Targeting GPCRs in Neuroscience: Advances in Drug Discovery Strategies

The complexity of the central nervous system (CNS) poses significant challenges for therapeutic intervention, particularly in the treatment of neurological disorders and neurodegenerative diseases such as depression, Parkinson’s disease, schizophrenia, and Alzheimer’s. Among the many molecular targets under investigation, G protein-coupled receptors (GPCRs) stand out. They play a central role in neuronal signaling and are among the most successful therapeutic targets in drug discovery in neuroscience. These receptors mediate the effects of key neurotransmitters and neuromodulators, making them essential players in brain signaling and plasticity. Furthermore, the integration of fluorescent ligands into screening workflows is accelerating the identification and validation of GPCR targets, offering unprecedented sensitivity and specificity. 

The Central Role of GPCRs in Neurological Disorders

GPCRs form the largest family of membrane receptors and regulate a wide range of CNS functions. They are essential for processes such as neurotransmission, synaptic plasticity, and the regulation of mood, cognition, motor control, and sensory perception. Dysregulation of GPCR function is implicated in numerous neurodegenerative diseases, such as Alzheimer’s and Parkinson’s, as well as psychiatric conditions like anxiety and depression.

More than 30% of FDA-approved drugs target GPCRs, and many of these are prescribed for CNS-related conditions. Their involvement in diverse physiological processes and their druggability, owing to their cell surface localization and ability to modulate intracellular signaling, make them indispensable in GPCR drug discovery for CNS applications. However, the development of GPCR therapeutics for neurological disorders requires a deep understanding of the receptor subtypes involved, their distribution in the brain, and their downstream effects.

GPCR Structure, Activation, and Signaling Pathways in the Brain

Understanding GPCR structure is fundamental to designing effective drugs. GPCRs are characterized by their seven-transmembrane domain architecture and their ability to transduce extracellular signals into intracellular responses through interaction with G-proteins.

What happens when a GPCR is activated? 

When a signaling molecule (e.g., neuromodulators or neurotransmitters) binds to the extracellular domain of the GPCR, it causes conformational changes that enable the activation of intracellular signaling cascades. These GPCR signaling pathways can lead to the activation of second messengers like cyclic adenosine monophosphate (cAMP) or inositol trisphosphate (IP3), ultimately influencing gene expression, neurotransmitter release, and synaptic plasticity.

Figure 1. GPCR signaling: (A) an orthosteric ligand (orange) binds an inactive GPCR, the β2 adrenergic receptor (β2AR; PDB ID: 2RH1); (B) A ligand-bound GPCR undergoes a conformational change to its active state (PDB ID: 3SN6); and (C) an active GPCR binds a G protein (PDB ID: 3SN6), which subsequently promotes nucleotide release from, and activation of, the G protein α-subunit. Source: Latorraca NR, Venkatakrishnan AJ, Dror RO. GPCR Dynamics: Structures in Motion. Chem Rev. 2017 Jan 11;117(1):139-155.

In the CNS, these pathways regulate critical functions such as pain modulation, memory consolidation, and motor coordination. Furthermore, the emergence of biased agonism, where ligands preferentially activate specific signaling routes, has opened new possibilities for fine-tuning therapeutic effects while minimizing side effects.

Emerging GPCR Therapeutic Targets in CNS Drug Discovery

One of the most promising areas in GPCR drug discovery in neuroscience is the identification of novel orphan GPCR targets implicated in CNS pathologies. Orphan receptors like GPR6, GPR37, and GPR139 have attracted growing interest due to their roles in motor control, neuroprotection, and metabolic regulation. These orphan GPCRs are particularly intriguing because their endogenous ligands and physiological functions are not yet fully understood, opening new avenues for therapeutic intervention.

GPR6, for instance, has been associated with neuroprotective functions and is under investigation for its role in Parkinson’s disease and neuropathic pain. Similarly, GPR37 has been linked to the rate of progression of Parkinson’s disease, while GPR139 has been implicated in schizophrenia and inattention in ADHD patients.

