At the first synaptic level of the vertebrate retina, photoreceptor light responses are transmitted to second order neurones through a chemical synapse based on a tonic release of neurotransmitter modulated by graded changes of presynaptic potential. The possibility that such synapses could work through a Ca2+-independent process had been proposed by previous authors, based on the persistence of transmission process in low Ca2+ media containing Co2+ or Ni2+ ions. Recently, we were able to explain these results within the framework of the classical calcium-hypothesis of synaptic transmission by taking into account the modifications of presynaptic surface potential brought about by changes of divalent cation concentrations. Here we report data showing how a surface-charge hypothesis could account for several apparently paradoxical effects of divalent cation manipulations such as: the enhancement of neurotransmitter release induced by low Ca2+ media; the transmission "unblocking" effect of Zn2+, Co2+ and Ni2+; and the reversal of transmission polarity induced by application of low Ca2+ media containing Cd2+ or Mg2+ ions.
Manipulation of synaptic sign and strength with divalent cations in the vertebrate retina: pushing the limits of tonic, chemical neurotransmission / Piccolino, M.; Vellani, Vittorio; Rakotobe, La; Pignatelli, A.; Barnes, S.; Mcnaughton, P.. - In: EUROPEAN JOURNAL OF NEUROSCIENCE. - ISSN 0953-816X. - STAMPA. - 11(11):(1999), pp. 4134-4138.