The "Mechanostat" Principle and the Osteoprotegerin-OPG/RANKL/RANK System PART II. The Role of the Hypothalamic-Pituitary Axis

Recent literature provides us with the indication that the ratio of OPG/RANKL concentrations fluctuate in a non-linear (bell-shaped) mode, according to Frost's "mechanostat" theory, during the application of mechanical strain. A similar cellular signal transduction mechanism to mechanical loading, that may also regulate bone resorptive activities, has nowdays been proposed. Since the skeleton is considered to be an endocrine organ, as such, it may likewise be subject to hypothalamic control. This central regulatory system may continually modulate the adaptation of the bone homeostatic system to external challenges. The hypothalamic-pituitary-gonadal axis may play a role in the regulation of OPG/RANKL secretion. The "mechanostat" principle may well characterize the oscillatory properties of the signal transduction mechanism that controls hard tissue metabolism and includes biochemical apart from mechanical stimuli. J. Cell. Biochem. 118: 962-966, 2017. © 2016 Wiley Periodicals, Inc.

Decreased Serotonin Levels and Serotonin-Mediated Osteoblastic Inhibitory Signaling in Patients With Ankylosing Spondylitis

Evidence suggests that serotonin is an inhibitor of bone formation. We aimed to assess: 1) serum serotonin levels in patients with ankylosing spondylitis (AS), a prototype bone-forming disease, compared with patients with rheumatoid arthritis (RA) and healthy subjects; 2) the effect(s) of TNFα blockers on serum serotonin levels in patients with AS and RA; and 3) the effect(s) of serum of AS patients on serotonin signaling. Serum serotonin levels were measured in 47 patients with AS, 28 patients with RA, and 40 healthy subjects by radioimmunoassay; t test was used to assess differences between groups. The effect of serum on serotonin signaling was assessed using the human osteoblastic cell line Saos2, evaluating levels of phospho-CREB by Western immunoblots. Serotonin serum levels were significantly lower in patients with AS compared with healthy subjects (mean ± SEM ng/mL 122.9 ± 11.6 versus 177.4 ± 24.58, p = 0.038) and patients with RA (mean ± SEM ng/mL 244.8 ± 37.5, p = 0.0004). Patients with AS receiving TNFα blockers had significantly lower serotonin levels compared with patients with AS not on such treatment (mean ± SEM ng/mL 95.8 ± 14.9 versus 149.2 ± 16.0, p = 0.019). Serotonin serum levels were inversely correlated with pCREB induction in osteoblast-like Saos-2 cells. Serotonin levels are low in patients with AS and decrease even further during anti-TNFα treatment. Differences in serotonin levels are shown to have a functional impact on osteoblast-like Saos-2 cells. Therefore, serotonin may be involved in new bone formation in AS.

The developmental basis of epigenetic regulation of HTR2A and psychiatric outcomes

The serotonin receptor 5-HT2A (encoded by HTR2A) is an important regulator of fetal brain development and adult cognitive function. Environmental signals that induce epigenetic changes of serotonin response genes, including HTR2A, have been implicated in adverse mental health outcomes. The objective of this perspective article is to address the medical implications of HTR2A epigenetic regulation, which has been associated with both infant neurobehavioral outcomes and adult mental health. Ongoing research has identified a region of the HTR2A promoter that has been associated with a number of medical outcomes in adults and infants, including bipolar disorder, schizophrenia, chronic fatigue syndrome, borderline personality disorder, suicidality, and neurobehavioral outcomes. Epigenetic regulation of HTR2A has been studied in several different types of tissues, including the placenta. The placenta is an important source of serotonin during fetal neurodevelopment, and placental epigenetic variation of HTR2A has been associated with infant neurobehavioral outcomes, which may represent the basis of adult mental health disorders. Further analysis is needed to identify intrinsic and extrinsic factors that modulate HTR2A methylation, and the mechanism by which this epigenetic variation influences fetal growth and leads to altered brain development, manifesting in psychiatric disorders.

