Although most neuronal synapses operate in an 'all-or-nothing' fashion, some — for example, in the vertebrate retina or the insect ocellus — transmit graded signals. A new study by Peter Simmons, published in Neuron, describes how such transmission at one type of synapse shows an unprecedented dependency on the rate at which the presynaptic neuron is depolarized, rather than on the amplitude of the presynaptic depolarization.

The study focused on three large second-order neurons in the locust ocellar system, which is thought to be used to maintain stability during flight by providing rapid information about changes in the position of the horizon. The neurons — called L1–L3 — form reciprocal inhibitory synapses that produce graded postsynaptic depolarizations, but transmit only transiently, regardless of the duration of presynaptic depolarization.

Simmons found that, although spikes of different amplitudes in the presynaptic neuron produced inhibitory postsynaptic potentials (IPSPs) of different amplitudes in the postsynaptic cell, the size of the presynaptic depolarization was not the determining factor. He used a voltage clamp to depolarize the presynaptic neuron at different rates, and found that the rate of depolarization determined how fast the IPSP developed, and that each IPSP peaked at a fixed time after it had begun. So, because the IPSP can grow for only a limited time and because the rate of growth depends on the rate of presynaptic depolarization, the amplitude of the IPSP is dictated by the rate at which the presynaptic neuron depolarizes.

The reason for this behaviour is that the synapse depresses extremely rapidly. By measuring inhibitory postsynaptic currents under voltage clamp, Simmons found that transmitter was released at the synapse for a maximum of 2.1 ms. No other synapse has been found that depresses so rapidly and completely. When the ocellus is operating naturally, rapid decreases in light intensity produce fast rebound spikes, which would be rapid enough to produce IPSPs in the L neurons. These IPSPs might lead to synchrony of spiking in the three neurons. Simmons proposes that this mechanism allows the ocellar system to provide accurate information about the timing of changes in ocellar illumination, allowing the insect to monitor the horizon and maintain flight stability.

The mechanism of the rapid depression of these synapses is unclear, but it might involve a reduction in the sensitivity of presynaptic vesicle release to calcium entry, as has been found at other invertebrate synapses. Further studies of the dynamics of graded synaptic transmission might lead to similar insights into how these synapses are specialized to function in specific behavioural contexts, such as vision.