Differential effects of neuroexcitatory amino acids on corticotropin-releasing hormone-41 and vasopressin release from rat hypothalamic explants.

We have investigated the direct effects of different neuroexcitatory amino acids (EAA) on the secretion of CRH-41 and arginine vasopressin (AVP) from the rat hypothalamus maintained in vitro. CRH-41 and AVP released in the medium were assayed by RIA before and after incubation with N-methyl-D-aspartate (NMDA), N-methyl-D,L-aspartic acid, kainate (KA), and quisqualate in the concentration range 1 nM to 1 mM in either the absence or the presence of 1 mM Mg2+ in the medium. In the case of NMDA, the effect of the addition of glycine (1 and 10 microM) to the incubation medium was also studied. Finally, we investigated whether different periods of exposure (up to 100 min) of hypothalamic explants to NMDA and KA would affect CRH-41 release. While no EAA was able to induce CRH-41 release under any of the above conditions, 20-min incubations with NMDA in the dose range of 1 nM to 1 mM in the absence of added Mg2+ significantly stimulated AVP release in a dose-related fashion; the maximum effect occurred at a concentration of 1 mM [ratio of stimulated collection/basal collection: NMDA, 1.51 +/- 0.10, controls, 0.86 +/- 0.05 (mean +/- SEM); P < 0.001]. KA also showed a dose-related stimulatory effect in the dose range of 1 nM to 1 mM, with maximal AVP stimulation at 10 microM (KA, 1.91 +/- 0.28; controls, 0.90 +/- 0.03; P < 0.01). The effects of both NMDA and KA on AVP were completely reversed by the competitive antagonists D,L-2-amino-5-phosphonovaleric acid and 6-cyano-7-nitroquinoxaline-2,3 dione, respectively, at doses 10 times higher than those of the agonists. N-Methyl-D,L-aspartic acid stimulated AVP secretion only at a dose of 10 mM (P < 0.01), whereas quisqualate was ineffective at any concentration. The addition of 1 mM Mg2+ to the medium blocked the effect of NMDA, while attenuating AVP stimulation induced by KA. The stimulatory effect of KA on AVP was significantly reduced by D-L-2-amino-5-phosphonovaleric acid (P < 0.05), suggesting that KA may also act through NMDA receptors. Moreover, the presence of glycine in the incubation medium did not result in any effect of NMDA on CRH-41 secretion, nor did it appear to potentiate NMDA-induced AVP release.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacological characterization of MCCG and MAP4 at the mGluR1b, mGluR2 and mGluR4a human metabotropic glutamate receptor subtypes.

The two reported metabotropic glutamate receptor (mGluR) antagonists, alpha-methyl-cyclopropyl glycine (MCCG) and alpha-methyl-aminophosphonobutyrate (MAP4) were tested on the mGluR1b, mGluR2 and mGluR4a subtypes of human mGluRs. Neither MCCG (500 microM) nor MAP4 (500 microM) antagonized the activation of mGluR1b by 10 microM quisqualate. MCCG was found to potently antagonize the action of 30 microM (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S,3R)-ACPD] at mGluR2 (IC50 = 87.5 microM; apparent Kd = 25 microM) but did not block the action of 1 microM S-2-amino-4-phosphonobutyric acid at mGluR4a (IC50 >> 1 mM). MAP4 was found to be a weak antagonist or partial agonist at mGluR4a (IC50 > 500 microM) and, less potently, also antagonized the action of 30 microM (1S,3R)-ACPD) at mGluR2 (IC50 approximately 2 mM).

Pharmacological analysis of 4-carboxyphenylglycine derivatives: comparison of effects on mGluR1 alpha and mGluR5a subtypes.

