The goal of the proposed project is to determine the basic mechanisms of synaptic plasticity and functional circuitry in the electrosensory lobe (ELL) of mormyrid electric fish. The ELL is a cerebellum-like structure where the primary afferent fibers from electroreceptors terminate. The ELL of the mormyrid electric fish and similar cerebellumlike structures in other fish are adaptive sensory processors in which memory-like predictions about sensory input are generated and subtracted from current sensory input, allowing neural responses to unexpected or novel input to stand out. The predictions are based on prior associations between sensory input and various central signals conveyed by parallel fibers such as corollary discharge signals linked to motor commands. Plasticity at the excitatory synapse between parallel fibers and ELL cells has been established in in vitro slices. This synaptic plasticity is probably at least partly responsible for the systems-level generation of memory-like predictions. The plasticity is observed after a few minutes of pairing a presynaptic parallel fiber input with a postsynaptic spike. The plasticity includes an associative depression that depends on the relative timing of pre- and postsynaptic spikes and a non-associative potentiation. The potentiation can reverse the depression and vice versa, indicating that both processes affect a common cellular site. Recent findings suggest that this common site of action is in the presynaptic terminal of parallel fibers.
The Specific Aims of the proposed project are: 1) to determine the basic mechanisms of the plasticity established at parallel fiber synapses in ELL; 2) to determine whether plasticity also occurs at inhibitory synapses in ELL; and 3) to determine basic features of ELL circuitry. The results of this research will enhance our understanding of synaptic plasticity at the cellular level and the role of such plasticity in memory-like functions. The results may also be a source of insight into other cerebellum-like structures, such as the dorsal cochlear nucleus of mammals or the cerebellum itself.
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