Effect of 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline (CNQX) on dorsal root-, NMDA-, kainate- and quisqualate-mediated depolarization of rat motoneurones in vitro

Morphine selectively depresses the slowest, NMDA-independent component of C-fibre-evoked synaptic activity in the rat spinal cord in vitro

The effects of morphine on the depolarizing synaptic responses produced in motoneurons by electrical stimulation of primary sensory neurones have been recorded in hemisected spinal cord preparations (8- to 12-day-old rat pups). Morphine at concentrations of 0.1-20 microM reduced a slow, long-lasting (latency greater than 1 s, duration up to 10 s) component of the ventral root potential (VRP) evoked by C-fibre strength stimulation of the dorsal root. At 2 microM the reduction in area of this slow synaptic potential was 71.7 +/- 0.9% of control values (n = 15). The earliest components of the C-fibre strength VRP (the first 100 ms) and the responses to A beta strength stimuli were unaffected by the opioid even at 10-20 microM. The intermediate, NMDA receptor antagonist (D-AP5, 40 microM)-sensitive component (which lasts 100-1000 ms) was reduced by 34 +/- 2.2% of control (n = 15), which was significantly less than the reduction of the later NMDA-independent component (P < 0.001). Morphine (0.1-20 microM) also depressed the cumulative depolarization generated by the temporal summation of synaptic responses evoked by brief trains of C-fibre strength stimuli (1 or 10 Hz). A significantly greater reduction at the lower frequency of stimulation (56.3 +/- 2.0%) than at the higher (20.3 +/- 1.69%, n = 10, measured at 2 microM morphine) was found (P < 0.005). The effects of morphine were reversible upon wash-out or superfusion with the opioid receptor antagonist naloxone (2 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Correlates of anesthetic properties in isolated spinal cord: cyclobutanes

Two halogenated cyclobutanes, one anesthetic and one not, were compared on receptor-specific pathways in isolated neonatal rat spinal cord. The anesthetic 1-chloro-1,2,2-trifluorocyclobutane depressed the monosynaptic reflex (glutamate non-NMDA receptors) and abolished a slow ventral root potential (glutamate NMDA, non-NMDA and tachykinin receptors). This compound slightly enhanced the muscimol-evoked dorsal root potential (GABAA) but reversibly depressed the dorsal root potential elicited by dorsal root stimulation. The non-anesthetic 1,2-dichlorohexafluorocyclobutane increased monosynaptic reflex, depressed slow ventral root potential approximately 50%, had little effect on muscimol-evoked dorsal root potential, and irreversibly depressed dorsal root-evoked dorsal root potential. Hypoxia accounts for slow ventral root potential depression, but not monosynaptic reflex enhancement. In this preparation and for this pair of compounds, anesthetic properties are related to blockade of transmission at glutamate synapses, with a small component of GABAA enhancement. Monosynaptic reflex increase may be related to the non-anesthetic cyclobutane's convulsant and anti-anesthetic properties.

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.

Light and electron microscopic immunocytochemical localization of AMPA-selective glutamate receptors in the rat spinal cord

