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M Raiteri

Publications and source records attributed to M Raiteri.

At least 19 recordsLinked to original sources
PubMed1998

Neurosteroids may differentially affect the function of two native GABA(A) receptor subtypes in the rat brain.

Hippocampal noradrenergic and cerebellar glutamatergic axon terminals are known to possess GABA(A) receptors mediating, respectively, enhancement of noradrenaline (NA) and glutamate release. It has been recently found that the hippocampal receptor is benzodiazepine-sensitive, whereas the cerebellar receptor is insensitive to benzodiazepine agonists. We here tested the effects of neurosteroids on these two native GABA(A) receptors using superfused rat hippocampal and cerebellar synaptosomes. Allopregnanolone (3alpha,5alpha-P), at nanomolar concentrations, potentiated the GABA-induced [3H]-NA release from superfused hippocampal synaptosomes; in the absence of GABA, the steroid was ineffective up to 10 microM. The enhancement by GABA of the K+-evoked [3H]-D-aspartate release from cerebellar synaptosomes also was potentiated by nanomolar 3alpha,5alpha-P; in addition, at 1-10 microM, the steroid increased [3H]-D-aspartate release in the absence of GABA. Both in hippocampus and cerebellum the potentiations of the GABA effects produced by nanomolar 3alpha,5alpha-P were abolished by dehydroepiandrosterone sulphate (DHEAS). Added up to 10 microM, DHEAS could not inhibit the effects of GABA alone. The enhancement of [3H]-D-aspartate release elicited by 3 microM 3alpha,5alpha-P in the absence of added GABA was antagonized completely by bicuculline and picrotoxin and halved by DHEAS. To conclude, 3alpha,5alpha-P, at nanomolar concentrations, behaves as a positive allosteric GABA modulator at both the GABA(A) receptors under study. Low micromolar 3alpha,5alpha-P can directly activate the cerebellar receptor, whereas the hippocampal GABA(A) receptor is insensitive to the neurosteroid alone. DHEAS appears to be a pure antagonist at the neurosteroid allosteric sites. Along with the previously observed differential sensitivity to benzodiazepines, the present data strengthen the idea that the two receptors investigated represent native subtypes of the GABA(A) receptor having distinct pharmacology, neuronal localization and function.

Animals↗
PubMed1998

The human immunodeficiency virus-1 envelope protein gp120 binds through its V3 sequence to the glycine site of N-methyl-D-aspartate receptors mediating noradrenaline release in the hippocampus.

Recent results show that the HIV-1 protein gp120 can enhance N-methyl-D-aspartate receptor-mediated release of noradrenaline from CNS nerve endings. We now investigate the mechanism of this action, including the structural determinants of the gp120 effect and the nature of its binding sites. The N-methyl-D-aspartate-evoked release of [3H]noradrenaline from rat hippocampal synaptosomes was potentiated similarly by gp120 and gp160; gp41 was ineffective. The regions of gp120 involved appear to be outside the CD4-binding domain of the protein, because gp120 retained its activity after pretreatment with N-carbomethoxycarbonyl-D-prolyl-D-phenylalanine, a compound known to inhibit binding of gp120 to CD4 receptors. Moreover, sequences of gp120 critical for binding to CD4 did not mimic the effect of gp120. Preincubation of synaptosomes with anti-galactocerebroside antibodies did not affect gp120 activity. The protein effect was retained by peptides mimicking its V3 sequence, including the cyclic V3 "universal peptide" and the linear V3 sequence BRU-C-34-A, but not RP-135 (a central portion of BRU-C-34-A). The block of the N-methyl-D-aspartate-induced [3H]noradrenaline release by 7-chlorokynurenate, an antagonist at the N-methyl-D-aspartate receptor glycine site, was competitively reversed by glycine, by V3 and by BRU-C-34-A. When added with N-methyl-D-aspartate, V3 was three to four orders of magnitude more potent than glycine (EC50 values: about 20 pM and 150 nM, respectively) in enhancing [3H]noradrenaline release. Gp120 did not release glycine or serine from synaptosomes, thus excluding indirect actions through these agents. To conclude, gp120 may act following recognition by its V3 sequence of a high-affinity site possibly coincident with the glycine site of N-methyl-D-aspartate receptors present on hippocampal terminals of noradrenergic neurons. Considering the importance of N-methyl-D-aspartate receptor activation and of noradrenaline in cognitive processes, the effects of gp120 and V3 described here may be relevant to the pathology of AIDS dementia.