Signal hyperintensities on brain magnetic resonance imaging in patients with primary Sjögren syndrome and frequent episodic tension-type headache: relation to platelet serotonin level and disease activity

OBJECTIVE

To examine differences in number and size of signal hyperintensities (SH) on magnetic resonance imaging (MRI) between patients with primary Sjögren syndrome (pSS) and controls who all had frequent episodic tension-type headache (FETH), and to investigate their relation to platelet serotonin level (PSL), patient age, disease duration, and activity.

METHODS

SH in 22 pSS patients with FETH were compared to 20 aged-matched controls with FETH, using the modified semiquantitative rating scale. Spectrofluorimetry was used for determination of PSL, and the European League Against Rheumatism SS Disease Activity Index (ESSDAI) for disease activity assessment.

RESULTS

Statistically significant differences in the total number of SH were noted infratentorially (p = 0.025) and in the basal ganglia for lesions of diameter > 5 mm (p = 0.048). Significant correlations were found between disease duration and number of overall lesions > 5 mm (p = 0.04) and subcortical lesions of diameter 2-5 mm (p = 0.035). Number of periventricular SH inversely correlated to PSL (p = 0.019) and to patient age (p = 0.004), without association with markers of immunoinflammation and ESSDAI.

CONCLUSION

Our study showed that SH on brain MRI are more common in specific regions of the brain in pSS patients with FETH than in controls with FETH, signifying a more widespread cerebral vasculopathy in SS patients with FETH. Periventricular SH seem to be associated to increased platelet serotonin release in pSS patients with FETH and correlated with disease duration, without correlation to the actual ESSDAI and markers of immunoinflammation, and might be linked with chronic immunoinflammation of low-grade intensity and vasculitis in pSS.

Is reserpine tranquilization linked to change in brain serotonin or brain norepinephrine?

Reserpine, when administered to animals stressed by exposure to cold, does not induce sedation or appreciably lower brain serotonin, but markedly lowers brain norepinephrine. Reserpine in cold-exposed hypophysectomized rats elicits sedation and releases both amines equally. The results support the view that the tranquilizing action of reserpine is not related to brain norepinephrine loss but rather to change in the level of brain serotonin.

Studies on wound healing

Factors affecting the rate of healing of experimental skin wounds in rats have been investigated. The effectiveness of healing was measured by determining the tensile strengths of the incised skin after various time intervals. When the skin histamine content was lowered by treatment with polymyxin B or with compound 48/80, retardation of the healing process was evident from the reduced tensile strengths. When the skin 5-hydroxytryptamine content was lowered by treatment with reserpine, retardation of healing was also found. Heparin increased the rate of healing and more rapid healing was obtained by giving histamine before each dose of heparin. On the other hand, some glucocorticoids markedly inhibited the healing process. Of the constituents of the tissue mast cells, heparin appears to be more important than histamine and 5-hydroxytryptamine in promoting the healing of experimental skin wounds in rats.

The action of drugs on the circular muscle strip from the guinea-pig isolated ileum

The circular muscle strip is a new preparation for examining the action of drugs on the circular muscle of the guinea-pig isolated intestine. The preparation differed from the longitudinal muscle in that it was insensitive to drugs which act on autonomic effector tissues but, after inhibition of cholinesterase, it responded readily to choline esters, 5-hydroxytryptamine, histamine and nicotine. This behaviour necessitated the treatment of each strip with the anticholinesterase NN-diisopropylphosphodiamidic fluoride (mipafox) before each experiment. The contractions of the strip by 5-hydroxytryptamine, histamine and nicotine were abolished by procaine, botulinum toxin (Type A), morphine and hemicholinium, whilst the actions of acetylcholine and methacholine were unaffected. Contractions of the strip in response to each of the drugs were abolished by atropine and hyoscine. The action of nicotine was specifically antagonized by hexamethonium, that of 5-hydroxytryptamine by desensitization of the tissue to 5-hydroxytryptamine, and that of histamine either by desensitization of the tissue to histamine or by mepyramine. It is postulated that 5-hydroxytryptamine, histamine and nicotine stimulate specific receptor sites within the intramural nerve plexuses of the guinea-pig isolated ileum. Finally, botulinum toxin (Type A), morphine or hemicholinum, acting on the neuronal elements of the intramural plexuses, depressed the contractions of the circular muscle strip due to histamine or nicotine more readily than those due to 5-hydroxytryptamine.

