Neurochemically-evoked activity in slice preparations of the octopus arm nerve cord

Octopus arms contain circuits that support local sensorimotor integration and autonomy, responsible for fast and flexible behaviors, but their population dynamics remain unknown. The axial nerve cord (ANC) is a series of sucker-associated ganglia whose cortex layer houses multiple classes of intermingled neurons. Here we use calcium imaging in ex vivo slices from arms of Octopus bocki, to visualize how these networks respond to controlled application of neurotransmitters and neuromodulators. Glutamate and dopamine are dominant excitatory drivers that activate most neurons in the ANC cortex, with a substantial overlapping populations and smaller transmitter-specific subsets. Glutamate continues to excite additional neurons at higher concentration, whereas dopamine responses saturate. Serotonin alone evokes mixed responses but, when applied first, consistently reduces glutamate- and dopamine-driven activation, revealing a state-dependent modulatory role. GABA and octopamine yield weak heterologous effects, and high-dose acetylcholine sharply suppresses global ANC neuronal activity while inducing muscular contraction, consistent with inhibitory cholinergic receptors in arm ganglia. Across conditions, responsive neurons show no evidence of spatial structure, with no clear anatomical segregation by transmitter response profiles. These results provide the first link between neurochemical architecture and real-time firing dynamics in a semi-autonomous circuit.