The glutamate receptor of the Qp-type activates protein kinase C and is regulated by protein kinase C

An Essential Role of Fyn in the Modulation of Metabotropic Glutamate Receptor 1 in Neurons

Fyn is a member of the Src family of nonreceptor tyrosine kinases and is broadly expressed in the CNS. As a synapse-enriched kinase, Fyn interacts with and phosphorylates local substrates to regulate synaptic transmission and plasticity, although our knowledge of specific targets of Fyn at synaptic sites remains incomplete and the accurate role of Fyn in regulating synaptic proteins is poorly understood. In this study, we initiated an effort to explore the interaction of Fyn with a metabotropic glutamate receptor (mGluR). We found that recombinant Fyn directly binds to mGluR1a at a consensus binding motif located in the intracellular C-terminus (CT) of mGluR1a in vitro. Similarly, endogenous Fyn interacts with mGluR1a in adult rat cerebellar neurons in vivo. Active Fyn phosphorylates mGluR1a at a conserved tyrosine residue in the CT region. In cerebellar neurons and transfected HEK293T cells, the Fyn-mediated tyrosine phosphorylation of mGluR1a is constitutively active and acts to facilitate the surface expression of mGluR1a and to potentiate the mGluR1a postreceptor signaling. These results support mGluR1a to be a novel substrate of Fyn. Fyn, by binding to and phosphorylating mGluR1a, potentiates surface expression and signaling of the receptors.

Synaptic ERK2 Phosphorylates and Regulates Metabotropic Glutamate Receptor 1 In Vitro and in Neurons

A synaptic pool of extracellular signal-regulated kinases (ERK) controls synaptic transmission, although little is known about its underlying signaling mechanisms. Here, we found that synaptic ERK2 directly binds to postsynaptic metabotropic glutamate receptor 1a (mGluR1a). This binding is direct and the ERK-binding site is located in the intracellular C-terminus (CT) of mGluR1a. Parallel with this binding, ERK2 phosphorylates mGluR1a at a cluster of serine residues in the distal part of mGluR1a-CT. In rat cerebellar neurons, ERK2 interacts with mGluR1a at synaptic sites, and active ERK constitutively phosphorylates mGluR1a under normal conditions. This basal phosphorylation is critical for maintaining adequate surface expression of mGluR1a. ERK is also essential for controlling mGluR1a signaling in triggering distinct postreceptor signaling transduction pathways. In summary, we have demonstrated that mGluR1a is a sufficient substrate of ERK2. ERK that interacts with and phosphorylates mGluR1a is involved in the regulation of the trafficking and signaling of mGluR1.

Phosphorylation and feedback regulation of metabotropic glutamate receptor 1 by calcium/calmodulin-dependent protein kinase II

The metabotropic glutamate receptor 1 (mGluR1) is a Gα(q)-protein-coupled receptor and is distributed in broad regions of the mammalian brain. As a key element in excitatory synaptic transmission, the receptor regulates a wide range of cellular and synaptic activities. In addition to regulating its targets, the receptor itself is believed to be actively regulated by intracellular signals, although underlying mechanisms are essentially unknown. Here we found that a synapse-enriched protein kinase, Ca²⁺/calmodulin-dependent protein kinase IIα (CaMKIIα), directly binds to the intracellular C terminus (CT) of mGluR1a. This binding is augmented by Ca²⁺ in vitro. The direct interaction promotes CaMKIIα to phosphorylate mGluR1a at a specific threonine site (T871). In rat striatal neurons, the mGluR1 agonist triggers the receptor-associated phosphoinositide signaling pathway to induce Ca²⁺-dependent recruitment of CaMKIIα to mGluR1a-CT. This enables the kinase to inhibit the response of the receptor to subsequent agonist exposure. Our data identify an agonist-induced and Ca²⁺-dependent protein-protein interaction between a synaptic kinase and mGluR1, which constitutes a feedback loop facilitating desensitization of mGluR1a.

Intracellular messengers involved in spontaneous pain, heat hyperalgesia, and mechanical allodynia induced by intrathecal dihydroxyphenylglycine

We investigated the role of two intracellular second messengers, extracellular signal-regulated protein kinase (ERK) and protein kinase C (PKC), in a model of persistent pain using intrathecal (i.t.) (R,S)-3,5-dihydroxyphenylglycine (DHPG). Spontaneous nociceptive behaviours (SNBs), mechanical allodynia (von Frey thresholds) and heat hyperalgesia (plantar test latencies) induced by DHPG were measured in animals pretreated i.t. with membrane permeable inhibitors of ERK (PD 98059) and PKC (GF 109203X). Spinal administration of PD 98059 dose-dependently reduced SNBs, and attenuated both mechanical allodynia and heat hyperalgesia induced by DHPG. GF 109203X treatment also reduced SNBs and heat hyperalgesia, but did not affect mechanical allodynia induced by DHPG. Neither PD 98059, nor GF 109203X, altered mechanical or thermal thresholds in saline-injected control rats. These results suggest that both ERK and PKC are involved in persistent pain associated with the i.t. administration of DHPG.

Agents that act by different mechanisms modulate the activity of protein kinase CβII isozyme in the rat spinal cord during peripheral inflammation

Hyperalgesia following unilateral complete Freund's adjuvant-induced inflammation was characterized by paw withdrawal latency to thermal stimulus. Paw withdrawal latencies were significantly shorter on the complete Freund's adjuvant-treated paw than on the contralateral paw of the complete Freund's adjuvant- and the sham-treated rats. Total cytosolic protein kinase C activity in the lumbar enlargement was unchanged on the sides of the spinal cord ipsi- and contra-lateral to the inflamed paw. Membrane-associated activities of protein kinase Calpha, protein kinase CbetaI and protein kinase Cgamma did not change significantly on the sides of the cord ipsi- and contra-lateral to the inflammation. However, membrane-associated activity of protein kinase CbetaII was increased in the cord section ipsilateral to the inflammation, suggesting that increased translocation/activation of protein kinase CbetaII is related to thermal hyperalgesia. Dextrorphan (an N-methyl-D-aspartate receptor antagonist), L-703,606 (an NK-1 receptor antagonist) and an antisense oligodeoxynucleotide for a selective knockdown of protein kinase Cbeta, reduced complete Freund's adjuvant-induced hyperalgesia, and reversed significant changes in the membrane activity of protein kinase CbetaII on the spinal cord section ipsilateral to the inflamed paw. Dextrorphan and protein kinase Cbeta antisense oligodeoxynucleotide were effective in reversing complete Freund's adjuvant-induced increase in the activity of protein kinase CbetaII ipsilateral to the inflammation at all the doses tested, but L-703,606 was effective only at the highest dose. Furthermore, in the presence of inflammatory stimulus, dextrorphan and L-703,606 did not alter the activities of membrane-associated protein kinase Calpha, protein kinase CbetaI, and protein kinase Cgamma in the section of the spinal cord ipsi- and contra-lateral to the inflammation. Protein kinase Cbeta antisense oligodeoxynucleotide had no significant effect on the membrane-associated activities of protein kinase Calpha and protein kinase Cgamma, but decreased the activities of both protein kinase CbetaI and protein kinase CbetaII and the expression of protein kinase Cbeta isozyme in the spinal cord. The data provide evidence that a common molecular event that converges to initiate and maintain hyperalgesia may include the translocation and activation of protein kinase CbetaII in the spinal dorsal horn.

Differential regulation of metabotropic glutamate receptor 5-mediated phosphoinositide hydrolysis and extracellular signal-regulated kinase responses by protein kinase C in cultured astrocytes

The metabotropic glutamate receptor 5 (mGluR5) exhibits a rapid loss of receptor responsiveness to prolonged or repeated agonist exposure. This receptor desensitization has been seen in a variety of native and recombinant systems, and is thought to result from receptor-mediated, protein kinase C (PKC)-dependent phosphorylation of the receptor, uncoupling it from the G protein in a negative feedback regulation. We have investigated the rapid PKC-mediated desensitization of mGluR5 in cortical cultured astrocytes by measuring downstream signals from activation of mGluR5. These include activation of phosphoinositide (PI) hydrolysis, intracellular calcium transients, and extracellular signal-regulated kinase 2 (ERK2) phosphorylation. We present evidence that PKC plays an important role in rapid desensitization of PI hydrolysis and calcium signaling, but not in ERK2 phosphorylation. This differential regulation of mGluR5-mediated responses suggests divergent signaling and regulatory pathways which may be important mechanisms for dynamic integration of signal cascades.

