S-Glutamate: its interactions with spinal neurons

Enhanced excitability of thalamic sensory neurons and slow-wave EEG pattern after stimuli that induce spinal long-term potentiation

Spinal nociceptive neurons are well known to undergo a process of long-term potentiation (LTP) following conditioning by high-frequency sciatic nerve stimulation (HFS) at intensities recruiting C-fibers. However, little if any information exists as to whether such HFS conditioning that produces spinal LTP affects sensory transmission at supraspinal levels. The present study explored this possibility. Conventional extracellular recording methods were used to examine the consequences of HFS versus sham HFS conditioning on individual wide-dynamic range thalamic neurons located in the ventro-postero-lateral (VPL) nucleus in isoflurane-anesthetized rats. Following HFS, the ongoing firing rate and stimulus-evoked (brush, pinch, sciatic nerve) responses were markedly enhanced as were responses to juxtacellular, microiontophoretic applications of glutamate. These HFS-induced enhancements lasted throughout the recording period. Sham stimuli had no effect on VPL neuron excitability. Cortical electroencephalographic (EEG) wave activities were also measured around HFS in conjunction with VPL neuron recordings. The cortical EEG pattern under baseline conditions consisted of recurring short duration bursts of high-amplitude slow waves followed by longer periods of flat EEG. Following HFS, the EEG shifted to a continuous large-amplitude, slow-wave pattern within the 0.5-8.0 Hz bandwidth lasting throughout the recording period. Sham HFS did not alter EEG activity. Sciatic nerve conditioning at A-δ fiber strength, known to reverse spinal LTP, did not alter enhanced neuronal excitability or the EEG slow-wave pattern induced by HFS. These data support the concept that HFS conditioning of the sciatic nerve, which leads to spinal LTP, is associated with distinct, long-lasting changes in the excitability of neurons comprising thalamocortical networks.

Activation of N-methyl-d-aspartate Receptors Induces Endogenous Rhythmic Bursting Activities in Nucleus Tractus Solitarii Neurons: An Intracellular Study on Adult Rat Brainstem Slices

A brainstem slice preparation and intracellular recording techniques were used to examine the effects of N-methyl-d-aspartate (NMDA) application on neurons within the swallowing area of the nucleus tractus solitarii (NTS). According to their cellular properties, NTS neurons were classified into type I and type II neurons. The most striking difference was the occurrence of delayed excitation in type I but not in type II neurons, when they were depolarized from membrane potentials more negative than -60 mV. Bath application of NMDA (30 - 60 microM) elicited depolarization and triggered stable repetitive firing in all the NTS neurons but one. During the NMDA-induced depolarization, hyperpolarization below -60 mV elicited, in some type I neurons, a rhythmic bursting pattern. The duration of the bursts (300 - 1000 ms) and their frequency (0.5 - 2 Hz) depended on the membrane potential. With hyperpolarizations below -75 mV, rhythmic bursting was converted into rhythmic single discharges, a pattern elicited directly in the other type I neurons. In all cases, rhythmic patterns were superimposed on cyclic depolarizations of the membrane potential characterized by an initial ramp-shaped phase. In type II neurons, rhythmic bursting discharges, superimposed on rhythmic oscillations of the membrane potential, were also obtained upon hyperpolarization during the NMDA-induced depolarization. In all type I neurons tested, NMDA-induced cyclic ramp-shaped depolarizations continued after addition of tetrodotoxin to the medium. Rhythmic bursting was not elicited by bath application of kainate (10 - 20 microM). Application of d-2-amino-5-phosphonovalerate (50 microM) blocked NMDA-induced depolarizations without modifying those elicited by kainate, which were selectively depressed by 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM). Moreover, removal of Mg2+ from the medium suppressed NMDA-induced cyclic depolarizations. Results demonstrate that both NMDA and non-NMDA receptors are present in NTS neurons and that selective activation of NMDA receptors induced rhythmic bursting and/or rhythmic single discharges. Rhythmic patterns were not driven by synaptic mechanisms but originated from endogenous properties of NTS neurons activated by NMDA. Thus, NTS neurons can be considered as conditional pacemakers. According to the location of the neurons, the conditional properties shown in these in vitro experiments might be involved in vivo in the generation of rhythmic motor activities set up at the NTS level, such as swallowing.

On the reduction of spontaneous and glutamate-driven spinocerebellar and spinoreticular tract neuronal activity during active sleep

The present study was performed to provide evidence that dynamic neural processes underlie the reduction in dorsal spinocerebellar tract and spinoreticular tract neuron activity that occurs during active sleep. To ascertain the effect of local inhibition on the spontaneous and glutamate-evoked spike discharge of sensory tract neurons, preliminary control tests were performed during the state of quiet wakefulness, where GABA or glycine was co-administered in a sustained fashion during pulsatile release of glutamate to dorsal spinocerebellar tract (n=3) or spinoreticular tract (n=2) neurons. Co-administration of GABA or glycine also resulted in a significant marked suppression of spontaneous spike activity and glutamate-evoked responses of these cells. Extracellular recording experiments combined with juxtacellular application of glutamate were then performed on 20 antidromically identified dorsal spinocerebellar tract and spinoreticular tract neurons in the chronic intact cat as a function of sleep and wakefulness. The glutamate-evoked activity of a group of 10 sensory tract neurons (seven dorsal spinocerebellar tract, three spinoreticular tract), which exhibited a significant decrease in their spontaneous spike activity during active sleep, was examined. Glutamate-evoked activity in these cells was significantly attenuated during active sleep compared with wakefulness. In contrast, the glutamate-evoked activity of a second group of eight sensory tract neurons (four dorsal spinocerebellar tract, four spinoreticular tract), which exhibited a significant increase in their spontaneous spike activity during active sleep, was not significantly altered in a state-dependent manner. These data indicate that, during natural active sleep, a dynamic neural process is engaged onto certain dorsal spinocerebellar tract and spinoreticular tract neurons, which in turn dampens sensory throughput to higher brain centers.

