In vivo photopharmacology with a caged mu opioid receptor agonist drives rapid changes in behavior

Inputs to the locus coeruleus from the periaqueductal gray and rostroventral medulla shape opioid-mediated descending pain modulation

The supraspinal descending pain modulatory system (DPMS) shapes pain perception via monoaminergic modulation of sensory information in the spinal cord. However, the role and synaptic mechanisms of descending noradrenergic signaling remain unclear. Here, we establish that noradrenergic neurons of the locus coeruleus (LC) are essential for supraspinal opioid antinociception. While much previous work has emphasized the role of descending serotonergic pathways, we find that opioid antinociception is primarily driven by excitatory output from the ventrolateral periaqueductal gray (vlPAG) to the LC. Furthermore, we identify a previously unknown opioid-sensitive inhibitory input from the rostroventromedial medulla (RVM), the suppression of which disinhibits LC neurons to drive spinal noradrenergic antinociception. We describe pain-related activity throughout this circuit and report the presence of prominent bifurcating outputs from the vlPAG to the LC and the RVM. Our findings substantially revise current models of the DPMS and establish a supraspinal antinociceptive pathway that may contribute to multiple forms of descending pain modulation.

Genetically Encoded Photocatalysis Enables Spatially Restricted Optochemical Modulation of Neurons in Live Mice

Light provides high temporal precision for neuronal modulations. Small molecules are advantageous for neuronal modulation due to their structural diversity, allowing them to suit versatile targets. However, current optochemical methods release uncaged small molecules with uniform concentrations in the irradiation area, which lack spatial specificity as counterpart optogenetic methods from genetic encoding for photosensitive proteins. Photocatalysis provides spatial specificity by generating reactive species in the proximity of photocatalysts. However, current photocatalytic methods use antibody-tagged heavy-metal photocatalysts for spatial specificity, which are unsuitable for neuronal applications. Here, we report a genetically encoded metal-free photocatalysis method for the optochemical modulation of neurons via deboronative hydroxylation. The genetically encoded photocatalysts generate doxorubicin, a mitochondrial uncoupler, and baclofen by uncaging stable organoboronate precursors. The mitochondria, nucleus, membrane, cytosol, and ER-targeted drug delivery are achieved by this method. The distinct signaling pathway dissection in a single projection is enabled by the dual optogenetic and optochemical control of synaptic transmission. The itching signaling pathway is investigated by photocatalytic uncaging under live-mice skin for the first time by visible light irradiation. The cell-type-specific release of baclofen reveals the GABABR activation on NaV1.8-expressing nociceptor terminals instead of pan peripheral sensory neurons for itch alleviation in live mice.

A "double-edged" role for type-5 metabotropic glutamate receptors in pain disclosed by light-sensitive drugs

Knowing the site of drug action is important to optimize effectiveness and address any side effects. We used light-sensitive drugs to identify the brain region-specific role of mGlu5 metabotropic glutamate receptors in the control of pain. Optical activation of systemic JF-NP-26, a caged, normally inactive, negative allosteric modulator (NAM) of mGlu5 receptors, in cingulate, prelimbic and infralimbic cortices and thalamus inhibited neuropathic pain hypersensitivity. Systemic treatment of alloswitch-1, an intrinsically active mGlu5 receptor NAM, caused analgesia, and the effect was reversed by light-induced drug inactivation in in the prelimbic and infralimbic cortices, and thalamus. This demonstrates that mGlu5 receptor blockade in the medial prefrontal cortex and thalamus is both sufficient and necessary for the analgesic activity of mGlu5 receptor antagonists. Surprisingly, when light was delivered in the basolateral amygdala, local activation of systemic JF-NP-26 reduced pain thresholds, whereas inactivation of alloswitch-1 enhanced analgesia. Electrophysiological analysis showed that alloswitch-1 increased excitatory synaptic responses in prelimbic pyramidal neurons evoked by stimulation of BLA input, and decreased feedforward inhibition of amygdala output neurons by BLA. Both effects were reversed by optical silencing and reinstated by optical reactivation of alloswitch-1. These findings demonstrate for the first time that the action of mGlu5 receptors in the pain neuraxis is not homogenous, and suggest that blockade of mGlu5 receptors in the BLA may limit the overall analgesic activity of mGlu5 receptor antagonists. This could explain the suboptimal effect of mGlu5 NAMs on pain in human studies and validate photopharmacology as an important tool to determine ideal target sites for systemic drugs.

In vivo photopharmacology with light-activated opioid drugs

Traditional methods for site-specific drug delivery in the brain are slow, invasive, and difficult to interface with recordings of neural activity. Here, we demonstrate the feasibility and experimental advantages of in vivo photopharmacology using "caged" opioid drugs that are activated in the brain with light after systemic administration in an inactive form. To enable bidirectional manipulations of endogenous opioid receptors in vivo, we developed photoactivatable oxymorphone (PhOX) and photoactivatable naloxone (PhNX), photoactivatable variants of the mu opioid receptor agonist oxymorphone and the antagonist naloxone. Photoactivation of PhOX in multiple brain areas produced local changes in receptor occupancy, brain metabolic activity, neuronal calcium activity, neurochemical signaling, and multiple pain- and reward-related behaviors. Combining PhOX photoactivation with optical recording of extracellular dopamine revealed adaptations in the opioid sensitivity of mesolimbic dopamine circuitry in response to chronic morphine administration. This work establishes a general experimental framework for using in vivo photopharmacology to study the neural basis of drug action.

