Stress increases endogenous benzodiazepine receptor ligand-monoamine oxidase inhibitory activity (tribulin) in rat tissues

Low sodium intake induces an increase in renal monoamine oxidase activity in the rat. Involvement of an angiotensin II dependent mechanism

AIMS

The interplay between natriuretic dopamine and antinatriuretic angiotensin II represents an important mechanism for the regulation of renal sodium and water excretion. Monoamine oxidase is the main metabolizing pathway for dopamine in the renal cortex. In this study, we have analysed the effect of low sodium feeding and AT1 receptor blockade on renal dopamine metabolism by monoamine oxidase.

METHODS

Four groups of rats were studied: 1, normal salt diet (NS); 2, low salt diet (LS); 3, NS receiving Losartan (Los, specific AT1 receptor antagonist, 20 mg kg(-1) bwt day(-1), NS + Los); 4, LS receiving Los (LS + Los).

RESULTS

Urinary dopamine excretion was lower in LS than in NS rats (543 +/- 32 vs. 680 +/- 34 ng day(-1) 100 g(-1) bwt, P < 0.05). When treated with Los, DOPAC excretion and urinary DOPAC/dopamine ratio fell significantly in the LS + Los group as compared with the LS group (1199 +/- 328 vs. 3081 +/- 681 ng day(-1) 100 g(-1) bwt and 1.90 +/- 0.5 vs. 5.7 +/- 1.2, respectively, both P < 0.02). Losartan increased hydroelectrolyte excretion in the LS group. No changes were found in the NS + Los group. Aromatic L-amino acid decarboxylase activity in cortex was similar in NS and LS rats. Instead, monoamine oxidase activity was higher in cortical homogenates from LS rats (in nmol mg tissue(-1) h(-1): NS 7.66 +/- 0.52; LS 9.82 +/- 0.59, P < 0.05) and this difference was abolished in LS + Los rats (7.34 +/- 0.49 nmol mg tissue(-1) h(-1), P < 0.01, vs. LS).

CONCLUSIONS

We have concluded that low levels of dopamine in the urine of LS rats are because of an increase in the activity of renal monoamine oxidase and that angiotensin II mediates this increase through stimulation of AT1 receptors.

Isatin, an endogenous MAO inhibitor, and a rat model of Parkinson's disease induced by the Japanese encephalitis virus

A single dose of isatin (indole-2,3-dione)(i.p.), an endogenous MAO inhibitor, significantly increased norepinephrine and 5-hydroxytryptamine concentrations in the rat brain and also significantly increased acetylcholine and dopamine (DA) levels in the rat striatum. Urinary isatin concentrations in patients with Parkinson's disease tend to increase according to the severity of disease. We have developed a rat model of Parkinson's disease induced by the Japanese encephalitis virus (JEV). The distribution of the pathological lesions of JEV-rats resemble those found in Parkinson's disease. Significant behavioral improvement was observed in JEV-rats after isatin, L-DOPA and selegiline administration using a pole test. Both isatin and selegiline prevented the decrease in striatum DA levels of JEV-rats. The increased turnover of DA (DOPAC/DA) induced by JEV was significantly inhibited by isatin, but not selegiline. These findings suggest that JEV-infected rats may serve as a model of Parkinson's disease and that exogenously administered isatin and selegiline can improve JEV-induced parkinsonism by increasing DA concentrations in the striatum.

