Effects of cyclo-oxygenase inhibition upon glutamate release from rat cerebellum

Effect of cyclooxygenase inhibitors on pentylenetetrazol (PTZ)-induced convulsions: Possible mechanism of action

Cyclooxygenase (COX) is reported to play a significant role in neurodegenerative and neuropsychiatric disorders, and may play a significant role in the pathogenesis of epilepsy. Various neurotransmitter abnormalities, especially of GABA and glutamate, have been reported to play a key role in the pathophysiology of epilepsy. The objective of the present study was to elucidate the effect of cyclooxygenase inhibitors on pentylenetetrazol (PTZ)-induced (80 mg/kg) convulsions in mice with possible mechanism of action. Various COX-inhibitors were administered 45 min prior to the PTZ administration. Onset, duration of clonic convulsions and percentage mortality/recovery were recorded. Pretreatment with COX-inhibitors aspirin (10 and 20 mg/kg, p.o.), naproxen (7 and 14 mg/kg, p.o.), nimesulide (1-5 mg/kg, p.o.) or rofecoxib (1-4 mg/kg, p.o.) dose-dependently showed protection against PTZ-induced convulsions. COX-2 inhibitors were more effective as compared to non-selective COX-inhibitors. Rofecoxib (1 mg/kg) or nimesulide (1 mg/kg) also enhanced the sub-protective effect of diazepam or muscimol showing GABAergic modulation of COX-2 inhibitors. COX-2 inhibitors also antagonized the effect of flumazenil (4 mg/kg)- against PTZ-induced convulsions further confirming the GABAergic mechanism. In conclusion, the results of the present study strongly suggest the possible role of cyclooxygenase isoenzymes in the pathophysiology of epilepsy and the use of COX-inhibitors as an adjuvant therapy in the treatment of epilepsy.

Regulation of neurotransmitter release by metabotropic glutamate receptors

The G protein-coupled metabotropic glutamate (mGlu) receptors are differentially localized at various synapses throughout the brain. Depending on the receptor subtype, they appear to be localized at presynaptic and/or postsynaptic sites, including glial as well as neuronal elements. The heterogeneous distribution of these receptors on glutamate and nonglutamate neurons/cells thus allows modulation of synaptic transmission by a number of different mechanisms. Electrophysiological studies have demonstrated that the activation of mGlu receptors can modulate the activity of Ca(2+) or K(+) channels, or interfere with release processes downstream of Ca(2+) entry, and consequently regulate neuronal synaptic activity. Such changes evoked by mGlu receptors can ultimately regulate transmitter release at both glutamatergic and nonglutamatergic synapses. Increasing neurochemical evidence has emerged, obtained from in vitro and in vivo studies, showing modulation of the release of a variety of transmitters by mGlu receptors. This review addresses the neurochemical evidence for mGlu receptor-mediated regulation of neurotransmitters, such as excitatory and inhibitory amino acids, monoamines, and neuropeptides.

The effect of losigamone (AO-33) on electrical activity and excitatory amino acid release in mouse cortical slices

1. Losigamone is a novel anticonvulsant the mechanism of action of which is not known. This study investigated the effect of losigamone on spontaneous, NMDA- and AMPA-induced depolarizations in the cortical wedge preparation of the DBA/2 mouse (which are susceptible to sound-induced seizures) and on endogenous amino acid release from BALB/c mouse cortical slices. 2. Cortical wedges exhibit spontaneous depolarizations in magnesium-free medium and losigamone was effective in significantly reducing these spontaneous depolarizations at concentrations of 100 microM and above. 3. NMDA-induced depolarizations were significantly reduced by losigamone at concentrations of 25 microM and above. Losigamone had no effect on AMPA-induced depolarizations. 4. Veratridine (20 microM) and potassium (60 mM) were used to stimulate the release of amino acids from mouse cortex. Veratridine-stimulated release of glutamate was significantly reduced by losigamone at concentrations of 100 microM and above, while potassium-stimulated release was significantly reduced by losigamone at 200 microM. 5. NMDA antagonism and inhibition of excitatory amino acid release may contribute to the anticonvulsant effect of losigamone.

