Local anesthetics are effective in suppressing pain sensation, but most of these compounds act non-selectively, inhibiting the activity of all neurons. Moreover, their actions abate slowly, preventing precise spatial and temporal control of nociception. We have developed a photoisomerizable molecule named QAQ (Quaternary ammonium – Azobenzene – Quaternary ammonium) that enables rapid and selective optical control of nociception. QAQ is membrane-impermeant and it has no effect on most cells, but it infiltrates pain-sensing neurons through endogenous ion channels that are activated by noxious stimuli, primarily TRPV1. After QAQ accumulates intracellularly, it blocks voltage-gated ion channels in the trans but not the cis form. QAQ enables reversible optical silencing of mouse nociceptive neuron firing without exogenous gene expression and can serve as a light-sensitive analgesic in rats in vivo. Moreover, because intracellular QAQ accumulation is a consequence of nociceptive ion channel activity, QAQ-mediated photosensitization provides a new platform for understanding signaling mechanisms in acute and chronic pain.

Photochromic channel blockers provide a conceptually simple and convenient way to modulate neuronal activity with light. We have recently described a family of azobenzenes that function as tonic blockers of K(v) channels but require UV-A light to unblock and need to be actively switched by toggling between two different wavelengths. We now introduce red-shifted compounds that fully operate in the visible region of the spectrum and quickly turn themselves off in the dark. Furthermore, we have developed a version that does not block effectively in the dark-adapted state, can be switched to a blocking state with blue light, and reverts to the inactive state automatically. Photochromic blockers of this type could be useful for the photopharmacological control of neuronal activity under mild conditions.

The optical neuroscience revolution is transforming how we study neural circuits. By providing a precise way to manipulate endogenous neuronal signaling proteins, it also has the potential to transform our understanding of molecular neuroscience. Recent advances in chemical biology have produced light-sensitive compounds that photoregulate a wide variety of proteins underlying signaling between and within neurons. Chemical tools for optopharmacology include caged agonists and antagonists and reversibly photoswitchable ligands. These reagents act on voltage-gated ion channels and neurotransmitter receptors, enabling control of neuronal signaling with a high degree of spatial and temporal precision. By covalently attaching photoswitch molecules to genetically tagged proteins, the newly emerging methodology of optogenetic pharmacology allows biochemically precise control in targeted subsets of neurons. Now that the tools for manipulating endogenous neuronal signaling proteins are available, they can be implemented in vivo to enhance our understanding of the molecular bases of brain function and dysfunctions.

Light work: Studies into the mechanism of AAQ, a photoswitchable blocker of voltage‐gated K+ channels, led to the discovery and development of photochromic ligands that act at the internal tetraethylammonium binding site (see picture). These molecules can be applied from the extracellular side to impart long‐lasting photosensitivity on K+ channels in living cells and thereby afford photocontrol of action potential firing in neurons.

Light-controllable tools provide powerful means to manipulate and interrogate brain function with low invasiveness and high spatiotemporal precision. Although optogenetics permits neuronal excitation or inhibition at the network level, other technologies such as optopharmacology (also known as photopharmacology) have emerged that provide molecular level control by endowing light-sensitivity to endogenous biomolecules. In this Review, we discuss the challenges and opportunities of photocontrolling native neuronal signaling pathways, focusing on ion channels and neurotransmitter receptors. We describe existing strategies for rendering receptors and channels light-sensitive and provide an overview of the neuroscientific insights gained from such approaches. At the cross roads of chemistry, protein engineering and neuroscience, optopharmacology offers great potential in understanding the molecular basis of brain function and behavior, with promises for future therapeutics.

Advances in synthetic chemistry, structural biology, molecular modelling and molecular cloning have enabled the systematic functional manipulation of transmembrane proteins. By combining genetically manipulated proteins with light-sensitive ligands, innately ‘blind’ neurobiological receptors can be converted into photoreceptors, which allows them to be photoregulated with high spatiotemporal precision. Here, we present the optochemical control of neuronal nicotinic acetylcholine receptors (nAChRs) with photoswitchable tethered agonists and antagonists. Using structure-based design, we produced heteromeric α3β4 and α4β2 nAChRs that can be activated or inhibited with deep-violet light, but respond normally to acetylcholine in the dark. The generation of these engineered receptors should facilitate investigation of the physiological and pathological functions of neuronal nAChRs and open a general pathway to photosensitizing pentameric ligand-gated ion channels.

Cholinergic neurotransmission affects decision-making, notably through the modulation of perceptual processing in the cortex. In addition, acetylcholine acts on value-based decisions through as yet unknown mechanisms. We found that nicotinic acetylcholine receptors (nAChRs) expressed in the ventral tegmental area (VTA) are involved in the translation of expected uncertainty into motivational value. We developed a multi-armed bandit task for mice with three locations, each associated with a different reward probability. We found that mice lacking the nAChR β2 subunit showed less uncertainty-seeking than their wild-type counterparts. Using model-based analysis, we found that reward uncertainty motivated wild-type mice, but not mice lacking the nAChR β2 subunit. Selective re-expression of the β2 subunit in the VTA was sufficient to restore spontaneous bursting activity in dopamine neurons and uncertainty-seeking. Our results reveal an unanticipated role for subcortical nAChRs in motivation induced by expected uncertainty and provide a parsimonious account for a wealth of behaviors related to nAChRs in the VTA expressing the β2 subunit.

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