RPEP-00040 · 1987Using a "pan-opioid" antibody that detects all opioid peptides, researchers screened 108 tumors. Every adrenal pheochromocytoma (15 of 15), thyroid medullary carcinoma (6 of 6), and pituitary adenoma (5 of 5) stained positive for opioid peptides.
Most parathyroid adenomas (8 of 9), pancreatic islet-cell tumors (7 of 10), and carcinoid tumors (18 of 26) also stained positive.
Zero non-neuroendocrine tumors showed opioid staining. Lung small-cell carcinomas, skin Merkel-cell tumors, and neuroblastomas were all negative.
The same opioid peptides were found in normal versions of these tissues (adrenal medulla, pancreatic islets, pituitary), suggesting the tumors simply overproduce a normal cellular product.
Bostwick, D G; Null, W E; Holmes, D; Weber, E; Barchas, J D; Bensch, K G · Cross Sectional
RPEP-00045 · 1987A single injection of kainic acid (1 microgram) into rat brains caused seizures lasting 3 to 6 hours. During seizures, hippocampal met-enkephalin dropped 31% and dynorphin A dropped 63%, suggesting these opioid peptides were being released.
By 24 hours, levels returned to normal. By 48 hours, met-enkephalin surged to 270% of normal and dynorphin to 150%. The brain had ramped up production to replace what was used.
The biosynthetic machinery confirmed this: mRNA for preproenkephalin (the genetic template for making enkephalin) jumped to 400% of control at 6 hours. The actual precursor protein followed at 24 hours, reaching 300% of control.
The seizure-induced shaking behavior (wet-dog shakes) was directly linked to enkephalin. Naloxone (an opioid blocker) reduced the shaking. Anti-enkephalin antibodies also reduced it. Injecting enkephalin peptides into the hippocampus mimicked the shaking.
Hong, J S; Grimes, L; Kanamatsu, T; McGinty, J F · Animal Study
RPEP-00046 · 1987All four tested opioid peptides (beta-endorphin, dynorphin, MEAP, and DADLE) increased plasma corticosterone (a stress hormone) in normal rats. The effects were dose-dependent and blocked by naloxone, confirming they work through opioid receptors.
Cross-tolerance experiments revealed which receptor each peptide uses. Rats made tolerant to one opioid were tested with others. Dynorphin(1-13) and MEAP (Met-Enk-Arg-Phe) acted at kappa-opioid receptors. The synthetic peptide DADLE acted at delta-opioid receptors.
Beta-endorphin was the surprise. It did not fit neatly into the mu, delta, or kappa categories. Its corticosterone-releasing effect was maintained even in rats tolerant to all three receptor types. The researchers proposed it acts at an epsilon-opioid receptor, a debated fourth receptor type.
Iyengar, S; Kim, H S; Wood, P L · Animal Study
RPEP-00049 · 1987Beta-endorphin (0.5 microgram injected into the brain), dynorphin (1 microgram), and U50-488H (a kappa opioid drug, 10 mg/kg) all stimulated prolactin (PRL) release more in the afternoon (4-5 PM) than in the morning (8-9 AM).
Morphine (10 mg/kg subcutaneous), met-enkephalin (200 micrograms), and D-Met-Pro-Enk (0.5 microgram) did not show this daily rhythm. Their prolactin response was the same regardless of time.
For corticosterone (a stress hormone), the pattern was different. Morphine, D-Met-Pro-Enk, met-enkephalin, and dynorphin could only increase corticosterone in the morning, when baseline levels were low. In the afternoon, when baseline corticosterone was already high, there was no room for further increase.
Removing the adrenal glands eliminated the circadian rhythm in prolactin responses, suggesting that natural cortisol cycling drives the time-of-day differences.
Kiem, D T; Kanyicska, B; Stark, E; Fekete, M I · Animal Study
RPEP-00050 · 1987Rats drinking 2% salt solution showed progressive increases in three neuropeptide mRNAs in the hypothalamus (a brain region controlling hormones):
Vasopressin, oxytocin, and dynorphin mRNAs all increased in the magnocellular neurons of the supraoptic and paraventricular nuclei. These are the brain cells that make these peptide hormones.
Enkephalin mRNA was not detectable in these brain areas under normal conditions. It only appeared after 12 days of salt loading or after the acute stress of a salt injection into the abdomen.
Lactating mother rats (10 days postpartum) showed a very large increase in oxytocin mRNA, with smaller increases in vasopressin and dynorphin. No enkephalin or CRF (corticotrophin-releasing factor) changes were seen in lactating rats.
