Somatostatin-induced histamine secretion in mast cells. Characterization of the effect

Mast cells in the autonomic nervous system and potential role in disorders with dysautonomia and neuroinflammation

Mast cells (MC) are ubiquitous in the body, and they are critical for not only in allergic diseases but also in immunity and inflammation, including having potential involvement in the pathophysiology of dysautonomias and neuroinflammatory disorders. MC are located perivascularly close to nerve endings and sites such as the carotid bodies, heart, hypothalamus, the pineal gland, and the adrenal gland that would allow them not only to regulate but also to be affected by the autonomic nervous system (ANS). MC are stimulated not only by allergens but also many other triggers including some from the ANS that can affect MC release of neurosensitizing, proinflammatory, and vasoactive mediators. Hence, MC may be able to regulate homeostatic functions that seem to be dysfunctional in many conditions, such as postural orthostatic tachycardia syndrome, autism spectrum disorder, myalgic encephalomyelitis/chronic fatigue syndrome, and Long-COVID syndrome. The evidence indicates that there is a possible association between these conditions and diseases associated with MC activation. There is no effective treatment for any form of these conditions other than minimizing symptoms. Given the many ways MC could be activated and the numerous mediators released, it would be important to develop ways to inhibit stimulation of MC and the release of ANS-relevant mediators.

Role of SARS-CoV-2 Spike-Protein-Induced Activation of Microglia and Mast Cells in the Pathogenesis of Neuro-COVID

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes coronavirus disease 2019 (COVID-19). About 45% of COVID-19 patients experience several symptoms a few months after the initial infection and develop post-acute sequelae of SARS-CoV-2 (PASC), referred to as "Long-COVID," characterized by persistent physical and mental fatigue. However, the exact pathogenetic mechanisms affecting the brain are still not well-understood. There is increasing evidence of neurovascular inflammation in the brain. However, the precise role of the neuroinflammatory response that contributes to the disease severity of COVID-19 and long COVID pathogenesis is not clearly understood. Here, we review the reports that the SARS-CoV-2 spike protein can cause blood-brain barrier (BBB) dysfunction and damage neurons either directly, or via activation of brain mast cells and microglia and the release of various neuroinflammatory molecules. Moreover, we provide recent evidence that the novel flavanol eriodictyol is particularly suited for development as an effective treatment alone or together with oleuropein and sulforaphane (ViralProtek®), all of which have potent anti-viral and anti-inflammatory actions.

Myalgic Encephalomyelitis/Chronic Fatigue Syndrome-Metabolic Disease or Disturbed Homeostasis due to Focal Inflammation in the Hypothalamus?

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a complex disease characterized by debilitating fatigue, lasting for at least 6 months, with associated malaise, headaches, sleep disturbance, and cognitive impairment, which severely impacts quality of life. A significant percentage of ME/CFS patients remain undiagnosed, mainly due to the complexity of the disease and the lack of reliable objective biomarkers. ME/CFS patients display decreased metabolism and the severity of symptoms appears to be directly correlated to the degree of metabolic reduction that may be unique to each individual patient. However, the precise pathogenesis is still unknown, preventing the development of effective treatments. The ME/CFS phenotype has been associated with abnormalities in energy metabolism, which are apparently due to mitochondrial dysfunction in the absence of mitochondrial diseases, resulting in reduced oxidative metabolism. Such mitochondria may be further contributing to the ME/CFS symptomatology by extracellular secretion of mitochondrial DNA, which could act as an innate pathogen and create an autoinflammatory state in the hypothalamus. We propose that stimulation of hypothalamic mast cells by environmental, neuroimmune, pathogenic and stress triggers activates microglia, leading to focal inflammation in the brain and disturbed homeostasis. This process could be targeted for the development of novel effective treatments.

The mast cell in innate and adaptive immunity

Mast cells (MCs) were once considered only as effector cells in pathogenic IgE- and IgG-mediated responses such as allergy. However, developments over the last 15 years have suggested that MCs have evolved in vertebrates as beneficial effector cells that are involved in the very first inflammatory responses generated during infection. This pro-inflammatory environment has been demonstrated to be important for initiating innate responses in many different models of infection and more recently, in the development of adaptive immunity as well. Interestingly this latter finding has led to the discovery that small MC-activating compounds can behave as adjuvants in vaccine formulations. Thus, our continued understanding of the MC in the context of infectious disease is likely to not only expand our scope of the MC in the normal processes of immunity, but provide new therapeutic targets to combat disease.

