Regulation of striatal dopamine release by metabotropic glutamate receptors

This study examined the temporal relationship between aminoglycoside ototoxicity and the onset of auditory function in the rat. A single dose of gentamicin sulfate (200 mg/kg) and furosemide (100 mg/kg) was administered on postnatal day 6 (P6), P7, P8, P9, or P10, just before the onset of auditory function. Ototoxicity was assessed by the elevation of auditory brain stem response (ABR) thresholds, recorded once the rats had matured. The ABRs were evoked by acoustic clicks and tone pips. The thresholds of control and P6- and P7-treated animals did not differ significantly from each other. Thresholds of some P8- and all P9-treated animals were elevated. The P10-treated animals were deafened, according to these ABR criteria. These data suggest that the potential for aminoglycoside ototoxicity develops rapidly between P8 and P10, just before the onset of auditory function.     

Modulation of excitatory synaptic transmission in locus coeruleus by multiple presynaptic metabotropic glutamate receptors

Metabotropic glutamate receptors have been implicated in modulation of synaptic transmission in many different systems. This study reports the effects of selective activation of metabotropic glutamate receptors on synaptic transmission in intracellularly recorded locus coeruleus neurons in brain slice preparations. Perfusion of either L-2-amino-4-phosphonobutyric acid (L-AP4; 0.1-500 microM) or (+/-)-1-aminocyclopentane-trans-1,3,dicarboxylic acid (t-ACPD; 0.1-500 microM) caused a depression of excitatory postsynaptic potentials in a dose-dependent fashion to about 70% inhibition. Both agonists exerted their effects at relatively low concentrations with estimated EC50s of 2.6 microM and 11.5 microM for L-AP4 and t-ACPD, respectively. This inhibition was not observed with the potent group I metabotropic glutamate receptor agonist (RS)-3,5-dihydroxyphenylglycine (DHPG; 100 microM). Conversely, (R)-4-carboxy-3-hydroxyphenyl-glycine (4C-3H-PG), a group I antagonist/group II agonist, and 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (APDC), a novel and specific group II agonist, also caused an inhibition of excitatory postsynaptic potentials. Both t-ACPD and L-AP4 produced an increase in paired-pulse facilitation, and failed to change the locus coeruleus response to focally applied glutamate, indicating a presynaptic locus of action. The L-AP4 inhibition was antagonized by (S)-amino-2-methyl-4-phosphonobutanoic acid (MAP4: group III antagonist) but not by (RS)-alpha-methyl-4-carboxyphenylglycine [(RS)-MCPG; mixed antagonist], suggesting that this agonist acts through a type 4 metabotropic glutamate receptor. Conversely, t-ACPD was antagonized by MCPG and by ethyl glutamate (group II antagonist), but not by aminoindan dicarboxylic acid (AIDA; group I antagonist) or MAP4, suggesting that this agonist acts on a type 2 or 3 metabotropic glutamate receptor. Taken together, these results suggest that two pharmacologically distinct presynaptic metabotropic glutamate receptors function in an additive fashion to inhibit excitatory synaptic transmission in locus coeruleus neurons. These receptors may be involved in a feedback mechanism and as such may function as autoreceptors for excitatory amino acids.     

Regulation of phosphoinositide turnover in neonatal rat cerebral cortex by group I- and II- selective metabotropic glutamate receptor agonists

