Characterisation of the binding of [3H]FR115427, a novel non-competitive NMDA receptor antagonist, to rat brain membranes

The binding of [3H]FR115427 ([3H](+)-1-methyl-1-phenyl-1,2,3,4- tetrahydroisoquinoline) to rat cortical synaptosomal membranes was investigated. Binding was optimal at pH 7.4-8.0, and temperature had little effect on specific binding. Binding reached equilibrium within 30 min at 25 degrees C, and was reversible in the presence of excess unlabelled FR115427. [3H]FR115427 bound to a single population of non-interacting sites with an affinity of 45.4 +/- 3.9 nM, and a binding site density of 9.12 +/- 0.52 pmol/mg protein. The affinities of other N-methyl-D-aspartate (NMDA) receptor channel blockers for [3H]FR115427 binding sites were consistent with binding to a similar site to that occupied by dizocilpine. Binding was potentiated by L-glutamate and glycine with EC50 values of around 80 nM. In the presence of L-glutamate (10 microM), specific binding was increased 4-fold, whilst addition of glycine (10 microM) increased specific binding 2-fold. FR115427 exhibited marked stereoselectivity; (+)-FR115427 has 100-fold higher affinity than (-)-FR115427. This ligand may therefore be useful for the pharmacological investigation of the NMDA receptor ion channel.

Neurochemical and electrophysiological studies on FR115427, a novel non-competitive NMDA receptor antagonist

The pharmacological profile of FR115427 has been examined using ligand binding and electrophysiological techniques. Binding of [3H]dizocilpine in the presence of L-glutamate was inhibited by the (+) isomers of dizocilpine and FR115427. The corresponding (-) isomers were less active, and stereoselectivity was particularly marked in the case of FR115427. In contrast to dizocilpine, the affinity of FR115427 for [3H]dizocilpine binding sites was little affected by addition of either L-glutamate and/or glycine. In a cortical wedge preparation, FR115427 inhibited N-methyl-D-aspartate (NMDA)-induced responses in a non-competitive, use-dependent manner. Intracellularly recorded excitatory synaptic responses in hippocampal neurones were only partially inhibited by FR115427 thereby confirming a selective effect on the NMDA-mediated component of neuronal excitation induced by the endogenous neurotransmitter. The data suggest that FR115427 is a non-competitive, use-dependent NMDA receptor antagonist with more pronounced stereoselectivity and less marked use dependence than dizocilpine.

Binding of 5-HT1A receptor and 5-HT transporter ligands in rat cortex and hippocampus following cholinergic and serotonergic lesions

The cellular localisation of 5-HT1A receptor and 5-HT transporter binding sites in the rat cortex and hippocampus has been examined. Lesions of either basal forebrain neurones or serotonergic neurones did not affect [3H]8-OH-DPAT binding, suggesting that 5-HT1A binding sites are not localised on cholinergic or serotonergic nerve terminals. The binding of the 5-HT transporter ligand, [3H]citalopram was unaffected by the cholinergic lesion whereas binding was reduced in both the hippocampus and cortex following serotonergic lesions. A reduction in binding site density rather than an alteration in affinity was responsible for this effect. While these data suggest that [3H]citalopram binding sites are located on serotonergic nerve terminals, the abolition of hippocampal binding sites contrasted with a 50% loss in cortical tissue.

Synaptosomal plasma membrane transport of excitatory sulphur amino acid transmitter candidates: Kinetic characterisation and analysis of carrier specificity

