Antagonism of presynaptically mediated depressant responses and cyclic AMP-coupled metabotropic glutamate receptors

Activation of group III metabotropic glutamate receptors is neuroprotective in cortical cultures

(RS)-alpha-Methyl-4-phosphonophenylglycine (MPPG) and (S)-alpha-methyl-3-carboxyphenylalanine (M3CPA), two novel preferential antagonists of group III metabotropic glutamate (mGlu) receptors, antagonized the neuroprotective activity of L-2-amino-4-phosphono-butanoate (L-AP4) or L-serine-O-phosphate in mice cultured cortical cells exposed to a toxic pulse of N-methyl-D-aspartate. In contrast, MPPG did not influence the neuroprotective activity of the selective group II mGlu receptor agonist, (2S,1'R,2'R,3'R)-2-(2,3-dicarboxy-cyclopropyl) glycine (DCG-IV). These results indicate that activation of group III mGu receptors exerts neuroprotective activity against excitotoxic neuronal death. At least one of the two major group III mGlu receptor subtypes, i.e. mGlu4 receptor, is expressed by cultured cortical neurons, as shown by immunocytochemical analysis with specific polyclonal antibodies.

Agonists of cyclic AMP-coupled metabotropic glutamate receptors in adult rat cortical slices

A number of potential Group 2 and Group 3 metabotropic glutamate receptor (mGlu receptor) agonists were investigated in adult rat brain cerebrocortical slices. The rank order of their potency in inhibiting forskolin-stimulated adenylyl cyclase was found to be: (S)-2-amino-2-methyl-4-phosphonobutyric acid (MAP4) > (2S,1'S,2'S)-2-(2-carboxycyclopropyl)glycine (L-CCG-I) > (1S,3S)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3S-ACPD) > (1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid (1S,3R)-ACPD) > (2S,1'R,2'R,3'R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV) > (S) -2-methylglutamate ((S)-MG) > L-glutamate > (2S,1'S, 2'S)-2-(2-carboxycyclopropyl)alanine (MCCG) > L-2-amino-4-phosphonobutyric acid (L-AP4) > L-serine-O-phosphate (SOP). The finding that (S)-2-amino-2-methyl-4-phosphonobutyric acid was the most potent agonist at these metabotropic glutamate receptors is in contrast to its observed potent mGlu receptor antagonist action in the neonatal rat spinal cord.

Novel potent selective phenylglycine antagonists of metabotropic glutamate receptors

The metabotropic glutamate (mGlu) receptor antagonist properties of novel phenylglycine analogues were investigated in adult rat cortical slices (mGlu receptors negatively coupled to adenylyl cyclase), neonatal rat cortical slices and in cultured rat cerebellar granule cells (mGlu receptors coupled to phosphoinositide hydrolysis). (RS)-alpha-methyl-4-phosphonophenylglycine (MPPG), (RS)-alpha-methyl-4-sulphonophenylglycine (MSPG), (RS)-alpha-methyl-4-tetrazolylphenylglycine (MTPG), (RS)-alpha-methyl-3-carboxymethyl-4-hydroxyphenylglycine (M3CM4HPG) and (RS)-alpha-methyl-4-hydroxy-3-phosphonomethylphenylglycine (M4H3PMPG) were demonstrated to have potent and selective effects against 10 microM L-2-amino-4-phosphonobutyrate (L-AP4)- and 0.3 microM (2S,1'S,2'S)-2-(2-carboxycyclopropyl)glycine (L-CCG-1)-mediated inhibition of forskolin-stimulated cAMP accumulation in the adult rat cortex. In contrast, these compounds demonstrated either weak or no antagonism at mGlu receptors coupled to phosphoinositide hydrolysis in either neonatal rat cortex or in cultured cerebellar granule cells. These compounds thus appear to be useful discriminatory pharmacological tools for mGlu receptors and form the basis for the further development of novel antagonists.

Comparing the role of metabotropic glutamate receptors in long-term potentiation and in learning and memory

1. Neuronal plasticity has been suggested to be the physical substrate for changes underlying the expression of memory. One model which has attracted wide attention as a possible candidate of such neuronal plasticity is long-term potentiation (LTP), mainly investigated in the hippocampus of rodents. Moreover, various processes with different time constants may underlie LTP, and these phases show striking correspondence to different phases of memory. 2. Pharmacological evidence strongly implicates that the neurotransmitter glutamate plays a major role in LTP. Although the involvement of ionotropic glutamate receptors has been proven, the role of the newly discovered metabotropic glutamate receptors is still uncertain. 3. Metabotropic glutamate receptors (mGluRs) comprise a whole family with currently eight members grouped into three classes according to their amino acid sequence identity and pharmacological profile. They are G-protein coupled, either positively linked to phospholipase C (class I) or negatively linked to adenylate cyclase (class II and III), and among other effects are known to induce phosphorylation of ionotropic glutamate receptors as well as modulate the excitability of neurons. Finally, they are heterogeneously distributed throughout the brain. 4. In hippocampal slice preparations, mGluRs have been shown to be involved in the induction of LTP in CA1 and dentate gyrus by some investigators, but others have failed to reproduce such experiments, leaving the question: what are the appropriate conditions for mGluR-mediated LTP? 5. In vivo, metabotropic receptor antagonists have been shown to block, and agonists to facilitate, induction and maintenance of LTP, mainly at perforant path/dentate granule cell synapses. As demonstrated in behavioral investigations, mGluRs apparently play an important part in hippocampus-dependent learning paradigms. As in LTP, antagonists block memory formation; in contrast to LTP, agonists also prevent memory formation. In memory recall metabotropic receptors seem to play no role. 6. Based on current information the authors develop models for a role of mGluRs in both LTP and memory formation. Activation of metabotropic receptors plays a particular modulatory role when high frequency stimulation is weak. Strong tetanization may bypass mGluRs by stimulating other systems leading to, at least phenomenologically, similar LTP, Behaviorally, mGluRs possibly set the signal to noise ratio of the hippocampal circuit.

alpha-Methyl derivatives of serine-O-phosphate as novel, selective competitive metabotropic glutamate receptor antagonists

The antagonist selectivity and potency of two novel serine-O-phosphate derivatives (RS)-alpha-methylserine-O-phosphate (MSOP) and the monophenylester (RS)-alpha-methylserine-O-phosphate monophenyl-phosphoryl ester (MSOPPE) was investigated against L-2-amino-4-phosphonobutyrate (L-AP4)- and (1S,3S)-1-aminocyclopentane-1, 3-dicarboxylate (ACPD)-induced depressions of the monosynaptic excitation of neonatal rat motoneurones, mediated via metabotropic glutamate receptors (mGLuRs). MSOP was shown to be a selective antagonist for the L-AP4-sensitive presynaptic mGluR, displaying an apparent KD of 51 microM, compared to > 700 microM for the (1S,3S)-ACPD-sensitive presynaptic mGluR. In contrast, MSOPPE displayed antagonist activity at both presynaptic mGluR, with a three times greater selectivity for the (1S,3S)-ACPD-sensitive receptor over the L-AP4-sensitive mGluR (apparent KD values 73 microM and 221 microM, respectively). Therefore, on addition of an alpha-methyl group to the mGluR agonist serine-O-phosphate, we have developed an mGluR antagonist which is selective for the presynaptic L-AP4-sensitive receptor. In contrast, monoesterification of MSOP to give the monophenylphosphoryl ester (MSOPPE), confers a degree of selectivity for the (1S,3S)-ACPD-over the L-AP4-sensitive presynaptic mGluR. Neither MSOP nor MSOPPE had any activity on either postsynaptic mGLuRs or ionotropic receptors.

