Photoactivatable Dopamine and Sulpiride to Explore the Function of Dopaminergic Neurons and Circuits

Neuromodulator and neuropeptide sensors and probes for precise circuit interrogation in vivo

To determine how neuronal circuits encode and drive behavior, it is often necessary to measure and manipulate different aspects of neurochemical signaling in awake animals. Optogenetics and calcium sensors have paved the way for these types of studies, allowing for the perturbation and readout of spiking activity within genetically defined cell types. However, these methods lack the ability to further disentangle the roles of individual neuromodulator and neuropeptides on circuits and behavior. We review recent advances in chemical biology tools that enable precise spatiotemporal monitoring and control over individual neuroeffectors and their receptors in vivo. We also highlight discoveries enabled by such tools, revealing how these molecules signal across different timescales to drive learning, orchestrate behavioral changes, and modulate circuit activity.

Multi-Photon-Sensitive Chromophore for the Photorelease of Biologically Active Phenols

Phenols confer bioactivity to a plethora of organic compounds. Protecting the phenolic functionality with photoremovable protecting groups (PPGs) sensitive to two-photon excitation (2PE) can block the bioactivity and provide controlled release of these compounds in a spatially and temporally restricted manner by photoactivation with IR light. To develop an efficient 2PE-sensitive PPG for releasing phenols, the (8-cyano-7-hydroxyquinolin-2-yl)methyl (CyHQ) chromophore was functionalized at the C4 position with methyl, morpholine, methoxy, para-tolyl, and 3,4,5-trimethoxyphenyl groups to provide 4-methyl-CyHQ (Me-CyHQ), 4-morpholino-CyHQ (Mor-CyHQ), 4-methoxy-CyHQ (MeO-CyHQ), 4-(p-tolyl)-CyHQ (pTol-CyHQ), and 4-(3,4,5-trimethoxyphenyl)-CyHQ (TMP-CyHQ) PPGs. The probes possess attributes useful for biological use, including high quantum yield (Φu), hydrolytic stability, and good aqueous solubility in physiological conditions. The MeO-CyHQ PPG enhanced the two-photon uncaging action cross section (δu) of dopamine 3.5-fold (0.85 GM) compared to CyHQ (0.24 GM) at 740 nm and 1.49 GM at 720 nm. MeO-CyHQ was used to mediate photoactivation via 2PE of serotonin, rotigotine, N-vanillyl-nonanoylamide (VNA) (a capsaicin analogue), and eugenol. The constructs except rotigotine showed excellent efficiency in 2PE with δu ranging from 0.75 to 1.01 GM at 740 nm and from 1.31 to 1.36 GM at 720 nm high yielding release of the payloads. These probes also performed well by using conventional single photon excitation (1PE). The spatially and temporally controlled release of dopamine from CyHQ-DA and MeO-CyHQ-DA and serotonin (5-HT) from MeO-CyHQ-5HT was quantified in cell culture by using genetically encoded sensors for dopamine and serotonin, respectively. Calcium imaging was employed to quantify the release of VNA and eugenol (EG) from MeO-CyHQ-VNA and MeO-CyHQ-EG, respectively. These tools will enable experiments to understand the intricate mechanisms involved in neurological signaling and the roles played by neurotransmitters, such as dopamine and serotonin, in the activation of their respective receptors.

Photoactivatable Agonist-Antagonist Pair as a Tool for Precise Spatiotemporal Control of Serotonin Receptor 2C Signaling

Orthogonal recreation of the signaling profile of a chemical synapse is a current challenge in neuroscience. This is due in part to the kinetics of synaptic signaling, where neurotransmitters are rapidly released and quickly cleared by active reuptake machinery. One strategy to produce a rapid rise in an orthogonally controlled signal is via photocaged compounds. In this work, photocaged compounds are employed to recreate both the rapid rise and equally rapid fall in activation at a chemical synapse. Specifically, a complementary pair of photocages based on BODIPY were conjugated to a 5-HT2C subtype-selective agonist, WAY-161503, and antagonist, N-desmethylclozapine, to generate "caged" versions of these drugs. These conjugates release the bioactive drug upon illumination with green light (agonist) or red light (antagonist). We report on the synthesis, characterization, and bioactivity testing of the conjugates against the 5-HT2C receptor. We then characterize the kinetics of photolysis quantitatively using HPLC and qualitatively in cell culture conditions stimulating live cells. The compounds are shown to be stable in the dark for 48 h at room temperature, yet photolyze rapidly when irradiated with visible light. In live cells expressing the 5-HT2C receptor, precise spatiotemporal control of the degree and length of calcium signaling is demonstrated. By loading both compounds in tandem and leveraging spectral multiplexing as a noninvasive method to control local small-molecule drug availability, we can reproducibly initiate and suppress intracellular calcium flux on a timescale not possible by traditional methods of drug dosing. These tools enable a greater spatiotemporal control of 5-HT2C modulation and will allow for more detailed studies of the receptors' signaling, interactions with other proteins, and native physiology.

