Optogenetics and the Endocannabinoid System: The 2026 Nobel Prize

On October 5, 2026, the Nobel Prize in Physiology or Medicine was awarded to Karl Deisseroth, Peter Hegemann, and Georg Nagel “for their discoveries related to light-gated ion channels and optogenetics” (Nobel Assembly at Karolinska Institutet, 2026). Behind a seemingly technical phrase lies one of the most profound conceptual shifts in contemporary neuroscience: the possibility of moving beyond merely observing which brain regions become active during a given behavior to experimentally asking whether activating a specific population of neurons can produce that behavior, or if inhibiting it can make it disappear.

From the discovery of light-sensitive proteins to the birth of optogenetics

The story begins far from psychiatry, neurology, and even mammals. Peter Hegemann was studying how the single-celled alga Chlamydomonas reinhardtii detected light and moved in response to it. From this investigation emerged light-sensitive proteins capable of altering ion flow across the cell membrane. In 2003, Nagel and colleagues demonstrated that channelrhodopsin-2, ChR2, functioned as a directly light-gated cation channel and that its expression could make other cells electrically responsive to illumination (Nagel et al., 2003).

The next leap was extraordinary. What if it were possible to introduce this protein into neurons and then control their activity with pulses of light? In 2005, Boyden, Zhang, Bamberg, Nagel, and Deisseroth demonstrated exactly that. The expression of ChR2 in mammalian neurons allowed action potentials to be controlled with millisecond precision (Boyden et al., 2005). Genetics came to determine which cells would respond, while light determined when they would respond. The foundation of what we now call optogenetics was born.

The difference compared to previous techniques is fundamental. Observing that the amygdala shows greater activity during a fear response does not demonstrate that this activity is responsible for the fear. With optogenetics, it becomes possible to activate a defined neuronal population and check whether a specific behavior emerges. Afterwards, one can inhibit the same population and observe whether the behavior diminishes or disappears. This allows researchers to investigate whether a circuit is necessary, sufficient, or both for a given biological response. In other words, neuroscience gained an extraordinary tool to move beyond correlation and test causality.

How optogenetics helps unravel the endocannabinoid system

It is precisely at this point that the 2026 Nobel meets the endocannabinoid system.

Much of neuronal communication occurs in the classical direction: the presynaptic terminal releases a neurotransmitter, which crosses the synaptic cleft and acts on the postsynaptic cell. Endocannabinoid signaling introduces a particularly elegant logic into this system. When certain conditions increase postsynaptic cell activity, 2-arachidonoylglycerol, 2-AG, can be synthesized on demand, primarily through a pathway involving phospholipase C, diacylglycerol, and diacylglycerol lipase α, DAGLα. 2-AG then travels retrogradely across the synapse and activates CB1 receptors located predominantly on presynaptic terminals (Kano et al., 2009; Tanimura et al., 2010).

The result is a reduction in the probability of neurotransmitter release. But this sentence must be interpreted carefully. CB1 is not simply a “receptor that inhibits the brain.” It reduces the release of the neurotransmitter from the terminal on which it is located. If it is on a glutamatergic terminal, its activation can decrease glutamate release and, consequently, excitatory transmission. If it is on a GABAergic terminal, it can decrease GABA release and produce disinhibition of the postsynaptic neuron. Therefore, the functional outcome of CB1 activation depends on which cell is involved, which neurotransmitter is being released, which projection is being modulated, and which physiological state the circuit is in (Kano et al., 2009).

This retrograde signaling underlies classical phenomena such as depolarization-induced suppression of inhibition and depolarization-induced suppression of excitation. The demonstration that the loss of DAGLα profoundly impairs these forms of synaptic modulation confirmed the central role of 2-AG as a retrograde messenger of the endocannabinoid system (Tanimura et al., 2010).

This is where the combination with optogenetics becomes particularly powerful. It is no longer enough to ask whether CB1 is involved in stress, anxiety, reward, or feeding behavior. Today it is possible to ask: CB1 in which neuronal population? In which terminal? In a glutamatergic or GABAergic projection? Connecting which brain structures? At what moment during the stress response?

With optogenetic tools, researchers can select a specific neural projection, stimulate only those terminals, and combine this manipulation with electrophysiology, genetic alterations of CB1, manipulation of enzymes like DAGLα or MAGL, and, more recently, biosensors capable of tracking the dynamics of endocannabinoids themselves. This approach is making it possible to build increasingly precise maps of how the endocannabinoid system modulates affective behavior, anxiety, and stress adaptation at the cellular and circuit levels (Loomba & Patel, 2025).