Figure 2. Orphan GPCRs related to neurodegenerative disorders. Source: Kim J, Choi C. Orphan GPCRs in Neurodegenerative Disorders: Integrating Structural Biology and Drug Discovery Approaches. Curr Issues Mol Biol. 2024 Oct 19;46(10):11646-11664.

In parallel, cannabinoid receptors (CBRs) such as CB₁R and CB₂R are gaining ground as emerging therapeutic targets in neurological disorders and neurodegenerative diseases, particularly Parkinson’s. Recent advances have demonstrated the potential of fluorescence-based assays for targeting these receptors. For example, in a recent study, the dual fluorescent ligand CELT-335 has been employed to develop a robust and efficient Tag-lite® binding assay for CB₁R and CB₂R. This platform not only enhances the detection of binding events but also eliminates the need for radioactive materials, offering a safer, more cost-effective, and environmentally friendly approach for hit and lead identification in drug discovery and neuroscience.

These developments underscore how both orphan and well-characterized GPCRs continue to provide untapped opportunities for CNS drug development, especially as traditional targets like dopamine and serotonin receptors reach their therapeutic limits. Combined with insights from genomics and transcriptomics, this new generation of targets and screening tools is paving the way for more precise, mechanism-based treatments.

Advantages of Fluorescent Ligands in GPCR Drug Screening for CNS

The success of GPCR drug discovery relies not only on selecting the right targets but also on using the right tools to characterize them. In recent years, fluorescent ligands have become indispensable in GPCR ligand screening, offering several advantages over traditional methods like radioligand binding or antibody staining.

• Live-cell imaging: Enables visualization of receptor-ligand interactions in real-time without cell fixation.
• Greater specificity: Allows selective tracking of receptor subtypes in complex brain tissues.
• Reduced background noise: Improves the signal-to-noise ratio, especially important in CNS assays.
• Faster assay development: Facilitates rapid screening and GPCR target validation.
• Avoid safety concerns and regulatory hurdles: Offers a non-radioactive alternative for early-phase drug screening.

In the context of CNS drug development, where receptor localization and real-time signaling are crucial, fluorescent ligands offer a powerful and adaptable solution. Their integration into drug discovery neuroscience workflows helps accelerate GPCR target identification, characterization, and lead optimization.

At Celtarys, we specialize in the design and synthesis of custom fluorescent ligands tailored to your target of interest that can help you identify and validate novel GPCR targets for neurological disorders and neurodegenerative diseases. 

Contact us to find out how we can support your research!

References

Latorraca NR, Venkatakrishnan AJ, Dror RO. GPCR Dynamics: Structures in Motion. Chem Rev. 2017 Jan 11;117(1):139-155. doi: 10.1021/acs.chemrev.6b00177

Alavi MS, Shamsizadeh A, Azhdari-Zarmehri H, Roohbakhsh A. Orphan G protein-coupled receptors: The role in CNS disorders. Biomed Pharmacother. 2018 Feb;98:222-232. doi: 10.1016/j.biopha.2017.12.056

Azam S, Haque ME, Jakaria M, Jo SH, Kim IS, Choi DK. G-Protein-Coupled Receptors in CNS: A Potential Therapeutic Target for Intervention in Neurodegenerative Disorders and Associated Cognitive Deficits. Cells. 2020 Feb 23;9(2):506. doi: 10.3390/cells9020506 

Kim J, Choi C. Orphan GPCRs in Neurodegenerative Disorders: Integrating Structural Biology and Drug Discovery Approaches. Curr Issues Mol Biol. 2024 Oct 19;46(10):11646-11664. doi: 10.3390/cimb46100691 

Navarro G, Sotelo E, Raïch I, Loza MI, Brea J, Majellaro M. A Robust and Efficient FRET-Based Assay for Cannabinoid Receptor Ligands Discovery. Molecules. 2023 Dec 15;28(24):8107. doi: 10.3390/molecules28248107