Pharmacological antagonism of the actions of group II and III mGluR agonists in the lateral perforant path of rat hippocampal slices

1. An understanding of the physiological and pathological roles of metabotropic glutamate receptors (mGluRs) is currently hampered by the lack of selective antagonists. Standard extracellular recording techniques were used to investigate the activity of recently reported mGluR antagonists on agonist-induced depressions of synaptic transmission in the lateral perforant path of hippocampal slices obtained from 12-16 day-old rats. 2. The group III specific mGluR agonist, (S)-2-amino-4-phosphonobutanoate (L-AP4) depressed basal synaptic transmission in a reversible and dose-dependent manner. The mean (+/-s.e. mean) depression obtained with 100 microM L-AP4 (the maximum concentration tested) was 74 +/- 3% and the IC50 value was 3 +/- 1 microM (n = 5). 3. The selective group II mGluR agonists, (1S,3S)-1-aminocyclopentane-1, 3-dicarboxylate ((1S,3s)-ACPD) and (2S, 1'R, 2'R, 3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) also depressed basal synaptic transmission in a reversible and dose-dependent manner. The mean depression obtained with 200 microM (1S,3S)-ACPD was 83 +/- 8% and the IC50 value was 12 +/- 3 microM (n = 5). The mean depression obtained with 1 microM DCG-IV was 73 +/- 7% and the IC50 value was 88 +/- 15 nM (n = 4). 4. Synaptic depressions induced by the actions of 20 microM (1S,3S)-ACPD and 10 microM L-AP4 were antagonized by the mGluR antagonists (+)-alpha-methyl-4-carboxyphenylglycine ((+)-MCPG), (S)-2-methyl-2-amino-4-phosphonobutanoate (MAP4), (2S,1'S,2'S)-2-methyl-2(2'-carboxycyclopropyl)glycine (MCCG), (RS)-alpha-methyl-4-tetrazolylphenylglycine (MTPG), (RS)-alpha-methyl-4-sulphonophenylglycine (MSPG) and (RS)-alpha-methyl-4-phosphonophenylglycine (MPPG) (all tested at 500 microM). 5. (+)-MCPG was a weak antagonist of both L-AP4 and (1S,3S)-ACPD-induced depressions. MCCG was selective towards (1S,3S)-ACPD, but analysis of its effects were complicated by apparent partial agonist activity. MAP4 showed good selectivity for L-AP4-induced effects. 6. The most effective antagonist tested against 10 microM L-AP4 was MPPG (mean reversal 90 +/- 3%; n = 4). In contrast, the most effective antagonist tested against 20 microM (1S,3S)-ACPD induced depressions was MTPG (mean reversal 64 +/- 4%; n = 4). Both antagonists produced parallel shifts in agonist dose-response curves. Schild analysis yielded estimated KD values of 11.7 microM and 27.5 microM, respectively. Neither antagonist had any effect on basal transmission or on depressions induced by the adenosine receptor agonist, 2-chloroadenosine (500 nM; n = 3). 7. We conclude that both group II and group III mGluRs can mediate synaptic depressions induced by mGluR agonists in the lateral perforant path. The mGlur antagonists MTPG, MPPG and MAP4 should be useful in determining the roles of group II and III mGluRs in the central nervous system.

Decrease in [Ca2+]c but not in cAMP Mediates L-AP4 inhibition of glutamate release: PKC-mediated suppression of this inhibitory pathway

We investigated the mechanism of the inhibition of glutamate release by (L)-2-amino-4-phosphonobutyrate ((L)-AP4) in cerebrocortical nerve terminals from young rats (3 weeks of age). The Ca(2+)-dependent release of glutamate was reduced by (L)-AP4 in a concentration-dependent manner. This inhibitory effect was prevented by pertussis toxin, insensitive to staurosporine and associated with a reduction both in the depolarization-evoked increase in the cytoplasmic free Ca(2+) concentration ([Ca(2+)](c)) and in forskolin-stimulated cAMP formation. However, the reduction in [Ca(2+)](c) but not in cAMP seemed to be responsible for the decrease in release, since inhibition by (L)-AP4 can also be observed in the absence of detectable changes in cAMP The inhibitory modulation by (L)-AP4 was suppressed by the activation of protein kinase C with phorbol esters. The nerve terminals from young rats also exhibited a facilitatory pathway of glutamate release which was mediated by protein kinase C. Interestingly, stimulation of this pathway with the glutamate agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate in the presence of arachidonic acid also abolished the inhibitory action of (L)-AP4. The dominance of the facilitatory pathway in its interaction with the (L)-AP4-mediated inhibitory control may provide some clues to understand the presynaptic changes during synaptic plasticity.

Structure-activity relationships of new agonists and antagonists of different metabotropic glutamate receptor subtypes

1. We investigated the agonist and antagonist activities of 22 new phenylglycine and phenylalanine derivatives for metabotropic glutamate receptors (mGluRs) by examining their effects on the signal transduction of mGluR1, mGluR2 and mGluR6 subtypes expressed in Chinese hamster ovary cells. This analysis revealed several structural characteristics that govern receptor subtype specificity of the agonist and antagonist activities of phenylglycine derivatives. 2. Hydroxyphenylglycine derivatives possessed either an agonist activity on mGluR1/mGluR6 or an antagonist activity on mGluR1. 3. Carboxyphenylglycine derivatives showed an agonist activity on mGluR2 but an antagonist activity on mGluR1. 4. alpha-Methylation or alpha-ethylation of the carboxyphenylglycine derivatives converts the agonist property for mGluR2 to an antagonist property, thus producing antagonists at both mGluR1 and mGluR2. 5. Structurally-corresponding phenylalanine derivatives showed little or no agonist or antagonist activity on any subtypes of the receptors. 6. This investigation demonstrates that the nature and positions of side chains and ring substituents incorporated into the phenylglycine structure are critical in determining the agonist and antagonist activities of members of this group of compounds on different subtypes of the mGluR family. 7. We also tested two alpha-methyl derivatives of mGluR agonists. (2S, 1'S, 2'S)-2-(2-Carboxycyclopropyl)glycine (L-CCG-I) is a potent agonist for mGluR2 but alpha-methylation of this compound changes its activity to that of an mGluR2-selective antagonist. In contrast, alpha-methylation of L-2-amino-4-phosphonobutyrate (L-AP4) results in retention of an agonist activity on mGluR6. Thus, alpha-methylation produces different effects, depending on the chemical structures of lead compounds and/or on the subtype of mGluR tested.