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors are probably the most widespread excitatory neurotransmitter receptors of the central nervous system, and they play a role in most normal and pathological neural activities. However, previous detailed studies of AMPA subunit distribution have been limited mainly to the brain. Thus, a comprehensive study of AMPA receptor subunit distribution was carried out on sections of rat spinal cord and dorsal root ganglia, which were immunolabeled with antibodies made against peptides corresponding to C-terminal portions of the AMPA receptor subunits: GluR1, GluR2/3, and GluR4. In the spinal cord, labeling was most prominent in the superficial dorsal horn, motoneurons, and nuclei containing preganglionic autonomic neurons. Immunostaining also was observed in neurons in other regions including those known to contain Renshaw cells and Ia inhibitory cells. Although overall immunostaining was lighter with antibody to GluR1 than with GluR2/3 and 4, there were neurons that preferentially stained with antibody to GluR1. These "GluR1 intense" neurons were usually fusiform and most concentrated in lamina X. In dorsal root ganglia, immunostaining of ganglion cell bodies was moderate to dense with antibody to GluR2/3 and light to moderate with antibody to GluR4. Possible neuroglia in the spinal cord (mainly GluR2/3 and 4) and satellite cells in dorsal root ganglia (GluR4) were immunostained. Electron microscopic studies of the superficial dorsal horn and lateral motor column showed staining that was restricted mainly to postsynaptic densities and associated dendritic and cell body cytoplasm. In dorsal horn, colocalization of dense-cored vesicles with clear, round synaptic vesicles was observed in unstained presynaptic terminals apposed to stained postsynaptic densities. Subsynaptic dense bodies (Taxi-bodies) were associated with some stained postsynaptic densities in both the superficial dorsal horn and lateral motor column. Based on several morphological features including vesicle structure and presence of Taxi-bodies, it is likely that at least some of the postsynaptic staining seen in this study is apposed to glutamatergic input from primary sensory afferent terminals.

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

The depression of monosynaptic excitation of neonatal rat motoneurones by (1S,3S)- and (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD) and by L-2-amino-4-phosphonobutyrate (L-AP4), which is probably presynaptically mediated, is antagonized by (+/-)- and (+)-alpha-methyl-4-carboxyphenylglycine (MCPG). The same phenylglycine derivatives also antagonize the depression of forskolin-stimulated cyclic AMP synthesis effected by the two ACPD stereoisomers and by L-AP4. These results support previous suggestions that presynaptic depression is mediated by metabotropic glutamate receptors negatively coupled to adenylyl cyclase activity. MCPG is the first antagonist to be reported for these receptors.

Excitatory amino acid receptor-mediated neurotransmission from cutaneous afferents in rat dorsal horn in vitro

1. The cutaneous mechanoreceptive fields (RFs) of forty-two lumbar dorsal horn neurones have been examined intracellularly using the hemisected spinal cord-hindlimb preparation of 10 to 14-day-old rats. The neurones were classified into three groups on the basis of their excitatory responses to innocuous and noxious mechanical stimulation; the majority (25/42) were activated by noxious and innocuous stimuli and were classed as 'wide-dynamic' type (WDR). 'Nociceptive-specific' neurones (NS) which were excited by noxious stimuli made up the next largest group (12/42) followed by 'low-threshold' neurones (LT, 5/42) which responded only weakly to noxious stimuli. Another fourteen neurones which did not respond to peripheral stimuli were used to test antagonist selectivity against excitatory amino acid agonists. 2. The response to light touch or pinch consisted of an initial EPSP and cell firing followed by subthreshold EPSPs. The mean +/- S.E.M. values for the amplitude (mV) and the duration (s) of the EPSP produced by noxious pinch were significantly greater than those to touch; in WDR neurones the respective values were 14.3 +/- 0.9 vs. 11.5 +/- 0.7 mV (P < 0.01) and 11.9 +/- 1.8 vs. 4.8 +/- 0.6 s (P < 0.01). 3. The non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 3-5 microM) antagonized DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA)-induced depolarizations. The amplitude and duration of the EPSPs produced in response to low- and high-threshold mechanical stimulation were potently attenuated and cell firing was abolished in WDR, NS, LT neurones. A similar profile of antagonism was produced in five WDR neurones superfused with ACSF containing 1 mM D-serine. 4. The NMDA-receptor antagonist D-aminophosphonovalerate (D-AP5, 50 microM) attenuated the EPSP amplitude and duration but never abolished cell firing produced by low- and high-intensity cutaneous mechanical stimulation. A preferential effect of D-AP5 against the EPSP duration resulted in failure of longer latency spikes. 5. The data indicate that non-NMDA receptors contribute substantially to dorsal horn neurotransmission and somatosensory processing of noxious and innocuous cutaneous stimuli, while the role of NMDA receptors is restricted to longer latency synaptic components.