Lightman, S L; Young, W S · Animal Study
RPEP-00051 · 1987Severe brain injury (3.0 to 4.0 atmospheres of pressure) caused dynorphin to increase significantly in five brain regions: striatum, frontal cortex, parietal cortex, pons, and medulla. These were the same regions with the worst tissue damage.
Beta-endorphin decreased in the hypothalamus after severe injury but increased in the anterior pituitary after both mild and severe trauma. Enkephalin levels did not change at any injury level.
The dynorphin increase in the medulla was significantly correlated with falling mean arterial blood pressure after severe injury. This suggests dynorphin release may contribute to the cardiovascular collapse that worsens outcomes after head trauma.
McIntosh, T K; Head, V A; Faden, A I · Animal Study
RPEP-00052 · 1987Dynorphin A accumulated in injured brain regions after fluid-percussion brain injury in cats. These same regions showed significant decreases in cerebral blood flow. Enkephalin did not accumulate.
The opioid antagonist Win-(-), given 15 minutes after injury, significantly improved multiple outcomes: mean arterial blood pressure increased, EEG amplitude (brain wave activity) improved, regional cerebral blood flow recovered, and both the severity and incidence of brain hemorrhage decreased. Survival after injury was significantly better.
The control was elegant: Win-(+), the mirror-image version of the same drug that cannot bind opioid receptors, had no effect. Neither did saline. This confirmed the benefits came specifically from blocking opioid receptors.
McIntosh, T K; Hayes, R L; DeWitt, D S; Agura, V; Faden, A I · Animal Study
RPEP-00053 · 1987Electrical stimulation of the periaqueductal grey (PAG, a brain region that controls pain) initially produced strong pain relief in rats. With repeated stimulation, this relief gradually disappeared.
The tolerance was not from opioid depletion. Brain levels of beta-endorphin, met-enkephalin, and dynorphin were unchanged in tolerant rats compared to controls.
It was not a learned (conditioned) response either. Tolerant rats exposed to all the cues associated with stimulation showed no compensatory pain increase. And repeated exposure to the cues without stimulation did not restore pain relief.
The tolerance behaved exactly like drug tolerance: the dose-response curve shifted right (needed more stimulation for the same effect), it was reversed by naloxone, and it recovered spontaneously over time. Tolerant rats also showed cross-tolerance to morphine, confirming the same opioid receptor system was involved.
Rats showed no signs of stress during the protocol: normal body weight, food intake, temperature, adrenal weight, and hormone levels.
Millan, M J; Członkowski, A; Herz, A · Animal Study
RPEP-00054 · 1987Human cerebrospinal fluid (CSF) from pain-free surgery patients contained Peak B at a concentration equivalent to about 1.4 picomoles of morphine per milliliter.
Injected into mouse brains, Peak B produced dose-dependent pain relief at 0.06 and 0.12 picomoles of morphine equivalents. Naloxone (an opioid blocker) reversed this effect, confirming it works through opioid receptors.
In a muscle tissue assay (mouse vas deferens), Peak B's activity was blocked by low naloxone concentrations but not high ones. This unusual pattern suggests it interacts with opioid receptors differently than known opioids.
Most surprisingly, Peak B was not destroyed by trypsin or alpha-chymotrypsin, protein-digesting enzymes that break down all known opioid peptides. This means Peak B is either not a peptide or has an unusual structure that protects it from enzymes.
Miller, B E; Lipman, J J; Byrne, W L · In Vitro
RPEP-00059 · 1987Peripheral mu-opioid receptors in the gut can independently slow gastrointestinal transit. The peptide agonist PL017 slowed gut transit at 0.37 mg/kg but did not produce pain relief until 30 mg/kg, about 80 times higher. This proves gut mu receptors work independently of brain receptors.
Delta and kappa opioid receptors in the gut also slowed transit, but less powerfully than mu receptors.
The key finding: a peptide opioid antagonist that cannot cross the blood-brain barrier blocked morphine's gut-slowing effect without reducing its painkilling activity. This pharmacological separation means constipation and pain relief work through different receptor populations (gut vs brain).
Shook, J E; Pelton, J T; Hruby, V J; Burks, T F · Animal Study
RPEP-00061 · 1987The new method combined HPLC (high-performance liquid chromatography) with radioimmunoassay (RIA) to measure opioid peptides in human cerebrospinal fluid, plasma, and tissues.
The key advantage: HPLC first separates beta-endorphin from its precursor beta-lipotropin and from other opioid peptides. Then RIA measures each fraction individually. This eliminates cross-reactivity, where antibodies confuse one peptide for another.
