The article, written by Erika Bistran, M. Evelyn Carpenter and Daniel J. Sprague of the Medical University of South Carolina, focuses on bioaminergic signaling, the system of neurotransmitters that includes serotonin, dopamine, histamine, tyramine and octopamine. These molecules have been implicated in flatworm neuromuscular control for decades, but the authors contend that the field has been stuck cataloging which amines are present and which compounds make worms twitch. What is needed, they say, is mechanistic depth: knowing exactly which receptor detects the signal, which second messenger carries it inside the cell, which tissue hosts the signaling, and which parasite behavior changes when the pathway is disrupted.

To organize that effort, the researchers propose a pathway-to-phenotype framework built on six interconnected areas of investigation. These are the bioamine ligand itself, the receptor or transporter that detects or clears it, the downstream signaling pathway it engages, the cell type or tissue where signaling occurs, the experimental perturbation used to test function, and the resulting parasite phenotype. No single experiment needs to cover all six domains, the authors stress, but each experimentally supported connection between them raises the resolution of the pathway model, moving the field from association toward mechanism.

The clearest proof that this approach works is the serotonin receptor Sm5HTR in Schistosoma mansoni, the blood fluke responsible for schistosomiasis, a disease that afflicts hundreds of millions of people. Serotonin has long been known to excite schistosome movement and potentiate body wall muscle contraction. The characterization of Sm5HTR turned that observation into a complete molecular story. The receptor is a serotonin-responsive G protein-coupled receptor distantly related to human 5-HT7 receptors. When expressed in heterologous systems, it responds to serotonin by raising intracellular cAMP, consistent with a Gαs-coupled mechanism. Immunolocalization places it throughout the parasite nervous system, including the cerebral ganglia, the longitudinal nerve cords and the peripheral innervation of body wall muscle. Crucially, knocking the receptor down with RNA interference in larvae and adults causes marked hypoactivity and blunted responses to exogenous serotonin.

Sm5HTR has also proven pharmaceutically fruitful. Screening campaigns have identified novel agonists and antagonists, including parasite-selective inhibitor classes, and efforts have been made to distinguish schistosome-specific from human-specific ligands. For the authors, this makes Sm5HTR the benchmark against which every other aminergic system in flatworms should be judged: ligand, receptor, second messenger, anatomy, perturbation and phenotype all connected in one system.

Other aminergic pathways in schistosomes are promising but incomplete. Two dopamine-responsive GPCRs, SmD2 and SmGPR-3, are associated with the parasite neuromusculature, yet the full chain from receptor activation to parasite phenotype has not been closed. The dopamine system also delivers a cautionary lesson about assuming that parasite receptors behave like their mammalian counterparts. SmGPR-3 shows a mixed pharmacological profile in which a mammalian dopamine receptor antagonist acts as an agonist. SmD2, meanwhile, resembles human D2 receptors by sequence, but while the mammalian D2 family couples to inhibitory Gαi/o proteins, SmD2 appears to couple to Gαs instead. Such divergences are exactly what the authors argue could create parasite-selective drug targets.

Histamine adds a further twist: not all flatworm aminergic receptors converge on cAMP. A histamine-responsive receptor called SmGPR-1, expressed mainly in body wall muscles, is linked to intracellular calcium mobilization rather than cAMP accumulation, suggesting a Gαq-like pathway. This matters practically, because receptor screens built solely on cAMP readouts will miss or misclassify calcium-linked receptors. A second histamine receptor, SmGPR-2, is upregulated in schistosomula and expressed in the subtegumental nerve plexus, and it carries a striking molecular anomaly. The conserved aspartate residue at position 3.32 in transmembrane helix 3, a linchpin of ligand activation in most Class A GPCRs, is replaced by an asparagine, and many mammalian histamine antagonists are ineffective against it. Histamine is also conceptually intriguing because parasites may encounter it both as their own signal and as a host-derived inflammatory cue, raising the possibility that some aminergic pathways serve in host sensing rather than classical neurotransmission.

Octopamine and tyramine may offer the largest selectivity window of all, because these tyrosine-derived amines are prominent in invertebrate neuromodulation but peripheral to mammalian neurobiology. Anatomical, phenotypic and pharmacological evidence supports octopamine signaling in S. mansoni, and octopamine-related compounds alter schistosomulum motility and length. But no octopamine or tyramine receptor has yet been functionally profiled, leaving the field unable to say whether these signals travel through cAMP, calcium, inhibitory G proteins or biased pathways. Until the receptors are deorphanized and compared with host targets, the authors note, parasite selectivity remains a hypothesis rather than a fact.

The perspective also warns against a schistosome-centric view. Serotonin-responsive GPCRs have been cloned and characterized from tapeworms such as Echinococcus and Mesocestoides, including a set of 5-HT7-like receptors that stimulate cAMP production and a novel class of 5-HT1-like receptors deorphanized through a promiscuous Gα15 effector and calcium-flux assays. Serotonin boosts larval motility in cestodes, and receptor localization near the suckers of Echinococcus granulosus larvae hints that the same signaling may influence attachment as well as movement. In liver flukes, recent work in Fasciola hepatica has localized serotonin receptors and transporters and used RNAi to probe their function, while dopaminergic antagonists disrupt bile chemotaxis and egg production in the carcinogenic fluke Clonorchis sinensis, even though no aminergic receptor from that parasite has been cloned. Comparing these lineages, the authors argue, will reveal which signaling features are broadly conserved targets and which are lineage-specific opportunities.

Transporters form the underdeveloped half of the story. By clearing serotonin, dopamine and other amines from the extracellular space, transporters control signal duration and receptor desensitization, and inhibiting them could disrupt multiple receptor pathways at once. Candidate and characterized transporters exist in schistosomes, F. hepatica and C. sinensis, but their substrate specificity, inhibitor sensitivity, expression patterns and whole-parasite consequences remain poorly mapped. Any transporter-directed strategy will demand careful selectivity profiling, since host monoamine transporters are the pharmacologically sensitive targets of drugs such as antidepressants. Looking forward, the authors highlight three unresolved questions: what intracellular signaling machinery parasite receptors actually use outside heterologous systems, whether the worms are sensing their own neurotransmitters or host-derived chemical cues, and how aminergic pathways crosstalk with neuropeptidergic and cholinergic networks. The most promising future drug targets, they conclude, will combine functional importance, parasite-selective pharmacology and measurable whole-parasite consequences, and the Sm5HTR story provides the template for finding them.

Subject of Research: Bioaminergic GPCR signaling pathways and drug discovery in parasitic flatworms

Article Title: Bioaminergic signaling in parasitic flatworms: a perspective for linking GPCRs to behavior and drug discovery

Article References: Bioaminergic signaling in parasitic flatworms: a perspective for linking GPCRs to behavior and drug discovery. (n.d.). https://doi.org/10.3389/fpara.2026.1933406

Image Credits: AI Generated

DOI: 10.3389/fpara.2026.1933406

Keywords: parasitic flatworms, schistosomiasis, G protein-coupled receptors, serotonin, dopamine, histamine, octopamine, neurotransmitter transporters, anthelmintics, drug discovery, Schistosoma mansoni, trematodes