Drosophila serotonergic varicosities are not distributed in a regular manner

Neurons of the brain form complex tree-like structures that are critical for function. Here we examine the spatial pattern of serotonergic varicosities, the synaptic sites of serotonin release in the central nervous system (CNS). These varicosities are thought to form largely nonjunctional-type connections that partition in a grid-like manner in order to distribute evenly the neuromodulatory neurotransmitter serotonin. We describe the neuropil distribution of serotonergic varicosities in the brain and ventral nerve cord (VNC) of the larval Drosophila CNS. In the brain, we find evidence for avoidance between varicosities at distances lower than 1.75 μm. However, in the VNC, we find a clustered distribution. A similar clustered pattern is found in the Xenopus brain. This pattern produces many varicosities that are clustered together but also includes some varicosities that are very isolated. These isolated varicosities are not found along particular topological sections of the neurite tree or in particular locations in the CNS. In addition, the pattern breaks down when serotonergic branches of adjacent segments invade each other's territory. The pattern is similar to those described by a power law.