The anterior pretectal nucleus participates as a relay station in the glutamate-, but not morphine-induced antinociception from the dorsal raphe nucleus in rats

The anterior pretectal nucleus (APtN) and the dorsal raphe nucleus (DRN) are involved in descending pathways that control noxious inputs to the spinal cord and participate in the normal physiological response to noxious stimulation. Evidence has also been provided for the involvement of the APtN acting as a relay station through which the DRN partly modulates spinal nociceptive messages. In the present study, the effects of microinjecting glutamate or morphine into the DRN on the latency for the tail withdrawal reflex after noxious heating of the skin were examined in rats in which hyperbaric lidocaine (5%), naloxone (a non-selective opioid antagonist) or methiothepin (a non-selective 5-HT(1) antagonist) was previously microinjected into the APtN. Microinjection of glutamate (38 nmol/0.25 microl) into the DRN evoked strong but short-lasting antinociception that was fully inhibited by the previous administration of lidocaine (0.25 microl), naloxone (2.7 nmol/0.25 microl), or methiothepin (1 nmol/0.25 microl). A smaller dose of methiothepin (0.5 nmol/0.25 microl) significantly reduced the effect of glutamate. Microinjection of morphine (7.5 nmol/0.25 microl) into the DRN evoked strong and long-lasting antinociception that was not significantly changed by previous microinjection of lidocaine into the APtN. These results confirm that APtN integrity is at least in part necessary for the antinociceptive effects of stimulating the DRN, and that at least opioid and 5-HT1 mechanisms in the APtN participate as neuromodulators in the DRN-APtN connection. The results demonstrate that the antinociceptive effects of stimulating the DRN-APtN path depend on the activation of cell bodies in the DRN that can be excited by the local administration of glutamate, but not morphine. The study also further supports the notion that the DRN is involved in both descending and ascending pain inhibitory systems.

Presynaptic dopaminergic inhibition of the spinal reflex in rats

Dopaminergic influence on spinal monosynaptic transmission was examined in rats. Monosynaptic mass reflex (MMR) was recorded from the ventral root L6 following supramaximal stimulation (0.2 Hz; 0.1 ms) to the ipsilateral dorsal root L6 in spinalized rat under pentobarbitone sodium (40 mg/kg, i.p.) anaesthesia. MMR was inhibited by intravenous administration of the dopaminergic agonist, apomorphine (50-200 ug/kg) in a dose-dependent manner. The attenuatory effect of apomorphine (200 ug/kg i.v.) on the reflex could be reversed by the dopaminergic antagonist haloperidol (0.5 mg/kg, i.v.). Under tetanic stimulation (200 Hz; 15s), the pretetanic relative inhibition induced by apomorphine (200 ug/kg, i.v.) was increased only for a short period immediately after the cessation of tetanic stimulation. The results indicate existence of presynaptic dopamine receptors on the afferent terminals converging on the motoneurone which may functionally modulate the spinal motor output.

Respiratory failure without pulmonary edema following injection of a glutamate agonist into the ventral medullary raphe of the rat

Injection of ibotenic acid (IA), a glutamate agonist, into the ventral medullary raphe (VMR; especially the nucleus raphe magnus) of the rat produced respiratory failure and death following a predictable course of events. The response to the IA injection was characterized initially by increased respiratory frequency and was followed by pulmonary arterial hypertension, systemic arterial hypoxemia, acidosis, and hypothermia. Within 90 min apnea occurred as a terminal event in all animals. Gravimetric, bronchoalveolar lavage protein, and histological analyses revealed no evidence of pulmonary edema. Intracerebral (VMR) pretreatment with PPP, a sigma receptor agonist, or scopolamine, a muscarinic cholinergic antagonist, prevented pulmonary failure and death even though postmortem histological analysis showed VMR cell loss and gliosis consequent to the cytotoxic IA injection. Based on the results of the study, it is suggested that the VMR has a role in regulation of pulmonary blood flow. Preliminary pharmacological studies suggested that a disruption of glutamatergic and cholinergic mechanisms mediates the lethal pulmonary phenomenon.

Evidence that activation of N-methyl-D-aspartate (NMDA) and non-NMDA receptors within the nucleus tractus solitarii triggers swallowing

Swallowing is a patterned motor activity generated by neurons located within the nucleus tractus solitarii (NTS). Previous experiments have shown that administration of excitatory amino acids within the NTS induces swallowing. The present study was undertaken to identify the receptor subtypes involved in this effect. Pressure microinjections of L-glutamate (10-100 pmol), quisqualate (0.1-10 pmol) and N-methyl-D-aspartate (NMDA, 0.1-10 pmol) were performed into the NTS of decerebrate rats. Glutamate and quisqualate microinjections elicited short series of swallows while NMDA microinjections induced long-lasting, rhythmic swallowing. Pretreatment with the selective NMDA antagonist, DL-2-amino-5-phosphonovalerate (50 pmol), almost completely suppressed the response elicited by NMDA (10 pmol) but did not induce a significant modification of swallowing triggered by either glutamate (25 pmol) or quisqualate (10 pmol). Pretreatment with 6-cyano-7-nitroquinoxaline-2,3-dione (50 pmol), a selective blocker of non-NMDA receptors, suppressed the swallows elicited by glutamate and strongly inhibited the response elicited by quisqualate microinjections. The same pretreatment induced only a slight modification of the swallowing elicited by NMDA. These data demonstrate that deglutition can be triggered by activating either NMDA or non-NMDA receptors localized within the NTS, and therefore suggest that both receptor subtypes may be involved in swallowing elicited under physiological conditions.

Pharmacology of spinal peptides affecting sensory and motor functions: dynorphins, somatostatins and tachykinins

In recent years, the pharmacological activity of dynorphins and somatostatins on spinal sensory transmission has been intensively investigated with a view to developing new agents for pain control. Similarly, a series of tachykinin-related peptides with apparent receptor antagonist activity on endogenous substance P and neurokinins has been investigated. However, a number of observations suggest that these peptides, injected intrathecally in laboratory animals, not only exert a direct effect on nociceptive transmission but also affect a broader range of spinal somatomotor and autonomic functions and may cause peculiar neurotoxic effects that are not elicited by a large number of peptides affecting spinal neurotransmission. This article makes a critical review of their pharmacological activity on spinal sensory and motor functions and briefly touches on their anatomical and functional organization in the spinal cord.

The quantitative autoradiographic distribution of [3H]MK-801 binding sites in the normal human spinal cord

The distribution of NMDA receptors in the normal human spinal cord has been investigated using the non-competitive channel blocking agent MK-801. Specific [3H]MK-801 binding was present throughout the spinal grey matter at all segmental levels, the greatest density of binding being found in the substantia gelatinosa. Focal areas of high binding were also found in a distribution corresponding to lower motor neurones in the ventral horns. This study provides anatomical evidence that NMDA receptors are likely to be important in motor as well as sensory spinal synaptic transmission. The anatomical distribution of NMDA receptors in relation to motor neurone somata may have important implications in selective vulnerability to excitotoxic injury.

