Opiate receptors: enkephalins and endorphins

Enkephalins and Pain Modulation: Mechanisms of Action and Therapeutic Perspectives

Enkephalins, a subclass of endogenous opioid peptides, play a pivotal role in pain modulation. Enkephalins primarily exert their effects through opioid receptors located widely throughout both the central and peripheral nervous systems. This review will explore the mechanisms by which enkephalins produce analgesia, emotional regulation, neuroprotection, and other physiological effects. Furthermore, this review will analyze the involvement of enkephalins in the modulation of different pathologies characterized by severe pain. Understanding the complex role of enkephalins in pain processing provides valuable insight into potential therapeutic strategies for managing pain disorders.

Plasma proenkephalin A and incident chronic kidney disease and albuminuria in the REasons for Geographic And Racial Differences in Stroke (REGARDS) cohort

BACKGROUND

Plasma proenkephalin A (PENK-A) is a precursor of active enkephalins. Higher blood concentrations have been associated with estimated glomerular filtration rate (eGFR) decline in European populations. Due to the significant disparity in incident chronic kidney disease (CKD) between White and Black people, we evaluated the association of PENK-A with incident CKD and other kidney outcomes among a biracial cohort in the U.S.

METHODS

In a nested cohort of 4,400 participants among the REasons for Geographic And Racial Differences in Stroke, we determined the association between baseline PENK-A concentration and incident CKD using the creatinine-cystatin C CKD-EPI 2021 equation without race coefficient, significant eGFR decline, and incident albuminuria between baseline and a follow-up visit 9.4 years later. We tested for race and sex interactions. We used inverse probability sampling weights to account for the sampling design.

RESULTS

At baseline, mean (SD) age was 64 (8) years, 49% were women, and 52% were Black participants. 8.5% developed CKD, 21% experienced ≥ 30% decline in eGFR and 18% developed albuminuria. There was no association between PENK-A and incident CKD and no difference by race or sex. However, higher PENK-A was associated with increased odds of progressive eGFR decline (OR: 1.12; 95% CI 1.00, 1.25). Higher PENK-A concentration was strongly associated with incident albuminuria among patients without diabetes mellitus (OR: 1.29; 95% CI 1.09, 1.53).

CONCLUSION

While PENK-A was not associated with incident CKD, its associations with progression of CKD and incident albuminuria, among patients without diabetes, suggest that it might be a useful tool in the evaluation of kidney disease among White and Black patients.

Framing and understanding the whole aspect of oral sex from social and health perspectives: a narrative review

Historically, oral sex is part of human sexual behaviour even though some consider it taboo, and its expression is deemed inappropriate. Human rights also stand out as technology becomes more sophisticated, leading to its practice disclosure in the 21st century. The large-scale oral sex discussion in media encourages people to express it, and there is feedback on whether it is right or not. It all depends on each person’s values. We found this sexual behaviour occurs everywhere regardless of religion, culture and race. Pop culture influences this behaviour considerably. It can be seen in music, movies and television programmes that allude to oral sex. Numerous motivations underlying this behaviour include sexual pleasure for the sake of psychological well-being. It is undeniable that this behaviour is still controversial. There is a risk of it causing disease, but it reportedly provides many benefits. Oral sex is not a new behaviour crossing boundaries, according to our theory. It is an old behaviour that has been highlighted because of the factors that support it. This behaviour, still considered taboo, has its disadvantages, such as sexually transmitted disease and oral cancer, but also has benefits, such as preventing preeclampsia.

Psycho-Neuro-Endocrine-Immunological Basis of the Placebo Effect: Potential Applications beyond Pain Therapy

The placebo effect can be defined as the improvement of symptoms in a patient after the administration of an innocuous substance in a context that induces expectations regarding its effects. During recent years, it has been discovered that the placebo response not only has neurobiological functions on analgesia, but that it is also capable of generating effects on the immune and endocrine systems. The possible integration of changes in different systems of the organism could favor the well-being of the individuals and go hand in hand with conventional treatment for multiple diseases. In this sense, classic conditioning and setting expectations stand out as psychological mechanisms implicated in the placebo effect. Recent advances in neuroimaging studies suggest a relationship between the placebo response and the opioid, cannabinoid, and monoaminergic systems. Likewise, a possible immune response conditioned by the placebo effect has been reported. There is evidence of immune suppression conditioned through the insular cortex and the amygdala, with noradrenalin as the responsible neurotransmitter. Finally, a conditioned response in the secretion of different hormones has been determined in different studies; however, the molecular mechanisms involved are not entirely known. Beyond studies about its mechanism of action, the placebo effect has proved to be useful in the clinical setting with promising results in the management of neurological, psychiatric, and immunologic disorders. However, more research is needed to better characterize its potential use. This review integrates current knowledge about the psycho-neuro-endocrine-immune basis of the placebo effect and its possible clinical applications.

