Summary: Researchers have rewired the neural circuits of one species and given its connections to a new species. The study answers a number of questions about evolution of neural circuits and behavior.
Source: Georgia State University.
Scientists at Georgia State University have rewired the neural circuit of one species and given it the connections of another species to test a hypothesis about the evolution of neural circuits and behavior.
Neurons are connected to each other to form networks that underlie behaviors. Drs. Akira Sakurai and Paul Katz of Georgia State’s Neuroscience Institute study the brains of sea slugs, more specifically nudibranchs, which have large neurons that form simple circuits and produce simple behaviors. In this study, they examined how the brains of these sea creatures produce swimming behaviors. They found that even though the brains of two species – the giant nudibranch and the hooded nudibranch – had the same neurons, and even though the behaviors were the same, the wiring was different.
The researchers blocked some of the connections in the giant nudibranch using curare, a paralyzing poison used on blow darts by indigenous South Americans. This prevented the brain of the giant nudibranch from producing the pattern of impulses that would normally cause the animal to swim. Then, they inserted electrodes into the neurons to create artificial connections between the brain cells that were based on connections from the hooded nudibranch. The brain was able to produce rhythmic, alternating activity that would underlie the swimming behavior, showing these two species produce their swimming behavior using very different brain mechanisms.
The findings are published in the journal Current Biology.
“Behaviors that are homologous and similar in form would naturally be assumed to be produced by similar neural mechanisms,” said Katz, co-author of the study and a Regent’s Professor in the Neuroscience Institute at Georgia State. “This and previous studies show that connectivity of the neural circuits of two different species of sea slugs differ substantially from each other despite the presence of homologous neurons and behaviors. Thus, the evolution of microcircuitry could play a role in the evolution of behavior.”
The study’s results are significant for several reasons. First, they show that over the course of evolution, behaviors might be conserved, but the underlying neural basis for the behaviors could shift.
In addition, other work by these researchers and Katz’s lab has underscored the conclusion that neurons are conserved, but differ in function across species. This has implications for extrapolating results across species in general and means caution must be taken in assuming that neural mechanisms are conserved even though brain regions and behaviors are present.
Sakurai is first author of the study and a research scientist in the Neuroscience Institute at Georgia State.
Funding: The research was funded by the National Science Foundation.
Source: Natasha De Veauuse Brown – Georgia State University
Image Source: NeuroscienceNews.com image is credited to Ralph and Dale Marie Gonzales.
Original Research: Abstract for “Artificial Synaptic Rewiring Demonstrates that Distinct Neural Circuit Configurations Underlie Homologous Behaviors” by Akira Sakurai and Paul S. Katz in Current Biology. Published online June 1 2017 doi:10.1016/j.cub.2017.05.016
Abstract for “Variations on a theme: Species differences in synaptic connectivity do not predict central pattern generator activity” by Charuni A. Gunaratne, Akira Sakurai, and Paul S. Katz in Journal of Neurophysiology. Published online May 24 2017 doi:10.1152/jn.00203.2017
Artificial Synaptic Rewiring Demonstrates that Distinct Neural Circuit Configurations Underlie Homologous Behaviors
•Two nudibranchs have homologous behaviors produced by distinct network mechanisms
•Homologous neurons have different synaptic connectivity
•Artificial synapses replaced those blocked by curare to restore motor pattern
•Artificial rewiring using other species’ connections restored an impaired circuit
Behavioral homology is often assumed to involve similarity in underlying neuronal mechanisms. Here, we provide a counterexample where homologous behaviors are produced by neurons with different synaptic connectivity. The nudibranch molluscs Melibe leonina and Dendronotus iris exhibit homologous swimming behaviors, consisting of alternating left and right body flexions. The swim central pattern generators (CPGs) in both species are composed of bilaterally symmetric interneurons, which are individually identified and reciprocally inhibit their contralateral counterparts, contributing to left-right burst alternation in the swim motor patterns. In Melibe, the swim CPG contains two parts that interact to produce stable rhythmic bursting; one part is the primary half-center kernel, and the other part, which consists of a bilateral pair of neurons called Si3, regulates period length. The Dendronotus swim CPG is simpler, with Si3 being part of the primary half-center oscillator. Application of curare (d-tubocurarine) selectively blocked the Si3 synapses in both species. In Melibe, curare application caused the burst duration of the swim motor pattern to lengthen, whereas in Dendronotus, curare halted bursting altogether. In both species, replacing the curare-blocked Si3 synapses with artificial synapses using dynamic clamp restored the original rhythmic bursting, thereby affirming the roles of those synapses. The curare-impaired bursting in Dendronotus was also restored by rewiring the homologous neurons into a Melibe-like primary half-center oscillator configuration, indicating that the connectivity itself could account for species differences in circuit responses to curare. The results suggest that synaptic connectivity diverged during evolution while behavior was conserved.
“Artificial Synaptic Rewiring Demonstrates that Distinct Neural Circuit Configurations Underlie Homologous Behaviors” by Akira Sakurai and Paul S. Katz in Current Biology. Published online June 1 2017 doi:10.1016/j.cub.2017.05.016
Variations on a theme: Species differences in synaptic connectivity do not predict central pattern generator activity
A fundamental question in comparative neuroethology is the extent to which synaptic wiring determines behavior versus the extent to which it is constrained by phylogeny. We investigated this by examining the connectivity and activity of homologous neurons in different species. Melibe leonina and Dendronotus iris (Mollusca, Gastropoda, Nudibranchia) have homologous neurons and exhibit homologous swimming behaviors consisting of alternating left-right (LR) whole body flexions. Yet, a homologous interneuron (Si1) differs between the two species in its participation in the swim motor pattern (SMP) and synaptic connectivity. Here we examine Si1 homologs in two additional nudibranchs: Flabellina iodinea, which evolved LR swimming independently of Melibe and Dendronotus, and Tritonia diomedea, which swims with dorsal-ventral (DV) body flexions. In Flabellina, the contralateral Si1s exhibit alternating rhythmic bursting activity during the SMP and are members of the swim central pattern generator (CPG), as in Melibe. The Si1 homologs in Tritonia do not burst rhythmically during the DV SMP, but are inhibited and receive bilaterally synchronous synaptic input. In both Flabellina and Tritonia, the Si1 homologs exhibit reciprocal inhibition as in Melibe. However, in Flabellina the inhibition is polysynaptic, whereas in Tritonia it is monosynaptic, as in Melibe. In all species, the contralateral Si1s are electrically coupled. These results suggest that Flabellina and Melibe convergently evolved a swim CPG that contains Si1; however, they differ in monosynaptic connections. Connectivity is more similar between Tritonia and Melibe, which exhibit different swimming behaviors. Thus, connectivity between homologous neurons varies independently of both behavior and phylogeny.
“Variations on a theme: Species differences in synaptic connectivity do not predict central pattern generator activity” by Charuni A. Gunaratne, Akira Sakurai, and Paul S. Katz in Journal of Neurophysiology. Published online May 24 2017 doi:10.1152/jn.00203.2017