The electrosensory system in mormyrid fish serves three functions: active electrolocation, in which external object are sensed by the distortions of the electric field of the fish’s electric organ discharge (EOD); el...The electrosensory system in mormyrid fish serves three functions: active electrolocation, in which external object are sensed by the distortions of the electric field of the fish’s electric organ discharge (EOD); electrocommunication, in which EODs convey information between fish; and passive electrolocation, in which the low frequency electric fields from other animals in the water are sensed. These functions are served by three classes of electroreceptors in the fish skin. The primary afferents from these electroreceptors terminate in different regions of the electrosensory lobe (ELL). ELL is a cerebellum like structure which is also strongly affected by descending electric organ corollary discharge signal (EOCD) associated with the motor command signals that drives EOD. Thus, the descending EOCD signal and the EOD electrosensory input converge onto the ELL. In vivo studies have indicated that the EOCD signal generates negative images of predictable features in the sensory inflow in mormyrid ELL. Addition of such negative images to the actual or concurrent sensory input minimizes the predictable features and allows novel or unexpected inputs to stand out more clearly. The generation of negative images is largely due to plasticity at the parallel fiber synapses onto Purkinje-like cell in ELL. The present study extended these in vivo findings to in vitro slice preparation. The basic features and mechanisms of such plasticity were studied in ELL slices with the intracellular recording method. The plasticity was rapidly established by pairing the electric activated parallel fiber epsp with a post synaptic broad spike evoked by depolarizing current pulse in a Purkinje like cell, at 1 Hz for 5 min at 0 to 50 ms delays. After such pairing, the parallel fiber epsp was depressed by 25%～30% for over 30 min. This depression was blocked by adding NMDA receptor antagonist AP5 to the bath or injecting calcium chelator BAPTA into the recorded cell. The depression could be reversed by stimulating the parallel fiber at 1 Hz without pairing with post-synaptic broad spike, this happens under natural physiological conditions. It is hypothesized that the broad spike leads to calcium influx through NMDA receptors during pairing. The elevation in post synaptic calcium in turn leads to the depression of synaptic transmission in the post synaptic cell by activating different protein kinases. Such reversibility and a narrow timing window are the most important features of this plasticity and has not been demonstrated in any system of its kind, including the mammalian cerebellum. The reversibility allows the neuron to “forget” the past event and get ready for a new task. The narrow timing window renders the cell able to selectively “remember” some information, not every single input signal.展开更多
文摘The electrosensory system in mormyrid fish serves three functions: active electrolocation, in which external object are sensed by the distortions of the electric field of the fish’s electric organ discharge (EOD); electrocommunication, in which EODs convey information between fish; and passive electrolocation, in which the low frequency electric fields from other animals in the water are sensed. These functions are served by three classes of electroreceptors in the fish skin. The primary afferents from these electroreceptors terminate in different regions of the electrosensory lobe (ELL). ELL is a cerebellum like structure which is also strongly affected by descending electric organ corollary discharge signal (EOCD) associated with the motor command signals that drives EOD. Thus, the descending EOCD signal and the EOD electrosensory input converge onto the ELL. In vivo studies have indicated that the EOCD signal generates negative images of predictable features in the sensory inflow in mormyrid ELL. Addition of such negative images to the actual or concurrent sensory input minimizes the predictable features and allows novel or unexpected inputs to stand out more clearly. The generation of negative images is largely due to plasticity at the parallel fiber synapses onto Purkinje-like cell in ELL. The present study extended these in vivo findings to in vitro slice preparation. The basic features and mechanisms of such plasticity were studied in ELL slices with the intracellular recording method. The plasticity was rapidly established by pairing the electric activated parallel fiber epsp with a post synaptic broad spike evoked by depolarizing current pulse in a Purkinje like cell, at 1 Hz for 5 min at 0 to 50 ms delays. After such pairing, the parallel fiber epsp was depressed by 25%～30% for over 30 min. This depression was blocked by adding NMDA receptor antagonist AP5 to the bath or injecting calcium chelator BAPTA into the recorded cell. The depression could be reversed by stimulating the parallel fiber at 1 Hz without pairing with post-synaptic broad spike, this happens under natural physiological conditions. It is hypothesized that the broad spike leads to calcium influx through NMDA receptors during pairing. The elevation in post synaptic calcium in turn leads to the depression of synaptic transmission in the post synaptic cell by activating different protein kinases. Such reversibility and a narrow timing window are the most important features of this plasticity and has not been demonstrated in any system of its kind, including the mammalian cerebellum. The reversibility allows the neuron to “forget” the past event and get ready for a new task. The narrow timing window renders the cell able to selectively “remember” some information, not every single input signal.
