Leslie J. Fowler

Regional Effects of Sodium Valproate on Extracellular Concentrations of 5-Hydroxytryptamine, Dopamine, and Their Metabolites in the Rat Brain: An In Vivo Microdialysis Study

Abstract: The effects of sodium valproate (VPA; 100, 200, and 400 mg/kg, i.p.) on ventral hippocampal and anterior caudate putamen extracellular levels ofdopamine (DA) and 5‐hydroxytryptamine (5‐HT) were examined using in vivo microdialysis. VPA induced dose‐related increases in dialysate DA, 3,4‐dihydroxyphenylacetic acid, and 5‐HT in the ventral hippocampus. Anterior caudate putamen dialysate 5‐HT was also dose dependently elevated by the drug, whereas DA levels tended to decrease with increasing VPA dose. In contrast, VPA (200, 400, and 800 mg/kg, i.p.) produced no significant elevation of DA in posterior caudate putamen dialysates, although 5‐HT levels were significantly elevated at the 400‐ and 800‐mg/kg doses. In all three regions studied, dialysate concentrations of 5‐hydroxyindoleacetic acid and homovanillic acid remained at basal levels following VPA treatments. The results are discussed with regard to the possible anticonvulsant mode of action of VPA.

MK-801 increases extracellular 5-hydroxytryptamine in rat hippocampus and striatum in vivo.

Abstract: The effect of MK‐801 (0.25 or 0.5 mg/kg) on the extracellular concentration of 5‐hydroxytryptamine (5‐HT) and 5‐hydroxyindoleacetic acid (5‐HIAA) in rat hippocampus and striatum was studied using intracerebral dialysis. The dialysate 5‐HT concentration was dose‐dependently increased by MK‐801 in both regions. In the hippocampus, at the higher drug dose a slow increase in the 5‐HIAA level was observed, and this became significant 3 h after treatment. In contrast to this, the extracellular 5‐HIAA content in the striatum was significantly decreased 150 min after administration of both doses of MK‐801. The data are discussed in the light of the known behavioural effects of MK‐801 and possible N‐methyl‐d‐aspartic acid receptor regulation of 5‐HT release.

ANALYSIS OF THE MAJOR AMINO ACIDS OF RAT BRAIN AFTER IN VIVO INHIBITION OF GABA TRANSAMINASE BY ETHANOLAMINE O-SULPHATE

The effect of in vivo inhibition of GABA transaminase by ethanolamine O‐sulphate on the content of the free amino acids in rat brain has been studied. Intracisternal injection of 2.0 mg/kg resulted in a progressive increase in GABA levels with time, to reach after 8 h a 100 per cent increase over saline‐injected control animals. The effect of injection of 0.5, 1.0 and 2.0 mg/kg was studied 24 h after injection and the results showed that the increased GABA levels were dependent on the dose of inhibitor employed. Apart from the substantial increase in the GABA concentration of the brain there were no significant changes in the content of the other amino acids except for a small but significant decrease in aspartic acid in one experiment. When the extent of inhibition of the transaminase was correlated with the rise in GABA concentration it was shown that no elevation occurred until more than half of the enzymic activity had been inhibited.

Lamotrigine, carbamazepine and phenytoin differentially alter extracellular levels of 5-hydroxytryptamine, dopamine and amino acids

