Figure 5.

A proposal for the protective vs. toxic effects of serotonin in the fly brain. Schematic representation of the dynamic changes in extracellular 5-HT content proposed to be occurring in the adult fly brain under the different conditions studied. Control flies exhibit a protective/trophic level of 5-HT content (blue line, gray zone), which allows the normal maturation and refinement of neuronal circuits that participate in motor behavior. In Pink1^B9 flies, no protective/trophic 5-HT levels are reached (red line, white zone), hindering the normal maturation and refinement processes, which favors the onset of Parkinsonian phenotypes in aged animals. A transient increase (fluoxetine treatment, yellow box) of extracellular 5-HT levels in mutant flies serves to reach the protective range (red dashed line), permitting the normal maturation and refinement of neuronal circuits and halting the onset of the PD symptomatic phase later in life. On the other hand, in control flies, the fluoxetine treatment (yellow box) results in extracellular 5-HT levels that increase over the normal range (blue dashed line, white zone), disturbing the normal maturation and refinement of circuits, leading to the onset of PD-like phenotypes as animals age.