Glutamatergic and non-glutamatergic responses evoked in neonatal rat lumbar motoneurons on stimulation of the lateroventral spinal cord surface

The effects on lumbar motoneurons of thoracic cord stimulation were investigated in the neonatal rat hemisected spinal cord in vitro using intracellular recording. Four responses were evoked--a fast, excitatory postsynaptic potential, a second component to the fast excitatory postsynaptic potential, a fast inhibitory postsynaptic potential and a slow excitatory postsynaptic potential. The fast (CNQX-sensitive) excitatory postsynaptic potential was probably monosynaptic, was blocked by CNQX, (10 microM) and showed a frequency-dependent run-down at stimulation frequencies between 0.1 and 1 Hz. A slower component to the fast excitatory postsynaptic potential ((+-)-2-amino-5- phosphono-valeric acid-sensitive excitatory postsynaptic potential) was blocked by (+-)-2-amino-5-phosphonovaleric acid (50 microM). Following fast excitatory postsynaptic potential blockade with both CNQX and (+-)-2-amino-5-phosphonovaleric acid, a fast inhibitory postsynaptic potential was revealed. This reversed at a membrane potential close to resting and was incompletely blocked by either bicuculline (30 microM) or strychnine (10 microM). The slow excitatory postsynaptic potential was a delayed depolarization associated with a small increase in input resistance (20%) and was insensitive to block by CNQX and/or (+/-)-2-amino-5-phosphonovaleric acid. It increased in amplitude on membrane depolarization and decreased on hyperpolarization and was potentiated by cocaine (3 microM) and citalopram (0.1 microM), but not by desipramine (5 microM). The slow excitatory postsynaptic potential was blocked by ketanserin (1 microM) and by LY 53857 (1 microM). It is concluded that a non-glutamatergic transmitter is involved in generating the slow excitatory postsynaptic potential possibly 5-hydroxytryptamine acting at 5-hydroxytryptamine 2 receptors.

Effects of excitatory amino acid receptor antagonists on a capsaicin-evoked nociceptive reflex: a comparison with morphine, clonidine and baclofen

The rat isolated spinal cord-tail preparation has been employed to examine the effects of several antinociceptive drugs and excitatory amino acid (EAA) receptor antagonists on nociceptive reflexes (recorded in ventral roots) stimulated by peripheral application of capsaicin (CAP). Non-nociceptive monosynaptic and polysynaptic dorsal root-evoked ventral root potentials (DR-VRPs) were also examined. Morphine (0.01-3 microM) and clonidine (0.03-1 microM) inhibited CAP-stimulated activity, but not the non-nociceptive dorsal root-evoked monosynaptic reflex (MSR) or polysynaptic (PSR) activity. These effects were antagonized by naloxone and efaroxan, respectively. The AMPA/KA receptor antagonists CNQX (0.1-100 microM) and DNQX (0.1-30 microM) blocked nociceptive activity and were 4-fold selective for CAP-evoked potentials compared to the monosynaptic reflex. Kynurenate (1-300 microM), DL-AP-4 (3-300 microM), L-AP-4 (3-300 microM), and the GABAB receptor agonist baclofen (0.1-10 microM), inhibited all evoked potentials with relatively little selectivity between nociceptive and non-nociceptive responses. NMDA receptor antagonism by AP-5 (100 microM) reduced nociceptive and non-nociceptive potentials by a maximum of 30-33%. These data indicate that AMPA/KA receptor-mediated synapses are involved in acute spinal nociceptive transmission and suggest that AMPA/KA receptor subtypes could provide novel analgesic targets.