Effect of Chaihu-jia-Longgu-Muli decoction on withdrawal symptoms in rats with methamphetamine-induced conditioned place preference

Traditional Chinese medicine detoxification prescription Chaihu-jia-Longgu-Muli decoction (CLMD) relieves depressive symptoms in patients withdrawing from methamphetamine. In the present study, we assessed the effects of CLMD on methamphetamine withdrawal in rats. A methamphetamine-intoxicated rat model was established. Rats were randomly divided into the control, model, high-dosage, medium-dosage, and low-dosage groups, receiving high, medium, and low doses of CLMD, respectively. Weekly body weight measurements revealed that rats treated with methamphetamine had the lowest body weight. The conditioned place preference (CPP) experiment revealed that methamphetamine-intoxicated rats stayed significantly longer in the drug-paired chamber than the control rats. However, after administering high-dosage CLMD, the amount of time the rats spent in the drug-paired chamber was significantly less than that of the model rats. Our open-field test revealed that the model group had lower crossing and rearing scores than the control group. Additionally, rats that received CLMD treatment exhibited higher crossing and rearing scores than the model rats. Striatal dopamine (DA), 5-hydroxytryptamine (5-HT), and endorphins (β-EP) and serum interleukin (IL)-1α and IL-2 concentrations were estimated. Rats in the model group had lower striatal DA, 5-HT, and β-EP and higher serum IL-1α and IL-2 concentrations than those in the control group. High-dosage CLMD administration significantly changed the concentrations of these molecules, such that they approached normal concentrations. In general, CLMD could prevent the development of methamphetamine-induced withdrawal symptoms in rats by increasing the DA, 5-HT, and β-EP and lowering the IL-1α and IL-2 concentrations.

Proenkephalin: A New Biomarker for Glomerular Filtration Rate and Acute Kidney Injury

Assessment of kidney function is primarily based on urine output and Creatinine (Cr)-based methods to estimate glomerular filtration rate (GFR). The latter is confounded as Cr is not exclusively filtered by the kidney and rises relatively late after the onset of acute kidney injury (AKI). This leads to delays in recognition of reduced kidney function and diagnosis of AKI, particularly in critically ill patients where kidney function can change rapidly. The gold standard methods of GFR determination, such as inulin or iohexol clearance, are labor intensive and unfeasible in acute clinical settings. Proenkephalin A 119-159 (PENK) has been intensively studied as a novel biomarker of kidney function. PENK belongs to the enkephalin peptide family and is freely filtrated in the glomerulus. Plasma PENK concentration appears to correlate strongly with GFR. Moreover, increased plasma PENK concentrations are found to be associated with long-term kidney outcomes and mortality. In this review, we summarize the role of PENK in assessment of kidney function and its capacity to predict various clinical outcomes.

Perspectives on Endogenous Opioids in Birds

The present review summarizes the state of knowledge of endogenous opioids in birds. Endogenous opioid peptides acts in a neuromodulatory, hormonal and paracrine manner to mediate analgesic and other physiological functions. These peptides act through specific G-protein coupled receptors. Opioid receptors consist of a family of four closely-related proteins. The three types of opioid receptors are the mu (MOR or μ), delta (DOR or δ), and kappa (KOR or κ) opioid receptor proteins. The role of the fourth member of the opioid receptor family, the nociceptin or orphanin FQ receptor (ORL), is not clear. The ligands for opioid receptors are: β –endorphin (MOR), Met- enkephalin, Leu-enkephalin (DOR) and dynorphin (KOR), together with probably endomorphins 1 and 2. In spite of long history of research on endogenous opioid peptides, there are no studies of endogenous opioids per se in wild birds and few in poultry species. β-endorphin is present in all birds investigated and there is close agreement between the structures of β-endorphin in different birds. Plasma concentrations of β-endorphin are increased by ether stress in geese. There is evidence that β-endorphin plays a role in the control of luteinizing hormone release in chickens. Met-enkephalin is present in tissues such as the retina, hypothalamus, pituitary gland, and adrenals together with circulation of birds. Stresses such as crowding and withholding water increase circulating concentrations of Met-enkephalin in chickens. The structures of chicken dynorphin A and B have been deduced from cDNA. What is missing are comprehensive studies of plasma concentrations and expression of the full array of endogenous opioids in multiple avian species under different situations. Also, what is not known is the extent to which circulating or locally released or intra-cellular Met-enkephalin influence physiological process in birds. Thus, there is considerable scope for investigation of the physiology of endogenous opioids in birds.

The Effects of Pre-emptive Administration of Ketamine and norBNI on Pain Behavior, c-Fos, and Prodynorphin Protein Expression in the Rat Spinal Cord after Formalin-induced Pain Is Modulated by the DREAM Protein

Background

We investigated the effects of pre-emptive administration of ketamine and norBNI on pain behavior and the expression of DREAM, c-Fos, and prodynorphin proteins on the ipsilateral side of the rat spinal cord at 2 and 4 hours after formalin injection.

Methods

Eighty-four male Sprague Dawley rats were divided into 4 major groups consisting of control rats (C) (n = 12), rats given only formalin injections (F) (n = 24), and rats treated with pre-emptive administration of either ketamine (K+F) (n = 24) or norBNI (N+F) (n = 24). The non-control groups were further divided into subgroups consisting of rats that were sacrificed at 2 and 4 hours (n = 12 for each group) after formalin injection. Pain behavior was recorded for 1 hour. After 2 and 4 hours, the rats were sacrificed and the spinal cords (L4-L5 sections) were removed for immunohistochemistry and Western blot analysis.

