Metabotropic glutamate receptor agonists potentiate a slow afterdepolarization in CNS neurons

Canonical transient receptor channel 5 (TRPC5) and TRPC1/4 contribute to seizure and excitotoxicity by distinct cellular mechanisms

Seizures are the manifestation of highly synchronized burst firing of a large population of cortical neurons. Epileptiform bursts with an underlying plateau potential in neurons are a cellular correlate of seizures. Emerging evidence suggests that the plateau potential is mediated by neuronal canonical transient receptor potential (TRPC) channels composed of members of the TRPC1/4/5 subgroup. We previously showed that TRPC1/4 double-knockout (DKO) mice lack epileptiform bursting in lateral septal neurons and exhibit reduced seizure-induced neuronal cell death, but surprisingly have unaltered pilocarpine-induced seizures. Here, we report that TRPC5 knockout (KO) mice exhibit both significantly reduced seizures and minimal seizure-induced neuronal cell death in the hippocampus. Interestingly, epileptiform bursting induced by agonists for metabotropic glutamate receptors in the hippocampal CA1 area is unaltered in TRPC5 KO mice, but is abolished in TRPC1 KO and TRPC1/4 DKO mice. In contrast, long-term potentiation is greatly reduced in TRPC5 KO mice, but is normal in TRPC1 KO and TRPC1/4 DKO mice. The distinct changes from these knockouts suggest that TRPC5 and TRPC1/4 contribute to seizure and excitotoxicity by distinct cellular mechanisms. Furthermore, the reduced seizure and excitotoxicity and normal spatial learning exhibited in TRPC5 KO mice suggest that TRPC5 is a promising novel molecular target for new therapy.

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.

Metabotropic glutamate receptors: electrophysiological properties and role in plasticity

Electrophysiological research on mGluRs is now very extensive, and it is clear that activation of mGluRs results in a large number of diverse cellular actions. Studies of mGluRs and on ionic channels has clearly demonstrated that mGluR activation has a widespread and potent inhibitory action on both voltage-gated Ca2+ channels and K+ channels. Inhibition of N-type Ca2+ channels, and inhibition of Ca(++)-dependent K+ current, IAHP, and IM being particularly prominent. Potentiation of activation of both Ca2+ and K+ channels has also been observed, although less prominently than inhibition, but mGluR-mediated activation of non-selective cationic channels is widespread. In a small number of studies, generation of an mGluR-mediated slow excitatory postsynaptic potential has been demonstrated as a consequence of the effect of mGluR activation on ion channels, such as activation of a non-selective cationic channels. Although certain mGluR-modulation of channels is a consequence of direct G-protein-linked action, for example, inhibition of Ca2+ channels, many other effects occur as a result of activation of intracellular messenger pathways, but at present, little progress has been made on the identification of the messengers. The field of study of the involvement of mGluRs in synaptic plasticity is very large. Evidence for the involvement of mGluRs in one form of LTD induction in the cerebellum and hippocampus is now particularly impressive. However, the role of mGluRs in LTP induction continues to be a source of dispute, and resolution of the question of the exact involvement of mGluRs in the induction of LTP will have to await the production of more selective ligands and of selective gene knockouts.

Action of a metabotropic glutamate receptor agonist in rat lateral septum: induction of a sodium-dependent inward aftercurrent