The method uses volatile solvents that evaporate cleanly without interfering with the antibody tests. This was the first combined method that could accurately measure endorphin, enkephalin, and dynorphin families in the same sample.
Preliminary results from chronic pain patients' CSF were presented, demonstrating the method's practical utility.
Venn, R F · In Vitro
RPEP-00064 · 1988After middle cerebral artery occlusion (a stroke model) in rats, the opioid antagonist WIN at doses of 0.4 to 400 micrograms/kg produced several benefits.
All WIN doses significantly increased mean arterial blood pressure compared to saline controls. At the optimal dose of 40 micrograms/kg, rats showed significantly greater EEG (brain wave) recovery and higher neurological scores at 24 hours compared to controls.
Neurological outcome correlated with brain wave recovery on the injured side, confirming the measures tracked together.
However, 24-hour mortality and infarct (dead tissue) size were not significantly different from controls. The drug improved functional recovery without reducing the actual area of brain death.
At 1 hour after stroke, there were no significant changes in dynorphin, enkephalin, or endorphin levels in the injured vs. uninjured brain hemisphere. This was unexpected given the changes seen in trauma models.
Andrews, B T; McIntosh, T K; Gonzales, M F; Weinstein, P R; Faden, A I · Animal Study
RPEP-00065 · 1988Beta-endorphin was 49% higher in the pituitary of hypertensive rats (SHR) but dramatically lower in peripheral organs: 92% lower in heart, 48% lower in adrenals, and 57% lower in kidneys compared to normal rats (WKY). In the brain, beta-endorphin was 71% lower in the striatum of SHR rats.
Dynorphin was 38% lower in the pituitary, 55% lower in the striatum, and 46% lower in the heart of SHR rats, but 33% higher in the hypothalamus.
Met-enkephalin showed the opposite pattern in some areas: 268% higher in adrenals, 40% higher in cortex, and 33% higher in pons/medulla of SHR rats, but 40% lower in the pituitary.
The changes were not uniform. Each opioid peptide had its own distinct pattern of increases and decreases across brain regions and organs.
Bhargava, H N; Matwyshyn, G A; Hanissian, S; Tejwani, G A · Animal Study
RPEP-00066 · 1988Enterochromaffin (EC) cells are the most abundant hormone-producing cells in the gut. They were known to make serotonin. Whether they also make peptides was debated for years.
Using antibodies against all three families of opioid peptide precursors, researchers tested gut tissue from dogs, guinea pigs, and humans. EC cells contained pro-dynorphin-derived peptides: dynorphin A and alpha-neo-endorphin.
They did not contain pro-opiomelanocortin derivatives (like endorphin) or pro-enkephalin derivatives. Previous reports of enkephalin in EC cells were likely due to antibody cross-reactivity.
The number and staining characteristics of opioid-positive EC cells varied considerably between species and between different segments of the gut, suggesting species-specific processing of the dynorphin precursor.
Cetin, Y · In Vitro
RPEP-00067 · 1988The myenteric plexus (the nerve network that controls gut movement) of guinea pig small intestine contained a well-defined mix of opioid peptides.
Met-enkephalin dominated at 405 pmol/g of tissue. Three other enkephalin-related peptides were present at 90 to 100 pmol/g each. Two rarer enkephalin forms (BAM-18 and Met-enkephalyl-Arg-Arg-Val-NH2) were present at 24 and 5 pmol/g.
Dynorphin-family peptides (alpha-neoendorphin, beta-neoendorphin, dynorphin A(1-8), and dynorphin B) were all present at 12 to 15 pmol/g. Full-length dynorphin A was very scarce at only 0.8 pmol/g.
No beta-endorphin was detected.
The two rare enkephalin forms (BAM-18 and MERV-NH2) had an unusual receptor profile: they preferred mu and kappa opioid receptors rather than the delta receptors that most enkephalins favor. This means they may have different functional roles in the gut.
Corbett, A D; McKnight, A T; Kosterlitz, H W · In Vitro
RPEP-00068 · 1988Morphine injected into the brain (intracerebroventricular) was 10 times more potent than intravenous morphine at increasing plasma ANP (atrial natriuretic peptide), a hormone released by the heart that lowers blood pressure and promotes sodium excretion.
Leu-enkephalin decreased plasma ANP concentrations, the opposite effect of morphine.
Dynorphin and beta-endorphin (given either into the brain or into the vein) did not change ANP levels at all.
All four opioids increased plasma norepinephrine and epinephrine (stress hormones), but morphine caused increases 10 to 50 times greater than the others.
