Presynaptic group 1 metabotropic glutamate receptors may contribute to the expression of long-term potentiation in the hippocampal CA1 region

In this study, we investigated the possible contribution of presynaptic group 1 metabotropic glutamate receptor activation to changes in synaptic efficacy by means of analysis of glutamate release in hippocampal synaptosomes. Data were interpreted in the context of group 1 metabotropic glutamate receptor involvement in synaptic plasticity in the CA1 region of freely moving rats. In synaptosomes, 3,5-dihydroxyphenylglycine enhanced diacylglycerol formation and facilitated vesicular Ca(2+)-dependent glutamate release, whereas trans-azetidine-2,4-dicarboxylic acid had no effect on these processes. Trans-azetidine-2,4-dicarboxylic acid enhanced glutamate release, but in a Ca(2+)-independent manner. This effect was mimicked by the L-glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid. (R,S)-alpha-Methyl-4-carboxyphenylglycine blocked the effects of 3,5-dihydroxyphenylglycine, but not trans-azetidine-2,4-dicarboxylic acid in synaptosomes. Short-term potentiation (100 Hz, three bursts of 10 stimuli, 0.1 ms stimulus duration, 10 s interburst interval) was induced in the CA1 region in vivo. The metabotropic glutamate receptor agonist 1S,3R-aminocyclopentane-2,3-dicarboxylic acid, or the group 1 metabotropic glutamate receptor agonists, 3,5-dihydroxyphenylglycine and trans-azetidine-2,4-dicarboxylic acid, dose-dependently facilitated short-term potentiation into long-term potentiation, which lasted > 24 h. The facilitation was inhibited by the metabotropic glutamate receptor antagonist, (R,S)-alpha-methyl-4-carboxyphenylglycine, and the group 1 metabotropic glutamate receptor antagonist, (S)-4-carboxy-phenylglycine, but not by the group 2 metabotropic glutamate receptor antagonist, (R,S)-alpha-methylserine-O-phosphate monophenyl ester. L-Trans-pyrrolidine-2,4-dicarboxylic acid dose-dependently facilitated short-term potentiation into long-term potentiation, which lasted < 4 h. These data suggest that activation of group 1 metabotropic glutamate receptors results in presynaptic modulation of glutamate release. This effect may contribute to group 1 metabotropic glutamate modulation of the expression of long-term potentiation in vivo.     

大鼠纹状体I组代谢型谷氨酸受体激动引起动物向对侧旋转

目的:研究代谢型谷氨酸受体(mGluRs)激动剂引起大鼠向对侧旋转时介导的受体亚型。方法:大鼠纹状体内微量注射mGluRs激动剂或拮抗剂,观察大鼠的意识、行为变化,并于给药后6h测定旋转活动。结果:mGluRs非亚型特异的激动剂tACPD(500、1000nmol)纹状体内注射引起大鼠向对侧旋转,mGluRs的非竞争性拮抗剂L-AP₃、竞争性拮抗剂MCPG及抑制细胞内钙释放的胆罗啉均可减轻tACPD引起的旋转。I组mGluRs的特异性激动剂DHPG(500nmol)纹状体内注射也引起大鼠向对侧旋转,MCPG及mGluR₁的拮抗剂LY367385及mGluR₅的拮抗剂MPEP均可拮抗DHPG引起的旋转。预先腹腔注射利血平(5mg/kg)可阻断DHPG的作用。结论:I组mGluRs激动引起大鼠向对侧旋转,此作用可能与细胞内钙释放有关及依赖于多巴胺的存在。     

A specific role for group II metabotropic glutamate receptors in hippocampal long-term depression and spatial memory

