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T W Stone

Publications and source records attributed to T W Stone.

At least 73 records · Page 4Linked to original sources
PubMed1998

The effects of adenine dinucleotides on epileptiform activity in the CA3 region of rat hippocampal slices.

Alpha, omega-adenine dinucleotides (Ap(n)A) consist of two adenosine molecules linked at the 5' position by phosphate groups, the number of which is denoted by n and can range from 2 to 6. The aim of this study was to investigate the effect of Ap4A and Ap5A on the rate of epileptiform activity. Hippocampal slices (450 microm), when perfused with a medium containing no added magnesium and 4-aminopyridine (50 microM), generate epileptiform activity of an interictal nature. Ap4A and Ap5A at 1 microM depressed the discharge rate to a significant extent. At this concentration adenosine (1 microM) did not produce any effect. However at 10 microM adenosine, Ap4A and Ap5A all decreased the burst frequency. Adenosine deaminase (0.2 U/ml) totally annulled the inhibition of epileptiform activity produced by 10 microM adenosine or 1 microM Ap4A and Ap5A. Adenosine deaminase did not significantly change the maximum depression of activity produced by 10 microM Ap4A and Ap5A. 8-cyclopentyl-1,3-dimethylxanthine, an A1, receptor antagonist, increased the basal rate of epileptiform activity and prevented the depression of burst discharges by Ap4A. 5'-adenylic acid deaminase converts AMP into IMP which is inactive. 5'-adenylic acid deaminase did not prevent the inhibitory effects of Ap4A. The results suggests that in the CA3 region of the hippocampus, Ap4A and Ap5A act partly by stimulating xanthine-sensitive receptors directly and partly through the formation of the metabolite, adenosine.

AMP Deaminase↗
PubMed1998

Extracellular levels of dopamine and its metabolite 3,4-dihydroxy-phenylacetic acid measured by microdialysis in the corpus striatum of conscious AS/AGU mutant rats.

The AS/AGU rat is a mutant derived from the Albino Swiss (AS) strain. It is characterized by an ungainly, staggering gait, hind limb rigidity, whole body tremor and, in older animals, difficulty in initiating movement. As and AS/AGU males aged three, six and nine months (n=6 per group) were used to estimate the levels of dopamine and its metabolites in the extracellular fluid of the caudate-putamen. The results indicate a profound loss of dopamine in the extracellular fluid at all age points examined, together with an increase in the concentration of the metabolite 3,4-dihydroxyphenylacetic acid. It is suggested that these changes reflect a defect of dopaminergic neuron function which may underlie the motor disorder seen in these animals.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1998

Epileptiform activity in supragranular and infragranular blocks of mouse neocortex.

Murine neocortical tissue blocks 450 microm in diameter displayed prolonged epileptiform bursts when exposed to magnesium-free medium in vitro; no evidence was found for a correlation between the ability to generate paroxysmal bursts and barrels, the specialized cortical modules or 'columns' found in rodent primary sensorimotor cortex. While previous reports suggested that preferential locations for the generation of epileptiform bursts exist within the six-layered structure of the neocortex, we find that cortical sections only containing either the supragranular or infragranular layers and an estimated 2000 neurones are capable of displaying epileptiform bursting. The subdivided blocks are markedly more sensitive to the inhibitory effects of bath-applied adenosine (100 microM) than intact blocks from the same animal. A similar enhancement of adenosine sensitivity is seen when the tissue is superfused with hypotonic ACSF (a model for injury-induced brain swelling), which by itself increases excitability. The increase in sensitivity to adenosine is a transient effect and may be related to its role as acute neuroprotective agent.

Adenosine↗
PubMed1998

Modulation by adenine nucleotides of epileptiform activity in the CA3 region of rat hippocampal slices.

