Glutamatergic modulation of noradrenaline release in the rat median preoptic area.

The present study was carried out to investigate whether glutamatergic receptor mechanisms modulate the release of noradrenaline (NA) in the region of the median preoptic nucleus (MnPO) using intracerebral microdialysis techniques in freely moving rats. Perfusion of N-methyl-d-asparatate (NMDA, 10 and 50µM) through the microdialysis probe significantly enhanced dialysate NA concentration in the region of the MnPO. Local perfusion of the NMDA antagonist dizocilpine (MK801, 10 and 50µM) did not change the basal release of NA in the MnPO area. MK801 (10µM) administered together with NMDA antagonized the stimulant effect of NMDA (50µM). Perfusion of the non-NMDA agonist quisqualic acid (QA, 10 and 50µM) or kainic acid (KA, 10 and 50µM) significantly increased the NA release in the MnPO area. Perfusion of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 and 50µM) had no effect on the NA release. CNQX (10µM) administered together with either QA (50µM) or KA (50µM) in the MnPO area prevented the stimulant effect of the agonists on the NA release. Nonhypotensive hypovolemia following subcutaneous injections of polyethylene glycol (PEG, 30%, 5ml) significantly elevated the NA level in the MnPO area. The PEG-induced elevation in the NA release was attenuated by perfusion of either MK801 (10µM) or CNQX (10µM). The present results suggest that glutamatergic synaptic inputs may act to enhance the release of NA in the MnPO area through both NMDA and non-NMDA receptors, and imply that these glutamatergic receptor mechanisms may be involved in the noradrenergic reguratory system for the body fluid balance.

Activation of metabotropic glutamate receptors induces periodic burst firing and concomitant cytosolic Ca2+ oscillations in cerebellar interneurons.

Little is known about the generation of slow rhythms in brain neuronal circuits. Nevertheless, a few studies, both from reconstituted systems and from hippocampal slices, indicate that activation of metabotropic glutamate receptors (mGluRs) could generate such rhythms. Here we show in rat cerebellar slices that after either release of glutamate by repetitive stimulation, or direct stimulation of type 1 mGluRs, molecular layer interneurons exhibit repetitive slow Ca(2+) transients. By combining cell-attached patch-clamp recording with Ca(2+) imaging, we show that the regular Ca(2+) transients (mean frequency, 35 mHz induced by 2 microm quisqualate in the presence of ionotropic glutamate receptor blockers) are locked with bursts of action potentials. Nevertheless, the Ca(2+) transients are not blocked by tetrodotoxin, indicating that firing is not necessary to entrain oscillations. The first Ca(2+) transient within a train is different in several ways from subsequent transients. It is broader than the subsequent transients, displays a different phase relationship to associated spike bursts, and exhibits a distinct sensitivity to ionic and pharmacological manipulations. Whereas the first transient appears to involve entry of Ca(2+) ions through transient receptor potential channel-like channels and secondarily activated L-type Ca(2+) channels, subsequent transients rely mostly on an exchange of Ca(2+) ions between the cytosol and D-myo-inositol-1,4,5-triphosphate-sensitive intracellular Ca(2+) stores. The slow, highly regular oscillations observed in the present work are likely to drive pauses in postsynaptic Purkinje cells, and could play a role in coordinating slow oscillations involving the cerebello-olivar circuit loop.

The relationship between agonist potency and AMPA receptor kinetics.

AMPA-type glutamate receptors are tetrameric ion channels that mediate fast excitatory synaptic transmission in the mammalian brain. When agonists occupy the binding domain of individual receptor subunits, this domain closes, triggering rearrangements that couple agonist binding to channel opening. Here we compare the kinetic behavior of GluR2 channels activated by four different ligands, glutamate, AMPA, quisqualate, and 2-Me-Tet-AMPA, full agonists that vary in potency by up to two orders of magnitude. After reduction of desensitization with cyclothiazide, deactivation decays were strongly agonist dependent. The time constants of decay increased with potency, and slow components in the multiexponential decays became more prominent. The desensitization decays of agonist-activated currents also contained multiple exponential components, but they were similar for the four agonists. The time course of recovery from desensitization produced by each agonist was described by two sigmoid components, and the speed of recovery varied substantially. Recovery was fastest for glutamate and slowest for 2-Me-Tet-AMPA, and the amplitude of the slow component of recovery increased with agonist potency. The multiple kinetic components appear to arise from closed-state transitions that precede channel gating. Stargazin increases the slow kinetic components, and they likely contribute to the biexponential decay of excitatory postsynaptic currents.

