physiological roles of mGluRs in the MOB were poorly understood. Over the past several years, mGluRs have been shown to play surprisingly powerful neuromodulatory roles at ON synapses and in dendrodendritic neurotransmission in the MOB. This chapter focuses on recent advances in our understanding of mGluR-mediated signaling components at dendrodendritic synapses.

Developmental regulation of metabotropic glutamate receptor 1 splice variants in olfactory bulb mitral cells

Alternative splicing of the metabotropic glutamate receptor 1 (mGluR1) receptor gene generates two major receptor isoforms, mGluR1a and mGluR1b, differing in intracellular function and distribution. However, little is known on the expression profiles of these variants during development. We examined the mRNA expression profile of mGluR1a/b in microdissected layers and acutely isolated mitral cells in the developing mouse olfactory bulb. This analysis showed that the two mGluR1 variants are differentially regulated within each bulb layer. During the first postnatal week, the mGluR1a isoform replaces GluR1b in the microdissected mitral cell layer (MCL) and in isolated identified mitral cells, coinciding with a developmental epoch of mitral cell dendritic reorganization. Although mGluR1a mRNA is expressed at high levels in both the adult external plexiform layer (EPL) and MCL, Western blotting analysis reveals a marked reduction of the mGluR1a protein in the MCL, where mitral cell bodies are located, and strong labeling in the EPL, which contains mitral cell dendrites. This suggests that there is increased dendritic trafficking efficiency of the receptor in adult. The temporal and spatial shift in mGluR1b/a expression suggests distinct roles of the mGluR1 isoforms, with mGluR1b potentially involved in the early mitral cell maturation and mGluR1a in dendritic and synapse function.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

Odorant deprivation reversibly modulates transsynaptic changes in the NR2B-mediated CREB pathway in mouse piriform cortex

The olfactory system is an outstanding model for understanding activity-dependent neuronal plasticity in mammals. Olfactory sensory neurons (OSNs) in the periphery project onto mitral/tufted cells in the olfactory bulb (OB) and these mitral/tufted cells in turn project to piriform cortex (PC). Numerous studies have examined changes in OB after a permanent OSN ablation, but little is known about "trans-transsynaptic" changes taking place in the PC. Permanent zinc sulfate lesion of the olfactory epithelium resulted in a selective loss of the NMDA receptor NR2B protein and mRNA expression in pyramidal cells in layer IIb of PC after 2-7 d. Regulatory elements affected by NR2B signaling, namely the phosphorylation of CREB, were also downregulated only in layer IIb neurons. These changes could be caused by OSN axon loss in the zinc sulfate lesion, or to a reduced activity. To test this hypothesis, we performed both permanent and reversible naris occlusion, which blocks odorant access to the nasal cavities and OSN activity. The expression of NR2B and phospho-CREB were downregulated 5 d after occlusion and this reduction was fully restored 10 d after reopening of the naris. Subsequently, we identified the subset of pyramidal cells in layer IIb that are especially sensitive to the loss of odor-evoked activity using double retrograde tracers. In summary, the present study provides an initial characterization of the molecular mechanisms associated with odor stimulation on second order neuronal plasticity and phenotype in the olfactory system.

Olfactory nerve-evoked, metabotropic glutamate receptor-mediated synaptic responses in rat olfactory bulb mitral cells

The group I metabotropic glutamate receptor (mGluR) subtype, mGluR1, is highly expressed on the apical dendrites of olfactory bulb mitral cells and thus may be activated by glutamate released from olfactory nerve (ON) terminals. Previous studies have shown that mGluR1 agonists directly excite mitral cells. In the present study, we investigated the involvement of mGluR1 in ON-evoked responses in mitral cells in rat olfactory bulb slices using patch-clamp electrophysiology. In voltage-clamp recordings, the average EPSC evoked by single ON shocks or brief trains of ON stimulation (six pulses at 50 Hz) in normal physiological conditions were not significantly affected by the nonselective mGluR antagonist LY341495 (50-100 microM) or the mGluR1-specific antagonist LY367385 (100 microM); ON-evoked responses were attenuated, however, in a subset (36%) of cells. In the presence of blockers of ionotropic glutamate and GABA receptors, application of the glutamate uptake inhibitors THA (300 microM) and TBOA (100 microM) revealed large-amplitude, long-duration responses to ON stimulation, whereas responses elicited by antidromic activation of mitral/tufted cells were unaffected. Magnitudes of the ON-evoked responses elicited in the presence of THA-TBOA were dependent on stimulation intensity and frequency, and were maximal during high-frequency (50-Hz) bursts of ON spikes, which occur during odor stimulation. ON-evoked responses elicited in the presence of THA-TBOA were significantly reduced or completely blocked by LY341495 or LY367385 (100 microM). These results demonstrate that glutamate transporters tightly regulate access of synaptically evoked glutamate from ON terminals to postsynaptic mGluR1s on mitral cell apical dendrites. Taken together with other findings, the present results suggest that mGluR1s may not play a major role in phasic responses to ON input, but instead may play an important role in shaping slow oscillatory activity in mitral cells and/or activity-dependent regulation of plasticity at ON-mitral cell synapses.