The transport kinetics of the excitatory sulphur-containing amino acid (SAA) transmitter candidates, L-cysteine sulphinate (L-CSA), L-cysteate (L-CA), L-homocysteine sulphinate (L-HCSA), and L-homocysteate (L-HCA), together with their plasma membrane carrier specificity, was studied in cerebrocortical synaptosome fractions by a sensitive high performance liquid chromatographic assay. A high affinity uptake system could be demonstrated for L-CSA (Km = 57 +/- 6 microM; Vmax = 1.2 +/- 0.1 nmol/min/mg protein) and L-CA (Km = 23 +/- 3 microM; Vmax = 3.6 +/- 0.1 nmol/min/mg protein), whereas L-HCSA (Km = 502 +/- 152 microM; Vmax = 6.1 +/- 1.3 nmol/min/mg protein) and L-HCA (Km = 1550 +/- 169 microM; Vmax = 10.3 +/- 1.1 nmol/min/mg protein) exhibited much lower affinity as transport substrates. In all cases, only a single, saturable Na(+)-dependent component of uptake could be identified, co-existing with a non-saturable, Na(+)-independent influx component. Plasma membrane carrier specificity of the SAAs was established following comparison with other high-affinity neurotransmitter systems. High-affinity L-CSA and L-CA transport and low-affinity L-HCSA and L-HCA transport demonstrate strong positive correlations in inhibition profiles when compared against each other or individually against the high-affinity transport of L-[3H]glutamate, L-[3H]aspartate, or D-[3H]aspartate. Moreover, the transport systems for the excitatory SAAs exhibited a negative correlation when compared in inhibition profiles with the high affinity transport of both [3H] gamma-aminobutyric acid (GABA) and [3H]taurine.(ABSTRACT TRUNCATED AT 250 WORDS)

Homocysteine-induced alterations in extracellular amino acids in rat hippocampus

The effects of DL-homocysteine, and DL-homocysteate, on extracellular levels of amino acids in the rat hippocampus have been studied using brain microdialysis. Hippocampal electroencephalogram activity was monitored simultaneously using an electrode attached to the dialysis probe. DL-Homocysteine (1200 mg/kg; i.p. injection) produced epileptic activity in hippocampus in an inconsistent manner. Alterations in electroencephalogram activity were not observed in urethane anaesthetized animals, whereas 50% of Hypnorm anaesthetized animals exhibited epileptic activity. DL-Homocysteate (2 mu mol; i.c.v.) induced epileptic activity in a majority of animals anaesthetized using urethane. Dialysate levels of aspartate were significantly elevated by homocysteine in both groups of animals. Conversely, dialysis levels of GABA were reduced. Dialysate levels of other amino acids measured (glutamate, glutamine, taurine, alanine and valine) were not affected significantly. Dialysate levels of taurine were increased significantly in animals injected with homocysteate. These data suggest that the imbalance in excitatory:inhibitory neurotransmission in the hippocampus caused by these alterations in extracellular levels of neuroexcitatory (i.e. aspartate) and neuroinhibitory (i.e. GABA) transmitters could underly the epileptic effect of homocysteine.

The NMDA receptor antagonist D-2-amino-5-phosphonopentanoate (D-AP5) impairs spatial learning and LTP in vivo at intracerebral concentrations comparable to those that block LTP in vitro

This series of experiments investigated whether the NMDA receptor antagonist D-2-amino-5-phosphonopentanoate (D-AP5) could induce impairments of spatial learning across a dose range comparable to its impairment of hippocampal long-term potentiation (LTP) in vivo. Estimations of the extracellular concentration of D-AP5 in hippocampus using microdialysis were also made to compare whether these impairments occur at concentrations similar to those required to impair LTP in the in vitro hippocampal slice. Rats were chronically infused with D-AP5 into the lateral ventricle at a range of concentrations (0-50 mM) via osmotic minipumps. They were first trained to find and escape onto a hidden platform in an open-field water maze task. After the behavioral learning, they were anesthetized with urethane and an attempt was made to evoke and monitor hippocampal LTP. Extracellular samples of D-AP5 in hippocampus were then taken using microdialysis, and finally, the animals were killed and tissue samples dissected. The microdialysis and tissue samples were analyzed for D-AP5 content using HPLC with fluorescence detection. The results established, first, that D-AP5 impairs spatial learning in a linear dose-dependent manner, highly correlated with its corresponding impairment of hippocampal LTP in vivo. No concentration of D-AP5 was observed to block LTP without affecting learning. Second, the microdialysis estimates indicated that, subject to certain assumptions, D-AP5 causes these impairments at extracellular concentrations comparable to those that impair LTP in vitro. Third, comparison of the whole tissue and microdialysis samples revealed a concentration ratio of approximately 30:1, indicating that 97% of the intracerebral D-AP5 is inaccessible to the dialysis probes. Infusion of 20 mM EGTA was found to cause a sevenfold increase in D-AP5 in the dialysis perfusates, suggesting that at least part of the inaccessible D-AP5 is trapped by a calcium-dependent mechanism. Two further behavioral control studies indicated that the D-AP5-induced impairment of spatial learning is unlikely to be secondary to a drug-induced motor disturbance, and that the performance of the D-AP5 group whose concentration was just sufficient to block hippocampal LTP completely was statistically indistinguishable from that of a group of rats with bilateral hippocampal lesions induced by ibotenic acid. Taken together, these findings offer support for the hypothesis that activation of NMDA receptors is necessary for certain kinds of learning.

