Lipotropin, melanotropin and endorphin: in vivo catabolism and entry into cerebrospinal fluid

Release of endogenous opioids from duodenal enteroendocrine cells requires Trpm5

BACKGROUND & AIMS

Enteroendocrine cells, the largest and most diverse population of mammalian endocrine cells, comprise a number of different cell types in the gut mucosa that produce, store, and secrete small molecules, peptides, and/or larger proteins that regulate many aspects of gut physiology. Little is known about less typical endocrine cells in the intestinal mucosa that do not contain secretory granules, such as brush or caveolated cells. We studied a subset of these enteroendocrine cells in duodenum that produce several peptides, including endogenous opioids, and that also express the Trpm5 cation channel.

METHODS

We studied expression patterns of Trpm5 and other molecules by immunohistochemical and enzyme-linked immunosorbent assay analyses of intestinal tissues from transgenic mice that express green fluorescent protein from the Trpm5 promoter, as well as wild-type and Trpm5-null mice.

RESULTS

We describe a type of enteroendocrine cell in mouse duodenum that is defined by the presence of Trpm5 and that does not contain typical secretory granules yet expresses endogenous opioids (beta-endorphin and Met-enkephalin) and uroguanylin in apical compartments close to the lumen of the gut.

CONCLUSIONS

Solitary chemosensory cells that coexpress beta-endorphin, Met-enkephalin, uroguanylin, and Trpm5 exist in mouse duodenum. These cells are likely to secrete the bioactive peptides into the intestinal lumen in response to dietary factors; release of the opioid peptides requires the Trpm5 ion channel.

No significant change of plasma beta-endorphin levels of psoriasis patients after synchronous balneophototherapy

BACKGROUND/PURPOSE

Previous studies suggested that beta-endorphin has a pathogenic role in psoriasis: its increased plasma concentration may play a role in the neuroimmunological processes in the pathomechanism of the disease, and plasma beta-endorphin levels should reflect the changes in the patients' skin status. The purpose of this study was to investigate the changes of peripheral blood beta-endorphin levels in psoriatic patients in conjunction with changes in their skin symptoms after synchronous balneophototherapy.

METHODS

With synchronous balneophototherapy, 12 patients with extended skin symptoms of psoriasis were treated. The therapy followed the Regensburg protocol, consisting of a basic course of 35 sessions. Patients' skin status was characterized by evaluating the Psoriasis Area and Severity Index score before and after the therapy course. Blood samples were taken before treatment, and 1 day after the last session, with symptom-free skin. Plasma beta-endorphin levels were measured by a specific radioimmunoassay developed by the authors.

RESULTS

There was no significant change in plasma levels of beta-endorphin after clinical clearance of psoriatic skin symptoms.

CONCLUSION

In this non-randomized, uncontrolled study no significant difference could be detected between plasma beta-endorphin levels before and after a basic course of synchronous balneophototherapy in patients with psoriasis. Although beta-endorphin has many neuroimmunological effects, the changes of its plasma level do not consistently reflect the skin status. Inflammation in psoriatic skin lesions is probably not mediated directly by circulating beta-endorphin.

The role of hypophyseal and adrenal mechanisms in the hypoalgesic response to non-contingent food delivery in the rat

Exposure to non-contingent food delivery has been shown to elicit an increase in nociceptive thresholds in rats. This hypoalgesia has previously been shown to be reversible by the opiate antagonist naloxone. Given that most opioid forms of SIA are found to rely on an intact pituitary-adrenal axis, this research examined the possibility that the hypoalgesic response to non-contingent food delivery is also hormonally mediated. Hypophysectomy (Expt. 1) but not bilateral adrenalectomy (Expt. 2) was found to completely attenuate the hypoalgesic response to non-contingent food delivery. Preliminary data from ongoing research is presented that points to the possible involvement of the hypophyseal peptides beta-endorphin and adrenocorticotropin hormone (ACTH).