Characterization of descending inhibition and facilitation from the nuclei reticularis gigantocellularis and gigantocellularis pars alpha in the rat

Descending influences on the spinal nociceptive tail-flick (TF) reflex produced by focal electrical stimulation and glutamate microinjection in the nuclei reticularis gigantocellularis (NGC) and gigantocellularis pars alpha (NGC alpha) were examined and characterized in rats lightly anesthetized with pentobarbital. Both inhibition and facilitation of the TF reflex were produced by electrical stimulation at identical sites in the NGC/NGC alpha; glutamate microinjection only inhibited the TF reflex. The chronaxie of stimulation for inhibition of the TF reflex was 169 +/- 28 microseconds. Inhibition of the TF reflex by stimulation was produced throughout the NGC and NGC alpha; intensities of stimulation for inhibition were least in the ventral NGC and in the NGC alpha. At threshold intensities of stimulation, inhibition of the TF reflex did not outlast the period of stimulation. Facilitation of the TF reflex was produced at many of the same sites at which stimulation inhibited the TF reflex, but always at lesser intensities of stimulation (mean, 10 microA vs. 43 microA for inhibition, n = 25). Stimulation in the NGC/NGC alpha at threshold intensities for facilitation or inhibition of the TF reflex did not significantly affect blood pressure. Strength-duration characterization of electrical stimulation and microinjection of glutamate into identical sites in the NGC and NGC alpha suggest that descending inhibition of the TF reflex results from activation of cell bodies in the NGC and NGC alpha.

Involvement of N-methyl-D-aspartate receptors in nociception and motor control in the spinal cord of the mouse: behavioral, pharmacological and electrophysiological evidence

The present study evaluated, using behavioral and electrophysiological methods, the involvement of the N-methyl-D-aspartate receptor in the processing of noxious information in the spinal cord of the mouse. The selectivity of the excitatory amino acid antagonists 2-amino-5-phosphonovalerate and glutamylaminomethylsulphonate was assessed behaviorally, using their ability to reverse the biting behavior elicited by intrathecal excitatory amino acid administration as a tool. 2-Amino-5-phosphonovalerate at concentrations up to 1 mM was shown to be selective for the N-methyl-D-aspartate receptor, while glutamylaminomethylsulphonate was selective for the kainate and quisqualate receptors at similar concentrations. At these concentrations, intrathecal administration of 2-amino-5-phosphonovalerate to awake mice produced significant analgesia on a battery of tests, as well as a dose-related motor impairment, while glutamylaminomethylsulphonate was without effect. Proof that 2-amino-5-phosphonovalerate exerts its effects via N-methyl-D-aspartate receptors is that glutamylaminomethylsulphonate, at concentrations which also block this receptor (greater than 1 mM), also produced analgesia and motor effects. Furthermore, N-methyl-D-aspartate, but not kainate or quisqualate, reversed the analgesic effects of 2-amino-5-phosphonovalerate. In fact, significant potentiation of analgesia could be seen with quisqualate. In accordance with the behavioral pharmacological data, topical application of 2-amino-5-phosphonovalerate onto the spinal cord of anesthetized mice significantly depressed the response of spinal sensory neurons to noxious mechanical and electrical stimulation, but did not affect the activity of neurons which showed no preferential reaction to noxious stimulation. Glutamylaminomethylsulphonate at non-analgesic concentrations was without effect. Based on these and other studies we conclude that N-methyl-D-aspartate receptor bearing interneurons participate in nociception, and that N-methyl-D-aspartate antagonists exert their analgesic and motor effect by changing the tone of spinal neural action in the spinal cord, rather than direct intervention in primary afferent transmission.

A microtest plate assay of functional excitatory amino acid receptors

A microtest plate assay of functional excitatory amino acid receptors present on cultured chick embryo retinal cells has been developed. It is based on measurements of excitatory amino acid-mediated increase in 22Na+ influx into retinal cells adhering to each of the 96 wells of microtest plates. Dose-dependent responses to L-glutamate, kainate and N-methyl-D-aspartate but not to quisqualate can be measured. These responses are selectively inhibited by antagonists of excitatory amino acids. The assay is reliable, fast to perform and parsimonious in terms of the volume and thus of amount of the drug applied. It allows a single investigator to perform, in one day, measurements of the effects of known or putative glutamatergic ligands in more than 100 different conditions.

Limitations of the technique of pressure microinjection of excitatory amino acids for evoking responses from localized regions of the CNS

The aim of this study, performed on anaesthetized cats and rabbits, was to test the assumption that pressure microinjections of excitatory amino acids cause long-lasting excitation of neurones located close to the injection site. Unitary action potentials or antidromic field potentials were recorded from respiratory or 'reticular' neurones in the medulla oblongata and from phrenic motoneurones at different distances from the injection site. Injection of 10-150 nl (5-150 nmol) of L-glutamate or DL-homocysteic acid into these areas resulted in complex and widespread neuronal events. Generally, more distant neurones (500-1300 microns) were excited for variable periods of time (3-15 min), while neurones in the vicinity of the injection site (0-500 microns) showed, after a brief period of excitation time, a long-lasting (up to 30 min) decrease in excitability or silencing of discharge, probably due to a depolarizing block and disturbances in the ionic composition of the extracellular space. These findings show that interpretation of physiological responses following such injections should not be based on an assumption of local neuronal excitation. Some recommendations regarding the use of this technique are made.

Effects of tizanidine (DS 103-282) on dorsal horn convergent neurones in the rat

The effects of tizanidine, a new muscle relaxant, 5-chloro-4-(2-imidazolin-2-yl-amino)-2,1,3-benzothiazole (DS 103-282) were studied on the activity of lumbar dorsal horn convergent neurons in anaesthetized paralysed rats. Following i.v. administration of tizanidine both the A- and C-fibre evoked responses were depressed in a dose-dependent manner in the 0.125-1.0 mg/kg range. The smaller dose employed (0.125 mg/kg) induced a significant depression of the C-fibre evoked responses (39.6 +/- 13.4% of the control responses) and a total recovery was observed 10 min after the injection: when the doses were increased, stronger and longer-lasting depressant effects were obtained. Identical but less powerful effects were observed on A-fibre responses. None of the depressive effects was correlated with variations in blood pressure. Microelectrophoretically applied tizanidine was found to depress current-dependently, the discharges of convergent neurones evoked by microelectrophoretically applied DL-homocysteic acid. In contrast, tizanidine (0.5, 1 mg/kg; i.v.) was found to be ineffective against the activities of non-nociceptive neurones triggered by mechanical stimulation of their receptive fields. It is concluded that tizanidine depresses specifically the activities of dorsal horn convergent neurones, probably in part by a post-synaptic inhibitory action. Owing to the role of convergent neurones in pain processes, the present result could explain, at least partially, the analgesic action of this compound.

Behavioral classification of excitatory amino acid receptors in mouse spinal cord

Intrathecal injections of excitatory amino acid (EAA) agonists to the spinal cord of mice produces behavioral activation manifest as biting and scratching of the hindquarters. The dose-response relationship of EAA (N-methyl-D-aspartate (NMDA), kainate, quisqualate and glutamate)-induced activation revealed qualitative and quantitative differences in their pattern of action, suggesting that these agonists act at distinct receptors. Evaluation of the blockade of EAA-induced bites by a series of antagonists: DL-2-amino-5-phosphonovalerate (APV), gamma-D-glutamyl glycine (DGG), kynurenate and glutamylaminomethylsulphonate (GAMS), indicated that selective activation of the NMDA, quisqualate and kainate receptors can be demonstrated using this behavior. The NMDA receptors could be subdivided on the basis of different sensitivity to kynurenate and APV. Antagonist-resistant components of both kainate and quisqualate action were also shown. Thus, the biting behavior induced by the administration of intrathecal EAA agonists can be used as a relatively selective behavioral tool for assessing the pharmacological profile of action of excitatory amino acid agonists and antagonists in the spinal cord.