Results

The pain behavior response was reduced in the K+F group compared to the other groups during the second phase of the formalin pain response. We detected an increase in the nuclear DREAM protein level in the K+F group at 2 and 4 hours and a transient decrease in the N+F group at 2 hours; however, it increased at 4 hours after injection. Fos-like immunoreactivity (FLI) and Prodynorphin-like immunoreactivity (PLI) neurons decreased in the K+F group but increased in the N+F group at 2 hours after injection. While FLI decreased, PLI increased in all groups at 4 hours after injection.

Conclusions

We suggest that NMDA and kappa opioid receptors can modulate DREAM protein expression, which can affect pain behavior and protein transcriptional processes at 2 hours and bring about either harmful or protective effects at 4 hours after formalin injection.

Characterization of the receptor binding residues of kisspeptins by positional scanning using peptide photoaffinity probes

Kisspeptins, endogenous peptide ligands for GPR54, play an important role in GnRH secretion. Since in vivo administration of kisspeptins induces increased plasma LH levels, GPR54 agonists hold promise as therapeutic agents for the treatment of hormonal secretion diseases. To facilitate the design of novel potent GPR54 ligands, residues in kisspeptins that involve in the interaction with GPR54 were investigated by kisspeptin-based photoaffinity probes. Herein, we report the design and synthesis of novel kisspeptin-based photoaffinity probes, and the application to crosslinking experiments for GPR54-expressing cells.

Naltrexone effects on cortisol secretion in women and men in relation to a family history of alcoholism: studies from the Oklahoma Family Health Patterns Project

Naltrexone evokes a cortisol response through its blockade of central opioid receptors on the hypothalamic-pituitary-adrenocortical axis (HPA). The magnitude of this cortisol response may be useful as a probe for central opioid activity in different groups of subjects. Accordingly, the present study examined the effect of opioid blockade on the HPA in 70 women and 58 men with (N=41) and without (N=87) a family history of alcoholism, using a randomized, placebo-controlled, double blind administration of oral naltrexone (50mg). Saliva cortisol was sampled at baseline prior to placebo or naltrexone and again every 30 min over the next 180 min. Women had significantly larger cortisol responses to naltrexone than did the men, F=6.88, p<0.0001. There were no significant differences in cortisol response between groups differing in family history of alcoholism, F=0.65, p>0.69. The present results confirm that women have much greater central opioid restraint on the HPA than men do and that this endogenous restraint is unmasked by opioid blockade. However the results provide no evidence of a differential central opioid tonus in persons with a family history of alcoholism at this dose of naltrexone. The cortisol response to naltrexone may be a useful probe for central opioid activity in women and to a lesser degree in men.

Endogenous kappa-opioid receptor systems inhibit hyperalgesia associated with localized peripheral inflammation

Peripheral inflammation evokes functional and biochemical changes in the periphery and spinal cord which result in central sensitization and hypersensitivity. Inhibitory control systems from the rostral ventromedial medulla (RVM) are also activated. The present study investigates whether endogenous kappa-opioid receptor (KOPr) systems contribute to these neuroadaptations. Inflammation was induced by intraplantar injection of complete Freund's adjuvant (CFA) into one hindpaw. Mechanical and thermal thresholds were determined using the Von Frey and radiant heat tests, respectively. KOPr gene deletion in mice or systemic administration of the long-acting KOPr antagonist, norbinaltorphimine (norBNI) significantly exacerbated mechanical and thermal hypersensitivity of the ipsilateral, inflamed paw. Thermal and mechanical thresholds of the non-inflamed, contralateral hindpaw were unaffected by CFA treatment. However, gene deletion as well as norBNI treatment resulted in mechanical, but not thermal hypersensitivity of the non-inflamed paw. Similar results were obtained when norBNI was administered intrathecally or into the RVM in rats. These data demonstrate a previously unrecognized role of endogenous KOPr systems in inhibiting hyperalgesia during inflammation. Furthermore, they demonstrate that decreased KOPr activity in either the spinal cord or RVM not only enhances mechanical and thermal hyperalgesia of the inflamed limb but also leads to an unmasking of mechanical hyperalgesia at a site remote from inflammation. The differential effects of KOPr antagonism on mechanical versus thermal thresholds for the non-inflamed paw support the notion that distinct neuroanatomical or neurochemical mechanisms modulate the processing of thermal versus mechanical stimuli.