The antagonist effects of the 4-carboxyphenylglycines: (S)-4-carboxy-3hydroxyphenylglycine (4C3HPG), (S)-4-carboxyphenylglycine (4CPG) and (+)-alpha-methyl-4-carboxyphenylglycine (M4CPG) were compared on functional responses of human metabotropic glutamate receptor (mGluR) subtypes mGluR1 alpha and mGluR5a. These receptors both belong to group 1 type mGluRs which couple to the phosphoinositide (PI) hydrolysis/[Ca2+]i mobilization signal transduction pathway and are closely related in both structure and agonist pharmacology. In this study, the IC50 values obtained for quisqualate induced PI hydrolysis responses show that although all the phenylglycines are antagonists for both mGluR1 alpha and mGluR5a, the compounds exhibit differential potencies at these receptor subtypes. The 4C3HPG derivative was the most potent antagonist for both mGluR1 alpha (IC50 range: 19-50 microM) and mGluR5a (IC50 range: 53-280 microM). 4CPG produced an IC50 range of 4r-72 microM for mGluR1 alpha and 150-156 microM for mGluR5a cells. The potency of the M4CPG could not be distinguished from that of 4CPG with IC50 ranges of 29-100 microM and 115-210 microM for mGluR1 alpha and mGluR5a respectively. Further characterization of the dose-response effects of the compounds on quisqualate induced [Ca2+]i mobilization showed that although the magnitude of phenylglycine inhibition was reduced for both mGluR subtypes compared to those observed for stimulation of PI hydrolysis (except for 4C3HPG on mGluR1 alpha), similar differences in the relative potencies of the phenylglycines between mGluR1 alpha (IC50s: 40 +/- 10 microM for 4C3HPG: 300-1000 microM for 4CPG and M4CPG) and mGluR5a (IC50s: > 1000 microM) were evident.(ABSTRACT TRUNCATED AT 250 WORDS)

Systemic administration of MK-801 protects against N-methyl-D-aspartate- and quisqualate-mediated neurotoxicity in perinatal rats.

MK-801, a non-competitive antagonist of N-methyl-D-aspartate-type glutamate receptors, was tested for its ability to antagonize excitotoxic actions of N-methyl-D-aspartate or quisqualic acid injected into the brains of seven-day-old rats. Stereotaxic injection of N-methyl-D-aspartate (25 nmol/0.5 microliters) or quisqualic acid (100 nmol/1.0 microliter) into the corpus striatum under ether anesthesia consistently produced severe unilateral neuronal necrosis in the basal ganglia, dorsal hippocampus and overlying neocortex. The distribution of the damage corresponded to the topography of glutamate receptors in the vulnerable regions demonstrated by previous autoradiographic studies. N-Methyl-D-aspartate produced severe, confluent neuronal destruction while quisqualic acid typically caused more selective neuronal necrosis. Intraperitoneal administration of MK-801 (0.1-1.0 mg/kg) 30 min before N-methyl-D-aspartate injection had a prominent dose-dependent neuroprotective effects as assessed morphometrically by comparison of bilateral striatal, hippocampal and cerebral hemisphere cross-sectional areas five days later. A 1 mg/kg dose of MK-801 given as pre-treatment completely protected the infant brain. The same dose of MK-801 was also completely protective when administered 30 or 40 min after N-methyl-D-aspartate and afforded partial protection when given 2 h later. MK-801 pre-treatment also prevented the electrically confirmed behavioral seizures induced by N-methyl-D-aspartate. The drug significantly reduced striatal but not hippocampal or neocortical injury when given as two doses (1 mg/kg) 30 min prior to and immediately following quisqualic acid injection. The data indicate that systemic administration of MK-801 can prevent N-methyl-D-aspartate-induced neuronal injury in perinatal rat brain even when administered after the initial insult. MK-801 also partially antagonized quisqualic acid-mediated neurotoxicity, suggesting that quisqualic acid-induced toxicity is, in part, mediated through N-methyl-D-aspartate receptor activation. The sensitivity of the developing brain to the toxicity of N-methyl-D-aspartate provides a sensitive and reproducible in vivo model for exploring these issues and for screening prospective neuroprotective drugs that act at the N-methyl-D-aspartate receptor-channel complex.