We have studied the effects of treatment with the anticonvulsants lamotrigine (LTG), phenytoin (PHN) and carbamazepine (CBZ) on basal and stimulated extracellular aspartate (ASP), glutamate (GLU), taurine (TAU), GABA, 5-hydroxytryptamine (5-HT) and dopamine (DA) in the hippocampus of freely moving rats using microdialysis. All of the drugs investigated have had inhibition of Na(+) channel activity implicated as their principal mechanism of action. Neither LTG (10-20 mg/kg), PHN (20-40 mg/kg) or CBZ (10-20 mg/kg) had an effect on the basal extracellular concentrations of any of the amino acids studied with the exception of glutamate, which was decreased at the highest LTG dose. However, when amino acid transmitter levels were increased with 50 microM veratridine, LTG was found to cause a dose-dependent decrease in dialysate levels of all four amino acids, with the effect being most pronounced for glutamate. In contrast, PHN decreased extracellular aspartate levels but had no effect on evoked-extracellular GLU, TAU or GABA. Somewhat unexpectedly, CBZ did not alter the stimulated increase in the excitatory amino acids, GLU and ASP, but, rather surprisingly for an antiepileptic drug, markedly decreased that of the inhibitory substances TAU and GABA. The three drugs had differing effects on basal extracellular 5-HT and DA. LTG caused a dose-dependent decrease in both, while CBZ and PHN both increased extracellular 5-HT and DA. When extracellular 5-HT and DA was evoked by veratridine LTG had no significant effect on this, while PHN but not CBZ increased stimulated extracellular 5-HT and both PHN and CBZ augmented DA. Thus, the effects of the three drugs studied seemed to depend on whether extracellular transmitter levels are evoked or basal and the particular transmitter in question. This suggests that there are marked differences in the neurochemical mechanisms of antiepileptic drug action of the three compounds studied.

Regional effects of MK-801 on dopamine and its metabolites studied by in vivo microdialysis.

The non-competitive N-methyl-D-aspartate receptor antagonist MK-801 was observed to have regionally specific effects on the extracellular concentration of dopamine and its metabolites. In rat anterior striatum, MK-801 transiently decreased extracellular dopamine, in spite of inducing intense circling behaviour which is generally associated with an increase in this neurotransmitter. In contrast, hippocampal extracellular dopamine was increased in a dose-related manner by MK-801. The possible significance of these data is discussed in relation to some of the known behavioural actions of MK-801.

The effects of sodium valproate on γ-aminobutyrate metabolism and behaviour in naive and ethanolamine-O-sulphate pretreated rats and mice

Sodium valproate was injected acutely (400 mg/kg i.p.) into naive and ethanoloamine-O-sulphate chronically pretreated rats and mice, in an attempt to gain further insight into the effects of this anticonvulsant on GABA metabolism. Sodium valproate significantly enhanced the activity of GAD in the medulla and pons, cerebellum and midbrain regions of rats, and partially relieved the suppression of GAD activity caused by chronic GABA-transaminase inhibition in whole mouse brain. In combination with EOS, sodium valproate caused behavioural excitation in mice which was similar to that sometimes seen with high doses of some GABA-T inhibitors. Pretreatment with EOS potentiated the characteristic abstinence behaviour caused by sodium valproate in rats, though no further significant rise in cerebral GABA levels was observed. In view of the neuronal location of GAD, the elevation of cerebral GABA levels at least in part by potentiation of GAD activity could be involved in the mediation of the anticonvulsant activity of sodium valproate.

An analysis of the kinetics of the inhibition of rabbit brain γ-aminobutyrate aminotransferase by sodium n-dipropylacetate and some other simple carboxylic acids

Abstract The kinetics of inhibition of GABA-transaminase by the antiepileptic drug sodium n -dipropylacetate have been studied using a preparation of the enzyme partially purified from rabbit brain and an assay method which determines one of the immediate products, i.e. succinic semialdehyde. It has been shown that the compound is a very weak inhibitor of the binding to the enzyme of both GABA ( K 1 = 42 mM) and α-ketoglutarate ( K 2 = 92 mM). Other simple carboxylic acids n -butyric, n -valeric and propionic acid have been shown to be more potent inhibitors of the enzyme. The significance of these results with regard to the mechanism of inhibition of GABA-transaminase by carboxylic acids has been discussed.

