Reversible associative depression and nonassociative potentiation at a parallel fiber synapse

The electrosensory robe (ELL) of mormyrid electric fish is one of several c erebellum-like sensory structures in fish that remove predictable features of the sensory inflow. This adaptive process obeys anti-Hebbian rules and a ppears to be mediated by associative depression at the synapses between par allel fibers and Purkinje-like cells of ELL. We show here that there is als o a nonassociative potentiation at this synapse that depends only on the re peated occurrence of the EPSP. The depression can be reversed by the potent iation and vice versa. Finally, we show that the associative depression req uires NMDA receptor activation, changes in postsynaptic calcium, and the oc currence of a postsynaptic dendritic spike within a few milliseconds follow ing EPSP onset.