Enkephalin: What It Is and Its Role in the Body

Enkephalins are small signaling molecules your body makes naturally to dull pain, shape mood, and fine-tune a surprising range of organ functions. Discovered in 1975, they were the first endogenous opioids ever identified, meaning they are the body’s own version of morphine-like chemicals. They come in two main forms, met-enkephalin and leu-enkephalin, and despite being only five amino acids long, their influence stretches from the spinal cord to the gut wall to the heart.

What Enkephalins Actually Are

Enkephalins belong to a broader family of molecules called endogenous opioid peptides, a group that also includes endorphins and dynorphins. What sets enkephalins apart is their tiny size and their slightly different receptor preferences. Each enkephalin is a chain of just five amino acids that begins with the same four-amino-acid sequence (tyrosine-glycine-glycine-phenylalanine). The two varieties differ only in their fifth amino acid: met-enkephalin ends with methionine, and leu-enkephalin ends with leucine. That single-residue difference is enough to give them subtly different binding profiles and biological half-lives, but both do roughly the same jobs.

Your body does not make enkephalins directly. Instead, cells produce a larger precursor protein called proenkephalin, which is then chopped up by specialized enzymes. The proenkephalin gene encodes four copies of met-enkephalin, two extended met-enkephalin sequences, and one copy of leu-enkephalin, each separated by pairs of basic amino acids that act as cut sites for processing enzymes.¹ The enzyme most responsible for snipping proenkephalin into its active opioid pieces is prohormone convertase 2 (PC2), which generates the small, free enkephalin peptides far more efficiently than the related enzyme PC1.² This processing happens inside neurons and certain other cell types, and the finished enkephalins are stored in vesicles until they are needed.

How Enkephalins Talk to Cells

Once released, enkephalins bind to opioid receptors on neighboring cells. They interact mainly with two receptor types: the delta opioid receptor and the mu opioid receptor, with a slightly higher affinity for the delta receptor.³ Mu receptors are the same ones that morphine, fentanyl, and other prescription opioids target, which is why the body’s own enkephalins produce many of the same effects as those drugs, just weaker and more localized. The delta receptor preference matters because delta signaling tends to produce analgesia and mood effects with fewer of the respiratory-depression and constipation problems that plague mu-heavy drugs.

Enkephalins are also extremely short-lived. Two enzymes, neprilysin and aminopeptidase N, break them down within seconds of release.⁴ That rapid degradation is actually a feature: it keeps enkephalin signaling tightly controlled and prevents the kind of prolonged receptor activation that leads to tolerance and dependence with synthetic opioids. When researchers want to study what happens when enkephalins stick around longer, they use drugs that block both of those degradation enzymes simultaneously. More on that approach later.

Pain Control From the Brain to the Spinal Cord

The role most people associate with enkephalins is pain relief, and the evidence for this is extensive. Your nervous system has a built-in pain-dampening circuit that runs from a region deep in the brainstem called the periaqueductal gray (PAG) down through the rostral ventromedial medulla (RVM) and into the spinal cord. Enkephalins are one of the key chemical messengers in this circuit.⁵ When you stub your toe or touch a hot pan, pain signals race up your spinal cord toward the brain. At the same time, neurons in the PAG and RVM send signals back down to dampen those incoming pain messages, and enkephalin is one of the chemicals they use to do it.

The spinal cord itself also has enkephalin-releasing neurons that act as local gatekeepers. These spinal enkephalin neurons receive direct input from pain-sensing nerve fibers and, when activated by a painful stimulus, release enkephalin to inhibit the onward transmission of that signal to the brain.⁶ They do this by acting on delta opioid receptors located both before and after the synapse, effectively turning down the volume on pain transmission at two points simultaneously. A separate study demonstrated that brainstem neurons projecting into the spinal cord use both enkephalin and the inhibitory neurotransmitter GABA in a coordinated way to gate mechanical pain signals presynaptically.⁷

One detail worth noting: enkephalin-containing nerve terminals in the spinal cord are distinct from the terminals of the thin, unmyelinated C-fibers that carry slow, burning pain. They also do not overlap with dynorphin-containing terminals. Researchers confirmed this by showing that enkephalin puncta in the spinal dorsal horn do not colocalize with markers for C-fiber terminals or with dynorphin, meaning the two opioid systems occupy separate anatomical compartments and likely serve different functions.⁸

Pleasure, Motivation, and Emotional Resilience

Enkephalins do more than silence pain. They play an active role in how you experience reward and pleasure, particularly within the nucleus accumbens, a brain region central to motivation and hedonic experience. Research on opioid “hedonic hotspots” within the nucleus accumbens has shown that stimulating delta opioid receptors (the ones enkephalins prefer) in a specific rostrodorsal zone of the medial shell can at least double the pleasurable reactions to a sweet taste, an effect comparable in strength to stimulating mu or kappa opioid receptors in the same spot.⁹ Enkephalins are one of the natural ligands that activate those delta receptors in everyday life, contributing to the small bursts of pleasure you get from food, social bonding, and other rewarding experiences.

