Effect of amino-oxyacetic acid (AOAA) on focal penicillin seizures

Concentration- and time-dependent effect of aminooxyacetic acid on cortical epileptogenicity

In the present electrophysiological study the effect of aminooxyacetic acid (AOAA) on the cortical epileptogenicity, and on the basic electro-cortical activity was investigated in anesthetized rats. AOAA did not induce spontaneous epileptiform discharges but modified the somato-sensory evoked responses and the cortical epileptogenicity (induced by 4-aminopyridine) in the same manner depending on its concentration. AOAA at low concentrations increased the amplitude of evoked responses and the ipsilateral manifestation of epileptiform activity, however, at high concentrations significantly suppressed both the evoked responses and the induction and expression of seizures discharges. The anticonvulsive effect of AOAA was time-dependent (reached its maximum after 2h AOAA pre-treatment) and reversible. AOAA at low concentrations probably increases the efficacy of the NMDA excitatory system and decreases GABA-synthesis, resulting neuronal hyperexcitation. However, AOAA at high concentrations can lead to an effective cortical inhibition through intra- and extracellular accumulation of GABA. The gradual GABA accumulation - up to a certain level - at the synapses could also explain the time-dependency of the anticonvulsive effect of AOAA.

Inhibition by gamma-aminobutyric acid system activation of epileptic seizures in spontaneously epileptic rats

The effects of muscimol, a gamma-aminobutyric acid (GABA)A-receptor agonist, and aminooxy-acetic acid (AOAA), an inhibitor of GABA-converting enzyme, on tonic and absence-like seizures in spontaneously epileptic rats (SER: zi/zi, tm/tm) were investigated to elucidate whether GABAergic function operates normally in these animals. Muscimol at doses of 1 and 3 mg/kg (i.p.) induced high-voltage slow waves in the cortical and hippocampal EEG of SER, although the behavioral observation suggested inhibition of absence-like seizures. Similar high-voltage slow waves were also observed in the cortical and hippocampal EEG of normal rats with muscimol (1 and 3 mg/kg). Tonic convulsions in SER were dose-dependently inhibited by muscimol. AOAA (3 and 10 mg/kg, i.v.) inhibited both tonic and absence-like seizures in SER, although there were no obvious changes in EEG pattern. The inhibitory effects of AOAA on tonic convulsions appeared more slowly and lasted longer than those on absence-like seizures. Cerebral, hippocampal and cerebellar GABA levels were significantly higher in SER than the normal Kyo:Wistar and zitter rat (zi/zi), which were both the parent strains. These findings suggest that GABA receptors and GABAergic neurons are functional in SER and that the GABA system is involved in the inhibition of both seizures.

Anterior thalamic mediation of generalized pentylenetetrazol seizures

The effects of microinjection of various neuroactive compounds into the anterior thalamic nucleus (AN) and other selected subcortical regions of guinea pig brain on the expression of pentylenetetrazol (PTZ)-induced behavioral and electrical seizure activity were examined. Excitatory agents, kainic acid (KA), bicuculline (BIC) or PTZ, injected into the AN or other thalamic nuclei, striatum, but not the mammillary bodies (MB), facilitated the EEG convulsant action of systemically administered PTZ. Injection of muscimol into the AN protected against the expression of PTZ-induced repetitive high-voltage EEG seizure discharges and inhibited the facilitatory effects of subcortically applied KA or BIC. Injection of muscimol into the AN was also able to terminate established ongoing seizure discharges. Unilateral application of muscimol to the AN did not prevent the repetitive hypersynchronous EEG discharges following systemic PTZ but did result in the delay in the onset of cortical hypersynchrony in the ipsilateral hemisphere. Muscimol injections into other thalamic nuclei, MB, cortex, striatum or directly into the CSF space had no anticonvulsant effect, however. Microinjection of gamma-vinyl-gamma-aminobutyric acid, a selective GABA transaminase inhibitor, resulted in protection from the behavioral convulsant action and lethal effects of PTZ when administered into the thalamus, especially the AN, but not when injected into the striatum or CSF. These data demonstrate that the AN is an important subcortical nucleus for the mediation of both cortical EEG synchrony and behavioral seizure expression induced by PTZ. In light of previous results establishing a role for the brainstem and diencephalon in PTZ seizure expression, the AN may serve, in part, as a gating mechanism for the propagation of paroxysmal activity between subcortical areas and the cerebral cortex.

Studies on the relation of gamma-hydroxybutyric acid (GHB) to gamma-aminobutyric acid (GABA). Evidence that GABA is not the sole source for GHB in rat brain

The effects of gamma-aminobutyric acid (GABA)-alpha-oxoglutarate aminotransferase (GABA-T) inhibitors, L-glutamic acid decarboxylase (GAD) inhibitors, and antipetit mal anticonvulsants on gamma-hydroxybutyric acid (GHB) and GABA were studied. Treatment with anticonvulsants and GABA-T inhibitors resulted in an increase in steady-state brain levels of both GHB and GABA. GAD inhibitors produced markedly decreased levels of brain GABA but no change in GHB concentrations. Studies of GHB derived exclusively from GABA showed that GABA-T inhibitors which produced an elevation of steady-state levels of GHB in brain also resulted in a decrease in GABA-derived GHB. Intracerebroventricular (i.c.v.) administration of GABA, putrescine, and 1,4-butanediol all produced significant elevations in brain GHB, but GABA-T inhibitors blocked this effect of GABA and putrescine. These data suggest that there may be another source for GHB in brain in addition to GABA and raise the possibility that 1,4-butanediol may be that source.

Focal seizures disrupt protein synthesis in seizure pathways: an autoradiographic study using [1-14C]leucine

We have used a new autoradiographic technique developed by Smith et al.22,33 for visualizing rates of incorporation of [1-14C]leucine into protein in brain. Focal seizures caused by topical convulsants resulted in a marked decrease in autoradiographic density. This was primarily confined to the seizure focus, especially marked in pyramidal cell layers, and to subcortical seizure pathways. There were no distinct changes in cortico-cortical pathways beyond the seizure focus. Pure orthodromic pathways through basal ganglia showed an 18% inhibition of leucine incorporation in caudate nucleus and substantia nigra, pars compacta (P less than 0.05). By contrast, thalamic nuclei connected both ortho- and antidromically to the focus showed a 30-63% inhibition (P less than 0.01). The topographic pattern and intensity of the thalamic changes were related to the site, size and intensity of the seizure focus. As seizures became severe there was a more generalized depression of metabolism beyond seizure pathways, especially in the ipsilateral hemisphere. The results suggest that seizures block incorporation of leucine into protein either by an increase oxidation of the precursor, and/or an inhibition of protein synthesis per se. The effect is most severe in neurons undergoing epileptic burst discharge in the focus and in thalamic neuronal beds connected reciprocally with the focus.