Involvement of NMDA and non-NMDA receptors in transmission of spinal visceral nociception in cat.
(49/1909)
AIM: To study the role of N-methyl-D-aspartic acid (NMDA) and non-NMDA receptors in processing nociceptive visceral information in the spinal cord. METHODS: The firing of spinal dorsal horn neurons to colorectal distension (3-15 kPa, 20 s) by inflation with air of latex balloon was recorded in 25 anesthetized cats. RESULTS: 1) According to the patterns of responses to colorectal distension, the neurons with increase and decrease in firing were classified as excitatory and inhibitory, respectively. The former consisted of 17 short-latency abrupt (SLA) neurons, 11 short-latency sustained (SLS) neurons, 9 long-latency (LL) neurons. The 15 inhibited (Inh) neurons were recorded. 2) Microelectrophoretic administration of NMDA, quisqualic acid (QA), and kainic acid (KA) activated 67.6%, 78.4%, and 59.5% of the colorectal distension-excited neurons tested. Also, 60%, 86.7%, and 53.3% of Inh neurons were activated by these 3 amino acids. 3) Colorectal distension-induced excitatory responses were reduced by 35% +/- 10% and 65% +/- 14% by a selective NMDA receptor antagonist d,l-2-amino-5-phosphonovalerate (APV) and a selective non-NMDA receptor antagonist 6,7-dinitro-quinoxaline-2,3-dione (DNQX), respectively. Such DNQX-induced inhibition was significantly more potent than that by APV (P < 0.05). Colorectal distension-induced inhibitory responses were partially relieved by 30%-50% in 3/7 Inh neurons by DNQX, but not APV. CONCLUSION: Both NMDA and non-NMDA receptors are involved in transmission and/or modulation of spinal visceral nociceptive information and non-NMDA receptors may play more important role than NMDA receptors. (+info)
Antagonism of the mGlu5 agonist 2-chloro-5-hydroxyphenylglycine by the novel selective mGlu5 antagonist 6-methyl-2-(phenylethynyl)-pyridine (MPEP) in the thalamus.
(50/1909)
Our previous work has shown that Group I mGlu receptors participate in thalamic sensory processing in vivo. However, unequivocal demonstration of mGlu5 participation has not been possible due to the lack of specific ligands. We have therefore made a preliminary study of the in vivo actions of the agonist (R,S)-2-Chloro-5-hydroxyphenylglycine [CHPG] and the novel mGlu5 antagonist 6-methyl-2-(phenylethynyl)-pyridine [MPEP] in order to characterize their suitability for functional studies. Iontophoretically administered MPEP selectively antagonized excitatory responses of single rat thalamic neurones to CHPG compared to the broad-spectrum mGlu agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate. In contrast, the established mGlu1 and mGlu5 antagonist (S)-4-carboxyphenylglycine reduced responses to both agonists. These findings are the first demonstration of an in vivo action of CHPG and its antagonism by a selective mGlu5 antagonist. Furthermore MPEP appears to be a good tool for functional studies of mGlu5. (+info)
Presynaptic inhibition preferentially reduces in NMDA receptor-mediated component of transmission in rat midbrain dopamine neurons.
