Reversible associative depression and nonassociative potentiation at a parallel fiber synapse.

The electrosensory lobe (ELL) of mormyrid electric fish is one of several cerebellum-like sensory structures in fish that remove predictable features of the sensory inflow. This adaptive process obeys anti-Hebbian rules and appears to be mediated by associative depression at the synapses between parallel fibers and Purkinje-like cells of ELL. We show here that there is also a nonassociative potentiation at this synapse that depends only on the repeated occurrence of the EPSP. The depression can be reversed by the potentiation and vice versa. Finally, we show that the associative depression requires NMDA receptor activation, changes in postsynaptic calcium, and the occurrence of a postsynaptic dendritic spike within a few milliseconds following EPSP onset.