Beta-endorphin and corticotropin immunoreactivity and specific binding in the neuromuscular system of obese-diabetic mice

The Clash of Two Epidemics: the Relationship Between Opioids and Glucose Metabolism

PURPOSE OF REVIEW

We are currently in the midst of a global opioid epidemic. Opioids affect many physiological processes, but one side effect that is not often taken into consideration is the opioid-induced alteration in blood glucose levels.

RECENT FINDINGS

This review shows that the vast majority of studies report that opioid stimulation increases blood glucose levels. In addition, plasma levels of the endogenous opioid β-endorphin rise in response to low blood glucose. In contrast, in hyperglycaemic baseline conditions such as in patients with type 2 diabetes mellitus (T2DM), opioid stimulation lowers blood glucose levels. Furthermore, obesity itself alters sensitivity to opioids, changes opioid receptor expression and increases plasma β-endorphin levels. Thus, opioid stimulation can have various side effects on glycaemia that should be taken into consideration upon prescribing opioid-based medication, and more research is needed to unravel the interaction between obesity, glycaemia and opioid use.

Antihyperglycaemic action of diosmin, a citrus flavonoid, is induced through endogenous β-endorphin in type I-like diabetic rats

Diosmin is one of the flavonoids contained in citrus and has been demonstrated to improve glucose metabolism in diabetic disorders. However, the mechanism(s) of diosmin in glucose regulation remain obscure. Therefore, we investigated the potential mechanism(s) for the antihyperglycaemic action of diosmin in streptozotocin-induced diabetic rats (STZ-diabetic rats). Diosmin lowered hyperglycaemia in a dose-dependent manner in STZ-diabetic rats. This action was inhibited by naloxone at a dose sufficient to block opioid receptors. Additionally, we determined the changes in plasma β-endorphin-like immunoreactivity (BER) using enzyme-linked immunosorbent assay (ELISA). Diosmin also increased BER dose-dependently in the same manner. Repeated treatment of STZ-diabetic rats with diosmin for 1 week resulted in an increase in the expression of the glucose transporter subtype 4 (GLUT 4) in the soleus muscle and a reduction in the expression of phosphoenolpyruvate carboxykinase (PEPCK) in the liver. These effects were also inhibited by naloxone at a dose sufficient to block opioid receptors. Bilateral adrenalectomy in STZ-diabetic rats eliminated the actions of diosmin, including both the reduction in hyperglycemia and the elevation of plasma BER. In conclusion, our results suggest that diosmin may act on the adrenal glands to enhance the secretion of β-endorphin, which can stimulate the opioid receptors to attenuate hepatic gluconeogenesis and increase glucose uptake in soleus muscle, resulting in reduced hyperglycemia in STZ-diabetic rats.

Antihyperglycemic action of rhodiola-aqeous extract in type1-like diabetic rats

Background

Rhodiola rosea (Rhodiola) is a plant in the Crassulaceae family that grows in cold regions of the world. It is mainly used in clinics as an adaptogen. Recently, it has been mentioned that Rhodiola increases plasma β-endorphin to lower blood pressure. Thus, the present study aims to investigate the antidiabetic action of Rhodiola in relation to opioids in streptozotocin-induced diabetic rats (STZ-diabetic rats).

Methods

In the present study, the plasma glucose was analyzed with glucose oxidase method, and the determination of plasma β-endorphin was carried out using a commercially available enzyme-linked immunosorbent assay. The adrenalectomy of STZ-diabetic rats was used to evaluate the role of β-endorphin. In addition, quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and western blotting analysis were performed to investigate mRNA and protein expressions.

Results

Rhodiola-water extract dose-dependently lowered the plasma glucose in STZ-diabetic rats and this action was reversed by blockade of opioid μ-receptors using cyprodime. An increase of plasma β-endorphin by rhodiola-water extract was also observed in same manner. The plasma glucose lowering action of rhodiola-water extract was attenuated in bilateral adrenalectomized rats. In addition, continuous administration of rhodiola-water extract for 3 days in STZ-diabetic rats resulted in an increased expression of glucose transporter subtype 4 (GLUT 4) in skeletal muscle and a marked reduction of phosphoenolpyruvate carboxykinase (PEPCK) expression in liver. These effects were also reversed by blockade of opioid μ-receptors.

Conclusions

Taken together, rhodiola-water extract improves hyperglycemia via an increase of β-endorphin secretion from adrenal gland to activate opioid μ-receptors in STZ-diabetic rats.