Changes in glutamate receptor subunit 4 expression in the deafferented olfactory bulb of zebrafish

The distribution of ionotropic glutamate receptor subunit 4 (iGluR4) was examined in both normal and deafferented olfactory bulbs of adult zebrafish, Danio rerio. With the exception of the olfactory nerve layer, there was extensive labeling with antibodies to iGluR4 in the olfactory bulbs, specifically in juxtaglomerular cell bodies and their processes. These results are consistent with previous work, which has suggested differential distribution of glutamate receptors in the vertebrate olfactory system. Analysis of bulbs following olfactory organ removal revealed a significant loss of iGluR4 immunoreactivity by 24 h post-deafferentation. At 48 h after denervation, iGluR4 labeling had returned to normal levels and was retained through 3 weeks post-surgery. Thus, afferent input plays a role in reduced labeling of this protein immediately following injury, but return of immunoreactivity can occur even without sensory innervation.

Distribution of GluR1 is altered in the olfactory bulb following neonatal naris occlusion

The olfactory system is well suited for studies of glutamate receptor plasticity. The sensory neurons are glutamatergic, and they turn over throughout life, and the olfactory bulb neurons that process their inputs express many of the known glutamate receptor subunits. Neonatal naris occlusion alters olfactory bulb development and the expression of certain neuroactive substances and receptors, at least in part due to loss of the sensory inputs. We therefore postulated that neonatal naris occlusion might alter glutamate receptor expression during postnatal development. Single nares of newborn mice were occluded on postnatal days 1-2, and the distribution of glutamate receptor subunits was evaluated using immunoperoxidase methods. Light microscopic examination on postnatal day 6 failed to reveal adult-like staining of neuronal cell bodies in the olfactory bulbs. By day 12, cell bodies that were immunoreactive (-IR) for the GluR1 subunit were visible in the external plexiform layer (EPL) of both sides. By day 18, many of the GluR1-IR cell bodies could be identified as cell types that had previously been reported to express homomeric GluR1 receptors. Analysis of single, mid-dorsal sections from 18-25-day-old mice showed that the medial EPL of the occluded side had a significantly lower density of these cell bodies. The GluR1 staining of the adjacent mitral cell layer (MCL) was also heavier on the occluded side, but no gross differences in staining for other glutamate receptor subunits were observed. Neonatal naris occlusion therefore appears to provide a new model for studying expression of GluR1 receptors during the development of a discrete population of olfactory bulb neurons.

Glutamate receptors and nociception: implications for the drug treatment of pain

Evidence from the last several decades indicates that the excitatory amino acid glutamate plays a significant role in nociceptive processing. Glutamate and glutamate receptors are located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Glutamate acts at several types of receptors, including ionotropic (directly coupled to ion channels) and metabotropic (directly coupled to intracellular second messengers). Ionotropic receptors include those selectively activated by N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate. Metabotropic glutamate receptors are classified into 3 groups based on sequence homology, signal transduction mechanisms and receptor pharmacology. Glutamate also interacts with the opioid system, and intrathecal or systemic coadministration of glutamate receptor antagonists with opioids may enhance analgesia while reducing the development of opioid tolerance and dependence. The actions of glutamate in the brain seem to be more complex. Activation of glutamate receptors in some brain areas seems to be pronociceptive (e.g. thalamus, trigeminal nucleus), although activation of glutamate receptors in other brain areas seems to be antinociceptive (e.g. periaqueductal grey, ventrolateral medulla). Application of glutamate, or agonists selective for one of the several types of glutamate receptor, to the spinal cord or periphery induces nociceptive behaviours. Inhibition of glutamate release, or of glutamate receptors, in the spinal cord or periphery attenuates both acute and chronic pain in animal models. Similar benefits have been seen in studies involving humans (both patients and volunteers); however, results have been inconsistent. More research is needed to clearly define the role of existing treatment options and explore the possibilities for future drug development.

