Command neuron for a half-center oscillator

Our latest paper just came out in the Journal of Neuroscience.

Command or obey? Homologous neurons differ in hierarchical position for the generation of homologous behaviors
Akira Sakurai and Paul S. Katz 
J Neurosci 17 June 2019, 3229-18
DOI: https://doi.org/10.1523/JNEUROSCI.3229-18.2019

Click here for the reprint

Nudibranchs have homologous neurons that can be identified across species. Cross-species comparisons of motor system organization provide fundamental insights into their function and origin. This paper shows that an identified cerebral ganglion neuron serves as a command neuron for the swimming behavior in a nudibranch species. The same neuron serves as a member of a central pattern generator (CPG) in another species. We described the synaptic and neuromodulatory mechanisms by which the command neuron initiates and accelerates rhythmic motor patterns.


Two sea slug species show homologous swimming behaviors
In motor systems, higher-level components issue commands that are carried out by lower-level circuits. In this paper, we describe the physiological actions of an identified neuron, which turned out to be a "command" neuron for the swimming behavior of a giant sea slug, Dendronotus iris. We determined which functional components of the swim CPG are modulated by the command inputs to initiate, maintain, and terminate the rhythmic activity of a central pattern generator circuit.

Among the swimming nudibranchs, two species Melibe leonina and Dendronotus iris show the homologous swimming behavior by flexing their bodies from left to right (Sakurai et al., 2011).
 



Homologous behaviors are produced by homologous neurons
Phylogenetic analysis indicates that the most recent common ancestor of these species likely swam in this manner, making the swimming behaviors homologous (Goodheart et al., 2015; Sakurai and Katz, 2017).

The brains of Melibe leonina (left) 
and Dendronotus iris (right)

Homologous behaviors are produced by distinct neural circuit designs
The neural circuits underlying their behaviors have been studied extensively in both species. All neurons in the swim CPGs have been identified, and their synaptic connections have been determined with careful pairwise electrophysiological recordings (Sakurai et al., 2014; Sakurai and Katz 2016). The two swim CPGs employ different network architectures for producing similar rhythmic motor patterns.

The swim CPGs of Melibe (left) and Dendronotus (right)
have distinct synaptic organizations 

Si1 as a command neuron in Dendronotus
In this study we found that Si1 neurons in Dendronotus iris serves as a neuromodulatory "command" neuron for the swim CPG.

We further revealed how such command actions were mediated by performing dynamic clamp experiments and electrophysiological manipulations.

Providing artificial synaptic boost of the Si3-to-Si2 synapse and tonic synaptic excitation of Si3 mimicked the command actions of Si1 neurons.


Synaptic and neuromodulatory actions underlie the command input
It turned out that the organization of the Dendronotus swim CPG closely resembles the model that was originally proposed for a half-center oscillator with excitatory drive arising from a command neuron (Friesen, 1994).

A classical model of the half-center oscillator (left) 
and the Dendronotus swim CPG (right)

The command neuron Si1 provides not only the overall excitatory drive but also the neuromodulation of synaptic potentiation within each half of the oscillator. Our results also suggest that the functional position of neurons in a motor hierarchy can shift from one level (CPG) to another (a command neuron) over evolutionary time.

• Friesen WO (1994) Reciprocal inhibition: a mechanism underlying oscillatory animal movements. Neuroscience and biobehavioral reviews 18:547-553.
• Goodheart JA, Bazinet AL, Collins AG, Cummings MP (2015) Relationships within Cladobranchia (Gastropoda: Nudibranchia) based on RNA-Seq data: an initial investigation. R Soc Open Sci 2:150196. 
• Sakurai A, Katz PS (2016) The central pattern generator underlying swimming in Dendronotus iris: a simple half-center network oscillator with a twist. J Neurophysiol 116:1728-1742.
• Sakurai A, Katz PS (2017) Artificial Synaptic Rewiring Demonstrates that Distinct Neural Circuit Configurations Underlie Homologous Behaviors. Curr Biol 27:1721-1734 e1723.
• Sakurai A, Newcomb JM, Lillvis JL, Katz PS (2011) Different roles for homologous interneurons in species exhibiting similar rhythmic behaviors. Curr Biol 21:1036-1043.
• Sakurai A, Gunaratne CA, Katz PS (2014) Two interconnected kernels of reciprocally inhibitory interneurons underlie alternating left-right swim motor pattern generation in the mollusc Melibe leonina. J Neurophysiol 112:1317-1328. 

Melibe and Dendronotus Dynamic Clamp paper

My latest paper will appear in Current Biology next weekend.

Artificial Synaptic Rewiring Demonstrates that Distinct Neural Circuit Configurations Underlie Homologous Behaviors
http://www.cell.com/current-biology/fulltext/S0960-9822(17)30552-3
by Akira Sakurai and Paul S. Katz

Behaviors can be homologous just like any other trait can be. This study directly compared neural circuit mechanisms underlying homologous behaviors in two closely-related species.