A comparison of the actions of agonists and antagonists at non-NMDA receptors of C fibres and motoneurones of the immature rat spinal cord in vitro

1. The shift in d.c. potential in dorsal roots (EC50 8.0 microM +/- 0.9 s.e. mean, n = 5) or depression of the C elevation of the compound action potential (EC50 3.0 microM +/- 0.3, n = 7) have been used to measure the depolarizing action of kainate on dorsal root C fibres of immature (3 to 5 day old) rats. Depolarization of motoneurones was measured from the shift in d.c. potential in ventral roots. 2. 6-Cyano-7-nitroquinoxaline,2-3,dione (CNQX) (pA2 5.78 +/- 0.06, n = 8) and 6-nitro-7-suplhamobenzo(f)quinoxaline-2,3-dione (NBQX) (pA2 5.75 +/- 0.04, n = 7) had similar potencies as antagonists of kainate at dorsal root fibres. The potency of NBQX as a kainate antagonist was similar also at motoneurones (pA2 5.72 +/- 0.07, n = 3). At motoneurones, NBQX was less potent as an antagonist of domoate (pA2 5.29 +/- 0.05) and more potent as an antagonist of S-alpha-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) (pA2 6.80 +/- 0.09) than as an antagonist of kainate. 3. Application of L-glutamate, quisqualate and RS-AMPA to dorsal roots produced only short lasting depolarizations but kainate concentration-effect plots were shifted to the right in the presence of these three agonists (pA2 5.08 +/- 0.08, (n = 3), 5.59 +/- 0.04, (n = 4) and 4.46 +/- 0.04 (n = 4) respectively). Slopes of dose-ratio against concentration were significantly less than one for the latter antagonism. 4. The amplitude of depolarizations induced by L-glutamate, AMPA and quisqualate were increased up to ten fold and those induced by kainate up to two fold following treatment of dorsal roots with concanavalin A. The duration of the responses was increased also by the latter treatment. Folowing 85 s applications of glutamate, quisqualate, AMPA and kainate the mean respective times (s +/- s.e.mean (n))taken for responses to decay to half the peak amplitude were increased from 63 +/- 7 (10), 86 +/- 17 (4),95 +/- 19 (4) and 135 +/- 3 (12) to 202 +/- 49 (10), 147 +/- 7 (4), 160 +/- 13 (6) and 163 +/- 10 (10). Under similar conditions the mean decay time of y-aminobutyric acid-induced responses was 145 +/- 7 (10). This was not significantly altered by concanavalin A treatment.5. Application to dorsal roots of L-aspartate at concentrations up to 5 mm (with or without concanavalin A treatment), the selective metabotropic agonist 1S,3R-trans-1-aminocyclopentane-1,3-dicarboxylate (1 mM,) and D-serine (20 pM) in the presence or absence of N-methyl-D-aspartate (NMDA,500 pM) neither depolarized the preparations nor shifted the kainate concentration-effect plot.6. It is concluded that primary afferent C fibres possess only one type of non-NMDA receptor which is activated strongly by domoate or kainate but only weakly by AMPA. This receptor is readily desensitized by glutamate, quisqualate or AMPA and it is less readily desensitized by kainate.

Antagonism of synaptic potentials in ventral horn neurones by 6-cyano-7-nitroquinoxaline-2,3-dione: a study in the rat spinal cord in vitro

1. The rat spinal cord in vitro has been used to assess the effect of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) on the dorsal root evoked extracellular ventral root reflex (DR-VRR) and the intracellular excitatory postsynaptic potential (e.p.s.p.) in ventral horn neurones and motoneurones. 2. CNQX (1-5 microM) produces a selective and dose-dependent reduction in the amplitude of the monosynaptic component of the DR-VRR recorded from lumbar spinal segments. 3. With low intensity dorsal root stimulation CNQX selectively attenuates the amplitude of the short latency intracellular e.p.s.p. (70% reduction, P < 0.005) and its rise-time (75%, P < 0.01) without affecting the half-time to decay. 4. When high intensity stimulation is used CNQX significantly attenuates the amplitude of the e.p.s.p. (56%, P < 0.005), rise-time (76%, P < 0.01) and abolishes the short latency spike. In addition longer latency synaptic components are attenuated and the half-time to decay significantly reduced (47%, P < 0.005). 5. The results with CNQX are compared to D-aminophosphonovalerate and discussed in relation to the recruitment of low versus high threshold afferents. The data supports an involvement of non-NMDA receptors in transmission through both mono- and polysynaptic pathways in the ventral horn.