The flip side is equally telling. In animal models of chronic stress, rats that developed anhedonia (a loss of interest in things they previously found pleasurable, like sugar water) showed a clear downregulation of enkephalin gene expression in the nucleus accumbens. The lower the enkephalin levels in the accumbens shell, the less interest the animals showed in sucrose, and this relationship was statistically tight.¹⁰ In other words, when chronic stress depletes your enkephalin supply in reward circuits, the capacity for everyday pleasure can shrink.

Enkephalins are also linked to anxiety and stress resilience through the amygdala. Experimentally knocking down enkephalin expression specifically in the posterior basolateral amygdala was enough to produce anxiety-like behavior in animals, mimicking the vulnerable phenotype seen after chronic unpredictable stress. Animals that maintained higher enkephalin levels in this region were more resilient.¹¹ The emerging picture is that enkephalins act as a kind of neurochemical buffer against the emotional fallout of prolonged adversity.

Beyond the Brain

Enkephalins are not confined to your central nervous system. They show up in the gut, the heart, the adrenal glands, and immune cells, each location suggesting a distinct functional role.

In the gastrointestinal tract, enkephalins are produced by neurons within the enteric nervous system, the mesh of nerve cells embedded in the gut wall. Acting through delta opioid receptors concentrated in the myenteric plexus, enkephalins slow intestinal motility and reduce secretion. During intestinal inflammation, enkephalin expression in gut neurons increases, which researchers interpret as a compensatory attempt to dial back the excessive contractions and diarrhea that inflammation triggers.¹² This same mechanism is why opioid drugs cause constipation: they activate the same receptors that enkephalins use to slow the gut, but much more powerfully and for much longer.

In the heart, an extended form of met-enkephalin called MEAP is abundant in the myocardium, particularly in the sinoatrial node, the heart’s natural pacemaker. When delivered to the sinoatrial node, MEAP inhibits vagal control of heart rate, contractile force, and coronary blood flow, likely by acting on opioid receptors located on vagal nerve terminals.¹³ When arterial blood supply to the sinoatrial node is temporarily reduced, the local concentration of endogenous opioids rises, a response that may help the heart adapt to ischemic stress.¹⁴

The adrenal glands represent another important source. Chromaffin cells in the adrenal medulla, the same cells that pump out adrenaline during a fight-or-flight response, also release met-enkephalin. When adrenal chromaffin-like cells are exposed to low oxygen conditions, they rapidly release both epinephrine and met-enkephalin, with enkephalin levels roughly doubling within fifteen minutes.¹⁵ This co-release during acute stress may help modulate the body’s alarm response, adding a pain-dampening or calming signal on top of the adrenaline surge.

Immune Modulation and a Surprising Cancer Connection

One of the less intuitive roles of enkephalins involves the immune system. Met-enkephalin has been shown to suppress the proliferation of T lymphocytes through a pathway involving what researchers call the opioid growth factor receptor (OGFr). In laboratory experiments, met-enkephalin reduced T cell numbers in a dose-dependent manner, and this immunosuppressive capability helps explain why the enkephalin system has been linked to the regulation of autoimmune diseases.¹⁶

At first glance, suppressing immune cells sounds like it would help tumors grow. But the relationship is more nuanced. Met-enkephalin has also been found to inhibit regulatory T cells (Tregs), a subset of immune cells that tumors co-opt to shield themselves from immune attack. By reducing Treg activity, met-enkephalin can actually slow tumor development in mouse models.¹⁷ The distinction matters: enkephalin does not globally shut down immunity so much as it reshapes the immune landscape, suppressing certain cell populations while freeing others to function. Whether this has practical therapeutic value for cancer patients is still being investigated, but the finding challenges the assumption that all immune suppression favors tumors.

Enkephalins and Disease

Disruptions in enkephalin signaling have been implicated in several neurological conditions. Huntington’s disease provides a particularly clear example. The brain’s striatum contains two main populations of medium spiny neurons, one expressing primarily enkephalin (the D2 receptor-bearing, “indirect pathway” neurons) and the other expressing primarily substance P and dynorphin (the D1 receptor-bearing, “direct pathway” neurons). In Huntington’s disease, the enkephalin-expressing D2 neurons are affected earlier and more severely than the D1 neurons, and the resulting loss of inhibitory output to a brain region called the globus pallidus leads to the involuntary, dance-like movements (chorea) that characterize the disease’s early stages.¹⁸ The selective vulnerability of enkephalin-rich neurons in the striatum has been a major clue in understanding why Huntington’s produces the specific symptom progression it does.