The mechanism by which (1S,3R)-ACPD, a metabotropic glutamate receptor agonist, induces burst firing in lateral septal neurons of the rat was investigated in coronal brainstem slices. Membrane currents were characterized in voltage clamp using whole-cell recordings. In the presence of (1S,3R)-ACPD, following depolarizing voltage jumps, repolarization towards the holding potential generated an inward aftercurrent. It could have a plateau-like phase and decayed exponentially. This (1S,3R)-ACPD-dependent inward aftercurrent was accompanied by an increase in cell conductance and was reduced following partial replacement of extracellular sodium by N-methyl-D-glucamine. It was unaffected by TEA or barium, and persisted in Cs-loaded neurons or following partial replacement of extracellular chloride by isethionate. This suggests that it was mainly carried by sodium. Loading neurons with the calcium chelator, BAPTA, or blocking transmembrane calcium currents, suppressed the (1S,3R)-ACPD-dependent aftercurrent. By contrast, partial replacement of extracellular sodium by lithium did not affect it. Thus, this current was dependent upon calcium influx but was not due to a sodium/calcium exchanger. It was probably mediated by G protein activation. Indeed, in neurons loaded with GTP-gamma-S, following depolarizing voltage jumps, repolarization towards the holding potential revealed an inward aftercurrent having properties similar to those of the (1S,3R)-ACPD-dependent current. We suggest that (1S,3R)-ACPD induced calcium-activated non-selective channels. In the presence of this agonist, a depolarization-evoked calcium influx could thus evoke a cationic inward current. This current probably promotes the burst firing observed in lateral septal neurons in current clamp.

The pharmacology of amino-acid responses in septal neurons

Septal cholinergic neurons are known to play an important role in cognitive processes including learning and memory through afferent innervation of the hippocampal formation and cerebral cortex. The septum contains not only cholinergic neurons but also various types of neurons including GABA (gamma-aminobutyric acid)-ergic neurons. Although synaptic transmission in the septum is mediated primarily by the activation of excitatory and inhibitory amino-acid receptors, it is possible that a distinct phenotype of neuron is endowed with a different type for each of the amino-acid receptors and thus they play different roles from each other, since it has been demonstrated within the septum that there is a regional distribution of various types of amino-acid receptor subunits, their expression as different combinations within a specific cell may produce receptor channels with disparate functional properties. As a first step towards knowing the various functions of septal cholinergic neurons, we characterized the functional properties of glutamate, GABA (type A; GABAA) and glycine receptor channels on cultured rat septal neurons which were histologically identified to be cholinergic. These were similar to those of receptor channels on other types of neurons, except for the actions of some neuromodulators. The septal N-methyl-D-aspartate receptor channel was distinct in being less sensitive to Mg2+ and in a voltage-dependent action of Zn2+. The septal GABAA receptor channel exhibited a lanthanide site whose activation resulted in a positive allosteric interaction with a binding site of pentobarbital. The septal glycine receptor channel was only positively modulated by Zn2+; this action of Zn2+ was not accompanied by an inhibitory effect. Our data suggest that the amino-acid receptors on septal cholinergic neurons may play a distinct role compared to other types of neurons; this difference depends on the actions of neuromodulators and metal cations. It would be interesting to compare these effects recorded in tissue culture to those observed with septal cholinergic neurons in slice preparations.

Metabotropic glutamate receptor subtypes mediating slow inward tail current (IADP) induction and inhibition of synaptic transmission in olfactory cortical neurones