Differences in agonist and antagonist activities for two indices of metabotropic glutamate receptor-stimulated phosphoinositide turnover

1. The abilities of the four diastereoisomers of 1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) to stimulate, and the metabotropic glutamate receptor (mGluR) antagonist (+/-)-alpha-methylcarboxyphenylglycine (MCPG) to inhibit, phosphoinositide turnover in neonatal rat cerebral cortex have been studied. Two indices of phosphoinositide cycle activity were assessed; inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) mass accumulation, and total inositol phosphate [3H]-InsPx accumulation (in the presence of Li+) in myo-[3H]-inositol prelabelled slices. 2. The diastereoisomers of ACPD stimulated each response with a rank order of potency of 1S, 3R > 1R, 3R > 1S, 3S >> 1R, 3S. The response to 1R, 3R-ACPD was largely prevented by pre-addition of the NMDA-receptor antagonist, MK-801, or omission of extracellular Ca2+, suggesting that this isomer acts indirectly on phosphoinositide responses through activation of NMDA-type ionotropic glutamate receptors. In contrast, the responses to 1S, 3R- and 1S, 3S-ACPD were unaffected by prior addition of MK-801, but were blocked by MCPG. 3. The concentration of 1S, 3R-ACPD required to half-maximally stimulate the Ins(1,4,5)P3 response (-log EC50 (M), -4.09 +/- 0.10) was significantly higher than that required to exert a similar effect on [3H]-InsPx accumulation (-log EC50 (M), -4.87 +/- 0.07; P < 0.01; n = 4). A similar marked 8-9 fold discrepancy between these two values was observed for the 1S, 3S isomer, which elicited similar maximal responses to those caused by 1S, 3R-ACPD. 4. Significant differences were also observed with respect to the ability of (+/-)-MCPG (1 mM) to cause a rightward shift in the concentration-response relationships for 1S, 3R-ACPD-stimulated Ins(1,4,5)P3 (5.59 +/- 0.24 fold shift) and [3H]-InsPx (3.04 +/- 0.34 fold shift; P < 0.01; n = 4) responses, giving rise to Kd values of 218 and 490 microM for (+/-)-MCPG antagonism of the respective responses. 5. The potency difference between the 1S, 3R-ACPD-stimulated Ins(1,4,5)P3 and [3H]-InsPx responses was reduced when experiments were performed in nominally calcium-free medium ([Ca2+]e = 2 - 5 microM) and EC50 values were almost identical when extracellular calcium was reduced further by EGTA addition ([Ca2+]e < or = 100 nM). Similarly, the Kd value for (+/-)-MCPG antagonism of the 1S, 3R-ACPD-stimulated [3H]-InsPx response decreased under [Ca2+]e-free conditions, approaching those obtained for the 1S, 3R-ACPD-stimulated Ins(1,4,5)P3 response in the presence of normal [Ca2+]e. 6. These data suggest that estimates of the activities of mGluR agonists and antagonists, derived by measuring phosphoinositide turnover, can differ significantly depending on whether Ins(1,4,5)P3 mass or [3H]-InsPx responses are measured. In particular, the possibility that the mGluR-mediated [3H]-InsPx response may not simply reflect direct receptor/G protein/phosphoinositidase C (PIC) activation, but may also be the consequence of stimulation of a facilitatory Ca2+-influx pathway is discussed.

Potent antagonists at the L-AP4- and (1S,3S)-ACPD-sensitive presynaptic metabotropic glutamate receptors in the neonatal rat spinal cord

In this report we describe the actions of two novel compounds, (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG) and (S)-alpha-ethylglutamate (EGLU), which are potent antagonists at two types of presynaptic metabotropic glutamate (mGlu) receptors in the neonatal rat spinal cord. Selective activation of these receptors by L-2-amino-4-phosphonobutyrate (L-AP4) or (1S,3S)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3S)-ACPD) results in the depression of the monosynaptic component of the dorsal root-evoked ventral root potential (DR-VRP). CPPG produces rightward parallel shifts of the dose-response curves for both L-AP4- and (1S,3S)-ACPD, with Schild slope in each case close to unity, consistent with a competitive mechanism of antagonism. CPPG is the most potent antagonist yet described for both L-AP4- and (1S,3S)-ACPD-sensitive presynaptic mGlu receptors but displays a 30-fold selectivity for the L-AP4-sensitive receptor over the (1S,3S)-ACPD-sensitive receptor (KD values 1.7 microM and 53 microM, respectively). EGLU, on the other hand, is selective for the (1S,3S)-ACPD-sensitive receptor, displaying little or no activity at the L-AP4-sensitive site. EGLU produces a rightward parallel shift of the dose-response curve to (1S,3S)-ACPD, with Schild slope close to unity, again indicative of a competitive mode of antagonism (KD 66 microM). Both CPPG and EGLU displayed only weak or no antagonist activity at postsynaptic metabotropic and ionotropic glutamate receptors.

Differential desensitization of mu- and delta- opioid receptors in selected neural pathways following chronic morphine treatment

1. Morphine produces a plethora of pharmacological effects and its chronic administration induces several side-effects. The cellular mechanisms by which opiates induce these side-effects are not fully understood. Several studies suggest that regulation of adenylyl cyclase activity by opioids and other transmitters plays an important role in the control of neural function. 2. The aim of this study was to evaluate desensitization of mu- and delta- opioid receptors, defined as a reduced ability of opioid agonists to inhibit adenylyl cyclase activity, in four different brain structures known to be involved in opiate drug actions: caudate putamen, nucleus accumbens, thalamus and periaqueductal gray (PAG). Opiate regulation of adenylyl cyclase in these regions has been studied in control and morphine-dependent rats. 3. The chronic morphine treatment used in the present study (subcutaneous administration of 15.4 mg morphine/rat/day for 6 days via osmotic pump) induced significant physical dependence as indicated by naloxone-precipitated withdrawal symptoms. 4. Basal adenylyl cyclase in the four brain regions was not modified by this chronic morphine treatment. In the PAG and the thalamus, a desensitization of mu- and delta-opioid receptors was observed, characterized by a reduced ability of Tyr-D-Ala-Gly-(NMe)Phe-Gly-ol (DAMGO; mu), Tyr-D-Pen-Gly-Phe-D-Pen (DPDPE; delta) and [D-Ala2]-deltorphin-II (DT-II; delta) to inhibit adenylyl cyclase, activity following chronic morphine treatment. 5. The opioid receptor desensitization in PAG and thalamus appeared to be heterologous since the metabotropic glutamate receptor agonists, L-AP4 and glutamate, and the 5-hydroxytryptamine (5-HT)1A receptor agonist, R(+)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT), also showed reduced inhibition of adenylyl cyclase activity following chronic morphine treatment. 6. In the nucleus accumbens and the caudate putamen, desensitization of delta-opioid receptor-mediated inhibition without modification of mu-opioid receptor-mediated inhibition was observed. An indirect mechanism probably involving dopaminergic systems is proposed to explain the desensitization of delta-mediated responses and the lack of mu-opioid receptor desensitization after chronic morphine treatment in caudate putamen and nucleus accumbens. 7. These results suggest that adaptive responses occurring during chronic morphine administration are not identical in all opiate-sensitive neural populations.