Amino Acid Sequence↗
PubMed1998

Glutamate release in human cerebral cortex and its modulation by 5-hydroxytryptamine acting at h 5-HT1D receptors.

1. The release of glutamic acid and its modulation by 5-hydroxytryptamine (5-HT) in the human brain has been investigated in synaptosomal preparations from fresh neocortical samples obtained from patients undergoing neurosurgery to reach deeply sited tumours. 2. The Ca2+-dependent K+ (15 mM)-evoked overflow of glutamate was inhibited by 5-HT in a concentration-dependent manner (EC50 = 2.9 nM; maximal effect approximately 50%). The inhibition caused by 5-HT was antagonized by the 5-HT1/5-HT2 receptor antagonist methiothepin. The 5-HT1B/5-HT1D receptor agonist sumatriptan mimicked 5-HT (EC50 = 6.4 nM; maximal effect approximately 50%); the effect of sumatriptan was also methiothepin-sensitive. Selective 5-HT1A receptor antagonists could not prevent the inhibition of glutamate release by 5-HT. 3. The 5-HT1B/5-HT1D receptor ligand GR 127935 and the 5-HT2C/5-HT1B/5-HT1D receptor ligand metergoline were unable to prevent the 5-HT effect; instead they inhibited glutamate release, their effects being abolished by methiothepin. Some 5-HT1A receptor antagonists also displayed intrinsic agonist activity. 4. The effect of sumatriptan was prevented by ketanserin, a drug known to display much higher affinity for recombinant h 5-HT1D than for h 5-HT1B receptors. 5. We propose that neocortical glutamatergic nerve terminals in human brain cortex possess release-inhibiting presynaptic heteroreceptors that appear to belong to the h 5-HT1D subtype.

Adult↗
PubMed1998

Nerve growth factor and brain-derived neurotrophic factor increase neurotransmitter release in the rat visual cortex.

A number of experiments have shown that neurotrophins are involved in the development and plasticity of the visual cortex (Bonhoeffer, T., Curr. Op. Neurobiol., 6, 119, 1996). A possible mechanism underlying these effects is the neurotrophin modulation of synaptic transmission. We investigated whether nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) can modulate the release of neurotransmitter in the rat visual cortex at the peak of the critical period for plasticity (P23). The release of glutamate, acetylcholine and gamma-aminobutyric acid (GABA) from visual cortical synaptosomes was analysed in continuous perfusion conditions. We found that NGF enhances the depolarization-evoked release of glutamate (approximately 90%) and acetylcholine (approximately 35%) but not that of GABA. By contrast, BDNF enhances the depolarization-evoked release of all three neurotransmitters investigated (approximately 30%). BDNF and NGF were ineffective on basal release of neurotransmitters. The effect of NGF was not blocked by cholinergic antagonists atropine and mecamylamine. NGF and BDNF potentiation of transmitter release was strongly but not completely blocked by K252a, a tyrosine kinase inhibitor. The role of TrkA and p75NTR receptors was investigated in NGF-induced potentiation of glutamate release. Block of NGF binding to p75NTR using specific blocking antibodies (REX-IgG) slightly but significantly reduced the effect of NGF. Activation of TrkA in isolation by RTA-IgG, an antibody that specifically activates TrkA, was less effective than activation of both receptors by NGF. These results show that neurotrophin action on neurotransmitter release was mostly mediated by Trk receptors with p75NTR having a little but significant positive role. Antigen blot analysis showed the presence of TrkA, TrkB and p75NTR receptors in the visual cortex.

Animals↗
PubMed1998

Evidence for functional native NMDA receptors activated by glycine or D-serine alone in the absence of glutamatergic coagonist.