Activity-dependent and sustained alterations in synaptic efficacy are widely regarded as the cellular correlates underlying learning and memory. Metabotropic glutamate receptors (mGluRs) are intrinsically involved in both hippocampal synaptic plasticity and spatial learning. Group II mGluRs are required for persistent hippocampal long-term depression (LTD), but are not required for long-term potentiation (LTP) in the hippocampal CA1 region in vivo. The role of these receptors in spatial learning, and in synaptic plasticity in the dentate gyrus in vivo has not yet been the subject of close scrutiny. We investigated the effects of group II mGluR antagonism on LTP and LTD in the adult rat, at medial perforant path-dentate gyrus synapses, and on spatial learning in the eight-arm radial maze. Daily application of the group 2 mGluR antagonist (2S)-alpha-ethylglutamic acid (EGLU) resulted in impairment of long-term (reference) memory with effects becoming apparent 6 days after training and drug-treatment began. Short-term (working) memory was unaffected throughout the 10-day study. Acute injection of EGLU did not affect either LTD or LTP in the dentate gyrus in vivo. Following six daily applications of EGLU a clear impairment of LTD but not LTP was apparent however. These data support that prolonged antagonism of group II mGluRs results in an impairment of LTD that parallels the appearance of spatial memory deficits arising from group II mGluR antagonism. These findings support the importance of group II mGluRs for spatial memory formation and offer a further link between LTD and the encoding of spatial information in the hippocampus.     

Preso1 dynamically regulates group I metabotropic glutamate receptors

Group I metabotropic glutamate receptors (mGluRs), including mGluR1 and mGluR5, are G protein–coupled receptors (GPCRs) that are expressed at excitatory synapses in brain and spinal cord. GPCRs are often negatively regulated by specific G protein–coupled receptor kinases and subsequent binding of arrestin-like molecules. Here we demonstrate an alternative mechanism in which group I mGluRs are negatively regulated by proline-directed kinases that phosphorylate the binding site for the adaptor protein Homer, and thereby enhance mGluR–Homer binding to reduce signaling. This mechanism is dependent on a multidomain scaffolding protein, Preso1, that binds mGluR, Homer and proline-directed kinases and that is required for their phosphorylation of mGluR at the Homer binding site. Genetic ablation of Preso1 prevents dynamic phosphorylation of mGluR5, and Preso1(?/?) mice exhibit sustained, mGluR5-dependent inflammatory pain that is linked to enhanced mGluR signaling. Preso1 creates a microdomain for proline-directed kinases with broad substrate specificity to phosphorylate mGluR and to mediate negative regulation.     

Activation of group I metabotropic glutamate receptors modulates locomotor-related motoneuron output in mice

Fast glutamatergic transmission via ionotropic receptors is critical for the generation of locomotion by spinal motor networks. In addition, glutamate can act via metabotropic glutamate receptors (mGluRs) to modulate the timing of ongoing locomotor activity. In the present study, we investigated whether mGluRs also modulate the intensity of motor output generated by spinal motor networks. Application of the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) reduced the amplitude and increased the frequency of locomotor-related motoneuron output recorded from the lumbar ventral roots of isolated mouse spinal cord preparations. Whole cell patch-clamp recordings of spinal motoneurons revealed multiple mechanisms by which group I mGluRs modulate motoneuron output. Although DHPG depolarized the resting membrane potential and reduced the voltage threshold for action potential generation, the activation of group I mGluRs had a net inhibitory effect on motoneuron output that appeared to reflect the modulation of fast, inactivating Na(+) currents and action potential parameters. In addition, group I mGluR activation decreased the amplitude of locomotor-related excitatory input to motoneurons. Analyses of miniature excitatory postsynaptic currents indicated that mGluRs modulate synaptic drive to motoneurons via both pre- and postsynaptic mechanisms. These data highlight group I mGluRs as a potentially important source of neuromodulation within the spinal cord that, in addition to modulating components of the central pattern generator for locomotion, can modulate the intensity of motoneuron output during motor behavior. Given that group I mGluR activation reduces motoneuron excitability, mGluRs may provide negative feedback control of motoneuron output, particularly during high levels of glutamatergic stimulation.     