The effect of centrally acting myorelaxants on NMDA receptor-mediated synaptic transmission in the immature rat spinal cord in vitro

1. The effect of the myorelaxant drugs baclofen, diazepam and tizanidine have been compared on in vitro preparations of baby rat spinal cord and adult rat superior cervical ganglion. 2. Dorsal root-elicited long duration (time to half decay 9.71 +/- 0.29 s.e. mean, n = 31) ipsilateral ventral root reflexes (DR-VRP), measured as integrated area, of immature rat spinal cord preparations were abolished by RS-2-amino-5-phosphonopentanoate (AP5) (EC50 8.13 +/- 0.92 microM, n = 3). The initial short latency component of DR-VRP was resistant to AP5. 3. Baclofen abolished both components of the DR-VRP. Respective EC50 values for the AP5-insensitive and AP5-sensitive components were 237 +/- 68 nM (n +/- 7) and 57 +/- 10 nM (n = 7). These effects of baclofen were reversed by the GABAB antagonist, CGP35348. The apparent Kd values (16.7 +/- 6.4 microM, n = 3 and 14.3 +/- 3.9 microM, n = 6 respectively) for this reversal were not significantly different. 4. Tizanidine, clonidine and diazepam had no effect on the AP5-insensitive component of the DR-VRP. 5. The AP5-sensitive long duration component of the DR-VRP was depressed to respective maximal levels of 23.2 +/- 1.4% (n = 7), 18.8 +/- 3.8% (n = 4) and 47.6 +/- 1.6% (n = 5) of control (100%) levels by tizanidine (EC50 135 +/- 33 nM), clonidine (EC50 26.0 +/- 2.2 nM) and diazepam (EC25 114 +/- 12 nM, n = 4). The depressant effects of tizanidine and clonidine were reversed by idazoxan (1 microM). Flumazenil (I microM) failed to reverse the depressant effect of tizanidine. The depressant effect of diazepam was reversed by flumazenil (1 microM) but not by idazoxan (1 microM). Naloxone 1 M did not reverse the effects of either tizanidine or diazepam.6. In the presence of tetrodotoxin (0.1 SAM) which abolished synaptic activity, clonidine, tizanidine or diazepam (10, 100 and 101JM respectively) produced no significant antagonism of NMDA-induced depolarizations recorded from ventral roots.7. Control (100%) synaptic responses of rat superior cervical ganglion preparations were depressed respectively to near maximal levels of 60.0 +/- 5.2% (n = 4) and 60.7 +/- 5.6% (n = 5) by clonidine (0.5 JAM,EC25 15.3 +/- 3.0 nM) and tizanidine (1 JAM, EC25 227 +/- 83 nM). These depressant effects were reversed by idazoxan (1 AM). Baclofen (EC25 28.7 +/- 10.0, n = 3) depressed the postganglionic response to a maximum level of 71.8 + 2.4% (n = 4) control at a concentration of 100 microM. The latter depressant action was reversed by the GABAB receptor antagonist, CGP35348 (1 mM). Diazepam (1 microM) had no significant effect on ganglionic transmission.8. It is concluded that the activation of benzodiazepine or M2-noradrenaline receptors can modulate NMDA receptor-mediated excitatory synaptic pathways whereas synaptic excitation from primary afferent terminals, mediated by non-NMDA receptors, appears to lack the propensity for this type of modulation. The results show also that the isolated spinal preparation can be used to identify central myorelaxant actions that are mediated through functional benzodiazepine or X2-noradrenaline receptors.

