Drosophila serotonergic varicosities are not distributed in a regular manner.

Serotonin is a monoamine neurotransmitter that is released from serotonergic neurons in the central nervous system (CNS). There, serotonin plays a role in modulating most neuronal circuits, and therefore has influences on many human behaviors (Jacobs and Azmitia, 1992). Altered serotonin levels and wiring are thought to play key roles in the etiology of many neuropsychiatric illnesses. Although serotonin is a key player in many human behaviors and diseases, little is known about the branching and patterning of the serotonergic system. Whereas some parts of the brain are thought to use predominantly synaptic transmission of serotonin, most regions are thought to communicate with their targets primarily through volumetric transmission, whereby serotonin is released from nonjunctional presynaptic varicosities (Chazal and Ralston, 1987; Moukhles et al., 1997; Bunin and Wightman, 1998; De-Miguel and Trueta, 2005). Volumetric transmission is used by several other neuromodulatory neurotransmitter systems, including the dopaminergic system (Bunin and Wightman, 1999). Serotonin is believed to diffuse 5 μm from a presynaptic varicosity release site and still maintain a functional concentration (Bunin and Wightman, 1998). On the basis of serotonin receptor distribution, most areas of the brain are thought to require serotonin modulation. Serotonergic varicosities are therefore predicted to be distributed in a somewhat regular array so as to ensure that serotonin can diffuse evenly to all of its targets (Bunin and Wightman, 1998). Previous spatial point analysis of the distribution of nonserotonergic synapses has provided support for the idea that presynaptic boutons are regularly spaced (Meinertzhagen et al., 1998). However, the actual spatial patterning of serotonergic innervation remains unknown. Many systems in nature face a similar problem of space filling, including the human bronchial and arterial system. Both the bronchial airways and blood vessels in the arterial system must be efficiently distributed in order to achieve proper gas and nutrient exchange, respectively. Both systems begin with stereotypical branching patterns that break down into a fractal structure after several generations of branching. Although a regular organization would seem to provide the most efficient means of space filling, the majority of their branches are dominated by a clustered scale-free pattern, also referred to as a fractal (Mandelbrot, 1977; McNamee, 1991; Bassingthwaighte et al., 1994). In order to better understand the patterning of the serotonergic system, we have utilized the Drosophila serotonergic system because of its simplicity and genetic amenability. By using a variety of spatial analyses, we examine varicosity distribution in the brain and the ventral nerve cord (VNC). In one part of the brain, we find a partially regular pattern. However, in the VNC, we find a distribution that is clustered over a broad scale. In addition, whereas each branched structure within a segment is clustered, branches from adjacent segments break down the overall clustering, and the pattern becomes more random.