Ganglionic blockade (cutting the nerve connection from the brain to the heart) significantly reduced morphine's ability to increase ANP, confirming the effect requires an intact autonomic nervous system.
Crum, R L; Brown, M R · Animal Study
RPEP-00070 · 1988The three families of opioid peptides (dynorphins, endorphins, and enkephalins) all have cardiovascular effects. Dynorphins prefer kappa receptors, enkephalins prefer delta and mu receptors, and beta-endorphin prefers mu and delta receptors.
The review focused on mu-opioid receptors in the hypothalamus, a brain region that controls many body functions including blood pressure. While the opioid system's role in normal blood pressure regulation was not well understood, cardiovascular stress clearly activates the opioid system.
The review covered both normal cardiovascular regulation and pathological states like hypertension, hemorrhagic shock, and heart failure, where opioid system changes had been documented.
Feuerstein, G; Sirén, A L · Review
RPEP-00071 · 1988Morphine and other mu-opioid agonists decreased gastric acid secretion. They were more potent when injected into the brain (i.c.v.) than into the blood (i.v.), indicating a central mechanism.
The quaternary opioid antagonist naltrexone methylbromide (which cannot cross the blood-brain barrier) blocked brain-injected morphine's effect and partially blocked IV morphine's effect. This confirmed morphine reduces acid through brain receptors, even when given intravenously.
The kappa-selective agonist U-50,488H had the opposite effect: it increased gastric acid when given IV but not when given into the brain. This increase was blocked by naloxone, the muscarinic blocker atropine, and the M1-selective blocker pirenzepine.
The delta-selective agonist DPDPE did not affect acid secretion by either route.
Fox, D A; Burks, T F · Animal Study
RPEP-00072 · 1988Human placental aminopeptidase M completely degraded Met-enkephalin and Leu-enkephalin into their five constituent amino acids. The degradation rate was measured by tracking tyrosine release.
The degradation speed ranked: Met-enkephalin (fastest) > Leu-enkephalin > beta-neoendorphin > dynorphin > beta-endorphin (slowest).
Smaller, simpler peptides were degraded faster than larger, more complex ones. Met-enkephalin (5 amino acids) was destroyed faster than beta-endorphin (31 amino acids).
Furuhashi, M; Mizutani, S; Kurauchi, O; Kasugai, M; Narita, O; Tomoda, Y · In Vitro
RPEP-00073 · 1988In rabbit ear and saphenous (leg) arteries, both delta-selective opioid agonists (leu-enkephalin and met-enkephalin) and kappa-selective agonists (dynorphin-(1-13) and ethylketocyclazocine) inhibited nerve-stimulation-evoked contractions.
Delta agonists showed a vessel-specific difference: their maximum inhibitory effect was significantly less in the saphenous artery than in the ear artery. Kappa agonists worked similarly in both vessels.
Yohimbine (an alpha-2 adrenergic blocker) did not enhance opioid inhibitory effects at any stimulation frequency in either vessel. This disproved the hypothesis that opioid receptors work by boosting alpha-2 receptor inhibition of noradrenaline release.
Gan, E A; Duckles, S P · In Vitro
RPEP-00074 · 1988The mu-selective opioid agonist DAGO (Tyr-D-Ala-Gly-(Me)Phe-Gly-ol) increased food intake when injected into the central nucleus of the amygdala at doses of 1 and 3 nanomoles.
Neither DSLET (a delta-selective agonist) nor dynorphin A (a kappa-selective agonist) affected food intake at the same location, even at the highest dose of 3 nanomoles.
Dynorphin A did increase food intake when injected into the medial hypothalamus at 2 nanomoles. This shows that different brain regions use different opioid receptor types to control eating.
Bilateral injections of DAGO were no more effective than unilateral injections. Naloxone and the long-acting antagonist beta-chlornaltrexamine both blocked DAGO's feeding effect, confirming opioid receptor involvement.
Gosnell, B A · Animal Study
RPEP-00075 · 1988Six opioid-related substances were tested on isolated rat adipocytes (fat cells):
- Beta-endorphin, dynorphin, met-enkephalin, leu-enkephalin, and morphine (agonists)
- Naloxone (antagonist)
None altered insulin binding to fat cells. None changed 2-deoxy-glucose uptake (a measure of sugar absorption). None affected glucose incorporation into lipids (fat production). None showed lipolytic (fat-breaking) activity.
The conclusion: endogenous opioids do not directly affect fat cell metabolism. The glucose problems seen with opioid drugs must happen through other mechanisms, likely in the brain, liver, or pancreas.
