What is 5-HT1B, and why does it matter in Alzheimer’s?

5-HT1B is a G-protein-coupled receptor found on presynaptic neurons throughout the brain. It regulates the release of neurotransmitters like serotonin, dopamine, and glutamate, which are essential for communication between brain cells. In healthy conditions, this signaling is balanced, but when that balance is disrupted in AD, it’s thought that 5-HT1B may play a more complicated role.

Research has shown that the serotonergic system is one of the first to be affected in AD, with serotonin-producing neurons degenerating even before the appearance of hallmark amyloid plaques and tau tangles¹. Because 5-HT1B is closely tied to serotonin signaling, scientists are asking whether changes in this receptor could contribute to early disease processes like memory loss and neuroinflammation.

A role in synaptic plasticity and inflammation

Synaptic plasticity, the brain’s ability to strengthen or weaken synapses over time, plays a critical role in learning and memory. In AD, synaptic plasticity declines over time, contributing to disease progression, and there’s growing evidence that 5-HT1B influences this process. In animal models of AD, stimulating serotonin-producing neurons in the dorsal raphe nucleus improves synaptic plasticity in the hippocampus, and these effects seem to depend on 5-HT1B receptor activity².

5-HT1B may also influence neuroinflammation, another driver of AD progression. In transgenic mouse models, researchers have found that loss of 5-HT1B worsens the inflammatory response to amyloid-beta (Aβ), suggesting the receptor normally acts to keep inflammation in check³. While the exact pathways are still being mapped, this is a compelling lead. Serotonin receptors may be doing more than regulating mood—they could also be modulating how the brain responds to damage.

What animal and post-mortem studies tell us

Evidence from both animal models and post-mortem human brains points to altered 5-HT1B expression in AD. Studies in transgenic mice, such as those carrying APP mutations, show reduced 5-HT1B levels in the hippocampus and cortex, regions deeply involved in memory and learning⁴. Human studies show similar trends. Post-mortem analyses have found significantly lower 5-HT1B receptor density in the frontal and temporal cortex of AD patients compared to controls. Receptor levels have also been shown to correlate with cognitive performance, suggesting a possible link between 5-HT1B loss and symptom severity⁵.

Can we target 5-HT1B to treat Alzheimer’s?

Because 5-HT1B regulates neurotransmitter release, it’s a tempting target for cognitive enhancement. In theory, modulating 5-HT1B activity could help restore neurotransmitter balance in the AD brain. Some preclinical studies have shown that increasing serotonin signaling using SSRIs can reduce Aβ and tau pathology and improve cognition in cellular and mouse models^6,7. While these drugs don’t target 5-HT1B specifically, they demonstrate the influence the serotonin system can have over AD disease progression. While there are no 5-HT1B-specific therapies for AD in clinical trials yet, we’re still in the early days. More targeted approaches to serotonin and serotonin receptors are being explored for AD and other neurodegenerative conditions⁸.

The gaps in our understanding

While the data so far is promising, many questions remain. Before 5-HT1B-specific therapies can reach clinical trials, researchers need to get clarity on key knowledge gaps, including:

• We don’t have a clear picture of 5-HT1B’s role across the brain. 5-HT1B’s effects vary by brain region and cell type. It acts as an autoreceptor on serotonin neurons but also modulates glutamate and GABA release in other contexts, making its overall impact in the AD brain hard to predict.^9,10

• We don’t fully understand the downstream signaling pathways. 5-HT1B couples to Gi/o proteins, reducing cAMP levels and altering ion channel activity, but how these molecular effects influence tau phosphorylation, microglial function, or network activity in AD is still unclear.¹¹

• We don’t know whether 5-HT1B loss is a cause or consequence of AD. It’s possible that the reduction in 5-HT1B in AD may be a compensatory change to rebalance signaling rather than a causality.

Clarifying these unknowns will be essential for determining whether 5-HT1B is simply a marker of disease or a meaningful target for intervention.

Approaches to studying 5-HT1B in Alzheimer’s models

To match the range of unanswered research questions, the range of experimental approaches used to study 5-HT1B in the context of AD is also broad. Popular methods span from molecular detection to in vivo imaging, offering complementary insights into how the receptor functions and what changes during disease progression. Some commonly used approaches are summarised in the table below:

Each approach answers a slightly different question: Where is the receptor expressed? How does it behave when modulated? What changes over the course of the disease?

By combining molecular, pharmacological, and imaging techniques, researchers can begin to build a more complete picture of 5-HT1B’s role in AD.

Could 5-HT1B be part of the Alzheimer’s solution?

Alzheimer’s disease is incredibly complex, and there’s unlikely to be a single “magic bullet” treatment. But understanding the contribution of neurotransmitter systems, like serotonin, could help explain why some symptoms appear when they do, and how we might intervene earlier. 5-HT1B is druggable, measurable, and already implicated in brain disorders. While more research is needed to fully understand its role in AD, the evidence so far suggests that this receptor shouldn’t be overlooked. For scientists working on the front lines of neurodegeneration, 5-HT1B may yet reveal critical insights into how and when to intervene in Alzheimer’s disease.