1. The pharmacological features of the pre- and postsynaptic metabotropic glutamate receptors (mGluRs) present in the guinea-pig olfactory cortex, were examined in brain slices in vitro by use of a conventional intracellular current clamp/voltage clamp recording technique. 2. Bath-application of trans-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) (50 microM) produced a sustained membrane depolarization, increase in cell excitability and induction of a post-stimulus inward (after depolarizing) tail current (IADP) (measured under 'hybrid' voltage clamp) similar to those evoked by the muscarinic receptor agonist oxotremorine-M (OXO-M, 2 microM). 3. L-Glutamate (0.25 1 mM. in the presence of 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 100 microM-DL-amino-5-phosphono valeric acid (DL-APV)) or the broad spectrum mGluR agonists 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD, 10 microM), 1S,3S-ACPD (50 microM), ibotenate (Ibo; 25 microM. in the presence of 100 microM DL-APV), the selective mGluR I agonists (S)-3,5-dihydroxyphenylglycine ((S)-3,5-DHPG, 10 microM), (S)-3-hydroxyphenylglycine ((S)-3HPG, 50 microM), or quisqualate (10 microM, in the presence of 20 microM CNQX), but not the mGluR II agonist 2S,1'S,2'S-2-(2'-carboxycyclopropyl)-glycine (L-CCG1,1 microM) or mGluR III agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4, 1 mM), were all effective in producing membrane depolarization and inducing a post-stimulus IADP. Unexpectedly, the proposed mGluR II-selective agonist (2S,1'R,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)-glycine (DCG-IV, 10 microM, in the presence of 100 microM DL-APV) was also active. 4. The excitatory effects induced by 10 microM 1S,3R-ACPD were reversibly antagonized by the mGluR I/II antagonist (1)-alpha-methyl-4-carboxyphenylglycine ((+)-MCPG, 0.5 1 mM), as well as the selective mGluR I antagonists (S)-4-carboxyphenylglycine ((S)-4CPG) and (S)-4-carboxy-3-hydroxyphenyl glycine ((S)-4C3HPG) (both at 1 mM), but not the nonselective mGluR antagonist L(+)-2-amino-3-phosphonopropionic acid (L-AP3, 1 mM) or the selective mGluR III antagonist (S)-alpha-methyl-L-AP4 (MAP4, 1 mM). 5. The excitatory postsynaptic potentials (e.p.s.ps), induced by single focal stimulation of cortical excitatory fibre tracts, were markedly reduced by 1S,3R-ACPD or L-AP4 (both at 10 microM), and by the selective mGluR II agonists (mGluR 1 antagonists) (S)-4CPG or (S)-4C3HPG (both at 1 mM) but not (S)-3,5-DHPG or (S)-3HPG (both at 100 microM). 6. The inhibitory effects of 1S-3R-ACPD, but not L-AP4, were reversibly blocked by (+)-MCPG (1 mM), whereas those produced by L-AP4, but not 1S,3R-ACPD, were blocked by the selective mGluR III antagonist MAP4 (1 mM). 7. It is concluded that a group I mGluR is most likely involved in mediating excitatory postsynaptic effects, whereas two distinct mGluRs (e.g. group II and III) might serve as presynaptic inhibitory autoreceptors in the guinea-pig olfactory cortex.

Activation of a metabotropic excitatory amino acid receptor potentiates spike-driven calcium increases in neurons of the dorsolateral septum

(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), an agonist for metabotropic glutamate receptors (mGluRs), causes depolarization and burst firing in rat dorsolateral septal nucleus (DLSN) neurons and results in long-term potentiation (LTP) at DLSN synapses. In the present study, we investigated whether these actions of 1S,3R-ACPD are attributable to the release of calcium from an inositol triphosphate-sensitive store after activation of mGluRs coupled to phospholipase C. Our data demonstrated that the ACPD-induced depolarization was associated with a small but significant decrease, not an increase, in [Ca2+]i; however, changes of [Ca2+]i, during ACPD-induced bursting were up to seven times larger than those produced by regular firing. Depletion of internal calcium stores by thapsigargin or ryanodine had a small to insignificant effect on the maximum changes of [Ca2+]i, associated with ACPD-induced bursting. Thus, elevation of [Ca2+]i, during firing by 1S,3R-ACPD is likely attributable to enhancement of calcium influx through voltage-gated channels and not to calcium release from internal stores. ACPD-induced burst firing elevated somatic and dendritic calcium levels up to 3 and 6 microM, respectively. Such an increase may be the underlying mechanism for ACPD-induced LTP as well as ACPD-induced acute cell death in rat DLSN.