Acupuncture: role in comprehensive cancer care--a primer for the oncologist and review of the literature

In recent studies, patients have reported an increased use of complementary and alternative medicine (CAM). Acupuncture is a popular complementary therapy for patients with cancer. This article will provide current cancer treatment providers with information on acupuncture as well as the research conducted on cancer symptoms and side effects of cancer treatments. Antiemetic studies are the most prevalent and contain the most promising results. Several studies have found that acupuncture significantly reduces the number of emesis (vomiting) episodes for patients receiving chemotherapy. While studies on pain control vary due to the heterogeneity of pain, there are few studies investigating pain caused from cancer and the removal of cancerous tumors. These studies, while promising, provide basic results that need further investigation for more definitive results. Although relatively few studies have been done on anxiety and depression, several researchers have found acupuncture to be just as effective as or more effective than antidepressants for patients without cancer. Studies on breathlessness, while small, have shown acupuncture to have a significant positive effect on chronic obstructive pulmonary disease, breathlessness associated with end-stage cancer, and asthma. Researchers studying xerostomic individuals who have received salivary gland irradiation found significant positive results in salivary flow rates compared to baseline. Patients with hot flashes due to hormonal imbalance may benefit from the use of acupuncture. A recent pilot study showed improvement of chronic postchemotherapy fatigue following acupuncture treatments. Many individuals with cancer have turned to acupuncture because their symptoms persisted with conventional treatments or as an alternative or complement to their ongoing treatments. Despite the immense popularity in the community, few large randomized trials have been conducted to determine the effects acupuncture has on cancer symptoms and side effects of treatments. A majority of the current studies have shown beneficial effects that warrant further investigation with large trial sizes.

Multiple sources of endogenous opioid peptide involved in the hypoglycemic response to 15 Hz electroacupuncture at the Zhongwan acupoint in rats

A decrease in plasma glucose levels was observed in rats which received electroacupuncture (EA) stimulation at the Zhongwan acupoint. In the present study, the role of the adrenal gland in this hypoglycemic response to EA at high frequency (15 Hz) was investigated on adrenalectomized (ADX) normal rats. There was a sharper decrease in plasma glucose by EA stimulation in the fasting ADX group than in the fasting sham-operated group. Naloxone blocked this hypoglycemic response to EA stimulation in rats which received ADX. Stimulation of EA failed to elicit an increase in plasma beta-endorphin and insulin levels in ADX rats. Similar results were observed in sham and ADX mice. EA stimulation of ADX mice can reduce plasma glucose levels. Furthermore, naloxone abolished the hypoglycemic response to EA stimulation in mice. Such a hypoglycemic response to EA stimulation was also observed in micro-opioid receptor knockout mice (MOR-KOM). Mediation by another opioid peptide should also be considered in future experiments. We conclude that multiple sources of endogenous opioid peptide participated in the lowering of plasma glucose in rats induced by EA stimulation at higher frequency (15 Hz) at the Zhongwan acupoint. Increase in beta-endorphin levels from the adrenal gland enhances the secretion of insulin, there by reducing plasma glucose levels, and is partially involved in this EA stimulation.

Hypothalamo-pituitary-adrenal axis-regulated stress response and negative feedback sensitivity is altered by prenatal morphine exposure in adult female rats

It has been shown that adult female rats react to stressors more intensely than adult male rats. Our previous work demonstrated that the adrenocorticotropic hormone (ACTH) but not corticosterone (CORT) response to stress is altered by prenatal morphine exposure in adult male rats. Response of the hypothalamo-pituitary-adrenal (HPA) axis to stress is known to be sex specific and dependent on the hormonal fluctuation of the estrous cycle. Therefore, the present study examined the effect of prenatal morphine exposure on the levels of ACTH and CORT before and after restraint stress in adult female rats. Experiment 1 tested ACTH and CORT plasma levels before and after restraint stress in prenatally morphine- and saline-exposed, adult diestrus and proestrus female rats. Prenatal morphine exposure suppressed the restraint stress-induced ACTH levels in both diestrus and proestrus females, but did not have any effects on the basal or stress-induced CORT levels. Experiment 2 examined the sensitivity of negative feedback using the dexamethasone (DEX) (0.001, 0.01, 0.1 and 1.0 mg/kg) suppression test in adult, prenatally morphine- and saline-exposed female rats. In saline-exposed, proestrus but not diestrus females, all doses of DEX were effective in suppressing the restraint stress-induced increase in CORT levels. In both diestrus and proestrus, morphine-exposed females, only the two highest doses of DEX (0.1 and 1.0 mg/kg) were successfully suppressing the stress-induced CORT levels. The stress-induced increase in the ACTH level was suppressed only by the highest dose of DEX (1.0 mg/kg) in both saline- and morphine-exposed, diestrus and proestrus females. Thus, the present study demonstrates that prenatal morphine exposure alters the HPA axis-regulated stress response and the sensitivity of negative feedback that are affected by the fluctuation of ovarian hormones.

Release of beta-endorphin from adrenal gland to lower plasma glucose by the electroacupuncture at Zhongwan acupoint in rats

We found that electroacupuncture (EA) at the Zhongwan acupoint released beta-endorphin to induce an insulin-dependent hypoglycemia in rats. The present study investigated the source of beta-endorphin. EA at 2 Hz for 30 min in rats decreased plasma glucose that could be abolished by naloxone. A similar effect of EA was also observed in wild-type mice but disappeared in mu-opioid receptor knockout mice. Mediation of the mu(1)-opioid receptor is considered from the blockade of response to EA by naloxonazine in rats. Otherwise, adrenalectomy abolished not only the hypoglycemic response to EA in rat and mouse but also the increase of plasma beta-endorphin and insulin by EA in rats. In conclusion, the increase of plasma beta-endorphin by EA at 2 Hz is mainly from the adrenal gland.