Evaluation of the pathologic characteristics of excitotoxic spinal cord injury with MR imaging.

BACKGROUND AND PURPOSE: Although high-resolution MR imaging is a valuable diagnostic tool, in vivo MR imaging has not yet been compared with in vitro MR imaging and histologic techniques following experimental spinal cord injury (SCI). The goal of the present study was to evaluate the feasibility of using in vivo MR imaging, in vitro MR imaging, and histologic techniques to study pathologic changes associated with excitotoxic SCI at a single time point. These results are important for future research using in vivo MR imaging to study the temporal profile of pathologic changes following SCI. METHODS: Rats received intraspinal injections of quisqualic acid at the T12-L2 spinal level. In vivo T1- and T2-weighted and dynamic contrast-enhanced MR images were collected 17-24 days postinjury. Once completed, spinal cords were removed and in vitro MR microscopy and histologic assessment were performed. MR images were collected using 4.7-T (in vivo) and 14.1-T magnets (in vitro). RESULTS: Pathologic changes--including hemorrhage, neuronal loss, cavities, and central canal expansion--were visible in T2-weighted in vivo MR images. Evaluation of the blood-spinal cord barrier after injury with contrast agent enhancement showed no disruption at the time points evaluated. In vitro MR images and histologic evaluation confirmed pathologic details observed in vivo. CONCLUSION: Results show that high-resolution in vivo MR imaging has the potential to be used in studying the progression of pathologic changes at multiple time points following SCI. This strategy may provide a way of studying structure-function relationships between therapeutic interventions and different pathologic characteristics of the injured spinal cord.

Effects of quisqualic acid on retinal ZENK expression induced by imposed defocus in the chick eye.

PURPOSE: Expression of the transcription factor ZENK in glucagon amacrine cells of the chicken retina is enhanced after treatment with positive spectacle lenses and reduced after treatment with negative lenses. ZENK may, therefore, have an important role in emmetropization. To learn more about its regulation, we have studied its expression after retinal intoxication with quisqualic acid (QA, a glutamatergic excitotoxin). METHODS: Lenses of either +7 or -7 D power were placed in front of the eyes of young chickens 6 days after intravitreal QA injections. By this time, QA had caused severe damage to the retina. After 2 hours of lens wearing, changes in ZENK immunoreactivity were measured by means of double staining. In another experiment, lenses were worn for 4 days to study the residual function of emmetropization. RESULTS: QA injections caused a massive loss of cells in the inner nuclear layer and the ganglion cell layer but left the numbers of glucagon cells unchanged. Four of six QA-injected eyes became more myopic in response to wearing positive lenses, and all eyes with negative lenses also became myopic. QA caused a general reduction in ZENK expression, and there was no clear evidence that ZENK expression was still controlled by the sign of imposed defocus. CONCLUSIONS: After severe destruction of the inner retina by QA, retinal image processing appeared to be reduced to blur detection with no sign, causing myopia with both types of lenses. QA must remove synaptic input to the glucagon cells, which is necessary to transmit the information on the sign of imposed defocus.

Expression of the nerve growth factor receptors TrkA and p75NTR in the visual cortex of the rat: development and regulation by the cholinergic input.