Reduction of Potassium Conductances Mediated by Metabotropic Glutamate Receptors in Rat CA3 Pyramidal Cells Does Not Require Protein Kinase C or Protein Kinase A

Metabotropic glutamate receptors, unlike ionotropic receptors, exert their actions on ion channels via G-proteins coupled to second messenger systems. In the hippocampus stimulation of metabotropic receptors can lead to decreased potassium channel conductance, decreased accommodation of cell firing and inhibition of the slow calcium-dependent afterhyperpolarizing current (IAHP). Using the single-electrode voltage-clamp technique in hippocampal slice cultures of the rat, the role of protein kinases in mediating these metabotropic glutamate responses was investigated. In the presence of staurosporin, protein kinase C activation by phorbol esters and protein kinase A activation by 8-bromo-cyclic adenosine monophosphate were blocked. Under these conditions, the inhibition of IAHP by 1-amino-cyclopentyl-trans-dicarboxylate (ACPD), a metabotropic agonist, was unchanged, whilst the inward current elicited by ACPD was enhanced. These results demonstrate that, in the hippocampus, metabotropic glutamate responses persist during inhibition of protein kinase A and C activation. Furthermore, these responses are insensitive to pertussis toxin, confirming previous observations.

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.

Opposing effects of protein kinase C and protein kinase A on metabotropic glutamate receptor signaling: selective desensitization of the inositol trisphosphate/Ca2+ pathway by phosphorylation of the receptor-G protein-coupling domain

Signaling by the metabotropic glutamate receptor 1alpha (mGluR1alpha) can lead to the accumulation of inositol 1,4, 5-trisphosphate (InsP(3)) and cAMP and to the modulation of K(+) and Ca(2+) channel opening. At present, very little is known about how these different actions are integrated and eventually turned off. Unraveling the molecular mechanisms underlying these functions is crucial for understanding mGluR-mediated regulation of synaptic transmission. It has been shown that receptor-induced activation of the InsP(3) pathway is subject to feedback inhibition mediated by protein kinase C (PKC). In this study, we provide evidence for a differential regulation by PKC and protein kinase A of two distinct mGluR1alpha-dependent signaling pathways. PKC activation selectively inhibits agonist-dependent stimulation of the InsP(3) pathway but does not affect receptor signaling via cAMP. In contrast, protein kinase A potentiates agonist-independent signaling of the receptor via InsP(3). Furthermore, we demonstrate that the selectivity of PKC action on receptor signaling rests on phosphorylation of a threonine residue located in the G protein-interacting domain of the receptor. Modification at Thr(695) selectively disrupts mGluR1alpha-G(q/11) interaction without affecting signaling through G(s). Together, these data provide insight on the mechanisms by which selective down-regulation of a specific receptor-dependent signaling pathway can be achieved and on how cross-talk between different second messenger cascades may contribute to fine-tune short- and long-term receptor activity.

Choline availability to the developing rat fetus alters adult hippocampal long-term potentiation

Supplementation with choline during pregnancy in rats causes a long-lasting improvement of visuospatial memory of the offspring. To determine if the behavioral effects of choline are related to physiological changes in hippocampus, the effect of perinatal choline supplementation or deficiency on long-term potentiation (LTP) was examined in hippocampal slices of 6-8 and 12-14 month old rats born to dams consuming a control, choline-supplemented, or a choline-free diet during pregnancy. Stimulating and recording electrodes were placed in stratum radiatum of area CA1 to record extracellular population excitatory postsynaptic potentials (pEPSPs). To induce LTP, a theta-like stimulus train was generated. The amplitude of the stimulus pulses was set at either 10% or 50% of the stimulus intensity which had induced the maximal pEPSP slope on the input/output curve. We found that at both ages, a significantly smaller percentage of slices from perinatally choline-deficient rats displayed LTP after 10% stimulus intensity (compared with control and choline-supplemented rats), and a significantly larger percentage of slices from choline-supplemented rats displayed LTP at 50% stimulus intensity (compared with control and choline-deficient rats). Results reveal that alterations in the availability of dietary choline during discrete periods of development lead to changes in hippocampal electrophysiology that last well into adulthood. These changes in LTP threshold may underlie the observed enhancement of visuospatial memory seen after prenatal choline supplementation and point to the importance of choline intake during pregnancy for development of brain and memory function.

Potentiation of NMDA and AMPA responses by the specific mGluR5 agonist CHPG in spinal cord motoneurons

The specific metabotropic glutamate receptor (mGluR)5 agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) is able to potentiate NMDA and AMPA responses recorded from ventral roots of the isolated hemisected baby rat spinal cord. Previously we have demonstrated that activation of group I mGluRs (mGluR1 and mGluR5) with the broad spectrum mGluR agonist 1S,3R-1-amino-1,3-cyclopentanedicarboxylate (ACPD) produced potentiation of ionotropic glutamate responses. In contrast to ACPD-induced potentiation, however, no evidence for an involvement of protein kinase C (PKC) is found in the CHPG-induced potentiation of both NMDA and AMPA depolarization because the PKC blockers chelerythrine chloride or calphostin C did not antagonize this effect. Moreover, in the absence of Ca2+ in the perfusing medium or depleting intracellular Ca2+ stores with thapsigargin or dantrolene did not modify the CHPG-induced enhancement of NMDA depolarizations. Phorbol-12,13-diacetate (PDA), on the other hand, was able to attenuate this effect, which was reversed by chelerythrine chloride. These results suggest that both mGluR5 and mGluR1 may act to enhance ionotropic glutamate responses but the two types of mGluRs may have different intracellular mechanisms of action.

Nitric oxide mediates the central sensitization of primate spinothalamic tract neurons

Nitric oxide (NO) has been proposed to contribute to the development of hyperalgesia by activating the NO/guanosine 3',5'-cyclic monophosphate (cGMP) signal transduction pathway in the spinal cord. We have examined the effects of NO on the responses of primate spinothalamic tract (STT) neurons to peripheral cutaneous stimuli and on the sensitization of STT cells following intradermal injection of capsaicin. The NO level within the spinal dorsal horn was increased by microdialysis of a NO donor, 3-morpholinosydnonimine (SIN-1). SIN-1 enhanced the responses of STT cells to both weak and strong mechanical stimulation of the skin. This effect was preferentially on deep wide dynamic range STT neurons. The responses of none of the neurons tested to noxious heat stimuli were significantly changed when SIN-1 was administered. Intradermal injection of capsaicin increased dramatically the content of NO metabolites, NO-2/NO-3, within the dorsal horn. This effect was attenuated by pretreatment of the spinal cord with a nitric oxide synthase (NOS) inhibitor, NG-nitro-L-arginine methyl ester (L-NAME). Sensitization of STT cells induced by intradermal injection of capsaicin was also prevented by pretreatment of the dorsal horn with the NOS inhibitors, L-NAME or 7-nitroindazole. Blockade of NOS did not significantly affect the responses of STT cells to peripheral stimulation in the absence of capsaicin injection. The data suggest that NO contributes to the development and maintenance of central sensitization of STT cells and the resultant mechanical hyperalgesia and allodynia after peripheral tissue damage or inflammation. NO seems to play little role in signaling peripheral stimuli under physiological conditions.

