Presynaptic depolarization rate controls transmission at an invertebrate synapse.
Second-order neurons L1-3 of the locust ocellar pathway make inhibitory synapses with each other. Although the synapses transmit graded potentials, transmission depresses rapidly and completely so that a synapse only transmits when the presynaptic terminal depolarizes rapidly. The rate at which a presynaptic neuron depolarizes determines the rate at which a postsynaptic neuron hyperpolarizes, and neurotransmitter is only released during a fixed 2 ms long period. Consequently, the amplitude of a postsynaptic potential depends on the rate rather than the amplitude of a presynaptic depolarization. Following a postsynaptic potential, a synapse recovers from depression over about a second. The synapse recovers from depression even if the presynaptic terminal is held depolarized.