Effects of hypoxia on fetal rat brain metabolism studied in utero by 31P-NMR spectroscopy

An animal model of perinatal asphyxia, in which near-term fetal rats are subjected to-short periods of hypoxia, has been investigated by 31P-NMR spectroscopy. Changes in the high-energy phosphates and intracellular pH of the fetal rat brain were measured in utero following ligation of the placental blood vessels, and during reperfusion after a 20-min period of occlusion. The hypoxia-induced changes observed in the fetal brain were substantially slower than in the adult, and were completely reversible after 20 min of hypoxia.

Correlation of the extracellular glutamate concentration with extent of blood flow reduction after subdural hematoma in the rat

The excitatory neurotransmitters glutamate and aspartate are an important factor in the causation of ischemic brain damage. The concentration of glutamate and aspartate was serially measured in extracellular fluid using in vivo microdialysis after induction of a subdural hematoma or after a sham operation in the rat. Measurements were made in the cortex underlying the hematoma and in the ipsilateral hippocampus, and these findings were correlated with regional cerebral blood flow (CBF), measured autoradiographically 2 hours after hematoma induction. In the severely ischemic cortex underlying the hematoma (mean CBF less than 25 ml/100 gm/min), glutamate and aspartate content increased more than 750% over basal levels. In individual animals the magnitude of glutamate release correlated with the extent of the focal ischemic zone under the hematoma (r = 0.907). Hippocampal glutamate levels rose 339%, yet regional CBF was preserved (114 ml/100 gm/min). This accords with focal hypermetabolism in this model, and may imply a glutamate-mediated "excitotoxic" process after subdural hematoma.

Intracerebral distribution of DL-2-amino-phosphonopentanoic acid (AP5) and the dissociation of different types of learning

Chronic intraventricular infusion of the selective NMDA receptor antagonist AP5 appears to cause an impairment of spatial but not visual discrimination learning. However, Goddard (1986) has questioned whether this dissociation in task-selectivity reflects a difference in the underlying neural mechanisms or differential drug diffusion. Two experiments conducted to address this issue established (a) that chronic intraventricular infusion of AP5, at a dose sufficient to cause a spatial learning impairment, results in a relatively uniform distribution of the drug across the brain, and (b) that chronic bilateral intracortical infusion at sites very close to visual cortex also fails to impair visual discrimination learning. These findings argue against differential diffusion being a major cause of the sensitivity of spatial but not visual discrimination tasks to AP5, and raises the possibility that representational and procedural memory tasks may depend upon distinct cell-biological mechanisms of plasticity.