Coactivation of metabotropic glutamate receptors and of voltage-gated calcium channels induces long-term depression in cerebellar Purkinje cells in vitro

Summary Using an in vitro slice preparation, we studied the effects, on parallel fiber (PF)-mediated EPSPs, of coactivation of metabotropic-glutamate receptors and of voltage-gated calcium (Ca) channels of Purkinje cells (PCs) by bath application of 50 M trans-1-aminocyclopentyl-1,3-dicarboxylate (trans-ACPD) and by direct depolarization of the cells, respectively. These effects were compared with changes in synaptic efficacy obtained when -amino-3hydroxy-5-methylisoxalone-4-propionate (AMPA) receptors of PCs were also activated through stimulation of PFs during the pairing protocol, as well as when similar experiments were performed without trans-ACPD in the bath. In a control medium, pairing for 1 min of PF-mediated EPSPs evoked at 1 Hz with Ca spikes evoked by steady depolarization of PCs (n = 13) led to LTD of synaptic transmission in 9 cases whereas for the others EPSPs were not affected. No LTD occurred in 9 out of 10 other cells tested when PF stimulation was omitted during the 1 min period of Ca spike firing of PCs. Bath application of 50 M trans-ACPD, in conjunction with the same pairing protocol as before (n = 8), led to a significantly larger LTD of PF-mediated EPSPs after washing out of this drug. Moreover, a clearcut LTD of PF-mediated EPSPs was also observed in 5 of the 8 other cells, when PF stimulation was omitted during Ca spike firing in the presence of trans-ACPD. As trans-ACPD alone induced fully reversible depressions of EPSPs, coactivation of metabotropic-glutamate receptors and of voltage-gated Ca channels is therefore likely to be sufficient to induce LTD of PF-mediated EPSPs.     

Developmental shift from long-term depression to long-term potentiation in the rat medial vestibular nuclei: role of group I metabotropic glutamate receptors

The effects of high frequency stimulation (HFS) of the primary vestibular afferents on synaptic transmission in the ventral part of the medial vestibular nuclei (vMVN) were studied during postnatal development and compared with the changes in the expression of the group I metabotropic glutamate receptor (mGluR) subtypes, mGluR1 and mGluR5. During the first stages of development, HFS always induced a mGluR5- and GABA_A-dependent long-term depression (LTD) which did not require NMDA receptor and mGluR1 activation. The probability of inducing LTD decreased progressively throughout the development and it was zero at about the end of the second postnatal week. Conversely, long-term potentiation (LTP) appeared at the beginning of the second week and its occurrence increased to reach the adult value at the end of the third week. Of interest, the sudden change in the LTP frequency occurred at the time of eye opening, about the end of the second postnatal week. LTP depended on NMDA receptor and mGluR1 activation. In parallel with the modifications in synaptic plasticity, we observed that the expression patterns and localizations of mGluR5 and mGluR1 in the medial vestibular nuclei (MVN) changed during postnatal development. At the earlier stages the mGluR1 expression was minimal, then increased progressively. In contrast, mGluR5 expression was initially high, then decreased. While mGluR1 was exclusively localized in neuronal compartments and concentrated at the postsynaptic sites at all stages observed, mGluR5 was found mainly in neuronal compartments at immature stages, then preferentially in glial compartments at mature stages. These results provide the first evidence for a progressive change from LTD to LTP accompanied by a distinct maturation expression of mGluR1 and mGluR5 during the development of the MVN.     

Activation of presynaptic group I metabotropic glutamate receptors enhances glutamate release in the rat spinal cord substantia gelatinosa

The activation of group I metabotropic glutamate receptors (mGluRs) produces a long-term potentiation of sensory transmission in the substantia gelatinosa (SG) region of the spinal cord (Prog. Brain Res. 129 (2000) 115). The mechanism(s) responsible for the induction of this potentiation is not known. Using rat spinal cord slice preparation and patch-clamp recordings, here we show, that the activation of the group I mGluRs by (S)-3,5-dihydroxyphenylglycine (DHPG, 1 microM), the mGluR1/5 agonist, increased the frequency of both activity-dependent spontaneous EPSCs, and activity-independent miniature EPSCs (mEPSCs). However, DHPG did not affect amplitude of mEPSCs. The effects of DHPG were not seen in the presence of the preferential mGluR1 antagonist CPCCOEt (10 microM). On the other hand, 2-methyl-6-(phenylethynyl)-pyridine (10 microM), a selective mGluR5 antagonist, blocked the DHPG facilitation present during the wash-out of the drug. This novel facilitating effect of the group I mGluR activation on glutamate release is the first report of a direct facilitatory action of both mGluR1 and mGluR5 subtypes on sensory transmission in the spinal cord SG region. These results indicate the potential contribution of synaptic activation of these facilitatory autoreceptors in plasticity of primary afferent neurotransmission.     