Inhibition of primate spinothalamic tract neurons by spinal glycine and GABA is modulated by guanosine 3',5'-cyclic monophosphate

Our recent work has suggested that the nitric oxide/guanosine 3', 5'-cyclic monophosphate (NO/cGMP) signal transduction system contributes to central sensitization of spinothalamic tract (STT) neurons in part by influencing the descending inhibition of nociception resulting from stimulation in the periaqueductal gray. This study was designed to examine further whether activation of the NO/cGMP cascade reduces the inhibition of the activity of STT neurons mediated by spinal inhibitory amino acid (IAA) receptors. Responses of STT cells to noxious cutaneous stimuli were inhibited by iontophoresis of glycine and GABA agonists in anesthetized monkeys. Administration of 8-bromoguanosine-3',5'-cyclophosphate sodium (8-bromo-cGMP), a membrane permeable analogue of cGMP, either by microdialysis or by iontophoresis reduced significantly the IAA-induced inhibition of wide dynamic range (WDR) STT cells in the deep layers of the dorsal horn. The reduction in inhibition lasted for up to 1-1.5 h after the cessation of drug infusion. In contrast, IAA-induced inhibition of WDR STT cells in the superficial dorsal horn and high-threshold (HT) cells in superficial or deep layers was not significantly changed during 8-bromo-cGMP infusion. Iontophoresis of 8-bromo-cGMP onto STT cells produced the same actions as produced by microdialysis of this agent, but the effect was not as long-lasting nor as potent. Finally, an attenuation of the IAA receptor-mediated inhibition of STT cells produced by iontophoretic release of a NO donor, 3-morpholinosydnonimine, could be blocked by pretreatment of the spinal cord with a guanylate cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. These results suggest that an increased spinal cGMP level contributes to the sensitization of WDR STT neurons in the deep dorsal horn in part by down-regulating spinal IAA receptors. However, no evidence is provided in this study that the NO/cGMP cascade regulates IAA receptors on HT and superficial WDR neurons. Combined with the preceding studies, our data support the view that NO and cGMP function in the same signal transduction cascade and play an important role in central sensitization.

Functional switch from facilitation to inhibition in the control of glutamate release by metabotropic glutamate receptors

We have investigated the role of metabotropic glutamate receptors linked to phosphoinositide hydrolysis in the control of glutamate release in cerebrocortical nerve terminals. The activation of these receptors with the agonist 3,5-dihydroxyphenylglycine enhanced intra-synaptosomal diacylglycerol and facilitated both the depolarization-induced increase in the cytosolic free Ca2+ concentration and the release of glutamate. However, 5 min after receptor activation, a second stimulation of the pathway with the agonist failed to produce diacylglycerol and to facilitate glutamate release. Interestingly, during the period in which the diacylglycerol response was desensitized, a strong agonist-induced inhibition of Ca2+ entry and glutamate release was observed. This change in the presynaptic effects of 3,5-dihydroxyphenylglycine is reversible since 30 min after the first stimulation, the agonist-induced inhibition of release disappeared, whereas both the production of diacylglycerol and the facilitation of glutamate release were recovered. The tonic elevation of the extracellular glutamate concentration from basal levels (0.8 microM) up to 5 microM also produced the switch from facilitation to inhibition in the receptor response. The existence of this activity-dependent switch in the presynaptic control of glutamate release suggests that release facilitation is limited to conditions under which an appropriate clearance of synaptic glutamate exists, probably to prevent the neurotoxic accumulation of glutamate in the synapse.

Evidence for functional metabotropic glutamate receptors in the dorsal cochlear nucleus

The parallel fibers (PFs) of the dorsal cochlear nucleus (DCN) molecular layer use glutamate as a neurotransmitter. Although metabotropic glutamate receptors (mGluRs) have been identified on cells postsynaptic to the PFs, little is known about the effects of mGluR activation in PF synaptic transmission in the DCN. To investigate these effects, PF-evoked field potentials were recorded from the DCN in guinea pig brain stem slice preparations. The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated components of the field response were reversibly depressed by bathing the slice in the mGluR agonists (+/-)-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) or (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD]. A similar depression was produced by the mGluR1/5 agonist (RS)-3,5-dihydroxyphenylglycine, but not by the mGluR2/3 agonist (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine or by the mGluR4/6/7/8 agonist L(+)-2-amino-4-phosphonobutyric acid. In addition to the AMPA component, an N-methyl-D-aspartate (NMDA) receptor-dependent component of the field potentials could be identified when the slices were bathed in a low magnesium solution. Under these conditions, the ACPD-induced depression of the AMPA component did not completely recover, whereas the depression of the NMDA component usually recovered and potentiated in some slices. Intracellular recordings of PF-evoked responses were obtained to ascertain which neuronal populations were affected by mGluR activation. Activation of mGluRs produced a reversible depression of PF-evoked responses in cartwheel cells that was not accompanied by any changes in paired-pulse facilitation. The PF-evoked responses recorded from pyramidal cells were unaffected by mGluR activation. Both cell types exhibited a reversible depolarization during (1S,3R)-ACPD application. Subsequent experiments explored the involvement of protein kinases in mediating the effects of mGluRs. The protein kinase C (PKC) activator phorbol-12,13-diacetate partially inhibited the mGluR-mediated depression of the field response; however, the PKC inhibitor 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimide or the protein kinase A inhibitor N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide had little effect on the actions of (1S,3R)-ACPD. These results demonstrate that functional mGluRs are present at PF synapses and are capable of modulating PF synaptic transmission in the DCN.

Persistent phosphorylation parallels long-term desensitization of cerebellar purkinje cell AMPA-type glutamate receptors

This study is aimed at testing the hypothesis that sustained phosphorylation underlies long-term desensitization of AMPA receptors, which is thought to be the mechanism of long-term synaptic depression in cerebellar Purkinje cells (PCs). We induced long-term desensitization of AMPA receptors in rat cerebellar slices by (1) a 4-min bath application of quisqualate (0.1 mM) or (2) a 15-min bath application of a protein kinase C (PKC) activator, phorbol-12,13-diacetate (0.5 microM) or -dibutyrate (0.6 microM), followed by a 4-min AMPA (0.1 mM) application. In slices so treated, labeling with an antibody (12P3) against a peptide corresponding to part of AMPA receptor subunit GluR2 including serine 696 and phosphorylated at this serine site revealed phosphorylation of the AMPA receptors in PC dendrites that was sustained for at least 1 hr. At an early phase, within 20 min after the chemical stimulation, the phosphorylation was resistant to an Ca2+ chelator (BAPTA-AM), a metabotropic glutamate receptor antagonist (MCPG), and a PKC inhibitor (calphostin C), whereas at a late phase, 30 min or more after the chemical stimulation, it was blocked by these reagents similarly to long-term desensitization of AMPA receptors. Taken together with data obtained previously using different protocols of chemical stimulation, the present results strongly support the above-mentioned hypothesis.

An electrophysiological study of neurons in the horizontal limb of the diagonal band of Broca

We examined the morphological and electrophysiological properties of neurons within the horizontal limb of the diagonal band of Broca (hDBB) and investigated the role of excitatory amino acid mediated synaptic transmission in this region. Whole cell patch-clamp recordings were obtained from hDBB neurons in rat forebrain slices. The hDBB cells examined in this study display a morphological and electrophysiological profile that is consistent with the type B, noncholinergic cell type. Cable analysis reveals that hDBB neurons are electrotonically compact and may therefore function as efficient relays for transmission of inputs to other forebrain target sites. Application of agonists for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate, N-methyl-D-aspartate (NMDA), and metabotropic receptors all evoke inward currents in hDBB neurons. Pharmacological analyses of synaptic events indicate that evoked excitatory postsynaptic currents (EPSC) are either mediated by non-NMDA receptors alone or a combination of non-NMDA and NMDA receptors. In some neurons, the metabotropic receptor agonist, 1-aminocyclopentane-trans-1, 3-dicarboxylic acid, reduced EPSC amplitude without altering postsynaptic input conductance, thus suggesting a presynaptic locus of action. The electrical and pharmacological properties described for hDBB neurons may be physiologically relevant for the effective transmission of excitatory synaptic inputs to sites that receive projections from the hDBB.

Possible role of protein kinase C in the sensitization of primate spinothalamic tract neurons

The responsiveness of spinal cord nociceptive neurons to innocuous mechanical stimuli can be increased by the release of excitatory amino acids (EAAs) and peptides attributable to an injury-induced barrage of impulses. This sensitization of spinal dorsal horn neurons can also result from administration of phorbol ester by microdialysis, presumably by direct activation of protein kinase C (PKC). This study was designed to examine the effects of central sensitization of spinothalamic tract (STT) neurons produced by intradermal injection of capsaicin on the descending inhibition driven from the periaqueductal gray (PAG) and the possible role of PKC in this process in anesthetized monkeys. Sensitization of responses of STT cells to mechanical stimuli was induced by intradermal injection of capsaicin. PAG inhibition was significantly attenuated when sensitization of responses to mechanical stimuli occurred. However, perfusion of the spinal cord with NPC15437 (a selective PKC inhibitor) by microdialysis could prevent the sensitization of the responses to mechanical stimuli and the reduction in PAG inhibition of these responses induced by capsaicin injection. Results similar to those produced by capsaicin injection were observed when a PKC activator, phorbol ester (12-O-tetradecanoylphorbol-13-acetate), was infused within the dorsal horn by microdialysis. An inactive phorbol ester (4 alpha-phorbol 12,13-didecanoate) had no effect. These results provide evidence that the activation of PKC contributes to the development of central sensitization in dorsal horn neurons produced by chemical stimulation with capsaicin. Attenuation of the effectiveness of PAG inhibition takes place when the sensitization of dorsal horn cells develops, and PKC may play a significant role in this process.