Dual action of metabotropic glutamate receptor agonists on neuronal excitability and synaptic transmission in spinal ventral horn neurons in vitro

A dual action of selective metabotropic glutamate receptor agonists on neuronal excitability and dorsal root-evoked excitatory (DR-EPSPs) and inhibitory (DR-IPSPs) neurotransmission is described for immature rat ventral horn neurons in vitro. Trans-1-Aminocyclopentane-1,3 -dicarboxylate (trans-ACPD), its stereoisomer (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate ((1S,3R)-ACPD) and (2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine (L-CCG-1) produced a concentration-related and alpha-methyl-4 -carboxyphenylglycine (MCPG)-sensitive depolarisation. An (1S,3R)-ACPD- or L-CCG-1-induced increase in intrinsic neuronal excitability was apparently independent of the depolarisation and was observed as (a) a fall in the threshold current required to elicit regenerative excitation and (b) an increased number of spikes to a fixed amplitude step depolarisation. The spike after-hyperpolarisation (AHP) duration and amplitude were reduced, suggesting an mGluR agonist action on potassium channels. Synaptic responses were depressed by the mGluR agonists. (1S,3R)-ACPD or L-CCG-1 reduced the mean +/- S.E.M. peak amplitude of a subthreshold EPSP elicited by low-intensity stimuli likely to recruit only low-threshold sensory afferents. The peak amplitude of longer-latency EPSPs elicited by higher-intensity stimuli likely to recruit high-threshold afferents in addition was attenuated. (1S,3R)-ACPD- or L-CCG-1 reduced the peak amplitude of an IPSP evoked by dorsal root stimulation. These effects on synaptic transmission were likely to be due to the combined activation of postsynaptic and presynaptic metabotropic glutamate receptors. The implications of these data for the physiological role of spinal mGluRs is discussed.

Metabotropic glutamate group II receptors are responsible for the depression of synaptic transmission induced by ACPD in the dentate gyrus

The functional role of metabotropic glutamate (mGlu) receptors in the rat dentate gyrus was investigated. By using extracellular recording techniques in slices, it was found that the depression induced by the mGlu receptor agonist (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylate (ACPD) was mediated through the mGlu group II receptors. The mGlu receptor antagonist alpha-methyl-4-carboxyphenylglycine (MCPG) (500 mu M), active at group I and group II subtype receptors, was effective in antagonizing the ACPD (30 mu M) - induced depression of the excitatory field potentials. An antagonist selective for group I, (S)-4-carboxyphenylglycine (4CPG), did not block the effects induced by ACPD, but by itself produced a dose-dependent depression of the field potentials. This ACPD-like effect shown at high concentrations of 4CPG (300 mu M) is explained by its group II receptor agonistic properties and was blocked by bath application of MCPG (500 mu M). A selective agonist of group I, (S)-3-hydroxyphenylglycine (3-HPG), did not cause any depression of synaptic transmission. However, the selective mGlu group II receptor agonist, (2S,3S,4S)-alpha-(carboxycyclopropyl)glycine (L-CCG-I), induced a marked dose-dependent depression and its action was blocked by MCPG (500 mu M). Furthermore, the selective mGlu group III receptor antagonist, alpha-methyl-L-2-amino-4-phosphonobutyrate (MAP4) (500 mu M), was not able to antagonize the depression induced by ACPD (30 mu M), but was effective in blocking the action induced by the selective mGlu group III agonist, L-2-amino-4-phosphonobutyrate (L-AP4) (100 mu M). These results indicate that mGlu group II receptors, but not groups I or III, are involved in the depression of synaptic transmission in the dentate area of the hippocampus induced by ACPD.

Structure-activity relationships for a series of phenylglycine derivatives acting at metabotropic glutamate receptors (mGluRs)

1. The actions of a series of twelve phenylglycine derivatives at metabotropic glutamate receptors (mGluRs) linked to both phosphoinositide hydrolysis (PI) and cyclic AMP were investigated. 2. PI hydrolysis was determined by the accumulation of [3H]-inositol-monophosphate ([3H]-IP1) in neonatal ral cortical slices prelabelled with [3H]-myo-inositol. The non-selective mGluR agonist (1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid ((1S,3R)-ACPD) produced a concentration-dependent increase in [3H]-IP1 (EC50 approximately 20 microM). This agonist was subsequently used to investigate potential antagonist activity of the phenylglycine derivatives. Of the compounds tested (+)-alpha-methyl-4-carboxyphenylglycine (M4CPG) and (RS)-alpha-ethyl-4-carboxyphenylglycine (E4CPG) were the most active with KP values of 0.184 +/- 0.04 mM and 0.367 +/- 0.2 mM respectively. 3. Activity at adenylyl cylase-coupled mGluRs was investigated by determining the accumulation of [3H]-cyclic AMP in adult rat cortical slices. [3H]-cyclic AMP accumulation, elicited by 30 microM forskolin, was inhibited by (2S,3S,4S)-alpha-(carboxycyclopropyl)glycine (L-CCG-1) and L-2-amino-4-phosphonobutanoate (L-AP4) with respective EC50 values of 0.3 microM and 10 microM. Neither agonist was able to inhibit completely forskolin stimulated cyclic AMP accumulation; this is evidence that not all adenylyl cyclase is susceptible to modulation by mGluRs. Phenylglycine derivatives were examined for their ability to antagonize the inhibition of [3H]-cyclic AMP accumulation by L-CCG-1 or L-AP4 at their EC50 concentrations. 4. A rank order of potency of the phenylglycine derivatives as antagonists of L-AP4 and L-CCG-1 was obtained. The most effective compound. (RS)-alpha-methyl-3-carboxymethylphenylglycine (M3CMPG) had IC50 values in the order of 1 microM against L-AP4 and 0.4 microM against L-CCG-1. 5. The results from this study indicate that phenylglycine-derived compounds can discriminate between groups of metabotropic glutamate receptors and may also display some selective activity between subtypes within groups. Future work based on these findings may lead to the development of more selective and potent compounds as important pharmacological tools.