This helps correct an oversimplification that is still very common when talking about cannabis and the brain. The endocannabinoid system is not merely a set of receptors that exist to respond to compounds derived from the plant Cannabis sativa. It is an extremely sophisticated endogenous modulatory system capable of tuning synaptic transmission in a spatial, temporal, and context-dependent manner. Optogenetics offers precisely the experimental resolution needed to begin understanding where, when, and how this modulation occurs.

Why is optogenetics not yet a psychiatric treatment?

Yet perhaps the most inevitable question following the Nobel is another: if we can turn brain circuits on and off with light, why is this not yet being used to treat depression, anxiety, post-traumatic stress disorder, or other psychiatric conditions?

The answer begins with anatomy.

The retina, for instance, offers an extraordinarily favorable circumstance for the clinical translation of optogenetics because it is neural tissue naturally accessible to light. And indeed, optogenetics has already moved beyond research in animal models. In 2021, Sahel and colleagues reported in Nature Medicine the partial recovery of visual function in an individual with advanced retinitis pigmentosa. An adeno-associated viral vector was used to induce the expression of the opsin ChrimsonR in retinal ganglion cells. Subsequently, specially developed goggles captured visual information from the environment and projected light patterns capable of stimulating these cells. With the treated eye and using the goggles, the individual was able to perceive, locate, count, and touch objects that they previously could not detect visually. Electroencephalographic recordings also demonstrated cortical activity associated with object perception (Sahel et al., 2021).

This does not mean, however, that optogenetics is currently a routinely available clinical treatment. It means there is already a clinical demonstration in humans that an optogenetic intervention can partially restore a lost function. It is an extraordinarily important proof of principle.

The deep brain, however, presents a very different challenge from the retina. To directly treat a brain circuit, one would first need to get specific cells to express a light-sensitive protein, which involves gene therapy strategies. Next, it would be necessary to ensure that expression occurs in the correct cells, rather than across an entire anatomical region. Then, light would need to be delivered to those cells, which, for many deep structures, still requires implantable devices or light delivery technologies currently under development. Added to this are questions regarding vector safety, immunogenicity, stability of gene expression, tissue heating, phototoxicity, device biocompatibility, and the effects of sustained neural modulation over years (Lüscher et al., 2025).

In psychiatry, there is yet another, perhaps more complex challenge: there is no single “depression neuron” or single “anxiety circuit.” Psychiatric diagnoses encompass biologically heterogeneous individuals, and different symptom dimensions can emerge from the interaction among multiple distributed circuits. Before installing an optical switch in the human brain, one would need to know which cell population must be modified, in what direction, for how long, and in which individuals. Paradoxically, the technical precision of optogenetics makes it even clearer how imprecise our current diagnostic categories can be from a neurobiological standpoint (Deisseroth et al., 2015; Lüscher et al., 2025).

The indirect impact of optogenetics on medicine

This does not mean, however, that psychiatry must wait until it is possible to implant optical fibers in the human brain to benefit from this revolution. Perhaps the most important clinical impact of optogenetics is precisely indirect.

By enabling the demonstration of causality between cells, circuits, and behaviors, optogenetics can identify which pathways truly matter for a clinical manifestation. This knowledge can guide the development of new drugs, reveal more specific receptors and molecular mechanisms, point to biomarkers of therapeutic response, and improve neuromodulation strategies such as transcranial magnetic stimulation, deep brain stimulation, focused ultrasound, and closed-loop stimulation systems. In addiction models, for example, principles obtained through optogenetic manipulation have already been used to refine deep brain stimulation protocols to reproduce, via electrically applicable means, synaptic changes initially identified with light (Creed et al., 2015). A broad perspective published in Nature Neuroscience in 2025 terms precisely this process the direct and indirect translation of optogenetics into human medicine (Lüscher et al., 2025).

What optogenetics could change in psychiatric pharmacotherapy

It is in this transition that a provocative discussion on psychiatric pharmacotherapy emerges.

Optogenetics will probably not “take off-label drugs off the market,” and such phrasing would be scientifically inaccurate. Off-label use means utilizing a medication outside the indications, populations, doses, or conditions formally approved by a regulatory agency. Some off-label uses are well supported scientifically and may be clinically necessary. The real issue lies elsewhere: the frequency with which psychiatry still relies on trial and error, pharmacological combinations, and prescriptions for symptoms or conditions where mechanistic support—and sometimes clinical evidence itself—is limited.