N-Methyl-d-aspartate receptors modulate extracellular 5-hydroxytryptamine concentration in rat hippocampus and striatum in vivo

The effects of infusing N-methyl-D-aspartate (NMDA) and the specific NMDA receptor antagonist D-2-amino-5-phosphonopropionic acid (D-AP5) into rat hippocampus and striatum on extracellular 5-hydroxytryptamine (5-HT) and its metabolite 5-hydroxy-indoleacetic acid (5-HIAA) were studied using intracerebral microdialysis. In striatum, NMDA (1-100 microM) caused a concentration-dependent increase in 5-HT. D-AP5 (10 microM) infusion caused increased extracellular 5-HT. When the two drugs were co-infused, no effect on extracellular 5-HT was seen. D-AP5 alone was found to cause a delayed but sustained increase in dialysate 5-HIAA. In hippocampus, NMDA infusion caused a dose-dependent decrease in extracellular 5-HT while D-AP5 produced a transitory increase in 5-HT level. NMDA caused a decrease in dialysate 5-HIAA. In striatum, the effect of 10 microM NMDA infusion was abolished by co-infusion with tetrodotoxin (TTX; 1 microM). In hippocampus, 1 microM TTX caused a slight but non-significant augmentation of the effect of 10 microM NMDA alone. These data indicate that NMDA receptors mediate control over 5-HT release and metabolism in different brain regions and may in part explain the behavioural effects of non-competitive NMDA receptor antagonists.

γ-Aminobutyric acid metabolism in rat brain following chronic oral administration of ethanolamine O-sulphate

Abstract Chronic, oral EOS ∗ administration resulted in a marked inhibition of rat whole-brain GABA-T activity and a significant increase in brain GABA concentrations. The maximum degree of GABA-T inhibition attained was 83 per cent, when GABA levels were 200 per cent of control values. A fixed dose of EOS produced a steady fall in GABA-T activity over the first 7 days of administration, when enzyme activity appeared to stabilize at 20–25 per cent of control values. Concurrently, GABA levels rose to a steady maximum value of approximately 240 per cent of control values. These changes were accompanied by significant reductions in whole-brain GAD activity. Chronic EOS also produced small but significant increases in brain content of alanine and taurine. No behavioural changes were seen following chronic EOS administration.

Effects of acute and chronic lamotrigine treatment on basal and stimulated extracellular amino acids in the hippocampus of freely moving rats.

The antiepileptic drug lamotrigine (LTG) is a relatively novel anticonvulsant frequently used in polytherapy and increasingly in monotherapy. LTG is believed to act by reducing excitatory glutamate (GLU) release due to an inhibition of Na(+) channels. In the present study, we have investigated the effects of acute and chronic (up to 21 days) treatment with LTG on basal and either veratridine- or KCl-stimulated release of aspartate (ASP), GLU, taurine (TAU) and GABA in the hippocampus of freely moving rats using microdialysis. Additionally, we have measured LTG concentrations in the plasma, whole brain and extracellular fluid of rats at the same time points. LTG significantly reduced basal ASP and GLU but only at the highest dose used (20 mg/kg) and was entirely without effect on basal TAU or GABA. When either veratridine or 100 mM KCl were added to the infusion medium amino acid release was evoked although the extent of this varied from one amino acid to another. LTG (10 mg/kg) reduced veratridine-evoked release of all four amino acids studied, although this was most marked in the case of GLU. LTG had no effect on KCl-stimulated amino acid release. When given for up to 21 days (2 x 5 mg/kg/day), LTG had no effect on basal amino acid levels. In contrast, LTG demonstrated over the time period studied an increasingly inhibitory effect on veratridine-evoked amino acid release. This effect of the drug was proportionally much greater in the case of GLU than for the other three amino acids studied. Measurement of plasma, whole brain tissue and extracellular LTG showed that in each of these compartments, it had reached an apparent steady state within 4 days of commencement of treatment and appeared to mirror the neurochemical changes measured. Our estimate of plasma LTG indicates that during chronic study, this was well within the therapeutic range, suggesting that the current neurochemical observations are clinically relevant.