Flupirtine increases the levels of glutathione and Bc1-2 in hNT (human Ntera/D1) neurons: mode of action of the drug-mediated anti-apoptotic effect

Flupirtine is a triaminopyridine analogue which has been successfully applied in clinics as a non-opiate analgesic drug. Previously we described that flupirtine acts like an N-methyl-D-aspartate (NMDA) receptor antagonist in neuronal cells both in vitro and in vivo. Here we show that flupirtine displays its anti-apoptotic effect also in hNT (human Ntera/D1) neurons. hNT neurons were induced to apoptosis applying glutamate (Glu; at concentrations > or = 1 mM) or NMDA (> or = 1 mM). During Glu/NMDA-mediated apoptosis the levels of the intracellular anti-apoptotic agents Bc1-2 and glutathione dropped by more than 50%. Flupirtine completely abolished this reduction of Bc1-2 and glutathione level at a concentration of 10 microM. In the presence of 3 microM flupirtine a > 6-fold increase of the Bc1-2 (B-cell leukemia/lymphoma-2) level was observed in hNT neurons. At the same concentration, the intracellular level of glutathione increased to 200%. We conclude that the Glu/NMDA-mediated neuronal cell death in vitro is controlled at least partially by Bc1-2 and glutathione. Neuronal cell death by Glu or NMDA in vitro can be overcome applying the drug flupirtine which is in clinical use.

Prostaglandins play a role in memory consolidation in the chick

Previous studies showed that inhibitors of cyclooxygenases have amnesic effects in chicks in a passive avoidance task. The onset of amnesia has a delay of 2 h post-training. To investigate if this effect is due to the inhibition of induction of the enzyme during learning, the release of cyclooxygenase products into the extracellular fluid was measured at 1, 2 and 3 h post-training. A cyclooxygenase inhibitor, ibuprofen, inhibited the training-dependent increase of cyclooxygenase products only 2 h and 3 h after learning when injected pre-training, as did dexamethasone which prevents cyclooxygenase induction, and SC58125 (1,2-diarylcyclopentene), an inhibitor of inducible cyclooxygenase. Injections 30 min post-training showed the same effect with the exception of dexamethasone. Injecting SC58125, ibuprofen, indomethacin, or dexamethasone i.c. before training showed amnesic effects for training on a one-trial passive avoidance task at 2 h but not 1 h after training. Injections 30 min post-training produced the same effects with the exception of dexamethasone. I conclude that cyclooxygenases are induced during training and that cyclooxygenase products are of importance in memory formation of the chick.

The effect of the desglycinyl metabolite of remacemide hydrochloride (FPL 12495AA) and dizocilpine (MK-801) on endogenous amino acid release from mouse cortex

1. In this study the effect of FPL 12495AA, the desglycinyl metabolite of remacemide hydrochloride and dizocilpine (MK-801), on potassium- and veratridine-stimulated release of neurotransmitter amino acids from mouse cortical slices was investigated. 2. Veratridine (20 microM) and potassium (60 mM) produced a preferential release of glutamate and aspartate. Potassium-stimulated release was calcium-dependent, while veratridine-stimulated release was only partially affected by removal of calcium from the medium. 3. FPL 12495AA significantly inhibited veratridine- and potassium-stimulated release of glutamate and aspartate. Lower concentrations of FPL 12495AA were needed to inhibit veratridine-stimulated release of glutamate (12.5 microM) than potassium-stimulated release (100 microM). 4. Dizocilpine significantly inhibited veratridine- and potassium-stimulated release of glutamate and aspartate at concentrations of 100 microM and above. 5. FPL 12495AA and dizocilpine both have an affinity for the ion channel subsite of the N-methyl-D-aspartate (NMDA) receptor. The reduction of potassium-stimulated release of glutamate and aspartate by FPL 12495AA and dizocilpine is probably due to NMDA receptor blockade. 6. FPL 12495AA inhibited veratridine-stimulated release at a concentration of 12.5 microM while dizocilpine was effective only at a concentration of 100 microM. This difference in efficacy is probably due to the higher affinity of FPL 12495AA compared to dizocilpine at the veratridine-binding site on the sodium channel.

Inhibitors of cyclooxygenases produce amnesia for a passive avoidance task in the chick

Arachidonic acid is a putative messenger in synaptic transmission which presumably plays a role in learning and memory. Previous experiments showed that inhibitors of phospholipase A2-dependent release of arachidonic acid cause amnesia in a one-trial passive avoidance task in the chick. To test if arachidonic acid is metabolized to other messengers, the effects of inhibitors of enzymes which metabolize arachidonic acid were tested in the same task. The cyclooxygenase inhibitors indomethacin, naproxen and ibuprofen caused amnestic effects at all concentrations tested when injected intracerebrally before training. Injections were 5 microliters of 5-20 mmolar solutions per hemisphere. The onset of amnestic effects was always 2 h after training, independently of drug type, concentration, and injection time before training. The delay of 2 h after training suggests that the drugs prevent induction of cyclooxygenase synthesis. Post-training injections had no effect. Control tests showed little effect of the drugs on motor control and motivation. Caffeic acid and esculetin, inhibitors of lipoxygenases, and sodium furegrelate, a thromboxane synthase inhibitor, had no effect on performance of chicks in the task at all concentrations or time points tested. The results indicate that cyclooxygenase products, but not lipoxygenase or thromboxane synthase products, play a role in memory consolidation in the chick when learning this task.