Adrenal Enkephalins: Opioid Peptides from Your Stress Glands
ACTH-Cortisol Stress Axis
10-20x increase
Denervation of the adrenal gland triggers a 10- to 20-fold increase in enkephalin-containing peptides, revealing how tightly the nervous system controls adrenal opioid production.
Yoburn et al., Life Sciences, 1987
Yoburn et al., Life Sciences, 1987
View as imageEvery time your body mounts a fight-or-flight response, your adrenal glands release more than adrenaline. Packed inside the same chromaffin granules as epinephrine and norepinephrine are small opioid peptides called enkephalins. These five-amino-acid molecules flood into the bloodstream alongside catecholamines during acute stress, providing a built-in pain suppression system at the exact moment you are most likely to be injured. The broader hormonal cascade that triggers this response begins with the ACTH-cortisol axis, a peptide signaling chain that connects the brain to the adrenal glands.
In 1975, John Hughes and Hans Kosterlitz identified two pentapeptides from pig brain with potent opiate agonist activity, naming them met-enkephalin (Tyr-Gly-Gly-Phe-Met) and leu-enkephalin (Tyr-Gly-Gly-Phe-Leu).[1] That discovery proved the body produces its own morphine-like compounds. Within five years, researchers found that the adrenal medulla contains some of the highest enkephalin concentrations outside the brain.
Key Takeaways
- Hughes and Kosterlitz identified met-enkephalin and leu-enkephalin in 1975 as the first endogenous opioid peptides, isolated from pig brain tissue
- Yang et al. (1980) found enkephalin-like peptides in adrenal glands across every species examined, with dogs and cattle showing the highest concentrations
- Livett et al. (1981) demonstrated that enkephalin and catecholamines are co-released from adrenal chromaffin cells by nicotine in a calcium-dependent manner
- Lewis et al. (1982) showed that adrenal demedullation reduces opioid stress analgesia in rats, while reserpine (which increases adrenal enkephalin levels) potentiates it
- Yoburn et al. (1987) found a 10- to 20-fold increase in enkephalin-containing peptides after adrenal gland denervation, blocked by hypophysectomy and partially restored by glucocorticoids
Discovery: The Body's Own Morphine
The story of enkephalins begins with a question that occupied pharmacologists for decades: if the brain has receptors for opium, the body must produce its own molecules that bind them. Hughes et al. (1975) answered that question by isolating two pentapeptides from pig brain extracts that competed with radiolabeled opiates for receptor binding sites.[1] The two peptides differ by a single amino acid at position 5: methionine in met-enkephalin and leucine in leu-enkephalin. Both share the N-terminal tyrosine residue that serves as the pharmacophore for all endogenous opioid peptides.
Both enkephalins derive from proenkephalin A (PENK), a 267-amino-acid precursor protein that contains four copies of met-enkephalin, one copy of leu-enkephalin, and two extended enkephalin sequences (met-enkephalin-Arg-Phe and met-enkephalin-Arg-Gly-Leu). Leu-enkephalin also appears within prodynorphin, linking the enkephalin and dynorphin opioid systems at the precursor level. Weihe et al. (1988) demonstrated that individual spinal cord neurons can co-express both proenkephalin- and prodynorphin-derived peptides, revealing overlap between these opioid families within single cells.[2]
Enkephalins preferentially activate delta-opioid receptors (DOR), though they retain affinity for mu-opioid receptors. This receptor preference distinguishes them from beta-endorphin (which prefers mu receptors) and dynorphin (which prefers kappa receptors). Delta-receptor activation produces analgesia with less respiratory depression and constipation than mu-receptor activation, making delta-selective compounds attractive therapeutic targets. The three endogenous opioid families represent parallel peptide systems with overlapping but distinct pharmacological profiles.
Why Adrenal Glands Store Opioid Peptides
Yang et al. (1980) characterized opioid peptides in adrenal glands across multiple species, finding enkephalin-like immunoreactive peptides in every species examined.[3] Dogs and cattle showed the highest adrenal enkephalin concentrations. The peptides exist in multiple molecular forms: low molecular weight forms (below 1,000 daltons) capable of binding opiate receptors directly, and high molecular weight forms (above 1,000 daltons) that contain enkephalin sequences within larger precursor polypeptides and require enzymatic processing to become active.
The adrenal medulla's chromaffin cells are derived from neural crest tissue, the same embryonic origin as sympathetic neurons. This shared lineage explains why chromaffin cells synthesize both catecholamines and neuropeptides. Enkephalins are packaged alongside epinephrine and norepinephrine within chromaffin granules, the secretory vesicles that fuse with the cell membrane during exocytosis. The ratio of enkephalin to catecholamine content is approximately fixed within each granule, meaning every secretory event releases both classes of molecules in proportion.
