Recombinant or transgenic non-human mammals are described having a mutant tryptophan hydroxylase 2 (Tph2) gene resulting in altered synthesis of 5-hydroxytryptophan and serotonin in the brain. In some embodiments the mutant tryptophan hydroxylase 2 gene contains mouse R439H and/or P447R functional mutations, or their corresponding mutations in other species. Congenic non-human mammals having mutant tryptophan hydroxylase 2 genes are also provided. Methods of screening a compound for serotonergic activity or activity in treating a serotonergic neurotransmission dysregulation disorder are provided, which include administering a test compound to a recombinant non-human mammal and then detecting the presence or absence of serotonergic activity, or activity in treating a serotonergic neurotransmission dysregulation disorder, in the mammal. A cell such as a nerve cell (e.g., a central nervous system neuron) isolated from a transgenic or congenic mammal is also disclosed, along with cell cultures containing these cells. Such mammals and cells and cell cultures are useful in vitro for screening the activity of candidate compounds for their effect on serotonergic neurotransmission and for their activity in treating serotonergic neurotransmission dysregulation disorders.