The inhibition of agonist- or depolarisation-evoked formation of inositol phosphate by excitatory amino acids in rat cerebral cortex is due to the neurotoxic action of this class of neurotransmitter and is mediated by sodium influx.

Previous work has shown that excitatory amino acids inhibit agonist or depolarisation evoked formation of inositol phosphate in brain. In this paper, possible mechanisms by which this may be occurring have been investigated. The inhibition of carbachol-stimulated formation of inositol phosphate by kainic acid (KA) was abolished if the tissue was incubated in a sodium-free medium. The sodium channel activator, veratridine (10 microM) and the sodium ionophore, monensin (3 microM), also inhibited the response of inositol phosphate to carbachol; tetrodotoxin (300 nM) reversed the effect of veratridine but not monensin or KA. Incubation with cadmium (0.3 mM) or removal of extracellular calcium did not alter the effects of KA, monensin or veratridine. The effects of KA were significantly reduced with the Na+/K(+)-ATPase inhibitor, ouabain (10-100 microM). Inhibition by KA was still observed in tissue that had been prestimulated with KA and then washed to remove the agonist. Incorporation of [3H]inositol into inositol lipids was significantly reduced by KA, in the absence or presence of carbachol. It is suggested that the inhibition of the turnover of stimulated phosphoinositide, by excitatory amino acids, is related to the neurotoxic actions of these transmitters and is mediated by Na+ influx, with a consequent activation of Na+/K(+)-ATPase, depletion of cellular ATP and reduction in synthesis of inositol lipid.

Chronic carbamazepine down-regulates adenosine A2 receptors: studies with the putative selective adenosine antagonists PD115,199 and PD116,948.

Carbamazepine (CBZ), an anticonvulsant with psychotropic and anti-pain properties, has been reported to displace ligands at adenosine binding sites. This paper describes biochemical and behavioural studies in rodents comparing CBZ to the adenosine agonists L-phenylisopropyl-adenosine (L-PIA) and N-ethylcarboxamido-adenosine (NECA), the new antagonists PD116,948 and PD115,199 which are also relatively A1 and A2 specific respectively, and the mixed antagonists theophylline and caffeine, attempting to determine functional correlates of the binding studies. Changes in cAMP synthesis and behavioural syndromes produced by the drugs, alone and in combination, were monitored. Classification of the observed effects in terms of A1 and A2 activity was complex, probably due to functional interactions between A1 and A2 subtypes. Nevertheless, it was found that chronic CBZ administration (0.25% in food for 3 days, followed by 0.5% for 11 days) produced a pattern of interaction identical to that of PD115,199 (10-100 mg/kg IP). Thus, both treatments attenuated the behavioural syndrome produced by L-PIA (0.1 or 0.5 mg/kg SC), but did not affect that produced by NECA (0.03 mg/kg SC). CBZ mildly increased hypoactivity after clonidine (0.2 mg/kg IP) which was used as a control. By contrast, the A1 antagonist PD116,948 (0.1-10 mg/kg IP) antagonised both behavioural syndromes. Similarly in the biochemical experiments both chronic CBZ and PD115,199 (10-100 microM) reduced stimulation of cAMP synthesis by L-PIA (confirming that this is mediated by A2 receptors), while only basal cAMP synthesis was affected by PD116,948 (10 microM) and theophylline (60 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of non-NMDA antagonists and phorbol esters on excitatory amino acid stimulated inositol phosphate formation in rat cerebral cortex.

The effects of various modulators of excitatory amino acid stimulated inositol phosphate (IP) formation were investigated in slices of rat cerebral cortex. Both quisqualic acid (Quis) and ibotenic acid (Ibo) stimulated IP formation in a dose-dependent manner. Quis (0.3mM) stimulated the rapid formation of inositol bis, tris and tetrakis-phosphates, with a slower linear rise in the monophosphate which plateaued after 20 min. The responses to both Ibo (0.3 mM) and Quis (0.3 mM) were dose-dependently inhibited by phorbol dibutyrate (PDBu); the Ibo response was particularly sensitive to PDBu, with a 90% inhibition of the response at 1 ?M and an IC(50) of 200 nM; Quis stimulated IP formation was less sensitive with a 50% inhibition observed at 10 ?M PDBu. PMA (1 ?M) and dioctanoylglycerol (30 ?M) also inhibited Ibo-stimulated IP formation though 4-?-phorbol (10 ?M) was ineffective. The inhibition by PDBu was reversed with staurosporine (10 ?M) and polymyxin (10 ?M), and both protein kinase C inhibitors potentiated the Ibo response in the absence of PDBu. The putative Quis-kainate receptor antagonists 6,7-dinitroquinoxaline-2,3-dione and 6-cyano-7-nitroquinoxaline-2,3-dione did not inhibit the responses to either Quis or Ibo at a concentration of 100 ?M. Both responses were, however, inhibited by dl-2-amino-4-phosphonobutyric acid and O- phospho- l -serine (IC(50)s were 1-2 mM), and also by the putative Quis receptor agonist dl-?-amino-3-hydroxy-5-methyl-4-isoxalone propionic acid (AMPA; IC(50) 10?M). Our data suggest that excitatory amino acid stimulated IP formation in rat cerebral cortex is via a site distinct from previously defined amino acid receptors.

Differential effects of lithium on agonist-stimulated inositol polyphosphate formation in rat cerebral cortex slices: Selective actions on muscarinic cholinergic responses.

The effect of lithium ions on agonist-stimulated inositol polyphosphate production was investigated in slices of rat cerebral cortex. LiCl potentiated the formation of inositol monophosphate and inositol bisphosphate following stimulation with a variety of agonists including carbachol (1 mM), noradrenaline (NA, 300 ?M), 5-hydroxytryptamine (5-HT, 100 ?M) and quisqualic acid (Quis, 100 ?M), the EC(50) for these effects was in the range 0.5-5 mM. The production of inositol trisphosphate and inositol tetrakisphosphate following NA, 5-HT or Quis stimulation was not significantly affected by LiCl, though there was a delayed but striking inhibition of both the inositol 1,4,5-trisphosphate and, particularly, inositol tetrakisphosphate responses to carbachol, which was even greater in buffer containing elevated (20 mM) KCl. The possible mechanisms underlying this effect of LiCl are discussed in relation to previously observed effects of this ion on phosphoinositide metabolism.

Alteration of inositol phospholipid metabolism in rat cortex by lithium but not carbamazepine.

The effect of carbamazepine on inositol phospholipid metabolism was investigated in rat cerebral cortex. The stimulation of inositol phosphate formation by 5-HT (10 microM), carbachol (100 microM) and noradrenaline (30 microM) was not significantly altered by carbamazepine in vitro. 14 days treatment of rats in vivo with carbamazepine was similarly without effect on these parameters. In contrast 14 days treatment with LiCl reduced the agonist responses by 25-50%. The results are discussed in relation to the therapeutic actions of these drugs.