Plasma glucose lowering mechanisms of catalpol, an active principle from roots of Rehmannia glutinosa, in streptozotocin-induced diabetic rats

Catalpol is one of the active principles from roots of Rehmannia glutinosa Steud (Scrophulariaceae) that is widely used to treat diabetic disorders in Chinese traditional medicine using the name of Di-Huang, which is used to investigate the mechanisms for lowering of plasma glucose in streptozotocin-induced diabetic rats (STZ-diabetic rats). Catalpol decreased plasma glucose in a dose-related manner, and this action was reduced by pretreatment with naloxone or naloxonazine. An increase of plasma β-endorphin by catalpol was also observed in parallel. The plasma glucose lowering action of catalpol was deleted in bilateral adrenalectomized rats. Moreover, catalpol enhanced β-endorphin release from the isolated adrenal medulla of STZ-diabetic rats. Otherwise, plasma glucose lowering action of catalpol failed to produce in opioid μ-receptor knockout mice. Also, repeated administration of catalpol for 3 days in STZ-diabetic rats resulted in a marked reduction of phosphoenolpyruvate carboxykinase (PEPCK) expression in liver and an increased expression of glucose transporter subtype 4 (GLUT 4) in skeletal muscle. These effects were also reversed by blockade of opioid μ-receptors. Our results suggested that catalpol increased glucose utilization through increase of β-endorphin secretion from adrenal gland in STZ-diabetic rats.

Mediation of Endogenous β-Endorphin in the Plasma Glucose-Lowering Action of Herbal Products Observed in Type 1-Like Diabetic Rats

Recently, there have been advances in the development of new substances effective in managing diabetic disorders. Opioid receptors couple multiple systems to result in various biological effects, although opioids are best known for analgesia. In the present review, we used our recent data to describe the advance in plasma glucose-lowering action of herbal products, especially the mediation of β-endorphin in glucose homeostasis of insulin-deficient diabetes. In type 1-like streptozotocin-induced diabetic rats, we identified many products purified from herbs that show a dose-dependent plasma glucose-lowering action. Increase in β-endorphin secretion from the adrenal gland may activate peripheral opioid μ-receptors (MOR) to enhance the expression of muscle glucose transporters and/or to reduce hepatic gluconeogenesis at the gene level, thereby leading to improved glucose utilization in peripheral tissues for amelioration of severe hyperglycemia. It has also been observed that stimulation of α(1)-adrenoceptors (α(1)-ARs) in the adrenal gland by some herbal products is responsible for the increase in β-endorphin secretion via a phospholipase C-protein kinase dependent pathway. However, an increase in β-endorphin secretion from the adrenal gland by herbal products can function via another receptor. New insights into the mediation of endogenous β-endorphin activation of peripheral MOR by herbal products for regulation of glucose homeostasis without the presence of insulin have been established. Therefore, an increase in β-endorphin secretion and/or direct stimulation of peripheral MOR via an insulin-independent action might serve as the potential target for development of a therapeutic agent or promising adjuvant in intensive plasma glucose control.

Mediation of beta-endorphin in andrographolide-induced plasma glucose-lowering action in type I diabetes-like animals

In the present study, we investigated the mechanism(s) for glucose-lowering action of andrographolide in streptozotocin-induced diabetic rats (STZ-diabetic rats). Andrographolide lowered plasma glucose concentrations in a dose-dependent manner and increased plasma beta-endorphin-like immunoreactivity (BER) dose-dependently in diabetic rats. Both of these responses to andrographolide were abolished by the pretreatment of animals with prazosin or N-(2-(2-cyclopropylmethoxy) ethyl) 5-choro-alpha-dimethyl-1H-indole-3-thylamine (RS17053) at doses sufficient to block alpha1-adrenoceptors (ARs). Also, andrographolide enhanced BER release from isolated rat adrenal medulla in a concentration-related manner that could be abolished by alpha1-ARs antagonists. Bilateral adrenalectomy in STZ-diabetic rats eliminated the activities of andrographolide, including the plasma glucose-lowering effect and the plasma BER-elevating effect. Andrographolide failed to lower plasma glucose in the presence of opioid micro-receptor antagonists and in the opioid micro-receptor knockout diabetic mice. Treatment of STZ-diabetic rats with andrographolide resulted in the reduced expression of phosphoenolpyruvate carboxykinase (PEPCK) in liver and an increased expression of the glucose transporter subtype 4 (GLUT 4) in soleus muscle. These effects were also blocked by opioid micro-receptor antagonists. In conclusion, our results suggest that andrographolide may activate alpha1-ARs to enhance the secretion of beta-endorphin which can stimulate the opioid micro-receptors to reduce hepatic gluconeogenesis and to enhance the glucose uptake in soleus muscle, resulting in a decrease of plasma glucose in STZ-diabetic rats. However, the roles of other endogenous opioid peptides or the mixture of several opioid peptides in the activation of opioid micro-receptors associated with the plasma glucose-lowering action of andrographolide, should be considered and need more investigation in the future.