Mast cells: the immune gate to the brain

Mast cells were originally considered wandering histiocytes, but are now known to derive from the bone marrow and enter the tissues as immature or precursor cells which then differentiate under micro-environmental influences such as interleukin-3. At least three types of mature mast cells have been identified as serosal (lung, peritoneal, skin), mucosal (nasal, gastrointestinal) and brain (dural, perivascular, parenchymal) with their own distinct biochemical, morphological and functional characteristics. Mast cells are necessary for immediate hypersensitivity reactions where they release numerous biologically powerful mediators in response to immunoglobulin E (IgE) and antigen (Ag), and appear to be required for delayed reactions. Anaphylatoxins, basic peptides and drugs, as well as certain neuropeptides and hormones, can also trigger mast cell secretion. Recent evidence indicates that mast cells are found in close proximity to neurons, an association which may be regulated by nerve growth factor. Moreover, mast cells may be capable of selective release of mediators which could, in turn, regulate further secretion. This information suggests that mast cells may serve as a link between the immune, endocrine and nervous systems and could have an important role in the access of lymphocytes and pathogens to the brain. The possible role of such interactions in the pathophysiology of specific neuroinflammatory conditions is also discussed.

Immunoreactive Calcitonin Gene-Related Peptide, Vasoactive Intestinal Polypeptide, and Somatostatin in Developing Chicken Spinal Cord Motoneurons

The distribution of calcitonin gene-related peptide (CGRP)-, vasoactive intestinal polypeptide (VIP)-, and somatostatin (SOM)-like immunoreactivities (-LI) in neurons of the spinal cord of developing chickens was characterized by use of the indirect immunofluorescence technique, and the findings related to a possible role for these peptides in the development of muscles and motor endplates. CGRP-LI in presumptive motoneurons of the ventral horn was first observed at embryonic day 6. During the following days the number of CGRP-immunoreactive (IR) cells increased reaching high numbers between days 12 and 18 of incubation, and thereafter decreasing in numbers until hatching. SOM-LI was first observed on embryonic day 7, while VIP-LI appeared on days 12 - 13. The number of SOM- and VIP-IR cells was considerably lower than that observed for CGRP-LI, but they also exhibited an initial increase followed by a decrease towards hatching. Intrathecal administration of colchicine increased the number of CGRP-IR motoneurons at days 7 and 30 after hatching and of VIP-IR ones at day 7, while at day 30 no expression of VIP-LI was found. Colchicine treatment did not cause any significant change in the number of SOM-IR motoneurons after hatching. The effect of VIP, SOM, and CGRP on cAMP accumulation in primary cultures of chick muscle cells was determined after labelling of the cells by (2-3H) adenine and by radioimmunoassay. All three peptides stimulated the accumulation of cAMP. However, the development of the pharmacological response of each of the peptides followed a different time course during in vitro differentiation of the primary cultures. The response of CGRP was the latest to develop and did not significantly decrease after the maximal response had been reached around day 3. The data are discussed in terms of 'trophic' effects of these neuropeptides upon muscle and spinal cord differentiation and synaptogenesis.

Effect of antianaphylactic agents on substance-P induced histamine release from rat peritoneal mast cells

Substance P is known to be a potent histamine liberator for mast cells. The influence of antianaphylactic agents, disodium cromoglycate (DSCG), ketotifen, and tranilast was studied on substance-P and compound 48/80-induced histamine release from rat peritoneal mast cells. Substance-P induced histamine release was inhibited by these agents, while compound 48/80-induced histamine release was not inhibited by tranilast. Our findings suggest that these antianaphylactic agents are assumed to be effective for cutaneous diseases which might be concerned with substance P and histamine.

Histamine secretion from mast cells stimulated with somatostatin

Histamine release from isolated mast cells stimulated with somatostatin resembled that induced by other basic agents. The process was rapid, independent of added calcium or phospholipids, non-cytotoxic, species and tissue specific, not mediated through cell-fixed antibodies or glucoreceptors, and inhibited by antagonists of the polyamine receptor. Somatostatin and other polycations may then act through a common receptor or binding site on the mast cell membrane.

Pharmacological studies of somatostatin and somatostatin-analogues: therapeutic advances and perspectives

This article is aimed at reviewing and analyzing studies that are related to the possible therapeutic use of a potent and ubiquitously-distributed hormone--somato-statin (SS-14), and its analogues. Administration of these substances has provided beneficial effects in treating acromegaly, gastro-intestinal hemorrhagic and hypersecretory disorders, acute pancreatitis, diabetes mellitus, and certain types of cancer. Further studies with SS-14-analogues might provide new therapies for treating certain life-threatening disorders of man.