(51/1909)
We used patch pipettes to record whole-cell currents from single dopamine neurons in slices of rat midbrain. Pharmacological methods were used to isolate EPSCs evoked by focal electrical stimulation. Baclofen was significantly more potent for inhibiting NMDA receptor-mediated EPSCs (IC50=0.24 microM) compared with inhibition of EPSCs mediated by AMPA receptors (IC50=1.72 microM). The increased potency of baclofen for inhibiting the NMDA component persisted in superfusate that contained zero Mg2+ and when postsynaptic K+ conductances were reduced by Cs+ and QX-314. Effects of baclofen on EPSCs were blocked by the GABA(B) receptor antagonist CGP-35348. Adenosine was 20 fold more potent for reducing the NMDA component of transmission (IC50=31 microM) compared with inhibition of AMPA receptor-mediated EPSCs (IC50=654 microM). Effects of adenosine on EPSCs were blocked by the A1 receptor antagonist DPCPX. Both baclofen and adenosine significantly increased the ratio of EPSCs in paired-pulse studies, suggesting presynaptic sites of action. Although adenosine (1 mM) did not reduce currents evoked by exogenous NMDA (10 microM), baclofen (1 microM) reduced NMDA currents by 29%. Neither baclofen nor adenosine altered currents evoked by exogenous AMPA (1 microM). We conclude that adenosine acts at presynaptic A1 receptors to cause a preferential reduction in the NMDA component of synaptic transmission. In contrast, baclofen preferentially reduces NMDA EPSCs by acting at both pre- and postsynaptic GABA(B) receptors. By regulating NMDA receptor function, A1 and GABA(B) receptors may play important roles in regulating the excitability of dopamine neurons. (+info)
This study is aimed at testing the hypothesis that sustained phosphorylation underlies long-term desensitization of AMPA receptors, which is thought to be the mechanism of long-term synaptic depression in cerebellar Purkinje cells (PCs). We induced long-term desensitization of AMPA receptors in rat cerebellar slices by (1) a 4-min bath application of quisqualate (0.1 mM) or (2) a 15-min bath application of a protein kinase C (PKC) activator, phorbol-12,13-diacetate (0.5 microM) or -dibutyrate (0.6 microM), followed by a 4-min AMPA (0.1 mM) application. In slices so treated, labeling with an antibody (12P3) against a peptide corresponding to part of AMPA receptor subunit GluR2 including serine 696 and phosphorylated at this serine site revealed phosphorylation of the AMPA receptors in PC dendrites that was sustained for at least 1 hr. At an early phase, within 20 min after the chemical stimulation, the phosphorylation was resistant to an Ca2+ chelator (BAPTA-AM), a metabotropic glutamate receptor antagonist (MCPG), and a PKC inhibitor (calphostin C), whereas at a late phase, 30 min or more after the chemical stimulation, it was blocked by these reagents similarly to long-term desensitization of AMPA receptors. Taken together with data obtained previously using different protocols of chemical stimulation, the present results strongly support the above-mentioned hypothesis. (+info)
Group III metabotropic glutamate receptors control corticothalamic synaptic transmission in the rat thalamus in vitro.
(53/1909)
1. Corticothalamic (CT) EPSPs evoked at <= 0.1 Hz were recorded from thalamocortical neurones in the rat dorsal lateral geniculate nucleus in vitro, with both GABAA and GABAB receptors blocked. 2. The group III metabotropic glutamate (mGlu) receptor agonists L-2-amino-4-phosphono-butyric acid (L-AP4) and O-phospho-L-serine (L-SOP) both caused a concentration-dependent depression of the CT EPSP. The maximum depression and EC50 values for these effects were 64.4 +/- 3.8 % and 88.0 +/- 24.7 microM for L-AP4, and 42.0 +/- 2.5 % and 958 +/- 492 microM for L-SOP, respectively (means +/- s.e.m.). Neither agonist had any effect on membrane potential or input resistance. 3. The depression of the CT EPSP caused by L-AP4 was reversed using the group III antagonist (S)-2-amino-2-methyl-4-phosphonobutanoic acid (MAP4, 1 mM), and the group II/III antagonist LY341495 (3 microM), but not using the group II antagonist (2S)-alpha-ethylglutamic acid (300 microM). The potencies of L-AP4, L-SOP and LY341495 indicate that this action of L-AP4 is mediated via mGlu7 and mGlu8 and not mGlu4 receptors. 4. Neither MAP4 nor LY341495 had any effect on the CT EPSPs evoked by 10 Hz trains of five stimuli, indicating the lack of endogenous activation of group III mGlu receptors in the thalamus during short bursts of cortical input. However, the magnitude of the depression caused by L-AP4 indicates that any physiological activation of group III mGlu receptors would have a profound effect on the CT input to the thalamus, and hence cortical control of thalamic function. (+info)
N-methyl-D-aspartate excitotoxicity: relationships among plasma membrane potential, Na(+)/Ca(2+) exchange, mitochondrial Ca(2+) overload, and cytoplasmic concentrations of Ca(2+), H(+), and K(+).