Effects of captopril on opioid peptides during exercise and quality of life in normal subjects

In a placebo-controlled, randomized, crossover, double-blind study of 17 normal volunteers, we examined the effects of captopril on the concentration of opioid peptides during bicycle exercise and on quality of life after a 2-week treatment period. Two exercise tests (progressive exercise and constant work rate exercise) were performed. Maximum oxygen uptake and blood lactate concentrations were measured in progressive exercise tests. The exercise intensities corresponding to a 1/2 lactate threshold, a lactate threshold, and a 4 mmol/L lactate concentration were determined. Constant work rate exercise at selected work loads for 20 minutes was carried out to measure the concentrations of opioid peptides and other hormones. Quality of life was assessed after the 2-week treatment period. Captopril treatment had no effect on the exercise response of blood pressure, heart rate, maximum VO2, and maximum work loads. The plasma concentrations of lactate, epinephrine, norepinephrine, and aldosterone increased during exercise and captopril did not change them. Beta-endorphin levels and plasma renin activity also increased during exercise, and the increases were greater with captopril treatment. Met-enkephalin and leu-enkephalin concentrations did not increase during exercise. According to responses in the quality of life questionnaires, administration of captopril improved the physiologic state more than the placebo did. These findings suggest that captopril may act on the central nervous system involving an increase in the beta-endorphin level.

An adrenal-mediated, naloxone-reversible increase in reaction time in the tail-flick test following intrathecal administration of substance P at the lower thoracic spinal level in the rat

We have shown previously that intrathecal administration of substance P to the lower thoracic vertebral level increases sympathetic output and increases the adrenal output of catecholamines. As opioid peptides are co-released with catecholamines from the adrenal medullae, experiments were done to determine whether the intrathecal administration of substance P to the eighth thoracic vertebral level would alter reaction time in the tail-flick test. Intrathecal administration of substance P (6.5 nmoles in artificial cerebrospinal fluid) in the awake restrained rat increased the reaction time at 1 and 6 min after injection to about 130-140% of the preadministration values; reaction time returned to preadministration values by 11 min. Similar administration of cerebrospinal fluid was without effect. Administration of 6.5 nmoles of thyrotropin-releasing hormone or oxytocin, peptides which also increase sympathetic output, failed to mimic the effects of substance P. The substance P-induced increase in reaction time was absent in rats which had been medullectomized and in rats pretreated with naloxone (10 mg/kg). Pretreatment with 10 mg/kg of either phentolamine or the quaternary opiate antagonists, nalorphine methochloride and naloxone methobromide, had no effect on the substance P-induced increase in reaction time. These results suggest that substance P given intrathecally to the eighth thoracic vertebral level may activate spinal sympathetic neurons to the adrenal medullae to cause the release of an opioid into the circulation. This circulating opioid may in turn play a role in the regulation of the tail-flick reflex by a centrally-mediated depression.

beta-Endorphin levels in the cerebrospinal fluid of male talapoin monkeys in social groups related to dominance status and the luteinizing hormone response to naloxone

beta-Endorphin-like immunoreactivity was measured in the cerebrospinal fluid of 20 male talapoin monkeys living in mixed-sex social groups. It was shown that beta-endorphin was the major immunoreactive peptide; there was no evidence for high molecular weight precursors, or for either N-acetyl or C-shortened metabolites. Dominant males (those at the top of the social hierarchy) had lower levels of beta-endorphin than those of intermediate rank; subordinate males had higher levels than either of the other two ranks--about three times those measured in dominants. There were significant negative correlations between beta-endorphin in cerebrospinal fluid and both the amount of aggression given and sexual behaviour shown towards females. The response of the hypothalamo-pituitary system to opiate blockade was tested by giving the males naloxone in doses of 0.125, 0.25, 0.5, 1.0 and 5.0 mg/kg and assaying serum levels of luteinizing hormone 20 min later. Dominant males released significant amounts of luteinizing hormone at doses of 0.25 and higher; there was no release in either intermediate or subordinate monkeys at any dose. These findings show that an animal's rank in the social group in which it lives is strongly correlated with beta-endorphin levels in the cerebrospinal fluid, and with changes in the neuroendocrine response to opiate blockade. Altered opiate neural activity may be responsible for the depressed levels of sexual behaviour and gonadal function observed in monkeys at the bottom of the hierarchy.

The in vitro and in vivo effects of enkephalins and beta-endorphin on ACE activity in mice

The in vitro and in vivo effects of naloxone (NAL) and endogenous opioids namely methionine and leucine enkephalins (MET-ENK, LEU-ENK) and beta-endorphin (BETA-END) on the brain and lung angiotensin converting enzyme (ACE) activities were investigated. All three peptides dose -dependently inhibited ACE activity in vitro except 10(-5) M concentration of BETA-END which increased the lung ACE activity. NAL which intensified the in vitro inhibitory effect of the used opioids showed an antagonistic effect on the in vivo suppressive effect of BETA-END on both brain and lung ACE activities whereas it had neither antagonistic nor synergistic effect on the in vivo inhibiting effect of MET-ENK and LEU-ENK on the lung ACE activity. The results were consistent with those obtained by using morphine and NAL. As a result the possible contributory action of the excessively released endogenous opioids to overcome shock via their inhibiting effect on ACE was discussed.