Gastrointestinal somatostatin: distribution, secretion and physiological significance

Somatostatin-like immunoreactivity (SLI) has been found throughout the gastrointestinal tract in all species examined. In the stomach it is mainly present in endocrine-type D-cells whereas in the intestine there is also an extensive distribution in enteric neurones. In all regions of the gastrointestinal tract multiple forms of somatostatin exist. A precursor (prosomatostatin) has been partially sequenced, three forms with 20 (SS-20), 25 (SS-25) and 28 (SS-28) amino acids completely sequenced, and somatostatin-14 (SS-14) demonstrated by radioimmunoassay. Both SS-14 and SS-28 exert a wide range of actions on the gastrointestinal tract and there is strong supportive evidence for a role in the regulation of gastric acid and gastrin secretion, gastrointestinal motility and intestinal transport. Both in vivo and in vitro studies on the secretion of gastric SLI into the vasculature have shown that nutrients initiate the process but that subsequent events are regulated by a complex interplay between hormonal and neuronal pathways. GIP is one of the most potent hormonal secretagogues. In the stomach, acetylcholine, opioid peptides and substance P are probably involved in parasympathetic inhibitory pathways and gastrin releasing peptide in stimulatory pathways. The sympathetic nerves are also stimulatory. Regulation of secretion of intestinal SLI has not been so extensively studied. Although SLI is also found in the gastrointestinal lumen the significance is unclear. Despite these advances the exact route of delivery of somatostatin to its target organs is uncertain and paracrine, endocrine and neural pathways may all be involved.

On the actions of substance P, somatostatin, and vasoactive intestinal polypeptide on rat peritoneal mast cells and in human skin

Substance P (SP), somatostatin (Som), and vasoactive intestinal polypeptide (VIP) induced a concentration-dependent release of histamine from isolated rat peritoneal mast cells. The release of histamine induced by these neuropeptides was inhibited by preincubation of the cells with the SP analogue [D-Pro4,D-Trp7,9,10]-SP4-11 (SP-A) (10 microM), and also by benzalkonium chloride (10 microM). In addition, SP-A inhibited histamine release induced by compound 48/80, whilst that induced by goat anti-(rat-IgE) was unaffected. In human skin, intradermal injection of SP, Som, or VIP produced flare and wheal responses. The flares to all three peptides were inhibited by preinjection of the skin with SP-A (25 pmol), whilst the wheal responses were unaffected. It is concluded that the receptors mediating histamine release and the flare response are similar, and that SP, Som, and VIP are acting at a similar receptor to produce these effects. It is probable that this receptor is also the site of action of compound 48/80.

Hydrocortisone inhibits phorbol ester stimulated release of histamine and arachidonic acid from rat mast cells

Purified rat peritoneal mast cells were incubated overnight with or without hydrocortisone (3 X 10(-6) M) and then stimulated with anti-IgE, somatostatin or a phorbol ester-ionophore combination, i.e., 12-O-tetradecanoyl-phorbol-13-acetate and A23187. The release of both histamine and [1-14C]arachidonic acid and its metabolites was determined. Hydrocortisone treatment markedly inhibited both anti-IgE and TPA-A23187 stimulated release, but not release stimulated by somatostatin. These results suggest that anti-inflammatory steroids may alter histamine release through an action involving the activation of the phosphatidylserine-calcium dependent protein kinase or its substrates.

Potency of various peptides as histamine liberators in the rat hind limb

The potency of several peptides and drugs as histamine liberators was assessed using the rat isolated hind limb preparation. Neurotensin (NT) and compound 48/80 (C48/80) were effective in concentrations as low as 10(-9) M and 10(-8) M, respectively. Threshold concentrations of vasoactive intestinal peptide (VIP) and substance P (SP) varied between 5 X 10(-7) to 5 X 10(-6) M while somatostatin (SS) was barely active at 6 X 10(-6) M. No histamine release could be detected following the use of high concentrations of thyrotropin releasing hormone (TRH) (6 X 10(-6) M), dynorphin (DYN) (6 X 10(-6) M) bradykinin (BK), des-Arg9-BK or bombesin (BB) (at 10(-5) M). Poly-L-Lysine and the calcium ionophore A23187 were about 100 times less active than NT. Concanavalin A (Con A) was inactive at 10(-6) M. These results indicate that NT is more potent (on a molar basis) as histamine liberator in the rat hind limb preparation (which contains a large population of cutaneous and subcutaneous mast cells) than any of the other compounds tested. Histamine release by NT was inhibited by preexposure of the rat hind limb mast cells to a high concentration of SP (1.5 X 10(-6) M). This result adds further support to the hypothesis suggesting that NT and SP might share a common mechanism of action and/or act through common receptors at least in rat mast cells.

