Abstract
A common CNS architecture is observed in all chordates, from vertebrates to basal chordates like the ascidian Ciona. Ciona stands apart among chordates in having a complete larval connectome. Starting with visuomotor circuits predicted by the Ciona connectome, we used expression maps of neurotransmitter use with behavioral assays 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 responds to changes in ambient light and mediates an escape response. This circuit uses GABAergic photoreceptors which project to GABAergic interneurons, and then to cholinergic interneurons. Our observations on the behavior of larvae either treated with a GABA receptor antagonist or carrying a mutation that eliminates photoreceptors indicate the second circuit is disinhibitory.
Highlights
Ascidians, including members of the widely-studied Ciona genus, have a biphasic life cycle
We find that the PR-I circuit is an excitatory relay from glutamatergic photoreceptors signaling to AMPA receptors on cholinergic photoreceptor RNs (prRN)
The Ciona connectome provides a detailed description of chemical synapse connectivity but it provides no information on neurotransmitter (NT) use
Summary
Ascidians, including members of the widely-studied Ciona genus, have a biphasic life cycle. The larval Ciona CNS is simple, having ~170 neurons (Ryan et al, 2016). Despite this simplicity, Ciona larvae display a range of integrated behaviors, including negative gravitaxis and phototaxis, and a response to dimming ambient light, all mediated by central sensory neurons. The MG is thought to be homologous to the vertebrate hindbrain and/or spinal cord, and contains ten motor neurons (MN) as well as a number of interneurons – including the two descending decussating neurons (ddN) which have been equated with vertebrate Mauthner cells which mediate the startle response
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