In this study we have examined the effects of N-methyl-D-aspartate (NMDA) receptor activation on the release of cholecystokinin and somatostatin from rat neocortical nerve endings. The release of cholecystokinin-like immunoreactivity (CCK-LI) and of somatostatin-like immunoreactivity (SRIF-LI) elicited by 12 mM K+ from superfused synaptosomes, but not the spontaneous release, was increased by NMDA in a concentration-dependent manner. The effects of NMDA could be prevented by antagonists selective for the glutamate recognition site, the receptor channel and the glycine site of the NMDA receptor. In the absence of NMDA, glycine increased on its own and in a concentration-dependent manner the depolarization-evoked release of both CCK-LI and SRIF-LI. This effect of glycine was strychnine-insensitive and could be mimicked by D-serine, a stereoselective agonist at the NMDA receptor glycine site. Antagonists selective for the glycine site or for the NMDA receptor channel prevented the effects of glycine/D-serine; these effects were, however, insensitive to blockade of the glutamate recognition site of the NMDA receptor, suggesting that glutamate released from synaptosomes or present as contaminant was not involved. The neuropeptide release elicited by D-serine was strongly inhibited by ifenprodil (0.3 microM) and by Zn2+ ions (50 nM), selective ligands at the NR2B and NR2A subunits of NMDA receptors, respectively. It is concluded that nerve terminals of CCK- and SRIF-releasing neurons possess non-conventional NMDA receptors whose channels can be operated by glycine or D-serine without apparent activation of the glutamatergic coagonist site. These receptors may display the triple subunit combination NR1/NR2A/NR2B.

Animals↗
PubMed1998

Time- and region-specific variations in somatostatin release following amygdala kindling in the rat.

Somatostatin biosynthesis is activated during and following kindling epileptogenesis. The aim of this study was to investigate whether this phenomenon translates into enhanced release of the peptide and whether it is involved in kindling maintenance. A marked increase in somatostatin-like immunoreactivity (somatostatin-LI) was observed in hilar interneurons of the hippocampus and in their presumed projections to the outer molecular layer 1 week, but not 1 month, after the last kindled seizure. No overt changes were observed in the striatum or in the cortex. Compared with sham-stimulated controls, (a) in the hippocampus, high-K+-evoked somatostatin-LI release was unchanged in synaptosomes taken from rats killed 7 days after the last kindled seizure but was bilaterally reduced after 30 days; (b) in the striatum, it was increased (mainly ipsilaterally to stimulation) 7, but not 30, days after the last seizure; and (c) in the cortex, somatostatin-LI release was bilaterally increased in synaptosomes taken from kindled rats 30, but not 7, days after the last seizure. This study shows that distinct changes occur in synaptosomal somatostatin-LI release after kindling acquisition, depending on the brain area analyzed and on the time elapsed from the last generalized seizure.

Amygdala↗
PubMed1998

Trazodone is a potent agonist at 5-HT2C receptors mediating inhibition of the N-methyl-D-aspartate/nitric oxide/cyclic GMP pathway in rat cerebellum.

The effects of trazodone on the cyclic GMP elevation elicited by N-methyl-D-aspartate in rat cerebellar slices were analyzed. Trazodone inhibited in a concentration-dependent manner (EC50 = 0.82 nM) the cyclic GMP response evoked by 0.1 microM N-methyl-D-aspartate. The inhibition was near complete at 10 nM trazodone. The effect of 10 nM trazodone was unaffected by 0.3 microM spiperone or rauwolscine, antagonists with selectivity for the 5-HT(serotonin)2A or the 5-HT2B subtype, respectively, but it was totally prevented by 0.01 microM mesulergine, a 5-HT2A/5-HT2B/5-HT2C receptor antagonist. Trazodone was potently counteracted (IC50 = 2.7 nM) by the selective 5-HT2B/5-HT2C receptor antagonist N-(1-methyl-5-indolyl)-N-(3-pyridil) urea HCl and, less potently (IC50 = 95 nM), by ketanserin, a 5-HT2A/5-HT2C receptor blocker. It is concluded that trazodone behaves as a potent full agonist at the 5-HT2C receptor mediating inhibition of the cerebellar N-methyl-D-aspartate/nitric oxide/cyclic GMP system.

