Gap Junctions Compensate for Sublinear Dendritic Integration in an Inhibitory Network

 By Koen Vervaeke, Andrea Lorincz, Zoltan Nusser, R. Angus Silver


This intense study show an example of how dendritic synaptic signals are integrated into action potentials in the electrically-coupled Golgi cells.
Major punch line results were drawn from a computer model.

Background:
1) A Golgi cell has big dendrites in one side and axon on the other side of the soma.
2) Parallel fibers make excitatory synapses on distal dendrites, whereas mossy fibers make excitatory synapses near the cell body.
3) Golgi cells are electrically coupled by gap junctions.

I. Local application of Glu to see how the synaptic signals were integrated and conducted to the soma.
This gave an imprecation of active conductance somewhere along the path (axon).

II. Indeed, proximal region from the soma has V-dependent potassium (outward) conductance.

III. Sponteneous EPSPs get bigger when recorded at more distal dendrite. The same EPSPs appear smaller in the soma, but the action potentials were always evoked near the soma.

IV. Because of the locational difference, MF and PF have different effects: MF can evoke spikes more efficiently whereas PF depolarized the dendrites more.

V. How gap-junction play a role in MF/PF-evoked excitation (model study)
 - Gap-junctions reduced overall Rinput - on average it reduces the responsiveness (Fig. 4D) by reduction of responsiveness in directly-innervated GoCs.
 - However, there is remarkable increase in coupled-firing in GoCs that are not innervated but coupled to the innervated ones. More in PF-generated firing.

VI. Discussion:
 - GJ-mediated lateral excitation counteracts the effects of sublinear dendritic behavior by enabling distal inputs to drive network activity more effectively.
 - The combination of passive dendrites and gap junctions facilitates wide-spread excitation among the interneurons whether or not each of them are innervated.
 - No need to boost synaptic potentials with active dendritic conductance.
 - How gap junction contribute spatial averaging and gain control of inhibitory interneurons.
 - The coupled interneurons exhibit features of a syncitium.
 - Similar mechanisms must be going on in other cortex inhibitory neurons, granule cells, retina and other sensory systems.



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