Enhancement of excitant amino acid release from rat brain slices by the convulsant 3-mercaptopropionic acid

An investigation into the pharmacokinetics of 3-mercaptopropionic acid and development of a steady-state chemical seizure model using in vivo microdialysis and electrophysiological monitoring

OBJECTIVES

The goal of the present study was to develop a chemical seizure model using the convulsant, 3-mercaptopropionic acid (3-MPA). A pharmacodynamics approach was taken, combining in vivo microdialysis sampling with electrophysiological methods to simultaneously monitor, in real-time, the 3-MPA concentration in the brain and the corresponding electrocorticographic (ECoG) activity.

METHODS

The 3-MPA was administered in two doses (50 and 100 mg/kg) in order to study its pharmacokinetics. Microdialysis samples were collected from the striatum, hippocampus, and jugular vein every 5 min. The microdialysates were analyzed using high-performance liquid chromatography with electrochemical detection (HPLC-EC). The ECoG activity was monitored via screws placed onto the cortex. Noncompartmental pharmacokinetics analysis was performed to obtain the elimination constants (K(e)), the maximum concentration (C(max)), the time to achieve maximum concentration (T(max)), and the area under the concentration-time curves (AUC(inf)).

RESULTS

The average brain K(e) for the 50 and the 100mg/kg doses were 0.060 and 0.018 min(-1), respectively. The brain AUC(inf) for the 50 and 100mg/kg doses were 353 and 2168 mg min(-1)mL(-1), respectively. This led to a 67-fold increase in the observed number of seizures in the higher dose with the average seizure intensity double that of the smaller dose. These data led to the dosing scheme for the chemical seizure model of administering a 3-MPA loading dose of 60 mg/kg followed by a constant infusion of 50 mg/(kg min(-1)).

CONCLUSIONS

This study describes, to our knowledge, the first successful attempt to combine in vivo microdialysis with electrophysiology to monitor in real-time, the concentration and effects of 3-MPA in the brain. This led to the development of a steady-state chemical seizure model.

Increased aspartic acid release from the iron-induced epileptogenic focus

Evidence has been growing in recent years for the involvement of excitatory neurotransmitter amino acids in the etiology of epilepsy. The precise mechanism of this involvement, however, remains unknown. In the present study, in vitro release and uptake of [3H]aspartic acid and [3H]glutamic acid were investigated in focal cerebral cortex in the iron-induced model of post-traumatic epilepsy in the rat. The animals were injected with FeCl3 or saline into the cerebral cortex and release and uptake studied in cortical slices from both acute and chronic foci (30 min and 3 weeks post injection, respectively), using a superfusion system. The results showed: (a) a significant increase in K(+)-stimulated aspartic acid release from the acute iron injected focus as compared to the corresponding saline injected cortex; and (b) no significant differences in the release of glutamic acid or in the uptake of glutamic acid and aspartic acid between the iron injected and the saline injected cortex. The finding of increased aspartic acid release suggests that this amino acid may play a role in the mechanism of iron-induced epilepsy in the rat.

The effect of intracerebroventricular 3-mercaptopropionic acid on blood pressure and heart rate in the rat

Intracerebroventricular (i.c.v.) injection of 3-mercaptopropionic acid (3-MP, 250 micrograms/10 microliter) elicited an elevation of blood pressure in a dose-dependent manner in anaesthetized rats. This elevation of blood pressure could be blocked by treatment with amino-oxyacetic acid (AOAA, 25 mg/kg, i.p.) 5 h prior to the i.c.v. injection of 3-MP. Given intraperitoneally (i.p.), 3-MP (75 mg/kg) had no effect on blood pressure or heart rate. I.c.v. injection of 3-MP produced a selective decrease in GABA content in the diencephalon and mesencephalon, whereas i.p. injection of 3-MP caused a decrease in GABA content in all regions of brain, including telencephalon, diencephalon, mesencephalon, pons and medulla. The results suggest that the elevation of blood pressure induced by i.c.v. injection of 3-MP may be the result of a selective depletion of GABA in diencephalon and/or mesencephalon. Decreases in GABA content in brain regions other than diencephalon and mesencephalon may lead to an opposite effect in modulating blood pressure.