Animals↗
PubMed1997

Gp120 can revert antagonism at the glycine site of NMDA receptors mediating GABA release from cultured hippocampal neurons.

The effects of the human immunodeficiency virus type 1 envelope protein gp120 on the release of GABA elicited by N-methyl-D-aspartate (NMDA) from rat hippocampal neurons in primary culture has been investigated. NMDA (1-300 microM) increased in a concentration-dependent manner (EC50 =37.9+/-12 microM) the release of [3H]-GABA. The effect of 100 microM NMDA was prevented by 30 microM of the GABA transport inhibitor N-(4,4-diphenyl-3-butenyl)guvacine (SKF 100330A). Glycine (10 microM) or gp120 (0.01 microM) affected neither the basal nor the NMDA-evoked [3H]-GABA release. The NMDA (100 microM)-evoked release was prevented by 5,7-dichloro-kynurenic acid (5,7-DCKA), a selective antagonist at the glycine site of the NMDA receptor, in a concentration-dependent manner (IC50 approximately 0.3 microM). Glycine (3-10 microM) or gp120 (0.003-0.01 microM) produced reversal of the 5,7-DCKA antagonism in a way that suggested competition at a same site; gp120 was at least 3 orders of magnitude more potent than glycine. It is suggested that gp120 may mimic glycine at NMDA receptors.

Animals↗
PubMed1997

D-serine modulates the NMDA receptor/nitric oxide/cGMP pathway in the rat cerebellum during in vivo microdialysis.

Several lines of investigation indicate that D-serine may be an endogenous ligand for the glycine site of N-methyl-D-aspartate (NMDA) receptors in some CNS regions. We here studied the in vivo effects of D-serine on the NMDA receptor/nitric oxide/cGMP pathway by monitoring extracellular cGMP in the cerebellum of freely-moving rats subjected to transcerebral microdialysis. Local application of NMDA (200, 500 microM) through the dialysis probe for 20 min evoked transient, concentration-dependent cGMP responses which peaked in the fraction of drug administration, the nucleotide levels returning to basal values after 40 min. The NMDA-induced elevation of the extracellular nucleotide was completely inhibited by the selective receptor channel blocker dizocilpine (MK-801) locally co-perfused at the concentration of 10 microM. The non-competitive antagonist had no effect on its own suggesting that endogenous glutamic acid does not tonically activate NMDA receptors. The effect of 200 microM NMDA was largely attenuated by 30 microM 7-chloro-kynurenic acid and completely abrogated when the concentration of the strychnine-insensitive glycine receptor antagonist was raised to 100 microM. D-serine (300 microM), perfused in the presence of 7-chloro-kynurenate (30 microM), was able to fully restore the NMDA (200 microM)-induced increase of cGMP extracellular levels. On the other hand, the D-amino acid directly potentiated in a concentration-dependent manner (0.3, 1 and 10 mM) the NMDA (200 microM)-evoked cGMP production whereas it was inactive on its own. These data show that in vivo the activation of the strychnine-insensitive glycine site is essential for the functioning of the NMDA receptor complex and can be activated by the selective agonist D-serine. They also confirm that cerebellar NMDA receptors do not have their glycine sites saturated.

Animals↗
PubMed1997

Differential desensitization of ionotropic non-NMDA receptors having distinct neuronal location and function.