The role of the endogenous opioid system in the effects of acupuncture on mood, behavior, learning, and memory

Ancient and contemporary papers report that acupuncture and its variations have a considerable effect on psychological state and behavior. Evidence from experimental and clinical studies suggests that acupuncture and its variations exert a strong influence on endogenous opioids in the brain, and that the endogenous opioid system is involved in various mental functions. The author suggests that the endogenous opioid system can play the key role in the mediation of the effects of acupuncture and its variations on mood, behavior, learning, and memory. Clinical implications of this suggestion are discussed.

Hypersensitivity of the hypothalamic-pituitary-adrenal axis to naloxone in post-traumatic stress disorder

Naloxone, which increases endogenous corticotropin-releasing hormone (CRH) release by blocking an inhibitory opioidergic tone on the hypothalamic-pituitary-adrenal (HPA) axis, was administered in a dose-response protocol to seven healthy volunteers and 13 patients with treated posttraumatic stress disorder (PTSD). Six of the PTSD patients showed an increased hormonal response to the lowest naloxone dose (6 micrograms/kg) compared to both the control subjects and the other PTSD patients. This difference persisted on detailed subgroup analysis, although it was less marked at the highest naloxone dose (125 micrograms/kg). The responses of the other seven PTSD patients were indistinguishable from those of the control group. The greater responses of the six PTSD patients could not be explained on the basis of associated psychiatric illnesses or psychotropic drug therapy, and did not correlate with standard psychological testing or severity of PTSD. The results of this preliminary study therefore suggest that a hypersensitivity of the HPA axis to endogenous CRH stimulation may occur in PTSD.

Effect of naltrexone treatment on the treadmill exercise-induced hormone release in amenorrheic women

The effect of an acute physical stress on hormone secretions before and after a 10-day naltrexone treatment in untrained healthy and amenorrheic women was investigated. Plasma levels of pituitary (LH, FSH, prolactin, GH, ACTH, beta-endorphin) and adrenal (cortisol, androstenedione, testosterone) hormones were measured at rest and in response to 60 min of physical exercise. The test was done both before and after a 10-day naltrexone (50 mg/day) treatment. Graded levels of treadmill exercise (50, 70 and 90% of maximal oxygen uptake (VO2) every 20 min) was used as physical stressor. While mean +/- SE plasma LH levels in control women were higher than in amenorrheic patients and increased following the naltrexone treatment (p < 0.01), no significant differences of basal plasma hormonal levels were observed between amenorrheic and eumenorrheic women, both before and after naltrexone treatment. Physical exercise at 90% VO2 induced a significant increase in plasma GH, ACTH, beta-endorphin, cortisol, androstenedione and testosterone levels in controls before naltrexone treatment (p < 0.01). The mean increase in plasma androstenedione and testosterone levels in control women was significantly higher after naltrexone treatment (p < 0.01). In amenorrheic patients before naltrexone, physical exercise induced an increase in plasma prolactin and GH levels, but not in plasma ACTH, beta-endorphin, cortisol, testosterone and androstenedione. After naltrexone treatment, the exercise induced a significant plasma ACTH, beta-endorphin and cortisol levels, while the increase of plasma prolactin levels was significantly higher than before treatment (p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

L-type calcium channels and CRH-mediated ACTH and cortisol release in humans

1. The effect of pretreatment with nifedipine on naloxone-stimulated corticotrophin-releasing hormone (CRH)-induced adrenocorticotrophin (ACTH) release in humans was investigated. The mean peak plasma ACTH and cortisol levels and the mean peak change in cortisol levels from basal were significantly lower in the nifedipine/naloxone test than in the naloxone alone test. The integrated areas under the ACTH-time and cortisol-time curves were reduced by 33 and 49%, respectively, in the nifedipine/naloxone test compared with the naloxone alone test. These results correlate well with published in vitro studies. 2. Acute administration of oral nifedipine partially inhibited naloxone-stimulated ACTH and cortisol release, probably by blockade of plasma membrane voltage-dependent L-type calcium channels normally activated following binding of CRH to pituitary corticotroph receptors. 3. Naloxone-induced CRH release may replace insulin hypoglycaemia testing of pituitary ACTH reserve in humans.

Naloxone-induced ACTH release in man is inhibited by clonidine

1. Adrenergic mechanisms play an important role in regulation of ACTH release. We used the alpha 2-adrenergic agonist, clonidine, as a central nervous system inhibitor of ACTH release to see if it would alter naloxone-induced ACTH secretion in normal human volunteers. 2. There was a significant blunting of the mean peak level of ACTH and the mean peak change of ACTH from basal as well as the area under the ACTH-time curve when clonidine was given prior to naloxone. 3. We conclude that clonidine, by blocking central noradrenergic pathways, which stimulate corticotropin-releasing hormone secretion, inhibits naloxone-induced ACTH secretion. This suggests that naloxone causes ACTH release through these same central noradrenergic pathways.