Medullary substrates mediating antinociception produced by electrical stimulation of the vagus

Electrical stimulation of afferents of the right cervical vagus inhibited the tail-flick reflex elicited by noxious heat in barbiturate-anesthetized rats. This inhibitory effect was eliminated in rats receiving local anesthetic blockade of either the nucleus tractus solitarii (NTS), the lateral reticular nuclei, the nucleus raphe magnus-medullary reticular formation, or nucleus raphe obscurus regions of the medulla. Similarly, the vasodepressor and bradycardic effects of vagal stimulation were either attenuated or eliminated by local anesthetic blockade of these regions. Microinjection of the non-specific glutamate antagonist gamma-D-glutamylglycine (DGG) into the NTS region also eliminated vagally evoked inhibition of the tail-flick reflex, hypotension, and bradycardia. Conversely, microinjection of glutamate into the NTS region resulted in inhibition of the tail-flick reflex, hypotension, and bradycardia. These findings with DGG and glutamate are consistent with the view that glutamate serves as a neurotransmitter of the primary vagal afferents mediating these antinociceptive and cardiovascular responses. These results are discussed in terms of vagal afferent influences on somatosensory, somatomotor, and cardiovascular function.

Contralateral head movements produced by microinjection of glutamate into superior colliculus of rats: evidence for mediation by multiple output pathways

One of the major efferent pathways of the superior colliculus crosses midline to run caudally in the contralateral predorsal bundle, innervating targets in the brain stem and eventually reaching the cervical spinal cord. A variety of evidence suggests that this tecto-reticulo-spinal pathway may mediate the orienting movements that can be evoked by tectal stimulation. However, we have recently found that orienting head movements can still be obtained in rats after section of the tecto-reticulo-spinal pathway, implying that additional pathways are also involved. The present study sought to test this implication, by taking advantage of the fact that in rats the cells of origin of the tecto-reticulo-spinal pathway are largely segregated within the lateral part of the stratum album intermediate. It is thus possible to find out whether orienting head movements can be produced by a cell-excitant from tectal regions that contain few cells of origin of the tecto-reticulo-spinal pathway. Hooded rats in an open field were filmed during microinjections of sodium L-glutamate (50 mM, 200 nl) into the superior colliculus, and the films analysed for the appearance of contralaterally directed movements of the head and body. Subsequent histological reconstruction of the injection sites indicated that such movements could be obtained from widespread areas within the superior colliculus, including not only lateral stratum album intermediale but also the deep layers, and parts of the medial superficial and intermediate layers. Moreover, sites in or close to lateral stratum album intermediate often gave circling movements with downward pointing head, whereas some sites outside lateral stratum album intermediale gave sustained immobility with the head pointing contralaterally and upwards. This evidence supports the view that tectal efferent pathways besides the tecto-reticulo-spinal pathway are involved in the control of head movement. In addition, at least some of these pathways are not collaterals of the tecto-reticulo-spinal pathway, since the movements were obtained from collicular regions with few tecto-reticulo-spinal pathway cells. Finally, the results are consistent with the view that different collicular output pathways mediate movements that have different functions.

Excitatory postsynaptic currents in response to different synaptic inputs of frog spinal motoneurons

Excitatory postsynaptic currents (EPSCs) evoked by the primary afferents (dorsal root; DR) and the descending lateral column (LC) fibers were studied in frog spinal motoneurons under voltage clamp with two separate electrodes. The average rise time and half-width of the EPSCs were shorter for LC-EPSCs than for DR-EPSCs, though the values of the parameters for LC- and DR-EPSCs were distributed within a similar range. The relation between the amplitudes of the EPSP and EPSC was almost linear. The amount of current required to generate a 1 mV increment in the EPSP was 5.0 +/- 2.3 nA for the DR-EPSC and 3.8 +/- 1.2 nA for the LC-EPSC. The decay time was shortened by hyperpolarization and prolonged by depolarization in DR- and LC-EPSCs and spontaneous EPSCs. The reversal potential ranged from -30 to -5 mV and was almost identical for DR- and LC-EPSCs and spontaneous EPSCs in individual motoneurons. The current-voltage relation was linear from -100 to +50 mV for these EPSCs. Spontaneous EPSCs became more prominent and frequent during a large hyperpolarization or a large depolarization. These results suggest that the ionic mechanisms underlying EPSC are similar for the functionally different excitatory synapses located on motoneurons.

Solubilization and characterization of kainate receptors from goldfish brain

The binding of [3H]kainate to goldfish brain membrane fragments was investigated. Scatchard analysis revealed a single class of binding sites in Tris-HCl buffer with a Kd of 352 nM and a Bmax of 3.1 pmol/mg wet weight. In Ringer's saline, [3H]kainate bound with a Bmax of 1.8 pmol/mg wet weight and a Kd of 214 nM. Binding in Ringer's saline, but not Tris-HCl buffer, displayed positive cooperativity with a Hill coefficient of 1.15. The [3H]kainate binding sites were solubilized in Ringer's saline using the nonionic detergent n-octyl-beta-D-glucopyranoside. Approximately 30-50% of the total number of membrane-bound binding sites were recovered on solubilization. The Kd of [3H]kainate for solubilized binding sites was approximately 200 nM. The rank order of potency for glutamatergic ligands at inhibiting [3H]kainate binding was identical and the competitive ligands had similar Ki values in both membranes and solubilized extracts. In membrane preparations, [3H]kainate displayed a two component off-rate with koff values of 0.97 min-1 and 0.07 min-1; in solubilized extracts, however, only a single off-rate (koff = 0.52 min-1) was observed. The hydrodynamic properties of n-octyl-beta-D-glucopyranoside solubilized [3H]kainate binding sites was investigated by sucrose density centrifugation. A single well defined peak was detected which yielded a sedimentation coefficient of 8.3 S. The results presented in this report suggest that goldfish brain may provide an ideal system in which to study kainate receptor biochemistry.

L-glutamate and N-methyl-D-asparatate actions on membrane potential and conductance of cat abducens motoneurones

The actions of L-glutamate and N-methyl-D-aspartate (NMDA) were studied with intracellular recordings from cat abducens motoneurones. Amino acids were electrophoresed extracellularly from the same 7-barreled electrode types as those used in the spinal cord. Depolarization development, conductance changes and firing pattern induced by amino acids were very similar to those described for spinal motoneurones. The shape of NMDA depolarization suggests a uniform distribution of the involved receptors on the membrane of the motoneurone.