The release of tritium from rat hippocampal synaptosomes prelabeled with [3H]noradrenaline ([3H]NA) or [3H]5-hydroxytryptamine ([3H]5-HT) and from rat neocortex synaptosomes prelabeled with [3H]choline and the release of endogenous GABA and glutamate from rat neocortex synaptosomes were monitored during superfusion with media containing varying concentrations of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or kainic acid. Concentration-dependent release potentiations were elicited by both excitatory amino acids (EAAs) in all the transmitter systems investigated. The releases evoked by 100 microM AMPA were, in all cases, almost totally dependent on external Ca2+ and sensitive to 6.7-dinitroquinoxaline-2,3-dione (DNQX), indicating involvement of non-NMDA receptors. When cyclothiazide, a drug able to prevent desensitization of AMPA-preferring receptors, was added to the superfusion medium (at 1 or 10 microM) concomitantly with 100 microM AMPA or kainate, the EAA-evoked release of [3H]NA was significantly enhanced. Concanavalin A, a lectin thought to prevent desensitization of kainate-preferring receptors, had no effect (up to 10 microM) on the release of [3H]NA evoked by AMPA or kainate. The effect of cyclothiazide was lost if, after an 8-min pretreatment, the drug was removed just before the AMPA stimulus. When added concomitantly with the EAAs, cyclothiazide potentiated the release of [3H]5-HT elicited by AMPA and, less so, that evoked by kainate. Concanavalin A was ineffective. Neither cyclothiazide (1 or 10 microM) nor concanavalin A (3 or 10 microM) could affect the release of [3H]ACh or endogenous GABA provoked by 100 microM AMPA or kainate, suggesting that the receptors involved do not desensitize. Exposure of neocortex synaptosomes to AMPA or kainate concomitantly with cyclothiazide caused endogenous glutamate release that did not differ from that evoked by the EAAs alone. In contrast, the effects of AMPA and kainate were potentiated by concanavalin A. The activity of the lectin (3 microM) persisted when it was applied for 8 min and then removed before the AMPA or kainate (100 microM) pulse. When hippocampal synaptosomes prelabeled with [3H]NA were subjected to three subsequent AMPA (100 microM) stimuli (S1, S2 and S3), the release of [3H]NA decreased dramatically from S1 to S3 (S3/S1 = 0.14 +/- 0.04); a significant 'protection' of the AMPA effect was offered by 1 microM cyclothiazide (S3/S1 = 0.36 +/- 0.06). This value did not differ from the S3/S1 ratio (0.38 +/- 0.04) obtained in parallel experiments with 12 mM K+. The release evoked by high-K+ was insensitive to cyclothiazide. Finally, the effect of AMPA on the release of [3H]ACh did not respond to cyclothiazide also during three subsequent stimuli with 100 microM AMPA. To conclude: a) ionotropic non-NMDA receptors mediating enhancement of NA, 5-HT, ACh, GABA and glutamate release exist on the corresponding nerve terminals; b) the receptors present on noradrenergic and serotonergic neurons are AMPA-preferring receptors, whereas the glutamate autoreceptors resemble most the kainate-preferring subtype; the receptors mediating ACh and GABA release can not be subclassified at present; c) desensitization may not be a property of all non-NMDA ionotropic receptors. The receptors here characterized represent five models of native non-NMDA receptors suitable for pharmacological and molecular studies.

Acetylcholine↗
PubMed1997

The glutamate receptor/NO/cyclic GMP pathway in the hippocampus of freely moving rats: modulation by cyclothiazide, interaction with GABA and the behavioural consequences.