A recent review showed that off-label use of antipsychotics remains frequent, affecting 30% to 60% of adults using these medications in different settings, with particular concern regarding prescriptions for anxiety and sleep symptoms and exposure to long-term adverse effects (Verdoux, 2025). The issue does not lie in the off-label concept itself, but in the difference between an unapproved prescription backed by robust evidence and empirical pharmacotherapy used because we do not yet sufficiently understand the causal mechanism we are trying to modify.

It is precisely here that a circuit-based neuroscience can shift the logic of treatment.

Much of current pharmacotherapy begins with a syndromic diagnosis, moves to a systemic drug, and, in the event of an insufficient response, progresses to switching molecules, increasing dosage, or combinations. The drug circulates throughout the body and acts on receptors distributed across numerous tissues and different brain circuits. Optogenetics offers an inverse experimental logic: first identify which circuit causes or sustains a given component of behavior, then investigate which molecular mechanisms can modulate it, and finally develop clinically viable ways to intervene in that mechanism.

This does not mean swapping antidepressants for lasers. It means progressively moving away from a medicine based solely on syndromes toward a mechanism-based medicine. If this trajectory succeeds, the outcome could be less empirical polypharmacy, fewer weakly supported off-label prescriptions, and pharmacological or neuromodulatory interventions more targeted to the circuits actually involved in each phenotype.

Raphael Mechoulam and the discovery of the endocannabinoid system

And this discussion inevitably leads to Raphael Mechoulam.

In 1964, Yehiel Gaoni and Raphael Mechoulam published the structural identification and partial synthesis of Δ9-tetrahydrocannabinol, THC, establishing one of the fundamental milestones in phytocannabinoid chemistry (Gaoni & Mechoulam, 1964). Decades later, Mechoulam’s group participated in the identification of anandamide as an endogenous ligand for the cannabinoid receptor (Devane et al., 1992) and, subsequently, the identification of 2-AG as another important endogenous ligand for cannabinoid receptors (Mechoulam et al., 1995). These studies helped transform an inquiry originally centered on plant chemistry into an entire field dedicated to understanding an endogenous physiological signaling system present within the body itself (Mechoulam et al., 2014).

Mechoulam died on March 9, 2023, at the age of 92, after more than six decades of contributions to cannabinoid chemistry and pharmacology (Crippa et al., 2023). This makes it impossible for him to receive the Nobel now. Under the statutes of the Nobel Foundation, the prize cannot be awarded posthumously, unless death occurs after the decision to award the laureate has already been made (Nobel Foundation, 2026).

Why didn't Raphael Mechoulam receive the Nobel Prize?

A historical question therefore remains: how did Raphael Mechoulam pass away without receiving a Nobel Prize?

There is no public answer to this question. Nominations, evaluations, and deliberations of the Nobel committees remain confidential for at least 50 years. Thus, we do not officially know whether Mechoulam was nominated, how many times he was considered, or how close he came to receiving the prize. Nor is it possible to state, as fact, that the stigma associated with cannabis prevented his recognition. This hypothesis can be discussed in a historical context, but not proven with available data.

Even so, the 2026 Nobel makes the question especially interesting.

Optogenetics was awarded because it provided neuroscience with an extraordinarily precise way to uncover how cells and circuits cause behaviors. The endocannabinoid system, in turn, represents one of the mechanisms by which the brain itself dynamically regulates the strength of communication between these cells. Today, optogenetics, genetics, electrophysiology, and biosensors are being combined precisely to reveal, circuit by circuit, how endocannabinoid signaling participates in fear, stress, reward, feeding, pain, memory, and other phenomena central to neuroscience and psychiatry (Loomba & Patel, 2025).

Perhaps this is one of the most fascinating ironies in the recent history of science. In 2026, the Nobel recognized a technology that makes it possible to illuminate brain circuits with unprecedented precision. And by illuminating these circuits, we are gaining an ever deeper understanding of a synaptic modulation system whose history was profoundly transformed by the discoveries of Raphael Mechoulam and his collaborators.

Light won the Nobel. The man who helped uncover one of the chemical languages used by the brain to modulate its own synapses died without it.

And perhaps it is precisely now, when we can literally turn neuronal circuits on and off, that the magnitude of what Mechoulam helped discover becomes even more evident.

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