Dendritic spike back propagation in the electrosensory lobe of Gnathonemus petersii

Spike timing-dependent plasticity that follows anti-Hebbian rules has been demonstrated at synapses between parallel fibers and inhibitory interneurons known as medium ganglionic layer (MG) neurons in the cerebellum-like electrosensory lobe of mormyrid fish. This plasticity is expressed when presynaptic activation is associated with a characteristically broad, postsynaptic action potential, lasting 7-15 ms, occurring within a window of up to 60-80 ms following synaptic activation. Since the site of plastic change is presumably in the apical dendrites, it is important to know where, when and how this broad spike is generated and the manner in which such events propagate within the intrinsic network of the electrosensory lobe. The electrosensory lobe has a strict layered organization that makes the preparation suitable for one dimension current source density analysis. Using this technique in an 'in vitro' interface slice preparation, we found that following either parallel fiber stimulation or an orthogonal field stimulus, a sink appeared in the ganglionic layer and propagated into the molecular layer. Intracellular records from MG somata showed these stimuli evoked broad action potentials whose timing corresponds to this sink. TTX application in the deep fiber layer blocked the synaptically evoked ganglionic layer field potential and the 'N3' wave of the outer molecular layer field potential simultaneously, while the molecular layer 'N1' and 'N2' waves corresponding to synaptic activation of the apical dendrites remained intact. These results confirm the hypothesis that the broad spikes of MG cells originate in the soma and propagate through the molecular layer in the apical dendritic tree, and suggest the possibility that this backpropagation may contribute to 'boosting' of the synaptic response in distal apical dendrites in certain circumstances.