Two main chromaffin cell populations exist: epinephrine-producing cells (approximately 80% of the medulla) and norepinephrine-producing cells (approximately 20%). Both populations store enkephalins, but the specific enkephalin peptide profile may differ between cell types. This heterogeneity means the opioid peptide mixture released during stress varies depending on which chromaffin cell populations are preferentially activated by splanchnic nerve input.
Co-Release: Enkephalins and Adrenaline Leave Together
Kilpatrick et al. (1980) provided direct evidence that enkephalins and enkephalin-containing polypeptides are released from perfused bovine adrenal glands by nicotine and barium, agents that trigger catecholamine secretion.[4] All the enkephalin-containing polypeptides normally found in the adrenal medulla were released by both secretagogues in approximately the same proportions as they exist in chromaffin granules. This proportional release confirmed that enkephalins exit the cell via the same exocytotic pathway as catecholamines, not through a separate mechanism.
Livett et al. (1981) extended this finding by demonstrating co-release from primary cultures of isolated chromaffin cells, eliminating the possibility that enkephalins were coming from splanchnic nerve terminals rather than the chromaffin cells themselves.[5] Nicotine triggered calcium-dependent co-release of both enkephalin and catecholamines from cultured bovine adrenal medullary chromaffin cells. The calcium dependence confirmed that release follows classical exocytotic mechanisms: acetylcholine from the splanchnic nerve activates nicotinic receptors on chromaffin cells, calcium influx triggers granule fusion with the plasma membrane, and the granule contents (catecholamines plus enkephalins) spill into the bloodstream.
This co-release creates a biological paradox with functional logic. The catecholamines prepare the body for physical action: increased heart rate, blood pressure, bronchodilation, glucose mobilization. The enkephalins simultaneously suppress pain perception through opioid receptors. The combination allows an organism to fight or flee while injured, deferring pain awareness until the acute threat has passed. Soldiers wounded in battle, athletes injured during competition, and accident victims frequently report feeling no pain until well after the threatening situation has resolved.
Stress-Induced Analgesia: The Adrenal Contribution
Lewis et al. (1982) tested whether adrenal enkephalins are the source of opioid stress analgesia by surgically removing the adrenal medulla in rats.[6] They found that different patterns of foot shock activate distinct analgesic mechanisms: some stressors produce opioid analgesia (blocked by the opioid antagonist naltrexone) and others produce nonopioid analgesia (unaffected by naltrexone). Adrenal demedullation and denervation reduced opioid stress analgesia but did not affect nonopioid stress analgesia. Reserpine, a drug that depletes catecholamines from chromaffin granules but increases the concentration of enkephalin-like peptides, potentiated opioid stress analgesia.
These findings placed adrenal enkephalins at the center of opioid stress analgesia. The adrenal medulla is not the only source of endogenous opioids during stress (the brain and spinal cord also contribute), but it is a major peripheral source that can be selectively manipulated experimentally. The Lewis et al. study demonstrated that the analgesic response to stress is not a single phenomenon but rather the sum of at least two distinct mechanisms, with the opioid component dependent on adrenal medullary function.
The interaction between the opioid and catecholamine arms of the stress response connects directly to how CRH, ACTH, and cortisol form a peptide feedback loop. CRH from the hypothalamus triggers ACTH release from the pituitary, which drives cortisol production from the adrenal cortex. Meanwhile, the same sympathetic activation that drives CRH release also activates the splanchnic nerve, triggering enkephalin and catecholamine release from the adrenal medulla. The cortisol and enkephalin arms of the stress response operate in parallel, with cortisol providing sustained metabolic support and enkephalins providing immediate pain suppression.
Glucocorticoid Regulation of Adrenal Enkephalins
Yoburn et al. (1987) revealed a remarkable regulatory relationship between the nervous system, the pituitary-adrenal axis, and adrenal enkephalin production.[7] Under normal conditions, the rat adrenal medulla contains low quantities of enkephalin-containing peptides. But when the adrenal gland is denervated (the splanchnic nerve is severed), enkephalin-containing peptides increase 10- to 20-fold. This massive increase consists mostly of the precursor proenkephalin, suggesting that without neural input, chromaffin cells ramp up proenkephalin synthesis but may not process it as efficiently into active enkephalin peptides.
