Abstract
Inhibitory interneurons (INs) in the lateral geniculate nucleus (LGN) provide both axonal and dendritic GABA output to thalamocortical relay cells (TCs). Distal parts of the IN dendrites often enter into complex arrangements known as triadic synapses, where the IN dendrite plays a dual role as postsynaptic to retinal input and presynaptic to TC dendrites. Dendritic GABA release can be triggered by retinal input, in a highly localized process that is functionally isolated from the soma, but can also be triggered by somatically elicited Ca2+-spikes and possibly by backpropagating action potentials. Ca2+-spikes in INs are predominantly mediated by T-type Ca2+-channels (T-channels). Due to the complex nature of the dendritic signalling, the function of the IN is likely to depend critically on how T-channels are distributed over the somatodendritic membrane (T-distribution). To study the relationship between the T-distribution and several IN response properties, we here run a series of simulations where we vary the T-distribution in a multicompartmental IN model with a realistic morphology. We find that the somatic response to somatic current injection is facilitated by a high T-channel density in the soma-region. Conversely, a high T-channel density in the distal dendritic region is found to facilitate dendritic signalling in both the outward direction (increases the response in distal dendrites to somatic input) and the inward direction (the soma responds stronger to distal synaptic input). The real T-distribution is likely to reflect a compromise between several neural functions, involving somatic response patterns and dendritic signalling.
Highlights
A single neuron may contain a dozen or more different types of ion channels, including the traditional action potentials (APs)-generating Na+- and delayed-rectifier K+-channels and different types of Ca2+-channels
We explored the effect of the distribution of T-channels (Tdistribution) on the following neural properties, R1: somatic, regular AP-firing R2: backpropagation of APs into distal dendrites, R3: somatic burst firing, R4: dendritic Ca2+-spikes, and R5: synaptic integration
The model was presented in six versions, each characterized by a different T-distribution (Fig. 1B), namely (i) the soma-distribution, Dsoma, where all T-channels were in the soma, (ii) the proximal-distribution, Dprox, where Tchannels were clustered in the proximaldendritic region and in the soma, (iii) the uniform-distribution, Duni, where T-channels were uniformly distributed over the membrane, (iv) the middledistribution, Dmid, where T-channels were clustered in dendritic regions halfway between the soma and the distal dendrites, (v) the linear-distribution, Dlin, where the density of T-channel increased linearly with distance from soma, and (vi) the distal-distribution, Ddist, where T-channels were clustered in distal dendrites
Summary
A single neuron may contain a dozen or more different types of ion channels, including the traditional AP-generating Na+- and delayed-rectifier K+-channels and different types of Ca2+-channels. The complex function of INs depends on the somatic firing properties as well as the two-way communication between the soma and distal dendritic sites, both of which are potentially influenced by the T-distribution.
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