PKC-dependent reduction of the acetylcholine-evoked inward Na current in Aplysia D-neurons: effect of injected PKC and PKC activators

The effect of elevated PKC activity on the membrane depolarization (D-response) evoked by extracellular ACh, applied on the soma of Aplysia neurons, was studied. Intracellularly injected PKC and certain PKC activators were used to elevate PKC activity. ACh-induced current was measured in voltage clamp. The neurons were treated extracellularly with the PKC activators: PDAc, SC-10, R-59949, (-)-ILV; or with purified PKC injected into the neuron through the recording electrode. PKC injection and treatment with any of the PKC activators caused a similar reduction of the ACh-induced inward Na current response (corresponding to D-response), while the non-activating alpha-PDD had no effect. The results provide evidence that a PKC-dependent reduction of receptor responses also exists in this kind of Aplysia neurons. Furthermore, they show that the reduction of ACh response is indeed due to PKC activation (and not to a direct action of the phorbol ester).

Characterization of metabotropic glutamate receptor-mediated facilitation of N-methyl-D-aspartate depolarization of neocortical neurones

1. Facilitation of the N-methyl-D-aspartate (NMDA) receptor-mediated depolarization of cortical neurones induced by metabotropic glutamate receptor (mGluR) agonists in the presence of tetrodotoxin has been examined by use of grease-gap recording. 2. Quisqualate (1-2 microM) and 10 to 100 microM 1S,3R-I-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) facilitated the NMDA-, but not the kainate-induced depolarization with an EC50 of 16 microM for 1S,3R-ACPD. The facilitation induced by quisqualate was reduced, but not blocked, by 4 microM 6-cyano-7-nitroquinoxaline-2,3-dione. 3. D,L-2-Amino-3-phosphonopropionic acid and D,L-2-amino-4-phosphonobutyric acid antagonized the 1S,3R-ACPD facilitation in a non-competitive manner with IC50 values of 0.24 microM and 4.4 microM respectively. 4. Homologous desensitization of the 1S,3R-ACPD induced facilitation was not observed. The facilitation was not altered by 10 nM staurosporine or 3 microM phorbol diacetate. 5. Substitution of 20 microM 8-bromo-cyclic adenosine monophosphate, 20 microM 8-bromo-cyclic guanosine monophosphate, or 10 microM arachidonic acid for 1S,3R-ACPD did not induce facilitation of the NMDA response. However, the 1S,3R-ACPD facilitation was potentiated by 10 mM myo-inositol and exhibited heterologous desensitization following exposure to 100 microM 5-hydroxytryptamine. 6. The 1S,3R-ACPD-induced facilitation persisted in both 10 microM nifedipine and nominally Ca(2+)-free medium and was only gradually eliminated following addition of 100 microM bis-(-o-aminophenoxy)-ethane-N,N,N,N-tetraacetic acid in Ca(2+)-free medium. Facilitation of the NMDA response induced by carbachol, but not phenylephrine, was also observed in nominally Ca(2+)-free medium. Perfusing 50 microM bis-(-aminophenoxy)-ethane-N,N,N,N-tetraacetic acid aminoethoxy eliminated the 1S,3R-ACPD facilitation. 7. These experiments have shown that mGluR agonists selectively facilitate the NMDA depolarization of cortical wedges, most likely by activating one or more mGluR subtypes that couple to phospholipase C. We conclude the facilitation results from a Ca(2+)-sensitive mechanism dependent on activation of phospholipase C and release of internal Ca2+. The facilitation is not contingent on activation of protein kinase C or entry of Ca2+ through nifedipine-sensitive Ca2+ channels.

Excitatory amino acid-, except 1S,3R-ACPD, induced transient high stimulation of phosphoinositide metabolism during hippocampal neuron development

Rat hippocampal neurons in culture extended their neurites until day 5 in vitro (DIV). Then, the mean neuritic length slightly decreased. Excitatory amino acid (EAA)-elicited inositol phosphate (IP) formation increased from 0.5 to 2 DIV, reached a plateau between 2 and 4-5 DIV, and then gradually decreased until 10 DIV. This decrease was likely not due to neuronal death. This developmental pattern was observed for N-methyl-D-aspartate, kainate, glutamate, ibotenate and quisqualate (QA). Interestingly, the 1S,3R-aminocyclopentane dicarboxylate (1S,3R-ACPD) response slightly increased during neuronal culture development. At 3 DIV, the ionotropic antagonists 6,7-dinitro-quinoxalin-2,3-dion and D-2-amino-5-phosphonopentanoate efficiently blocked N-methyl-D-aspartate and kainate-elicited IP formation, and partially inhibited glutamate and ibotenate responses. QA and 1S,3R-ACPD responses were not affected, suggesting a metabotropic action for these two compounds. Furthermore, QA and 1S,3R-ACPD potencies significantly increased between 3 and 10 DIV. The transient high activity periods induced by EAA, except for 1S,3R-ACPD, are not observed for norepinephrine, carbachol and potassium chloride responses. Taken together, these data suggest that: (i) QA and 1S,3R-ACPD can act on two different glutamate metabotropic receptors subtypes during development; and (ii) the EAA-induced transient peaks of IP stimulation, which are specific with respect to other neuroactive substances profiles, could be involved in the development of hippocampal neurons. Indeed, these transient high activities take place when the neuritic length regularly increases in vitro.

Metabotropic glutamate receptor subtypes differentially influence neuronal recovery from in vitro hypoxia/hypoglycemia in rat hippocampal slices

Hippocampal slices were transiently exposed to an oxygen- and glucose-free environment which causes a pronounced drop of both ATP and creatine phosphate, an anoxic depolarization, and an incomplete recovery of synaptically evoked population spike in the CA1 region after 1 h (48.5 +/- 3.6% of baseline values). This recovery could be markedly enhanced by the application of N-methyl-D-aspartate receptor antagonists. To examine the influence of metabotropic glutamate receptors on neuronal recovery from hypoxia/hypoglycemia, we applied various antagonists and agonists of the metabotropic glutamate receptors to the bath during the interval from 20 min before to 10 after hypoxia/hypoglycemia. The metabotropic glutamate receptor antagonists (+)-alpha-methyl-4-carboxyphenylglycine and L-2-3- amino-phosphonopropionic acid were both able to enhance the population spike recovery significantly. However, the mixed metabotropic glutamate receptor agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid also exhibited a protective effect on population spike recovery, leaving the anoxic depolarization and N-methyl-D-aspartate responses during the hypoxia/hypoglycemia untouched. With the help of more subtype-specific agonists, we found that an activation of phospholipase C coupled (class 1) metabotropic glutamate receptors prior to hypoxia/hypoglycemia may be responsible for the protective effect seen with 1S, 3R-1-aminocyclopentane-1,3-dicarboxylic acid, because the specific class 1 metabotropic glutamate receptor agonist trans-azetidine-2,4-dicarboxylic acid appeared to be highly protective, but only if it was applied 20 min before the hypoxia/hypoglycemia. An activation of class 2 metabotropic glutamate receptors by (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine, which inhibits adenylyl cyclase activity, led to a marked deterioration of the population spike recovery and even to a total prevention of the protective effect of the N-methyl-D-aspartate agonist D-2-amino-5-phosphonopentanoic acid. Our data suggest that prior activation of class 1 metabotropic glutamate receptors is beneficial, while their activation during hypoxia/hypoglycemia is detrimental. Furthermore, the activation of class 2 metabotropic glutamate receptors decreases the recovery from hypoxia/hypoglycemia.