Permeability of the blood-brain barrier to neuropeptides: the case for penetration

Evidence that peptides can cross the blood-brain barrier (BBB) is reviewed. Penetration is suggested by the observations that blood levels correlate with cerebrospinal fluid levels for many peptides and that peripheral administration of peptides results in effects on the CNS. Passage is confirmed by experiments involving administration of a peptide (immunoactive or radioactive) in one compartment and identification of its appearance in the other, supported by such methods as selective labeling, cross-reactivity with highly specific antibodies, and chromatography. The degree of passage varies among peptides and their analogs. The major route of passage is probably by a non-competitive, non-saturable mechanism, wih the physicochemical characteristics of the peptide (e.g. lipophilicity, charge, molecular weight, and protein binding) determining the degree of passage. A competitive transport mechanism also exists for some peptides. Penetration of the BBB via large pores or by pinocytosis does not appear to be of major importance for peptides. Permeability of the BBB to peptides, but not to the larger iodinated albumin, is affected by intraperitoneal administration of aluminum, apparently by an increase in the permeability of the membrane to lipophilic materials.

Acupuncture relief of chronic pain syndrome correlates with increased plasma met-enkephalin concentrations

In twenty patients with chronic pain syndrome, acupuncture treatment resulted in significant improvement of both pain and psychiatric symptoms and higher plasma concentrations of metenkephalin. Plasma beta-endorphin concentrations were unchanged. The degree of symptom relief was correlated with the increase in plasma met-enkephalin.

CSF-plasma relationships for DSIP and some other neuropeptides

The relationships between CSF and plasma hormonal levels of several peptides were studied in the same samples of simultaneously obtained plasma and CSF. A significant correlation existed between CSF and plasma levels of DSIP as well as gastrin. Preliminary results also showed a correlation between CSF and plasma levels of NT, but not VIP or calcitonin. CSF/plasma ratios and the effect of BBB disruption also varied from peptide to peptide. These diverse CSF/plasma relationships are not easily explained by models of nonspecific passage but may indicate individual systems or axes that could be involved in the central effects of peripherally administered peptides.

Minireview. Peptides and the blood-brain barrier

Most neuropeptides are known to occur both in the central nervous system and in blood. This, as well as the occurrence of central nervous peptide effects after peripheral administration, show the importance of studying the relationships between the peptides in the two compartments. For many peptides, such as the enkephalins, TRH, somatostatin and MIF-1, poor penetration of the blood-brain barrier was shown. In other cases, including beta-endorphin and angiotensin, peptides are rapidly degraded during or just after their entry into brain or cerebrospinal fluid. Some peptides, such as insulin, delta-sleep-inducing peptide, and the lipotropin-derived peptides, enter the cerebrospinal fluid to a slight or moderate extent in the intact form. Many peptide hormones, such as insulin, calcitonin and angiotensin, act directly on receptors in the circumventricular organs, where the blood-brain barrier is absent. Oxytocin, vasopressin, MSH, and an MSH-analog alter the properties of the blood-brain barrier, which may result in altered nutritient supply to the brain. In conclusion, the diffusion of most peptides across the brain vascular endothelium seems to be severely restricted. There are, however, several alternative routes for peripheral peptides to act on the central nervous system. The blood-brain barrier is a major obstacle for the development of pharmaceutically useful peptides, as in the case of synthetic enkephalin-analogs.

Shuttlebox avoidance in rhesus monkeys: effects on plasma cortisol and beta-endorphin

Groups of monkeys either extensively pretrained to avoid shocks in a shuttlebox or with minimal prior experience were compared for plasma cortisol and beta-endorphin levels immediately following: (1) an exposure to the box with no shock, (2) the box providing repeated inescapable shocks or (3) a re-exposure to the box, again with no shock presentation. Mere exposure to the unfamiliar box elevated plasma cortisol just as much as exposure + shock did when inexperienced monkeys were tested. However, animals with a history of previously successful shock avoidance showed smaller elevations when exposed to the box alone, than they did when inescapable shock was received. Plasma beta-endorphin levels following shuttlebox exposure showed only a sporadic pattern of elevations in either inexperienced or pretrained monkeys. However, levels of beta-endorphin as determined under control conditions in the home cage were lower in pretrained animals, as were plasma levels of cortisol. The results indicate that behavioral factors may effect plasma cortisol and beta-endorphin following both acute and chronic shuttlebox experience.

