Anatomical and functional organisation of the cholinergic nervous system in the tunicate Botryllus schlosseri

Cholinergic neurotransmission, a crucial component of neural communication, exhibits ancient evolutionary roots yet the intricate organization of cholinergic pathways within early chordates, such as the colonial tunicate Botryllus schlosseri, remains underexplored. This study delves into the spatial and functional organization of the cholinergic system in B. schlosseri, an organism characterized by continuous, cyclical neural regeneration.

Utilizing a combination of multi-modal approaches, including HCR-RNA FISH, in vivo pharmacology, brain electrophysiology, and quantitative reflex assays, researchers elucidate the core molecular machinery involved in acetylcholine (ACh) synthesis, transport, breakdown, and receptor signaling.

The spatial expression analysis delineates a compartmentalized cholinergic network encompassing central ganglia, peripheral sensory cells, and ciliated epithelia. Functionally, the study reveals that nicotinic receptors play a pivotal role in mediating rapid mechanosensory burst firing within the brain, initiating siphon reflex contractions, and causing ciliary arrest.

Conversely, muscarinic receptors modulate baseline siphon motility and regulate muscle tone. Pharmacological interventions targeting active siphon motor programs unveil a slower, vascular-coupled rhythmic motility, while inhibition of AChE results in paralysis, underscoring the indispensable role of regulated ACh breakdown and non-synaptic transmission.

In summary, the findings illuminate the sophisticated dual-effector nature of the B. schlosseri's cholinergic system, orchestrating both muscular and ciliary networks. This research provides significant insights into the evolutionary trajectory of chordate neuromuscular control mechanisms.