Cell signaling and trafficking of human melanocortin receptors in real time using two-photon fluorescence and confocal laser microscopy: differentiation of agonists and antagonists

Melanocortin hormones and neurotransmitters regulate a vast array of physiologic processes by interacting with five G-protein-coupled melanocortin receptor types. In the present study, we have systematically studied the regulation of individual human melanocortin receptor wild subtypes using a synthetic rhodamine-labeled human melanotropin agonist and antagonist, arrestins fused to green fluorescent protein in conjunction with two-photon fluorescence laser scanning microscopy and confocal microscopy. Stimulation of the melanocortin receptors by its cognate agonist triggered rapid arrestin recruitment and receptor internalization for all four human melanocortin receptors examined. Antagonists-bound melanocortin receptors, on the other hand, did not recruit beta-arrestins, and remained in the cell membrane even after long-term (30 min) treatment. Agonist-mediated internalization of all melanocortin receptor subtypes was sensitive to inhibitors of clathrin-dependent endocytosis, but not to caveolae inhibitors. In summary, agonist-mediated internalization of all subtypes of melanocortin receptors are dependent upon beta-arrestin-mediated clathrin-coated pits, whereas, beta-arrestin-2 conjugated green fluorescence protein (beta-arrestin-2-GFP) recruitment is not dependent on protein kinase A activation. Real time two-photon fluorescence laser scanning microscopy is a most powerful tool to study the dynamic processes in living cells and tissues, without inflicting significant and often lethal damage to the specimen.

Chronic stimulation of the peroneal nerve in rats upregulates the pro-opiomelanocortin gene in spinal motoneurones

Continuous unilateral stimulation of the peroneal nerve in rats for 8 h per day for 2 or 7 days caused significant increases in POMC mRNA and beta-endorphin immunoreactivity in both ipsilateral and contralateral motoneurones. Intermittent stimulation, for 10-min periods with 90-min rest periods, for 8 h per day for 2 days also caused upregulation of POMC mRNA. It is suggested that expression of POMC-derived peptides in motoneurones may be important for maintaining muscle contractile function.

David and Goliath - the slingshot that started the neuropeptide revolution

This review in honor of David de Wied summarizes the work done in my laboratory that first indicated that adrenocorticotropic hormone (ACTH) has a direct effect on the neuromuscular system. Cold stress or ACTH and its related peptides alpha-melanocyte-stimulating hormone (alpha-MSH ) and beta-lipotropin improve the electromechanical characteristics of adrenalectomized and hypophysectomized rats. ACTH-(1-39) accelerates the return of motor and sensory function and improves the morphological characteristics of the motor endplate after peripheral nerve crush. The non-corticotropic fragments ACTH-(4-10), alpha-MSH, the ACTH-(4-9) analogue Organon 2766 (Org 2766) or the ACTH-(4-10) analogue Biomeasure 22015 (BIM 22015) improve electrophysiological and morphological parameters of the regenerating neuromuscular system. ACTH-(4-10) immunoreactivity, present in ventral horn motor neurons in low levels, is decreased ipsilaterally following ipsilateral nerve crush but increases both ipsilaterally and contralaterally if injured animals are treated with ACTH-(4-10) indicating a neuroprotective action. Similarly, Org 2766 appears to have a protective action in the brain following nigrostriatal lesions. In developmental studies, perinatal exposure to ACTH peptides improves the structure of the neuromuscular junction, accelerates the maturation of electromechanical properties and enhances nerve-muscle integration and nerve regeneration. Perinatal exposure to these peptides decreases adult male sexual behavior, a change correlated with increased serotinergic input within the medial preoptic area. Similar changes occur in female rats and appear to be long-lasting. In tissue culture studies, both Org 2766 and BIM 22015 promote neurite outgrowth in the absence of nerve growth factor, indicating a neurotrophic role for these peptides.