Characterisation of methylphenidate and nomifensine induced dopamine release in rat striatum using in vivo brain microdialysis

The in vivo effects of methylphenidate (50 mg/kg) and nomifensine (20 mg/kg) on dopamine release and metabolism in the rat striatum were studied using brain microdialysis. Dialysate catecholamine content was measured by high pressure liquid chromatography with electrochemical detection. Both compounds increased the dialysate content of dopamine (430% and 320% of basal efflux, respectively). Release was inhibited by reducing extracellular calcium levels and by infusion of tetrodotoxin (2 microM) via the dialysis probe (calcium free-39 +/- 18% of control, methylphenidate; 40 +/- 17% of control, nomifensine; tetrodotoxin-35 +/- 19% of control, methylphenidate; 40 +/- 14% of control, nomifensine) and also by prior depletion of dopamine storage pools using reserpine (5 mg/kg) (15 +/- 12% of control, methylphenidate; 19 +/- 9% of control, nomifensine). Dialysate levels of dihydroxyphenylacetic acid were not altered by either drug whereas homovanillic acid levels increased. These data suggest that both drugs increase dialysate dopamine content by facilitating Ca2(+)-dependent vesicular release most probably by inhibition of dopamine reuptake.

Evidence for functional separation of alpha-1 and alpha-2 noradrenaline receptors by pre-synaptic terminal re-uptake mechanisms

Information transfer within the central nervous system is predominantly chemical in nature, and occurs both through synaptic specialisations and non-specific diffuse release. The localisation and description of receptors for these two types of neurotransmission is currently a contentious issue. In the present study, the noradrenaline reuptake inhibitor cocaine has been shown to overcome the inhibitory effects of idazoxan, a selective alpha-2 receptor antagonist, but not phentolamine, a non-selective alpha receptor antagonist, on eating following injection of noradrenaline into the rat hypothalamic paraventricular nucleus. Similarly, lesion by 6-hydroxydopamine of noradrenaline terminals in the paraventricular nucleus also reduced the efficacy of idazoxan in blocking eating induced by noradrenaline. These data confirm that postsynaptic alpha-2 receptors are involved in the feeding response to exogenous noradrenaline, but in addition, when taken in conjunction with previously published data, are used to suggest a differential distribution of NA receptors within the hypothalamic paraventricular nucleus. It is proposed that the alpha-2 subtype may be extrasynaptic, the alpha-1 subtype intrasynaptic.

Correlation between amino acid release and neuropathologic outcome in rat brain following middle cerebral artery occlusion

Using in vivo brain microdialysis, we studied amino acid release in the striatum and cortex of eight rats following permanent middle cerebral artery occlusion. We then processed all brains for histopathologic assessment of the volume of ischemic damage 4 hours after occlusion. Ischemic damage was varied by occlusion of the middle cerebral artery at a point either proximal (n = 4) or distal (n = 4) to the lenticulostriate vessels. Proximal occlusion elevated the dialysate contents of all amino acids. The largest increases occurred for the potentially neurotoxic amino acids aspartate and glutamate and for taurine (800-2,800% of basal efflux). We observed smaller increases for the "metabolic" amino acids (280-580% of basal efflux). Distal occlusion did not affect amino acid efflux in the striatum, and release in the cortex was significantly lower than that following proximal occlusion. We compared release data with acute histopathologic outcome. Proximal occlusion resulted in a large volume of ischemic damage in the cortex and striatum (25-48% of hemispheric volume). A smaller volume of ischemic damage was noted following distal occlusion (0-21% of hemispheric volume). The volume of ischemic damage and the amount of amino acid release were significantly correlated (p less than 0.05).

A dose-related impairment of spatial learning by the NMDA receptor antagonist, 2-amino-5-phosphonovalerate (AP5)

This paper describes an experiment which further tested the hypothesis that activation of N-methyl-D-aspartate receptors plays an important role in the neural mechanisms underlying certain kinds of learning and memory. The results demonstrate that a selective N-methyl-D-aspartate receptor antagonist, D-2-amino-5-phosphonovalerate, causes a dose-related and correlated impairment of both spatial learning and hippocampal long term potentiation in vivo. No dose of D-2-amino-5-phosphonovalerate was found to block long term potentiation without affecting spatial learning. These data therefore support the proposed link between long term potentiation and certain kinds of learning, and the role of N-methyl-D-aspartate receptors in both processes.