Effects of vasodilator drugs on normal and serotonin-constricted pulmonary vessels of the dog

Previous studies of the action of vasodilator drugs on the pulmonary blood vessels have yielded inconclusive results. Studies on the effects of continuous infusion of vasodilator agents, first on normal pulmonary vessels, and then on serotonin-constricted pulmonary vessels, suggest that the response of the pulmonary vasculature is dependent on the degree of vasoconstrictor tone present. Consistent vasodilator responses to acetylcholine, histamine and adenosinetriphosphate were elicited when the pulmonary vessels were constricted by continuous infusion of serotonin. Pulmonary vasomotor responses to drugs occur in both the small pulmonary vessels and the large pulmonary veins.

A study of antagonists of 5-hydroxytryptamine and catechol amines on the rat's blood pressure

The effects of 5-hydroxytryptamine on the blood pressure of anaesthetized rats depended on the dose and the initial level of blood pressure. At medium blood pressure levels, 5-hydroxytryptamine gave a depressor response and sometimes a pressor response which was more evident with large doses. The depressor effect was less apparent or even absent at low, and more pronounced at high, blood pressure levels, and the converse applied to the pressor components. Adenosine also gave a depressor and pressor response. Lysergic acid diethylamide, dihydroergotamine, 1-(3,4-dichlorophenyl)-2-isopropylaminoethanol (a dichloro analogue of isoprenaline), dibenamine and 1-benzyl-5-methoxy-2-methyltryptamine antagonized 5-hydroxytryptamine and catechol amines. Lysergic acid diethylamide and 1-benzyl-5-methoxy-2-methyltryptamine were more effective against 5-hydroxytryptamine, 1-(3,4-dichlorophenyl)-2-isopropylaminoethanol and dibenamine against catechol amines; dihydroergotamine was equally effective against both groups. These antagonists fell into two groups according to their action against the two types of effects (depressor and pressor) of 5-hydroxytryptamine: lysergic acid diethylamide and 1-(3,4-dichlorophenyl)2-isopropylaminoethanol acted preferentially against depressor effects; 1-benzyl-5-methoxy-2-methyltryptamine and dibenamine preferentially against pressor; dihydroergotamine was not assignable to either group. Adenosine was affected similarly, but less than 5-hydroxytryptamine.

Some neurochemical aspects of the depressant action of gamma-butyrolactone on the central nervous system

gamma-Butyrolactone, a depressant drug of the central nervous system, has been investigated for its ability to alter brain levels of 5-hydroxytryptamine, gamma-aminobutyric acid and acetylcholine in mice and rats; of these three compounds, only acetylcholine was changed in amount. Levels of acetylcholine in the cerebral cortex were increased by gamma-butyrolactone with a time-course which closely followed the depressed state of the animal. Indirect evidence has been presented to show that in the mid-brain and brain stem the change in acetylcholine level induced by gamma-butyrolactone is sharply localized in an area of the mesencephalon that contains the corpora quadrigemina.

Effects of 5-hydroxytryptophan on schizophrenia

The present investigation represents a logical continuation of a previous experiment where it was shown that the injection of 5-hydroxytryptophan (5HTP) alleviates the psychological effects of lysergic acid diethylamide (LSD-25) (Brengelmann, Pare and Sandler, 1958). LSD is of interest in psychiatry because the psychotomimetic effects of the drug bear a resemblance to schizophrenia clinically, in psychological tests and, according to Hoagland, Rinkel and Hyde (1955), in their abnormal excretion of urinary phosphate. The possibility presents itself that, in some cases of schizophrenia, the functional abnormality is similar to that produced by LSD.

The use of the guinea-pig ileum preparation for testing the activity of substances which imitate or antagonize the actions of 5-hydroxytryptamine and tryptamine

On the guinea-pig ileum, 5-hydroxytryptamine appeared to act in the same way as tryptamine on two types of receptor (morphine-sensitive and phenoxybenzamine(Dibenzyline)-sensitive). The actions of analogues of 5-hydroxytryptamine on the phenoxybenzamine-sensitive receptors resembled their actions on the rat uterus, and the actions on the morphine-sensitive receptors slightly resembled those on the rat fundus strip. The guinea-pig ileum preparation, however, did not appear to be more suitable than the rat uterus and rat fundus strip for testing the ability of compounds to imitate or antagonize 5-hydroxytryptamine.