Progesterone administration attenuates excitatory amino acid responses of cerebellar Purkinje cells.

We have previously shown that the sex steroid progesterone plays a modulatory role in amino acid physiology by suppressing excitatory responses of cerebellar Purkinje cells to glutamate and augmenting inhibitory responses of these neurons to GABA. In the present study using the rat, progesterone effects on neuronal responses to the specific excitatory amino acid agonists quisqualate, kainate and N-methyl-D-aspartate were tested using iontophoretic, extracellular single unit recording techniques. In addition, the effect of systemic administration of progesterone on quisqualate-evoked excitation was evaluated in the presence of the GABAA blocker bicuculline. Progesterone consistently attenuated excitatory neuronal responses to local application of all three excitatory amino acids by 40-51%, but exerted variable effects on combined administration of quisqualate and N-methyl-D-aspartate which were dependent on temporal and dose-related factors. Progesterone-induced attenuation of the quisqualate response was found to be mediated primarily by a non-N-methyl-D-aspartate receptor. In addition, bicuculline application did not block progesterone effects on quisqualate excitation, suggesting that the observed steroidal modulation of excitatory amino acid function is not secondary to progesterone-induced potentiation of GABA inhibition.

Regulation of D-aspartate release by glutamate and GABA receptors in cerebral cortical slices from developing and ageing mice.

The basal release of D-[3H]aspartate, an unmetabolized analogue of glutamate, from cerebral cortical slices remained at the same level from three-day-old to 24-month-old mice, but the response to K+ stimulation (50 mM) was smaller in young than in adult or aged mice. Kainate, N-methyl-D-aspartate and quisqualate (0.1 mM) stimulated the basal release of D-aspartate in the cerebral cortex of seven-day-old mice, the effects of kainate and N-methyl-D-aspartate being reduced by their antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and dizocilpine maleate, respectively, indicating that in the immature cerebral cortex the kainate and N-methyl-D-aspartate types of the glutamate receptor are involved in the basal release. The K(+)-stimulated release was not affected by glutamate agonists in developing mice, though they markedly attenuated the evoked release in adults. The inhibitory amino acids GABA, taurine and glycine depressed the K(+)-stimulated release only in the adult cerebral cortex. The action of GABA was abolished by bicuculline, demonstrating the involvement of presynaptic GABAA receptors. The glycine effect was strychnine-insensitive, characteristic of the glycine modulatory site in the N-methyl-D-aspartate receptor. This kind of regulation by both kainate and N-methyl-D-aspartate receptors could be of physiological significance, particularly in the immature cerebral cortex.

Intracerebral implantation of nerve growth factor-producing fibroblasts protects striatum against neurotoxic levels of excitatory amino acids.

With the exception of L-DOPA pharmacological treatment in Parkinson's disease, the neurodegenerative diseases lack effective treatment. Previous studies of neurodegenerative diseases suggest that symptoms arise secondary to defects in local neuronal circuitry and cannot be treated effectively with systemic drug delivery. Therefore, a promising treatment is the application of fetal or genetically engineering cells which protect or replace neurons in deficient regions. Engineered cells can be derived from cell lines or grown from recipient host fibroblasts or other cells, then modified to produce and secrete substances at a specific area of the brain. A previous study using parallel intracerebral infusions of nerve growth factor and an excitotoxic amino acid into the rat striatum demonstrated a protective effect of nerve growth factor on neurons [Aloe L. (1987) Biotechnology 5, 1085-1086]. In order to further test this paradigm, we have utilized a biological delivery system of nerve growth factor by implanting fibroblasts into the rat striatum which secrete high levels of nerve growth factor, prior to infusing the neurotoxins quinolinate or quisqualate. Animals in this group had smaller lesions than did a group implanted with a similar non-nerve growth factor-producing graft. In addition, marked neuronal sparing was noted within areas of lesions in those animals containing a nerve growth factor-producing graft. These results indicate that implantation of genetically engineered nerve growth factor-secreting cells can be used to protect neurons at a specific target from excitotoxin-induced lesions.

