Biology subjects

Kourakis, M.

Publications and source records attributed to Kourakis, M..

3 recordsLinked to original sources

bioRxiv2024

Deep homologies in chordate caudal central nervous systems

Invertebrate chordates, such as the tunicate Ciona, can offer insight into the evolution of the chordate phylum. Anatomical features that are shared between invertebrate chordates and vertebrates may be taken as evidence of their presence in a common chordate ancestor. The central nervous systems of Ciona larvae and vertebrates share a similar anatomy despite the Ciona CNS having [~]180 neurons. However, the depth of conservation between the Ciona CNS and those in vertebrates is not resolved. The Ciona caudal CNS, while appearing spinal cord-like, has hitherto been thought to lack motor neurons, bringing into question its homology with the vertebrate spinal cord. We show here that the Ciona larval caudal CNS does, in fact, have functional motor neurons along its length, pointing to the presence of a spinal cord-like structure at the base of the chordates. We extend our analysis of shared CNS anatomy further to explore the Ciona "motor ganglion", which has been proposed to be a homolog of the vertebrate hindbrain, spinal cord, or both. We find that a cluster of neurons in the dorsal motor ganglion shares anatomical location, developmental pathway, neural circuit architecture, and gene expression with the vertebrate cerebellum. However, functionally, the Ciona cluster appears to have more in common with vertebrate cerebellum-like structures, insofar as it receives and processes direct sensory input. These findings are consistent with earlier speculation that the cerebellum evolved from a cerebellum-like structure, and suggest that the latter structure was present in the dorsal hindbrain of a common chordate ancestor.

neuroscience↗

bioRxiv2023

A whole nervous system atlas of glutamate receptors reveals distinct receptor roles in sensorimotor circuits

A goal of connectomics is to reveal the links between neural circuits and behavior. Larvae of the primitive chordate Ciona are well-suited to make contributions in this area. The small size of the Ciona larval nervous system ([~]180 neurons) has facilitated the full characterization of a connectome. In addition, the larvae display a range of behaviors that are readily quantified in both normal and manipulated larvae. Moreover, the small number of neurons allows for a neuron-by-neuron characterization of attributes such as neurotransmitter use. We present here a nervous system-wide atlas of glutamate receptor expression. Included in the atlas are both ionotropic receptors (AMPA, NMDA, and Kainate), and metabotropic receptors. The expression of these receptors is presented in the context of known circuits driving behaviors such as phototaxis, mechanosensation, and looming shadow response. The expression of AMPA and NMDA receptors, in particular, helps to resolve the apparently paradoxical coproduction of GABA and glutamate by some photoreceptors. We find that the targets of these photoreceptors, midbrain relay neurons, primarily express NMDA receptors in the absence of AMPA receptors. This is in agreement with previous results indicating that GABA is the primary neurotransmitter from the photoreceptors evoking a behavioral response (swimming) through a disinhibition mechanism. We hypothesize that NMDA receptors have a modulatory effect in the relay neurons. Other findings reported here are more unexpected. For example, the targets of glutamatergic epidermal sensory neurons (ESNs) do not express any of the ionotropic receptors, yet the ESNs themselves express metabotropic receptors. Thus, we speculate that their production of glutamate may be for communication with neighboring ESNs, rather than to their interneuron targets.

neuroscience↗

bioRxiv2019

Parallel Visual Circuitry in a Basal Chordate

A common CNS architecture is observed in all chordates, from vertebrates to basal chordates like the ascidian Ciona. Currently Ciona stands apart among chordates in having a complete larval CNS connectome. Starting with visuomotor circuits predicted by the Ciona connectome, we used expression maps of neurotransmitter use with behavioral assays and pharmacology to identify two parallel visuomotor circuits that are responsive to different components of visual stimuli. The first circuit is characterized by glutamatergic photoreceptors and responds to the direction of light. These photoreceptors project to cholinergic motor neurons, via two tiers of cholinergic interneurons. The second circuit is responsive to changes in ambient light and mediates an escape response. This circuit starts with novel GABAergic photoreceptors which project to GABAergic interneurons, and then to cholinergic interneurons shared with the first circuit. Our observations on neurotransmitter use and the behavior of larvae lacking photoreceptors indicate the second circuit is disinhibitory.

neuroscience↗