Plasma leucine enkephalin is increased in liver disease

Plasma methionine enkephalin is increased in liver disease and may contribute to some of the clinical manifestations of hepatic failure. To determine if another 'small' opioid peptide is increased in the plasma of patients with liver disease, leucine enkephalin was measured by radioimmunoassay. Its plasma concentration was raised approximately five-fold in patients with acute liver disease (median 1490 pmol/l, range 830-2420) and three-fold in patients with cirrhosis with ascites (960 pmol/l, 470-2900), compared with disease controls (325 pmol/l, 180-740) and healthy controls (305 pmol/l, 180-560). The increase in plasma leucine enkephalin was proportional to the degree of liver damage, as judged in the patients with acute liver disease by its correlation with the prothrombin time (r = 0.691, p less than 0.01) and alanine aminotransferase (r = 0.502, p less than 0.05), and in the patients with cirrhosis by its negative correlation with the plasma albumin (r = -0.743, p less than 0.001). It is unclear whether the raised plasma leucine enkephalin in liver disease is a consequence of diminished hepatic inactivation, increased secretion from sympathetic nerves and adrenal glands, or both.

Release of platelet Met-enkephalin, but not serotonin, in migraine. A platelet response unique to migraine patients?

Plasma-induced release of platelet-serotonin and Met-enkephalin was studied by incubating platelets from migraineurs, tension headache patients and healthy volunteers with water, plasma collected during migraine attacks and with plasma collected during attack-free periods. Migraine-attack plasma, but not migraine attack-free plasma, released Met-enkephalin from platelets of migraineurs. Platelets of healthy controls or tension headache sufferers did not release their Met-enkephalin in response to either incubation medium. In contrast to Met-enkephalin, serotonin was not released from platelets in relevant quantities by migraine-attack plasma.

Neuroendocrine control of male reproductive function. The opioid system as a model of control at multiple sites

It is known that the same peptide can be identified in different secretory tissues and in the central nervous system (CNS). We now provide evidence that the same peptides can be found in different organs related to the control of a single function, and speculate on the possibility that this reflects a common neuroendocrine programming. Endogenous opioid peptides (EOP) inhibit the reproductive function acting via the CNS. EOP inhibit gonadotropin secretion in rodents and humans via inhibition of GnRH release and have direct inhibitory actions at the pituitary level via specific binding sites on the gonadotrophs. However, EOP can also be synthesized in the testis and in different compartments of the male genital tract. Several findings indicate that EOP of the reproductive tract have a local, paracrine role. These include: (1) the detection of significant beta-endorphin (beta-EP) production by rat Leydig cells (Lc) in cultures; (2) the hormonal regulation of Lc beta-EP production by positive (gonadotropins) and negative (steroids, glucocorticoids, GnRH) factors; (3) the presence of opioid binding sites (Kd in the nanomolar range) in tubular homogenates and Sertoli cells (Sc) in culture of adult and immature rat testes; (4) the inhibition of basal and FSH-stimulated ABP production by Sc in culture when chronically exposed to beta-EP treatment; (5) the detection of high levels of beta-EP and met-enkephalin in human semen with values 6-12 times higher than in plasma; (6) the evidence for inhibitory functions of seminal opioids on sperm motility, vas deferens muscle contraction and partner immune system. Thus the same peptides, i.e. EOP, may control the reproductive function at multiple sites, operating as a multimessenger system in which the central and peripheral level are unified by the common chemical and inhibitory nature of the message.

Is ascites caused by impaired hepatic inactivation of blood borne endogenous opioid peptides?

Methionine enkephalin and catecholamines were measured in carefully collected plasma samples from 25 patients with cirrhosis and ascites, and 25 with cirrhosis without ascites, 15 disease and 15 healthy controls. Methionine enkephalin was invariably raised in the ascites group, the median value being 4.6-6.9 times that of the other three groups. Similarly, in the ascites group, median noradrenaline was increased 2.5-4.2 and median adrenaline 1.8-2.5 times that of the other groups. Plasma methionine enkephalin is considerably raised in patients with cirrhotic ascites and has actions which could enable it to be an initiating factor of ascites formation.

Opioid receptors in rat neostriatum: radioautographic distribution at the electron microscopic level

The distribution of mu-opioid receptors, selectively labeled in vitro with a monoiodinated Met-enkephalin analog [( 125I]FK 33-824), was analyzed by light and electron microscopic radioautography in sections from the neostriatum of the rat. In the light microscope, patches of high receptor densities were detected amidst a moderately labeled matrix. The number of silver grains, as counted in 1-micron thick plastic-embedded sections, was 3 times greater inside the patches than in the intervening matrix. In both compartments, the proportion of labeled binding sites associated with the neuropil was significantly higher (greater than 70%) than that associated with nerve cell bodies or myelinated fascicles. Quantitative analyses of electron microscopic radioautographs revealed that the majority of silver grains corresponding to specifically bound [125I]FK molecules originated from radioactive sources associated with apposed neuronal membranes. Of the total number of specific binding sites, 53% was associated with axodendritic, 18% with axoaxonic and 3% with axosomatic interfaces. The occurrence of multiple labeled foci along the plasma membrane of certain perikarya and dendrites suggested that some of the binding sites might be associated with somato/dendritic elements. The high incidence of labeling along axoaxonic interfaces indicated that others were linked to the membrane of axons and/or axon terminals. A major finding of the present study was that only a small proportion of specific FK binding sites (7% of total) was associated with synaptic junctions. Labeled synapses were primarily of the asymmetric type and were found predominantly on dendritic branches and spines. A few were observed on nerve cell bodies. Labeled symmetric synapses were rare and encountered exclusively on dendritic branches. The high frequency with which specifically labeled binding sites were found to be associated with neuronal interfaces involving axonal processes strongly suggests that even if non-junctional these binding sites correspond to functional receptors. Whether these receptors are activated by endogenous ligand molecules released by the labeled terminals themselves or from terminals located at a distance from the labeled interfaces remains to be determined.