Arachidonic acid rescues hippocampal long-term potentiation blocked by group I metabotropic glutamate receptor antagonists

Although there is evidence that group I metabotropic glutamate receptors participate in long-term potentiation, the role of these receptors remains unclear. Among antagonists of group I metabotropic glutamate receptors, the mGluR5-selective 6-methyl-2-(phenylethynyl)-pyridine inhibited long-term potentiation in the CA1 region of hippocampal slices from 30-day-old rats, whereas (RS)-1-aminoindan-1,5-dicarboxylic acid and cyclopropan[b]chromen-1a-carboxylic acid ethylester, which are more selective for mGluR1, failed to inhibit long-term potentiation. Evidence also indicates that arachidonic acid is required for long-term potentiation, as inhibition of phospholipase A(2) blocks long-term potentiation. Administration of arachidonic acid immediately after tetanic stimulation restored long-term potentiation that had been inhibited by group I antagonists. Furthermore, arachidonic acid overcame inhibition of long-term potentiation by xestospongin C, an inositol triphosphate receptor channel blocker, or by thapsigargin, an agent that depletes intracellular calcium stores. However, arachidonic acid did not restore long-term potentiation blocked by N-methyl-D-aspartate receptor antagonists.Although it has been assumed that the source of the arachidonic acid necessary for long-term potentiation is N-methyl-D-aspartate receptor activation, our results suggest that during long-term potentiation group I metabotropic glutamate receptors cause arachidonic acid release by mobilization of intracellular calcium.     

Activation of group I metabotropic glutamate receptors depresses recurrent inhibition of motoneurons in the neonatal rat spinal cord in vitro

A variety of experimental studies have demonstrated the neuroprotective effects of melatonin, based on its antioxidant activity. In a prospective randomized study, the effects of melatonin were investigated in experimental head trauma-induced oxidative stress in rabbits. The experimental study was performed on 30 rabbits. The animals were divided into three groups. Group I (sham procedure): a right parietal craniotomy was performed on each animal, and the dura mater was left intact. Group II: experimental brain trauma (EBT) was performed on each animal using a 1 cm inner diameter × 10 cm long glass tube, through which a 20 g weight (0.5 cm diameter) was dropped onto the brain at the craniotomy site, causing a contusional head trauma. Group III: the same EBT model was performed, but 2.5 mg/kg melatonin was injected intraperitoneally four times (total dose 10 mg/kg); these injections were performed 20 min before the operation, during the trauma, 1 h later and 2 h later. The rabbits were sacrificed after the EBT at 24 h after the brain trauma. The activities of the three principal antioxidant enzymes—catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GSH-Px)—were determined, and the levels of malondialdehyde (MDA), a product of lipid peroxidation, and glutathione (GSH) were measured in brain homogenates. MDA levels were found to be higher in the EBT group than in the EBT+melatonin group or the sham procedure group. The SOD activity was found to be higher in the EBT group than in the sham procedure group. Enzymatic parameters (except for SOD) were significantly higher in melatonin-treated animals than in EBT animals. GSH levels in melatonin-treated animals were decreased compared with EBT animals. In conclusion, the data indicate that melatonin protects against free radical-mediated oxidative changes in brain tissue by boosting antioxidant enzymes, and in particular lowering lipid peroxidation in rabbits with EBT.     

Synaptically stimulated induction of group I metabotropic glutamate receptor-dependent long-term depression and depotentiation is inhibited by prior activation of metabotropic glutamate receptors and protein kinase C