1. Hippocampal slices (450 microm) generate epileptiform bursts of an interictal nature when perfused with a zero magnesium medium containing 4-aminopyridine (50 microM). The effect of adenine nucleotides on this activity was investigated. 2. ATP and adenosine depressed this epileptiform activity in a concentration-dependent manner, with both purines being equipotent at concentrations above 10 microM. 3. Adenosine deaminase 0.2 u ml(-1), a concentration that annuls the effect of adenosine (50 microM), did not significantly alter the depression of activity caused by ATP (50 microM). 4. 8-Cyclopentyl-1,3-dimethylxanthine (CPT), an A1 receptor antagonist, enhanced the discharge rate significantly and inhibited the depressant effect of both ATP and adenosine such that the net effect of ATP or adenosine plus CPT was excitatory. 5. Several ATP analogues were also tested: alpha, beta-methyleneATP (alpha, beta-meATP), 2-methylthioATP (2-meSATP) and uridine triphosphate (UTP). Only alpha, beta-meATP (10 microM) produced an increase in the frequency of spontaneous activity which suggests a lack of involvement of P2Y or P2U receptors. 6. Suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), P2 receptor antagonists, failed to inhibit the depression produced by ATP (50 microM). The excitatory effect of alpha, beta-meATP (10 microM) was inhibited by suramin (50 microM) and PPADS (5 microM). 7. ATP therefore depresses epileptiform activity in this model in a manner which is not consistent with the activation of known P1 or P2 receptors, suggesting the involvement of a xanthine-sensitive nucleotide receptor. The results are also indicative of an excitatory P2X receptor existing in the hippocampal CA3 region.

Adenine Nucleotides↗
PubMed1998

Adenosine monophosphate as a mediator of ATP effects at P1 purinoceptors.

1. When perfused with a medium containing no added magnesium and 4-aminopyridine (4AP) (50 microM) hippocampal slices generated epileptiform bursts of an interictal nature. We have shown in a previous study that adenosine 5'-triphosphate (ATP) depressed epileptiform activity and that this effect was blocked by the adenosine A1 receptor antagonist cyclopentyltheophylline but was not affected by adenosine deaminase. This implied that ATP might act indirectly at P1 receptors or at a xanthine-sensitive P2 receptor. The aim of the present study was to investigate further the action of ATP on epileptiform activity. 2. ATP can be metabolized by ecto-nucleotidases to adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP) and adenosine, respectively. Each of these metabolites can activate receptors in its own right: P2 receptors for ADP and P1 receptors for AMP and adenosine. 3. We now show that both AMP and ATP (50 microM) significantly decrease epileptiform discharge rate in a rapid and reversible manner. 5'Adenylic acid deaminase (AMP deaminase, AMPase) (0.2 u ml(-1)), when perfused alone did not significantly alter the discharge rate over the 10 min superfusion period used for drug application. When perfused concurrently with AMP (50 microM), AMP deaminase prevented the depressant effect of AMP on discharge rate. 4. AMP deaminase, at a concentration of 0.2 u ml(-1) which annulled the effect of AMP (50 microM), prevented the inhibitory activity of ATP (50 microM). A higher concentration of ATP (200 microM) depressed the frequency of spontaneous bursts to approximately 30% control and this response was also prevented by AMP deaminase. 5. Superfusion of the slices with 5'-nucleotidase also prevented the inhibitory activity of ATP on epileptiform discharges. 6. The results suggest that AMP mediates the inhibitory effects of ATP on epileptiform activity, a conclusion which can explain the earlier finding that cyclopentyltheophylline but not adenosine deaminase inhibited the effect of ATP. A corollary to this is that, when examining the pharmacology of ATP, care must be taken to inactivate AMP with AMP deaminase, as well as adenosine with adenosine deaminase, before a direct action of ATP on P1 receptors can be postulated. Failure to do so may have led to erroneous conclusions in some previous studies of nucleotide activity on nucleotide receptors.

5'-Nucleotidase↗
PubMed1997

Changes in the concentration of amino acids in serum and cerebrospinal fluid of patients with Parkinson's disease.

The concentrations of sixteen amino acids have been measured in the serum and cerebrospinal fluid (CSF) of patients with Parkinson's disease and compared with those of control subjects. The levels of most amino acids were not different between the two groups, but the level of glutamate in CSF was decreased significantly, while the level of glutamine was increased. The results may be consistent with an alteration of glutamate neurotransmission in Parkinson's disease.