Several lines of evidence have shown that nerve growth factor (NGF), the progenitor of the neurotrophin family of growth factors, plays a fundamental role in the developmental plasticity of the rat visual cortex. However, the expression of NGF receptors (NGFRs) TrkA and p75(NTR) and the possible sites of NGF action in the visual cortex remain to be elucidated so far. Using a highly sensitive ECL immunoblot analysis, we have been able to show, in the present study, that the TrkA protein is expressed in the rat visual cortex and that it is developmentally upregulated during the critical period for cortical plasticity. In contrast, the expression level of the low-affinity NGF receptor p75(NTR) seems to remain nearly constant throughout development. In the analysis of possible pathways involved in the regulation of NGFR expression, we found that neither blockade of the visual input nor NGF administration to the visual cortex resulted in a modulation of NGFR levels of expression. On the other hand, the selective destruction of cholinergic afferents to the visual cortex caused a dramatic, but not complete, reduction of the cortical NGFRs, which suggests that these receptors are located on cholinergic terminals predominantly. At the functional level, we found that, after the elimination of the cholinergic afferents to the visual cortex, the NGF-induced increase of both acetylcholine and glutamate release from cortical synaptosomes was strongly impaired. These results indicate that the cholinergic input is an important mediator of visual cortex responsiveness to NGF action.

Effects of interleukin-10 (IL-10) on pain behavior and gene expression following excitotoxic spinal cord injury in the rat.

Intraspinal injection of quisqualic acid (QUIS) produces excitotoxic injury with pathophysiological characteristics similar to those associated with ischemic and traumatic spinal cord injury (SCI). Responses to QUIS-induced injury include an inflammatory component, as well as the development of spontaneous and evoked pain behaviors. We hypothesized that QUIS-induced inflammation and subsequent gene expression contribute to the development and progression of pain-related behaviors and that blockade of inflammation-related gene expression leads to the amelioration of these behaviors. Using the QUIS model of spinal cord injury, we examined whether interleukin-10 (IL-10), a potent anti-inflammatory cytokine, is able to reduce mRNA levels of inflammatory and cell death-related genes leading to a reduction of pain behaviors. The results demonstrate that animals receiving systemic injection of IL-10, 30 minutes following QUIS-induced SCI, showed a significant delay in the onset of excessive grooming behavior, a significant reduction in grooming severity, and a significant reduction in the longitudinal extent of a pattern of neuronal loss within the spinal cord characterized as "grooming-type damage." QUIS injections also resulted in an increase in mRNA levels of interleukin-1 beta (IL-1 beta), cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS), CD95 ligand (CD95-L, also called FAS-L/APO-1L), and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Results of QUIS injury plus IL-10 treatment resulted in a significant downregulation of IL1-beta and iNOS mRNA and these results were supported by Western blot analysis of protein levels following IL-10 treatment. These data suggest that IL-10 reduces inflammation and that targeting injury-induced inflammation is an effective strategy for limiting the extent of neuronal damage following excitotoxic SCI and thus the onset and progression of injury-induced pain behaviors.

Lysosphingomyelin prevents behavioral aberrations and hippocampal neuron loss induced by the metabotropic glutamate receptor agonist quisqualate.

1. Excessive excitation of brain neurons by the excitatory neurotransmitter, glutamate, induces a cascade of events leading to increased intracellular Ca++, neuronal degeneration and death. 2. Recent in vitro research has demonstrated that a natural cationic amphiphile in the brain, lysosphingomyelin, may be able to prevent neuronal degeneration by repressing phosphosinositidase-C overactivation induced by excessive excitation of the metabotropic glutamate receptor. 3. This research tested the latter finding in vivo in a rat model of glutamate excitotoxicity. Intracerebroventricular (i.c.v.) administration of the Group 1 metabotropic glutamate receptor (mGluR) agonist, quisqualate, produced seizures, akinesia, destruction of hippocampal pyramidal cell dendritic microtubule-associated protein-2, and major loss of hippocampal CA sector neurons. 4. Prophylactic i.c.v. infusion of lysosphingomyelin powerfully attenuates these quisqualate-induced behaviors and prevents neuronal degeneration. 5. Lysosphingomyelin may be of clinical use in allaying progressive Group 1 mGluR-induced hippocampal cognitive and motor disorders including Alzheimer's disease, brain seizure, and stroke.

In vivo electrochemical studies of the dynamic effects of locally applied excitatory amino acids in the striatum of the anesthetized rat.

