5-HT uptake blockade prevents the increasing effect of K(ATP) channel blockers on electrically evoked [3H]-5-HT release in rat and mouse neocortical slices

To explore if prolonged--as opposed to acute--5-HT uptake blockade can lead to changes in the function of ATP-dependent potassium (K(ATP)) channels, we investigated in rat and mouse neocortical slices the effects of K(ATP) channel blockers on electrically evoked 3H]-serotonin (3H]-5-HT) release after short- and long-term exposure to 5-HT uptake blockers. Glibenclamide (1 microM), a K(ATP) channel blocker, enhanced the electrically evoked 3H]-5-HT release by 66 and by 77%, respectively, in rat and in mouse neocortex slices. This effect was confirmed in the rat by tolbutamide (1 microM), another K(ATP) channel antagonist. After short-term blockade (45 min) of 5-HT uptake, glibenclamide still increased the release of 3H]-5-HT in the rat. Glibenclamide, however, failed to enhance 3H]-5-HT release after long-term uptake blockade (210 min). In the mouse, however, both short- and long-term inhibition of 5-HT reuptake by citalopram (1 microM) prevented the facilitatory effect of glibenclamide. The Na(+)/K(+)-ATPase inhibitor ouabain (3.2 microM) abolished the glibenclamide-induced increase in 3H]-5-HT release in both rat and mouse, suggesting that an operative Na(+)/K(+)-ATPase is a prerequisite for activation of K(ATP) channels. The terminal 5-HT(1B) autoreceptor-mediated feedback control was involved in the glibenclamide-induced increase in (3)H]-5-HT release only in mouse neocortical tissue, as evident from the use of the 5-HT(1B) autoreceptor ligands metitepin (1 microM) and cyanopindolol (1 microM). These results suggest that in the rat long-term uptake blockade leads to an impaired activity of the Na(+)/K(+)-ATPase, which increases intracellular ATP and consequently closes K(ATP) channels. In the mouse, however, short-term uptake blockade seems to already reduce the activity of the Na(+)/K(+)-ATPase and thereby the consumption of ATP. Blockade of 5-HT transporters thus may close K(ATP) channels through increased intracellular ATP. The following slight depolarisation seems to facilitate 5-HT release. These results may contribute to a better understanding of the mechanisms involved in the clinical time latency of antidepressant efficacy of monoamine uptake blockers.