Pharmacology of metabotropic glutamate receptors at the mossy fiber synapses of the guinea pig hippocampus

We have tested the ability of several specific agonists of glutamate metabotropic receptors (mGluRs) to depress synaptic transmission at mossy fiber synapses in the CA3 region of the guinea pig hippocampus. 1S,3R-1-amino-cyclopentyl-1,3-dicarboxylate (ACPD) reversibly inhibited monosynaptic mossy fiber field potentials, presumably by a presynaptic mechanism, with an EC50 of 2.0 +/- 0.4 microM (n = 3), suggesting the presence of mGluRs on mossy fiber synaptic terminals of the group 1 or 2 category. L-2-amino-4-phosphono butanoate (L-AP4) also inhibited responses with an EC50 of 1.1 +/- 0.2 microM suggesting that mGluRs of the group 3 (mGluR4, 6, 7 and 8) category of receptors are also present on mossy fiber terminals. Both (2S,1'S,2'S)-2-(2'-carboxycyclopropyl)glycine (L-CCG1) and (S)-4-carboxy-3-hydroxy phenylglycine (4C3HPG) were also efficacious at blocking mossy fiber transmission, with an EC50 of 1.1 +/- 0.1 microM (n = 4) and 4.8 +/- 0.6 microM (n = 3) respectively. The latter finding indicates the involvement of mGluRs belonging to the group 2 (mGluR2, 3) category of receptors. The effects of L-AP4 and L-CCG1 were both antagonized by (+)-alpha-methyl-4-carboxyphenylglycine [(+)MCPG]. MAP4, an antagonist of group 3 mGluRs in other systems, blocked the effect of L-AP4, but not the effect of L-CCG1, while MCCG, an antagonist of group 2 mGluRs in other systems, blocked the effect of L-CCG1, but not the effect of L-AP4. These pharmacological findings provide strong evidence for the coexistence of group 2 and 3 mGluRs on the terminals of mossy fibers in the guinea pig.

Rapid agonist mediated phosphorylation of the metabotropic glutamate receptor 1 alpha by protein kinase C in permanently transfected BHK cells

Clonal BHK cells permanently transfected with the metabotropic glutamate receptor 1 alpha (mGluR1 alpha), which is coupled to phospholipase C, were used to study the phosphorylation state of the receptor. Cells were labelled with 32PO4(3-), lysed, the receptor immunoprecipitated with specific anti-peptide antibodies and the immunoprecipitates analysed by SDS-PAGE followed by autoradiography. A significant basal level of receptor phosphorylation was observed which was rapidly and transiently increased in response to agonist activation of the receptor. This agonist effect was found to be dose dependent with a rapid time course and could be abolished by the specific PKC inhibitor Ro318220, suggesting that PKC was responsible for the agonist mediated phosphorylation of the receptor.

Hippocampal long-term potentiation: transient increase but no persistent translocation of protein kinase C isoenzymes alpha and beta

Using a monoclonal antibody the translocation of the Ca(2+)-dependent protein kinase C (PKC) isoenzymes alpha/beta was studied in hippocampal slices after stimulation of glutamate receptors or induction of long-term potentiation. In submerged slices preincubated for 60 min in a medium usually used in electrophysiological studies, cytosolic PKC was not detectable and the amount of membrane-associated enzyme was increased. The treatment of these slices with 10(-6) M phorbol-12,13-dibutyrate induced a time-dependent translocation of alpha/beta PKC from the membrane-associated into the membrane-inserted state. The glutamatergic agonists N-methyl-D-aspartate, quisqualate and trans-ACPD did not cause a membrane insertion of alpha/beta PKC as observed for the phorbol ester when applied alone or in combination. Furthermore, 2 min and 15 min after induction of LTP in the Schaffer collateral-CA1 pathway the distribution of alpha/beta PKC between the two membrane fractions remained unchanged. An increase in the total amount of PKC immunoreactivity was measured immediately after tetanization (142.6% of controls). The data suggest that a membrane insertion of alpha/beta PKC is not a prerequisite for the LTP-induced increased phosphorylation of PKC substrates and that the enzyme might be recruited from a previously inactive pool.

The metabotropic glutamate receptors: structure and functions

Glutamate is the main excitatory neurotransmitter in the brain. For many years it has been considered to act only on ligand-gated receptor channels--termed NMDA, AMPA and kainate receptors--involved in the fast excitatory synaptic transmission. Recently, glutamate has been shown to regulate ion channels and enzymes producing second messengers via specific receptors coupled to G-proteins. The existence of these receptors, called metabotropic glutamate receptors, is changing our views on the functioning of fast excitatory synapses.

Effect of activity at metabotropic, as well as ionotropic (NMDA), glutamate receptors on morphine dependence

1. The contribution of various excitatory amino acid (EAA) receptors (NMDA, AMPA/kainate and metabotropic) in the brain to the development of morphine dependence was examined. This was performed by measuring the severity of the precipitated withdrawal syndrome following chronic subcutaneous (s.c.) morphine and intracerebroventricular (i.c.v.) EAA antagonist treatment. 2. Continuous subcutaneous (s.c.) treatment with morphine sulphate (36.65 mumol day-1) produced an intense and reliable naloxone-precipitated withdrawal syndrome. 3. Chronic i.c.v. treatment with antagonists selective for metabotropic and NMDA receptors, but not AMPA/kainate receptors, significantly attenuated abstinence symptoms. Conversely, EAA antagonists had very little effect on non-withdrawal behaviours. 4. These results suggest that, as well as changes elicited by activation of NMDA receptors, metabotropic receptors and intracellular changes in the phosphatidylinositol (PI) second-messenger system or the cyclic adenosine 3',5'-monophosphate (cAMP) second messenger system, to which EAA metabotropic receptors are linked, may be involved in the development of opioid dependence with chronic morphine treatment.

Postsynaptic integration of glutamatergic and dopaminergic signals in the striatum

The aim of this study was to achieve a better understanding of the integration in striatal medium-sized spiny neurons (MSNs) of converging signals from glutamatergic and dopaminergic afferents. The review of the literature in the first section shows that these two types of afferents not only contact the same striatal cell type, but that individual MSNs receive both a corticostriatal and a dopaminergic terminal. The most common sites of convergence are dendritic shafts and spines of MSNs with a distance between the terminals of less than 1-2 microns. The second section focuses on synaptic transmission and second messenger activation. Glutamate, the candidate transmitter of corticostriatal terminals, via different types of glutamate receptors can evoke an increase in intracellular free calcium concentrations. The net effect of dopamine in the striatum is a stimulation of adenylate cyclase activity leading to an increase in cAMP. The subsequent sections present information on calcium- and cAMP-sensitive biochemical pathways and review the regional and subcellular distribution of the components in the striatum. The specific biochemical reaction steps were formalized as simplified equilibrium equations. Parameter values of the model were chosen from published experimental data. Major results of this analysis are: at intracellular free calcium concentrations below 1 microM the stimulation of adenylate cyclase by calcium and dopamine is at least additive in the steady state. Free calcium concentrations exceeding 1 microM inhibit adenylate cyclase, which is not overcome by dopaminergic stimulation. The kinases and phosphatases studied can be divided in those that are almost exclusively calcium-sensitive (PP2B and CaMPK), and others that are modulated by both calcium and dopamine (PKA and PP1). Maximal threonine-phosphorylation of the phosphoprotein DARPP requires optimal concentrations of calcium (about 0.3 microM) and dopamine (above 5 microM). It seems favourable if the glutamate signal precedes phasic dopamine release by approximately 100 msec. The phosphorylation of MAP2 is under essentially calcium-dependent control of at least five kinases and phosphatases, which differentially affect its heterogeneous phosphorylation sites. Therefore, MAP2 could respond specifically to the spatio-temporal characteristics of different intracellular calcium fluxes. The quantitative description of the calcium- and dopamine-dependent regulation of DARPP and MAP2 provides insights into the crosstalk between glutamatergic and dopaminergic signals in striatal MSNs. Such insights constitute an important step towards a better understanding of the links between biochemical pathways, physiological processes, and behavioural consequences connected with striatal function. The relevance to long-term potentiation, reinforcement learning, and Parkinson's disease is discussed.