The selective cyclooxygenase-2 inhibitor rofecoxib suppresses brain inflammation and protects cholinergic neurons from excitotoxic degeneration in vivo.

Brain inflammatory processes underlie the pathogenesis of Alzheimer's disease, and non-steroidal anti-inflammatory drugs have a protective effect in the disease. The aim of this work was to study in vivo whether attenuation of brain inflammatory response to excitotoxic insult by the selective cyclooxygenase-2 inhibitor, rofecoxib, may prevent neurodegeneration, as a contribution to a better understanding of the role inflammation plays in the pathology of Alzheimer's disease. We investigated, by immunohistochemical methods, glia reaction, the activation of p38 mitogen-activated protein kinase (p38MAPK) pathway with an antibody selective for the phosphorylated form of the enzyme and the number of choline acetyltransferase-positive neurons and, by in vivo microdialysis, cortical extracellular levels of acetylcholine following the injection of quisqualic acid into the right nucleus basalis of adult rats. Seven days after injection, a marked reduction in the number of choline acetyltransferase-positive neurons was found, along with an intense glia reaction, selective activation of p38MAPK at the injection site and a significant decrease in the extracellular levels of acetylcholine in the cortex ipsilateral to the injection site. The loss of cholinergic neurons persisted for at least up to 28 days. Rofecoxib (3 mg/kg/day, starting 1 h prior to injection of quisqualic acid) treatment for 7 days significantly attenuated glia activation and prevented the loss of choline acetyltransferase-positive cells and a decrease in cortical acetylcholine release. The prevention of cholinergic cell loss by rofecoxib occurred concomitantly with the inhibition of p38MAPK phosphorylation. Our findings suggest an important role of brain inflammatory reaction in cholinergic degeneration and demonstrate a neuroprotective effect of rofecoxib, presumably mediated through the inhibition of p38MAPK phosphorylation.

Anion transport blockers inhibit DL-2-amino-4-phosphonobutyrate responses induced by quisqualate in the rat cerebral cortex.

1. Depolarizing responses to DL-2-amino-4-phosphonobutyrate (AP4) and related amino acids have been studied in the rat cerebral cortex slice following the application of quisqualate (Quis). 2. Before exposure to Quis, 500 microM DL-AP4 had little or no effect. However, following a single application of 40 microM Quis for 2 min, DL-AP4 produced depolarizing responses. With repeated applications of DL-AP4, there was a decline in response amplitude. A second application of Quis restored the depolarizing potency of DL-AP4 to a level above that for the first DL-AP4 response after the first Quis application. With a sequence of alternate applications of Quis and DL-AP4, the amplitude of DL-AP4 responses became maximal after the second Quis application. Responses to DL-AP4 could also be induced by the application of 1 microM Quis for 60 min, but were smaller in amplitude. 3. Responses to the normally inactive amino acids L-cysteine (Cys), L-cystathionine (CTN) and L-alpha-aminoadipate (AA) were also induced once Quis was applied. These responses were also maximized after a second application of Quis, except those to L-Cys, which failed to reach a plateau after three Quis applications. 4. The co-application of DL-AP4 with the first Quis application depressed the subsequent mean DL-AP4 response by 47%. Re-application of Quis restored the amplitude of DL-AP4 responses to levels comparable to control. L-alpha-AA also suppressed the induction of DL-AP4 responses, when co-applied with the first Quis exposure, reducing mean response amplitude by 98%. Unlike DL-AP4, however, the effect with L-alpha-AA persisted so that DL-AP4 responses were significantly suppressed compared to control, even after further applications of Quis. 5. The effects of the anion transport blockers, 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) and 4-acetoamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid (SITS) on the induction process and the DL-AP4 responses themselves were examined. DIDS (100 microM) significantly inhibited the DL-AP4 responses, and to a lesser extent the induction of the responses by 40 microM Quis (2 min), while SITS (300 microM) only inhibited the DL-AP4 responses. However, the induction of responses by 1 microM Quis (60 min) was significantly affected by this concentration of SITS. 6. DIDS (100 microM) had no effect on responses to alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA), but selectively potentiated those to Quis. Examination of the full concentration-response curve for Quis revealed that, while the Rmax remained constant, the Hill slope was increased and the EC50 was decreased in the presence of DIDS. SITS (300 microM), however, antagonized responses to AMPA, and had little effect on responses to Quis except at the highest concentration of Quis tested (20 microM), where a potentiation was observed, suggesting that it is a non-NMDA receptor antagonist.7. These observations indicate that the production of depolarizing responses to a number of amino acids, including DL-AP4, in the cerebral cortex is mediated via an anion transport mechanism sensitive to DIDS and SITS, and that the exchange of DL-AP4 for a sequestered excitatory amino acid receptor agonist, probably Quis, could underlie the production of these responses. Indeed, Quis is apparently sequestered via a similar process. However, the involvement of such a process in the induction of these responses remains inconclusive.