1. The interactive effects of different metabotropic glutamate (mGlu) receptor subtypes to regulate phosphoinositide turnover have been studied in neonatal rat cerebral cortex and hippocampus by use of agonists and antagonists selective between group I and II mGlu receptors. 2, The group II-selective agonist 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC; 100 microM) had no effect on basal total inositol phosphate ([3H]-InsPx) accumulation (in the presence of Li+) in myo-[3H]-inositol pre-labelled slices, but enhanced the maximal [3H]-InsPx response to the group I-selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) by about 100% in both hippocampus and cerebral cortex. In cerebral cortex the enhancing effect of 2R,4R-APDC occurred with respect to the maximal responsiveness and had no effect on EC50 values for DHPG (-log EC50 (M): control, 5.56+/-0.05; +2R,4R-APDC, 5.51+/-0.08). 2R,4R-APDC also caused a significant enhancement of the DHPG-stimulated inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) mass response over an initial 0-300 s time-course. 3. The enhancing effects of 2R,4R-APDC on DHPG-stimulated [3H]-InsPx accumulation were observed in both the presence and nominal absence of extracellular Ca2+, and irrespective of whether 2R,4R-APDC was added before, simultaneous with, or subsequent to DHPG. Furthermore, increasing the tissue cyclic AMP concentration up to 100 fold had no effect on DHPG-stimulated Ins(l,4,5)P3 accumulation in the absence or presence of 2R,4R-APDC. 4. 2R,4R-APDC and (2S, 1'R, 2'R, 3'R)-2-(2,3-dicarboxylcyclopropyl)glycine (DCG-IV), the latter agent in the presence of MK-801 to prevent activation of NMDA-receptors, each inhibited forskolin-stimulated cyclic AMP accumulation by about 50%, with respective EC50 values of 1.3 and 0.04 microM (-log EC 50 (M): 2R,4R-APDC, 5.87+/-0.09; DCG-IV, 7.38+/-0.05). In the presence of DHPG (30 microM), 2R,4R-APDC and DCG-IV also concentration-dependently increased [3H]-InsPx accumulation with respective EC50 values of 4.7 and 0.28 microM (-log EC50 (M): 2R,4R-APDC, 5.33+/-0.04; DCG-IV, 6.55+/-0.09) which were 3-7 fold rightward-shifted relative to the adenylyl cyclase inhibitory responses. 5. The group II-selective mGlu receptor antagonist LY307452 (30 microM) caused parallel rightward shifts in the concentration-effect curves for inhibition of forskolin-stimulated adenylyl cyclase, and enhancement of DHPG-stimulated [3H]-InsPx accumulation, by 2R,4R-APDC yielding similar equilibrium dissociation constants (KdS, 3.7+/-1.1 and 4.1+/-0.4 microM respectively) for each response. 6. The ability of 2R,4R-APDC to enhance receptor-mediated [3H]-InsPx accumulation appeared to be agonist-specific; thus although DHPG (100 microM) and the muscarinic cholinoceptor agonist carbachol (10 microM) stimulated similar [3H]-InsPx accumulations, only the response to the former agonist was enhanced by co-activation of group II mGlu receptors. 7. These data demonstrate that second messenger-generating phosphoinositide responses stimulated by group I mGlu receptors are positively modulated by co-activation of group II mGlu receptors in cerebral cortex and hippocampus. The data presented here are discussed with respect to the possible mechanisms which might mediate the modulatory activity, and the physiological and pathophysiological significance of such crosstalk between mGlu receptors.     

Development of metabotropic glutamate receptor-mediated synaptic inhibition

Modulation of hippocampal CA3-CA1 synaptic transmission during metabotropic glutamate receptor (mGluR) activation was investigated in juvenile (postnatal day (P) 15-21) and young adult rats (P28-35). The mGluR agonist 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (ACPD) depressed the EPSP slope more in young adults than juveniles. ACPD increased paired-pulse facilitation (PPF) at both ages. The group I mGluR antagonist (+)-alpha-methylcarboxyphenylglycine (MCPG) inhibited the ACPD-mediated depression of the EPSP slope and completely blocked the increase in PPF only in young adults. Minimal effects of MCPG on ACPD-dependent synaptic depression were observed in juveniles. These data suggest that presynaptic group I mGluR-mediated synaptic inhibition increases across late postnatal development. In addition, other mGluR subtypes, with the ability to depress presynaptic function, appear to be present in juveniles.     