Glutamate, GABA and epilepsy

The nature and value of various animal models of epilepsy for the study and understanding of the human epilepsies are reviewed, with special reference to the ILAE classification of seizures. Kindling as a model of complex-partial seizures with secondary generalisation is treated in detail, dwelling principally on the evidence that the neurotransmitters glutamate and GABA are centrally involved in the kindling process. Kindling in the entorhinal cortex-hippocampus system and its relationship to LTP are analysed in detail. Changes in amino acid content in animal and human brain tissue following onset of the epileptic state are reviewed with special reference to glutamate and GABA. Studies of changes in the extent of basal and stimulus-evoked release of glutamate and GABA both in vivo (microdialysis) and in vitro (brain slices) are evaluated. This includes both kindling and other models of epilepsy, and microdialysis of human patients with epilepsy. Experiments which study the influence of pre-synaptic metabotropic glutamate receptors on glutamate release, and consequently on the extent of electrical kindling, are described. This pre-synaptic control of glutamate release can be studied using synaptosomes. The significance of the ability of focal intracerebrally injected glutamate and NMDA to cause (chemical) kindling and the strong sensitivity of this process to pre-treatment with NMDA receptor antagonists is analysed. Electrical and chemical kindling effects are additive, indicating the existence of mechanisms in common. They are both sensitive to NMDA antagonists and the common mechanism is probably NMDA receptor activation due to the presence of exogenous (chemical) or endogenous (electrically-released) extracellular glutamate. The participation of the NMDA receptor in the generation of the spontaneous hyperactivity which characterises the chronic epileptic state is reviewed. This includes the entry of Ca2+ to stimulate various post-synaptic phosphorylation processes, and possible modulation of NMDA receptor population size and sensitivity. The question of whether neurotransmitter glutamate is involved in initiation and/or spread of seizures is discussed.

Blockade of both epileptogenesis and glutamate release by (1S,3S)-ACPD, a presynaptic glutamate receptor agonist

The influence of intracerebrally focally administered doses of a presynaptic metabotropic glutamate receptor agonist, (1S,3S)-ACPD, and of the post-synaptically targeted competitive NMDA receptor antagonist, D-CPPene (SDZ EAA 494), was tested on the development of amygdaloid kindling. The actions of these drugs, compared to that of D-CPP, was also tested on fully developed stage 5 amygdala kindled seizures. Both (1S,3S)-ACPD and D-CPPene dose-dependently increased the generalised seizure threshold in fully kindled animals. They showed a similar potency, with (1S,3S)-ACPD acting presynaptically and D-CPPene postsynaptically. Both drugs reversibly inhibited epileptogenesis at 10 nmol in 0.5 microliter of injection vehicle, keeping the kindling stage at or below stage 2. All animals reached stage 5 after withdrawal of the 2 drugs. Whereas (1S,3S)-ACPD inhibited depolarisation-induced release of [3H]L-glutamate and [3H]D-aspartate from cortical synaptosomes (IC50 63 microM and 50 microM, respectively), D-CPPene (postsynaptically active) was without effect. These findings suggest a new approach to the development of clinically effective anticonvulsants through the development of presynaptic glutamate receptor agonists which could be administered systemically to control the extent of synaptic release of glutamate.

Metabotropic and ionotropic glutamate receptors mediate opposite effects on periaqueductal gray matter

Microinjections, into the dorso-lateral periaqueductal gray matter, of N-methyl-D-aspartic acid (NMDA, 0.07-7 nmol/rat) significantly (P < 0.01) increased arterial blood pressure in a dose-related manner. Pretreatment, 5 min before NMDA (7 nmol/rat), in the same area with 2-amino-5-phosphonovaleric acid (2-APV, 5 nmol/rat), a selective antagonist of NMDA receptors, significantly (P < 0.01) reduced NMDA-induced arterial hypertension. trans-(+/-)-1-Amino-1,3-cyclopentanedicarboxylic acid (t-ACPD, 6-30 nmol/rat), an agonist of metabotropic glutamate receptors (mGlu receptors), significantly (P < 0.01) decreased arterial blood pressure when microinjected into the dorsal-lateral periaqueductal gray matter. Pretreatment, 5 min before t-ACPD (30 nmol/rat), in the same area with L-2-amino-3-phosphono-propionate (L-AP-3, 30 nmol/rat), a putative antagonist of the mGlu receptors, was not able to prevent t-ACPD-induced hypotension. Microinjections of L-AP-3 (30 nmol/rat) induced a hypotension similar to the one obtained with t-ACPD at the dose of 6 nmol/rat. From these data we can suggest that mGlu receptors act inversely to the NMDA receptors in the dorso-lateral periaqueductal gray area and that L-AP-3 is a partial agonist rather than an antagonist of mGlu receptors within the periaqueductal gray area.

Pre- and postsynaptic glutamate receptors at a giant excitatory synapse in rat auditory brainstem slices

1. Whole-cell patch recordings were used to examine the EPSC generated by the calyx of Held in neurones of the medial nucleus of the trapezoid body (MNTB). Each neurone receives a somatic input from a single calyx (giant synapse). 2. A slow NMDA receptor-mediated EPSC peaked in 10 ms and decayed as a double exponential with time constants of 44 and 147 ms. A fast EPSC had a mean rise time of 356 microseconds (at 25 degrees C), while the decay was described by a double exponential with time constants of 0.70 and 3.43 ms. 3. Cyclothiazide slowed the decay of the fast EPSC, indicating that it is mediated by AMPA receptors. The slower time constant was slowed to a greater extent than the faster time constant. Cyclothiazide potentiated EPSC amplitude, partly by a presynaptic mechanism. 4. The metabotropic glutamate receptor (mGluR) agonists, 1S,3S-ACPD, 1S,3R-ACPD and L-2-amino-4-phosphonobutyrate (L-AP4) reversibly depressed EPSC amplitude. A dose-response curve for 1S,3S-ACPD gave an EC50 of 7 microM and a Hill coefficient of 1.2. 5. Analysis of the coefficient of variation ratio showed that the above mGluR agonists acted presynaptically to reduce the probability of transmitter release. Adenosine and baclofen also depressed transmission by a presynaptic mechanism. 6. alpha-Methyl-4-carboxyphenylglycine (MCPG; 0.5-1 mM) did not antagonize the effects of 1S,3S-ACPD, while high concentrations of L-2-amino-3-phosphonopropionic acid (L-AP3; 1 mM) and 4-carboxy-3-hydroxyphenyglycine (4C3HPG; 500 microM) depressed transmission. 7. There was a power relationship between [Ca2+]o and EPSC amplitude with co-operativity values ranging from 1.5 to 3.4. 8. The mechanism by which mGluRs modulate transmitter release appeared to be independent of presynaptic Ca2+ or K+ currents, since ACPD caused no change in the level of paired-pulse facilitation or the duration of the presynaptic action potential (observed by direct recording from the terminal), indicating that the presynaptic mGluR transduction mechanism may be coupled to part of the exocytotic machinery. 9. Our data are not consistent with the presence at the calyx of Held of any one known mGluR subtype. Comparison of the time course and pharmacology of the fast EPSC with data from cloned AMPA receptors is consistent with the idea that GluR-Do subunits dominate the postsynaptic channels.