Monitoring of extracellular cGMP during intracerebral microdialysis in freely moving rats permits the study of the functional changes occurring in the glutamate receptor/nitric oxide (NO) synthase/guanylyl cyclase pathway and the relationship of these changes to animal behaviour. When infused into the rat hippocampus in Mg2+-free medium, cyclothiazide, a blocker of desensitization of the AMPA-preferring receptor, increased cGMP levels. The effect of cyclothiazide (300 microM) was abolished by the NO synthase inhibitor L-NARG (100 microM) or the soluble guanylyl cyclase inhibitor ODQ (100 microM). During cyclothiazide infusion the animals displayed a pre-convulsive behaviour characterized by frequent "wet dog shakes" (WDS). Neither L-NARG nor ODQ decreased the WDS episodes. Both cGMP and WDS responses elicited by cyclothiazide were prevented by blocking NMDA receptor function with the glutamate site antagonist CGS 19755 (100 microM), the channel antagonist MK-801 (30 microM) or Mg2+ ions (1 mM). The AMPA/kainate receptor antagonists DNQX (100 microM) and NBQX (100 microM) abolished the WDS episodes but could not inhibit the cyclothiazide-evoked cGMP response. DNQX or NBQX (but not MK-801) elevated, on their own, extracellular cGMP levels. The cGMP response elicited by the antagonists appears to be due to prevention of a glutamate-dependent inhibitory GABAergic tone, since infusion of bicuculline (50 microM) caused a strong cGMP response. The results suggest that (a) AMPA/kainate receptors linked to the NO/cGMP pathway in the hippocampus (but not NMDA receptors) are tonically activated and kept in a desensitized state by endogenous glutamate; (b) blockade of AMPA/kainate receptor desensitization by cyclothiazide leads to endogenous activation of NMDA receptors; (c) the hippocampal NO/cGMP system is under a GABAergic inhibitory tone driven by non-NMDA ionotropic receptors; (d) the pre-convulsive episodes observed depend on hippocampal NMDA receptor activation but not on NO and cGMP production.

Animals↗
PubMed1997

In vivo microdialysis study of GABA(A) and GABA(B) receptors modulating the glutamate receptor/NO/cyclic GMP pathway in the rat hippocampus.

Intrahippocampal perfusion of bicuculline (50 microM) in Mg2+-free medium caused elevation of extracellular cGMP and epileptic-like behaviour. Both effects were partially prevented by blocking NMDA receptors with MK-801 or Mg2+ ions. Similarly, the GABA(B) receptor antagonists CGP52432 (0.1-30 microM) and CGP35348 (0.3-1 mM) evoked increases of extracellular cGMP. CGP52432 also elicited behavioural responses ranging from wet dog shakes to convulsions. MK-801 or Mg2+ ions reduced the effects of CGP52432. Local application of muscimol (100-300 microM) or (-)baclofen (300 microM) caused inhibition of extracellular cGMP. Administration of the AMPA/kainate receptor antagonist NBQX (100 microM) caused cGMP elevation which was almost abolished by co-perfusion of muscimol and (-)baclofen. In the presence of physiological Mg2+, perfusion of AMPA (30 microM) failed to affect cGMP levels, although rats displayed wet dog shakes episodes. When AMPA was co-perfused with low concentrations of bicuculline or CGP52432, cGMP elevations were observed in 60% of the rats. Addition of both antagonists to AMPA resulted in 85% of rats displaying a cGMP response. To conclude: (a) extracellular hippocampal cGMP is controlled by inhibitory GABA(A) and GABA(B) receptors tonically activated through GABAergic interneurons receiving AMPA/kainate-mediated glutamatergic inputs; (b) the GABAergic receptors are not endogenously saturated and can be further stimulated by exogenous agonists; (c) blockade of the GABA-mediated inhibition causes increase of cGMP and epileptic-like behaviour, due largely to endogenous activation of NMDA receptors; (d) reproducible cGMP responses to AMPA can be observed when the inhibitory GABAergic inputs to the NO/guanylyl cyclase system are blocked, confirming the previously proposed existence of AMPA/kainate receptors able to increase the nucleotide synthesis.

Animals↗
PubMed1997

Neuropeptide Y release from cultured hippocampal neurons: stimulation by glutamate acting at N-methyl-D-aspartate and AMPA receptors.