Excitatory synaptic potentials dependent on metabotropic glutamate receptor activation in guinea-pig hippocampal pyramidal cells

1. Intracellular and extracellular recordings of CA1 and CA3 neurones were performed in guinea-pig hippocampal slices to examine synaptic activities dependent on metabotropic glutamate receptors (mGluRs). 2. Long burst activities were elicited by 4-aminopyridine in the presence of ionotropic glutamate receptor and GABAA receptor blockers (6-cyano-7-nitroquinoxaline-2,3-dione and 3-(RS-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid, and picrotoxin). Long bursts were also elicited by alpha-dendrotoxin. 3. Long bursts consisted of a 5-25 s depolarization with overriding action potentials and occurred rhythmically at intervals ranging from 1 to 20 min. Long bursts were generated in a population of CA3 neurones and the synchronized output elicited long bursts in CA1 cells. Depolarizing potentials underlying long bursts in CA1 cells had a reversal potential of -14.8 +/- 5.1 mV. 4. Long burst-associated depolarizations in CA1 neurones were suppressed by local application of L-(+)-2-amino-3-phosphonopropionic acid (L-AP3) and of the phenylglycine derivatives (+)-alpha-methyl-4-carboxyphenylglycine ((+)-MCPG), S-4-carboxyphenylglycine (S-4CPG) and S-4-carboxy-3-hydroxyphenylglycine (S-4C3HPG). (-)-MCPG or atropine application did not affect the long burst-associated depolarization. 5. Bath perfusion of (+)-MCPG (0.5 mM), S-4CPG (0.5 mM), S-4C3HPG (0.5 mM) or L-AP3 (1 mM) blocked the occurrence of long bursts. 6. The results suggest that the long burst-associated depolarizations are synaptic potentials dependent on mGluR activation. Activation of mGluRs may also be involved in the generation of synchronized long bursts in the CA3 region.

(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid-induced burst firing is mediated by a native pertussis toxin-sensitive metabotropic receptor at rat dorsolateral septal nucleus neurons

We have reported previously that a selective metabotropic glutamate receptor agonist, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), caused two primary postsynaptic membrane changes, namely, a slow membrane depolarization, and burst firing in rat dorsolateral septal nucleus neurons. In addition, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid also potentiates a slow after depolarization in rat dorsolateral septal nucleus neurons. We now report that, among all the postsynaptic membrane changes induced by (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid, only the burst firing was selectively blocked by pertussis toxin pretreatment. Thus, aminocyclopentane-1,3-dicarboxylic acid induced burst firing was mediated by a metabotropic receptor coupled to a pertussis toxin-sensitive GTP-binding protein, while the other induced cellular responses may be mediated by metabotropic glutamate receptors insensitive to pertussis toxin. We further characterized this receptor pharmacologically. This metabotropic receptor is activated by several metabotropic glutamate receptor agonists, but is insensitive to L-glutamate or L-aspartate. On the basis of its agonist activity profile, particularly the ineffectiveness of glutamate as an agonist, we have tentatively assigned the name aminocyclopentane-1,3-dicarboxylic acid metabotropic receptor, to this native, pertussis toxin-sensitive metabotropic receptor in the dorsolateral septal nucleus. Furthermore, this receptor is coupled to protein kinase C, probably via a phospholipase C independent pathway.

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.

Metabotropic glutamate response in acutely dissociated hippocampal CA1 pyramidal neurones of the rat

