ACTIVATION AND DESENSITIZATION OF ACETYLCHOLINE-RELEASE BY ZINC AT TORPEDO NERVE-TERMINALS

Treatment with 100 or 250 mu M ZnCl2 irreversibly blocked neurotransmi ssion in the Torpedo electric organ by inhibiting acetylcholine (ACh) release. In Zn2+-treated tissue, release failure did not result from i mpair ment of Ca2+ entry since stimulation still provoked an accumulat ion of Ca2+. Also pretreatment of isolated synaptosomes with Zn2+ inhi bited to the same extent the release elicited by KCl-evoked depolarisa tion and the release elicited by using the Ca2+ ionophore A23187. On t he other hand, after application of A23187, Zn2+ by itself efficiently triggered ACh release from synaptosomes. This dual effect of Zn2+ was also observed to occur in proteoliposomes equipped with mediatophore (a protein of the presynaptic membrane characterised by its capability to support Ca2+-dependent transmitter release). Hence, Zn2+ mimicked two fundamental actions of Ca2+ on nerve terminals, which are: (1) the immediate activation of release, and (2) a more slowly developing des ensitisation of release. Zn2+ was more powerful than Ca2+ for both act ions. It is concluded that the dual action of Zn2+ on the mediatophore protein accounts at least in part for its complex effects on neurotra nsmission.