NMDA and AMPA glutamate receptor subtypes in the thoracic spinal cord in lean and obese-diabetic ob/ob mice

Quantitative autoradiography was used to characterise the binding of selective radiolabelled antagonists for the N-methyl-D-aspartate (NMDA) receptor and the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor in the dorsal, intermediate and ventral subregions of the grey matter of the upper thoracic spinal cord in male and female lean and obese-diabetic (ob/ob) mice. The density of binding sites for both receptor subtypes was greater in diabetic mice, in all three subregions of the grey matter, than the corresponding subregions in the lean mice. The affinity of the binding site for the NMDA antagonist was significantly higher in obese mice than lean mice, consistent with the presence of two subpopulations of NMDA receptors with different ligand binding affinities in obese mice. The increase in expression of the glutamate receptor subtypes, and altered ligand affinity for the NMDA receptor subtype in the obese mice may be causally involved in the peripheral neuropathies which can accompany diabetes mellitus.

Expression of beta-endorphin and its receptors in the spinal cord of obese-diabetic ob/ob mice

Immunocytochemistry was used to demonstrate the presence of beta-endorphin, and quantitative autoradiography with [125I]beta-endorphin was used to study beta-endorphin binding sites, in spinal cord of lean and obese diabetic ob/ob mice. The proportion of beta-endorphin-positive neurones was approximately 6-fold higher in the ventral horn, and 2-fold higher in the dorsal horn of ob/ob mice than in lean controls. The maximum density of beta-endorphin binding sites was significantly higher in the dorsal horn and intermediate zone of ob/ob mice. The Kd value for the binding was similar in the ventral horn and intermediate zone in lean and ob/ob mice, but slightly lower in the dorsal horn of ob/ob mice. The findings indicate upregulation of both beta-endorphin and its receptors in spinal neurones of ob/ob mice.

Effect of beta-endorphin C-terminal peptides on glucose uptake in isolated skeletal muscles of the mouse

The uptake of a nonmetabolizable derivative of glucose, [3H]2-deoxy-D-glucose was examined in isolated slow (soleus) and fast (extensor digitorum longus, EDL) muscles of adult mice. An analogue of beta-endorphin (28-31), Ac-Lys-D-Lys-Sar-Glu, which is stable to proteolytic digestion, enhanced the uptake of glucose into the slow and fast muscles. The muscles of male mice were more sensitive to the peptide than those of female mice. The maximum uptake seen in the presence of the peptide was similar to that seen with insulin in the soleus muscle and greater than that seen with insulin in the EDL muscle.

Distribution of opioid peptide receptors in muscles of lean and obese-diabetic mice

Autoradiography was used to demonstrate beta-endorphin and delta-opioid receptors in muscles of normal and obese-diabetic mice. The density of the receptors was significantly higher in the obese-diabetic mice. In both normal and diabetic mice, glycolytic and oxidative fibers exhibited the beta-endorphin receptors. However, a significantly greater density of beta-endorphin receptors was observed in the extensor digitorum longus muscles than in the soleus muscles in the diabetic mice. In normal muscles the beta-endorphin receptors were largely restricted to regions where endplates were present, but in the obese-diabetic mice they were densely distributed along the length of the muscle fibers.

Effect of beta-endorphin on the contractile responses in mouse skeletal muscle

Tension development in response to direct and indirect electrical stimulation was studied in an isolated phrenic nerve hemidiaphragm preparation of the mouse. beta-Endorphin (beta-EP) caused an increase in the amplitude and a decrease in the time to peak of muscle contractions in response to low frequency stimulation of the nerve. Upon direct stimulation of the muscle the peptide had no effect. The actions of beta-EP were abolished in the presence of the opioid antagonist naloxone and mimicked by delta opioid agonists. Upon high frequency stimulation of the nerve, beta-EP caused an increase in the initial, maximum, and mean tension. It also prevented the fall in the final tension seen in the control preparations with repeated periods of stimulation. The findings are consistent with beta-EP having a role to improve neuromuscular function and delay fatigue, and indicate the possible therapeutic potential of opioid substances in conditions where muscle weakness is present.

