PRESYNAPTIC INITIATION BY ACTION-POTENTIALS OF RETROGRADE SIGNALS IN DEVELOPING NEURONS

Until recently, the only means by which electrical activity was believ ed to initiate retrograde signals was via postsynaptic events: modulat ed synthesis or release of trophic factors, We have evidence in chick embryos for a presynaptic initiation of retrograde signals from the re tina to the isthmo-optic nucleus, which is known to undergo 55% neuron death between embryonic days 12 and 17 and to become laminated during this period, Intraocular injections of saxitoxin just before embryoni c day 14 reduce neuron death and prevent lamination in the isthmo-opti c nucleus within as few as 6 hr. We show that these rapid effects are attributable to the direct action of saxitoxin on the isthmo-optic ter minals. Alternative possibilities, such as an indirect effect via the target cells, are ruled out by control experiments, Normally, action p otentials may lead to a chain of second messenger events in the axon t erminal that is signaled retrogradely via the transport of a long-live d second messenger.