Brain microdialysis studies on the control of dopamine release and metabolism in vivo

This paper studies the actions of drugs known to release dopamine from brain tissue. Most of the theoretical background to this work has been developed in experiments on slices of brain in vitro but using in vivo microdialysis we have elaborated and extended the ideas from the in vitro experiments and been able to make a direct comparison of the mechanism of action and source of dopamine released by 5 different manipulations. The mode of action of tyramine, amphetamine, veratrine, ouabain and potassium is discussed in the light of the computer model of the nerve terminal published by Justice et al. (1988). The data may lend themselves to such an interpretation, but they could be compatible with several other models.

Effects of selective monoamine oxidase inhibitors on the in vivo release and metabolism of dopamine in the rat striatum

Brain microdialysis was used to examine the in vivo efflux and metabolism of dopamine (DA) in the rat striatum following monoamine oxidase (MAO) inhibition. Relevant catecholamines and indoleamines were quantified by HPLC coupled with a electrochemical detection system. The MAO-B inhibitor selegiline only affected DA deamination at a dose shown to inhibit partially type A MAO. Alterations in DA and metabolite efflux were not observed when using the MAO-B-selective dose of 1 mg/kg of selegiline. At 10 mg/kg, selegiline reduced the efflux of DA metabolites to approximately 70% of basal values without affecting DA efflux. K(+)- and veratrine-stimulated DA efflux was not affected by selegiline. Experiments using amphetamine and the DA uptake inhibitor nomifensine demonstrated that the effect of selegiline on DA metabolism was unlikely to be mediated either by inhibition of DA uptake or by an indirect effect of its metabolite amphetamine. The possibility that the effect of selegiline is mediated via a nonspecific inhibition of MAO is discussed. In contrast, the MAO-A inhibitor clorgyline inhibited basal DA metabolism and increased basal and depolarisation-induced DA efflux. A 1 mg/kg dose of clorgyline reduced basal DA metabolite efflux (40-60% of control values) without affecting DA efflux. At 10 mg/kg of clorgyline, DA efflux increased to 253 +/- 19% of basal values, whereas efflux of DA metabolites was reduced to between 15 and 26% of control values. The release of DA induced by K+ and veratrine was not affected by 1 mg/kg of clorgyline but was increased by approximately 200% following pretreatment with 10 mg/kg of clorgyline. The nonselective MAO inhibitor pargyline caused similar but more pronounced alterations in these parameters.(ABSTRACT TRUNCATED AT 250 WORDS)

Hippocampal synaptic plasticity and NMDA receptors: a role in information storage?

There has recently been renewed interest in the idea that alterations in synaptic efficacy may be the neural basis of information storage. Particular attention has been focused upon long-term potentiation (LTP), a long-lasting, but experimentally induced synaptic change whose physiological properties point to it being a candidate memory mechanism. However, considerations of storage capacity and the possibility of concomitant activity-dependent synaptic depression make it unlikely that individual learning experiences will give rise to gross changes in field potentials similar to those that occur in LTP, even if learning and LTP utilize common neural mechanisms. One way of investigating the functional significance of LTP is to use selective antagonists of those excitatory amino acid receptors whose activation is essential for its induction. This paper discusses various design requirements for such experiments and reviews work indicating that the N-methyl-D-aspartate receptor antagonist AP5 causes a behaviourally selective learning impairment having certain common features to the behavioural profile seen after hippocampal lesions. Two new studies are described whose results show that AP5 has no effect upon the retrieval of previously established memories, and that the dose-response profile of the impairment of spatial learning occurs across a range of extracellular concentrations in hippocampus for which receptor selectivity exists. These experiments show that activation of NMDA receptors is essential for certain kinds of learning.