The actions of locally applied excitatory amino acids (EAAs) on dopamine nerve terminals in the striatum of the urethane-anesthetized rat were investigated. Rapid (5 Hz) in vivo electrochemical recording was used to measure the amplitude and duration of dopamine (DA) overflow elicited by the local application of glutamate, N-methyl-D- aspartate (NMDA), kainate, quisqualate, quinolinate and DL- alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid. EAAs were pressure ejected into the striatum via a pipette positioned 300 +/- 30 mum away from the electrochemical working electrode. Brief (5s) applications of the EAA agonists directly elicited DA-like electrochemical signals with amplitudes averaging about 2 microM. In some instances, putative increases in ascorbic acid-like signals were detected. Repeated applications of glutamate agonists in the same brain site resulted in diminished electrochemical responses, compared to the complete reproducibility seen after repeated applications of 120 mM potassium. Low doses of NMDA (10 mM barrel conc), which did not cause a detectable increase in the electrochemical baseline signal, significantly potentiated (50%) potassium-evoked DA overflow. These results indicate that low levels of endogenously released glutamate may modulate overflow when DA nerve terminals are depolarized, while higher concentrations of glutamate may directly elicit increases in extracellular levels of DA and/or ascorbic acid.

Effects of the polyamine spermidine on NMDA-induced arterial hypertension in freely moving rats.

We investigated the effect of the polyamine spermidine (SPD) (0.01-1 microgram/rat) on hypertension induced by N-methyl-D-aspartate (NMDA) (0.1 microgram/rat) microinjected into the latero-caudal periaqueductal gray (PAG) area of freely moving rats. Pretreatment with a low dose of SPD (0.01 microgram/rat) significantly increased NMDA-induced hypertension. On the contrary, higher doses of SPD (0.1 and 1 microgram/rat) significantly decreased NMDA-induced cardiovascular changes. SPD alone did not modify arterial blood pressure. Arcaine (1 microgram/rat), a putative antagonist at the polyamine recognition site on NMDA receptors, when microinjected into the PAG area, prevented the negative but not the positive modulatory effects of SPD on the NMDA-induced cardiovascular changes. Pretreatment with SPD did not affect cardiovascular effects induced by quisqualic acid (QUIS), a non-NMDA receptor agonist. These data, in agreement with the in vitro results, suggest that at the level of the PAG area, the polyamines also show multiple actions at NMDA receptors in vivo.

Nucleus basalis lesions suppress spike and wave discharges in rats with spontaneous absence-epilepsy.

Cholinergic drugs were shown to affect spike and wave discharges in a selected strain of Wistar rats with generalized non-convulsive absence epilepsy, named GAERS (Genetic Absence Epilepsy Rats from Strasbourg). The involvement of cholinergic transmission from the nucleus basalis in the control of absence seizures in GAERS was investigated in the present study, by examining the effects of unilateral excitotoxic lesions of this nucleus on the occurrence of spike-wave discharges. Ibotenate (0.01 M) and quisqualate (0.03 and 0.06 M)-induced lesions of the nucleus basalis suppressed spike-wave discharges in the cortex ipsilateral to the lesion. The suppression was associated with a disappearance of both acetylcholinesterase-fibres in the cerebral cortex and choline acetyltransferase immunopositive neurons within the nucleus basalis. Concomitantly, the background electroencephalographic activity was slowed. These results suggest that cholinergic innervation of the cerebral cortex by the nucleus basalis is involved in the occurrence of generalized non-convulsive seizures, in relation to the control of cortical activation.

Quisqualate injection into the nucleus basalis magnocellularis produces seizure-related brain damage that is prevented by MK-801.