Intracellular messengers contributing to persistent nociception and hyperalgesia induced by L-glutamate and substance P in the rat formalin pain model

The contribution of the intracellular messengers nitric oxide, arachidonic acid and protein kinase C to persistent nociception in response to tissue injury in rats was examined following the subcutaneous injection of formalin into the hindpaw. Formalin injury-induced nociceptive behaviours were reduced by intrathecal pretreatment with inhibitors of nitric oxide synthase (NG-nitro-L-arginine methyl ester, L-NAME), arachidonic acid (dexamethasone) or protein kinase C [protein kinase C (19-26) and 1-95-(isoquinolinesulphonyl)-2-methylpiperazine dihydrochloride, H-7]. Each of these agents affected the tonic, but not the acute, phase of the formalin response. Furthermore, none of these agents affected mechanical or thermal flexion reflex thresholds in rats not injected with formalin. Conversely, formalin-induced nociceptive responses were enhanced by stimulators of nitric oxide (sodium nitroprusside), arachidonic acid metabolism (arachidonic acid) or protein kinase C [(+/-)-1-oleoyl-2-acetyl-glycerol], and were slightly reduced by inositol trisphosphate. Mechanical flexion reflexes were also reduced by arachidonic acid, while thermal flexion reflexes were reduced after treatment with sodium nitroprusside, arachidonic acid or [(+/-)-1-oleoyl-2-acetyl-glycerol]. The enhancement of formalin nociceptive behaviours (hyperalgesia) in rats treated with L-glutamate or substance P was reversed by pretreatment with inhibitors of nitric oxide (L-NAME), arachidonic acid (dexamethasone) or protein kinase C (H-7). The results suggest that central sensitization and persistent nociception following formalin-induced tissue injury, and the hyperalgesia in the formalin test induced by L-glutamate and substance P, are dependent on the intracellular messengers nitric oxide, arachidonic acid and protein kinase C.

The metabotropic glutamate receptor (MGR): pharmacology and subcellular location

A pharmacological characterization of the metabotropic glutamate receptor (MGR) was performed in striatal neurons. Among the excitatory amino acid receptor antagonists tested, only D, L-2-amino-3-phosphonopropionate (D, L-AP3) inhibited QA-induced inositol phosphate (InsP) formation in a competitive manner (mean pKi = 4.45 +/- 0.43, n = 4). However, this drug was a partial agonist of MGR since it stimulated the inositol-phosphate formation. We found that D, L-AP3 also inhibited NMDA-induced calcium increase, in a competitive manner (mean pIC50 = 4.34 +/- 0.22, n = 8, and mean pKi = 3.7 +/- 0.11 n = 5). 1 mM of the ionotropic agonists alpha-amino-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainate (KA) or domoate (DO) (100 microM or higher) induced a significant InsP formation in striatal neurons. The InsP responses induced by all these agonists were totally blocked by the phorbol ester phorbol-12,13-dibutyrate (PdBu), but not by atropine or prazosin. Agonist-induced increases of intracellular calcium concentrations ([Ca2+]i) were insensitive to PdBu, suggesting that all these substances were able to stimulate the MGR in striatal neurons. Trans-1-amino-cyclopentyl-1,3-dicarboxylate (trans-ACPD) evoked dose-dependent inositol phosphate formations with an EC50 of 29 microM but had no significant effect on NMDA or AMPA receptors, as measured by the patch clamp technique. In the presence of 30 microM of AMPA, trans-ACPD induced a significant release of arachidonic acid (AA) in striatal neurons. No important AA release was observed by any of these agonists alone. 56 mM K+ did not mimic AMPA in this associative ionotropic/metabotropic effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of the dendritic fields of layer 3 pyramidal cells in the kitten's visual cortex

The cat's visual cortex is immature at birth and undergoes extensive postnatal development. For example, cells of layers 2 and 3 do not complete migration until about 3 weeks after birth. Despite the importance of dendritic growth for synaptic and functional development, there have been few studies of dendritic development in the cat's visual cortex to correlate with numerous studies of functional and synaptic development. Accordingly, we used the Golgi method to study the development of the dendrites of layer 3 pyramidal cells in the visual cortex of a series of cats ranging in age from 2 days to 3 years. Blocks of visual cortex were impregnated by the Golgi-Kopsch method and sectioned in the tangential plane. Layer 3 pyramidal cells were drawn with a camera lucida and analyzed by Sholl diagrams and vector addition. In kittens < 1 week old, these cells were very immature, with only an apical dendrite and no basal dendrites. Basal dendrites appeared during the second week. By 2 weeks, all of the basal dendrites had emerged from the soma, but they had few branches and were tipped with growth cones. By 4 weeks, they had finished branching but continued to grow in length until, by 5 weeks, they reached their adult size. Examination of the basal dendritic fields in the tangential plane revealed that their dendritic fields were more elongated at 2 weeks than at later ages, perhaps because of their smaller size. The distribution of dendritic field orientations was uniform at all ages except 3 and 4 weeks, when there was a preponderance of fields oriented in the rostrocaudal direction. Because dendritic growth and branching occurred very rapidly over a period that precedes and overlaps with the peak periods of synaptogenesis and of sensitivity to the effects of early visual experience, they may depend on afferent visual activity. The early emergence of primary dendrites, however, suggests that this process is independent of afferent activity. The coincident timing of dendritic branching with the presence of dendritic growth cones suggests that branching may occur at growth cones.

Excitotoxicity and neurological disorders: involvement of membrane phospholipids

Excitatory amino acids and their receptors play an important role in membrane phospholipid metabolism. Persistent stimulation of excitatory amino acid receptors by glutamate may be involved in neurodegenerative diseases and brain and spinal cord trauma. The molecular mechanism of neurodegeneration induced by excitatory amino acids is, however, not known. Excitotoxin-induced calcium entry causes the stimulation of phospholipases and lipases. These enzymes act on neural membrane phospholipids and their stimulation results in accumulation of free fatty acids, diacylglycerols, eicosanoids, and lipid peroxides in neurodegenerative diseases and brain and spinal cord trauma. Other enzymes, such as protein kinase C and calcium-dependent proteases, may also contribute to the neuronal injury. Excitotoxin-induced alterations in membrane phospholipid metabolism in neurodegenerative diseases and neural trauma can be studied in animal and cell culture models. These models can be used to study the molecular mechanisms of the neurodegenerative processes and to screen the efficacy of therapeutic drugs.

Rapid desensitization of the metabotropic glutamate receptor that facilitates glutamate release in rat cerebrocortical nerve terminals

The metabotropic autoreceptor of glutamatergic nerve terminals from the cerebral cortex of adult rats has been characterized. Receptor activation involves a rapid and transient increase in diacylglycerol, which is sensitive to L-2-amino-3-phosphonopropionate (L-AP3) and L-2-amino-4-phosphonobutanoic acid (L-AP4) and is partially blocked by pertussis toxin. Protein kinase C (PKC) has a negative feedback control in this transduction pathway because the activation of the kinase, either by phorbol esters or by the endogenous diacylglycerol produced by the receptor, results in a reversible receptor desensitization, with loss of the ability to further facilitate glutamate release. It is concluded that the facilitatory metabotropic receptor located at the glutamatergic nerve endings belongs to the subclass coupled to phosphoinositide hydrolysis and that the rapid and use-dependent desensitization of the facilitatory pathway may underlie a mechanism to prevent its permanent activation and thereby to avoid neurotoxicity.

Metabotropic glutamate receptors: an original family of G protein-coupled receptors

In 1985, we discovered a new glutamate receptor which was coupled to phospholipase C via a G protein and which was later termed metabotropic glutamate receptor (mGluR). In this review, both the diversity of mGluRs and the cellular events they control are discussed, as well as their roles in physiological regulation and brain function.

Glutamate activates phospholipase D in hippocampal slices of newborn and adult rats

Phospholipase D (PLD) is activated by many neurotransmitters in a novel signal transduction pathway. In the present work, PLD activity was studied comparatively in hippocampal slices of newborn and adult rats. Basal PLD activity in adult rats was almost three times higher than in newborn rats. In newborn rats, L-glutamate and 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) time- and concentration-dependently enhanced the formation of [3H]phosphatidylpropanol ([3H]PP) and of [3H]phosphatidic acid in the presence of 2% propanol. N-Methyl-D-aspartate and kainate (both 1 mM) caused small, but significant increases (approximately 50%), whereas alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (100 microM) was ineffective. Maximally effective concentrations of glutamate (1 mM) and of 1S,3R-ACPD (300 microM) increased the PLD activity to almost 300% of basal activity; the EC50 values were 199 and 47 microM, respectively. Glutamate receptor antagonists, such as DL-2-amino-3-phosphonopropionic acid (AP3), DL-2-amino-5-phosphonovaleric acid, and kynurenate (all 1 mM) did not inhibit the glutamate-evoked increase of PP formation. In slices of adult rats, the response to 1S,3R-ACPD was significant, but small, whereas glutamate was effective only in the presence of the glutamate uptake inhibitor L-aspartate-beta-hydroxamate. It is concluded that glutamate activates PLD in rat hippocampus through an AP3-resistant metabotropic receptor. This effect is subject to ontogenetic development, with one important factor being glutamate uptake.