Modulation of adenosine-induced cAMP accumulation via metabotropic glutamate receptors in chick optic tectum

Changes on cyclic adenosine monophosphate (cAMP) levels in response to adenosine and glutamate and the subtype of glutamate receptors involved in this interaction were studied in slices of optic tectum from 3-day-old chicks. cAMP accumulation mediated by adenosine (100 microM) was abolished by 8-phenyltheophylline (15 microM). Glutamate and the glutamatergic agonists kainate or trans-D, L-1-aminocyclopentane-1,3-dicarboxylic acid (trans-ACPD) did not evoke cAMP accumulation. Glutamate blocked the adenosine response in a dose-dependent manner. At 100 microM, glutamate did not inhibit the effect of adenosine. The 1 mM and 10 mM doses of glutamate inhibited adenosine-induced cAMP accumulation by 55% and 100%, respectively. When glutamatergic antagonists were used, this inhibitory effect was not affected by 200 microM 6,7-dihydroxy-2,3,dinitroquinoxaline (DNQX), an ionotropic antagonist, and was partially antagonized by 1 mM (RS)-alpha-methyl-4-carboxyphenylglycine [(RS)M-CPG], a metabotropic antagonist, while 1 mM L-2-amino-3-phosphonopropionate (L-AP3) alone, another metabotropic antagonist, presented the same inhibitory effect of glutamate. Kainate (10 mM) and trans-ACPD (100 microM and 1 mM) partially blocked the adenosine response. This study indicates the involvement of metabotropic glutamate receptors in adenylate cyclase inhibition induced by glutamate and its agonists trans-ACPD and kainate.

The effects of L-AP4 and L-serine-O-phosphate on inhibition in primary somatosensory cortex of the adult rat in vivo

The effects of two iontophoretically applied Group III mGluR agonists were studied on the inhibition in neocortex produced by natural stimulation of vibrissae. The agonists L-AP4 and L-serine-O-phosphate (L-SOP) were shown to produce qualitatively similar effects on the inhibition. Forty-four percent of neurones (total n = 57) displayed disinhibition during application of the agonists. The disinhibitory effects often outlasted the offset of the agonist application by at least 10 min. Concurrent application of the mGluR antagonist (+)-alpha-methyl-4-carboxyphenylglycine ((+)-MCPG) appeared to reverse the disinhibitory effects of L-AP4 and L-SOP in 3 out of 5 neurones tested. However (+)-MCPG itself was found to have disinhibitory effects in some neurones. Some neurones (n = 7) showed increases in inhibition during either L-AP4 or L-SOP application. These appeared most pronounced in those neurones where the initial (pre-drug) inhibition was minimal, perhaps suggesting that the agonists were disinhibiting a local disinhibition. The data obtained in the experiments suggest that the disinhibitory effects are mediated by a heteroreceptor on inhibitory terminals, action at which depresses the release of inhibitory transmitter. The possible role of the modulation of inhibition by presynaptic mGluRs is discussed.

Pharmacological tools for the investigation of metabotropic glutamate receptors (mGluRs): phenylglycine derivatives and other selective antagonists--an update

Glutamate is known to produce many of its pre- and post-synaptic effects through interaction with at least three groups of G-protein-coupled metabotropic receptors. While molecular biological approaches have revealed a great deal about the nature of these receptors and their neuroanatomical localization, elucidation of their role in both physiological and pathological processes has been hampered by the lack of appropriate pharmacological agents. However, the situation is rapidly changing with the discovery of antagonist phenylglycine derivatives, and other compounds. Not only is it now possible to discriminate between the individual metabotropic glutamate receptor groups but, in several cases, between individual group members. The future development of potent and subtype-specific antagonists will greatly facilitate the advancement of our understanding of these receptors as well as providing the potential for novel therapeutic approaches in a variety of neuropathological states.

Novel agonists for metabotropic glutamate receptors: trans- and cis-2-(2-carboxy-3-methoxymethylcyclopropyl)glycine (trans- and cis-MCG-I)

New derivatives of 2-(carboxycyclopropyl)glycine (CCG), (2S,1'S,2'R,3'S)- and (2S,1'S,2'R,3'R)-2-(2-carboxy-3-methoxymethylcyclopropyl) glycine (trans- and cis-MCG-I), effectively inhibited forskolin-stimulated cyclic AMP formation in a concentration dependent manner in cultured spinal neurones of rats. They effectively depressed monosynaptic excitation in the spinal reflex of newborn rats with IC50 values of 0.3 and 3 microM, respectively, which was sensitive to (+)-MCPG. They did not cause any depolarization even when the concentration was increased up to 0.3 mM. However, after treatment with quisqualate, cis-MCG-I caused a depolarization of motoneurones in the newborn rat spinal cord in a concentration dependent manner with a threshold concentration of 1 microM (quisqualate effect). The depolarizing activity developed after quisqualate treatment gradually decreased but lasted for more than 2 hr. The depolarization induced by cis-MCG-I seemed pharmacologically similar to that of phosphonate-containing analogues of glutamate such as L-AP4 or L-AP6 under the "quisqualate effect". These novel CCG derivatives would be expected to provide useful probes for elucidating the physiological function of mGluRs.