Philanthotoxin blocks quisqualate-, AMPA- and kainate-, but not NMDA-, induced excitation of rat brainstem neurones in vivo.

1. The effect of electrophoretic ejection of philanthotoxin (the polyamine toxin, from the Egyptian digger wasp) was tested on responses of brainstem and spinal neurones in the pentobarbitone-anaesthetized rat to excitatory amino acids. 2. Philanthotoxin caused a dose-dependent reduction of responses to quisqualate, alpha-amino-3-hydroxy-5-phenyl-4-isoxazolepropionate (AMPA) and kainate with little effect on those to N-methyl-D-aspartate (NMDA). 3. The time-course of this antagonist action was slow. In particular the rate of recovery was dependent on frequency of ejection of the agonist. This agonist-dependent recovery suggests that philanthotoxin has a channel blocking mode of action on mammalian central neurones.

L-NAME blocks responses to NMDA, substance P and noxious cutaneous stimuli in cat dorsal horn.

The nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), administered i.v. (50 mg kg-1) or by iontophoresis, was tested on the responses of spinal dorsal horn neurones in cats anaesthetized with alpha-chloralose and spinally transected at the L1 level. Extracellular, single-unit recordings were obtained from functionally identified dorsal horn cells. All units included in this study were wide dynamic range neurones. L-NAME significantly reduced the responses of (i) twelve neurones to noxious thermal stimulation of the receptive field, (ii) nine neurones to noxious pinch, (iii) nine neurones to iontophoretic application of N-methyl-D-aspartate (NMDA) and (iv) ten neurones to iontophoretic application of substance P. The inhibition usually lasted for 50-70 min following i.v. administration and for 5-8 min after iontophoretic application of L-NAME. The responses of four neurones to iontophoretic application of quisqualate were not affected by L-NAME. The results suggest the possible involvement of nitric oxide in the mediation of the spinal effects of NMDA and substance P, and in the transmission of thermal and mechanical nociceptive imputs.

Effects of N-methyl-D-aspartate on quisqualate-stimulated phosphoinositide hydrolysis in slices of kitten striate cortex.

Stimulation of phosphoinositide (PI) hydrolysis by excitatory amino acids (EAAs) was studied in coronal slices of kitten visual cortex. Coincubation with N-methyl-D-aspartate (NMDA) markedly reduced the stimulation by quisqualate, however, this inhibition developed with a latency of > 10 min and occurred even when the NMDA exposure preceded, but did not overlap with, incubation in quisqualate. This time-course of NMDA inhibition of EAA-stimulated PI turnover places new constraints on its possible mechanism of inhibition.

