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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A slug on focus

One of our collaborator trying to do motion capture of swimming slugs.

The white beads glued on the skin fell off because this shy slug sweated a lot of slime.







It did perform an excellent swim, though.

Interesting SfN posters

It is a bit late but this is about the SfN meeting. During cleaning up my desk near the end of year I rescued a bunch of notes I took during the meeting. Most of them were hard to read. Here are some.


497.06 Presentation Title: Endogenous neuropeptide facilitates INap pacemakers and acts on low respiratory frequency
*A. DOI, J. M. RAMIREZ

I often heard of Dr. Doi's work in Dr. Ramirez's talks. This was actually my first time visiting his poster. He explained to me in very clear logic in English and then we chatted in Japanese. He studied the role of endogenous substance P (SP) and toic activation of neurokinin1 (NK1) receptors in the generation of eupneic respiratory rhythm in pre-Bötzinger complex (pre-BötC) by blocking of NK1 receptors.
Both Cd²⁺-sensitive (CS) and Cd²⁺-insensitive (CI) pacemakers received endogenous SP release, but CI pacemaker activities were only tonically facilitated by activation of NK1 receptors. This seems to be due to the difference in voltage-sensitivity between CS pacemakers and CI pacemakers. The former mediated by non-selective cation channel (ICaN) whereas the latter by persistent Na⁺ channel (INaP).


585.09 Presentation Title: Electrical coupling in Aplysia bag cell neurons: Biophysical characterization and role in synchronous firing.
P. L. W. COLMERS, *N. S. MAGOSKI
The role of gap-junctions among the Aplysia bag cell neurons were examined. Meclofenamic acid is an electrical coupling blocker.


699.20 Presentation Title: Developmental reconfiguration of gaze control commands from spinal locomotor circuitry during Xenopus metamorphosis.
*D. COMBES, G. VON UCKERMANN, D. LE RAY, F. LAMBERT, H. STRAKA, J. SIMMERS
Dr. Denis Combes' poster is always one of my favorite in this meeting. This is a true NEUROBIOLOGY work. He studied the developmental changes in the neural control of eye movement in tadpole. During metamorphosis, Xenopus switches its mode of locomotion from tail-based undulatory movements to bilaterally-synchronous hindlimb kick propulsion, with intervening stages where these two modes co-exist.
There is a developmental transition in spinal efference copy control of extraocular motor output during Xenopus metamorphosis that corresponds to the change in locomotor strategy.


818.15 Evolution of a behavior mediated by the lateral line system adapts Astyanax to life in darkness.
*M. YOSHIZAWA, W. R. JEFFERY
This is also a very interesting work that grasped the moment of evolution of cavefish traits during the adaptation processes. Showing physiological and genetic basis of an adaptive behavior, vibration attraction behavior (VAB) inAstyanax mexicanus, which has an eyed surface (surface fish) and a blind cave (cavefish) dwelling forms.
Quantitative laboratory assays indicate that VAB is common in cavefish but rarely observed and much less robust in surface fish. VAB is beneficial for feeding in the dark. Increase in superficial neuromast (SN) during development is important forVAB.

Aplysia brasiliana

Charuni, a grad student in our lab, is now studying the swimming behavior of a sea hare, Aplysia brasiliana. Compared to other sea slugs, this one looks very intelligent. Especially when they are swimming. Peeping over the water, flipping over, making quick turns... They are not sluggish at all. Their playful behaviors make me even think like these creatures are almost as intelligent as dolphins.

The Dendronotus and Melibe paper in Current Biology

My latest paper appeared in Current Biology!

Different Roles for Homologous Interneurons in Species Exhibiting Similar Rhythmic Behaviors

This paper describes the differences in synaptic properties and organization of the swim CPGs in closely-related nudibranch species, Melibe leonina and Dendronotus iris. These animals show very similar swimming behaviors with left-right body flexions. However, their CPGs have quite different network organization; they have different synaptic connectivity and responded differently when perturbed by current injections. Thus, similarity in species-typical behavior is not necessarily predictive of common neural mechanisms.



We also discuss about how species-specific behaviors have developed through the animal evolution. We showed that, even though closely-related animals with similar neuronal architectures exhibit similar behaviors, some degree of divergence can be found in the underlying neural circuitry. A pair of homologous neurons in one species have regulatory role to modify the motor rhythm, whereas in the other species they are part of the CPG and reinforce the rhythm regularity. We don't know what is the functional significance; maybe such difference will be hidden until some time when it becomes more critical in the face of some environmental perturbations.

Plateau potentials in sacrocaudal motoneurons of chronic spinal rats, recorded in vitro

David J. Bennet, Yunru Li, and Merek Siu
J. Neurophysiol. 86, 2001

Jerky limb movements often develop after spinal cord injury. This general spasticity syndrome is caused at least in part by enhanced plateau potentials in the MNs. Depolarization in MN triggers the plateau potential and causes its sustained firing, which lead to exaggerated muscle contraction. This paper describes the basic properties of this plateau potential and the sustained firing in MNs after spinal cord transection. The recurrent GABAergic and Glycinergic inhibitions may play a role in suppressing this enhanced plateau potential. This will be studied in following paper.

A new pump from Pentair Aquatics

A few weeks ago on Saturday I installed a new pump in one of our animal tanks. It is Quiet One Aquarium Pump 3000 - 780 gph from Pentair Aquatics. It can directly connect to Lifegard Module Filter System. But I didn't do that. I didn't want to do the plumbing job for all the circuitry of the PVC pipes under the tank on weekend. I just used plastic tubes to put it in.



Little Giant pumps were originally installed when Paul purchased them in 1997. Since then we had been using them with no specific reason despite their frequent failures. I like this new one better. It's real quiet. It is waterproof. And it's cheap (~$60)! Little Giant pumps usually cost more than $200. I will switch from Little Giant to Quiet One for all tanks.