Aged↗
PubMed1997

The attenuation of kainate-induced neurotoxicity by chlormethiazole and its enhancement by dizocilpine, muscimol, and adenosine receptor agonists.

Systemically administered kainate (10 mg.kg-1) caused neuronal loss in both the hippocampus and the entorhinal regions of the rat brain. This resulted in a loss of 68.3 +/- 13.8 and 53.3 +/- 12.8% of pyramidal neurones in the hippocampal CA1 and CA3a regions, respectively. Chlormethiazole attenuated the loss of neurones in the hippocampal cell layers CA1 (cell loss 10 +/- 3.2%) and CA3a (cell loss 10 +/- 7.7%). The neuroprotective activity of chlormethiazole was apparent in the presence or absence of a low dose of clonazepam (200 micrograms.kg-1 i.p.). The kainate-induced damage could also be measured by the increase in binding of the peripheral benzodiazepine ligand ([3H]PK11195) in the hippocampus. In kainate-treated rats there was a 350-500% increase in binding indicative of reactive gliosis. Chlormethiazole prevented this elevation in a dose- and time-dependent manner, with an ED50 of 10.64 mg.kg-1 and an effective therapeutic window from 1 to 4 h posttreatment. Dizocilpine also attenuated damage significantly. The GABAA agonist muscimol was also able to attenuate the increase in [3H]PK11195 binding in a dose-dependent manner, with an ED50 of approximately 0.1 mg.kg-1. If muscimol, dizocilpine, or the adenosine A1 receptor agonist R-N6-phenylisopropyl-adenosine were administered together with chlormethiazole at their respective ED25 doses, a potentiation was apparent in the degree of neuroprotection. It is concluded that the combination of neuroprotective agents with different mechanisms of action can lead to a synergistic protection against excitotoxicity.

Adenosine↗
PubMed1997

Alkylxanthine adenosine antagonists and epileptiform activity in rat hippocampal slices in vitro.

Despite its potent proconvulsant effects in vitro, the adenosine A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) does not induce seizures when administered in vivo. This contrasts with the effects of less selective adenosine antagonists such as theophylline or cyclopentlytheophylline, and led us to reexamine the nature of DPCPX-induced epileptiform activity. In the present study, we report that proconvulsant effects of bath-applied DPCPX in rat hippocampal slices are only observed after a preceding stimulus such as NMDA receptor activation or brief tetanic stimulation. While this may be due to the absence of a basal "purinergic tone", the relatively high interstitial concentrations of adenosine present in the slice suggest that access of the drug to A1 receptors may instead be prevented by tightly coupled endogenous adenosine, with the ternary adenosine-A1 receptor-G protein complex stabilised in the high-affinity conformation by a coupling cofactor. This implies that a substantial percentage of adenosine A1 receptors are inactive under physiological conditions, but that access of adenosine A1 receptor antagonists may be facilitated under pathological conditions. Once induced, DPCPX-evoked spiking persists for long periods of time. A "kindling" effect of A1 receptor blockade is unlikely, since persistent spiking is not usually observed with less selective A1 antagonists even after prolonged application. Alternatively, endogenous adenosine released during increased neuronal activity may activate A2 receptors during selective A1 blockade. The most important factor determining the duration of DPCPX-induced spiking, however, may be a persistence of the drug in the tissue and subsequent access to the A1 receptor via a membrane-delineated pathway, since DPCPX-induced spiking could be shown to decrease markedly after a transient superfusion of theophylline. This hypothesis, which implies that the apparent affinity of adenosine antagonists for the A1 receptor is in part a function of their membrane partitioning coefficient, is supported by a close correlation between alkylxanthine logP values obtained from the literature and their Ki value at A1 receptors, but not at the enzyme phosphodiesterase, whose xanthine binding site is presented to the cytosol. The implications for the therapeutic value of purinergic drugs are discussed.

Animals↗
PubMed1997

Purine modulation of dizocilpine effects on spontaneous alternation.