We have investigated metaplasticity of the group I metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) and depotentiation (DP) induced by physiological synaptic stimulation in the medial perforant path of the dentate gyrus in vitro. Group I mGluR-LTD/DP was inhibited by prior preconditioning brief high frequency stimulation (HFS) if the preconditioning HFS induced long-term potentiation (LTP) or if the induction of LTP was inhibited by an NMDA receptor antagonist. The inhibitory effect of the preconditioning HFS on LTD/DP was dependent upon activation of mGluRs, as it was blocked by the presence of the mGluR antagonist (S)-alpha-methyl-4-carboxyphenylglycine during the preconditioning stimulation. The inhibitory effect of the preconditioning HFS involved stimulation of PKC, as the presence of the PKC inhibitor bisindolylmaleimide (BIS) during the preconditioning stimulation prevented the inhibitory effect of such preconditioning stimulation. Activation of PKC was also necessary for the induction of mGluR-LTD itself, as the PKC inhibitor BIS prevented the induction of the mGluR-LTD. We suggest that the physiological stimulation of mGluRs by the preconditioning stimulation produces a PKC-dependent inactivation of subsequent group I mGluR functioning and thereby an inhibition of induction of group I mGluR-dependent LTD/DP induction.     

Reduced activity of hippocampal group-I metabotropic glutamate receptors in learning-prone rats

Following the hypothesis of the "signal-to-noise" ratio we examined whether changes in the activity of group-I metabotropic glutamate (mGlu) receptors in the hippocampus are associated with a condition that specifically enhances the learning capacity in rats. As a model, we used rats that had been nursed by mothers drinking a solution of corticosterone (13.5 mg of daily intake of corticosterone hemisuccinate) during the lactation period. These rats were prone to learn, as indicated by a better performance in a passive avoidance test. Stimulation of polyphosphoinositide (PI) hydrolysis by the mGlu receptor agonist, 1S,3R-1-amino-cyclopentan-1,3-dicarboxylic acid (1S,3R-ACPD), was attenuated in hippocampal slices prepared from corticosterone-nursed male and female rats at 30 or 60 days of postnatal life, an age at which an increased learning capacity could be demonstrated. This effect was specific because the PI response to carbamylcholine was unchanged. A reduced PI hydrolysis in corticosterone-nursed rats was also observed when group-I mGlu receptors (i.e. mGlu1 and -5 receptors) were selectively activated using 3,5-dihydroxyphenylglycine or 1S,3R-APCD combined with the selective group-II mGlu receptor antagonist, 2S-2-amino-2-(1S,2S-2-carboxycyclopropan-1-yl)-3-(xanth-9-yl)propionate. Western blot analysis showed a selective reduction in the expression of mGlu1a receptor protein in the hippocampus of corticosterone-nursed rats, whereas expression of mGlu5 and mGlu2/3 receptors was unchanged. The reduction in mGlu-receptor mediated PI hydrolysis in the hippocampus may contribute to the greater learning capacity of corticosterone-nursed rats by reducing the background noise over which a specific signal must be superimposed during learning. This hypothesis was supported by the evidence that mGlu-receptor stimulated PI hydrolysis was amplified in hippocampal slices from rats subjected to a passive avoidance learning paradigm, and that this amplification was greater in slices from corticosterone-nursed rats of both sexes.     

Cooperativity between activation of metabotropic glutamate receptors and NMDA receptors in the induction of LTP in hippocampal CA1 neurons

In CA1 neurons of guinea pig hippocampal slices, long-term potentiation (LTP) was induced by 10 min application of 10 microM aminocyclopentane-1S, 3R-dicarboxylic acid (ACPD), the metabotropic glutamate receptor (mGluR) agonist, in the presence of test synaptic inputs (once every 20 s). In contrast, long-term depression (LTD) was induced by application of 10 microM ACPD in the absence of test inputs. When 10 microM ACPD was applied in the presence of test inputs, co-application of the N-methyl-D-aspartate (NMDA) receptor antagonist, D,L-2-amino-5-phosphonovalerate resulted in LTD induction when used at 50 microM. In ACPD-induced LTP, the delivery of test synaptic inputs to CA1 neurons could be replaced by co-application of NMDA (100 nM) during ACPD perfusion. These results suggest that, in CA1 neurons, a co-operative effect involving the activation of both mGluRs and NMDA receptors is required to trigger the process involved in ACPD-induced LTP. In addition, ACPD-induced LTD was blocked by co-application of an inositol 1,4,5-trisphosphate (IP3) receptor inhibitor, 2-aminotheoxydiphenyl borate (10 microM), which had no effect on ACPD-induced LTP. The results of the present study, therefore, indicate that ACPD-induced LTP involves NMDA receptors, but not IP3 receptors, whereas the converse applies to ACPD-induced LTD.     

