Neuronal wiring diagram of an adult brain

A statistical prediction of the small molecule neurotransmitter (GABA (γ-aminobutyric acid), glutamate, acetylcholine, serotonin, dopamine and octopamine) secreted by each neuron is available. A number of validations suggest that the predictions are highly accurate in aggregate, but for any given synapse the prediction could be wrong. We assume that every neuron secretes a single small molecule neurotransmitter and combine the predictions for all outgoing synapses to an estimate that we assign to all outgoing synapses of a neuron—that is, we provisionally assume that neurons obey Dale’s law, although it is known that co-transmission does occur in the fly brain.

GABAergic neurons had higher degrees on average than glutamatergic and cholinergic neurons (median in- and out degrees of intrinsic neurons: GABA, 14 incoming and 16 outgoing partners; glutamate, 11 incoming and 13 outgoing partners; acetylcholine, 10 incoming and 13 outgoing partners; Extended Data Fig. 3j). Across all neuron categories, we found that GABAergic neurons were on average longer than glutamatergic and cholinergic neurons (median length of intrinsic neurons: GABA, 0.88 mm; glutamate, 0.85 mm; acetylcholine, 0.63 mm; Extended Data Fig. 3k).

As a rule, we assume that cholinergic neurons are excitatory and GABAergic and glutamatergic neurons are inhibitory. Lin et al. identified all GABAergic and glutamatergic neurons that are bidirectionally coupled with large numbers of cholinergic neurons. This reciprocal inhibitory–excitatory motif is widespread throughout the fly brain.