L-Glutamate, N-methyl-D-aspartate, DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate increased the release of neuropeptide Y-like immunoreactivity from primary cultures of rat hippocampal neurons incubated in Mg2+(1.2 mM)-containing medium. The neuropeptide Y-like immunoreactivity released by 100 microM glutamate was mainly accounted for by neuropeptide Y (1-36), but consisted in part (about 20%) of peptide YY. The effect of 100 microM glutamate on neuropeptide Y-like immunoreactivity release was largely (about 70%) prevented by the N-methyl-D-aspartate receptor antagonist dizocilpine maleate (10 microM), while the remainder (about 30%) was sensitive to the AMPA/ kainate receptor antagonist 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2-3-dione (10 microM). The AMPA(100 microM)-evoked release of neuropeptide Y-like immunoreactivity was strongly antagonized by 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2-3-dione and by 1-aminophenyl-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine, but it was in part (15-20%) sensitive to dizocilpine. The releases of neuropeptide Y-like immunoreactivity elicited by glutamate, N-methyl-D-aspartate, AMPA and kainate were all strongly Ca(2+)-dependent. Tetrodotoxin (1 microM) abrogated the N-methyl-D-aspartate-evoked release and partly inhibited the release caused by glutamate, but did not modify significantly AMPA- or kainate-evoked release. Removal of Mg2+ from the medium caused increase of neuropeptide Y-like immunoreactivity release, an effect prevented by dizocilpine maleate or 7-Cl-kynurenate. Cyclothiazide (10 microM), a drug known to prevent AMPA receptor desensitization, enhanced the neuropeptide Y-like immunoreactivity release elicited by 100 microM AMPA, but not that caused by 100 microM kainate. However, when used at a lower concentration (50 microM), kainate elicited a response that was potentiated significantly by cyclothiazide. It is concluded that glutamate can stimulate Ca(2+)-dependent release of neuropeptide Y from hippocampal neurons mainly through N-methyl-D-aspartate receptors and, less so, by activating cyclothiazide-sensitive receptors of the AMPA-preferring type.

Animals↗
PubMed1997

Pharmacologically distinct GABAB receptors that mediate inhibition of GABA and glutamate release in human neocortex.

1. The release of endogenous gamma-aminobutyric acid (GABA) and glutamic acid in the human brain has been investigated in synaptosomal preparations from fresh neocortical samples obtained from patients undergoing neurosurgery to reach deeply located tumours. 2. The basal outflows of GABA and glutamate from superfused synaptosomes were largely increased during depolarization with 15 mM KCl. The K(+)-evoked overflows of both amino acids were almost totally dependent on the presence of Ca(2+) in the superfusion medium. 3. The GABAB receptor agonist (-)-baclofen (1, 3 or 10 microM) inhibited the overflows of GABA and glutamate in a concentration-dependent manner. The inhibition caused by 10 microM of the agonist ranged from 45-50%. 5. The effect of three selective GABAB receptor antagonists on the inhibition of the K(+)-evoked GABA and glutamate overflows elicited by 10 microM (-)-baclofen was investigated. Phaclofen antagonized (by about 50% at 100 microM; almost totally at 300 microM) the effect of (-)-baclofen on GABA overflow but did not modify the inhibition of glutamate release. The effect of (-)-baclofen on the K(+)-evoked GABA overflow was unaffected by 3-amino-propyl (diethoxymethyl)phosphinic acid (CGP 35348; 10 or 100 microM); however, CGP 35348 (10 or 100 microM) antagonized (-)-baclofen (complete blockade at 100 microM) at the heteroreceptors on glutamatergic terminals. Finally, [3-[[(3,4-dichlorophenyl) methyl]amino]propyl] (diethoxymethyl) phosphinic aid (CGP 52432), 1 microM, blocked the GABAB autoreceptor, but was ineffective at the heteroreceptors. The selectivity of CGP 52423 was lost at 30 microM, as the compound, at this concentration, inhibited completely the (-)-baclofen effect on both GABA and glutamate release. 5. It is concluded that GABA and glutamate release evoked by depolarization of human neocortex nerve terminals can be affected differentially through pharmacologically distinct GABAB receptors.

Aged↗
PubMed1997

Activation of brain nitric oxide synthase in depolarized human temporal cortex slices: differential role of voltage-sensitive calcium channels.