Heterotopic activation of A delta and C fibres triggers inhibition of trigeminal and spinal convergent neurones in the rat

1. Extracellular recordings were made from fourteen non-noxious only and nineteen convergent neurones in trigeminal nucleus caudalis of halothane-anaesthetized rats. All the neurones studied were excited by the continuous micro-electrophoretic ejection of an excitatory amino acid, DL-homocysteic acid (DLH), with mean currents of 38.0 +/- 7.2 and 39.8 + 6.5 nA producing steady discharges of 35.0 +/- 3.3 and 31.8 +/- 1.3 spikes/s from the non-noxious only and convergent neurones respectively. 2. The repeated percutaneous application (100 trials; 0.66 Hz) of single square-wave stimuli (10 mA; 2 ms) to the tail always induced a biphasic depression of the activity of the convergent, but never of the non-noxious only, neurones. Both the early and late components of this inhibition occurred at shorter latencies when the base rather than the tip of the tail was stimulated. Differences in latencies from the two sites of stimulation (100 mm apart) were used to estimate the conduction velocities of the peripheral fibres which were triggering the inhibitions. 3. The cumulated results showed that, for the onset of the earlier component of the inhibition, the mean difference between the latencies from the two sites of stimulation was 13.6 +/- 1.9 ms, corresponding to a peripheral conduction velocity of 7.3 +/- 0.3 m/s, which is in the A delta-fibre range. For the onset of the late component of inhibition, the mean difference was 147.7 +/- 14.9 ms, corresponding to a peripheral conduction velocity of 0.68 +/- 0.07 m/s, which is in the C-fibre range. 4. When currents of different intensities were applied percutaneously to the two stimulation sites, the thresholds for obtaining the A delta component were in the range 0.25-0.5 mA whereas the C component appeared with currents 1-2 mA. A clear relationship between current intensity and magnitude of inhibition was observed in the 0.25-2 mA range for the A delta component and in the 1-5 mA range for the C component. 5. In an additional series of experiments recordings were made from eleven convergent neurones in the dorsal horn of the lumbar spinal cord. By using essentially the same experimental procedure the effects of repetitive application (100 trials, 0.66 Hz) of percutaneous electrical stimuli (1 or 10 mA, 2 ms) applied to the muzzle, were studied on the steady discharges (42.3 +/- 5.4 spikes/s) induced by DLH. The application of the 10 mA stimuli induced a biphasic depression of activity, whereas only an early component was observed following 1 mA stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

A study of amino acid-activated currents recorded from frog motoneurones in vitro

Superfusion of the excitatory amino acids glutamate (1-2 mM), quisqualate (15-30 microM) and N-methyl-D-aspartate (NMDA: 15-30 microM) induced inward currents in voltage-clamped motoneurones, in vitro. Typically the NMDA and quisqualate currents had prolonged time courses relative to glutamate currents. No desensitization was apparent during repeated agonist application. D-2-Amino-5-phosphonovalerate (10 microM) selectively antagonized the NMDA current without affecting the quisqualate current; the glutamate current was partially reduced reflecting its mixed agonist properties.

The action of excitatory amino acids on chick spinal cord neurones in culture

1. Membrane currents evoked by N-methyl-D-aspartate (NMDA), L-aspartate, L-glutamate, quisqualate and kainate were studied in cultured neurones from the embryonic chick spinal cord by the patch-clamp technique and by employing a quasi-step microperfusion technique. 2. Application of NMDA, aspartate, glutamate and quisqualate induced currents which exhibited an initial peak which declined to a plateau level with a time constant of 2 s and then remained constant or slowly decreased. The discontinuation of the application was followed by an after-current. The individual components of the responses were insensitive to TTX (2 X 10(-6) M) and were present in neurones which did not exhibit any sign of synaptic activity. The responses induced by kainate were monophasic and declined slowly during long-lasting application. 3. The responses induced by NMDA, aspartate and glutamate were voltage dependent, while those induced by kainate were linear between -80 and +80 mV. The equilibrium potential for all components of the responses to all excitatory amino acids was close to zero. 4. From dose-response curves the half-maximum effective dose (ED50) for glutamate and kainate was 3 X 10(-5) and 2 X 10(-4) M respectively. The Hill coefficients for the glutamate and the kainate were calculated to be 1.8 +/- 0.1 (n = 4) and 1.9 +/- 0.5 (n = 4) respectively. Thus two molecules may be interacting with each of the receptor-activated ion channels. 5. Interaction between kainate and quisqualate or kainate and NMDA was studied at both negative and positive holding potentials. No summation of the responses was found when kainate at concentrations close to those required for evoking the maximum response was applied simultaneously with quisqualate or NMDA. On the contrary, a diminution of the membrane currents was observed. A marked decrease in membrane currents was also observed when glutamate (10(-4) M) was applied simultaneously with aspartate (10(-4) M). 6. Glutamate-activated single-channel currents were recorded in the cell-attached configuration with electrodes filled with glutamate (20 microM) in five neurones and a conductance approximately 50 pS was found. 7. It is suggested that differences in the potency of the different excitatory amino acids as open-channel blockers may be one of the mechanisms which contribute to the diversity in the action of excitatory amino acids and that at least some of the effects of NMDA, aspartate, glutamate, quisqualate and kainate may be mediated by a common receptor-channel complex.

Ontogeny of adenosine 3',5'-monophosphate metabolism in guinea pig cerebral cortex. II. Development of responses to L-glutamate in the presence of adenosine or histamine

The effects of L-glutamate and other dicarboxylic amino acids on the accumulation of adenosine 3',5'-monophosphate (cyclic AMP) in slices of cerebral cortex from strain 2 guinea pigs were examined using tissue from animals at 39 days gestation to 7 days after birth. Responses to glutamate were inhibited completely by adenosine deaminase or theophylline unless histamine was present. When tested in the presence of adenosine, glutamate increased cyclic AMP accumulation up to 10-fold at 39 days gestation; the response was maximal at 52 days gestation, and both the efficacy and potency of glutamate declined thereafter. While the effects of glutamate were smaller in the presence of histamine plus theophylline, the developmental pattern was similar to that in the presence of adenosine. The relative potencies of D-aspartate, kainate, and alpha-methyl-DL-glutamate were much greater in fetal than in adult tissue. Glutamic acid diethyl ester, N-acetyl glutamate or 2,3-diaminopropionate had no effect in fetal tissue either in the presence or absence of glutamate. Responses to glutamate in adult tissue were much more dependent upon the presence of calcium ions than were those in fetal tissue. It was concluded that responses to glutamate involve mechanisms that differ in fetal and adult tissue.