Hauner, H; Glatting, G; Ditschuneit, H H; Pfeiffer, E F · In Vitro
RPEP-00076 · 1988After seizures (from electroshock, kainic acid, or kindling), both dynorphin and enkephalin initially decrease in the hippocampus, indicating release. But their recovery patterns differ dramatically.
Enkephalin shows a rapid, sustained rebound above normal levels after all three seizure types. Dynorphin shows a large, sustained decrease after electroshock and kindling, with slow recovery.
Mu-opioid receptor antagonists shortened the kindling process (where repeated mild stimulation eventually produces seizures). This strongly suggests brain opioid peptides are involved in how seizure susceptibility develops.
Enkephalin in the hippocampus appears to mediate wet-dog shakes (a specific seizure behavior seen in rodents), connecting it to the opioid withdrawal syndrome.
Hong, J S; McGinty, J F; Grimes, L; Kanamatsu, T; Obie, J; Mitchell, C L · Review
RPEP-00077 · 1988Cortisol at 25 mg/kg given 24 hours before testing decreased prolactin release triggered by all four opioid peptides (dynorphin, beta-endorphin, met-enkephalin, D-Met-Pro-enkephalinamide) injected into the brain.
Actinomycin D pretreatment blocked cortisol's inhibitory effect. This means cortisol works by triggering new protein synthesis, not by directly blocking opioid receptors.
In adrenalectomized (no adrenal glands) rats, cortisol's inhibitory effect was even stronger. Maximum inhibition was reached at only 5 mg/kg, lower than the 25 mg/kg used in intact rats.
Cortisol did not affect opioid-induced corticosterone release. This selectivity means cortisol specifically targets the prolactin pathway, not all opioid effects.
Kiem, D T; Kanyicska, B; Stark, E; Fekete, M I · Animal Study
RPEP-00078 · 1988A commercially purchased synthetic dynorphin A-(1-13) raised cytosolic calcium in rat pituitary cells. However, this effect was NOT caused by dynorphin itself.
When the same preparation was purified by HPLC, the calcium-raising effect disappeared. HPLC combined with LHRH radioimmunoassay revealed contamination with a LHRH-like compound (luteinizing hormone-releasing hormone).
The LHRH antagonist blocked the calcium effect, confirming the contamination. Pure dynorphin A-(1-13), dynorphin A-(2-13), leu-enkephalin, beta-endorphin, morphine, and U50,488H all had zero effect on pituitary calcium.
LHRH itself raised calcium by about 50 nM, which was blocked by the LHRH antagonist.
Knepel, W; Schöfl, C; Wesemeyer, G; Götz, D M · In Vitro
RPEP-00079 · 1988Endogenous opioid peptides inhibit the release of both vasopressin and oxytocin from the posterior pituitary. They also affect anterior pituitary hormones through the hypothalamic portal blood system.
Dynorphin mRNA co-exists in the same neurons as vasopressin. Stimuli that increase vasopressin secretion (like salt loading) also increase dynorphin mRNA accumulation. This parallel regulation strongly suggests dynorphin has a genuine role alongside vasopressin.
Pro-enkephalin A mRNA co-exists with CRF in a different group of hypothalamic cells. Stresses that increase CRF mRNA also increase pro-enkephalin mRNA in the same area.
The co-existence of opioid peptides with established hormones in the same cells, combined with their coordinated regulation, provides powerful evidence that opioids are genuine neuroendocrine regulators, not incidental bystanders.
Lightman, S L · Review
RPEP-00081 · 1988Within 24 hours of inflammation, dynorphin increased specifically in the ipsilateral (same-side) dorsal horn of the spinal cord. Met-enkephalin, leu-enkephalin, and opioid receptor densities did not change.
The inflamed paw showed pronounced supersensitivity to morphine's painkilling effect against pressure pain, starting at 24 hours and increasing at 1 week.
Opioid antagonists (naloxone and the kappa-preferring MR 2266) made the pain worse, suggesting endogenous opioids were already partially controlling the pain.
By 3-5 weeks, inflammation had spread to the opposite paw. Dynorphin was now elevated bilaterally and in the cervico-thoracic cord. Met-enkephalin and leu-enkephalin also rose bilaterally. Adrenal glands were enlarged and plasma beta-endorphin was elevated, signs of chronic stress.
No changes in mu, delta, or kappa receptor density were found at any time point.
Millan, M J; Członkowski, A; Morris, B; Stein, C; Arendt, R; Huber, A; Höllt, V; Herz, A · Animal Study
RPEP-00083 · 1988Alpha-helical CRF9-41 (a CRF receptor blocker) applied to hypothalamic slices in vitro caused beta-endorphin and met-enkephalin to drop significantly within 10 minutes. At the same time, LHRH (luteinizing hormone-releasing hormone) rose significantly.