The role of excitatory amino acid receptors and intracellular messengers in persistent nociception after tissue injury in rats

Increased pain sensitivity (hyperalgesia) and persistent nociception following peripheral tissue injury depends both on an increase in the sensitivity of primary afferent nociceptors at the site of injury (peripheral sensitization), and on an increase in the excitability of neurons in the central nervous system (central sensitization). We will review evidence that central sensitization, and the persistent nociception it leads to, are dependent on an action of glutamate and aspartate at excitatory amino acid (EAA) receptors. Additional evidence will be presented implicating a role of various intracellular second messengers that are coupled to EAA receptors (nitric oxide, arachidonic acid, and protein kinase C) to central sensitization and persistent nociception following tissue injury. Finally, we will examine the evidence for a contribution of molecular events, including noxious stimulus-induced expression of immediate-early genes such as c-fos to persistent nociception.

Facilitatory effect of phorbol ester on 2-deoxyglucose uptake in rat hippocampal slices

It is well known that synaptic potentiation in the hippocampus can be produced by phorbol ester, a protein kinase C activator. The 2-deoxyglucose uptake is an index of regional glucose utilization which predominantly reflects activity in the axonal terminal of neuronal pathways. In the present experiment, therefore, we examined whether application of phorbol ester produces a facilitatory effect on 2-deoxyglucose uptake by the rat hippocampus in vitro. The application of phorbol-12,13-dibutyrate (PdBU) produced an elevation of 2-deoxyglucose uptake, while pretreatment with PdBU for 60 min eliminated the pdBU-induced elevation. Pretreatment with protein kinase C inhibitors, K252a (0.1 and 1 microM) or staurosporine (0.1 and 1 microM), was found to block significantly the PdBU-induced elevation of 2-deoxyglucose uptake. In addition, the facilitatory effect of glutamate, quisqualate and carbachol on 2-deoxyglucose uptake was reduced by pretreatment with PdBU. In the present experiment, we demonstrated that application of phorbol ester caused an elevation of 2-deoxyglucose uptake, which is linked in turn to neuronal activity, suggesting a positive relationship between protein kinase C activation and energy consumption.

Co-activation of metabotropic glutamate and N-methyl-D-aspartate receptors is involved in mechanisms of long-term potentiation maintenance in rat hippocampal CA1 neurons

Slices of hippocampal area CA1 in the rat were employed to test the hypothesis that the activation of metabotropic glutamate receptors during tetanization is necessary for the late maintenance of long-term potentiation. If the metabotropic glutamate receptor antagonist L-2-amino-3-phosphonopropionate was present during tetanization, post-tetanic and early long-term potentiation of the population spike as well as field excitatory postsynaptic potential developed almost normally. However, 100 min after tetanization, long-term potentiation of the field excitatory postsynaptic potential decreased in an irreversible manner. The same concentration of D-2-amino-3-phosphonopropionate was ineffective. If L-2-amino-3-phosphonopropionate was applied 120 min after tetanization, it did not influence long-term potentiation. The presence of the metabotropic glutamate receptor agonist trans-D,L-1-aminocyclopentane-1,3-dicarboxylic acid during tetanization weakly enhanced the slope of field excitatory postsynaptic potential long-term potentiation. The influence of L-2-amino-3-phosphonopropionate and D,L-1-aminocyclopentane-1,3-dicarboxylic acid on ionotropic glutamate receptors was studied using whole-cell voltage-clamp and pressure application techniques. No effect of L-2-amino-3-phosphonopropionate on either early or late components of excitatory postsynaptic currents could be detected at the concentration used to block long-term potentiation. It is therefore unlikely that the effect of L-2-amino-3-phosphonopropionate on long-term potentiation is due to an interaction with N-methyl-D-aspartate receptors or alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors. However, bath-applied 1S,3R-D,L-1-aminocyclopentane-1,3-dicarboxylic acid facilitated the N-methyl-D-aspartate-induced depolarization in response to N-methyl-D-aspartate pressure application in a reversible manner. These data suggest that besides the involvement of N-methyl-D-aspartate receptors the activation of a 2-amino-3-phosphonopropionate-sensitive metabotropic glutamate receptors during or immediately after tetanization is necessary for subsequent mechanisms responsible for the maintenance of long-term potentiation. A link between metabotropic glutamate receptors and protein kinase C activation during long-term potentiation is discussed considering the similar time course of long-term potentiation blockade after application of L-2-amino-3-phosphonopropionate and protein kinase C inhibitors.

Brain protein kinase C assay using MARCKS substrate reveals no translocation due to profound insulin-induced hypoglycemia

Hypoglycemia sufficient to produce EEG isoelectricity or coma leads to neuronal death by an excitotoxic mechanism due to elevated extracellular levels of glutamate, aspartate and increased intracellular calcium. Since an elevated intracellular calcium concentration is known to translocate protein kinase C (PKC) from the cytosol to the membrane (a process thought to represent the in vivo activation of the enzyme), the objective of this investigation was to determine if calcium-dependent isoforms of PKC were translocated in specific brain regions of rats subjected to 40 min of insulin-induced hypoglycemic coma. The caudate nucleus and hippocampus (regions damaged by hypoglycemia showing extensive neuronal necrosis), and cerebellum (an undamaged, control region) of hypoglycemic rats were microdissected. Soluble and detergent (Triton X-100)-solubilized particulate fractions were partially purified by DEAE-Sephacel chromatography. PKC activity in both fractions was then measured using a novel assay based on the calcium- and lipid (phosphatidylserine and diolein)-dependent phosphorylation of the specific substrate myristoylated alanine rich C kinase substrate (MARCKS). The percentage distribution of PKC in the soluble and particulate (membrane-bound) fractions of all the brain regions from hypoglycemic rats was not significantly different from that in the control brains, indicating that 40 min of hypoglycemia does not result in PKC translocation as measured in subcellular fractions from brain tissue.

Transduction mechanism for glutamate-induced potassium current in neurones of the mollusc Planorbarius corneus

1. The potassium currents evoked by glutamate agonists on isolated and identified neurones of molluscan pedal ganglia were investigated using the voltage clamp technique. 2. Glutamate responses were not modified by increasing intracellular cyclic nucleotide concentrations (treatment with 8-Br-cAMP, 8-Br-cGMP, forskolin and/or the phosphodiesterase inhibitor isobutylmethylxantine, IBMX), whereas inward-going currents induced by the nucleotides were observed. It follows that glutamate currents are independent of intracellular cyclic nucleotide control. 3. Protein kinase C activation with phorbol esters or oleoylacetylglycerol induced a slowly developing outward current and reduced glutamate response amplitude. Staurosporine itself did not affect the glutamate responses but completely prevented the effects of phorbol esters and oleoylacetylglycerol. This indicated that protein kinase C was not involved in the transduction mechanism for the potassium component of the glutamate response. 4. The possible involvement of inositol-1,4,5-trisphosphate seems to be improbable because the glutamate responses were independent of intracellular calcium concentration. Intracellular injection of calcium buffer BAPTA, failed to affect any of the glutamate currents, although it effectively blocked the after-hyperpolarization following directly evoked action potentials. 5. Nordihydroguaiaretic acid (NDGA) and indomethacin, inhibitors of the lipoxygenase and cyclo-oxygenase pathways of arachidonic acid metabolism, correspondingly, did not change the glutamate responses of these neurones. 6. The failure to demonstrate the involvement of any known secondary messenger systems in glutamate response transduction favours two assumptions: (1) the receptor-G protein complex controls the potassium channel directly; or (2) some still unknown transduction system is used.