Diversity of calcium signaling by metabotropic glutamate receptors

During prolonged application of glutamate (20 min), patterns of increase in intracellular Ca2+ concentration ([Ca2+]i) were studied in HEK-293 cells expressing metabotropic glutamate receptor, mGluR1alpha or mGluR5a. Stimulation of mGluR1alpha induced an increase in [Ca2+]i that consisted of an initial transient peak with a subsequent steady plateau or an oscillatory increase in [Ca2+]i. The transient phase was largely attributed to Ca2+ mobilization from the intracellular Ca2+ stores, but the sustained phase was solely due to Ca2+ influx through the mGluR1alpha receptor-operated Ca2+ channel. Prolonged stimulation of mGluR5a continuously induced [Ca2+]i oscillations through mobilization of Ca2+ from the intracellular Ca2+ stores. Studies on mutant receptors of mGluR1alpha and mGluR5a revealed that the coupling mechanism in the sustained phase of Ca2+ response is determined by oscillatory/non-oscillatory patterns of the initial Ca2+ response but not by the receptor identity. In mGluR1alpha-expressing cells, activation of protein kinase C selectively desensitized the pathway for intracellular Ca2+ mobilization, but the mGluR1alpha-operated Ca2+ channel remained active. In mGluR5a-expressing cells, phosphorylation of mGluR5a by protein kinase C, which accounts for the mechanism of mGluR5a-controlled [Ca2+]i oscillations, might prevent desensitization and result in constant oscillatory mobilization of Ca2+ from intracellular Ca2+ stores. Our results provide a novel concept in which oscillatory/non-oscillatory mobilizations of Ca2+ induce different coupling mechanisms during prolonged stimulation of mGluRs.     

Characterization of an intracellular alkaline shift in rat astrocytes triggered by metabotropic glutamate receptors

The modulation of intracellular pH by activation of metabotropic glutamate receptors was investigated in cultured and acutely dissociated rat astrocytes. One minute superfusion of 100 microM (1S,3R)-1-aminocyclopentane-1, 3-dicarboxcylic acid (ACPD) evoked an alkaline shift of 0.13 +/- 0. 013 (mean +/- SE) and 0.16 +/- 0.03 pH units in cultured (cortical or cerebellar) and acutely dissociated cortical astrocytes, respectively. Alkalinizations were elicited by concentrations of ACPD as low as 1 muM. The ACPD response was mimicked by S-3-hydroxyphenylglycine (3-HPG) and by (s)-4-carboxy-3-hydroxyphenylglycine (4C-3HPG) but was not blocked by alpha-methyl-4-carboxyphenylglycine (MCPG) or (RS)-1-aminoindan-1, 5-dicarboxcylic acid (AIDA), features consistent with an mGluR5 receptor-mediated mechanism. The ACPD-evoked alkaline shift was insensitive to amiloride, 4,4'-diisothiocyanostilbene-2, 2'-disulfonic acid (DIDS), and the v-type ATPase inhibitors 7-chloro-4-nitrobenz-2-oxa-1,3-diazol (NBD-Cl), bafilomycin, and concanamycin. The alkaline response persisted in Na+- or Cl--free saline, but was reversibly blocked in bicarbonate-free, N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES)-buffered solutions. A bicarbonate-dependent and Na+-independent alkaline shift could also be elicited by either 3 mM caffeine or 1 muM ionomycin. These data suggest that a rise in cytosolic Ca2+ activity is instrumental in triggering the alkalinizing mechanism and that this response is independent of the classic depolarization-induced alkalinization mediated by electrogenic sodium-bicarbonate cotransport.     