Optical Control of Dopamine D2-like Receptors with Cell-Specific Fast-Relaxing Photoswitches

Dopamine D2-like receptors (D2R, D3R, and D4R) control diverse physiological and behavioral functions and are important targets for the treatment of a variety of neuropsychiatric disorders. Their complex distribution and activation kinetics in the brain make it difficult to target specific receptor populations with sufficient precision. We describe a new toolkit of light-activatable, fast-relaxing, covalently taggable chemical photoswitches that fully activate, partially activate, or block D2-like receptors. This technology combines the spatiotemporal precision of a photoswitchable ligand (P) with cell type and spatial specificity of a genetically encoded membrane anchoring protein (M) to which the P tethers. These tools set the stage for targeting endogenous D2-like receptor signaling with molecular, cellular, and spatiotemporal precision using only one wavelength of light.

Gait Abnormalities and Aberrant D2 Receptor Expression and Signaling in Mice Carrying the Human Pathogenic Mutation DRD2I212F

A dopamine D2 receptor mutation was recently identified in a family with a novel hyperkinetic movement disorder. That allelic variant D2-I212F is a constitutively active and G protein-biased receptor. We now describe mice engineered using CRISPR-Cas9-mediated gene editing technology to carry the D2-I212F variant. Drd2I212F mice exhibited gait abnormalities resembling those in other mouse models of chorea and/or dystonia and had striatal D2 receptor expression that was decreased approximately 30% per Drd2I212F allele. Electrically evoked inhibitory postsynaptic conductances in midbrain dopamine neurons and striatum from Drd2I212F mice, caused by G protein activation of potassium channels, exhibited slow kinetics (e.g., approximately four- to sixfold slower decay) compared with Drd2 +/+ mice. Current decay initiated by photolytic release of the D2 antagonist sulpiride from CyHQ-sulpiride was also ∼fourfold slower in midbrain slices from Drd2I212F mice than Drd2 +/+ mice. Furthermore, in contrast to Drd2 +/+ mice, in which dopamine is several-fold more potent at neurons in the nucleus accumbens than in the dorsal striatum, reflecting activation of Gα o versus Gα i, dopamine had similar potencies in those two brain regions of Drd2I212F mice. Repeated cocaine treatment, which decreases dopamine potency in the nucleus accumbens of Drd2 +/+ mice, had no effect on dopamine potency in Drd2 I212F mice. The results demonstrate the pathogenicity of the D2-I212F mutation and the utility of this mouse model for investigating the role of pathogenic DRD2 variants in early-onset hyperkinetic movement disorders. SIGNIFICANCE STATEMENT: The first dopamine receptor mutation to cause a movement disorder, D2-I212F, was recently identified. The mutation makes receptor activation of G protein-mediated signaling more efficient. To confirm the pathogenesis of D2-I212F, this study reports that mice carrying this mutation have gait abnormalities consistent with the clinical phenotype. The mutation also profoundly alters D2 receptor expression and function in vivo. This mouse model will be useful for further characterization of the mutant receptor and for evaluation of potential therapeutic drugs.

Co-activation of GPCRs facilitate GIRK-dependent current

The activity of dopamine neurons is dependent on both intrinsic properties and afferent projections. One potent form of inhibition is mediated by the activation of two inhibitory G protein-coupled receptors, D2 and GABA_B receptors. Each of these receptors activates G protein-coupled inwardly rectifying potassium (GIRK) channels. Recordings in brain slices have shown that co-activation using saturating concentrations of agonists results in occlusion of the GIRK current. The present study examined the interaction between D2 and GABA_B receptors using transient applications of sub-saturating concentrations of agonists where the co-application of one agonist resulted in both facilitation and inhibition (desensitization) of the other. The heterologous facilitation was modelled based on the known cooperative interaction between the G protein βγ subunits and GIRK channels. The results indicate that a low tonic level of G βγ results in facilitation of GIRK current and a high level of G βγ results in occlusion. The kinetics of the current induced by transient receptor activation is prolonged in each case. The results suggest that the cooperative interaction between G βγ subunits and GIRK channels determines both the amplitude and kinetics of GPCR-dependent current. KEY POINTS: Inhibitory D2 and GABA_B receptors modulate dopamine neuron activity through shared G protein-coupled inwardly rectifying potassium (GIRK) channels. This study reports robust bidirectional interactions between these two converging receptor pathways. Coincident activation of D2 and GABA_B receptors leads to facilitation of GIRK channel currents, augmenting both amplitude and prolonging the duration of phasic responses. Activation of either D2 or GABA_B receptors also acutely desensitized the GIRK channel current induced by D2 receptor activation that rapidly recovers following termination of desensitizing stimulus. Results demonstrate that the activity of either G protein-coupled receptor system must be considered in the context of other G protein-coupled receptors.

