Small_1989_Science_243_1603

Tail shock produces transient presynaptic inhibition and longer lasting presynaptic facilitation of the siphon sensory neurons in Aplysia. The facilitation undergoes activity-dependent enhancement that is thought to contribute to classical conditioning of the gill- and siphon-withdrawal reflex. Inhibition of the sensory neurons has now also been shown to undergo activity-dependent enhancement when action potential activity in the sensory neurons is paired with inhibitory transmitter. This effect appears to involve an amplification of the same cellular mechanisms that are involved in normal presynaptic inhibition. These results suggest that activity-dependent enhancement may be a general type of associative cellular mechanism.