Novel dopaminergic neurotransmission in the Octopus visual system

Coleoid cephalopods such as the common octopus have a complex visual system, with a camera-type eye and a large optic lobe, that evolved independently from its counterpart in vertebrates. However, the molecular and neurochemical basis of signalling in Octopus visual circuits is not well understood. Through heterologous expression of ligand-gated ion channel genes from Octopus vulgaris, we have identified and characterised novel ionotropic receptors for two of the major optic lobe neurotransmitters, dopamine and acetylcholine. One of these, DopC1, defines a new class of dopamine-gated cation channels that is broadly conserved in many invertebrates. Dopamine robustly stimulates neural activity in the optic lobe, particularly in layers where DopC1 is expressed, consistent with a role for this channel in excitatory neurotransmission. A second protein, AChRB1, forms acetylcholine-gated anion channels and is implicated in inhibition of dopaminergic amacrine cells in the outer granular layer (OGL). This combination of inhibitory cholinergic input and excitatory dopaminergic output may allow OGL amacrine cells to carry out lateral inhibition; thus, despite their very different functional and neurochemical properties, Octopus amacrine cells may execute similar computations to amacrine and horizontal cells in the vertebrate retina.