The common ancestor of fruit flies and humans diverged 700 million years ago¹. While their appearances are vastly different, the underlying genes and cellular mechanisms are remarkably conserved^{2, 3}. Utilizing the fruit fly model, we have explored the effects of first-line ADHD medications on different brain cell types and proposed new ideas for drug repurposing⁴.

The mechanisms of action for most psychiatric drugs are not fully clear. For instance, the first-line medication for ADHD, methylphenidate (MPH), is thought to act directly on dopamine transporters, increasing synaptic dopamine levels and thus achieving therapeutic effects; MPH can also affect the norepinephrine transporter, indirectly influencing synaptic dopamine levels⁵. Correspondingly, atomoxetine (ATX) can inhibit the norepinephrine transporter and is also effective in treating ADHD⁶. However, these drugs only work for some patients. It is unclear which brain cell types and genetic pathways are affected by these drugs, which cannot be thoroughly investigated through direct human experimentation. Early studies indicate that the fruit fly model exhibits ADHD-like behaviors, which can be alleviated by MPH^{7, 8}.

Expansion of the “Increased Synaptic Dopamine” Theory in Fruit Fly Experiments

Traditionally, ADHD medications are thought to increase synaptic dopamine levels by inhibiting its reuptake, among other mechanisms, to achieve therapeutic effects. However, most drugs are only partially effective and it’s unclear how increased dopamine levels alleviate ADHD symptoms. Our research with fruit flies has found that both glial cells and neurons respond to medication. The glial response is stronger than that of neurons, with communication between them and dopaminergic neurons primarily mediated by the tumor necrosis factor-alpha pathway. Among many neurons, cholinergic neurons exhibit the strongest response. We observed changes in dopamine metabolism and signaling, the impact on synaptic transmission, and downregulation of several neurotransmitter receptors. Changes in glutamate receptors, for instance, were also observed in postmortem brain tissue samples from ADHD patients⁹. This suggests that the increase in synaptic dopamine affects a variety of cell types, including glia and neurons, to alleviate ADHD symptoms. Glial and cholinergic cells and receptors will be the focus of future research.

New Ideas for Drug Repurposing

In experiments, we identified a list of genes that respond to medication, which includes potential therapeutic targets. We have mapped these response genes to their human counterparts, and integrated them with genome-wide association results to create a drug repurposing dataset, available through an online interface (http://adhdrug.cibr.ac.cn), offering potential ADHD treatment targets and corresponding small molecule drugs. In fact, drugs currently used for clinical ADHD treatment are significantly enriched in our drug repurposing dataset. This model of drug genomics research, combining animal models, offers new insights into drug repurposing.

Behavior-based High-throughput Drug Screening in Fruit Fly Models

Obviously, mouse and even non-human primate models can provide richer information, such as whether different brain areas have different drug responses or whether the drug-responsive brain areas are the same as the core circuit regions. These animal models provide the detailed behavior and molecular and cellular mechanisms that the fruit fly model lacks. However, in today’s age of high-throughput molecular drug design, we urgently need downstream high-throughput screening platforms. Cell line-based high-throughput screening has had some success, as demonstrated by pioneers like Recursion Pharmaceuticals. But cell lines do not address issues of microenvironments and behavior. Due to costs, mice and non-human primates are practically out of reach for high-throughput screening. The fruit fly model is a potential solution for high-throughput screening scenarios involving microenvironments and behavioral phenotypes, such as those addressed by Vivan Therapeutics.

References

1. Hedges SB, Dudley J, Kumar S. TimeTree: a public knowledge-base of divergence times among organisms. Bioinformatics 2006; 22(23): 2971-2972.
2. Lichtneckert R, Reichert H. Insights into the urbilaterian brain: conserved genetic patterning mechanisms in insect and vertebrate brain development. Heredity (Edinb) 2005; 94(5): 465-477.
3. Hirth F, Reichert H. Conserved genetic programs in insect and mammalian brain development. Bioessays 1999; 21(8): 677-684.
4. Qu S, Zhou X, Wang Z, Wei Y, Zhou H, Zhang X et al. The effects of methylphenidate and atomoxetine on Drosophila brain at single-cell resolution and potential drug repurposing for ADHD treatment. Molecular Psychiatry 2023.
5. Berridge CW, Devilbiss DM, Andrzejewski ME, Arnsten AF, Kelley AE, Schmeichel B et al. Methylphenidate preferentially increases catecholamine neurotransmission within the prefrontal cortex at low doses that enhance cognitive function. Biol Psychiatry 2006; 60(10): 1111-1120.
6. Bymaster FP, Katner JS, Nelson DL, Hemrick-Luecke SK, Threlkeld PG, Heiligenstein JH et al. Atomoxetine increases extracellular levels of norepinephrine and dopamine in prefrontal cortex of rat: a potential mechanism for efficacy in attention deficit/hyperactivity disorder. Neuropsychopharmacology 2002; 27(5): 699-711.
7. van der Voet M, Harich B, Franke B, Schenck A. ADHD-associated dopamine transporter, latrophilin and neurofibromin share a dopamine-related locomotor signature in Drosophila. Mol Psychiatry 2016; 21(4): 565-573.
8. Klein M, Singgih EL, van Rens A, Demontis D, Borglum AD, Mota NR et al. Contribution of Intellectual Disability-Related Genes to ADHD Risk and to Locomotor Activity in Drosophila. Am J Psychiatry 2020; 177(6): 526-536.
9. Sudre G, Gildea DE, Shastri GG, Sharp W, Jung B, Xu Q et al. Mapping the cortico-striatal transcriptome in attention deficit hyperactivity disorder. Mol Psychiatry 2023; 28(2): 792-800.