Quisqualate (Quis) and other excitotoxins such as ibotenate and N-methyl aspartate, have been used to destroy neurons in the area of the nucleus basalis magnocellularis (NBM) in order to study the relationship between loss of cholinergic neurons in the basal forebrain and various behavioral deficits, including learning and memory impairments. The results of several studies suggest that although Quis NBM lesions may produce greater depletions in cortical choline acetyltransferase levels than ibotenate lesions, the learning/memory deficits tend to be milder following Quis lesions. In these studies, it was often assumed that the lesions induced by Quis were restricted to the local vicinity of the injection. However, in the present study, we found that an injection of Quis into the NBM/substantia inominata (SI) region often induces limbic seizures and disseminated brain damage. Specifically, we found that an injection of Quis into the NBM/SI area of female rats at a dose (120 nmol) used by others in previous behavioral studies produced massive damage in areas distant from the lesion site, particularly in the amygdala and piriform cortex. This disseminated damage occurred in 50% of the rats treated with Quis, was typically more severe than damage at the injection site, and was often accompanied by equally severe "mirror" lesions in the contralateral amygdala and piriform cortex. Injecting rats with MK-801 (1 mg/kg) 30 min before the Quis injection protected against the disseminated damage. These data underscore the need for careful histological evaluation of excitotoxic lesions and for caution in interpreting the relationship between altered transmitter markers and learning/memory impairment seen following these lesions.

Inhibitory effects of salmon calcitonin on the tail-biting and scratching behavior induced by substance P and three excitatory amino acids.

We have examined the effects of salmon calcitonin (SCT), injected into the cerebral ventricle (i.c.v.), on the tail-biting and scratching behavior induced by the intrathecal injection of different types of nociceptive agents, i.e., substance P, N-methyl-D-aspartate (NMDA), kainate (KA), and quisqualate (Quis). Tail-biting and scratching behavior induced by the 4 substances was significantly inhibited by SCT (i.c.v.) in the same manner: the dose-response curves were U-shaped, and the most effective dose was 0.1 IU/mouse in all cases. SCT did not, however, completely inhibit tail-biting and scratching behavior. At its most effective dose, the percent inhibition of substance P-, NMDA-, KA- and Quis-induced behavior were 77.9%, 40.2%, 49.4%, and 52.9%, respectively. These results suggest that SCT has the inhibitory effects of substance P- and glutamate receptor agonists-induced nociceptive response in vivo.

Globus pallidus and motor initiation: the bilateral effects of unilateral quisqualic acid-induced lesion on reaction times in monkeys.

The results of many experimental studies have shown that the globus pallidus (GP) is involved in the control of motor activities, particularly during motor execution. Whether or not the GP is involved in the initiation phase is still a matter of controversy, however. This question was investigated in the present study in Papio papio monkeys after GP lesion using a simple reaction time (RT) task, focusing particularly on the initiation phase. The monkeys were trained to perform this task, which consisted of raising their hand as quickly as possible in response to a visual signal. The RT and its premotor and motor components were measured. In addition, the distribution of the RTs was analyzed in order to assess the number of long latency responses. After making unilateral GP cell lesions by locally injecting small amounts of the excitatory amino acid quisqualic acid, a bilateral increase was observed in RT. This lengthening involved both the premotor and the motor phases of the RT when the task was performed with the contralateral limb and only the premotor phase when it was performed with the ipsilateral one. A significant increase was observed in the percentage of long latency responses recorded in the contralateral limb after the GP lesion but not in the ipsilateral one. Increases in the RT and in the percentage of long latency responses are thought to constitute two indices of the akinesia observed in our task involving speed constraints, which suggests that the GP may participate in motor initiation. A complete recovery of the RT was observed within one month, whereas the increase in the percentage of long latency responses persisted. These two indices of akinesia seemed therefore to result from an impairment involving both motor and nonmotor processes. These data suggest that the GP may be involved in the control of postural adjustment, motivation, and/or the control of the initial isometric part of movements. The time course of the recovery from the deficits observed after GP lesion shows the existence of mechanisms which seem to have been operative particularly in the case of impairments affecting motor processes.

Mnemonic deficits in the double Y-maze are related to the effects of nucleus basalis injections of ibotenic and quisqualic acid on choline acetyltransferase in the rat amygdala.