In vivo mechanisms underlying dopamine release from rat nigrostriatal terminals: II. Studies using potassium and tyramine

The brain microdialysis technique has been used to examine the in vivo effects of potassium and tyramine on dopamine (DA) release and metabolism in the striatum of halothane-anaesthetised rats. Increasing the concentration of potassium perfusing the dialysis probe (30-120 mM) induced a dose-related efflux of DA. A dose-related release of DA was also observed following addition of tyramine (1-100 microM) to the perfusing buffer. High concentrations of potassium were found to reduce the dialysate content of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid and the serotonin metabolite 5-hydroxyindoleacetic acid. No such effect was observed even when using the highest concentration of tyramine tested. Potassium-evoked DA release was facilitated by pretreatment with the DA uptake inhibitor nomifensine, was inhibited by depletion of extracellular calcium, and was not significantly affected by tetrodotoxin (TTX). The effect of tyramine on DA efflux was inhibited by nomifensine and was insensitive to both TTX and calcium depletion. These data suggest that potassium and tyramine induce release of DA via different mechanisms. Potassium-induced DA release involves a carrier-independent process and may utilise an exocytotic release mechanism. On the other hand, tyramine-induced DA release would appear to involve a carrier-dependent process. Depletion of vesicular stores of DA by pretreatment with reserpine did not significantly affect potassium-induced DA release, whereas a marked inhibition of the effects of tyramine was noted. However, in reserpinised animals the potassium-induced release of DA was inhibited by nomifensine, a result suggesting that a carrier-dependent release mechanism operates in the absence of vesicular DA.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo mechanisms underlying dopamine release from rat nigrostriatal terminals: I. Studies using veratrine and ouabain

The in vivo mechanisms underlying the dopamine (DA)-releasing actions of veratrine and ouabain in the striatum of halothane-anaesthetised rats have been investigated using brain microdialysis. Relevant catecholamines and indoleamines were separated and quantified using HPLC combined with an electrochemical detection system. Veratrine (10 micrograms/ml-1 mg/ml) and ouabain (10 microM-1 mM) were added to the medium perfusing the dialysis probes. Both compounds increased dialysate DA content in a dose-related manner. Dialysate levels of the DA metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid and the serotonin metabolite 5-hydroxyindoleacetic acid were reduced by both veratrine and ouabain. Veratrine-induced DA efflux was maximal in the first 20-min sample collected after drug infusion began, whereas the maximal effect of ouabain was not observed until 20-40 min after administration began. Veratrine-induced DA efflux was unaffected by systemic injection of the DA uptake inhibitor nomifensine but was inhibited by either coperfusion of tetrodotoxin (TTX) or removal of calcium from the perfusing buffer. These data suggest that veratrine induces release of DA via a carrier-independent mechanism, perhaps involving an exocytotic release process. In contrast, ouabain-induced DA release was reduced by nomifensine but was inhibited to a lesser degree by calcium depletion and TTX. Detailed analyses of these data suggest that although ouabain initially induces release of DA via a carrier-dependent mechanism, an exocytotic process may also be involved. The finding that ouabain-induced DA efflux exhibits a degree of TTX and calcium sensitivity suggests that membrane depolarisation caused by Na+,K(+)-ATPase blockade opens voltage-gated sodium channels and initiates an exocytotic release of DA. The intracellular pools of DA involved in the release of DA induced by veratrine and ouabain were also examined. Depletion of vesicular pools of DA by pretreatment with reserpine reduced the amount of DA release induced by both agents, although this effect was only significant in the case of veratrine. However, in reserpinised animals the residual amount of DA release induced by veratrine was inhibited by nomifensine, a result suggesting that DA may be released via a carrier-dependent process in the absence of vesicular DA. Newly synthesised pools of DA were also depleted by pretreatment with the DA synthesis inhibitor alpha-methyl-p-tyrosine. Under these conditions, both veratrine- and ouabain-induced DA efflux was reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Dopamine release and metabolism in the rat striatum: an analysis by 'in vivo' brain microdialysis

Brain microdialysis studies on the mechanisms underlying dopamine release in the rat striatum provide evidence that both exocytotic and carrier-dependent processes operate in vivo. While several releasers (potassium, veratrine, amphetamine, ouabain) utilize newly synthesized stores of dopamine, tyramine is uniquely sensitive to depletion of vesicular storage by reserpine. Extracellular DOPAC is closely associated with the newly synthesized pool of dopamine and experiments with selective monoamine oxidase inhibitors suggest that DOPAC is formed mainly by MAO-A. Recent work on the two dopamine receptors suggest that release by different mechanisms may selectively activate D1 or D2 receptor subtypes.