Pharmacological aspects of excitatory synaptic transmission to second-order vestibular neurons in the frog

Synaptic excitation of second-order vestibular neurons is mediated by two principal afferents: vestibular afferents projecting into the brain via the VIIIth cranial nerve and commissural afferents from the contralateral vestibular nuclear complex. The shape of the excitatory postsynaptic potentials (EPSPs) generated by selectively activating these two inputs differs qualitatively, such that ipsilateral VIIIth nerve afferents generate a faster-rising EPSP than do the commissural afferents. We have investigated the synaptic pharmacology of these two inputs in the isolated, intact medulla of the frog in order to determine the nature of the transmitter substances released by the afferents and the nature of the subsynaptic receptors with which these transmitters interact. Electrical stimulation of the ipsilateral VIIIth cranial nerve evokes in the region of the vestibular nuclear complex a field potential that exhibits a presynaptic (afferent volley) and a postsynaptic (slow negativity) component. Bath application of glutamate receptor antagonists, such as kynurenic acid (KENYA), blocks the postsynaptic component of this field potential in a dose-dependent manner, without affecting the presynaptic volley, suggesting that the VIIIth nerve afferent releases glutamate and/or similar substances as its neurotransmitter. A comparison of the actions of various glutamate receptor antagonists to block this postsynaptic negativity gives a rank order of effectiveness such that KENYA greater than gamma-D-glutamylglycine (gamma DGG) = gamma-D-glutamylaminomethylsulfonic acid (GAMS) greater than gamma-D-glutamyltaurine (gamma DGT) much greater than gamma-D-glutamylaminomethylphosphonic acid (GAMP) greater than D-2-amino-5-phosphonovaleric acid (D-APV) greater than D,L-APV greater than D-2-amino-7-phosphonoheptanoic acid (APH). This rank order of effectiveness suggests that the VIIIth nerve transmitter activates second-order neurons through kainate (KA)/quisqualate (QUIS) synaptic receptors. Intracellular studies support these conclusions. Chemically mediated EPSPs evoked from ipsilateral VIIIth nerve stimulation are completely blocked by high concentrations of KENYA (greater than or equal to 1 mM). Occasionally an extremely short-latency, probably electrically mediated, component to these EPSPs persists in the presence of KENYA. The slower-rising EPSPs evoked from contralateral VIIIth nerve or contralateral vestibular nuclear complex stimulation are also completely blocked by KENYA, suggesting that the transmitter released by the commissural afferents is also glutamate and/or related compounds.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurotransmitter glutamate: its clinical importance

Excitatory amino acid glutamate has several important functions in the mammalian central nervous system (CNS). This review focuses on the transmitter role of glutamate and discusses anatomical and pharmacological data of clinical neurological relevance. Experimental and clinical conditions which have been associated with altered content, uptake, membrane binding or release of glutamate in the CNS are discussed. Such conditions include, epilepsy, disorders of the basal ganglia, cerebral ischemia, hypoxia, hypoglycemia, metabolic encephalopathies, olivopontocerebellar atrophy and cerebellar ataxias, amino acidopathies, mental and other neurological disorders. With the exception of a few fibre systems, it is very difficult to differentiate between glutamate and aspartate as CNS transmitters. The term glutamate is, thus, used in the sense glutamate and/or aspartate unless specifically stated.

Characterization of L-glutamate binding sites in rat spinal cord synaptic membranes: evidence for multiple chloride ion-dependent sites

The effects of various ions on L-glutamate (L-Glu) binding sites (Na+-dependent, Cl(-)-dependent, and Cl(-)-independent) in synaptic plasma membranes (SPM) isolated from rat spinal cord and forebrain were examined. Cl(-)-dependent binding sites were over twofold higher in spinal cord (Bmax = 152 +/- 34 pmol/mg protein) as compared to forebrain SPM (Bmax = 64 +/- 12 pmol/mg protein). Na+-dependent binding, on the other hand, was nearly sixfold less in spinal cord (Bmax = 74 +/- 10 pmol/mg protein) compared to forebrain SPM (408 +/- 26 pmol/mg protein). Uptake of L-Glu (Na+-dependent) was also eightfold less in the P2 fraction from spinal cord relative to forebrain (Vmax of 2.89 and 22.3 pmol/mg protein/min, respectively). The effects of Na+, K+, NH4+, and Ca2+ on L-Glu binding sites were similar in both regions of the CNS. In addition, in spinal cord membranes, Br-, I-, and NO3- were equivalent to Cl- in their capacity to stimulate L-Glu binding, whereas F- and CO3- were less effective. Cl(-)-dependent L-Glu binding in spinal cord membranes consisted of two distinct sites. The predominant site (74% of the total) had characteristics similar to the Cl(-)-dependent binding site in forebrain membranes [i.e., Ki values of 5.7 +/- 1.4 microM and 119 +/- 38 nM for 2-amino-4-phosphonobutyric acid (AP4) and quisqualic acid, (QUIS), respectively]. The other Cl(-)-dependent site was unaffected by AP4 but was blocked by QUIS (Ki = 14.2 +/- 4.8 microM).

First-order nociceptive synapses in rat dorsal horn are blocked by an amino acid antagonist

An antagonist to amino acid evoked excitation, gamma-D-glutamylglycine (gamma-DGG) inhibits, dose-dependently, nociceptive C-fibre evoked field potentials and neuronal discharges in single nociceptive neurons in rat spinal cord. It is concluded that amino acids are possible transmitter substance(s) in nociceptive afferent C-fibres. These results might be useful for development of centrally acting analgesic drugs.

Excitatory amino acid recognition sites coupled with inositol phospholipid metabolism: developmental changes and interaction with alpha 1-adrenoceptors

Glutamate, aspartate, ibotenate, and quisqualate activate inositol phospholipid hydrolysis in hippocampal slices prepared from brains of 6- to 8-day-old rats. The stimulation by glutamate and aspartate progressively declines during postnatal development and is negligible after the 24th day of life. In contrast, the stimulation of inositol phospholipid hydrolysis by norepinephrine is low in hippocampal slices from newborn animals and increases during development, reaching mature values after the 35th day of life. In adult hippocampal slices, the stimulation of inositol phospholipid hydrolysis elicited by norepinephrine is inhibited by glutamate in a concentration-dependent fashion. This inhibition can also be brought about by aspartate, 2-amino-4-phosphonobutanoate, and L-phosphoserine, a product of endogenous phosphatidylserine hydrolysis.