The metabotropic glutamate receptors of the vestibular organs

This research sought to test the presence and function of metabotropic excitatory amino acid receptors (mGluR) in the frog semicircular canal (SCC). The mGluR agonist +/- 1-aminocyclopentane-trans-1,3-dicarboxylate (ACPD) produced an increase in afferent firing rates of the ampullar nerve of the intact posterior canal. This increase was not due to a stimulation of cholinergic efferent terminals or the acetylcholine (ACh) receptor, since atropine, in concentrations which blocked the response to exogenous acetylcholine, did not affect the response to ACPD. Likewise, ACPD effects were not due to stimulation of postsynaptic NMDA receptors, since the NMDA antagonist D(-)-2-amino-5-phosphonopentanoate (AP-5) did not affect the response to ACPD, reinforcing the reported selectivity of ACPD for mGluRs. When the SCC was superfused with artificial perilymph known to inhibit hair cell transmitter release (i.e. low Ca-high Mg), ACPD failed to increase afferent firing. This suggests that the receptor activated by ACPD is located on the hair cell. Pharmacological evidence suggested that the mGluRs involved in afferent facilitation belong to Group I (i.e. subtypes 1 and 5). In fact, the Group III agonist AP-4 had no effect, and the ACPD facilitatory effect was blocked by the Group I mGluR antagonists (S)-4-carboxyphenylglycine (CPG) and (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA). Additional pharmacological evidence supported the presence of Group I mGluRs. Interestingly, the mGluR antagonists, AIDA and 4CPG, by themselves did not affect the resting firing rates of ampullar afferents. This may suggest that the mGluRs are not involved in resting activity but perhaps only in evoked activity (as suggested in Guth et al. (1991) Hear. Res. 56, 69-78). In addition, the mRNA for the mGluR1 has been detected in hair cells of both SCC, utricle, and saccule. In summary, the evidence points to an mGluR localized to the hair cell (i.e. an autoreceptor) which may be activated to produce a positive feedback augmentation of evoked but not resting transmitter release and thus affect afferent activity.     

Postsynaptic current mediated by metabotropic glutamate receptors in cerebellar Purkinje cells

In rat cerebellar slices, repetitive parallel fiber stimulation evokes an inward, postsynaptic current in Purkinje cells with a fast component mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors and a slower component mediated by metabotropic glutamate receptors (mGluR). The mGluR-mediated excitatory postsynaptic current (mGluR-EPSC) is evoked selectively by parallel fiber stimulation; climbing fiber stimulation is ineffective. The mGluR-EPSC is elicited most effectively with increasing frequencies of parallel fiber stimulation, from a threshold of 10 Hz to a maximum response at approximately 100 Hz. The amplitude of the mGluR-EPSC is a linear function of the number of stimulus pulses without any apparent saturation, even with >10 pulses. Thus mGluRs at the parallel fiber-Purkinje cell synapse can function as linear detectors of the number of spikes in a burst of activity in parallel fibers. The mGluR-EPSC is present from postnatal day 15 and persists into adulthood. It is inhibited by the generic mGluR antagonist (RS)-a-methyl-4-carboxyphenylglycine and by the group I mGluR antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid at a concentration selective for mGluR1. Although the intracellular transduction pathway involves a G protein, the putative mediators of mGluR1 (phospholipase C and protein kinase C) are not directly involved, indicating that the mGluR-EPSC studied here is mediated by a different and still unidentified second-messenger pathway. Heparin, a nonselective antagonist of inositol-trisphosphate (IP3) receptors, has no significant effect on the mGluR-EPSC, suggesting that also IP3 might be not required for the response. Buffering intracellular Ca2+ with a high concentration of bis-(o-aminophenoxy)-N,N,N', N'-tetraacetic acid partially inhibits the mGluR-EPSC, indicating that Ca2+ is not directly responsible for the response but that resting Ca2+ levels exert a tonic potentiating effect on the mGluR-EPSC.     