The denervation-induced rise in enkephalin peptides is blocked by hypophysectomy (removal of the pituitary gland) and partially restored by treatment with corticosterone, dexamethasone, or ACTH. This establishes that glucocorticoids exert a permissive effect on adrenal enkephalin biosynthesis. The pituitary-adrenal axis does not directly drive enkephalin production under normal conditions, but without glucocorticoid signaling, the denervation response cannot occur. This means adrenal enkephalin levels are co-regulated by two systems: neural input from the splanchnic nerve (which controls acute release) and hormonal input from the HPA axis (which sets the baseline capacity for synthesis).
The 10- to 20-fold magnitude of the denervation response also reveals that neural input normally keeps adrenal enkephalin synthesis suppressed. This is the opposite of what intuition might suggest. The splanchnic nerve triggers enkephalin release during stress, but it also restrains the rate of enkephalin production. When that restraint is removed, production surges.
Enkephalins Beyond the Adrenal Gland
While the adrenal medulla is the primary source of circulating enkephalins, proenkephalin is expressed throughout the nervous system and gut. Enkephalinergic neurons populate the spinal cord dorsal horn (pain transmission), brainstem (cardiovascular regulation), striatum (motor control and reward), and enteric nervous system (gut motility).
Hoyle and Burnstock (1990) demonstrated that enkephalins modulate inhibitory neuromuscular transmission in the circular muscle of the human colon through delta-opioid receptors.[8] This provides the mechanistic basis for the constipating effect of opioids and for enkephalins' role in normal gut motility. Szymaszkiewicz et al. (2019) reviewed the therapeutic potential of enkephalinase inhibitors for diarrhea-predominant irritable bowel syndrome, proposing that extending the local action of endogenous enkephalins in the gut could slow motility and reduce secretion without systemic opioid side effects.[9] The enkephalinase inhibitor racecadotril, which blocks neprilysin from degrading enkephalins in the gut wall, is approved in many countries for acute diarrhea. It represents the most successful clinical application of enkephalin biology to date.
The immune system also participates in enkephalin signaling. Immune cells express opioid receptors, and circulating enkephalins released from the adrenal medulla can modulate immune cell activity. Research has shown met-enkephalin enhances natural killer cell cytotoxicity through delta-opioid receptor activation. During acute stress, this immune modulation may serve a protective function: enhanced innate immunity at the moment of greatest vulnerability to injury and infection. Chronic stress, however, produces sustained opioid release and receptor desensitization, potentially contributing to the well-documented link between chronic stress and immune suppression. Neuropeptide Y, another peptide released during stress, modulates a parallel immune regulatory circuit.
What Remains Unresolved
Quantifying the contribution of adrenal enkephalins to circulating opioid levels in conscious humans during stress remains technically difficult. Enkephalins have plasma half-lives of only 1-2 minutes due to rapid degradation by aminopeptidases and enkephalinases (particularly neprilysin). By the time blood is drawn and processed, much of the released enkephalin has been broken down.
Delta-opioid-selective analgesics derived from enkephalin pharmacology have been explored for decades without a breakthrough drug. Delta agonists produce analgesia with less addiction potential than mu agonists, but they have a narrower therapeutic window and can provoke seizures at high doses. Whether chronic dysregulation of the enkephalin system contributes to conditions like chronic pain syndromes, addiction vulnerability, or stress-related disorders is under active investigation but lacks definitive clinical evidence. Proenkephalin levels in cerebrospinal fluid and plasma have been proposed as biomarkers for pain processing and infection severity (elevated proenkephalin correlates with poor sepsis outcomes), but standardized clinical assays are not yet established.
The interaction between adrenal enkephalins and food-derived opioid peptides that activate the same receptors also warrants investigation. Whether dietary opioid peptides influence the same delta-receptor pathways that endogenous enkephalins use, and whether this has functional consequences for pain modulation or gut motility, remains an open question. Similarly, the role of adrenal enkephalins in substance P-mediated pain circuits, where opioid and tachykinin systems converge to modulate the emotional dimension of pain, is an active research area.
The Bottom Line
Adrenal enkephalins are five-amino-acid opioid peptides co-stored and co-released with catecholamines from chromaffin cells during the stress response. Discovered in 1975 and found in adrenal glands across all species examined, they provide stress-induced analgesia through delta-opioid receptors, with their synthesis regulated by both splanchnic nerve input and glucocorticoids from the HPA axis. The enkephalinase inhibitor racecadotril is the most successful clinical application of enkephalin biology, while delta-selective analgesics remain unrealized despite decades of research.