(54/1909)
A high cytoplasmic Na(+) concentration may contribute to N-methyl-D-aspartate (NMDA)-induced excitotoxicity by promoting Ca(2+) influx via reverse operation of the Na(+)/Ca(2+) exchanger (NaCaX), but may simultaneously decrease the electrochemical Ca(2+) driving force by depolarizing the plasma membrane (PM). Digital fluorescence microscopy was used to compare the effects of Na(+) versus ions that do not support the NaCaX operation, i.e., N-methyl-D-glucamine(+) or Li(+), on: PM potential; cytoplasmic concentrations of Ca(2+), H(+), and K(+); mitochondrial Ca(2+) storage; and viability of primary cultures of cerebellar granule cells exposed to NMDA receptor agonists. In the presence of Na(+) or Li(+), NMDA depolarized the PM and decreased cytoplasmic pH (pH(C)); in the presence of Li(+), Ca(2+) influx was reduced, mitochondrial Ca(2+) overload did not occur, and the cytoplasm became more acidified than in the presence of Na(+). In the presence of N-methyl-D-glucamine(+), NMDA instantly hyperpolarized the PM, but further changes in PM potential and pH(C) were Ca-dependent. In the absence of Ca(2+), hyperpolarization persisted, pH(C) was decreasing very slowly, K(+) was retained in the cytoplasm, and cerebellar granule cells survived the challenge; in the presence of Ca(2+), pH(C) dropped rapidly, the K(+) concentration gradient across the PM began to collapse as the PM began to depolarize, and Ca(2+) influx and excitotoxicity greatly increased. These results indicate that the dominant, very likely excitotoxic, component of NMDA-induced Ca(2+) influx is mediated by reverse NaCaX and that direct Ca(2+) influx via NMDA channels is curtailed by Na-dependent PM depolarization. (+info)
We showed previously that the induction of neural crest (NC) and neural tube (NT) defects is a general property of N-methyl-D-aspartate receptor (NMDAR) antagonists. Since homocysteine induces NC and NT defects and can also act as an NMDAR antagonist, we hypothesized that the mechanism of homocysteine-induced developmental defects is mediated by competitive inhibition of the NMDAR by homocysteine. If this hypothesis is correct, homocysteine-induced defects will be reduced by NMDAR agonists. To test the hypothesis, we treated chicken embryos during the process of neural tube closure with sufficient homocysteine thiolactone to induce NC and NT defects in approximately 40% of survivors or with homocysteine thiolactone in combination with each of a selected set of NMDAR agonists in 0. 05-5000 nmol doses. Glutamate site agonists selected were L-glutamate and N-methyl-D-aspartate. Glycine site agonists were glycine, D-cycloserine, and aminocyclopropane-carboxylic acid. Glycine was the most effective overall, reducing defects significantly at two different doses (each P>0.001). These results support the hypothesis that homocysteine may affect NC and NT development by its ability to inhibit the NMDAR. One potentially important consequence of this putative mechanism is that homocysteine may interact synergistically with other NMDAR antagonists to enhance its effect on development. (+info)
Role of the hippocampus in contextual memory after classical aversive conditioning in pigeons (C. livia).
(56/1909)
We investigated the effects of hippocampal lesions with ibotenic acid (IBO) on the memory of the sound-context-shock association during reexposure to the conditioning context. Twenty-nine adult pigeons were assigned to a non-lesioned control group (CG, N = 7), a sham-lesioned group (SG, N = 7), a hippocampus-lesioned experimental group (EG, N = 7), and to an unpaired nonlesioned group (tone-alone exposure) (NG, N = 8). All pigeons were submitted to a 20-min session in the conditioning chamber with three associations of sound (1000 Hz, 85 dB, 1 s) and shock (10 mA, 1 s). Experimental and sham lesions were performed 24 h later (EG and SG) when EG birds received three bilateral injections (anteroposterior (A), 4.5, 5.25 and 7.0) of IBO (1 microl and 1 microg/microl) and SG received one bilateral injection (A, 5.25) of PBS. The animals were reexposed to the training context 5 days after the lesion. Behavior was videotaped for 20 min and analyzed at 30-s intervals. A significantly higher percent rating of immobility was observed for CG (median, 95.1; range, 79.2 to 100.0) and SG (median, 90.0; range, 69.6 to 95.0) compared to EG (median, 11.62; range, 3.83 to 50.1) and NG (median, 7.33; range, 6.2 to 28.1) (P<0.001) in the training context. These results suggest impairment of contextual fear in birds who received lesions one day after conditioning and a role for the hippocampus in the modulation of emotional aversive memories in pigeons. (+info)