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. 

Calcium-Induced Calcium Release during Action Potential Firing in Developing Inner Hair Cells

by Radu Iosob, Daniele Avitabile, Lisa Grant, Krasimira Tsaneva-Atanasova, and Helen J. Kennedy
https://doi.org/10.1016/j.bpj.2014.11.3489

- They studied how Ca²⁺-induced Ca²⁺ release (CICR) contributes to the spiking activity of an inner hair cell.

- CICR is a quite ubiquitous mechanism for amplification in the intracellular signaling cascade. A brief increase in intracellular calcium triggers calcium release from sarcoplasmic reticulum through ryanodine-sensitive channels (RyRs). This cascade is often triggered by the Ca²⁺  influx of from extracellular space through voltage-gated Ca²⁺ channels

- They concluded that CICR plays a pivotal role in shaping action potentials through activating the SK2 channel. SK2 is a calcium-dependent potassium channel. Blockade of CICR or SK2 broadens the spikes.

- They also stated that CICR indirectly affects the amount of transmitter release by changing the spike duration. However, they concluded that it has no direct effect on the transmitter release machinery at the presynaptic terminal.


-  They performed only a few electrophysiological experiments with ryanodine of two different concentrations. And then mathematical modeling. Nevertheless, this is a straightforward paper — a good read for early learners studying signal transduction of sensory neurons.

Journal club: The cortex as a central pattern generator

NATURE REVIEWS NEUROSCIENCE 6(6) 2005

by Rafael Yuste, Jason N. MacLean, Jeffrey Smith and Anders Lansner

A vast number of neurons in the brain cortex together generates synchronized oscillatory activity. In this review paper, the authors discuss how the neocortex neural network shares basic designs with the central pattern generator circuits in the spinal cord and brain stem.

For rhythmogenesis in a neural network, it seems like the general rule that self-boosting excitation within a functional unit is more important than reciprocal inhibition.  In the neocortical circuit, a population of neurons forms an excitatory kernel by being interconnected through recurrent excitation. Such excitatory neuronal population is capable of producing synchronized oscillatory activity through their active membrane properties and short-term synaptic plasticity. This circuit architecture is similar to the CPGs in the spinal cord and brain stem of vertebrates. The inhibitory connections are necessary for the regulation of the rhythm and spatiotemporal pattern of the network output.

The extent of spatial distribution is one of the major differences between the cortical network and the CPG in the brain stem/spinal cord.  In the cortex, the recurrent excitatory network is widely spread out over the neocortex, whereas in those for locomotion and breathing have bilaterally-grouped functional units. The other difference is high plasticity in the cortical network in contrast to the hard-wired CPGs in the spinal cord and brain stem. The connectivity in the cortex develops through activity-dependent Hebbian plasticity. We can call the cortex network as a learning CPG that is based on Hebbian assemblies and is specialized for learning and storing or retrieving memories.  Because of these properties, the neocortex network is highly fluidic.

Journal Club: Fast Silencing Reveals a Lost Role for Reciprocal Inhibition in Locomotion

By Peter R. Moult, Glen A. Cottrell, and Wen-Chang Li
Neuron Volume 77, Issue 1, 9 January 2013, Pages 129-140

Reciprocal inhibition is considered as a fundamental building block in neural circuits for rhythmic motor pattern generation. Despite commonality in many rhythmic systems, however, the reciprocal inhibition is often found to be not a necessary component for rhythmogenesis. This is because a network is often capable of exhibiting rhythmic activity even when the reciprocal inhibition was removed mechanically or pharmacologically. The excitatory synaptic components within each half of the network are more crucial.  In this study, Wen-Chang Li's group challenged this idea by using optogenetics to hyperpolarize the entire population of neurons in one side of the spinal cord in a tadpole.

When the entire half of the spinal cord was suppressed by flashing yellow light, the other half also stopped bursting. This indicates that, in this reciprocally inhibitory network, the excitation in one side is necessary for the excitation of the other. Firing in one side would cause a bombardment of IPSPs, which induce post-inhibitory rebound. The authors also get a similar result by giving a strong hyperpolarizing current into one of the reciprocally inhibitory neurons. They concluded that the reciprocal inhibition is necessary in the tadpole swim circuit; it induces rebound excitation in the contralateral neurons.

In this paper, it is noteworthy that the author also mentioned about the "functional homeostasis" of the neural circuit. They said the rhythmic activity recovers after half an hour by itself without the reciprocal inhibition. The mechanism of the recovery still unknown.

Journal club: An increased extrasynaptic NMDA tone inhibits A-type K+ current and increases excitability of hypothalamic neurosecretory neurons in hypertensive rats.

by Meng Zhang, Vinicia C. Bianchardi and Javier E. Stern
DOI: 10.1113/JP274327

In this paper the authors tried to elucidate the mechanism of how the magnocellular neurosecretory neurons become hyperexcitable under pathological condition that could lead to high blood pressure and heart failure.

   Magnocellular neurosecretory cells (MNCs) in hypothalamus release vasopressin and oxytocin, which control fluid balance, and cardio-vascular and reproductive homeostasis. The MNC activity is fine-tuned by intrinsic membrane property, by extrasynaptic inputs, and by the activity of neighbouring astrocytes. MNCs have A-current, which pulls down MNCs from firing action potentials.
   In this study, the authors showed that the activation of eNMDARs inhibited A-current in sham MNCs, but not in MNCs from renovascular hypertensive (RVH) rats. However, neither the exogenously evoked NMDA current nor the expression of NMDAR subunits were altered in RVH rats.  Instead, a larger endogenous glutamate tone, which was not due to blunted glutamate transport activity, led to the sustained activation of eNMDARs. This tonically inhibited A-current and contributed in turn to higher firing activity in RVH rats.

   In this study, they did not show the inactivation curve for the A-current. If I were a referee, I would ask them to show it, because the Glu tone should act very slowly. A-current can inactivated under such condition.

   So, the basal Glu level appears to be very important in regulating the homeostatic functions. It might not be just in the brain. If the extrasynaptic GluRs play similar roles in other nervous system, I can speculate that chronic consumption of MSG might have some bad effects on the peripheral nervous systems such as those controlling guts.

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.

Journal club: The role of electrical coupling in generating and modulating oscillations in a neuronal network

by Christina Mouser, Amitabha Bose and Farzan Nadim
Mathematical Biosciences 278 (2016) 11–21

Melissa Coleman showed in her 1995 Nature paper that there is an electrical connection between MCN1 and LG. I was a grad student back then. I really liked that paper, but I have forgotten about this electrical connection.  This electrical synapse is voltage-sensitive. It gets stronger when LG was depolarized. It looks opposite to regular EPSPs which usually become smaller when depolarized. This voltage-sensitive electrical synapse seems to act as a positive feedback to LG activity.

   In this paper, Mouser et al. showed that this electrical coupling is very important for LG bursting by using a mathematical model. LG forms a Half-Center Oscillator with Int1 by forming mutually inhibitory synapses. This HCO configuration itself can generate a rhythm, which is influenced by periodic inhibition from pyrolic pacemaker (AB). However, the major finding in this paper is that the LG oscillation can also be generated without the reciprocal inhibition, but with the voltage-dependent electrical coupling between MCN1 and LG. This also indicates that the half-center configuration is not the main element that produces the rhythm; rather it is to form a pattern of activity in left-right alternation. For rhythmogenesis, some sorts of positive feedback system seems more important than mutual inhibition.

   I don't know how electrical coupling gets voltage-sensitive. I assume it not caused by changes in the coupling coefficient, but rather by overall depolarization at dendrites, which may trigger Ca influx and hence enhancing spike width.