1. Nitric oxide (NO) synthase activity was studied in slices of human temporal cortex samples obtained in neurosurgery by measuring the conversion of L-[3H]-arginine to L-[3H]-citrulline. 2. Elevation of extracellular K+ to 20, 35 or 60 mM concentration-dependently augmented L-[3H]-citrulline production. The response to 35 mM KCl was abolished by N(G)-nitro-L-arginine (100 microM) demonstrating NO synthase specific conversion of L-arginine to L-citrulline. Increasing extracellular MgCl2 concentration up to 10 mM also prevented the K+ (35 mM)-induced NO synthase activation, suggesting the absolute requirement of external calcium ions for enzyme activity. 3. However, the effect of high K+ (35 mM) on citrulline synthesis was insensitive to the antagonists of ionotropic and metabotropic glutamate receptors dizocilpine (MK-801), 6-nitro-7-sulphamoylbenzo(f)-quinoxaline-2-3-dione (NBQX) or L-2-amino-3-phosphonopropionic acid (L-AP3) as well as to the nicotinic receptor antagonist, mecamylamine. 4. The 35 mM K+ response was insensitive to omega-conotoxin GVIA (1 microM) and nifedipine (100 microM), but could be prevented in part by omega-agatoxin IVA (0.1 and 1 microM). The inhibition caused by 0.1 microM omega-agatoxin IVA (approximately 30%) was enhanced by adding omega-conotoxin GVIA (1 microM) or nifedipine (100 microM). Further inhibition (up to above 70%) could be observed when the three Ca2+ channel blockers were added together. Similarly, synthetic FTX 3.3 arginine polyamine (sFTX) prevented (50% at 100 microM) the K+-evoked NO synthase activation. This effect of sFTX was further enhanced (up to 70%) by adding 1 microM omega-conotoxin GVIA plus 100 microM nifedipine. No further inhibition could be observed upon addition of MK-801 or/and NBQX. 5. It was concluded that elevation of extracellular [K+] causes NO synthase activation by external Ca2+ entering cells mainly through channels of the P/Q-type. Other Ca2+ channels (L- and N-type) appear to contribute when P/Q-channels are blocked.

Alanine↗
PubMed1997

Serotonin inhibition of the NMDA receptor/nitric oxide/cyclic GMP pathway in rat cerebellum: involvement of 5-hydroxytryptamine2C receptors.

Previous studies have shown that 5-hydroxytryptamine (5-HT) can potently inhibit glutamatergic transmission in rat cerebellum through the activation of multiple 5-HT receptors. The aim of this study was to subclassify the 5-HT2 receptor mediating inhibition of the cyclic GMP response elicited by N-methyl-D-aspartate in adult rat cerebellar slices. Seven receptor antagonists, endowed with relative selectivities for the 5-HT2A, 5-HT2B, and 5-HT2C subtypes, differentially affected the inhibition by (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane of the cyclic GMP response, suggesting that the receptor involved belongs to the 5-HT2C subtype.

Adrenergic alpha-Antagonists↗
PubMed1997

Increased presynaptic protein kinase C activity and glutamate release in rats with a prenatally induced hippocampal lesion.

We have previously shown that protein kinase C (PKC) activity is up-regulated in nerve terminals of animals that have been subjected to targeted cellular ablation of cortical and hippocampal neurons by treatment with methylazoxymethanol (MAM), which results in impaired long-term potentiation (LTP) and cognitive deficit. In this study we investigated the consequences of increased membrane-bound PKC in the regulation of release of glutamate, the major excitatory transmitter involved in LTP. We show that nerve terminals of MAM-treated rats show higher PKC activity, as monitored by the in situ phosphorylation of B-50/GAP-43, in both basal and phorbol ester-stimulated conditions. In these animals, hippocampal nerve endings release a greater amount of glutamate than those of controls, both in basal conditions and when synaptosomes are stimulated with KCl or 3,4-diaminopyridine. The potentiation observed in MAM-treated rats was counteracted by the PKC blocker H-7 and the clostridial tetanus toxin. On the contrary, GABA release was not significantly up-regulated, either in basal or in depolarization-evoked conditions. Therefore our data show that the increase in synaptosomal PKC activity is paralleled by increased glutamate but not GABA release in this animal model. Whether this reflects specific up-regulation of membrane PKC activity in glutamatergic terminals or an alteration in the regulation of glutamate release remains to be determined.

Animals↗