Chronic pain conditions also involve changes in the enkephalin system, though the direction of those changes can vary. In some chronic pain states, prolonged nociceptive input can exhaust local enkephalin stores or downregulate receptor expression, reducing the body’s ability to modulate its own pain. This is one reason why chronic pain tends to self-perpetuate: the natural braking system weakens under sustained demand.

Dual Enkephalinase Inhibitors

Perhaps the most exciting pharmacological development related to enkephalins is the creation of a class of drugs called dual enkephalinase inhibitors, or DENKIs. The logic is straightforward: rather than flooding opioid receptors with a foreign molecule like morphine, why not just prevent the body’s own enkephalins from being broken down so quickly? DENKIs block both neprilysin and aminopeptidase N, the two enzymes responsible for degrading enkephalins, allowing your natural supply to linger longer at the synapse and produce stronger effects.⁴

The appeal of this approach goes beyond pain relief. Because DENKIs amplify endogenous enkephalin signaling rather than imposing a blanket activation of all opioid receptors everywhere, they produce analgesia with fewer adverse effects than traditional opioids. Early evidence also suggests they could help manage acute opioid withdrawal and serve as alternatives to opioid substitution therapy, potentially offering an exit ramp for people with opioid use disorder.¹⁹ The rationale is that boosting a person’s own enkephalins might ease withdrawal symptoms without the abuse potential that comes with introducing another exogenous opioid. Clinical development is still in relatively early stages, but the concept represents a genuinely different strategy from anything currently available for chronic pain or addiction.

An Ancient Molecular System

Enkephalins are not unique to humans, or even to mammals. The proenkephalin gene is present across the entire radiation of jawed vertebrates (gnathostomes), from fish to amphibians to reptiles to birds to mammals. Across all of these groups, the gene maintains a remarkably consistent organizational plan, encoding seven opioid-active sequences: five pentapeptides, one met-enkephalin-7, and one met-enkephalin-8.²⁰ That kind of conservation over hundreds of millions of years of evolution is a strong indicator that the enkephalin system is doing something essential. A gene that is merely useful tends to drift and diversify. A gene that stays nearly identical from sharks to sparrows to humans is under serious selective pressure to keep working exactly the way it does.²¹

This evolutionary depth also helps explain why the opioid system is so deeply embedded in basic survival functions like pain avoidance, feeding motivation, and stress response. These are not luxuries that appeared late in mammalian evolution. They are foundational tools that vertebrates have relied on since long before anything resembling a human brain existed.

Enkephalins Versus Endorphins Versus Dynorphins

People often use “endorphins” as a catch-all for the body’s feel-good chemicals, but the three main endogenous opioid families are distinct in their origins, receptor preferences, and functions. Enkephalins come from the proenkephalin gene, endorphins (specifically beta-endorphin) come from the proopiomelanocortin (POMC) gene, and dynorphins come from the prodynorphin gene. Each precursor is expressed in different brain regions and processed differently.

Beta-endorphin is a much larger peptide (31 amino acids) and binds preferentially to mu opioid receptors. It is released in large bursts during intense exercise and acute stress, which is why it gets credit for “runner’s high.” Dynorphins bind primarily to kappa opioid receptors and tend to produce dysphoria rather than pleasure; they are more associated with stress and negative emotional states. Enkephalins sit in the middle of this spectrum: preferring delta receptors, modulating everyday pain and small pleasures, and operating at a more local, fine-grained level than the long-range bursts of beta-endorphin.

The anatomical separation between enkephalin and dynorphin terminals in the spinal cord, confirmed by the colocalization studies described earlier, underscores that these are not interchangeable systems doing the same thing. They occupy different neural real estate, activate different receptor populations, and produce different downstream effects, even though they all belong to the opioid family.

Why Enkephalins Do Not Get You High

Given that enkephalins activate the same receptor family as heroin and morphine, a reasonable question is why your own enkephalins do not produce an opioid-like high under normal conditions. The answer lies in concentration, duration, and spatial precision. Enkephalins are released in tiny quantities at specific synapses and degraded within seconds by neprilysin and aminopeptidase N. The resulting receptor activation is brief, local, and modest compared to the overwhelming, sustained flood that an injected opioid produces across the entire brain.

Synthetic opioids bypass all of the body’s built-in checks. They arrive in concentrations far exceeding anything endogenous, they resist the degradation enzymes that would normally clear enkephalins, and they activate receptors body-wide rather than at a handful of targeted synapses. The DENKI approach mentioned earlier is interesting precisely because it tries to respect the body’s spatial and temporal controls. By simply slowing degradation rather than adding an external molecule, DENKIs raise enkephalin levels only where and when the body is already releasing them. The result is a gentler amplification of natural signaling rather than a pharmacological override. Whether that gentleness will be enough to provide meaningful clinical analgesia in severe pain remains an open question, but the principle is sound and the safety profile so far looks promising.