Peripheral group I metabotropic glutamate receptors modulate nociception in mice

The metabotropic glutamate receptors (mGluRs) are found throughout the central nervous system, where they modulate neuronal excitability and synaptic transmission. Here we report the presence of phospholipase C-coupled group I mGluRs (mGluR1 and mGluR5) outside the central nervous system on peripheral unmyelinated sensory afferents. Given their localization on predominantly nociceptive afferents, we investigated whether these receptors modulate nociceptive signaling, and found that agonist-induced activation of peripheral group I mGluRs leads to increased sensitivity to noxious heat, a phenomenon termed thermal hyperalgesia. Furthermore, group I mGluR antagonists not only prevent, but also attenuate established formalin-induced pain. Taken together, these results suggest that peripheral mGluRs mediate a component of hyperalgesia and may be therapeutically targeted to prevent and treat inflammatory pain.     

Focal photolysis of caged glutamate produces long-term depression of hippocampal glutamate receptors

Separating contributions of pre- and postsynaptic factors to the maintenance of long-term potentiation (LTP) and long-term depression (LTD) has been confounded by their experimental interdependence. To isolate the postsynaptic contribution, glutamate-receptor-mediated currents were elicited by localized photolysis of caged glutamate in small spots along the dendrites of CA1 hippocampal pyramidal cells. With synaptic transmission blocked, pairing depolarization of pyramidal cells with repeated photolysis of caged glutamate at one site markedly and persistently depressed subsequent responses to glutamate; responses at a second, unpaired site were unchanged. Like synaptically induced LTD at the CA3-CA1 synapse, this depression was site specific, NMDA-receptor dependent and blocked by protein-phosphatase inhibitors. Thus, robust, persistent alterations of postsynaptic glutamate receptor efficacy can occur without presynaptic neurotransmitter release.     

Excitation of cerebellar interneurons by group I metabotropic glutamate receptors

Cerebellar basket and stellate neurons (BSNs) provide feed-forward inhibition to Purkinje neurons (PNs) and thereby play a principal role in determining the output of the cerebellar cortex. During low-frequency transmission, glutamate released at parallel fiber synapses excites BSNs by binding to AMPA receptors; high-frequency transmission also recruits N-methyl-d-aspartate (NMDA) receptors. We find that, in addition to these ligand-gated receptors, a G-protein-coupled glutamate receptor subtype participates in exciting BSNs. Stimulation of metabotropic glutamate receptor 1alpha (mGluR1alpha) with the mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) leads to an increase in spontaneous firing of BSNs and indirectly to an increase in the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded in PNs. Under conditions in which ligand-gated glutamate receptors are blocked, parallel fiber stimulation generates a slow excitatory postsynaptic current (EPSC) in BSNs that is inhibited by mGluR1alpha-selective antagonists. This slow EPSC is capable of increasing BSN spiking and indirectly increasing sIPSCs frequency in PNs. Our findings reinforce the idea that distinct subtypes of glutamate receptors are activated in response to different patterns of activity at excitatory synapses. The results also raise the possibility that mGluR1alpha-dependent forms of synaptic plasticity may occur at excitatory inputs to BSNs.     

Desensitization of group I metabotropic glutamate receptors in rat sympathetic neurons

Desensitization of heterologously expressed metabotropic glutamate receptor 5a (mGluR5a) was examined in rat sympathetic neurons. Calcium currents in cells expressing mGluR5a exhibited substantial inhibition in response to glutamate exposure. In the continued presence of glutamate, inhibition attenuated rapidly over the course of about a minute. Desensitization was eliminated when a nonhydrolyzable ATP analogue was substituted for ATP in the pipette solution, suggesting that desensitization was mediated by a phosphorylation event. Next, pharmacological agents were used to investigate the nature of the kinase involved in desensitization. Desensitization was sensitive to the nonspecific kinase inhibitor, staurosporine, but not H-7, another nonspecific kinase inhibitor. Inhibitors of myosin light chain kinase and calmodulin-dependent kinase were without effect on desensitization. However, desensitization was sensitive to the protein kinase C inhibitor bisindolymaleimide. In contrast, G?6976, a selective inhibitor of conventional protein kinase C isoforms, was without effect. In addition, desensitization persisted in the presence of 10 mM intracellular bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid, a fast Ca(2+) chelator. Finally, overexpression of wild-type calmodulin, which can bind mGluR5 and inhibit phosphorylation, did not alter mGluR desensitization. Two Ca(2+)-binding-deficient calmodulin mutants were also without effect. These data indicate a role for nonconventional protein kinase C isoforms as a mediator of mGluR5 desensitization and that the phosphorylation of mGluR5a that competes with calmodulin binding does not mediate desensitization.     