Regional changes in transmitter amino acids during focal and generalized seizures in rats

Measurement of glucose utilization after 20 minutes of seizure activity induced by the systemic administration of convulsants in paralyzed, ventilated rats demonstrates that the initial metabolic activation during kainic acid seizures is confined to the hippocampus. L-allylglycine-induced generalized seizures result in a global metabolic activation, whereas bicuculline seizures in fasted rats have little effect on brain glucose utilization after 20 minutes of seizure activity. Other metabolic changes associated with kainic acid seizures also predominate in the hippocampus, were decreases in the levels of aspartate, glutamate, taurine and glutamine, and an increase in the level of GABA are observed after 20 minutes of seizure activity. L-allylglycine seizures are associated with generalized decreases in regional GABA levels, and increases in regional glutamine levels. Bicuculline-induced changes include increases in hippocampal GABA and taurine levels, and increases in cerebellar glutamine and taurine levels. These changes can be tentatively explained in terms of known biochemical and neurophysiological mechanisms of these convulsants.

Interactions of some anaesthetic, convulsant, and anticonvulsant drugs at GABA-benzodiazepine receptor-ionophore complexes in rat brain synaptosomal membranes

The effects of several anaesthetic, convulsant and anticonvulsant drugs were studied upon high affinity [3H]GABA and [3H]diazepam binding to rat brain synaptosomal membranes in chloride-containing incubation buffers at 25 degrees C, conditions under which pentobarbitone extensively enhanced binding of both ligands to GABA-benzodiazepine-receptor-ionophore complexes. Of the compounds studied, only (+)-etomidate enhanced both GABA and diazepam binding; the sedative-hypnotic glutethimide weakly enhanced GABA binding while inhibiting diazepam binding. Several drugs, including beta-butyl-beta-methyl-glutarimide, phenobarbitone, pentylenetetrazole, and ketamine reversed the enhancement of GABA binding by pentobarbitone (500 microM) while not altering basal GABA or diazepam binding. Enhancement of high affinity GABA binding does not appear to be a general property of sedative or anticonvulsant drugs.

Effect on cerebral D-[3H]aspartate release of 3-mercaptopropionic acid and methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate

Minislices of rat brain cortex were preloaded with D-[3H]aspartate and spontaneous and potassium-induced (25 mM KCl) efflux of labelled compound into superfusate measured. 3-Mercaptopropionic acid, 100-220 microM did not modify the spontaneous efflux of D-[3H]aspartate, but augmented the K+-induced efflux. Methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate, 10 or 100 microM, enhanced both spontaneous and K+-induced efflux of D-[3H]aspartate. The convulsant action of both compounds may be due to enhanced synaptic release of aspartate or a related excitatory neurotransmitter.

Effects of the glutamic acid decarboxylase inhibitor 3-mercaptopropionic acid on the synthesis of brain GABA in vivo and postmortally

The effects of the glutamic acid decarboxylase inhibitor 3-mercaptopropionic acid (MPA) on the concentration of GABA in the mouse brain were studied. MPA completely inhibited the postmortem increase in GABA. This effect was used in order to achieve a maximal inhibition of the GABA synthesis in vivo during 67.5 minutes before killing by giving the drug repeatedly (50 mg/kg + 6 X 10 mg/kg i.p.) to mice pretreated with chloral hydrate (100 mg/kg i.p., 65 min before killing). Such a treatment with MPA markedly reduced the accumulation of GABA following inhibition of the GABA transaminase by aminooxyacetic acid but it did not change the endogenous concentration of GABA. This discrepancy might be due to inhibition of the impulse--evoked release of GABA following MPA.