GDP beta S antagonizes whole-cell current responses to excitatory amino acids.

Guanine nucleotides have been predicted to be NMDA antagonists in tissue binding studies. Using the patch-clamp technique, we now show that the guanine nucleotide GDP beta S produces a rapidly reversible antagonism of NMDA, kainate, and, less potently, quisqualate whole-cell current responses. Furthermore, it does not appear that this antagonism is effected intracellularly. Our results suggest a novel extracellular role for guanine nucleotides apart from their traditional intracellular actions on G-proteins.

An analogue of Joro spider toxin selectively suppresses hippocampal epileptic discharges induced by quisqualate.

The anticonvulsant effect of 1-naphthylacetyl spermine, an analogue of Joro spider toxin (JSTX), was studied against seizures induced by quisqualate (QUIS), a non-NMDA agonist, as assessed electrophysiologically and behaviorally in freely moving rats. Electrodes were implanted into right dorsal hippocampus and an injection cannula for drugs into right ventricle. The pretreatment with JSTX analogue significantly inhibited both of QUIS-induced hippocampal discharges (80-11%) and generalized tonic clonic seizures (100-33%) in a dose-dependent manner, whereas JSTX had no effect on seizures induced by quinolinate, a NMDA agonist. The paper provides the first direct evidence that the JSTX analogue exerts a potent and selective suppression of hippocampal epileptic discharges mediated by non-N-methyl-D-aspartate (non-NMDA) receptors.

Use-dependent pentobarbital block of kainate and quisqualate currents.

The effects of pentobarbital on whole-cell excitatory amino acid-induced currents were studies in cultured rat cortical neurons. Currents evoked by 40 microM kainate were reversibly inhibited by pentobarbital with an IC50 value of 50 microM. The block of the kainate response by pentobarbital was use dependent, requiring kainate stimulation. In the absence of kainate activation, 10 min perfusions of 100 microM pentobarbital inhibited kainate-induced currents less than 10%. Recovery from pentobarbital block also exhibited use dependence, reversing in 5-10 s with kainate stimulation, while persisting 10 min or more in the absence of agonist. Pentobarbital inhibition of the kainate response was not voltage dependent. Responses evoked by 10 microM quisqualate consisted of a peak current desensitizing to a smaller steady-state current. The co-application of 100 microM pentobarbital reduced the steady-state current by 49 +/- 5%. The peak current before desensitization, however, was inhibited less than 10%. Currents evoked by 25 microM N-methyl-D-aspartate were not significantly inhibited by co-application of 100 microM pentobarbital. The results suggest that the pentobarbital-induced inhibition of kainate responses involves open channel block and that the block of quisqualate currents primarily involve non-desensitizing receptor channels that generate steady-state currents.

Effects of sigma ligands on mouse cerebellar cyclic guanosine monophosphate (cGMP) levels in vivo: further evidence for a functional modulation of N-methyl-D-aspartate (NMDA) receptor complex-mediated events by sigma ligands.