Substance P and NMDA receptors mediate a slow nociceptive ventral root potential in neonatal rat spinal cord

Substance P and glutamate actions have separately been implicated in the generation of nociceptive-related slow ventral root potentials (slow VRPs). We report that slow VRPs are dependent on both substance P and NMDA receptor-mediated neurotransmission. Slow VRPs of 10-40 s duration were evoked by electrically stimulating a lumbar dorsal root and recorded at the corresponding ipsilateral ventral root in spinal cords isolated from 1- to 5-day-old rats; the monosynaptic reflex was also recorded. The NMDA receptor antagonist APV (5-20 microM) and the substance P antagonist spantide (10-20 microM) both reversibly depressed the slow VRP without affecting the monosynaptic reflex; spantide and APV applied together nearly abolished the slow VRP. The quisqualate-kainate receptor antagonist CNQX (1-5 microM) reduced the monosynaptic reflex and an early component of the slow VRP. A slow VRP could be elicited by brief (0.1-1.0 s) focal applications of either substance P (2-20 microM) or NMDA (10 microM), and also by CGRP (2-20 microM). Substance P-evoked and NMDA-evoked responses were blocked by their respective antagonists spantide and APV. Each was also cross-sensitive to the other antagonist. Both excitatory amino acids, acting on an NMDA receptor, and substance P, acting on a tachykinin receptor, thus appear to be involved in generating this slow potential. Both NMDA and tachykinin receptors are necessary to generate a full response.

Structure-activity analysis of binding kinetics for NMDA receptor competitive antagonists: the influence of conformational restriction

1. The kinetics of action of 17 structurally related NMDA receptor competitive antagonists were measured under voltage clamp in mouse hippocampal neurones. Analysis of the response to rapid changes in antagonist concentration during constant application of agonist was used to estimate microscopic association (kon) and dissociation (koff) rate constants for antagonist binding, assuming a two-equivalent site model for competitive antagonism. Dose-inhibition curves were analysed to estimate antagonist equilibrium dissociation constants. 2. For a series of 11 omega-phosphono, alpha-amino acids kon and koff varied 26 and 107 fold respectively. Rapid association and dissociation rate constants were obtained for flexible antagonist molecules such as D-2-amino-7-phosphonoheptanoic acid (D-AP7): kon 1.4 x 10(7) M-1 s-1; koff 20.3 s-1. For conformationally restrained molecules such as 3S,4aR,6S,8aR-6-phosphonomethyl-decahydroisoquinoline- 3-carboxylic acid (LY 235959), association and dissociation rate constants were much slower: kon 1.1 x 10(6) M-1 s-1; koff 0.2 s-1. For the D- and L-isomers of 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) estimates for kon were similar, but for the L-isomer koff was 10 fold faster than for the D-isomer. 3. For 2-amino-5-phosphonopentanoic acid (AP5) and its piperidine derivative cis-4-(phosphonomethyl)piperidine-2-carboxylic acid (CGS 19755), an increase in chain length of two methylene groups between the omega-phosphono and alpha-carboxylate moieties caused a 1.6 to 1.8 fold decrease in kon with little change in koff. In contrast, for AP5, CPP and its omega-carboxylate analogue, addition of a double bond close to the phosphonate moiety caused a 1.3 to 1.6 fold increase in kon. 4. For antagonists with an omega-tetrazole moiety, kon and koff were 2.8-4.6 times faster than for the parent omega-phosphono compounds. A similar, but smaller increase in kon and koff was observed for antagonists with an omega-carboxylate moiety. 5. The slow kinetics of action of potent NMDA receptor antagonists were not an artefact of buffered diffusion. In neurones equilibrated with 200 microM D-AP7, 2 microM LY 235959 and 10 microM NMDA, a transient agonist response was recorded following a rapid switch to D-AP7-free solution. This can only be explained by differences in the binding kinetics of AP7 and LY 235959, since at equilibrium, with these concentrations, either antagonist essentially eliminates the agonist response to 10 microM NMDA. 6. For all antagonists studied, the ratio koff/k0. was consistent with equilibrium Ki values obtained under similar experimental conditions, over a 40 fold range of potency. Comparison of these values with Ki estimates determined from both agonist ([3H]-glutamate), and antagonist ([3H]-CGS 19755 and [3H]- CPP) radioligand competition studies revealed good correlation between data from voltage clamp and binding experiments. However, Ki values obtained in antagonist binding assays showed on average 6.5 fold higher affinity than those obtained in voltage clamp experiments; in contrast Ki values obtained in agonist binding assays showed only 1.4 fold higher affinity. 7. The insights gained from our experiments may be of use for predicting the structural features required to generate more potent NMDA receptor antagonists, and suggest that novel acyclic compounds will have greater potential for high potency than derivatives of conformationally rigid compounds with piperazine, piperidine or bicyclic ring structures.