Pharmacological analysis of 4-carboxyphenylglycine derivatives: comparison of effects on mGluR1 alpha and mGluR5a subtypes

The antagonist effects of the 4-carboxyphenylglycines: (S)-4-carboxy-3hydroxyphenylglycine (4C3HPG), (S)-4-carboxyphenylglycine (4CPG) and (+)-alpha-methyl-4-carboxyphenylglycine (M4CPG) were compared on functional responses of human metabotropic glutamate receptor (mGluR) subtypes mGluR1 alpha and mGluR5a. These receptors both belong to group 1 type mGluRs which couple to the phosphoinositide (PI) hydrolysis/[Ca2+]i mobilization signal transduction pathway and are closely related in both structure and agonist pharmacology. In this study, the IC50 values obtained for quisqualate induced PI hydrolysis responses show that although all the phenylglycines are antagonists for both mGluR1 alpha and mGluR5a, the compounds exhibit differential potencies at these receptor subtypes. The 4C3HPG derivative was the most potent antagonist for both mGluR1 alpha (IC50 range: 19-50 microM) and mGluR5a (IC50 range: 53-280 microM). 4CPG produced an IC50 range of 4r-72 microM for mGluR1 alpha and 150-156 microM for mGluR5a cells. The potency of the M4CPG could not be distinguished from that of 4CPG with IC50 ranges of 29-100 microM and 115-210 microM for mGluR1 alpha and mGluR5a respectively. Further characterization of the dose-response effects of the compounds on quisqualate induced [Ca2+]i mobilization showed that although the magnitude of phenylglycine inhibition was reduced for both mGluR subtypes compared to those observed for stimulation of PI hydrolysis (except for 4C3HPG on mGluR1 alpha), similar differences in the relative potencies of the phenylglycines between mGluR1 alpha (IC50s: 40 +/- 10 microM for 4C3HPG: 300-1000 microM for 4CPG and M4CPG) and mGluR5a (IC50s: > 1000 microM) were evident.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of the effects of selective metabotropic glutamate receptor agonists on synaptically evoked whole cell currents of rat spinal ventral horn neurones in vitro

1. Whole cell synaptic currents were recorded under voltage clamp from a total of 54 ventral horn neurones held near to their resting potential by the patch clamp technique in immature rat spinal cord preparations in vitro. Twenty eight neurones were identified, by antidromic invasion from ventral roots, as motoneurones. Excitatory postsynaptic currents (e.p.s.cs) of peak amplitude -480 pA +/- 66 s.e. mean and -829 +/- 124 pA were evoked respectively from the unidentified ventral horn neurones and the motoneurones in response to maximal activation of the segmental dorsal root. 2. The e.p.s.cs were depressed reversibly by the metabotropic glutamate agonists 1S3S-1-aminocyclopentane-1,3-dicarboxylate (1S3S-ACPD) (EC50 17.1 microM +/- 0.3 s.e. mean, n = 14) and L-2-amino-4-phosphonobutanoate (L-AP4) (EC50 = 2.19 +/- 0.19 microM, n = 15). Since both agonists independently produced more than 90% depression it is likely that the receptors that mediate their effects are present on the same presynaptic terminals. 3. When the Mg2+ concentration was raised from 0.75 mM to 2.75 mM together with the addition of 50 microM D-2-amino-5-phosphonopentanoate (AP5), a treatment which would increase the proportion of monosynaptic component in the e.p.s.c. the concentration-effect plots for both 1S3S-ACPD (EC50 1.95 +/- 0.4 microM, n = 8) and L-AP4 (EC50 0.55 +/- 0.20 microM, n = 7) were shifted to the left, suggesting that monosynaptic e.p.cs of primary afferents to ventral horn neurones are more susceptible to L-AP4 and 1S3S-ACPD than are other synapses in polysynaptic pathways. 4. lS3S-ACPD (20 and 50 microM) also caused mean sustained inward currents of 95 +/- 31 pA (n = 6) and248 +/- 49 pA (n = 10) respectively. In the combined presence of AP5 (50 microM) and Mg2+ (2.75 mM) themean response to 50 microM lS3S-ACPD was reduced to 106+/- 18 pA (n = 4). In the presence of tetrodotoxin(1 microM) the corresponding value was 48 +/- 6 pA (n = 4). Similar sustained inward currents produced by N-methyl-D-aspartate (NMDA) were almost abolished to < 10 pA in the presence of AP5 and 2.75 mMMg2+. In the presence of tetrodotoxin the maximum inward current produced by NMDA was undiminished. Thus a large component of the excitatory action of lS3S-ACPD was mediated at non-NMDA receptors both directly at the patch-clamped neurones and indirectly by synaptic relay.

New phenylglycine derivatives with potent and selective antagonist activity at presynaptic glutamate receptors in neonatal rat spinal cord

The depression of the monosynaptic excitation of neonatal rat motoneurones produced by the metabotropic glutamate receptor (mGluR) agonists (1S,3S)-1-aminocyclopentane-1, 3-dicarboxylate (ACPD) or L-2-amino-4-phosphonobutyrate (L-AP4) was antagonized by three novel phenylglycine analogues: (RS)-alpha-methyl-4-sulphonophenylglycine (MSPG), (RS)-alpha-methyl-4-phosphonophenylglycine (MPPG) and (RS)-alpha-methyl-4-tetrazolylphenylglycine (MTPG). The potencies of all the new compounds were greater than that of the previously reported (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG). For L-AP4-sensitive presynaptic mGluRs, the order of antagonist potency found was MPPG > MSPG > MTPG > MCPG. In contrast, the order of antagonist potency found for (1S,3S)-ACPD-sensitive presynaptic mGluRs was MTPG > MPPG > MSPG > MCPG. To date, MPPG (KD 9.2 microM) is the most potent L-AP4-sensitive receptor antagonist yet tested on the neonatal rat spinal cord. In addition, MTPG (KD 77 microM) is the most potent antagonist yet tested for (1S,3S)-ACPD-sensitive receptors in this preparation.

Identification of 2-amino-2-methyl-4-phosphonobutanoic acid as an antagonist at the mGlu4a receptor

2-Amino-2-methyl-4-phosphonobutanoic acid (MAP4) was tested for interactions with the mGlu4a receptor which when expressed in baby hamster kidney (BHK570) cells couples to inhibition of forskolin-stimulated cAMP production. MAP4 had no agonist activity at this receptor and caused a concentration-dependent inhibition of the reduction in forskolin-stimulated cyclic AMP formation elicited by L-2-amino-4-phosphonobutanoic acid (L-AP4). Inhibition by MAP4 was consistent with a competitive mechanism of action (Schild slope = 1.2) with a Ki of 190 microM. MAP4 is the first antagonist identified for the mGlu4a receptor.

Metabotropic glutamate receptor agonists inhibit endogenous glutamate release from rat striatal synaptosomes

A striatal synaptosomal preparation was used to assess the action of metabotropic glutamate receptor (mGlu receptor) agonists on 4-aminopyridine (2 mM)-stimulated endogenous glutamate release. 4-Aminopyridine alone increased basal glutamate release by 6.89 +/- 0.74 nmol/mg. The mGlu receptor agonists L-2-amino-4-phosphonobutyric acid (L-AP4) (IC50 approximately 0.2 microM) and (1S,3S)-1-amino-cyclopentane-1,3-dicarboxylic acid (IC50 approximately 0.2 microM) inhibited 4-aminopyridine-stimulated release. The putative mGlu receptor antagonist (S)-alpha-methyl-L-AP4, which itself inhibited 4-aminopyridine-stimulated release (IC50 approximately 10 microM), did not inhibit the effects of the two agonists.