Dynorphin also decreased but the change was not statistically significant.
When the antagonist was removed, the pattern reversed within 10 minutes: beta-endorphin, met-enkephalin, and dynorphin rose back up while LHRH fell back down.
The same results were obtained in vivo using push-pull perfusion of the arcuate-median eminence region in anesthetized rats. This confirmed the in vitro findings in living animals.
Nikolarakis, K E; Almeida, O F; Sirinathsinghji, D J; Herz, A · Animal Study
RPEP-00084 · 1988GABA at concentrations from 10^-8 to 10^-4 M caused a dose-dependent increase in GnRH release from rat hypothalamic slices. The GABA-A receptor agonist isoguvacine replicated this, while the GABA-B agonist baclofen had no effect. The GABA-A antagonist SR95103 blocked the GABA effect.
Blockade of nerve conduction with tetrodotoxin abolished GABA's stimulatory effect, meaning it works through neural circuits, not directly on GnRH cells.
Naloxone (opioid blocker) prevented GABA-induced GnRH release. The CRF antagonist also blocked it. CRF itself decreased GnRH release, and GABA could not reverse this.
GABA stimulated the release of beta-endorphin, dynorphin, and met-enkephalin from the same tissue, connecting opioid peptide release to the control of reproduction.
Nikolarakis, K E; Loeffler, J P; Almeida, O F; Herz, A · In Vitro
RPEP-00085 · 1988DADLE (D-Ala2-D-Leu5 enkephalin), a delta-opioid receptor agonist, at 1.50 mg/kg/day induced summer hibernation comparable to that caused by natural Hibernation Induction Trigger (HIT).
Morphine (1.50 mg/kg/day), morphiceptin (0.82 mg/kg/day), and dynorphin A (0.82 mg/kg/day) did not induce hibernation.
Strikingly, morphine, morphiceptin, and dynorphin A actually antagonized HIT-induced hibernation. They blocked the natural hibernation signal.
This suggests delta-opioid receptors promote hibernation while mu and kappa opioid receptors promote the arousal (waking) state. Natural hibernation may involve a shift in the balance between these receptor systems.
Oeltgen, P R; Nilekani, S P; Nuchols, P A; Spurrier, W A; Su, T P · Animal Study
RPEP-00086 · 1988Naloxone (100 to 1000 micrograms/kg) did not affect the initial stomach response to vagus nerve stimulation. But it dose-dependently enhanced the delayed response (from vagal afferent fibers). This means endogenous opioids normally restrain the delayed stomach excitation.
Met-enkephalin was markedly more potent than the mu-agonist DAGO and the kappa-agonist dynorphin A(1-13) at inhibiting both types of vagal stomach responses.
The delta-selective agonist DPDPE mimicked met-enkephalin's inhibitory effects. The delta-selective antagonist ICI 174,864 blocked DPDPE's effects. This confirmed delta-opioid receptors mediate the gastric inhibition.
The conclusion: natural opioid peptides (likely enkephalins) tonically restrain vagus nerve-driven stomach excitation through delta receptors.
Okamoto, T; Kurahashi, K; Fujiwara, M · Animal Study
RPEP-00087 · 1988Met-enkephalin, leu-enkephalin, dynorphin (1-13), DSLET (delta agonist), and DAGO (mu agonist) were tested on human NK cells from healthy donors after 18-hour incubation.
The results were bidirectional: populations with low baseline NK activity showed enhancement. Populations with high baseline NK activity showed suppression. This immunoregulatory pattern was consistent across different opioid peptides.
The effect was confirmed in a serum-free system with recombinant interferon-alpha, ruling out serum-factor interference.
Naloxone displayed both antagonist properties (blocking opioid effects) and direct immunomodulatory effects. This suggested lymphocytes themselves may produce opioid peptides in culture, creating an autocrine signaling loop.
Oleson, D R; Johnson, D R · In Vitro
RPEP-00088 · 1988Regardless of which nerve was cut (right or left sciatic, both sciatic, right brachial plexus, saphenous, or sural), the same brain-wide pattern emerged.
Beta-endorphin decreased significantly in all brain areas except the striatum. Met-enkephalin increased in all brain areas and in the affected spinal cord segments. Substance P, somatostatin, and dynorphin were unaffected.
The changes appeared within 24 hours of surgery and persisted for at least 4 months, indicating a long-lasting reorganization of the opioid system.
There was no lateralization: unilateral nerve cuts produced bilateral brain changes. This means local nerve damage triggers a global brain response.