Somatostatin-induced histamine release in mast cell incubates from thymusaplastic nude rats

Both cyclic and linear somatostatin-14 are potent histamine secretagogues in rat mast cell incubates. Linear somatostatin-14 is more effective than cyclic somatostatin-14 in a concentration range up to 10(-6) mol/l, in normal Sprague-Dawley rats, and even more responsive in sensitized hooded Lister rats. This additional mast cell stimulation capacity of linear somatostatin-14 depends on circulating IgE levels. No differences are observed in athymic rat mast cell incubates between the linear and the cyclic isomer of somatostatin-14. A direct unspecific interaction of linear somatostatin with cell surface antibodies can be concluded.

Suppression of passive cutaneous anaphylaxis by pertussis toxin, an islet-activating protein, as a result of inhibition of histamine release from mast cells

Passive cutaneous anaphylaxis (PCA) produced by antigen challenge to antibody-sensitized rats were interfered with by prior treatment with pertussis toxin, an islet-activating protein (IAP). The degree of interference was dependent on the dose and injection time of IAP; the effect of IAP developed slowly, with a maximal effect being observed 3 days later. Inhibition of PCA by IAP was associated with a decrease in histamine release from peritoneal mast cells, making it very likely that the process affected was mast cell secretion. Much less histamine was discharged in vitro, in response to certain membrane receptor (e.g. IgE receptor) stimulation, from mast cells that had been exposed to IAP than from the cells not exposed. Such an inhibitory effect of IAP was not observed when histamine release was provoked by a calcium ionophore without mediation of membrane receptors. IAP was a stronger inhibitor of histamine release than beta-adrenergic agonists. Further inhibition was produced when a beta-agonist was added to IAP-treated mast cells. The increase in the cellular content of cyclic AMP was associated with beta-agonist-induced, but not with IAP-induced, inhibition of histamine release. Thus, IAP inhibited histamine release by a mechanism in which metabolism of cyclic AMP was not directly involved.

Calcium requirement for the inhibition by theophylline of histamine release from mast cells

When added to Ca2+-free Hanks' solution, Ca2+ (0.1-2.5 mM) had no significant effect on antigen-induced histamine release from rat mast cells, but Sr2+ (1.0-3.0 mM) dose-dependently increased the release. Ba2+ (1.0 and 2.0 mM) also enhanced the release. Ca2+ and Ba2+ inhibited compound 40/80-induced histamine release, in a dose-dependent manner. In ordinary Hanks' medium, theophylline and 3-isobutyl-1-methylxanthine (IBMX) dose-dependently inhibited the antigen-induced histamine release but these drugs were ineffective in Ca2+-free medium. Theophylline (1.0 mM) also inhibited compound 48/80-induced histamine release in the presence but not absence of Ca2+. There was an optimal Ca2+ concentration for the theophylline effect. Sr2+ but not Ba2+ could substitute for Ca2+ in supporting the theophylline effect. Theophylline (1.0 mM) and IBMX (1.0 mM) increased mast cell cyclic AMP levels both in the presence and absence of Ca2+. These results suggest that Ca2+ is required in the interaction of theophylline and specific sites on mast cells or in the mast cell response to theophylline which probably does not involve the cyclic AMP increase and is linked to the inhibition of histamine release.

Evaluation of the contribution of mast cell mediators to the hypotensive activity of various peptides in rats

We have tested the effects of intravenous injections of substance P (SP), bradykinin (BK), somatostatin (SS) and vasoactive intestinal peptide (VIP) on the blood pressure, histaminemia and hematocrit in pentobarbital-anesthetized rats. The four peptides elicited a decrease of the mean arterial blood pressure which varied both in amplitude and in duration depending both on the peptide and on the doses utilized. The hypotensive effects of SP and VIP were more persistent than those caused by BK or SS. Only SP evoked an increase of histaminemia. Both SP and BK caused an increase of hematocrit. The change of hematocrit was more prominent and of longer duration after Sp than after BK. Pretreatment of rats with the antiinflammatory drug dexamethasone inhibited markedly the changes of blood pressure, histaminemia and hematocrit caused by SP. The hypotensive effects of BK, SS and VIP as well as the transient change of hematocrit evoked by BK were not affected by dexamethasone. The results suggest that part of the hypotensive activity and changes of hematocrit evoked by SP in rats is due to the release and action of histamine and possibly of other vasoactive substances, of mast cell origin. The results also indicate that mast cell mediators, particularly histamine, are unlikely to be instrumental in the hypotensive activity of BK, SS or VIP in rats.