Role of Protein Damage Inflicted by Dopamine Metabolites in Parkinson's Disease: Evidence, Tools, and Outlook

Dopamine is an important neurotransmitter that plays a critical role in motivational salience and motor coordination. However, dysregulated dopamine metabolism can result in the formation of reactive electrophilic metabolites which generate covalent adducts with proteins. Such protein damage can impair native protein function and lead to neurotoxicity, ultimately contributing to Parkinson's disease etiology. In this Review, the role of dopamine-induced protein damage in Parkinson's disease is discussed, highlighting the novel chemical tools utilized to drive this effort forward. Continued innovation of methodologies which enable detection, quantification, and functional response elucidation of dopamine-derived protein adducts is critical for advancing this field. Work in this area improves foundational knowledge of the molecular mechanisms that contribute to dopamine-mediated Parkinson's disease progression, potentially assisting with future development of therapeutic interventions.

Caged Oxime Reactivators Designed for the Light Control of Acetylcholinesterase Reactivation†

Despite its promising role in the active control of biological functions by light, photocaging remains untested in acetylcholinesterase (AChE), a key enzyme in the cholinergic family. Here, we describe synthesis, photochemical properties and biochemical activities of two caged oxime compounds applied in the photocontrolled reactivation of the AChE inactivated by reactive organophosphate. Each of these consists of a photocleavable coumarin cage tethered to a known oxime reactivator for AChE that belongs in an either 2-(hydroxyimino)acetamide or pyridiniumaldoxime class. Of these, the first caged compound was able to successfully go through oxime uncaging upon irradiation at long-wavelength ultraviolet light (365 nm) or visible light (420 nm). It was further evaluated in AChE assays in vitro under variable light conditions to define its activity in the photocontrolled reactivation of paraoxon-inactivated AChE. This assay result showed its lack of activity in the dark but its induction of activity under light conditions only. In summary, this article reports a first class of light-activatable modulators for AChE and it offers assay methods and novel insights that help to achieve an effective design of caged compounds in the enzyme control.

The residence of synaptically released dopamine on D2 autoreceptors

Neuromodulation mediated by synaptically released endogenous transmitters acting in G-protein-coupled receptors (GPCRs) is slow primarily because of multistep downstream signaling. What is less well understood is the spatial and temporal kinetics of transmitter and receptor interaction. The present work uses the combination of the dopamine sensor, dLight, to detect the spatial release and diffusion of dopamine and a caged form of a D2-dopamine receptor antagonist, CyHQ-sulpiride, to rapidly block the D2 autoreceptors. Photoactivation of the CyHQ-sulpiride blocks receptors in milliseconds such that the time course of dopamine/receptor interaction is mapped onto the downstream signaling. The results show that highly localized release, but not dopamine diffusion, defines the time course of the functional interaction between dopamine and D2 autoreceptors, which determines downstream inhibition.

Photoactivatable AMPA for the study of glutamatergic neuronal transmission using two-photon excitation

We report a photoactivatable agonist of the AMPA subtype of ionotropic glutamate receptors, TMP-CyHQ-AMPA, which was designed to study the fast excitatory transmission between neurons. Upon visible light excitation, TMP-CyHQ-AMPA quantitatively released AMPA in high quantum yield on an ultra-short timescale. Intriguingly, the photolyisis can be carried out using 2-photon excitation (2PE) with remarkable efficiency, giving a two-photon uncaging action cross section (δu) value of 1.71 GM. TMP-CyHQ-AMPA is soluble in pysiological buffer and no hydrolysis was detected in the absence of light. Molecular docking experiments indicated that the photocaging strategy abolishes the affinity of AMPA for the GluR2 receptor and no GABAergic effects (as commonly observed in caged glutamates) are expected. TMP-CyHQ-AMPA can be used to study glutamatergic neuronal transmission with exceptional spatial-temporal resolution in complex tissue preparations.