Pharmacological characterization of metabotropic glutamate receptors coupled to phospholipase D in the rat hippocampus

1. Phospholipase D (PLD) is the key enzyme in a signal transduction pathway leading to the formation of the second messengers phosphatidic acid and diacylglycerol. In order to define the pharmacological profile of PLD-coupled metabotropic glutamate receptors (mGluRs), PLD activity was measured in slices of adult rat brain in the presence of mGluR agonists or antagonists. Activation of the phospholipase C (PLC) pathway by the same agents was also examined. 2. The mGluR-selective agonist (1S,3R)-l-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD] induced a concentration-dependent (10-300 microM) activation of PLD in the hippocampus, neocortex, and striatum, but not in the cerebellum. The effect was particularly evident in hippocampal slices, which were thus used for all subsequent experiments. 3. The rank order of potencies for agonists stimulating the PLD response was: quisqualate > ibotenate > (2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine > (1S,3R)-ACPD > L-cysteine sulphinic acid > L-aspartate > L-glutamate. L-(+)-2-Amino-4-phosphonobutyric acid and the ionotropic glutamate receptor agonists N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and kainate failed to activate PLD. (RS)-3,5-dihydroxyphenylglycine (100300 microM), an agonist of mGluRs of the first group, stimulated PLC but inhibited the PLD response elicited by 100 microM (1S,3R)-ACPD. 4. (+)-alpha-Methyl-4-carboxyphenylglycine (0.1-1 mM), a competitive antagonist of mGluRs of the first and second group, elicited a significant PLD response. L-(+)-2-Amino-3-phosphonopropionic acid (1 mM), an antagonist of mGluRs of the first group, inhibited the 100 microM (1S,3R)-ACPD-induced PLC response but produced a robust stimulation of PLD. 5. 12-O-Tetradecanoylphorbol 13-acetic acid and phorbol 12,13-dibutyrate (PDBu), activators of protein kinase C, at 1 microM had a stimulatory effect on mGluRs linked to PLD but depressed (1S,3R)-ACPD-induced phosphoinositide hydrolysis. The protein kinase C inhibitor, staurosporine (1 and 10 microM) reduced PLD activation induced by 1 microM PDBu but not by 100 microM (1S,3R)-ACPD. 6. Our results suggest that PLD-linked mGluRs in rat hippocampus may be distinct from any known mGluR subtype coupled to PLC or adenylyl cyclase. Moreover, they indicate that independent mGluRs coupled to the PLC and PLD pathways exist and that mGluR agonists can stimulate PLD through a PKC-independent mechanism.     

Homer regulates the association of group 1 metabotropic glutamate receptors with multivalent complexes of homer-related, synaptic proteins

OBJECTIVE: The study aimed to assess the frequency, indications, and outcome of additional ocular procedures after initial treatment of vitrectomy (VIT) or tap-biopsy (TAP) for patients with endophthalmitis after cataract extraction. DESIGN: The study design was an analysis of observational data collected as part of a multicenter, randomized clinical trial. PARTICIPANTS: Of the 420 patients enrolled in the Endophthalmitis Vitrectomy Study, the 148 who had additional procedures were compared with the 272 who did not. MAIN OUTCOME MEASURES: The types, indications, and number of additional ocular procedures were assessed. A masked examiner measured visual acuity 9 to 12 months after study entry. RESULTS: Within 1 week of study entry, 8% of VIT eyes and 13% of TAP eyes underwent additional procedures, 14% for complications of the initial procedure and 86% for worsening ocular inflammation or infection. Cultures were obtained in 33 of the 38 eyes operated on for worsening inflammation or infection and were positive in 42%. Cultures obtained from the early additional procedures were positive more frequently in eyes with an initial TAP (71%) than in eyes with an initial VIT (13%). Both virulence of initial microbiologic organism isolated and poor presenting vision were risk factors for requirement of reoperation. In all cases in which a single organism was cultured at the initial procedure, when the reculture was positive, it was the same organism. Late additional procedures (after 7 days) were required in 27% of patients. Visual outcome was much worse for eyes that had an additional procedure compared to eyes that did not, and this was especially the case for eyes that had an early additional procedure. Only 15% of eyes that had an early additional procedure achieved 20/40 visual acuity as compared to 57% of eyes that did not. CONCLUSION: Need for an additional procedure was a marker of more severe disease, and patients who underwent additional procedures achieved poorer visual acuity at final follow-up.     