In the present investigation, the effects of sigma ligands [WY-47384 [8-fluoro-2,3,4,5-tetrahydro-2[3-(3-pyridinyl)propyl)1H- pyrido(4,3b)indole], (+)-pentazocine, (+)-SFK 10,047 (N-allylnormetazocine), mafoprazine, opipramol, dextromethorphan, dextrorphan, (+)-3-PPP [3-(3-hydroxyphenyl)-N-propylpiperidine], (-)-butaclamol, DTG [1,3-di(2-tolyl)guanidine], rimcazole, ifenprodil and BMY-14802 [alpha-(fluorophenyl)-4-(5-fluoropyrimidinyl)-1-piperazine butanol]] on harmaline-, pentylenetetrazol (PTZ)-, methamphetamine (MA)- and D-serine-induced increases in mouse cerebellar levels of cGMP were determined. Ifenprodil, BMY-14802, dextromethorphan, dextrorphan, (+)-SKF 10,047, opipramol and mafoprazine reversed harmaline-, PTZ-, MA- and D-serine-induced increases in levels of cGMP. Rimcazole reversed only the harmaline-induced response. WY-47384 reversed harmaline-, MA-, D-serine-, but not PTZ- or quisqualate-induced increases in levels of cGMP. (+)-Pentazocine attenuated harmaline- and D-serine-, but not PTZ- and MA-induced cGMP responses. Haloperidol did not affect harmaline- and D-serine-induced cGMP responses. (+)-3-PPP and (-)-butaclamol did not affect any of the responses studied. Furthermore, (+)-3-PPP-induced increases in levels of cGMP were reversed by the competitive N-methyl-D-aspartate (NMDA) antagonist, CPP]3-(2-carboxypiperazin-4-yl)propyl- 1-phosphonic acid, the non-competitive NMDA antagonist, (+)-MK-801 (dizocilipine maleate), the NMDA-associated glycine receptor antagonist, HA-966 (3-amino-1-hydroxypyrrolidin-2-one), the partial glycine agonist, DCS (D-cycloserine) as well as by the sigma ligands, ifenprodil, WY-47384, (+)-pentazocine, (+)-SKF 10,047, dextromethorphan and dextrorphan but not by rimcazole.(ABSTRACT TRUNCATED AT 250 WORDS)

Injections of excitatory amino acid antagonists into the median raphe nucleus produce hippocampal theta rhythm in the urethane-anesthetized rat.

The median raphe nucleus (MR) exerts a pronounced desynchronizing influence on the hippocampal EEG. MR stimulation disrupts theta, while MR lesions produce constant uninterrupted theta. The MR receives pronounced excitatory amino acid (EAA)-containing afferents that have been implicated in several MR-mediated behaviors. The present study examined the effects on the hippocampal EEG of MR injections of the following EAA antagonists in the urethane-anesthetized rat: 2-amino-7-phosphonoheptanoate (AP-7), dizocilpine maleate (MK-801), and gamma-glutamyl-aminomethylsulfonic acid (GAMS). MR injections of the competitive (AP-7) and non-competitive (MK-801) N-methyl-D-aspartic acid (NMDA) receptor antagonists produced theta at short latencies (2.86 min; 4.02 min, respectively) and for long durations (116.1 min; 66.8 min, respectively). It was further shown that the theta-eliciting effects of AP-7 injections could be reliably and temporarily reversed with MR injections of NMDA. MR injections of the kainate/quisqualate receptor antagonist (GAMS) also produced theta at relatively short latencies (6.5 min) and for long durations (60.5 min) indicating that EAA effects on the MR are not NMDA receptor specific. Injections of each of the foregoing EAA antagonists into regions of the brainstem adjacent to the MR including the dorsal raphe nucleus and the medullary or pontine reticular formation generated theta at very long latencies or were without effect. The present findings indicate EAA afferents to the MR normally exert an excitatory influence on the MR in its desynchronization of the hippocampal EEG, whereas the removal of EAA inputs to MR produces the opposite: a reduction of MR activity and hence the elicitation of theta.(ABSTRACT TRUNCATED AT 250 WORDS)

Streptomycin blocks the postsynaptic effects of excitatory amino acids on the vestibular system primary afferents.