Activation of group II metabotropic glutamate receptors results in long-term potentiation following preconditioning stimulation in the dentate gyrus.

The role of group II metabotropic glutamate receptors in the induction/expression of long-term potentiation has been investigated in the medial perforant path of the outer (infrapyramidal) blade of the rat dentate gyrus in vitro. Activation of group II metabotropic glutamate receptors by perfusion of the selective agonist LY354740 did not induce long-term potentiation or long-term depression in control. However, LY354740, applied following the induction of long-term potentiation by high frequency stimulation, resulted in additional long-term potentiation. LY354740 was only found to cause additional long-term potentiation if the pre-existing high frequency stimulation-induced long-term potentiation was sub-maximal. Although activation of metabotropic glutamate receptors was not required for induction of high frequency stimulation-induced long-term potentiation, activation of both group I and group II metabotropic glutamate receptors was required during high frequency stimulation-induced long-term potentiation in order for subsequent application of LY354740 to result in additional long-term potentiation. Thus, the long-term potentiation caused by application of LY354740 following high frequency-induced long-term potentiation was prevented if the high frequency stimulation was given in the presence of (S)-alpha-methyl-4-carboxyphenylglycine or the selective group I or group II metabotropic glutamate receptor antagonists 1-aminoindan-1,5-dicarboxylic acid or (2S)-alpha-ethylglutamic acid respectively. The long-term potentiation caused by LY354740 was also dependent upon activation of N-methyl-D-aspartate receptors during the high frequency stimulation, being blocked if high frequency stimulation was given in the presence of the N-methyl-D-aspartate receptor antagonist, D(-)-2-amino-5-phosphonopentanoic acid. The long-term potentiation resulting from activation of group II metabotropic glutamate receptors could be due either to the enhancement of the expression level of the high frequency stimulation-induced long-term potentiation, or alternatively, to a direct novel induction of long-term potentiation. In either theory, the long-term potentiation resulting from activation of group II metabotropic glutamate receptors is dependent upon prestimulation of group I and group II metabotropic glutamate receptors and N-methyl-D-aspartate receptors during the 'preconditioning high frequency stimulation'.     

Modulation of afterpotentials and firing pattern in guinea pig CA3 neurones by group I metabotropic glutamate receptors

Activation of group I metabotropic glutamate receptors (mGluRs) alters the firing patterns of individual CA3 pyramidal cells in guinea pig hippocampal slices. Following addition of the selective group I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) to the bathing solution, pyramidal cells initially firing regular, single action potentials switched to firing in brief bursts. This change in firing pattern resulted from modulation by mGluRs of three afterpotentials. The medium and slow afterhyperpolarizations (m and sAHPs) were blocked by mGluR activation. In addition, a voltage-dependent afterdepolarization (ADP) was induced. Recordings from mutant mice lacking phospholipase C_β1 (PLC_β1) showed that mGluR block of the mAHP, as well as induction of the ADP, depended on the phosphoinositide hydrolysis pathway. Block of the sAHP, however, was partly spared in the absence of PLC_β1. Optical recordings of postspike intracellular Ca²⁺ rises showed that mGluR block of the AHP was not mediated by alterations of action potential-associated Ca²⁺ increases (Ca²⁺ transients). The mGluR induction of an ADP was also independent of any changes in the Ca²⁺ transient. The mGluR-induced change in the firing pattern of hippocampal pyramidal cells is thus the result of multiple mechanisms, including suppression of both m and sAHPs and activation of an ADP, that act together to produce a specific excitatory effect, namely an increased likelihood that a single action potential will lead immediately to one or more following action potentials.