Serotonergic Neurons Transiently Require a Midline-Derived FGF Signal

In the grasshopper CNS, serotonergic growth cones cross the midline early in development and initiate expression of serotonin uptake activity, or SERT. To test if the midline contains an activity that induces SERT, cuts were made that separated serotonergic cell bodies from the midline. SERT activity is completely lost when the midline is separated but is then rescued by bath-applied FGF2 (fibroblast growth factor 2), which can activate the heartless FGF receptor. heartless is expressed specifically in serotonergic neurons. A candidate FGF-like molecule was identified that is expressed in a subset of midline glia. SERT-expressing severed growth cones continue to migrate to their correct targets, which indicates that by the time SERT is activated, the serotonergic growth cones are committed to target-directed growth.