It has been suggested that streptomycin might be an antagonist of the glutamate receptors, and that it selectively blocks quisqualic acid receptors. We studied whether streptomycin blocks the responses to excitatory amino acid agonists on the vestibular system primary afferents, and if it allows us to differentiate between kainate (KA) and quisqualate (QA) receptor mediated responses. The experiments were performed in the axolotl (Ambystoma tigrinum). Intra- and extracellular records of the electrical activity of semicircular canal afferent fibers were obtained. Drugs were applied by pressure ejection in volumes of 20 microliters in a 10 ml bath. Streptomycin (0.01-10 mM), induced a dose dependent reversible inhibition of the basal spike discharge of the afferent fibers. This coincided with a reduction in the amplitude of excitatory postsynaptic potentials (EPSP) recorded intracellularly in the afferent fibers. Streptomycin also blocked the excitatory action produced by KA and QA; increasing concentrations of streptomycin produced a rightward shift in the concentration-response curves for both KA and QA. This action persisted even in a high Mg2+ (10 mM), low Ca2+ (0.09 mM) Ringer solution, indicating its postsynaptic nature. These results show that streptomycin might be a non-selective excitatory amino acid (EAA) receptor antagonist.

Differential effects of NBQX on the distal and local toxicity of glutamate agonists administered intra-hippocampally.

The ability of the non-NMDA glutamate antagonist NBQX (2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline) to protect the brain against the neuronal death caused by glutamate agonists was examined. Glutamate agonists and NBQX were co-injected into the dorsal region of the rat hippocampus and 4 days later the brain was examined histochemically for the loss of neurons. 95 nmol NBQX prevented the toxicity of glutamate agonists acting on the AMPA receptor (quisqualate and AMPA [L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate]), except for the higher dose of AMPA where toxicity was only partially reduced. This dose of NBQX also prevented about 50% of the toxicity of kainate, but produced a slight increase in the size of the lesions caused by NMDA (N-methyl-D-aspartate). With 190 nmol NBQX, a variable degree of non-specific damage resulted, but was mainly confined to the dentate region. Allowing for this damage, almost complete protection against the toxicity of non-NMDA glutamate agonists was obtained, with a partial protection against NMDA toxicity. Kainate, and a high dose of AMPA (2 nmol), consistently caused neuronal death in other limbic regions of the brain in addition to the hippocampal damage. About 50% of rats treated with 15 nmol quisqualate also showed damage to limbic regions. Both doses of NBQX prevented this distal damage caused by quisqualate, but not that caused by kainate. With AMPA, only the high dose of NBQX blocked the distal toxicity. Diazepam also blocked the distal toxicity of AMPA, but had only a minor effect on the hippocampal damage.(ABSTRACT TRUNCATED AT 250 WORDS)

Intrathecal administration of non-NMDA receptor agonists increases arterial pressure and heart rate in the rat.

We have found that spinal NMDA receptors are involved in control of sympathetic output in pathways to the heart and vessels. The present study was done to determine whether spinal non-NMDA excitatory amino acid receptors participate in cardiovascular regulation. Experiments were done on urethane-anesthetized Sprague-Dawley rats, giving the non-NMDA receptor agonists, quisqualate and kainate, and the antagonist, kynurenate, intrathecally at the spinal T9 level. Both quisqualate (30 nmol; n = 7; to activate AMPA receptors) and kainate (2 nmol; n = 6; to activate K receptors) increased arterial pressure and heart rate. The responses were characterized by a rapid onset, achieving, in most cases, greater than 80% of the maximum response within 1-4 min, and a persistence throughout the remaining 20-24 min of the experiment. I.v. injection of hexamethonium (10 mg/kg) prevented the effects of intrathecal administration of quisqualate (n = 5) but not of kainate (n = 7). To determine whether the hexamethonium-resistant effects of kainate were due to a peripheral action, kainate was given i.v. (n = 6); it was found to be without effect on arterial pressure or heart rate. The increases in arterial pressure and heart rate produced by intrathecal administration of quisqualate (30 nmol; n = 6), kainate (2 nmol; n = 6), glutamate (1 mumol; n = 6) and NMDA (2 nmol; n = 6) but not carbachol (27.4 nmol; n = 6) were prevented by similar preadministration of kynurenate (125 nmol). Intrathecal administration of kynurenate (125 nmol; n = 6; 500 nmol; n = 7) decreased arterial pressure and/or heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)