Serotonin and downstream leucokinin neurons modulate larval turning behavior in Drosophila.

Serotonin (5-HT) is known to modulate motor outputs in a variety of animal behaviors. However, the downstream neural pathways of 5-HT remain poorly understood. We studied the role of 5-HT in directional change, or turning, behavior of fruit fly (Drosophila melanogaster) larvae. We analyzed light- and touch-induced turning and found that turning is a combination of three components: bending, retreating, and rearing. Serotonin transmission suppresses rearing; when we inhibited 5-HT neurons with Shibire or Kir2.1, rearing increased without affecting the occurrence of bending or retreating. Increased rearing in the absence of 5-HT transmission often results in slower or failed turning, indicating that suppression of rearing by 5-HT is critical for successful turning. We identified a class of abdominal neurons called the abdominal LK neurons (ABLKs), which express the 5-HT1B receptor and the neuropeptide leucokinin, as downstream targets of 5-HT that are involved in the control of turning. Increased rearing was observed when neural transmission or leucokinin synthesis was inhibited in these cells. Forced activation of ABLKs also increased rearing, suggesting that an appropriate level of ABLK activity is critical for the control of turning. Calcium imaging revealed that ABLKs show periodic activation with an interval of ∼15 s. The activity level of ABLKs increased and decreased in response to a 5-HT agonist and antagonist, respectively. Our results suggest that 5-HT modulates larval turning by regulating the activity level of downstream ABLK neurons and secretion of the neuropeptide leucokinin.