A Quantal Analysis of the Synaptic Depression Underlying Habituation of the Gill-Withdrawal Reflex in Aplysia

Habituation, one of the simplest behavioral paradigms for studying memory, has recently been examined on the cellular level in the gill-withdrawal reflex in the mollusc Aplysia and in the escape response in cray-fish. In both cases short-term habituation involved a decrease in excitatory synaptic transmission at the synapses between the sensory neurons and their central target cells. To analyze the mechanisms of the synaptic depression in Aplysia, we applied a quantal analysis to synaptic transmission between the sensory and motor neurons of the gill-withdrawal reflex. Our results indicate that short-term habituation results from a presynaptic mechanism: a decrease in the number of transmitter quanta released per impulse. The sensitivity of the postsynaptic receptor remains unaltered.