A.M. Paoletti

Systemic Administration of Nω-Nitro-l-Arginine Methyl Ester and Indomethacin Reduces the Elevation of Brain PGE2Content and Prevents Seizures and Hippocampal Damage Evoked by LiCl and Tacrine in Rat☆

Administration of tacrine (5 mg/kg i.p.), an anticholinesterase agent, in rats pretreated (24 h beforehand) with lithium chloride (LiCl; 12 mEq/kg i.p.) enhances the expression of neuronal nitric oxide (NO) synthase (NOS), increases NO, and causes seizures and hippocampal damage. Here we report immunohistochemistry evidence showing that in rat LiCl and tacrine enhance the expression of cyclooxygenase type 2 (COX-2) enzyme protein in the dorsal hippocampus and elevate brain PGE2 content during the preconvulsive period. The latter effect, but not enhanced COX-2 expression, is inhibited by previous (30 min before tacrine) administration of N omega-nitro-L-arginine-methyl ester (L-NAME; 10 mg/kg i.p.), an inhibitor of NO synthesis, thus implicating NO in the mechanism of stimulation of COX activity leading to elevation of brain PGE2 content. Indomethacin (10 mg/kg given i.p. 30 min before tacrine), an inhibitor of COX activity, prevented brain PGE2 elevation and abolished the expression of seizures and hippocampal damage thus supporting a role for this metabolite of the arachidonic acid cascade in the mechanisms of LiCl and tacrine-evoked neurotoxicity in rat.

Systemic administration of lithium chloride and tacrine but not kainic acid augments citrulline content of rat brain

The effects of tacrine (5 mg/kg i.p.) in lithium chloride (LiCl; 12 mEq/kg i.p.)-pretreated (24 h beforehand) animals and of kainate (10 mg/kg i.p.) on brain citrulline, the co-product of nitric oxide (NO) synthesis, were studied in rats. High performance liquid chromatography analysis of whole brain tissue homogenates from rats treated with LiCl and tacrine revealed a significant increase in citrulline content before the onset of seizures. This effect was prevented in a stereoselective manner by N omega-nitro-L-arginine methyl ester (10 mg/kg i.p., given 20 min before tacrine), an inhibitor of NO synthase. By contrast, kainic acid (10 mg/kg i.p.) did not affect significantly brain citrulline during the pre-convulsive period. In conclusion, our data indicate that in rats seizures induced by LiCl and tacrine but not kainic acid are triggered by excessive NO production in the brain.

Adenosine receptors in rat basophilic leukaemia cells: transductional mechanisms and effects on 5‐hydroxytryptamine release

1 The presence of adenosine receptors linked to adenylate cyclase activity and their functional role in calcium‐evoked 5‐hydroxytryptamine (5‐HT) release was investigated in rat basophilic leukaemia (RBL) cells, a widely used model for studying the molecular mechanisms responsible for stimulus‐secretion coupling. 2 In [3H]‐5‐HT‐loaded cells triggered to release by the calcium ionophore A23187, a biphasic modulation of 5‐HT secretion was induced by adenosine analogues, with inhibition of stimulated release at nm and potentiation at μm concentrations, suggesting the presence of adenosine receptor subtypes mediating opposite effects on calcium‐dependent release. This was also confirmed by results obtained with other agents interfering with adenosine pharmacology, such as adenosine deaminase and the non‐selective A1/A2 antagonist 8‐phenyl‐theophylline. 3 Similar biphasic dose‐response curves were obtained with a variety of adenosine analogues on basal adenylate cyclase activity in RBL cells, with inhibition and stimulation of adenosine 3′:5′‐cyclic monophosphate (cyclic AMP) production at nm and μm concentrations, respectively. The rank order of potency of adenosine analogues for inhibition and stimulation of adenylate cyclase activity and the involvement of G‐proteins in modulation of cyclic AMP levels suggested the presence of cyclase‐linked A1 high‐affinity and A2‐like low‐affinity adenosine receptor subtypes. However, the atypical antagonism profile displayed by adenosine receptor xanthine antagonists on cyclase stimulation suggested that the A2‐like receptor expressed by RBL cells might represent a novel cyclase‐coupled A2 receptor subtype. 4 Micromolar concentrations of adenosine analogues could also increase inositol phospholipid hydrolysis and inositol tris‐phosphate formation in both unstimulated cells and in cells triggered to release by the calcium ionophore. The stimulation was constant, small and additive to that exerted by the calcium ionophore. 5 It is concluded that RBL cells express both A1 and A2‐like adenosine receptors which exert opposite effects on 5‐HT release and intracellular cyclic AMP levels. However, besides modulation of cyclic AMP levels, additional transduction pathways, such as modulation of phospholipase C activity, may contribute to the release response evoked by adenosine analogues in this cell‐line.