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.     

Activation of metabotropic glutamate receptors induce differential effects on synaptic transmission in the dentate gyrus and CA1 of the hippocampus in the anaesthetized rat

Activation of ACPD-sensitive metabotropic receptors induced differential effects on synaptic transmission and the induction of LTP in CA1 and the dentate gyrus of the hippocampus i.c.v. injections of (1.S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD] induced enduring potentiation of the fEPSP in CA1, which occluded tetanically induced LTP. In contrast, ACPD induced a dose-dependent biphasic effect on the fEPSP in the dentate gyrus, consisting of an initial short lasting potentiation, followed by enduring depression of the response, and blockade of LTP. These two effects are likely to be mediated by two different classes of the receptor as in the dentate gyrus the selective class I agonist, (RS)-3,5-dihydroxyphenylglycine (DHPG) induced sustained potentiation of the fEPSP, whereas the mixed mGluR2 agonist-mGluR1 antagonist, (S)-4-carboxy-3-hydrophenylglycine((S)-4C3H-PG) induced only depression. Increasing the concentration of calcium directly in the dentate gyrus prior to, and in conjunction with, injections of ACPD induced sustained potentiation rather than depression. The differential effects indicate that the second messenger cascades the subtypes of receptors are linked with, mediate different forms of synaptic plasticity within the hippocampus and have important implications for their role in learning.     

Localization of ionotropic and metabotropic glutamate receptors in distinct neuronal elements of the rat substantia nigra

PURPOSE: The contingent valuation method (CVM) is a survey-based approach for eliciting consumer's monetary valuations for programme benefits for use in cost-benefit analysis (CBA). We used the conceptual framework of O'Brien and Gafni (1996) to classify and critically appraise health care CVM studies. METHODS: Search of computerized health care and economic citation databases (e.g. MEDLINE, ECONLIT) and manual search for papers published between 1984 1996 reporting primary data valuing health programme benefits in monetary units by CVM using willingness-to-pay (WTP) or accept (WTA). We classified studies using both empirical (i.e. who was surveyed and how) and conceptual criteria (i.e. which measure of consumer utility was measured and why). RESULTS: 48 CVM studies were retrieved; the majority (42) undertook money valuation in the context of cost benefit analysis (CBA), with the remainder being pricing/demand studies. Among the 42 CBA studies, the consumer utility being measured (i.e. compensating (CV) vs. equivalent variation (EV) was explicitly stated in only three (7%) studies). WTP was measured in 95% of studies and WTA in 5%. By cross-tabulation, 42 (91%) studies were designed as WTP/CV, two (4%) were WTP/EV, two (4%) were WTA/CV and no studies used WTA/EV. Most studies were administered by mail (52%) with 38% being in-person interviews. Value elicitation techniques included open-ended questions (38%), payment cards (19%) discrete choice questions (26%) or bidding games (29%). Some form of construct validation tests, particularly associations between WTP and income, were done in 21 studies (50%). CONCLUSIONS: (i) The number of health care CVM studies is growing rapidly and the majority are done in the context of CBA; (ii) there is wide variation among health care CVM studies in terms of the types of questions being posed and the elicitation formats being used; (iii) classification and appraisal of the literature is difficult because reporting of methods and their relationship with the conceptual framework of CBA is poor; (iii) the applicability to health care of the CVM guidelines issued by the National Oceanic and Atmospheric Administration (NOAA) panel for environmental economics is unclear.     

