Variable action potential backpropagation during tonic firing and low-threshold spike bursts in thalamocortical but not thalamic reticular nucleus neurons.

William M. Connelly, Vincenzo Crunelli and Adam C. Errington

(2017) The Journal of Neuroscience, 37(21) 5319-5333.

In neurons throughout the brain it has been shown that backpropagation of action potentials into the dendritic tree is important for a number of physiological processes in neurons, particularly plasticity processes. In both thalamocortical (TC) and thalamic reticular nucleus (TRN) neurons the full extent of action potential backpropagation has remained unclear despite calcium imaging and computational modelling studies. In this study we have used 2-photon targeted dendritic recordings to directly measure the backpropagation of action potentials into TC and TRN neuron dendrites. In these neurons we find that action potentials are initiated in the region close to the soma, most likely the axon initial segment, and are able to backpropagate into the dendritic tree. However, compared with other cell types backpropagation into the dendritic tree in not particularly efficient in TC and TRN neurons. We also show that in TC neurons the extent of action potential backpropagation varies between low-threshold spike bursts and tonic firing but this does not occur in TRN neurons where bAPs are unaffected by firing mode. These data support earlier findings using 2-photon Ca2+-imaging and demonstrate that there is a clear difference in dendritic signalling in thalamic neurons during tonic firing, which is prevalent during wakefulness, and low-threshold spike bursts that occur during lower vigilance states.