TPH1 and attention deficit-hyperactivity disorder: Furthermore, many genes previously implicated in ADHD [9] are expressed in the synapse (i.e., DBH, SLC6A2, ADRA2A, HTR1B, HTR2A, TPH1/2, MAOA, CHRNA4, SNAP25, and BDNF), suggesting the involvement of synaptic function in the etiology of ADHD.