Abstract

In nervous systems, there are two main modes of transmission for the propagation of activity between cells. Synaptic transmission relies on close contact at chemical or electrical synapses while volume transmission is mediated by diffusible chemicals and does not require direct contact. In principle, it is possible to wire arbitrarily complex neuronal networks by both chemical and synaptic transmission. Both types of networks are ubiquitous in nervous systems, leading to the question which of the two appeared first in evolution. Last year, I proposed a scenario where chemically organized cellular networks appeared before synapses in evolution. This was supported by the presence of complex peptidergic signalling in all animals except sponges. More recently, we have discovered the first neuropeptide-like molecules in sponges and choanoflagellates, giving more support to this chemical brain hypothesis. We also carried out large-scale pharmacological screens and identified several cnidarian and placozoan neuropeptide receptors, indicating extensive peptidergic transmission in these non-bilaterian animals. In our work on the larval neurosecretory center of the annelid Platynereis, we are experimentally investigating how synaptic and chemical networks interact. We have reconstructed the full synaptic connectome of the Platynereis larva and densely mapped neuromodulators to the connectome. Through calcium imaging, CRISPR knockouts and behavioural work we aim to understand how the chemical and the synaptic brain work together to generate circuit activity during behaviour.