Signaling-Biased and Constitutively Active Dopamine D2 Receptor Variant

A dopamine D2 receptor mutation was recently identified in a family with a novel hyperkinetic movement disorder. Compared to the wild type D2 receptor, the novel allelic variant D2-I²¹²F activates a Gα_i1β₁γ₂ heterotrimer with higher potency and modestly enhanced basal activity in human embryonic kidney (HEK) 293 cells and has decreased capacity to recruit arrestin3. We now report that omitting overexpressed G protein-coupled receptor kinase-2 (GRK2) decreased the potency and efficacy of quinpirole for arrestin recruitment. The relative efficacy of quinpirole for arrestin recruitment to D2-I²¹²F compared to D2-WT was considerably lower without overexpressed GRK2 than with added GRK2. D2-I²¹²F exhibited higher basal activation of Gα_oA than Gα_i1 but little or no increase in the potency of quinpirole relative to D2-WT. Other signs of D2-I²¹²F constitutive activity for G protein-mediated signaling, in addition to basal activation of Gα_i/o, were enhanced basal inhibition of forskolin-stimulated cyclic AMP accumulation that was reversed by the inverse agonists sulpiride and spiperone and a ∼4-fold increase in the apparent affinity of D2-I²¹²F for quinpirole, determined from competition binding assays. In mouse midbrain slices, inhibition of tonic current by the inverse agonist sulpiride in dopamine neurons expressing D2-I²¹²F was consistent with our hypothesis of enhanced constitutive activity and sensitivity to dopamine relative to D2-WT. Molecular dynamics simulations with D2 receptor models suggested that an ionic lock between the cytoplasmic ends of the third and sixth α-helices that constrains many G protein-coupled receptors in an inactive conformation spontaneously breaks in D2-I²¹²F. Overall, these results confirm that D2-I²¹²F is a constitutively active and signaling-biased D2 receptor mutant and also suggest that the effect of the likely pathogenic variant in a given brain region will depend on the nature of G protein and GRK expression.

Design, Synthesis, and Biological Evaluation of Light-Activated Antibiotics

The spatial and temporal control of bioactivity of small molecules by light (photopharmacology) constitutes a promising approach for study of biological processes and ultimately for the treatment of diseases. In this study, we investigated two different "caged" antibiotic classes that can undergo remote activation with UV-light at λ = 365 nm, via the conjugation of deactivating and photocleavable units through a short synthetic sequence. The two widely used antibiotics vancomycin and cephalosporin were thus enhanced in their performance by rendering them photoresponsive and thereby suppressing undesired off-site activity. The antimicrobial activity against Bacillus subtilis ATCC 6633, Staphylococcus aureus ATCC 29213, S. aureus ATCC 43300 (MRSA), Escherichia coli ATCC 25922, and Pseudomonas aeruginosa ATCC 27853 could be spatiotemporally controlled with light. Both molecular series displayed a good activity window. The vancomycin derivative displayed excellent values against Gram-positive strains after uncaging, and the next-generation caged cephalosporin derivative achieved good and broad activity against both Gram-positive and Gram-negative strains after photorelease.

In Situ Electrochemical Monitoring of Caged Compound Photochemistry: An Internal Actinometer for Substrate Release

Caged compounds are molecules that release a protective substrate to free a biologically active substrate upon treatment with light of sufficient energy and duration. A notable limitation of this approach is difficulty in determining the degree of photoactivation in tissues or opaque solutions because light reaching the desired location is obstructed. Here, we have addressed this issue by developing an in situ electrochemical method in which the amount of caged molecule photorelease is determined by fast-scan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes. Using p-hydroxyphenyl glutamate (pHP-Glu) as our model system, we generated a linear calibration curve for oxidation of 4-hydroxyphenylacetic acid (4HPAA), the group from which the glutamate molecule leaves, up to a concentration of 1000 μM. Moreover, we are able to correct for the presence of residual pHP-Glu in solution as well as the light artifact that is produced. A corrected calibration curve was constructed by photoactivation of pHP-Glu in a 3 μL photoreaction vessel and subsequent analysis by high-performance liquid chromatography. This approach has yielded a linear relationship between 4HPAA concentration and oxidation current, allowing the determination of released glutamate independent of the amount of light reaching the chromophore. Moreover, we have successfully validated the newly developed method by in situ measurement in a whole, intact zebrafish brain. This work demonstrates for the first time the in situ electrochemical monitoring of caged compound photochemistry in brain tissue with FSCV, thus facilitating analyses of neuronal function.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.