Delta-opioid peptide receptors in muscles from obese diabetic and normal mice

Autoradiography was used to study the opioid receptors in soleus and extensor digitorum longus (EDL) muscles from normal mice and mice with type II diabetes. Binding sites for [125I]beta-endorphin were present on the surface membranes in muscles from normal mice. The density of receptors was higher in muscles from obese diabetic mice. The specific delta-opioid ligands DPDPE and [D-Ala2]deltorphin-II inhibited the binding of [125I]beta-endorphin whereas mu and kappa agonists did not. Therefore, the opioid receptor present in skeletal muscle fibers of the mouse is of the delta subtype and the number of these receptors is increased in type II diabetes in the mouse.

Endogenous opiates: 1992

This paper is the fifteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1992 involving the behavioral, non-analgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

Melanotropins as growth factors

Peptides that regulate the growth of tissues, whether in a positive or negative manner, are termed growth factors. The melanocortins, neurotrophic sequences that correspond to peptide fragments contained within ACTH-(1-13), beneficially affect neural growth during development and regeneration. Analogues of ACTH-(4-9) (Org 2766) and ACTH-(4-10) (BIM 22015) are capable of sustaining neurite outgrowth from cultured dorsal root ganglion and spinal cord cells in the absence of nerve growth factor. The development of sexually dimorphic behavior in both male and female rats is influenced by perinatal administration of ACTH. This change appears to be correlated with changes in the growth and metabolism of developing serotonergic and dopaminergic systems in the hypothalamic nuclei associated with male and female sexual behavior. Similar melanotropic influences are found in the developing neuromuscular system. Neuromuscular development is accelerated by perinatal administration of melanocortins, provoking both nerve and muscle to attain early maturation. However, the responding tissue varies pivotally with age: early in gestation, embryonic muscle is acutely sensitive to peptide exposure; but once innervation has occurred, only the developing nerve reacts to melanocortin treatment. Melanocortins have little if any effect on the normal, adult neuromuscular system. Following peripheral nerve injury or pathology, melanotropins once again become effective growth factors, accelerating and enhancing nerve regeneration and muscle reinnervation. Electrophysiological, morphological, biochemical, and functional tests all indicate that ACTH-(4-10), Org 2766, BIM 22015, and alpha-MSH improve various facets of nerve regeneration, the degree to which the specific parameter is improved being dependent on the peptide fragment, its dosage, and pattern of administration. BIM 22015, while less effective as a neurotrophic factor, has potent myotrophic effects that the other peptides lack. Org 2766 may provide some protective action to the injured CNS as demonstrated by tests of cognitive function following brain lesions, although evaluation of recovery is sometimes enigmatic. Recovery from destruction of the nigrostriatal system is more easily measured through tests of motor function and open field behavior, both of which support a protective role for Org 2766. Compensatory mechanisms, including the presence of increased tyrosine hydroxylase and greater density of dopaminergic fibers, may be involved. Melanocortins are effective growth factors in sciatic nerve regeneration in neonatal rats. Both alpha-MSH and ACTH-(4-10) favor the formation of morphologically normal end plates despite the trauma following nerve crush at postnatal day 2.(ABSTRACT TRUNCATED AT 400 WORDS)

Pro-opiomelanocortin neuropeptide receptors on developing and dystrophic muscle fibers

Autoradiography was used to investigate the presence of corticotropin and beta-endorphin receptors on soleus, extensor digitorum longus, and diaphragm muscles of normal and dystrophic adult mice and developing mice. In both adult and developing mice, specific binding sites for both [125I]ACTH and [125I]beta-endorphin were present in some fibers in all of the muscles examined. The specific binding sites appeared to be distributed over the length of the surface membrane in the fibers that expressed them. There were significantly higher proportions of fibers exhibiting the specific beta-endorphin and the specific ACTH binding sites in the three muscle types in mice of 5 d of age compared to the muscles of the normal adult. There was also a higher proportion of fibers with the putative ACTH and beta-endorphin receptors in the three muscle types in dystrophic mice.

Effect of IDPN on the expression of POMC-derived peptides in rat motoneurones

Immunocytochemistry was used to detect beta-endorphin and alpha-melanotropin (alpha-MSH) in lumbar spinal motoneurones in rats treated with beta,beta'-iminodiproprionitrile (IDPN), a neurotoxicant that targets motoneurones or corn oil, which has no known neurotoxicity. After IDPN treatment most of the motoneurones were immunoreactive for both peptides but after corn oil treatment immunostaining was negligible. It is suggested that increased expression of the POMC-derived peptides may be part of the regenerative repertoire of the damaged motoneurone regardless of the cause of the lesion. Alternatively the peptides may simply accumulate in the motoneurones as a result of impaired axoplasmic transport.