Actions of agonists of metabotropic glutamate receptors on synaptic transmission and transmitter release in the olfactory cortex

1. The effects of agonists of on the evoked N-wave complex in slices of mouse have been studied: most experiments were carried out using slices perfused with Mg(2+)-free solution to which 10 microM of either 6,7-dinitroquinoxaline-2,3-dione or 6-cyano-7-nitroquinoxaline-2,3-dione was applied. 2. Following agonist washout, a slowly developing, long lasting potentiation of the complex occurred which was confined to the mediated component of the potential. The relative agonist potencies were 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD, 5-250 microM) = quisqualate (5-50 microM) > 1RS,3RS-cis-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD, 25-1000 microM) > L-glutamate (0.25-2.5 mM); NMDA, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) and L-aspartate were inactive. 3. Potentiation of the NMDA receptor-mediated component by 1S,3R-ACPD (0.1 mM) was non-competitively antagonised by S-(+)- but not R-(-)-2-amino-3-phosphonopropionate (AP3, 0.125 mM), equally by D-(-) and L-(+)-2-amino-4-phosphonobutyrate (0.25 mM) and also by the protein kinase C inhibitors sphingosine, (25 microM), sangivamycin (25 microM) and 5-(isoquinolinylsulphonyl)-3-methylpiperazine (50 microM). 4. In a series of input-output experiments, 1S,3R-ACPD (0.1 mM) reversibly reduced the latency to peak of the NMDA receptor-mediated component at submaximal stimulus intensities, an effect blocked by S-(+)-AP3 (0.125 mM). On agonist washout, there was an increase in the area of the receptor-mediated component over all stimulus intensities, an effect blocked by the inhibitors of protein kinase C and by S-(+)-AP3 (0.125mM). 4-beta-Phorbol-12,13-diacetate (2.5 muM) also potentiated the component, an action inhibited by protein kinase C inhibitors but not by S-(+)-AP3. IS,3R-ACPD (0.1mM) had no significant effect on postsynaptic responses evoked by NMDA, AMPA and kainate, but significantly reversed a partial antagonism of NMDA responses produced by 7-chlorokynurenate (2.5 muM). The K+evoked release of glycine was selectively and significantly increased in the presence 0.1mM 1S,3R-ACPD(antagonized by 0.125 mM S-(+)-AP#) whereas following agonist washout, release of glycine fell to control levels but there was a significant increase in release of aspartate(antagonized by 25 muM sangivamycin and 0.125 muM S-(+)-AP3). It is concluded that mediate (i) a reduction in the latency of the mediated component of potentials by a mechanism that is independent of protein kinase C but which may depend on increased glycine release release and (ii) a long lasting increase in the total area of the potential by increasing transmitter (possibly aspartate) release by a mechanism that is protein kinase C-dependent.

The expression of presynaptic t-ACPD receptor in rat cerebellum

The expression of a receptor subtype for one type of excitatory amino acid agonist, t-ACPD, was examined in developing Purkinje cells of cerebellar slices. The t-ACPD-induced responses were compared with those induced by QA in current response, single cell Ca2+ imaging and changes in the miniature currents in the same preparation. It was found that t-ACPD induced a single component of inward current, and an increase in the frequency of miniature currents associated with the presence of external Ca2+, but was ineffective at mobilizing intracellular Ca2+ even in the presence of external Ca2+. The present study suggests the expression of at least two types of metabotropic receptors in the Purkinje cell region, one of which, expressed in the Purkinje cell dendrites, is highly sensitive to QA, but relatively insensitive to t-ACPD, and the other of which is a t-ACPD-sensitive receptor expressed on the presynaptic terminals of the neurons making synapses onto Purkinje cells.

Positive feedback of glutamate exocytosis by metabotropic presynaptic receptor stimulation

Glutamate is important in several forms of synaptic plasticity such as long-term potentiation, and in neuronal cell degeneration. Glutamate activates several types of receptors, including a metabotropic receptor that is sensitive to trans-1-amino-cyclopenthyl-1,3-dicarboxylate, coupled to G protein(s) and linked to inositol phospholipid metabolism. The activation of the metabotropic receptor in neurons generates inositol 1,4,5-trisphosphate, which causes the release of Ca2+ from intracellular stores and diacylglycerol, which activates protein kinase C. In nerve terminals, the activation of presynaptic protein kinase C with phorbol esters enhances glutamate release. But the presynaptic receptor involved in this protein kinase C-mediated increase in the release of glutamate has not yet been identified. Here we demonstrate the presence of a presynaptic glutamate receptor of the metabotropic type that mediates an enhancement of glutamate exocytosis in cerebrocortical nerve terminals. Interestingly, this potentiation of glutamate release is observed only in the presence of arachidonic acid, which may reflect that this positive feedback control of glutamate exocytosis operates in concert with other pre- or post-synaptic events of the glutamatergic neurotransmission that generate arachidonic acid. This presynaptic glutamate receptor may have a physiological role in the maintenance of long-term potentiation where there is an increase in glutamate release mediated by postsynaptically generated arachidonic acid.

Characterization of a metabotropic glutamate receptor: direct negative coupling to adenylyl cyclase and involvement of a pertussis toxin-sensitive G protein

We have characterized a G-protein-coupled glutamate receptor in primary cultures of striatal neurons. Glutamate, quisqualate, or trans-1-aminocyclopentane-1,3-dicarboxylate inhibited by 30-40% either forskolin-stimulated cAMP production in intact cells or forskolin plus vasoactive intestinal peptide-activated adenylyl cyclase assayed in neuronal membrane preparations. These inhibitory effects were suppressed after treatment of striatal neurons with Bordetella pertussis toxin, suggesting the involvement of a heterotrimeric guanine nucleotide-binding protein (G protein) of the G(i)/G(o) subtype. The pharmacological profile of this glutamate receptor negatively coupled to adenylyl cyclase was different from that of the metabotropic Qp glutamate receptor coupled to phospholipase C in striatal neurons and from that of the recently cloned "mGluR2" glutamate receptor, which is negatively coupled to adenylyl cyclase when expressed in non-neuronal cells.

Actions of a metabotropic glutamate receptor agonist in immature and adult rat cerebellum

The electrophysiological actions of the metabotropic glutamate receptor agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) on Purkinje and granule cells were studied in immature and adult cerebellar slices. ACPD elicited a depolarising response when applied to Purkinje cells (EC50 approximately 20 microM). Granule cells hyperpolarised when exposed to low (3-10 microM) concentrations of ACPD; higher concentrations produced a depolarisation (EC50 approximately 40 microM) that was rapidly curtailed by a hyperpolarisation. The hyperpolarisation was abolished when Ca2+ was removed. In Purkinje cells, the amplitude of the depolarisation was greater in adult slices compared to those in immature slices. The responses were not blocked by ionotropic glutamate receptor antagonists or (L)-2-amino-3-phosphonopropionate (AP3).

Interactions between phospholipase C-coupled and N-methyl-D-aspartate receptors in cultured cerebellar granule cells: protein kinase C mediated inhibition of N-methyl-D-aspartate responses

The N-methyl-D-aspartate (NMDA) receptor of rat cerebellar granule cells in primary culture is inhibited by phospholipase C-coupled receptor activation. In the absence of ionotropic agonist, cells modulate their cytoplasmic free Ca2+, [Ca2+]c, in response to stimulation of M3 muscarinic receptors, metabotropic glutamate receptors, and endothelin receptors by the respective agonists carbachol, trans-1-amino-1,3-cyclopentanedicarboxylic acid, and endothelin-1. The response is consistent with the ability of phospholipase C-coupled receptors to release a pool of intracellular Ca2+ and induce a subsequent Ca2+ entry into the cell; both of these responses can be abolished by discharge of internal Ca2+ stores with low concentrations of ionomycin or thapsigargin. In the case of cells stimulated with NMDA, the [Ca2+]c response to the phospholipase C-coupled agonists is complex and agonist dependent; however, in the presence of ionomycin each agonist produces a partial inhibition of the NMDA component of the [Ca2+]c signal. This inhibition can be mimicked by the protein kinase C activator 4 beta-phorbol 12,13-dibutyrate. It is concluded that NMDA receptors on cerebellar granule cells are inhibited by phospholipase C-coupled muscarinic M3, glutamatergic, and endothelin receptors via activation of protein kinase C.