Calcium-independent release of [H-3]spermine from chick retina

Spermine has been shown to influence NMDA receptor function through an inte raction at the coagonist site for glycine in the central nervous system (CN S) and the retina. In order to support a role for spermine as neurotransmit ter or neuromodulator in the chick retina, specific stimulated-release of s permine should be demonstrated. Isolated chick retinas, preloaded with [H-3 ]spermine, were stimulated with 1 mM NMDA and other glutamate agonists at i onotropic receptors, in a continuous superfusion system. [H-3]spermine was released from the retina by depolarization with 50 mM KCl, in a Ca2+-indepe ndent manner. Inhibition of Na+/K+-ATPase by ouabain or digitoxigenin also induced spermine release following 36 min in the presence of the drugs; suc h effect seems unrelated to changes in Na+ electrochemical gradients, since nigericin and veratrine did not induce release in Na+ containing medium. T he lack of effect of glutamate, NMDA and kainate at 1 mM concentration, sug gests that release of spermine in the retina is mediated by the reversal of uptake and not necessarily linked to EAA-receptor activation. (C) 2000 Els evier Science B.V. All rights reserved.