Many researchers have reported that the magnitude of decrease in cortical choline acetyltransferase (ChAT) following excitotoxic lesions of the nucleus basalis magnocellularis (nbm) is unrelated to the degree of cognitive impairment. Recently, an explanation has been offered for this lack of correlation: different excitotoxins, when injected into the nbm, differentially affected cholinergic projections to the cortex and amygdala, and those excitotoxins previously reported to produce the greatest mnemonic deficits produced the largest decreases in amygdaloid ChAT. The present study evaluated the role of amygdalofugal cholinergic projections in memory by comparing the effects of intra-nbm ibotenic and quisqualic acid on cortical and amygdaloid ChAT and on mnemonic performance in the double Y-maze. Rats were trained in the double Y-maze until working and reference memory choice accuracy stabilized to a criterion of > or = 78% correct. Rats then were given either bilateral quisqualic acid (60 nmol in 0.5 microliter), bilateral ibotenic acid (50 nmol in 0.5 microliter), or sham (0.9% saline in 0.5 microliter) lesions of the nbm, and again were tested on the maze. Quisqualate produced a selective impairment of working memory, a large (51%) decrease in cortical ChAT and a small (17%) decrease in amygdaloid ChAT; ibotenate, on the other hand, produced a greater impairment of working memory, an impairment of reference memory, a similar (51%) decrease in cortical ChAT, but a greater (30%) decrease in amygdaloid ChAT. These results suggest that the cholinergic projections from the nbm to the cortex and amygdala play an important role in memory. They suggest that excitotoxins producing greater depletions of amygdaloid ChAT produce greater mnemonic deficits.

Regulation of dopamine levels in intrastriatal grafts of fetal mesencephalic cell suspension: an in vivo voltammetric approach.

An in vivo voltammetric technique was used to monitor dopamine (DA) release in the 6-hydroxydopamine (6-OHDA)-lesioned rat striatum reinnervated by grafts of ventral mesencephalon containing DA neurons. Extracellular levels of DA were measured during the administration of D1 or D2 DA receptor antagonists. In addition, changes in DA levels induced by agonists and antagonists of excitatory amino acid (EAA) receptors were studied to verify the possible existence of a host glutamatergic control on the grafted DA cells in the 'transplanted' rats. Two months after the grafts were performed, the voltammetric signal measured under baseline conditions in the grafted striata was found to be almost similar to that recorded on the contralateral control side. Likewise, in another group of transplanted rats, the turnover of the amine, as expressed by the DO-PAC/DA tissue level ratio, was found to have become "normalized" after grafting, compared with the lesion-only group. The increase in the voltammetric signal observed after administering the D2 antagonist sulpiride (100 mg/kg i.p.) was lower in the grafted striata than on the contralateral side, however. This suggests that some D2 autoreceptor subsensitivity may have helped to maintain the baseline level of dopaminergic transmission. Adaptive processes of this kind might compensate for the partial DA reinnervation of the host striatum found to occur on the basis of the tyrosine hydroxylase immunostaining patterns. After administration of either the D1 antagonist SCH 23390 (0.1 mg/kg s.c.), or injection of EAA receptor agonists--1-glutamate, quisqualate and N-methyl-D-aspartate (all 10 nmol i.c.v.)--and antagonists--amino-phosphono-valeric acid (10 nmol i.c.v.) and dizocilpine (MK801, 0.2 mg/kg i.p.)--no significant differences between the two striata were detected in the voltammetric signals. These results suggest that, in the grafted rats, neurons belonging to the host population, such as the striatal cells bearing D1 receptors or the corticostriatal afferents presumed to contain glutamate, might modulate the DA levels, as was found to occur under normal conditions.

The mechanosensitivity of spinal sensory neurons following intraspinal injections of quisqualic acid in the rat.

The mechanoreceptive properties of rat spinal sensory neurons were evaluated in segments adjacent to those injected with the excitatory amino acid agonist quisqualic acid. Following survival periods of 7-36 days cells recorded in quisqualate injected animals had an increased level of background activity, increased sensitivity to mechanical stimuli, and an increase in the duration of afterdischarge responses. It is suggested that a central mechanism that alters the functional state of neurons may be responsible for the sensory abnormalities, e.g. allodynia and hyperalgesia, that occur following excitotoxic induced cell death associated with ischemic and traumatic spinal cord injury.