Oxidation of 5-hydroxytryptamine and related compounds by Mytilus gill plates

Homogenates of gill plates of Mytilus edulis L. used oxygen when 5-hydroxytryptamine was added. The oxidation of 5-hydroxytryptamine was not due to the presence of an amine oxidase, but to that of an enzyme that catalysed the oxidation of other 5-hydroxyindoles (5-hydroxytryptophan, bufotenine). The oxidation was cyanide-sensitive, but was not inhibited by iproniazid. In the reaction a yellowish-brown substance was formed. The occurrence of an amine oxidase in the anterior retractor muscle of the byssus and in the digestive gland was confirmed.

Stimulation and sensitization of the isolated Venus heart by cerebrospinal fluid

5-Hydroxytryptamine was not detected in animal cerebrospinal fluid after amine oxidase inhibitors, reserpine, or both. Both human and animal cerebrospinal fluid can produce a stimulating effect on the clam heart which is not due to 5-hydroxytryptamine. Cerebrospinal fluid also has the property of increasing the sensitivity of the clam heart to 5-hydroxytryptamine.

The subcellular distribution of histamine, slow-reacting substance and 5-hydroxytryptamine in the brain of the rat

Various extracts of brain were prepared with dilute and concentrated acid, acetone and n-butanol. When the extracts were applied to the guinea-pig isolated ileum, they produced a slow contraction which was not prevented by an antihistamine; in addition, the acid and acetone extracts inhibited the action of histamine. Histamine in extracts of brain was separated from other pharmacologically active substances by chromatography on a carboxylic-acid resin and estimated biologically. The bulk of this histamine was found in small particulate material, whereas slow-reacting substance was found in particulate material of greater density. 5-Hydroxytryptamine was in both the small and large particles. There was no relationship between the distribution of these substances and that of succinic dehydrogenase activity. The measurement of histamine in brain both by biological assay on the guinea-pig ileum and by chemical assay using a fluorimetric procedure gave mean values of 53 and 246 ng/g of wet tissue, respectively. The high values obtained by the chemical assay are attributable in part to substances other than histamine which become fluorescent after reaction with o-phthalaldehyde in this procedure.

An analysis of the direct and indirect actions of drugs on the isolated guinea-pig ileum

The responses of the isolated guinea-pig ileum to coaxial stimulation of its nerves, to histamine, acetylcholine, bradykinin, nicotine, tetramethylammonium, 1,1-dimethyl-4-phenyl-piperazinium iodide and 5-hydroxytryptamine were studied, before and during anoxia, cooling, or exposure to hyoscine, phenoxybenzamine hydrochloride, morphine or hexamethonium. Dose ratios were used to determine the amount of block induced by these procedures. With the response to coaxial nerve stimulation as an indication of the excitability of the nervous tissue, it was found that anoxia or cooling abolished the response to single shocks. Under these conditions the response of the ileum to histamine, acetylcholine and bradykinin was hardly affected, indicating a direct action of these substances on the muscle fibres. The effects of nicotine, tetramethylammonium, dimethylphenylpiperazinium and 5-hydroxytryptamine were reduced to various degrees, and we have concluded that their main actions are indirect, through stimulation of cholinergic nerve fibres. When these indirect actions were prevented, increasing the dose revealed a direct action, a larger increase in dose being required for 5-hydroxytryptamine and dimethylphenylpiperazinium than for tetramethylammonium and nicotine. Exposure of the ileum to hyoscine and phenoxybenzamine showed that these direct actions of nicotine and tetramethylammonium were not only on acetylcholine receptors but also on receptors insensitive to hyoscine but sensitive to phenoxybenzamine. The main action of 5-hydroxytryptamine was on nervous elements, yet treatment of the ileum with phenoxybenzamine gave a higher dose ratio for 5-hydroxytryptamine than did treatment with morphine. The meaning of this result is discussed in relation to the general belief that receptors sensitive to morphine are in nervous tissue and receptors sensitive to phenoxybenzamine are in smooth muscle. We have concluded that morphine is only a partial antagonist of 5-hydroxytryptamine receptors in nervous tissue and that phenoxybenzamine antagonizes more 5-hydroxytryptamine receptors than those in smooth muscle.