In vivo studies on the enhancement of cholecystokinin release in the rat striatum by dopamine depletion

The release of cholecystokinin-8-like (CCK) immunoreactivity from the rat striatum has been studied in vivo using brain microdialysis. A basal efflux of CCK-like immunoreactivity was not detectable in the majority of experiments. Intrastriatal infusion of veratrine (100 micrograms/ml) increased striatal dialysate levels of CCK-like immunoreactivity above detection limits, representing an overflow into the interstitial fluid. High concentrations of potassium caused similar but less consistent effects. Extracellular dopamine depletion using alpha-methyl-p-tyrosine or reserpine also increased the dialysate content of CCK-like immunoreactivity. In contrast, inhibition of peptidases reported to hydrolyse CCK in vitro did not affect either basal or evoked efflux of CCK-like immunoreactivity. These data demonstrate that CCK-like immunoreactivity may be released from neuronal elements within the striatum by depolarizing stimuli in vivo, and suggest that increased overflow of CCK-like immunoreactivity is associated with dopamine depletion.

On the epileptogenic effects of kainic acid and dihydrokainic acid in the dentate gyrus of the rat

The in vivo effects of the acidic amino receptor agonist, kainic acid and the inhibitors of the uptake of glutamate, dihydrokainic acid and threo-3-hydroxyaspartate, on spontaneous activity and perforant path evoked field potentials were examined in the dentate gyrus of the rat. The effect of these compounds on extracellular levels of endogenous amino acids in the hippocampus was assessed simultaneously using in vivo microdialysis. Kainic acid (10-100 microM) and dihydrokainic acid (1-10 mM) both evoked epileptiform activity and an apparent loss of recurrent inhibition (as assessed using the paired-pulse technique). Extracellular increases in taurine, alanine and phosphoethanolamine were noted following administration of kainate (100 microM) and dihydrokainate (1-10 mM). An increase in extracellular glutamate and aspartate was also noted in rats treated with dihydrokainate (100 microM-10 mM). In contrast, threo-3-hydroxyaspartate did not induce epileptiform activity, suggesting that the epileptogenic effects of dihydrokainate and kainate are not mediated by inhibition of uptake. The effect of the N-methyl-D-aspartate receptor antagonist, D-2-amino-5-phosphonovalerate on these responses was studied. This compound attenuated the epileptiform activity and reversed the apparent loss of recurrent inhibition in response to both kainic acid and dihydrokainic acid. These data suggest that activation of N-methyl-D-aspartate receptors underlies the epileptogenic effects of both compounds, and the possible mechanisms which might be involved in this response are discussed.

Amphetamine-induced dopamine release in the rat striatum: an in vivo microdialysis study