References

(1) Smith, G. S.; Protas, H.; Kuwabara, H.; Savonenko, A.; Nassery, N.; Gould, N. F.; Kraut, M.; Avramopoulos, D.; Holt, D.; Dannals, R. F.; Nandi, A.; Su, Y.; Reiman, E. M.; Chen, K. Molecular Imaging of the Association between Serotonin Degeneration and Beta-Amyloid Deposition in Mild Cognitive Impairment. NeuroImage Clin. 2023, 37, 103322. https://doi.org/10.1016/j.nicl.2023.103322.

(2) Yu, C.; Wang, X.; Wang, J.; Li, C.; Xiao, J.; Wang, X.; Han, R.; Wang, S.; Lin, Y.; Kong, L.; Du, Y. Electroacupuncture Alleviates Memory Deficits in APP/PS1 Mice by Targeting Serotonergic Neurons in Dorsal Raphe Nucleus. Curr. Med. Sci. 2024, 44 (5), 987–1000. https://doi.org/10.1007/s11596-024-2908-9.

(3) Metaxas, A.; Anzalone, M.; Vaitheeswaran, R.; Petersen, S.; Landau, A. M.; Finsen, B. Neuroinflammation and Amyloid-Beta 40 Are Associated with Reduced Serotonin Transporter (SERT) Activity in a Transgenic Model of Familial Alzheimer’s Disease. Alzheimers Res. Ther. 2019, 11 (1), 38. https://doi.org/10.1186/s13195-019-0491-2.

(4) Tajeddinn, W.; Persson, T.; Maioli, S.; Calvo-Garrido, J.; Parrado-Fernandez, C.; Yoshitake, T.; Kehr, J.; Francis, P.; Winblad, B.; Höglund, K.; Cedazo-Minguez, A.; Aarsland, D. 5-HT1B and Other Related Serotonergic Proteins Are Altered in APPswe Mutation. Neurosci. Lett. 2015, 594, 137–143. https://doi.org/10.1016/j.neulet.2015.03.064.

(5) Garcia-Alloza, M.; Hirst, W. D.; Chen, C. P. L.-H.; Lasheras, B.; Francis, P. T.; Ramírez, M. J. Differential Involvement of 5-HT1B/1D and 5-HT6 Receptors in Cognitive and Non-Cognitive Symptoms in Alzheimer’s Disease. Neuropsychopharmacology 2004, 29 (2), 410–416. https://doi.org/10.1038/sj.npp.1300330.

(6) Reddy, A. P.; Sawant, N.; Morton, H.; Kshirsagar, S.; Bunquin, L. E.; Yin, X.; Reddy, P. H. Selective Serotonin Reuptake Inhibitor Citalopram Ameliorates Cognitive Decline and Protects against Amyloid Beta-Induced Mitochondrial Dynamics, Biogenesis, Autophagy, Mitophagy and Synaptic Toxicities in a Mouse Model of Alzheimer’s Disease. Hum. Mol. Genet. 2021, 30 (9), 789–810. https://doi.org/10.1093/hmg/ddab091.

(7) Reddy, A. P.; Yin, X.; Sawant, N.; Reddy, P. H. Protective Effects of Antidepressant Citalopram against Abnormal APP Processing and Amyloid Beta-Induced Mitochondrial Dynamics, Biogenesis, Mitophagy and Synaptic Toxicities in Alzheimer’s Disease. Hum. Mol. Genet. 2021, 30 (10), 847–864. https://doi.org/10.1093/hmg/ddab054.

(8) Xing, C.; Chen, H.; Bi, W.; Lei, T.; Hang, Z.; Du, H. Targeting 5-HT Is a Potential Therapeutic Strategy for Neurodegenerative Diseases. Int. J. Mol. Sci. 2024, 25 (24), 13446. https://doi.org/10.3390/ijms252413446.

(9) Sari, Y. Serotonin1B Receptors: From Protein to Physiological Function and Behavior. Neurosci. Biobehav. Rev. 2004, 28 (6), 565–582. https://doi.org/10.1016/j.neubiorev.2004.08.008.

(10) Clark, E. A.; Wang, L.; Hanania, T.; Kretschmannova, K.; Bianchi, M.; Jagger, E.; Hu, T.; Li, F.; Gallero-Salas, Y.; Koblan, K. S.; Dedic, N.; Bristow, L. J. 5-HT1B Receptor Activation Produces Rapid Antidepressant-like Effects in Rodents. Pharmacol. Biochem. Behav. 2025, 247, 173917. https://doi.org/10.1016/j.pbb.2024.173917.

(11) (PDF) Serotonergic Signaling: Multiple Effectors and Pleiotropic Effects. ResearchGate. https://doi.org/10.1002/wmts.50.

(12) Cosgrove, K. P.; Kloczynski, T.; Nabulsi, N.; Weinzimmer, D.; Lin, S.-F.; Staley, J. K.; Bhagwagar, Z.; Carson, R. E. Assessing the Sensitivity of [11C]P943, a Novel 5-HT1B Radioligand, to Endogenous Serotonin Release. Synap. N. Y. N 2011, 65 (10), 1113–1117. https://doi.org/10.1002/syn.20942.