The Y-maze was used to assess spontaneous alternation behaviour in mice to examine possible interactions between the N-methyl-D-aspartate receptor channel blocker dizocilpine and purine receptor agonists and antagonists. Scopolamine reduced spontaneous alternation. Dizocilpine also produced a dose-dependent reduction in alternation scores, which was accompanied by an increase in locomotion. The selective A1 adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (CPX) had no effect when administered alone, or in combination with scopolamine. However, when co-administered with dizocilpine, CPX reversed both the deficit in alternation behaviour and also the increase in locomotion induced by dizocilpine. The A1 selective agonist N6-cyclopentyladenosine (CPA) had no effect on either locomotion or alternation scores when administered alone, but in combination with scopolamine, CPA attenuated the scopolamine-induced deficit. CPA had no significant effect on the dizocilpine-induced deficit. The A2 selective agonist N6-[2-(3, 5-dimethoxyphenyl)-2(2-methylphenyl)-ethyl]adenosine (DPMA), had no effect on spontaneous alternation when administered alone, but did cause a depression of locomotion. DPMA had no significant effect when co-administered with scopolamine, but reversed the deficit in spontaneous alternation, and the increase in locomotion induced by dizocilpine. The A2 selective antagonist 3,7-dimethyl-1-propargylxanthine (DMPX) had no effect when given alone or in combination with scopolamine, but when co-administered with dizocilpine, DMPX reversed the reduction in spontaneous alternation caused by dizocilpine. It is concluded that dizocilpine has a detrimental effect on spontaneous alternation which is mediated partly by A1 and A2 adenosine receptors.

Adenosine↗
PubMed1997

The involvement of adenosine receptors in the effect of dizocilpine on mice in the elevated plus-maze.

It has been claimed that blockade of receptors for N-methyl-D-aspartate (NMDA) can enhance adenosine receptor function on single neurones. Previous work has also indicated that the NMDA channel blocker dizocilpine, and the A1 selective agonist N6-cyclopentyladenosine (CPA) both had anxiolytic profiles in the elevated plus-maze. The anxiolytic effect of dizocilpine was accompanied by an increase in locomotor activity. In the present study, the elevated plus-maze has been used to determine whether dizocilpine's effects on behaviour are mediated through activation of adenosine receptors. When co-administered with dizocilpine (0.05 mg/kg), CPA (0.05 mg/kg) reduced the anxiolytic and locomotor effects of dizocilpine. The A1 selective antagonist 1,3-dipropyl-8-cyclopentylxanthine (CPX, 0.05 mg/kg) had no effect when administered alone. When co-administered with dizocilpine, CPX reversed the anxiolytic and increased locomotor effects induced by dizocilpine. The A2 receptor selective agonist N6-[2-(3,5-dimethoxyphenyl)-2(2-methylphenyl)ethyladenosine (DPMA) (1 mg/kg) reversed both the anxiolytic effect and the increased locomotion induced by dizocilpine, while the A2 selective antagonist 3,7-dimethyl-1-propargylxanthine (DMPX) (1 mg/kg) did not. It is concluded that at least part of the anxiolytic and locomotor stimulant properties of dizocilpine may be explained by the release of endogenous adenosine acting at A1, but not A2 receptors.

Adenosine↗
PubMed1997

Kainate-evoked release of adenosine from the hippocampus of the anaesthetised rat: possible involvement of free radicals.