   This work originates from two questions. First, we wanted to grasp a clue to figure out how species-specific behaviors have evolved. Mollusks are good models to study this because of their wide variety of speciation and the simplicity of the nervous system. In other animal models, functional elements of a neural circuit often consist of a population of neurons having the same function. This makes it difficult to manipulate because there are so many. In contrast, mollusks have one large neuron playing a key role in generating motor output for behavior. Their behaviors are also simple and reliable. The neuronal activity can be precisely manipulated so that one can easily relate one neuron to one behavior. By looking into molluscan species, we hoped we might be able to witness how a species-specific behavior has evolved.
   Secondly, we have been wondering what would happen if we swap a neural circuit of one species with that of the other species. There is a technique called "dynamic clamping", by which one can modify the strength of synapses or membrane conductances by injecting electrical current into neurons. The amount of the injected current is calculated in real-time by a computer based on the membrane potential that is being recorded. With this technique, we hoped we could reveal a crucial element that provides a neural circuit function.

   In this paper, we compared two neural circuits underlying swimming behaviors of two nudibranchs, Melibe leonina and Dendronotus iris. These sea slugs swim by flexing their body from left to right. These behaviors are likely to be homologous because both species belong to a clade that consists only of families that contain species that swim in the same way.
   We found that their swimming behaviors are produced by distinct neural circuit mechanisms. In Melibe, one of the neurons called Si3 makes an inhibitory synapse to fine-tune the rhythm made by other neurons.  In Dendoronotus, Si3 provides excitatory drive to other neurons to induce their rhythmic activities. When the Si3 synapses were blocked by curare, the swim rhythm slowed down in Melibe; whereas in Dendronotus, curare abolished the motor pattern. Replacing these synapses by artificial computer-generated synapses using "dynamic clamping" immediately restored the motor pattern. Using the dynamic clamp, we also rewired the Dendronotus circuit to the Melibe circuit. Then the Dendronotus neurons started to burst like the Melibe neurons in curare.

   From the results, we discussed that the neural mechanisms underlying homologous behaviors appear to have diverged but are still interchangeable in the other species. This has important significance for making inferences into neural mechanisms based on behavior. It also provides a real life instantiation for a prediction based on models, that there are multiple circuit architectures that can produce the same pattern of activity.

A simple half-center network oscillator with a twist

My latest paper came out from J Neurophys:

The central pattern generator underlying swimming in Dendronotus iris: A simple half-center network oscillator with a twist  

by Akira Sakurai and Paul S. Katz

DOI: 10.1152/jn.00150.2016

This paper describes the central oscillatory circuit underlying rhythmic swimming of a nudibranch sea slug, Dendronotus iris.

Dendronotus iris swims by rhythmically flexing its body to left and right.

The Dendronotus brain is a cluster of lobes or "ganglia." The neurons that produce the rhythmic motor output for swimming have their cell bodies in the pedal ganglia. They all project the axons toward the other side of the brain to synapse with their contralateral counterparts.  

The circuit is a typical "half-center oscillator" that consists of only two bilateral pairs of neurons. The paired neurons each inhibit their contralateral counterparts.  The circuit has a “twisted” organization; that is, a neuron in one pair is excitatory-coupled contralaterally to a neuron in the other pair.

The Dendronotus swim CPG is a half-center oscillator.  The left illustration shows actual synaptic connections. The left Si3 (L-Si3) neuron forms an excitatory synapse and electrical connection onto the right Si2 (R-Si2), forming a twisted configuration. A modified version is shown on the right. Coupled Si2 and Si3 neurons form a functional unit that works as a half-center to produce rhythmic bursting.

The half-center oscillator is the simplest design of a network oscillator, in which two neuronal elements with no endogenous rhythmicity form reciprocally inhibititory synapses. The half-center theory was first proposed by T. Graham Brown in 1911 after the historical experiment with a walking cat with transected spinal cord.

In addition to the reciprocal inhibition, each functional unit of the half-center oscillator generally contains the excitatory neurons that provide rhythmic excitatory drive onto the mutually-inhibitory neurons (eg., Clione, tadpole, lamprey, zebrafish, and mice). Because of high complexity with so many neurons involved, the role of the excitatory neurons have not been clearly understood.  Here, we found that the Dendronotus swim circuit consists of only 4 neurons. By using "Dynamic Clamping" technique, we manipulated the strength of the excitatory synapse and found that they play crucial roles for the circuit to function as the half-center oscillator.  To our knowledge, this is probably the simplest half-center oscillator described to date. Because of such simplicity, this circuit is also highly manipulable, and hence may provide a good system to study the fundamental properties of a network oscillator. 

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.

Dendronotus swim movie

OK, this is another nudibranch, Dendronotus iris.



Just like Melibe, it swims by lateral body flexions.