Synaptic excitation in cultures of mouse spinal cord neurones: receptor pharmacology and behaviour of synaptic currents

Fast monosynaptic excitatory post-synaptic potentials between spinal cord neurones in cell culture (s.c.-s.c. e.p.s.p.s) were studied with current-clamp and two-electrode voltage-clamp methods. The reversal potential, response to acidic amino acid antagonists, and behaviour of the synaptic current were examined. The amplitude of the e.p.s.p. increased with membrane potential hyperpolarization and decreased with depolarization. The reversal potential of the e.p.s.p. was +3.8 +/- 2.5 mV (mean +/- S.E. of mean). The reversal potential for responses to ionophoretically applied L-glutamate and L-aspartate was also near 0 mV. The acidic amino acid antagonist, cis-2,3-piperidine dicarboxylic acid (PDA, 0.25-1.0 mM) reversibly antagonized the monosynaptic e.p.s.p.s as well as responses to kainate (KA) or quisqualate (QA). The selective N-methyl-D-aspartate antagonist, (+/-) 2-amino-5-phosphonovaleric acid (APV), had little effect on either the monosynaptic e.p.s.p.s or responses to QA or KA at concentrations that abolished responses to L-aspartate. Under voltage clamp, the peak synaptic current (e.p.s.c.) was linearly related to the membrane potential, increasing in amplitude with hyperpolarization and decreasing with depolarization from the resting potential. The decay of a somatic e.p.s.c. was well fitted by a single exponential function with a time constant of 0.6 ms at 25 degrees C. E.p.s.c.s which had proximal dendritic locations had decay time constants of 1-2 ms. The decay time constant was voltage-insensitive between -80 and +10 mV. We suggest that an acidic amino acid receptor other than that for NMDA mediates excitatory transmission at the s.c.-s.c. synapse; and that the underlying conductance mechanism is voltage insensitive with an estimated mean channel lifetime of less than 1 ms.

Characterization of coeruleospinal inhibition of the nociceptive tail-flick reflex in the rat: mediation by spinal alpha 2-adrenoceptors

Several lines of evidence implicate bulbospinal noradrenergic pathways in antinociception and descending inhibition. In the present study, descending inhibition of the nociceptive tail-flick (TF) reflex by electrical stimulation in the dorsolateral pons (DLP) and the spinal neurotransmitter(s) mediating that inhibition were characterized in lightly pentobarbital-anesthetized rats. It was determined that 10 s of stimulation in the DLP prior to the application of heat to the tail resulted in optimum (lowest) thresholds for inhibition of the TF reflex. Conditioning-test studies indicated that the duration of the inhibitory effects produced by stimulation outlasted the 10-s period of stimulation by approximately 5 s. Systematic mapping studies revealed that inhibition of the TF reflex could be produced by stimulation throughout a large portion of the DLP; however, stimulation sites requiring the lowest intensities of stimulation (less than or equal to 25 microA) were in the locus coeruleus/subcoeruleus. Changes in blood pressure were not produced at this intensity and duration of stimulation. S-glutamate microinjections and stimulation strength-duration determinations suggest that inhibition of the TF reflex produced by stimulation in the locus coeruleus/subcoeruleus results from activation of cell bodies. The intrathecal administration of pharmacologic antagonists (phentolamine, yohimbine, prazosin, naloxone, methysergide, atropine and bicuculline) revealed that only the alpha-adrenergic antagonists phentolamine and yohimbine resulted in significant increases in stimulation thresholds in the locus coeruleus/subcoeruleus for inhibition of the TF reflex (83.1 and 93.9%, respectively). These results indicate that inhibition of the spinal nociceptive TF reflex produced by electrical stimulation in the locus coeruleus/subcoeruleus is at least in part a noradrenergic, postsynaptic alpha 2-adrenoceptor-mediated effect.

Excitatory amino acid receptors in hippocampal neurons: kainate fails to desensitize them

The excitatory responses to L-glutamate (L-Glu), quisqualate (QA) and kainate (KA) have been investigated in isolated pyramidal cells from rat hippocampus using intracellular perfusion and concentration clamp techniques. The responses to L-Glu and QA demonstrated rapid desensitization and complete cross-desensitization, while KA produced a non-desensitizing response. The activation of KA response was determined by the level of desensitization induced by L-Glu or QA pretreatment. It is concluded that QA, KA and L-Glu activate the same excitatory receptors with apparent Kd values of 9.3 X 10(-5) M, 5.0 X 10(-4) M and 1.1 X 10(-3) M, respectively.

Coupling of inositol phospholipid metabolism with excitatory amino acid recognition sites in rat hippocampus

Ibotenate, a rigid structural analogue of glutamate, markedly enhances the hydrolysis of membrane inositol phospholipids, as reflected by the stimulation of [3H]inositol monophosphate formation in rat hippocampal slices prelabeled with [3H]inositol and treated with Li+. Quisqualate, homocysteate, L-glutamate, and L-aspartate also induce a significant (albeit weaker) increase in [3H]inositol monophosphate formation, whereas N-methyl-D-aspartate, kainate, quinolinate, and N-acetylaspartylglutamate are inactive. The increase in [3H]inositol monophosphate formation elicited by the above-mentioned excitatory amino acids is potently and selectively antagonized by DL-2-amino-4-phosphonobutyric acid, a dicarboxylic amino acid receptor antagonist. These results suggest that, in the hippocampus, a class of dicarboxylic amino acid recognition sites is coupled with phospholipase C, the enzyme that catalyzes the hydrolysis of membrane inositol phospholipids.

The uptake and metabolism of L-glutamate by tissue slices of the cestode Hymenolepis diminuta

The in vitro uptake of L-[3H]glutamate by tissue slices of the cestode Hymenolepis diminuta, denuded of tegument, was investigated. Two sodium concentration-dependent mechanisms, one of high affinity (Kt 1.8 X 10(-5) M; Vmax 4.76 pMoles/min/mg wet weight) and another of low affinity (Kt 2.2 X 10(-4) M; Vmax 50.7 pMoles/min/mg wet weight), were identified, in addition to a sodium insensitive component. Exchange of preloaded [3H]glutamate did not occur in tissue slices incubated in dilute unlabelled glutamate. Acidic amino acids, imipramine and fluoxetine were effective inhibitors of high and low affinity uptake, while glutamate receptor ligands, neurotransmitters and some antihelminthics generally were not. The concentrations present in, and the metabolism of glutamate by, tissue slices was examined by HPLC. The significance of the three modes of glutamate uptake and their possible role in the physiology of H. diminuta are discussed.

Selective excitation of neurons in the mammalian spinal dorsal horn by aspartate and glutamate in vitro: correlation with location and excitatory input

The electrical activity of mammalian dorsal horn neurons was recorded with pipette microelectrodes in an in vitro spinal cord slice preparation with dorsal roots intact. Addition of relatively low concentrations of aspartate or glutamate to the superfusion solution or through the recording pipette with small iontophoretic currents excited only a subset of neurons. The majority of these excited neurons were located in the superficial dorsal horn (Rexed's laminae I and II) and a preponderance were excited by the C-fiber components of dorsal root volleys. These findings are consistent with the idea that aspartate or glutamate may function as a synaptic mediator for some neurons terminating in the superficial dorsal horn.

