Analysis of high intracellular [Na+]-induced release of [3(H)]noradrenaline in rat hippocampal slices

Our aim was to investigate the mechanisms involved in the high intracellula r sodium-induced transmitter release in the CNS through the characterisatio n of the veratridine-evoked (40 muM) noradrenaline release from rat hippoca mpal slices. The response to veratridine was completely inhibited by tetrod otoxin (1 muM). indicating that the effect is due to the activation of sodi um channels. Omission of Ca2+ from the superfusion fluid inhibited the vera tridine-evoked release by 72% showing that the majority of release results From external Ca2+-dependent exocytosis. The residual Ca2+-independent rele ase was not blocked by the intracellular Ca2+ chelator 1,2-bis(2-aminopheno xy)ethane-N,N,N,N-tetraacetic acid acetoxymethyl ester (100 muM) suggesting that intracellular Ca2+ stores are not involved in this component of verat ridine effect. The noradrenaline uptake blockers, desipramine (10 muM) and nisoxetine (10 CIR I). inhibited the external Ca2+-independent release by 5 0 and 46%, respectively, indicating that the release partly originates from the reversal of transporters (carrier-mediated release). In contrast to up take blockers, lowering the temperature. another possibility to inhibit tra nsporter function, completely inhibited the effect of veratridine in the ab sence of Ca2+. Further experiments revealed that low temperature (20 and 12 degreesC) reduces the veratridine-induced increase of intracellular sodium concentration ([Na+](i)) in rat cortical synaptosomes (68 and 78% inhibiti on, respectively). The clinical relevance of our data is that during ischemia a massive releas e of transmitters occurs mainly due to the elevation of [Na+](i), which con tributes to the development of ischemic brain injury. Our results show that low temperature may be a better therapeutic approach to the treatment of i schemia because it has a dual action on this process. Firstly. it inhibits the function of uptake transporters and hence reduces the carrier-mediated outflow of transmitters. Secondly it inhibits the sodium influx and therefo re prevents the unwanted elevation of [Na+](i). Our data also suggest that veratridine stimulation can be a suitable model for ischemic conditions. (C ) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.