Current Status and Future Strategies for Advancing Functional Circuit Mapping In Vivo

The human brain represents one of the most complex biological systems, containing billions of neurons interconnected through trillions of synapses. Inherent to the brain is a biochemical complexity involving ions, signaling molecules, and peptides that regulate neuronal activity and allow for short- and long-term adaptations. Large-scale and noninvasive imaging techniques, such as fMRI and EEG, have highlighted brain regions involved in specific functions and visualized connections between different brain areas. A major shortcoming, however, is the need for more information on specific cell types and neurotransmitters involved, as well as poor spatial and temporal resolution. Recent technologies have been advanced for neuronal circuit mapping and implemented in behaving model organisms to address this. Here, we highlight strategies for targeting specific neuronal subtypes, identifying, and releasing signaling molecules, controlling gene expression, and monitoring neuronal circuits in real-time in vivo Combined, these approaches allow us to establish direct causal links from genes and molecules to the systems level and ultimately to cognitive processes.

Inputs to the locus coeruleus from the periaqueductal gray and rostroventral medulla shape opioid-mediated descending pain modulation

The supraspinal descending pain modulatory system (DPMS) shapes pain perception via monoaminergic modulation of sensory information in the spinal cord. However, the role and synaptic mechanisms of descending noradrenergic signaling remain unclear. Here, we establish that noradrenergic neurons of the locus coeruleus (LC) are essential for supraspinal opioid antinociception. Unexpectedly, given prior emphasis on descending serotonergic pathways, we find that opioid antinociception is primarily driven by excitatory output from the ventrolateral periaqueductal gray (vlPAG) to the LC. Furthermore, we identify a previously unknown opioid-sensitive inhibitory input from the rostroventromedial medulla (RVM), the suppression of which disinhibits LC neurons to drive spinal noradrenergic antinociception. We also report the presence of prominent bifurcating outputs from the vlPAG to the LC and the RVM. Our findings significantly revise current models of the DPMS and establish a novel supraspinal antinociceptive pathway that may contribute to multiple forms of descending pain modulation.

Human OPRM1 and murine Oprm1 promoter driven viral constructs for genetic access to μ-opioidergic cell types

With concurrent global epidemics of chronic pain and opioid use disorders, there is a critical need to identify, target and manipulate specific cell populations expressing the mu-opioid receptor (MOR). However, available tools and transgenic models for gaining long-term genetic access to MOR+ neural cell types and circuits involved in modulating pain, analgesia and addiction across species are limited. To address this, we developed a catalog of MOR promoter (MORp) based constructs packaged into adeno-associated viral vectors that drive transgene expression in MOR+ cells. MORp constructs designed from promoter regions upstream of the mouse Oprm1 gene (mMORp) were validated for transduction efficiency and selectivity in endogenous MOR+ neurons in the brain, spinal cord, and periphery of mice, with additional studies revealing robust expression in rats, shrews, and human induced pluripotent stem cell (iPSC)-derived nociceptors. The use of mMORp for in vivo fiber photometry, behavioral chemogenetics, and intersectional genetic strategies is also demonstrated. Lastly, a human designed MORp (hMORp) efficiently transduced macaque cortical OPRM1+ cells. Together, our MORp toolkit provides researchers cell type specific genetic access to target and functionally manipulate mu-opioidergic neurons across a range of vertebrate species and translational models for pain, addiction, and neuropsychiatric disorders.

A Biomimetic C-Terminal Extension Strategy for Photocaging Amidated Neuropeptides

Photoactivatable neuropeptides offer a robust stimulus-response relationship that can drive mechanistic studies into the physiological mechanisms of neuropeptidergic transmission. The majority of neuropeptides contain a C-terminal amide, which offers a potentially general site for installation of a C-terminal caging group. Here, we report a biomimetic caging strategy in which the neuropeptide C-terminus is extended via a photocleavable amino acid to mimic the proneuropeptides found in large dense-core vesicles. We explored this approach with four prominent neuropeptides: gastrin-releasing peptide (GRP), oxytocin (OT), substance P (SP), and cholecystokinin (CCK). C-terminus extension greatly reduced the activity of all four peptides at heterologously expressed receptors. In cell type-specific electrophysiological recordings from acute brain slices, subsecond flashes of ultraviolet light produced rapidly activating membrane currents via activation of endogenous G protein-coupled receptors. Subsequent mechanistic studies with caged CCK revealed a role for extracellular proteases in shaping the temporal dynamics of CCK signaling, and a striking switch-like, cell-autonomous anti-opioid effect of transient CCK signaling in hippocampal parvalbumin interneurons. These results suggest that C-terminus extension with a photocleavable linker may be a general strategy for photocaging amidated neuropeptides and demonstrate how photocaged neuropeptides can provide mechanistic insights into neuropeptide signaling that are inaccessible using conventional approaches.