Naloxone and the ventilatory response to exercise in man

Endogenous opiate peptides are known to exert a depressant action on ventilation (VE), and their plasma levels have been shown to be elevated during a variety of exercise protocols. We investigated whether they might modulate the control of the hyperpnea of short-term constant-load (CLE) and incremental (IE) cycle-ergometer exercise. Four healthy subjects performed CLE tests at ca. 80% of the anaerobic threshold (theta an) for 5 min following a period of unloaded pedaling, and IE tests (10 or 20 W min-1) to the limit of tolerance. Normal saline (3 ml) or the opiate antagonist naloxone (1.2 mg in 3 ml) were administered intravenously prior to each test. Naloxone elicited no discernible effect on VE, alveolar gas tensions, or heart rate throughout the entire range of work rates; neither were theta an nor the maximum work rate affected. It is concluded that, for short-term exercise ranging in intensity from moderate to severe, the role played by endogenous opiate peptides in the control of the exercise hyperpnea appears to be negligible in man.

Effect of naloxone on cerebral perfusion and cardiac performance during experimental cerebral ischemia

Transient global cerebral ischemia (TGI) was induced in awake rats using the "four-vessel" occlusion model of Pulsinelli and Brierley. Blood pressure, arterial blood gases, cerebral blood flow, and cardiac output were measured during the acute (up to 2 hours) and chronic (2 to 72 hours) postischemic time periods. Coincident with the onset of TGI, cardiac output and caudate blood flow were depressed. The former returned to baseline within 30 minutes after the conclusion of TGI, and the latter progressed to hyperemia at 12 hours (81.8 +/- 4.9 vs 68.6 +/- 3.9 ml/min/100 gm tissue (mean +/- standard error of the mean] and oligemia at 72 hours (45.5 +/- 4.8 ml/min/100 gm tissue) post-TGI in the untreated control rats. Arterial blood gases and blood pressure were unchanged. Naloxone (1mg/kg) given at the time of TGI or as late as 60 minutes post-TGI and every 2 hours thereafter for 24 hours or bilateral cervical vagotomy prevented the depression in cardiac output and blocked the hyperemic-oligemic cerebral blood flow pattern that was predictive of stroke in this rat model. Changes in cardiac output after TGI in this model appear to be mediated by parasympathetic pathways to the heart from the brain stem. Opiate receptor blockade probably blocks endogenous opioid peptide stimulation of these brain-stem circulatory centers, which results in inhibition of parasympathetic activity and improvement in cardiac output. The usefulness of naloxone in the treatment of experimental stroke may be a function of its ability to improve cerebral perfusion in pressure-passive cerebrovascular territories. Variations in cardiac output during experimental stroke may explain the dissimilar responses to naloxone treatment reported by other investigators of experimental stroke.

Effects of naloxone on dental pain threshold following muscle exercise and low frequency transcutaneous nerve stimulation: a comparative study in man

Previous studies have shown that muscle exercise and low frequency transcutaneous nerve stimulation (TNS) give rise to an analgesic effect in humans and animals. Endorphin has been proposed to mediate this analgesia. In this investigation, the effect of muscle exercise and low frequency TNS, on dental pain thresholds was studied and the possible involvement of endorphinergic mechanisms was investigated using naloxone as an antagonist. Dental pain thresholds were measured in 11 volunteers following leg or arm exercise and after low frequency TNS of the hands or face. After exercise (20 min) or stimulation (30 min) either 0.8 mg naloxone (2 ml) or saline (2 ml) was injected i.v. in a double-blind fashion. Pain thresholds were measured repetitively before and after exercise or stimulation. Both leg and arm exercise increased pain threshold. Stimulation of the hands also increased pain threshold, but less than arm exercise. A marked increase in pain threshold was seen after face stimulation. These changes in pain threshold were unaffected following injections of either naloxone or saline, except for an early and short-lasting reduction when naloxone was injected following arm exercise. The increases in pain threshold following muscle exercise and after low frequency TNS, showed similarities suggesting that a common mechanism might be involved. The pain threshold increase after arm exercise could only be partially mediated by endorphinergic mechanisms.

