Action potential backpropagation and somato-dendritic distribution of ion channels in thalamocortical neurons

Thalamocortical (TC) neurons of the dorsal thalamus integrate sensory input s in an attentionally relevant manner during wakefulness and exhibit comple x network-driven and intrinsic oscillatory activity during sleep. Despite t hese complex intrinsic and network functions, little is known about the den dritic distribution of ion channels in TC neurons or the role such channel distributions may play in synaptic integration. Here we demonstrate with si multaneous somatic and dendritic recordings from TC neurons in brain slices that action potentials evoked by sensory or cortical excitatory postsynapt ic potentials are initiated near the soma and backpropagate into the dendri tes of TC neurons. Cell-attached recordings demonstrated that TC neuron den drites contain a nonuniform distribution of sodium but a roughly uniform de nsity of potassium channels across the somatodendritic area examined that c orresponds to approximately half the average path length of TC neuron dendr ites. Dendritic action potential backpropagation was found to be active, bu t compromised by dendritic branching, such that action potentials may fail to invade relatively distal dendrites. We have also observed that calcium c hannels are nonuniformly distributed in the dendrites of TC neurons. Low-th reshold calcium channels were found to be concentrated at proximal dendriti c locations, sites known to receive excitatory synaptic connections from pr imary afferents, suggesting that they play a key role in the amplification of sensory inputs to TC neurons.