Diverse modulation by systemic lidocaine of iontophoretic NMDA and quisqualic acid induced excitations on rat dorsal horn neurons.

The effects of systemic lidocaine (3-4 mg/kg) on the responses of 60 wide dynamic range neurons (WDR) to iontophoretically applied N-methyl-D-aspartic acid (NMDA) and quisqualic acid (QUIS) were studied in anesthetized, paralysed rats. The results show that lidocaine induced (i) potentiation of the NMDA excitation, reversible by 7-chloro-kynurenate (7-Cl-KYNA), a selective antagonist of the glycine binding site on the NMDA receptor; (ii) reduction of the QUIS excitation, reversible by strychnine (STRYCH), a glycine antagonist at its receptor. These findings, supporting a glycine-like action of lidocaine, are discussed together with data on the role of excitatory amino acids (EAAs) and the analgesic effect of lidocaine on neuropathic pain.

The triggering of spreading depression in the chicken retina: a pharmacological study.

Spreading depression (S.D.) can be reproducibly evoked in the retinas of 3- to 6-day-old chickens by K+ ions, N-methyl-D-aspartate (NMDA), kainate (KA), and quisqualate (QA). Specific NMDA antagonists inhibit S.D. evoked by all the above agents. The very selective non-NMDA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline (NBQX) blocks QA- and KA-evoked S.D. but not NMDA- or K(+)-evoked S.D. These findings indicate that NMDA receptor activation is the vital event in the triggering of S.D. in this tissue, and that QA and KA trigger S.D. indirectly via excitation of NMDA receptors. Tetrodotoxin, cadmium chloride, conotoxin, baclofen and adenosine agonists are all ineffective in blocking K(+)-, NMDA-, QA- or KA-evoked S.D. L-trans-Pyrrolidine-2,4-dicarboxylic acid, a glutamate uptake blocker, does inhibit QA-evoked S.D. It is therefore argued that a presynaptic release of vesicular, glutamate 'neurotransmitter stores' is not the mechanism of action of QA and KA. A mechanism involving a reversal of the glutamate uptake carrier is suggested.

The effects of intrathecal administration of excitatory amino acid agonists and antagonists on the initiation of locomotion in the adult cat.

Development of pharmacological strategies for the control of locomotion in patients with spinal cord injury or disease requires an understanding of the neuroactive substances involved in the activation of the spinal cord neural systems for the control of locomotion. Studies using the in vitro preparations of the lamprey, frog embryo, and newborn rat indicate that excitatory amino acids (EAAs) are involved in the initiation of locomotion. The present study determines whether spinal EAA receptors play a role in locomotion in an in vivo, adult mammalian preparation. Experiments were performed on precollicular, postmammillary decerebrate cats, some of which were spinalized at the 13th thoracic segment. Cannulas for drug infusions were positioned intrathecally in the lumbar region of the spinal cord. A ligature around the spinal cord at the level of the 13th thoracic segment prevented rostral diffusion of the drugs. Locomotion was monitored with electromyograms in treadmill locomotion experiments and electroneurograms in fictive locomotion experiments. Intrathecal infusion of either the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid or the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione blocked hindlimb treadmill and fictive locomotion induced by electrical stimulation of the mesencephalic locomotor region (MLR) of the midbrain. Intrathecal administration of NMDA elicited hindlimb fictive locomotion in resting animals similar to that evoked by electrical stimulation of the MLR. At lower concentrations, NMDA evoked either independent bursting activity in the various nerves or loosely organized rhythmicity showing little reciprocity between antagonists. In contrast, administration of the EAA uptake blocker dihydrokainic acid (DHK) evoked intermittent periods of bursting activity characterized by a variable duration and a high degree of reciprocity between flexors and extensors. Given together at low concentrations, NMDA and DHK produced a well-coordinated locomotor pattern. Kainate and quisqualate were ineffective in producing fictive locomotion. These results are consistent with the suggestion that EAAs play a role in the initiation of mammalian locomotion. Furthermore, the results are consistent with those obtained from the neonatal rat in vitro preparations.