Knockdown of spinal metabotropic glutamate receptor 1 (mGluR(1)) alleviates pain and restores opioid efficacy after nerve injury in rats

1. Nerve injury often produces long-lasting spontaneous pain, hyperalgesia and allodynia that are refractory to treatment, being only partially relieved by clinical analgesics, and often insensitive to morphine. With the aim of assessing its therapeutic potential, we examined the effect of antisense oligonucleotide knockdown of spinal metabotropic glutamate receptor 1 (mGluR(1)) in neuropathic rats. 2. We chronically infused rats intrathecally with either vehicle, or 50 microg day(-1) antisense or missense oligonucleotides beginning either 3 days prior to or 5 days after nerve injury. Cold, heat and mechanical sensitivity was assessed prior to any treatment and again every few days after nerve injury. 3. Here we show that knockdown of mGluR(1) significantly reduces cold hyperalgesia, heat hyperalgesia and mechanical allodynia in the ipsilateral (injured) hindpaw of neuropathic rats. 4. Moreover, we show that morphine analgesia is reduced in neuropathic rats, but not in sham-operated rats, and that knockdown of mGluR(1) restores the analgesic efficacy of morphine. 5. We also show that neuropathic rats are more sensitive to the excitatory effects of intrathecally injected N-methyl-D-aspartate (NMDA), and have elevated protein kinase C (PKC) activity in the spinal cord dorsal horn, two effects that are reversed by knockdown of mGluR(1). 6. These results suggest that activity at mGluR(1) contributes to neuropathic pain through interactions with spinal NMDA receptors and PKC, and that knockdown of mGluR(1) may be a useful therapy for neuropathic pain in humans, both to alleviate pain directly, and as an adjunct to opioid analgesic treatment.

Role of metabotropic glutamate receptor subtype mGluR1 in brief nociception and central sensitization of primate STT cells

G-protein coupled metabotropic glutamate receptors (mGluRs) are important modulators of synaptic transmission in the mammalian CNS and have been implicated in various forms of neuroplasticity and nervous system disorders. Increasing evidence also suggests an involvement of mGluRs in nociception and pain behavior although the contribution of individual mGluR subtypes is not yet clear. Subtypes mGluR1 and mGluR5 are classified as group I mGluRs and share the ability to stimulate phosphoinositide hydrolysis and activate protein kinase C. The present study examined the role of group I mGluRs in nociceptive processing and capsaicin-induced central sensitization of primate spinothalamic tract (STT) cells in vivo. In 10 anesthetized male monkeys (Macaca fascicularis) extracellular recordings were made from 20 STT cells in the lumbar dorsal horn. Responses to brief (15 s) cutaneous stimuli of innocuous (BRUSH) and barely and substantially noxious (PRESS and PINCH, respectively) intensity were recorded before, during, and after the infusion of group I mGluR agonists and antagonists into the dorsal horn by microdialysis. Cumulative concentration-response relationships were obtained by applying different concentrations for at least 20 min each (at 5 microl/min). The actual concentrations reached in the tissue are 2-3 orders of magnitude lower than those in the microdialysis fibers (values in this paper refer to the latter). The group I antagonists were also applied at 10-25 min after capsaicin injection. S-DHPG, a group I agonist at both mGluR1 and mGluR5, potentiated the responses to innocuous and noxious stimuli (BRUSH > PRESS > PINCH) at low concentrations (10-100 microM; n = 5) but had inhibitory effects at higher concentrations (1-10 mM; n = 5). The mGluR5 agonist CHPG (1 microM-100 mM; n = 5) did not potentiate but inhibited all responses (10-100 mM; n = 5). AIDA (1 microM-100 mM), a mGluR1-selective antagonist, dose-dependently depressed the responses to PINCH and PRESS but not to BRUSH (n = 6). The group I (mGluR1 > mGluR5) antagonist CPCCOEt (1 microM-100 mM) had similar effects (n = 6). Intradermal injections of capsaicin sensitized the STT cells to cutaneous mechanical stimuli. The enhancement of the responses by capsaicin resembled the potentiation by the group I mGluR agonist S-DHPG (BRUSH > PRESS > PINCH). CPCCOEt (1 mM) reversed the capsaicin-induced sensitization when given as posttreatment (n = 5). After washout of CPCCOEt, the sensitization resumed. Similarly, AIDA (1 mM; n = 7) reversed the capsaicin-induced sensitization and also blocked the potentiation by S-DHPG (n = 5). These data suggest that the mGluR1 subtype is activated endogenously during brief high-intensity cutaneous stimuli (PRESS, PINCH) and is critically involved in capsaicin-induced central sensitization.