Topical capsaicin pretreatment inhibits axon reflex vasodilatation caused by somatostatin and vasoactive intestinal polypeptide in human skin

1 Wheal and flare reactions are described following intradermal injections of somatostatin, vasoactive intestinal polypeptide, substance P and histamine in normal human forearm skin. Bombesin failed to produce a significant wheal and flare.2 Pretreatment of skin with capsaicin in all cases dramatically inhibited the flare but not the wheal. This result is in accord with the hypothesis that capsaicin blocks the effector side of the axon reflex, perhaps by depleting nerve terminals of vasodilatory peptide(s).

Histamine secretion in leukocyte incubates of patients with allergic hyperreactivity induced by somatostatin-14 and somatostatin-28

Somatostatin is a potent histamine secretagogue found not only in rat mast cells but also in human leukocyte preparations. In concentrations greater than or equal to 5 mumol/l, somatostatin-14 induces histamine release, which correlates with the basophilic blood cell count, as shown in samples from allergic patients suffering from slight basophilia. Somatostatin-14 is twice as effective as somatostatin-28 on a molar basis, and acylating the tetradecapeptide with N-hydroxysuccinimidyl-p-hydroxy-phenyl-propionate decreases significantly the potency of histamine release.

Release of histamine by neuropeptides from the perfused rat hindquarter

The release of histamine and serotonin by neuropeptides and capsaicin was measured in the isolated perfused rat hindquarter preparation. Substance P and two antagonistic peptides, [D-Pro2, D-Phe7, D-Trp9]-SP and [D-Pro2, D-Trp7,9)]-SP, release histamine, the SP(4-11) and SP(6-11) analogues did not. VIP and somatostatin released histamine and also serotonin. No amines were released by bombesin. Thus, all amine releasing peptides possessed at least two basic charges. However, the histamine releasing activity of the neuropeptides tested did not correlate with their reported ability to cause vasodilation and plasma extravasation. The SP(4-11) and SP(6-11) analogues which did not release histamine caused plasma extravasation. It is concluded that SP causes plasma extravasation by a direct action on blood vessels. Capsaicin released only serotonin but no histamine either in untreated rats and such desensitized with capsaicin as neonates. In rats desensitized with capsaicin 4 days prior to the experiment the substance P induced histamine release was as high as in untreated controls; it was, however, absent in rats desensitized with capsaicin as neonates. It is assumed that the sensitivity of mast cells to substance P is lost after degeneration of substance P containing primary sensory fibers.

Induction of mast cell secretion by parathormone

The biologically active fragment of human parathormone (PTH) and intact bovine PTH were found to induce secretion of both serotonin and histamine from rat peritoneal mast cells in vitro. Release of serotonin and histamine was demonstrated with 25 units/ml PTH or higher. This level is within the higher limits of the elevated PTH levels found in advanced uremia. Mast cell secretion by PTH was dose, time and energy dependent and was not cytotoxic. Although mast cell activation was independent of extracellular calcium, it required intracellular calcium, thus resembling the action of certain other peptide secretagogues. Intradermal injection of PTH induced immediate increases in vascular permeability suggesting that PTH could induce mast cell secretion in vivo. Light and electron microscopic observations confirmed mast cell degranulation by exocytosis. These results demonstrate that elevated levels of PTH can induce mast cell secretion in vitro and in vivo and suggest a possible role for mast cells in the pathophysiology of non-allergic disease states.

Mast cell histamine secretion in response to somatostatin analogues: structural considerations

Comparative studies of the activity of somatostatin and of several of its analogues in releasing histamine from rat mast cells suggest that the integrity of the positively charged amino terminus and of lysines at positions four and nine of somatostatin may be necessary to preserve its histamine releasing activity. D-Lys4 substitution reduced activity by 80% while D-Lys9 substitution increased it four fold. Replacement of the amino terminal Ala by Tyr or simultaneous removal of Ala1,Gly2 and the amino portion of the now terminal Cys3 inhibited the activity by about 95%. Finally, dihydrosomatostatin retained 33% activity while an analogue where both Cys sulfur atoms were permanently blocked by acetamidomethyl groups retained only about 13% activity. Using information from these studies and from the literature, two-dimensional and space-filling models approximating the conformation of somatostatin were constructed and compared with a plausible corresponding model of the hexameric form of 48/80, the most active congener of this classic mast cell secretagogue. By such modelling it was possible to show that the orientation of the cationic moieties in the two molecules could be similarly arranged thereby perhaps explaining the ability of these compounds to induce mast cell secretion.