Inhibition of Glutamate Release, but Not of Glutamine Recycling to Glutamate, Is Involved in Delaying the Onset of Initial Lithium-Pilocarpine-Induced Seizures in Young Rats by a Non-Convulsive MSO Dose.

Initial seizures observed in young rats during the 60 min after administration of pilocarpine (Pilo) were delayed and attenuated by pretreatment with a non-convulsive dose of methionine sulfoximine (MSO). We hypothesized that the effect of MSO results from a) glutamine synthetase block-mediated inhibition of conversion of Glu/Gln precursors to neurotransmitter Glu, and/or from b) altered synaptic Glu release. Pilo was administered 60 min prior to sacrifice, MSO at 75 mg/kg, i.p., 2.5 h earlier. [1,2-13C]acetate and [U-13C]glucose were i.p.-injected either together with Pilo (short period) or 15 min before sacrifice (long period). Their conversion to Glu and Gln in the hippocampus and entorhinal cortex was followed using [13C] gas chromatography-mass spectrometry. Release of in vitro loaded Glu surrogate, [3H]d-Asp from ex vivo brain slices was monitored in continuously collected superfusates. [3H]d-Asp uptake was tested in freshly isolated brain slices. At no time point nor brain region did MSO modify incorporation of [13C] to Glu or Gln in Pilo-treated rats. MSO pretreatment decreased by ~37% high potassium-induced [3H]d-Asp release, but did not affect [3H]d-Asp uptake. The results indicate that MSO at a non-convulsive dose delays the initial Pilo-induced seizures by interfering with synaptic Glu-release but not with neurotransmitter Glu recycling.

Selected Molecular Targets for Antiepileptogenesis.

The term epileptogenesis defines the usually durable process of converting normal brain into an epileptic one. The resistance of a significant proportion of patients with epilepsy to the available pharmacotherapy prompted the concept of a causative treatment option consisting in stopping or modifying the progress of epileptogenesis. Most antiepileptic drugs possess only a weak or no antiepileptogenic potential at all, but a few of them appear promising in this regard; these include, for example, eslicarbazepine (a sodium and T-type channel blocker), lamotrigine (a sodium channel blocker and glutamate antagonist) or levetiracetam (a ligand of synaptic vehicle protein SV2A). Among the approved non-antiepileptic drugs, antiepileptogenic potential seems to reside in losartan (a blocker of angiotensin II type 1 receptors), biperiden (an antiparkinsonian drug), nonsteroidal anti-inflammatory drugs, antioxidative drugs and minocycline (a second-generation tetracycline with anti-inflammatory and antioxidant properties). Among other possible antiepileptogenic compounds, antisense nucleotides have been considered, among these an antagomir targeting microRNA-134. The drugs and agents mentioned above have been evaluated in post-status epilepticus models of epileptogenesis, so their preventive efficacy must be verified. Limited clinical data indicate that biperiden in patients with brain injuries is well-tolerated and seems to reduce the incidence of post-traumatic epilepsy. Exceptionally, in this regard, our own original data presented here point to c-Fos as an early seizure duration, but not seizure intensity-related, marker of early epileptogenesis. Further research of reliable markers of early epileptogenesis is definitely needed to improve the process of designing adequate antiepileptogenic therapies.

Pretreatment with a glutamine synthetase inhibitor MSO delays the onset of initial seizures induced by pilocarpine in juvenile rats.

The contribution of glutamatergic transmission to generation of initial convulsive seizures (CS) is debated. We tested whether pretreatment with a glutamine synthetase (GS) inhibitor, methionine sulfoximine (MSO), affects the onset and progression of initial CS by cholinergic stimulus in juvenile rats. Male rats (24 days old, Sprague Dawley) sequentially received i.p. injections of lithium-carbonate, MSO, methyl-scopolamine, and pilocarpine (Pilo). Pilo was given 150 min after MSO. Animals were continuously monitored using the Racine scale, EEG/EMG and intrahippocampal glutamate (Glu) biosensors. GS activity as measured in hippocampal homogenates, was not altered by MSO at 150 min, showed initial, varied inhibition at 165 (15 min post-Pilo), and dropped down to 11% of control at 60 min post-Pilo, whereas GS protein expression remained unaltered throughout. Pilo did neither modulate the effect of MSO on GS activity nor affect GS activity itself, at any time point. MSO reduced from 32% to 4% the number of animals showing CS during the first 12 min post-Pilo, delayed by ~6 min the appearance of electrographic seizures, and tended to decrease EMG power during ~15 min post-Pilo. The results indicate that MSO impairs an aspect of glutamatergic transmission involved in the transition from the first cholinergic stimulus to the onset of seizures. A continuous rise of extracellular Glu lasting 60 min was insignificantly affected by MSO, leaving the nature of the Glu pool(s) involved in altered glutamatergic transmission undefined.