Excitatory amino acid receptors in normal and abnormal vestibular function

Although excitatory amino acid (EAA) receptors have been investigated extensively in the limbic system and neocortex, less is known of the function of EAA receptors in the brainstem. A number of biochemical and electrophysiological studies suggest that the synapse between the ipsilateral vestibular (VIIIth) nerve and the brainstem vestibular nucleus (VN) is mediated by an EAA acting predominantly on kainate or alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors. In addition, there is electrophysiological evidence that input from the contralateral vestibular nerve via the contralateral VN is partially mediated by N-methyl-D-aspartate (NMDA) receptors. Input to the VN from the spinal cord may also be partially mediated by NMDA receptors. All of the electrophysiological studies conducted so far have used in vitro preparations, and it is possible that denervation of the VN during the preparation of an explant or slice causes changes in EAA receptor function. Nonetheless, these results suggest that EAA receptors may be important in many different parts of the vestibular reflex pathways. Studies of the peripheral vestibular system have also shown that EAAs are involved in transmission between the receptor hair cells and the vestibular nerve fibers. A number of recent studies in the area of vestibular plasticity have reported that antagonists for the NMDA receptor subtype disrupt the behavioral recovery that occurs following unilateral deafferentation of the vestibular nerve fibers (vestibular compensation). It has been suggested that vestibular compensation may be owing to an upregulation or increased affinity of NMDA receptors in the VN ipsilateral to the peripheral deafferentation; however; at present, there is no clear evidence to support this hypothesis.

Potentiation of synaptic reflexes by D-serine in the rat spinal cord in vitro

6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX) (10 microM) depressed dorsal root-evoked ventral and dorsal root potentials of the in vitro immature rat spinal cord to 26.3 +/- 5.2 S.E.M. and 40.8 +/- 2.7% of control values respectively. These depressant effects of CNQX were partially reversed by D-serine (EC50 values 39.7 microM +/- 8.7 S.E.M. N = 6 and 34.9 +/- 12.5 microM, N = 5 for ventral root potential and dorsal root potential respectively). Under our experimental conditions, which included the presence of Mg2+ (0.75 mM) in the bathing medium, no measurable potentiation of these synaptic reflexes by D-serine was recorded in the absence of CNQX. These data indicate that CNQX, in addition to its depressant effect at non-NMDA receptors, depresses an NMDA receptor-mediated component of segmental transmission through its action at the glycine site of the NMDA receptor complex.

The influence of assay conditions on measurement of excitatory dibasic sulphinic and sulphonic alpha-amino acids in nervous tissue

Major improvements to the HPLC separation of fluorescent derivatives of excitatory sulphur-containing amino acids have been made. Quisqualate was used as the internal standard since no endogenous derivatives coeluted with it. The artefactual generation of sulphinic and sulphonic amino acids from the oxidation of cysteine (56 microM) and homocysteine (1.2 microM) has been investigated using deionised water, an acidic phosphate/methanol mixture, perchloric acid and trichloroacetic acid (TCA) as extraction media. Of the four extraction media examined, TCA in combination with ether extraction was shown to be the most potent oxidative treatment and resulted in 23% oxidation of original cysteine or homocysteine to sulphinic and sulphonic acids. This oxidation was partially resistant to the presence of physiological concentrations of glutathione (1.5 mM) such that in the case of cysteine, 6% oxidation was observed. A 10% (v/v) mixture of methanol in 75 mM phosphate solution (pH 4.6) was found to be the most artefact-free extraction method and in spinal cord tissue processed with this medium, cysteine sulphinic acid was the only excitatory sulphur-containing amino acid consistently detectable (0.24 +/- 0.01 pmol/mg wet weight, n = 6).