Developmental changes in gamma-glutamyl transpeptidase activity in nervous tissues with reference to amino acid transmission processes

In homogenate supernatants of hippocampal formation and cerebellum of the rat, gamma-glutamyl transpeptidase (gamma-GTP) activities increased about 6 times from postnatal day 6 (0.178 +/- 0.02 and 0.187 +/- 0.007 U/g wet wt., respectively) to day 100. In dorsal root ganglia (0.183 +/- 0.003 U/g at day 6) and superior cervical ganglia (0.188 +/- 0.019 U/g at day 6) in which apparently amino acidergic transmission processes do not occur, enzyme activities were seen to go up 4 times and 2.4 times, respectively. Based on protein, enzyme activities in both brain material and dorsal root ganglia showed a similar pattern, whereas the activity increase in superior cervical ganglia was somewhat gradual (1.4 times from day 6 to day 100). Postnatal changes in gamma-GTP activities indicated a functional correlation with the maturation of amino acidergic structures. Kainic acid added to hippocampal extracts (0.45 and 5.0 mM) and, for comparison, to those of kidney (5.0 mM) did not yield any statistically significant effect on gamma-GTP activity.

Ketamine and phencyclidine cause a voltage-dependent block of responses to L-aspartic acid

Excitatory amino acids depolarize central mammalian neurons by increasing membrane conductance. This increase in conductance can be voltage-dependent (i.e. N-methyl-D-aspartate or L-aspartic acid (L-ASP)) or voltage-independent (i.e. kainic acid (KA)) depending on whether or not the channel is blocked by Mg2+ [8,9]. Intracellular recordings were made from dissociated mouse spinal cord cells and conductance was calculated using constant current techniques. The dissociative anesthetics, ketamine and phencyclidine caused a selective depression in the change in conductance evoked by L-ASP but not that by KA. Under whole cell voltage-clamp (in the absence of extracellular Mg2+) this depression of responses to L-ASP was found to be highly voltage-dependent suggesting a blockade of the channel.

The variation in action of excitatory amino acids in relation to distance of iontophoretic application to spinal motoneurones

Intracellular recordings were made from lumbosacral motoneurones of barbiturate-anaesthetized cats. DL-homocysteate (DLH) and L-glutamate were iontophoresed extracellularly over a range of distances from the impaled motoneurone. Movement of the iontophoretic electrode unit was controlled by a micromanipulator which was advanced independently of that moving the intracellular electrode. Depolarizations to DLH were first detected at a greater distance from the impaled motoneurones (mean, 383 micron) than depolarizations to L-glutamate (mean, 165 micron). As the point of application approached the soma of the motoneurone, depolarizations developed more rapidly, were larger and the latent period of the L-glutamate depolarization became shorter. Dendritic 'hot-spots' of the depolarizing action of L-glutamate were not detected.

Controlled induction of paired helical filaments of the Alzheimer type in cultured human neurons, by glutamate and aspartate

One characteristic histopathological feature of brains of patients with Senile Dementia of the Alzheimer Type (SDAT) is a neuronal change known as neurofibrillary degeneration. Ultrastructurally, this degeneration presents as aggregates of Paired Helical Filaments (PHFs). We have reported (De Boni and Crapper 1978) that brain affected with SDAT contains a factor which induces PHFs similar to those occurring in Alzheimer's disease, in human CNS neurons in vitro. We now report that PHFs may be induced in cultured, fetal human spinal cord lesions by factors other than extracts from brain affected by SDAT. Specifically, ultrastructural analyses have shown that PHFs closely similar to those found in SDAT are induced by the excitotoxic aminoacids, glutamate (Glu) and aspartate (Asp). PHFs were found with Glu and Asp in combination at 1.1 and 0.45 mM, respectively and by Glu alone at 2.2 mM, when added to the culture medium. Cultures grown and maintained in MEM Eagle medium without Glu and Asp were invariably composed of tightly packed neuropil with abundant synaptic contacts, were free of vacuolated somata or processes and did not exhibit the presence of PHFs. In contrast, cultures exposed to Glu and Asp invariably responded with the presence of vacuolated neuronal somata and degenerating neuronal processes containing intermediate filaments, frequently paired into PHFs. These induced PHFs are composed of intermediate filaments morphologically identical (diameter 10 +/- 0.9 nm, n = 55) to neurofilaments. While the induced PHFs exhibit a range of periodicities (50-540 nm) with an overall mean period of 140 +/- 68 nm (n = 245), approximately 4% are morphologically closely similar (mean period 76 +/- 10 nm, n = 10) to those in SDAT.

The encoding of thermal stimuli applied to the tail of the rat by lowering the excitability of trigeminal convergent neurones

Recordings were made from convergent neurones in the nucleus caudalis of the trigeminal complex. They could be excited by both innocuous and noxious stimuli applied to their excitatory receptive field located on the ipsilateral part of the muzzle. The responses to A- and C-fibre activation induced by supramaximal transcutaneous electrical stimulation of their receptive fields were conditioned by thermal stimuli applied to the tail, and the relationship between the conditioning temperatures and their ability to induce inhibitions of the evoked discharges of convergent neurones were investigated. During sequences of repetitive stimulation, the posterior two-thirds of the tail were immersed in a waterbath at various temperatures (36-52 degrees C) and the resultant inhibitions of responses were calculated. The threshold for obtaining an inhibitory effect ranged between 40 and 44 degrees C. Above this level, there was a highly significant correlation between the conditioning temperature and the degree of inhibition, which was very strong for the highest temperature (i.e. for 52 degrees C: 54.4 +/- 7.3 and 92.3 +/- 3.7% inhibitions being obtained for A- and C-fibre-evoked responses respectively); such a correlation concerned the inhibitions observed both during immersion of the tail and after the removal of the conditioning stimuli. In further experiments, identical conditioning stimuli were tested upon responses of trigeminal convergent neurones to microelectrophoretic applications of an excitatory amino acid, DL-homocysteate, and very similar results were obtained: a threshold for inhibition in the 40-44 degrees C range and a significant correlation between the conditioning temperature and the degree of inhibition in the 44-52 degrees C range (52 degrees C giving inhibitions of 98.8 +/- 0.4%.(ABSTRACT TRUNCATED AT 250 WORDS)

Cations differentially affect subpopulations of L-glutamate receptors in rat synaptic plasma membranes

Several cations were examined for their ability to specifically affect one of the 3 L-glutamate (L-Glu) binding sites in rat forebrain synaptic plasma membranes (i.e. Na+-dependent, Cl--dependent and Cl--independent). Na+-dependent binding was potently inhibited by K+ and NH4+ ions. Other monovalent cations tested (Cs+, Li+, triethylammonium) had no effect on this binding site. Polyvalent cations (Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Cr3+) also had little effect on the Na+-dependent L-Glu binding site. Cl--dependent L-Glu binding was potently inhibited by Na+ ions but was not affected by other monovalent ions. All of the divalent cations were potent inhibitors of both Cl--dependent and -independent binding. The results show that these binding sites of L-Glu can be distinguished by their response to cations and suggest possible novel modes of regulation in vivo.