Manipulation of synaptic sign and strength with divalent cations in the vertebrate retina: pushing the limits of tonic, chemical neurotransmission.
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.
Piccolino
M
M
Dipartimento di Biologie, Sezione di Fisiologie Generale, Università di Ferrara, Ferrara, Italy. pic@unife.it
Vellani
V
V
Rakotobe
L A
LA
Pignatelli
A
A
Barnes
S
S
McNaughton
P
P
D000557:Ambystoma; D000818:Animals; D002118:Calcium / Q000494:pharmacology / Q000502:physiology*; D002413:Cations, Divalent / Q000494:pharmacology*; D003035:Cobalt / Q000494:pharmacology; D066298:In Vitro Techniques; D008027:Light; D008274:Magnesium / Q000494:pharmacology; D009474:Neurons / Q000187:drug effects / Q000502:physiology*; D009532:Nickel / Q000494:pharmacology; D020419:Photoreceptor Cells, Vertebrate / Q000187:drug effects / Q000502:physiology*; D012160:Retina / Q000166:cytology / Q000502:physiology*; D017948:Retinal Rod Photoreceptor Cells / Q000502:physiology; D013569:Synapses / Q000187:drug effects / Q000502:physiology*; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*; D014426:Turtles; D015032:Zinc / Q000494:pharmacology
D002413:Cations, Divalent; D003035:Cobalt; D009532:Nickel; D008274:Magnesium; D015032:Zinc; D002118:Calcium