Involvement of a cyclic-AMP pathway in group I metabotropic glutamate receptor responses in neonatal rat cortex

3,5-Dihydroxyphenylglycine (DHPG), (S)-3-hydroxyphenylglycine and (S)-4-carboxy-3-hydroxyphenylglycine (S-4C3HPG) stimulated phosphoinositide hydrolysis in neonatal rat cortical slices, but with lower maximal effect, in comparison with 2S,1'S,2'S-2-(2'-carboxycyclopropyl)glycine (L-CCG I) or (1S,3R)-1-aminocyclo-pentane-1,3-dicarboxylic acid (1S,3R-ACPD). DHPG, 1S,3R-ACPD, and S-4C3HPG also evoked a rapidly desensitizing increase in [Ca2+]i in cortical layers of neonatal brain slices. (R,S)-alpha-methyl-4-tetrazolyl-phenylglycine (MTPG), and (R,S)-alpha-methyl-4-phosphono-phenylglycine (MPPG) inhibited the increase of phosphoinositide hydrolysis elicited by 1S,3R-ACPD but not that by R,S-DHPG. In contrast, the selective group II receptor agonist (1S,2S,5R,6S)-2-amino-bicyclo-[3.1.0]-hexane-2,6-dicarboxylate (LY 354740) potentiated the response of R,S-DHPG. Finally, 8-(4-chlorophenylthio)-cAMP, a membrane permeant analogue of cAMP, reversed the stimulatory effect of 1S,3R-ACPD and S-4C3HPG on phosphoinositide hydrolysis and [Ca2+]i mobilization, without affecting the response induced by R,S-DHPG. These data suggest that, in neonatal rat cortex, the activation of group II metabotropic glutamate receptors potentiates the phosphoinositide hydrolysis and [Ca2+]i responses mediated by group I metabotropic glutamate receptors.     

Metabotropic glutamate receptor antagonists, like GABA(B) antagonists, potentiate dorsal root-evoked excitatory synaptic transmission at neonatal rat spinal motoneurons in vitro

Recordings of whole-cell synaptic current responses elicited by electrical stimulation of dorsal roots were made from motoneurons, identified by antidromic invasion, in isolated spinal cord preparations from five- to eight-day-old Wistar rats. Supramaximal electrical stimulation of the dorsal root evoked complex excitatory postsynaptic currents with mean latencies (+/- S.E.M.) of 6.1 +/- 0.26 ms, peak amplitude of -650 +/- 47 pA and duration of 4.30 +/- 0.46 s (n=34). All phases of excitatory postsynaptic currents were potentiated to approximately 20% above control levels in the presence of the metabotropic glutamate receptor antagonists S-2-amino-2-methyl-4-phosphonobutanoate (MAP4; 200 microM; n=15) and 2S, 1'S,2'S-2-methyl-2-(carboxycyclopropyl)glycine (MCCG; 200 microM; n=9). A similar level of potentiation was produced by the GABA(B) receptor antagonist 3-N[1-(S)-(3,4-dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl-P-benzyl-p hosphinic acid (CGP55845; 200 nM; n=5). MAP4 (200 microM) produced a six-fold rightward shift in the concentration-effect plot for the depressant action of the metabotropic glutamate receptor agonist S-2-amino-4-phosphonobutanoate (L-AP4), whereas CGP55845 produced no significant change in the potency of L-AP4. MAP4 did not antagonize the depressant actions of baclofen (n=8), 1S,3S-1-aminocyclopentane-1,3-dicarboxylate (n=4) or 2-S,1'S,2'S-2-(carboxycyclopropyl)glycine (n=4). The metabotropic glutamate receptor antagonists produced no change in the holding current of any of the neurons, indicating that they had no significant postsynaptic excitatory actions. These results are the first to indicate a possible physiological role for metabotropic glutamate receptors in the spinal cord. Like GABA(B) receptors, they control glutamatergic synaptic transmission in the segmental spinal pathway to motoneurons. This is likely to be a presynaptic control mechanism.     