Using microdialysis in the hippocampus of anaesthetised rats, the concentration of extracellular adenosine was estimated to be 0.8 microM. Kainic acid (0.1-25 mM) in the perfusate evoked a concentration-dependent release of adenosine with an EC50 of 940 microM. Two 5-min pulses of 1 mM kainic acid in the perfusate increased the dialysate levels with an S2/S1 ratio of 0.52 +/- 0.03. Kainate-evoked release of adenosine was reduced significantly by 10 microM tetrodotoxin and by a kappa-receptor agonist, U50, 488H (100 microM). The S2/S1 ratio was reduced by 4.5 microM 6-cyano-7-nitroquinoxaline-2,3-dione, a non-NMDA receptor antagonist, but not by the NMDA receptor blockers (+)-MK-801 (dizocilpine; 100 microM) or (+/-)-2-amino-5-phosphonopentanoic acid (1 mM), indicating a non-NMDA receptor-mediated process. The S2/S1 ratio was also reduced significantly by 10 mM ascorbic acid, 10 mM glutathione (a scavenger of hydroperoxides), and 1 mM oxypurinol (a xanthine oxidase inhibitor), indicating the possible involvement of free radicals. Neither the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (100 microM) nor the A1 adenosine receptor agonist R(-)-N6-(2-phenylisopropyl)adenosine (100 microM) affected release. Adenosine release evoked by kainic acid is therefore mediated by activation of non-NMDA receptors and may involve the propagation of action potentials and the production of free radicals.

Adenosine↗
PubMed1996

The contribution of adenosine to paired-pulse inhibition in the normal and disinhibited hippocampal slice.

The effects of the adenosine receptor antagonist 1,3-dimethyl-8-cyclopentylxanthine (cyclopentyltheophylline) and the enzyme adenosine deaminase have been examined on paired-pulse inhibition between orthodromic evoked field potentials in the CA1 region of the normal and disinhibited hippocampal slice. In the presence of the GABAA receptor antagonist (-)-bicuculline methobromide, cyclopentyltheophylline suppressed homosynaptic paired-pulse inhibition between stimuli 300 ms apart. Slices treated with (-)-bicuculline and cyclopentyltheophylline together tended to develop spontaneous burst potentials. In slices in which a surgical cut isolated the CA1 and CA3 areas, thereby preventing the development of bursts in CA1, the effect on paired-pulse inhibition was lessened but was still apparent. Adenosine deaminase, in the presence of (-)-bicuculline showed the same effect as cyclopentyltheophylline, decreasing substantially the amount of paired-pulse inhibition. These results suggest that adenosine may contribute to homosynaptic paired-pulse inhibition in disinhibited slices. For comparison, we also examined the effect of cyclopentyltheophylline in normal ((-)-bicuculline-free) slices. At 100 nM, cyclopentyltheophylline increased reversibly the size of orthodromically evoked synaptic population potentials in the CA1 region of the slices and also reduced reversibly the degree of homosynaptic paired-pulse inhibition between two stimuli delivered only 30 ms apart. This suggests that adenosine may also contribute to shorter latency paired-pulse inhibition in the normal hippocampal slice.

Adenosine↗
PubMed1996

Ascorbate attenuates the systemic kainate-induced neurotoxicity in the rat hippocampus.

The neuronal damage induced by systemic administration of kainic acid reproduces the cellular and regional pattern of damage produced by repeated seizures. The ability of kainic acid to induce lipid peroxidation, and the ability of free radical inhibitors to prevent ischaemically-induced cell death, has led us to examine the possible role of free radicals in kainate-induced injury. Ascorbic acid was able to reduce kainate-induced damage of the rat hippocampus, measured by means of the gliotic marker ligand [3H]PK11195. Ascorbate was significantly effective at doses of 30 mg kg-1 and above, with total protection against kainate at 50 mg kg-1. Histologically, ascorbate at 50 mg kg-1 was able to prevent kainate-induced neuronal loss in the hippocampal CA1 and CA3a cell layers. The antioxidant was also effective when administered simultaneously with, or 1 h before the kainate. Protection was also obtained by allopurinol, 175 mg kg-1 and by oxypurinol, 40 mg kg-1. Ascorbate did not modify synaptically evoked potentials or long-term potentiation in hippocampal slices, ruling out any blocking activity at glutamate receptors. It is concluded that the neuronal damage produced by systemically administered kainate involves the formation of free radicals.

Analysis of Variance↗
PubMed1996

Prevention by a purine analogue of kainate-induced neuropathology in rat hippocampus.