Possible involvement of endorphin withdrawal or imbalance in specific premenstrual syndromes and postpartum depression

Premenstrual and postpartum dysphoric changes are very prevalent. However, their etiology is still obscure. The authors hypothesize that changes in levels of endorphins may be involved in the pathophysiology of these changes. Studies of various endorphins indicate a possible relationship between levels of endorphins and depressive symptoms. In addition, some studies of naloxone and naltrexone suggest a relationship between a blockage in the action of endorphins and the development of a syndrome of dysphoric symptoms similar to the depressive features manifested premenstrually and postpartum by many women and frequently seen in some depressed outpatients. There is also some evidence that there may be a relationship between elevated levels of endorphins and other subtypes of depressive syndromes. Endorphins and estrogen levels have been shown to covary. During the postpartum and the premenstrual period, levels of both change rapidly and substantially. Therefore the link between changes in levels of endorphins and the dysphoric changes during the periods in focus is supported from three complementary directions: (1) the characteristic psychiatric symptomatology, (2) the reported hormonal changes, and (3) the possible involvement of endorphins in neuroendocrine regulation.

Chemistry and biosynthesis of pro-opiomelanocortin. ACTH, MSH's, endorphins and their related peptides

Studies of lipotropins, melanotropins and endorphins on one hand, and of adrenocorticotropin on the other, has given rise to the concept of a multipotent precursor molecule recently renamed proopiomelanocortin. The preferential sites of cleavage of the precursor to produce its biologically active components are made of pairs of basic amino acid residues as described for the biosynthesis of beta-MSH and pro-insulin. Such structural feature is also found in other pro-hormone molecules. Pulse chase experiments and secretory studies carried out in both anterior and intermediate lobes of rat pituitary glands revealed the transformation of different forms of the precursor into different end-products, the anterior lobe producing preferentially ACTH and beta-LPH while the intermediate produces mainly the alpha-MSH and beta-endorphin. The multiple forms of precursors seem to differ in their carbohydrate content although at least two different gene products are still possible. The presence of similar peptides in the hypothalamus makes it highly probable that neuropeptides are biosynthesized with similar process. Thus the model of beta-LPH precursor, proposed as early as in 1967, is now applicable to the biosynthesis of all other neuropeptides. Major advances in this field are expected in the 1980s.

Evidence for homologous actions of pro-opiocortin products

alpha-Melanocyte-stimulating hormone (alpha-MSH), a modified fragment of adrenocorticotropic hormone, derives from the same biosynthetic route as beta-endorphin and is stored by the same arcuate neurons. Microinjection of alpha-melanocyte-stimulating hormone and several related peptides into the periaqueductal gray matter significantly reduced responsiveness to pain and had a behavioral profile similar to that produced by beta-endorphin.

Adrenocorticotropic hormone may be transported directly from the pituitary to the brain

Experiments were designed to test the hypothesis that pituitary hormones may be delivered directly to the brain. Concentrations of adrenocorticotropic hormone (ACTH) in the plasma were determined in blood samles obtained simultaneously from the carotid artery, the sagittal sinus, and the jugular vein of three awake sheep. Seizures were induced electrically to stimulate ACTH secretion, and at precise intervals thereafter several simultaneous comparisons were made in each animal. In many of the post-seizure comparisons, the ACTH plasma concentrations within the sagital sinus exceeded those within the carotid artery as well as those within the jugular vein, indicating that this hormone was released from the pituitary and carried directly through capillary beds of brain to the venous blood within the sagittal sinus. The experiment was repeated in one hypophysectomized sheep and, in this animal, ACTH concentration in the plasma was reduced, but that in the sagittal sinus still was elevated after the seizure, an indication that some ACTH (or ACTH-like material) was released from the brain itself.

beta-Endorphin: stability, clearance behavior, and entry into the central nervous system after intravenous injection of the tritiated peptide in rats and rabbits