Persistent increase in dopamine release following activation of metabotropic glutamate receptors in the rat nucleus accumbens

We report two cases of severe hypertension and unilateral renal dysplasia. No renal artery stenosis and no other urogenital malformations were found. In both cases we found substantially enhanced secretion of renin from the dysplastic kidney. After nephrectomy both patients obtained a distinctive and permanent reduction or normalization of blood pressure. In the two cases reported, regional renin release induced by ischemia is a very likely etiological factor.     

3,5-dihydroxyphenylglycine is a highly selective agonist for phosphoinositide-linked metabotropic glutamate receptors in the rat hippocampus

Metabotropic glutamate receptors (mGluRs) are a heterogeneous family of G protein-coupled glutamate receptors that are linked to multiple second messenger systems in the CNS. In this study the selectivity of mGluR agonists for different mGluR second messenger effects was characterized in slices of the rat hippocampus. The mGluR agonists (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid and (2S,3S,4S)alpha-(carboxycyclopropyl)glycine produced multiple effects on second messengers that included enhanced phosphoinositide hydrolysis in both adult and neonatal rat hippocampus, inhibition of forskolin-stimulated cyclic AMP (cAMP) formation in adult tissue, and increases in basal cAMP formation in the neonatal hippocampus. In contrast, 3,5-dihydroxyphenylglycine was potent and effective in increasing phosphoinositide hydrolysis in both adult and neonatal hippocampus but unlike the other mGluR agonists did not inhibit forskolin-stimulated cAMP formation (in the adult) or substantially enhance basal cAMP formation (in the neonate). Thus, in the rat hippocampus mGluR agonist-mediated increases or decreases in cAMP formation are not secondary to mGluR-mediated changes in phosphoinositide hydrolysis. Furthermore, 3,5-dihydroxyphenylglycine can be used to activate subpopulations of mGluRs coupled to phosphoinositide hydrolysis with minimal effects on cAMP-mGluR second messenger systems.     

Group II and group III metabotropic glutamate receptors in the rat retina: distributions and developmental expression patterns

We have studied the distributions of group II metabotropic glutamate receptors, mGluR2 and mGluR3, and a group III metabotropic glutamate receptor, mGluR4, in the adult rat retina and during postnatal development using receptor specific anti-peptide antisera. Of the three receptors examined, mGluR3 was not expressed in the retina. MGluR2 showed a distinct stratification pattern in the inner plexiform layer (IPL). Double-labelling immunocytochemistry revealed that mGluR2 was localized in the processes of cholinergic amacrine cells. MGluR4 was found throughout the entire IPL. At the subcellular level, both mGluR2 and mGluR4 were found to be localized exclusively in processes postsynaptic to bipolar cell synapses in the IPL. During postnatal development, labelling for mGluR2 was detected at around postnatal day five. MGluR4 was already present at postnatal day one, prior to the establishment of synaptic connections in the IPL. The differential expression patterns of individual metabotropic glutamate receptors in the adult and developing rat retina suggest distinct roles for these receptors in retinal synaptic circuitry.     

Differential expression of mGluR5 metabotropic glutamate receptor mRNA by rat striatal neurons

The ability to control the degree and spatial distribution of cooling in biological tissues during a thermally mediated therapeutic procedure would be useful for several biomedical applications of lasers. We present a theory based on the solution of the heat conduction equation that demonstrates the feasibility of selectively cooling biological tissues. Model predictions are compared with infrared thermal measurements of in vivo human skin in response to cooling by a cryogen spurt. The presence of a boundary layer, undergoing a liquid-vapour phase transition, is associated with a relatively large thermal convection coefficient (approximately 40 kW m-2 K-1), which gives rise to the observed surface temperature reductions (30-40 degrees C). The degree and the spatial-temporal distribution of cooling are shown to be directly related to the cryogen spurt duration.