A method for the quantitative estimation of drugs on the isolated intact trachea

A method is described for the detection and comparison of substances active on tracheal smooth muscle. Several pharmacological agents affecting smooth muscle have been tested using the technique. Sensitivity was high, results reproducible and linear log dose-response relationships obtained over a wide dosage range. The method has been used to estimate the concentrations of adrenaline, acetylcholine and histamine in test solutions of these compounds.

An action of 5-hydroxytryptamine on adrenaline receptors

Contractions of isolated strips of cat spleen due to 5-hydroxytryptamine, adrenaline, histamine and acetylcholine were antagonized by phenoxybenzamine. Responses to both 5-hydroxytryptamine and adrenaline were not blocked in strips which were protected by a high concentration of either 5-hydroxytryptamine or adrenaline throughout exposure to phenoxybenzamine. The contraction due to a large dose of 5-hydroxytryptamine lasted less than 1 hr even when the drug was still present. Strips thus desensitized to 5-hydroxytryptamine responded normally to acetylcholine and histamine but did not respond to adrenaline. The actions of 5-hydroxytryptamine and adrenaline were blocked by 2-bromolysergic acid diethylamide or by dihydroergotamine. These results indicated that 5-hydroxytryptamine and adrenaline act on the same receptors. Cocaine potentiated the action of adrenaline but inhibited the action of 5-hydroxytryptamine. The sensitivity to 5-hydroxytryptamine of spleen strips from cats treated 24 hr earlier with reserpine was only one-fiftieth of that of normal strips. Cocaine potentiated the action of 5-hydroxytryptamine on strips from reserpine-treated cats. A high concentration of 5-hydroxytryptamine in spleen strips from reserpine-treated cats and in cocaine-treated strips prevented phenoxybenzamine from blocking the actions of adrenaline. The effects of tyramine on spleen strips almost exactly paralleled the effects of 5-hydroxytryptamine. Strips showing tachyphylaxis to tyramine did not respond to 5-hydroxytryptamine. It is concluded that 5-hydroxytryptamine has a dual action, viz., a major action due to release of stored noradrenaline and a minor direct action of adrenaline receptors.

The effect of iproniazid and imipramine on the blood platelet 5-hydroxytrptamine level in man

Observations are reported on the blood platelet 5-hydroxytryptamine content of six patients receiving imipramine, N-(gamma-dimethylaminopropyl)-iminodibenzyl hydrochloride. The response was a fall to a level of one-sixth of the original in three weeks, with little change thereafter. This is in sharp contrast to the action of iproniazid which caused a rise of some 200% in the blood platelet 5-hydroxytryptamine level over the same period. Imipramine in a concentration of 1 mg./ml. had no inhibitory action on 5-hydroxytryptophan decarboxylase; 8.0 mug./ml. of imipramine suppressed two-thirds of the in vitro uptake of 5-hydroxytryptamine (2.5 mug./ml.) by normal human platelets.

A method for assessing the effects of drugs on the central actions of 5-hydroxytryptamine

Injection of 5-hydroxytryptophan into mice produces a characteristic head-twitch. For a given period of observation this response may be assessed quantitatively either by observing the proportion of mice showing at least one head-twitch (a quantal response) or by counting the number of head-twitches for each mouse (a graded response). A method, based on the quantal response, of investigating the effect of centrally acting compounds on the head-twitch response is described. Evidence is presented that the head-twitches are due to a central action of 5-hydroxytryptamine formed by decarboxylation of 5-hydroxytryptophan. Head-twitches are potentiated by monoamine oxidase inhibitors and by phenytoin. Antagonists tested include an inhibitor of decarboxylase, antagonists of 5-hydroxytryptamine, some antihistamines, major tranquillizers and analgesic and sympathomimetic drugs. Drugs which neither potentiate nor inhibit the response include barbiturates and minor tranquillizers. The method may be valuable in the preliminary examination of compounds likely to have a central action.