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.

Actions of phenylglycine derivatives at L-AP4 receptors in retinal ON bipolar cells

Phenylglycine derivatives can act as agonists or antagonists at different metabotropic glutamate receptor (mGluR) subtypes, including subtypes sensitive to L-2-amino-4-phosphonobutyric acid (L-AP4). We examined the pharmacology of four phenylglycines at L-AP4 receptors in ON bipolar cells of the amphibian retina in situ. As previously shown for S-4-carboxy-3-hydroxyphenylglycine (S-4C3H-PG) (Thoreson W. B. and Miller R. F., J. Gen. Physiol. 103, 1019-1034, 1994), whole cell recordings indicate that S-3-carboxy-4-hydroxyphenylglycine (S-3C4H-PG) and S-4-carboxyphenylglycine (S-4C-PG) are L-AP4 receptor agonists in retina. Concentration-response curves for these compounds were obtained using the b-wave of the electroretinogram (ERG) as an assay for ON bipolar cell activity. The rank-order potency and IC50 values obtained were: S-4C-PG (204 microM) > S-4C3H-PG (399 microM) > or = S-3C4H-PG (558 microM). At 1 mM, RS-alpha-methyl-4-carboxyphenylglycine (RS-M4C-PG) suppressed the b-wave by less than 20%. This weak effect is attributed to agonist actions of RS-M4C-PG. The agonist actions of phenylglycines in retina are different from their effects at L-AP4 receptors in spinal cord or the expressed L-AP4-sensitive receptor subtype, mGluR4 (Kemp et al., Eur. J. Pharmac. Molec. Pharmac., 266, 187-192, 1994; Thomsen et al., Eur. J. Pharmac. Molec. Pharmac., 267, 77-84, 1994; Hayashi et al., J. Neurosci., 14, 3370-3377, 1994).(ABSTRACT TRUNCATED AT 250 WORDS)

Changes of pharmacological properties of (1S,3R)-ACPD-sensitive glutamate binding sites in developing mouse cerebellum

Autoradiography of [3H]glutamate binding to mouse cerebellar sections was used to study the distribution of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid-((1S,3R)-ACPD) sensitive [3H]glutamate binding sites and the sensitivity of these sites to drugs preferentially acting on one or few types of the metabotropic receptor family. The inhibitory effect of (1S,3R)-ACPD on [3H]glutamate binding and its estimated inhibition constant showed the presence of a different global metabotropic receptor population according to the region considered. During ontogeny, the (1S,3R)-ACPD binding site density increased in the molecular layer (ML), in contrast it decreased in the internal granular layer (IGL) and the deep cerebellar nuclei (DCN). In addition, different sensitivities to (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG), (S)-4-carboxyphenylglycine (4-CPG), (2S,3S,4S)-alpha-(carboxycyclopropyl)glycine (L-CCG-I) and L-2-amino-4-phosphonobutyric acid (L-AP4) were demonstrated according to the region and the age. In the DCN, the high (1S,3R)-ACPD binding site density at PND 10 seems to be also sensitive to L-CCG-I but not to MCPG, 4-CPG or L-AP4. In the ML, the MCPG-, the 4-CPG- and the L-AP4-sensitive [3H]glutamate binding sites appeared during ontogeny and the L-CCG-I-sensitive [3H]glutamate binding sites were already present at PND 10. In the IGL, L-CCG-I-sensitive binding sites disappeared in contrast to the L-AP4-sensitive binding sites which appeared during development even if the total (1S,3R)-ACPD binding site density was relatively weak in the adults. These results all reflected the multiplicity of the receptor subtypes included in the cerebellar metabotropic component.

Phenylglycine derivatives as antagonists of metabotropic glutamate receptors

Metabotropic glutamate receptors represent a family of G protein-coupled receptors that can be activated by L-glutamate, the principal excitatory neurotransmitter in the brain. Until recently, progress in identifying the physiological and pathological roles of metabotropic glutamate receptors has been hampered by the lack of selective antagonists. In this article, Jeff Watkins and Graham Collingridge describe the pharmacology of, and initial physiological studies using, certain phenylglycine derivatives and related substances--the first definitive antagonists of metabotropic glutamate receptors.

Actions of two new antagonists showing selectivity for different sub-types of metabotropic glutamate receptor in the neonatal rat spinal cord

1. The presynaptic depressant action of L-2-amino-4-phosphonobutyrate (L-AP4) on the monosynaptic excitation of neonatal rat motoneurones has been differentiated from the similar effects produced by (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate ((1S,3R)-ACPD), (1S,3S)-ACPD and (2S,3S,4S)-alpha-(carboxycyclopropyl)glycine (L-CCG-I), and from the postsynaptic motoneuronal depolarization produced by (1S,3R)-ACPD, by the actions of two new antagonists, alpha-methyl-L-AP4 (MAP4) and alpha-methyl-L-CCG-I (MCCG). Such selectivity was not seen with a previously reported antagonist, (+)-alpha-methyl-4-carboxyphenylglycine (MCPG). 2. MAP4 selectively and competitively antagonized the depression of monosynaptic excitation produced by L-AP4 (KD 22 microM). At ten fold higher concentrations, MAP4 also antagonized synaptic depression produced by L-CCG-I but in an apparently non-competitive manner. MAP4 was virtually without effect on depression produced by (1S,3R)- or (1S,3S)-ACPD. 3. MCCG differentially antagonized the presynaptic depression produced by the range of agonists used. This antagonist had minimal effect on L-AP4-induced depression. The antagonism of the synaptic depression effected by (1S,3S)-ACPD and L-CCG-I was apparently competitive in each case but of varying effectiveness, with apparent KD values for the interaction between MCCG and the receptors activated by the two depressants calculated as 103 and 259 microM, respectively. MCCG also antagonized the presynaptic depression produced by (1S,3R)-ACPD. 4. Neither MAP4 nor MCCG (200-500 microM) significantly affected motoneuronal depolarizations produced by (1S,3R)-ACPD. At the same concentrations the two antagonists produced only very weak and variable effects (slight antagonism or potentiation) on depolarizations produced by (S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartate (NMDA).5. It is concluded that MAP4 is a potent and selective antagonist for those excitatory amino acid(EAA) receptors on neonatal rat primary afferent terminals that are preferentially activated by L-AP4,and that MCCG is a relatively selective antagonist for different presynaptic EAA receptors that are preferentially activated by (1S,3S)-ACPD and (perhaps less selectively) by L-CCG-I. These receptors probably comprise two sub-types of metabotropic glutamate receptors negatively linked to adenylyl cyclase activity.