Systemic injection of kainic acid produces a characteristic regional and cellular pattern of neuronal loss in the central nervous system by mechanisms which may be relevant to an understanding of neurodegenerative disorders. It has previously been found, by measuring the binding of a glial marker ligand, that analogues of adenosine, such as R-N6-phenylisopropyladenosine (R-PIA), can prevent kainate-induced damage of the hippocampus at doses as low as 10 micrograms/kg, i.p. The use of gliotic markers, however, is open to misinterpretation, and the present work was designed to re-examine purine protection against kainate using histological methods. The results show that R-PIA, at a dose of 25 micrograms/kg i.p. in rats, can protect against the neuronal damage caused by kainate and that this protection could be completely prevented by the simultaneous administration of 1,3-dipropyl-8-cyclopentylxanthine, indicating the involvement of adenosine A1 receptors in the protection.

Animals↗
PubMed1996

Comparative sensitivity to adenosine of paired-pulse inhibition and single field potentials in the rat hippocampus.

If excitatory terminals onto inhibitory interneurones were more sensitive to adenosine than excitatory terminals onto pyramidal cells in the hippocampus it might explain the effect of adenosine to decrease paired-pulse inhibition and account for reported excitatory effects of low concentrations of adenosine. We have compared the concentration-response relationships for the effect of adenosine on single evoked field potentials and on paired-pulse inhibition in the CA1 area of the rat hippocampal slice in order to test this hypothesis. Adenosine caused a concentration-dependent decrease in both single evoked population spike size and in paired-pulse inhibition between potentials. The concentration-response relationships for both effects was very similar, ruling out the possibility that excitatory terminals onto inhibitory interneurones are more sensitive to adenosine than excitatory terminals onto pyramidal cells, and suggesting that the receptors located at the two sites may be indistinguishable.

Adenosine↗
PubMed1996

Effects of purine analogues on spontaneous alternation in mice.

The Y-maze was used to examine the effects of purines acting at A1 and A2 adenosine receptors upon spontaneous alternation, a model of working memory, in mice. In support of previous work, scopolamine produced a loss of spontaneous alternation behaviour to the 0.5 chance level. The A1 receptor selective agonist N6- cyclopentyladenosine (CPA) did not change spontaneous alternation behaviour alone, but it prevented the decrease of spontaneous alternation scores produced by scopolamine. The A1 receptor selective antagonist 1,3-dipropyl-8-cyclopentylxanthine (CPX) blocked the effect of CPA in combination with scopolamine but had no effect alone. The A2 receptor selective agonist (N6-[2-(3,5-dimethoxyphenyl)-2-(2- methylphenyl)ethyl] adenosine (DPMA), and the A2 receptor selective antagonist 3,7-dimethyl-1-propargylxanthine (DMPX) had no effect of alternation behaviour alone and did not modify the effect of scopolamine. The results indicate the ability of A1 but not A2 receptor activation to modify working memory deficits induced by scopolamine, but suggest that endogenous adenosine does not normally participate in working memory processes.

Adenosine↗
PubMed1996

Glutamate-independent long term depression in rat hippocampus by activation GABAA receptors.

Long-term depression (LTD) of synaptic transmission is a candidate for a neuronal model of forgetting and is considered to be important in learning and memory. The present study employed extracellular recording in the CA1 pyramidal cell layer of rat hippocampal slices following orthodromic stimulation of Schaffer collateral fibres in stratum radiatum. Muscimol induced a time and concentration-dependent LTD at a frequency of stimulation of 0.01 Hz or in the absence of stimulation. The LTD was reversed by stimulation at 1 Hz. The ability of muscimol to act via GABAA receptors was confirmed by the ability of bicuculline (5 microM) to reverse the LTD. The NMDA non-selective glutamate antagonists kynurenate, failed to modify the LTD. receptor antagonist 2-AP5, the selective metabotropic antagonist L(+)AP3 and the non-selective glutamate antagonists kynurenate, failed to modify the LTD. (1S,3R)-ACPD, a selective agonist at metabotropic receptors, did not induce LTD. The lack of involvement of glutamate receptors in muscimol induced LTD in our protocol may indicate a novel type of long-lasting synaptic depression

2-Amino-5-phosphonovalerate↗