Serotonergic drugs normalized beta-endorphin levels, suggesting the serotonin system mediates the opioid changes after nerve injury.
Panerai, A E; Sacerdote, P; Brini, A; Bianchi, M; Mantegazza, P · Animal Study
RPEP-00089 · 1988DBA mice had higher baseline pain thresholds than C57 mice. But C57 mice showed much greater stress-induced analgesia (pain relief from stress).
The opioid systems differed genetically: DBA mice had significantly more dynorphin in the hypothalamus and neurointermediate pituitary. C57 mice had lower KD values (better affinity) for spinal mu receptors but higher KD (lower affinity) for cerebral kappa receptors.
Under stress, the two strains activated different opioid systems. C57 mice: beta-endorphin decreased in hypothalamus and increased in neurointermediate pituitary lobe. DBA mice: dynorphin decreased in hypothalamus and beta-endorphin increased in the anterior pituitary.
Both strains showed spinal cord dynorphin decreases under stress.
Przewłocka, B; Vetulani, J; Lasoń, W; Dziedzicka, M; Silberring, J; Castellano, C; Przewłocki, R · Animal Study
RPEP-00090 · 1988Intrathecal (spinal cord) met-enkephalin at low doses markedly reduced the aversive behavior caused by intrathecal substance P. At these same low doses, met-enkephalin produced no measurable analgesia in the tail-flick pain test.
Met-enkephalin needed much higher doses to produce general analgesia.
Beta-endorphin and dynorphin-(1-17) were different. They blocked substance P-induced behavior and produced general analgesia at roughly the same doses. There was no separation between the two effects.
This suggests met-enkephalin has a specific, preferential interaction with substance P-mediated pain signaling in the spinal cord, separate from its general painkilling effects.
Sakurada, T; Takahashi, K; Sakurada, S; Kisara, K; Folkesson, R; Terenius, L · Animal Study
RPEP-00091 · 1988Radioactive beta-endorphin injected intravenously accumulated specifically in the lung and liver but not other tissues. Four lines of evidence confirmed this was specific receptor binding, not passive trapping.
First, adding excess unlabeled beta-endorphin reduced the labeled peptide in lung and liver.
Second, injecting unlabeled beta-endorphin via the femoral vein (which passes through the lungs first) rapidly increased blood levels of the pre-injected labeled peptide, displacing it from lung binding sites. Injection via the carotid artery (bypassing the lungs) did not have this effect.
Third, immunoreactive labeled peptide that had lost its receptor-binding ability did not accumulate in lung or liver.
Fourth, dynorphin (1-13) and ethylketocyclazocine (kappa agonist) displaced beta-endorphin from these sites, but DADLE (delta agonist) and naloxone (mu antagonist) did not. This pharmacological profile indicates kappa-type binding sites.
Sato, H; Sugiyama, Y; Sawada, Y; Iga, T; Hanano, M · Animal Study
RPEP-00092 · 1988Three opioid peptides caused dose-dependent contraction of isolated gallbladder muscle cells: met-enkephalin > dynorphin(1-13) > leu-enkephalin.
The contractions were blocked by the opioid antagonists naloxone and MR2266 but not by muscarinic, CCK/gastrin, or tachykinin antagonists. This confirmed the response was specifically through opioid receptors.
Differential sensitivity to preferential antagonists of mu (naloxone) and kappa (MR2266) receptors, combined with the different potencies of the three agonists, indicated the presence of mu, delta, and kappa opioid receptors on the gallbladder muscle cells.
These opioid peptides are known to be present in myenteric neurons of the gut, providing the natural source of opioid signals for gallbladder muscle.
Severi, C; Grider, J R; Makhlouf, G M · In Vitro
RPEP-00093 · 1988Three opioid peptide families showed distinct, non-overlapping distributions in the preoptic brain region.
Beta-endorphin fibers were mainly in the periventricular nucleus. Dynorphin B fibers were more uniformly distributed with few cell bodies. Enkephalin (peptide E) had hundreds of cell bodies, concentrated in specific nuclei.
The key sexual dimorphism: male rats had significantly more enkephalin cells in the anteroventral periventricular nucleus (AVPv) than females. This was despite the AVPv being physically larger in female rats.
This sex difference was at least partially dependent on perinatal gonadal steroids. Early hormone exposure during the critical developmental window permanently organized the number of enkephalin cells, a process called organizational hormone effects.
No sex difference was found in enkephalin cells in the anterodorsal preoptic nucleus, showing the dimorphism was region-specific, not a general property of enkephalin neurons.
