Calcium entry related to active zones and differences in transmitter release at phasic and tonic synapses

Synaptic functional differentiation of crayfish phasic and tonic motor neur ons is large. For one impulse, quantal release of neurotransmitter is typic ally 100-1000 times higher for phasic synapses. We tested the hypothesis th at differences in synaptic strength are determined by differences in synapt ic calcium entry. Calcium signals were measured with the injected calcium i ndicator dyes Calcium Green-1 and fura-2. Estimated Ca2+ entry increased al most linearly with frequency for both axons and was two to three times larg er in phasic terminals. Tonic terminal Ca2+ at 10 Hz exceeded phasic termin al Ca2+ at 1 Hz, yet transmitter release was much higher for phasic termina ls at these frequencies. Freeze-fracture images of synapses revealed on ave rage similar numbers of prominent presynaptic active zone particles (putati ve ion channels) for both neurons and a two- to fourfold phasic/tonic ratio of active zones per terminal volume. This can account for the larger calci um signals seen in phasic terminals. Thus, differences in synaptic strength are less closely linked to differences in synaptic channel properties and calcium entry than to differences in calcium sensitivity of transmitter rel ease.