Rabbits and rats were given intravenous injections of tritiated human beta-endorphin. The levels of beta-endorphin were followed by the decrease in radioactivity in the plasma of rats or rabbits and by the increase in radioactivity in the cerebrospinal fluid of the rabbit. The results were identical with the tritium label on either tyrosine-1 or -27. The plasma distribution times were 2 and 5 min in the rat and rabbit, respectively, with a later clearance time of approximately 1-8 hr. In the rat, approximately 50% of the radioactivity in the plasma was found to be intact human beta-endorphin 45 min after injection. Radioactivity appeared in the cerebrospinal fluid of the rabbit within 30 sec after injection and reached a plateau in approximately 60-90 min after injection. Approximately 75% of the radioactivity in the cerebrospinal fluid of the rabbit was intact human beta-endorphin. In the brain hemispheres of the rat and the rabbit, the only significant radiolabeled product was found to be radioactive tyrosine. Moreover, rat plasma levels of beta-endorphin decreased dramatically after hypophysectomy, which only slightly lowered the levels in the brain. It appears that beta-endorphin, upon entry into the plasma, is either not significantly taken up into the brain or is broken down with extreme rapidity upon entry into the brain, although it apparently does enter the cerebrospinal fluid.

The role of endorphins in stress: evidence and speculations

Several lines of evidence suggest that the endogenous opioid peptides endorphins may play a role in the defensive response of the organism to stress. The present paper summarizes these findings as well as evidence linking endorphins to the anterior pituitary polypeptide hormone adrenocorticotropin (ACTH). Evidence is presented that endorphins may function as trophic hormones in peripheral target organs such as the adrenal medulla and the pancreas. As such they may be part of the physiological mechanisms that mediate adrenaline and glucagon release in response to stress. Endorphins (enkephalins) are also suggested to play a role in the control of the pituitary gland during stress. In such capacity they may act as hormone-releasing or inhibiting factors. Finally, endorphins appear to play a role in the behavioral concomitants of stress. In such capacity endorphins are suggested to function as modulators of neural systems that mediate the elaboration and expression of the reactive/affective components of stress. Speculations on the mode of interaction between endorphins and ACTH in the global response to stress are discussed.

Evidence for a cholecystokinin gut-brain axis with modulation by bombesin

The relationship between cerebrospinal fluid and plasma cholecystokinin (CCK) levels was investigated by simultaneous withdrawal of CSF and blood from anesthetized mongrel dogs and measurement of CCK immunoreactivity by radioimmunoassay. A significant correlation occurred between CSF and plasma levels of CCK. During a CCK IV infusion, a statistically significant inverse correlation was noted between CSF and plasma values, while no significant relationship was noted during a bombesin (BBS) IV infusion. When infusion data were analyzed together with the appropriate baseline data, polynomial analysis revealed significant biphasic relationships for both CCK and BBS infusion studies. Intraventricular infusion of CCK did not alter plasma levels. These data suggest that existence of a mechanism relating CSF to plasma CCK levels (a gut to brain axis) with possible modulation or suppression by BBS.

Endogenous opiates: through 1978

The discovery of receptors in the brain for opiates and the structure of the endogenous peptides for these receptors has led to an explosion of interest in this field. The present review is the first of an annual series. It summarizes many of the highlights of research with opiate peptides published with a date of 1978 or earlier.

Additional evidence that small amounts of a peptide can cross the blood-brain barrier

It was determined that an antiserum against delta-sleep inducing peptide (DSIP) required eight of the nine constituent amino acids for antigenic activity. Measurement by this radioimmunoassay (RIA) or DSIP-like material in the rat brain, therefore, would necessarily involve almost the entire molecule present in essentially intact form. Injection of 200 microgram DSIP into the carotid artery of rats resulted in a doubling of brain levels of peptide as measured shortly afterwards by RIA. The brain tissue to plasma ratio of radioactivity in rats injected with labeled DSIP was much higher than that in rats injected with labeled inulin; this suggests that the increased amount of material measured by RIA was not merely trapped in the blood vessels. Thus, the results indicate that a small amount of essentially intact peptide can cross the blood-brain barrier. This could represent one of the mechanisms by which central effects of peripherally injected peptides can be exerted.

Pituitary-brain vascular relations: a new paradigm

Vascular casts of the pituitary gland have demonstrated a paucity of veins extending from the adenohypophysis to the systemic circulation and have suggested that some adenohypophyseal venous blood returns to the neurohypophysis. The neurohypophyseal capillary bed may function as a vascular switch and in this article a series of 14 questions are proposed regarding the vascular dynamics of the pituitary. Together these questions raise the larger question, namely, whether pituitary hormones are transported directly to the brain to modify brain function?