The effects of a number of biochemical and pharmacological manipulations on amphetamine (AMPH)-induced alterations in dopamine (DA) release and metabolism were examined in the rat striatum using the in vivo brain microdialysis method. Basal striatal dialysate concentrations were: DA, 7 nM; dihydroxyphenylacetic acid (DOPAC), 850 nM; homovanillic acid (HVA), 500 nM; 5-hydroxyindoleacetic acid (5-HIAA), 300 nM; and 3-methoxytyramine (3-MT), 3 nM. Intraperitoneal injection of AMPH (4 mg/kg) induced a substantial increase in DA efflux, which attained its maximum response 20-40 min after drug injection. On the other hand, DOPAC and HVA efflux declined following AMPH. The DA response, but not those of DOPAC and HVA, was dose dependent within the range of AMPH tested (2-16 mg/kg). High doses of AMPH (greater than 8 mg/kg) also decreased 5-HIAA and increased 3-MT efflux. Depletion of vesicular stores of DA using reserpine did not affect significantly AMPH-induced dopamine efflux. In contrast, prior inhibition of catecholamine synthesis, using alpha-methyl-p-tyrosine, proved to be an effective inhibitor of AMPH-evoked DA release (less than 35% of control). Moreover, the DA releasing action of AMPH was facilitated in pargyline-pretreated animals (220% of control). These data suggest that AMPH releases preferentially a newly synthesised pool of DA. Nomifensine, a DA uptake inhibitor, was an effective inhibitor of AMPH-induced DA efflux (18% of control). On the other hand, this action of AMPH was facilitated by veratrine and ouabain (200-210% of control). These results suggest that the membrane DA carrier may be involved in the actions of AMPH on DA efflux.

In vivo microdialysis studies on the effects of decortication and excitotoxic lesions on kainic acid-induced calcium fluxes, and endogenous amino acid release, in the rat striatum

The in vivo effects of kainate (1 mM) on fluxes of 45Ca2+, and endogenous amino acids, were examined in the rat striatum using the brain microdialysis technique. Kainate evoked a rapid decrease in dialysate 45Ca2+, and an increase in the concentration of amino acids in dialysates in Ca2+-free dialysates. Taurine was elevated six- to 10-fold, glutamate two- to threefold, and aspartate 1.5- to twofold. There was also a delayed increase in phosphoethanolamine, whereas nonneuroactive amino acids were increased only slightly. The kainic acid-evoked reduction in dialysate 45Ca2+ activity was attenuated in striata lesioned previously with kainate, suggesting the involvement of intrinsic striatal neurons in this response. The increase in taurine concentration induced by kainate was slightly smaller under these conditions. Decortication did not affect the kainate-evoked alterations in either dialysate 45Ca2+ or amino acids. These data suggest that kainate does not release acidic amino acids from their transmitter pools located in corticostriatal terminals.

DL-2-[3,4-3H]amino-4-phosphonobutyrate binding sites in the rat hippocampus: distribution and possible physiological role

Binding sites for the novel, glutamate-like radioligand DL-2-[3,4-3H]amino-4-phosphonobutyrate (DL-[3H]APB) on rat hippocampal synaptic membranes were identified and characterised. The existence of a single, saturable population of binding sites was demonstrated. These appeared to be indistinguishable, in terms of their pharmacological profile and ionic dependence, from those described previously in the striatum and whole brain. The distribution of these sites was also examined using a number of discrete neuronal lesions. A majority of sites (approx. 55%) were located on dentate gyrus granule cells. Smaller populations appeared to be situated on perforant path terminals and on pyramidal cells. However, L-APB was found to be ineffective as an inhibitor of basal and potassium evoked D-[3H]aspartate release from hippocampal slices. A presynaptic location can therefore presumably be ruled out. The likely postsynaptic location of DL-[3H]APB-binding sites in the hippocampus suggests that this site may be involved in synaptic neurotransmission. This possibility is discussed with regard to electrophysiological data concerning the synaptic pharmacology of neuronal connections within the hippocampus.

2-Amino-5-phosphonovalerate attenuates the severe hypoglycemia-induced loss of perforant path-evoked field potentials in the rat hippocampus

The effects of severe hypoglycemia on perforant path-evoked field potentials were examined in the rat hippocampus. Although a complete loss of this response was noted when blood glucose concentration fell below 1 mM, this occurred before cessation of electroencephalogram (EEG) activity. Both spontaneous and evoked responses recovered partially following glucose readministration. D-2-Amino-5-phosphonovalerate, an NMDA-sensitive acidic amino acid receptor antagonist, facilitated this recovery from the hypoglycemic challenge when administered via a dialysis probe.