1. The metabotropic glutamate (mGlu) response was investigated in dissociated rat hippocampal CA1 pyramidal neurones using conventional and nystatin-perforated whole-cell modes of the patch recording configuration. 2. In the perforated patch recording configuration, the application of glutamate (Glu), quisqualate (QA), aspartate (Asp) and N-methyl-D-aspartate (NMDA) induced a slow outward current superimposed on a fast ionotropic inward current, whereas alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and kainate (KA) induced only an ionotropic inward current at a holding potential (VH) of -20 mV. A specific agonist of the mGlu receptor (mGluR), trans-1-aminocyclopentane-1,3-dicarboxylate (tACPD), induced an outward current in approximately 80% of the neurones tested. Asp- and NMDA-induced outward currents were antagonized by D-2-amino-5-phosphonopentanoate (D-AP5) whereas Glu-, QA- and tACPD-induced outward currents were not antagonized by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 6,7-dinitroquinoxaline-2,3-dione (DNQX) and D-AP5, indicating that the mGlu response is an outward current component. 3. L-2-Amino-3-phosphonopropionate (L-AP3) and DL-2-amino-4-phosphonobutyrate (AP4) did not block the mGlu response. 4. The relative potencies of mGlu agonists were QA > Glu > tACPD. The threshold and EC50 values of metabotropic outward currents were 10-100 times lower than those of the ionotropic inward current (iGlu response). 5. The reversal potential of the mGlu response (EmGlu) was close to EK (K+ equilibrium potential), and it shifted 59.5 mV for a tenfold change in extracellular K+ concentration. 6. In Ca(2+)-free external solution, the mGlu response was elicited by an initial application of Glu, but subsequent applications failed to induce the response. There was also an increase in the intracellular free Ca2+ concentration ([Ca2+]i) during the application of Glu and QA but not of AMPA, indicating Ca2+ release from an intracellular Ca2+ store. 7. During the activation of a Ca(2+)-dependent K+ current (IK(Ca)) by inositol trisphosphate (IP3) in the internal solution, the mGlu response was suppressed. Addition of GDP-beta-S, neomycin or heparin to the internal solution also suppressed the mGlu response, but staurosporine had no effect. The mGlu response was abolished by pretreatment with either caffeine or ryanodine, but treatment with pertussis toxin (IAP) for 6-8 h had no effect. 8. The mGlu response was suppressed by tetraethylammonium, but not by either apamin or iberiotoxin, suggesting that intermediate-conductance Ca(2+)-dependent K+ (KCa+) channels are involved.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of trans-ACPD, a metabotropic excitatory amino acid receptor agonist, on the responses of primate spinothalamic tract neurons

The responses of primate spinothalamic tract (STT) neurons to innocuous and noxious mechanical stimuli applied to the skin can be enhanced for more than an hour following prolonged noxious stimulation. This increased responsiveness is thought to reflect sensitization of dorsal horn neurons and may help account for secondary hyperalgesia and mechanical allodynia. The proposal that central sensitization is due to the activation of second messenger system was tested in this study by examining the effect of trans-ACPD (trans-D,L-1-amino-1,3-cyclopentanedicarboxylic acid), an agonist of metabotropic excitatory amino acid (EAA) receptors, introduced into the dorsal horn by microdialysis. A low dose of trans-ACPD resulted in an increase in the responses of STT cells to an innocuous mechanical stimulus (BRUSH), but no increase in the responses to noxious mechanical and thermal stimuli or in the excitation produced by iontophoretically applied EAAs. A high dose of trans-ACPD caused a transient increase in background activity, but no change in the responsiveness of spinothalamic cells to any of the test stimuli. It is concluded that low doses of trans-ACPD can selectively enhance transmission through interneuronal pathways mediating tactile inputs to spinothalamic cells.

(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) induces burst firing via an inositol-1,4,5-triphosphate-independent pathway at rat dorsolateral septal nucleus

We have previously reported that a L-2-amino-3-phosphonopropionate (L-AP3)-sensitive metabotropic glutamate receptor was required for the induction of long-term potentiation (LTP) in rat dorsolateral septal nucleus neurons. (1S,3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), a selective agonist for metabotropic glutamate receptors, also causes burst firing of dorsolateral septal nucleus (DLSN) neurons. In this study, we investigated whether this response was mediated by a phospholipase C-(PLC) coupled metabotropic glutamate receptor. The threshold concentration of 1S,3R-ACPD for the induction of burst firing was about 5 microM, while 10 microM 1S,3R-ACPD produced a maximal effect. L-AP3 (50 microM) reduced the burst firing induced by 1S,3R-ACPD (5 microM). Although 1S,3R-ACPD stimulated the formation of inositol-1,4,5-triphosphate [Ins(1,4,5)P3] suggesting the presence of PLC-coupled metabotropic glutamate receptors, it was only effective in a higher (30-100 microM) concentration range. In addition, the 1S,3R-ACPD-stimulated formation of Ins(1,4,5)P3 level was not affected by L-AP3. These observations suggest that the 1S,3R-ACPD induced burst firing is not mediated by PLC-coupled metabotropic glutamate receptors.