Effects of high-dose and low-dose naloxone on plasma ACTH in patients with ACTH hypersecretion

The effect of a high (5.4 mg/h) and a low (0.8 mg/h) dose of naloxone (i.v. over a period of 90 min) on ACTH secretion was compared with placebo in patients with Addison's disease, congenital adrenal hyperplasia, Cushing's disease or Nelson's syndrome. In seven patients with primary adrenal insufficiency the high dose of naloxone provoked a significant increase of plasma ACTH concentrations (P less than 0.02) whereas the low dose of naloxone failed to influence ACTH secretion. In six patients with ACTH dependent Cushing's disease or Nelson's syndrome both doses failed to alter plasma ACTH levels. These results support the concept of inhibitory delta- or kappa-opiate receptors in the regulation of ACTH secretion. In patients with Cushing's disease or Nelson's syndrome ACTH secretion is insensitive to naloxone, presumably because of an autonomous pituitary adenoma or hypothalamic derangement.

Direct effects of catecholamines, thyrotropin-releasing hormone, and somatostatin on growth hormone and prolactin secretion from adenomatous and nonadenomatous human pituitary cells in culture

To determine the mechanism and the site of action of catecholamines as well as hormones including thyrotropin-releasing hormone (TRH)1 and somatostatin on pituitary hormone release in patients with acromegaly and in normal subjects, the effects of these substances on growth hormone (GH) and prolactin (PRL) secretion from adenomatous and nonadenomatous human pituitary cells in culture were examined. When dopamine (0.01-0.1 microM) or bromocriptine (0.01-0.1 microM) was added to the culture media, a significant inhibition of GH and PRL secretion from adenoma cells from acromegalic patients was observed. This inhibition was blocked by D2 receptor blockade with metoclopramide or sulpiride, but not by D1 receptor blockade. Similarly, dopamine suppressed GH and PRL release by nonadenomatous pituitary cells in a dose-dependent manner, which was again blocked by D2 receptor blockade. The minimum effective concentration of dopamine required for a significant inhibition of PRL secretion (0.01 microM) was lower than that for GH release (0.1 microM). Norepinephrine, likewise, caused a suppression of PRL secretion from adenomatous and nonadenomatous pituitary cells. This effect was blocked by sulpiride, phentolamine, however, was ineffective. When TRH was added to the media, both GH and PRL secretion were enhanced in adenoma cells, while only the stimulation of PRL release was observed in nonadenomatous pituitary cells. Coincubation of TRH and dopamine resulted in variable effects on GH and PRL secretion. Somatostatin consistently lowered GH and PRL secretion in both adenomatous and nonadenomatous pituitary cells and completely blocked the TRH-induced stimulation of GH and PRL secretion from adenoma cells. Opioid peptides (1 microM) failed to affect hormone release. These results suggest that no qualitative difference in GH and PRL responses to dopaminergic agonists or to somatostatin exists between adenoma cells of acromegalic patients and normal pituitary cells, and that the direct effect of catecholamines on GH and PRL secretion from human pituitary cells is mediated mainly through dopamine receptor activation.

Brain opiates and neuroendocrine function

Opioid peptides are found throughout the central nervous system, and have profound effects on neuroendocrine function. In man, exogenous opiates and opioids elevate circulating prolactin, GH and TSH, and suppress the release of the gonadotrophins and pro-opiocortin-related peptides. However, unlike in other species, there is substantial evidence for a physiological role of endogenous opioids only in the case of the gonadotrophins and ACTH/LPH. Most evidence suggests that LH and FSH are modulated via the hypothalamus or amygdala, where concentrations of opioids and opioid receptors are very high. Endogenous opioids appear to be principally concerned with the frequency-modulated release of GnRH, and this may be important clinically in patients presenting with amenorrhoea. ACTH/LPH are under tonic inhibition by endogenous opioids acting at hypothalamic and/or pituitary levels, and changes in this inhibition may be responsible for the release of these peptides in response to certain forms of stress. It has been reported that the opiate antagonist, naloxone, is clinically useful in paradoxically inhibiting the release of ACTH in patients with Nelson's syndrome, but this requires adequate confirmation. Vasopressin is under biphasic opiate control, but the principal effect is probably opiate-mediated inhibition of vasopressin release. The endogenous ligand for this response is likely to be dynorphin. Suppression of vasopressin release by opiates may become a useful therapy in the treatment of the 'Syndrome of inappropriate ADH'.

The hypothalamic-pituitary-adrenal axis

Anterior pituitary corticotrophin cells secrete ACTH as part of a larger precursor molecule, pro-opiomelanocortin. Post-translational cleavage of this precursor yields three major peptides: ACTH, beta-LPH and N-POMC. Experiments both in vivo and in vitro suggest that N-POMC may act as a prohormone amplifier for ACTH-induced adrenal steroidogenesis and as regulator of adrenocortical cell growth. The secretion of POMC is under the control of CRF. These findings are discussed in relation to the pathophysiology of corticotrophinoma. The primary defect in this condition appears to reside at the level of the anterior pituitary cell and is readily amenable to treatment by trans-sphenoidal microsurgery. The estimation of plasma ACTH concentrations is proving useful in the monitoring of various clinical conditions including Addison's disease and congenital adrenal hyperplasia.