Simerly, R B; McCall, L D; Watson, S J · Animal Study
RPEP-00094 · 1988Leu-enkephalin and leu-enkephalinamide inhibited electrical self-stimulation in the substantia nigra/ventral tegmental area (SN-VTA) but not the medial forebrain bundle/lateral hypothalamus (MFB-LH). However, injection into MFB-LH facilitated self-stimulation of the distant SN-VTA.
Ala-leu-enkephalin injected into SN-VTA actually facilitated its self-stimulation (opposite to leu-enkephalin). The same peptide in MFB-LH facilitated SN-VTA stimulation like the other enkephalins.
Dynorphin A(1-13) injected into SN-VTA facilitated its self-stimulation. Injection into MFB-LH had no effect.
Met-enkephalin had no direct or indirect effects in either region.
These differences likely reflect different receptor preferences in the SN-VTA neural organization.
Singh, J; Desiraju, T · Animal Study
RPEP-00095 · 1988Dynorphin A produced dose-related, naloxone-reversible analgesia via intracerebroventricular injection, confirming its function as an endogenous opioid painkiller acting through kappa receptors.
Tiseo, P J; Geller, E B; Adler, M W · Animal Study
RPEP-00096 · 1988CSF from rats that had just experienced an electroshock seizure significantly raised seizure thresholds in recipient rats when injected into their brain ventricles.
This anticonvulsant activity was blocked by high-dose naloxone and by the selective delta-opioid antagonist ICI 174,864. This identified delta-opioid receptors as the mediators.
The active substance was destroyed by heat (90°C) and by trypsin (a protein-digesting enzyme), confirming it is a peptide. It passed through 10,000 dalton membranes but not 5,000 dalton membranes, putting its size between 5,000-10,000 daltons.
Post-seizure CSF had increased beta-endorphin (31 amino acids, ~3,500 daltons) but not dynorphin A, leu-enkephalin, or met-enkephalin. However, hidden met-enkephalin sequences (released by trypsin digestion of larger peptides) were found.
The size (5,000-10,000 Da) and delta-receptor specificity do not match any known opioid peptide, suggesting an undiscovered anticonvulsant opioid peptide.
Tortella, F C; Long, J B · Animal Study
RPEP-00097 · 1988Cold swim stress caused opposite changes in opioid peptide levels depending on the body region. Beta-endorphin dropped in the pituitary but surged in blood plasma by over 3-fold.
Vaswani, K K; Richard, C W; Tejwani, G A · Animal Study
RPEP-00099 · 1988Chronic arthritis triggered spinal cord neurons to produce peptides from two different opioid precursor families in the same cells. All proenkephalin-positive cells also contained prodynorphin peptides.
Weihe, E; Millan, M J; Leibold, A; Nohr, D; Herz, A · Animal Study
RPEP-00102 · 1989Beta-endorphin had a dose-dependent biphasic effect on insulin secretion. Low doses inhibited and high doses stimulated insulin release. Met-enkephalin only inhibited. Dynorphin A had no effect.
Ahrén, B · Animal Study
RPEP-00103 · 1989Prostatic opioid innervation comes exclusively from proenkephalin-derived peptides. No prodynorphin or pro-opiomelanocortin products were detected in either species.
Aumüller, G; Jungblut, T; Malek, B; Konrad, S; Weihe, E · Cross Sectional
RPEP-00104 · 1989The opioidergic system (beta-endorphin, enkephalin, dynorphin) is involved in both the pharmacological actions of alcohol and the craving/genetic predisposition toward alcohol abuse.
Blum, K; Briggs, A H; Trachtenberg, M C · Review
RPEP-00106 · 1989Stress-induced analgesia in slugs appears mediated by endogenous opioid peptides, particularly enkephalins and beta-endorphin. The opioid pain system is evolutionarily ancient.
Dalton, L M; Widdowson, P S · Animal Study
RPEP-00107 · 1989The rat anterior pituitary contains at least six distinct high-molecular-weight intermediates of prodynorphin, and similar forms exist in spinal cord and hypothalamus.
Day, R; Akil, H · Animal Study
RPEP-00108 · 1989AtT-20 pituitary cells contain all the enzymes needed to process prodynorphin correctly, including at rare monobasic cleavage sites. The processed peptides were released in response to CRF stimulation.
Devi, L; Gupta, P; Douglass, J · In Vitro
RPEP-00109 · 1989Holostean fish have enkephalin peptides with a 3:1 met-to-leu ratio and a novel modified met-enkephalin form, but no detectable prodynorphin-derived peptides.
Dores, R M; McDonald, L K; Crim, J W · Animal Study