Influence of age on stress responses to metabolic cage housing in rats

1. We studied the effect of isolation stress in 3- and 12-month-old rats individually housed in metabolic cages for 7 days. Urine (24 hr) was collected daily from one group of animals of each age. The other group was tested in an open field and on a hot plate on days 1 and 7. 2. Total deambulation in the open-field test was lower in young than in older rats both on day 1 (54.7 +/- 9.9 vs 80 +/- 8.9 crossings/session; P < 0.04) and on day 7 (21 +/- 9 vs 48 +/- 7 crossings per session; P < 0.04) and decreased significantly in the two groups when tested on day 7 (P < 0.03). Latency to paw-licking in the hot-plate test was longer in young than in older animals on day 1 (14 +/- 2 vs 8 +/- 4 sec; P < 0.05) but was similar in the two groups on day 7. 3. Urinary excretions of norepinephrine (NE) and epinephrine (E) were determined by HPLC with electrochemical detection. Urinary NE in day 1 was similar in young and older animals (2627 +/- 828 vs 3069 +/- 598 ng/24 hr). In young animals NE excretion decreased along the study and was significantly (P < 0.02) lower than on day 1 during the last 3 days of the study. Conversely, in older animals urinary excretion of NE remained similar throughout the study. On day 7 urinary excretion of NE in older animals was about two fold that in young rats. Urinary E was similar in young and older rats (341 +/- 127 vs 532 +/- 256 ng/24 hr) on day 1 and showed a tendency to increase throughout the study. 4. Urinary monoamine oxidase inhibitory (IMAO) activity was determined by testing the ability of urine extracts to inhibit rat liver MAO activity in vitro and was higher in young than in older animals throughout the study (day 1, 54.8 +/- 4.2 vs 25.1 +/- 5.1%; P < 0.02). In young rats excretion of IMAO was significantly higher during the last 3 days of the study than on day 1 (P < 0.05). In older animals urinary IMAO showed a tendency to increase at the end of the study. 5. Isolation stress caused by housing rats in metabolic cages results in different behavioral and metabolic responses in young and older animals. Young animals exhibit a lower locomotor and analgesic response and excrete lower amounts of NE and higher IMAO activity in the urine than older rats. The metabolic and behavioral responses to isolation stress are highly dependent on the age of the animals tested. These results should be taken into consideration when designing experiments requiring the use of metabolic cages.

Ovarian steroids and stress produce changes in peripheral benzodiazepine receptor density

Although past research has described changes in the density of the peripheral benzodiazepine receptor in brain and in peripheral organs in response to stressors and steroid hormone exposure, their combined influence had yet to be determined. This study examined the effect of swim-stress as a function of ovarian hormone administration on the binding of an isoquinoline carboxamide derivative, [3H]PK 11195, in brain and peripheral tissues. In olfactory bulb and adrenal gland, stress increased peripheral benzodiazepine receptor density in ovariectomized rats with and without estradiol and progesterone replacement injection, even when compared with unstressed animals treated with hormones, where estradiol + progesterone decreased peripheral benzodiazepine receptor number in olfactory bulb, but estradiol and estradiol + progesterone increased it in adrenal gland. In frontal cortex, stress decreased peripheral benzodiazepine receptor number, an effect that was reversed by estradiol. In hippocampus estradiol decreased peripheral benzodiazepine receptor density in unstressed animals and estradiol + progesterone decreased peripheral benzodiazepine receptor number in unstressed and stressed animals. In cerebellum, stress, estradiol and estradiol + progesterone alone decreased peripheral benzodiazepine receptor density. In uterus of unstressed controls, estradiol + progesterone increased peripheral benzodiazepine receptor density, and stress produced a further increase in steroid-treated females. Stress did not affect peripheral benzodiazepine receptor density in kidney, except in animals that received estradiol + progesterone injections, where swim-stress produced a significant decrease in peripheral benzodiazepine receptor density. Thus, steroid hormones regulate peripheral benzodiazepine receptor density in endocrine organs and brain, and the hormonal state of the organism modifies the peripheral benzodiazepine receptor response to stress in a tissue- and brain region-specific manner, suggesting that the peripheral benzodiazepine receptor may play a pivotal role in an integrated response to stress.

Stress-induced increase in urinary isatin excretion in rats: reversal by both dexamethasone and alpha-methyl-P-tyrosine

The effects of acute food deprivation and acute cold exposure on 24-hr urinary isatin excretion in rats and a mechanism responsible for changes in urinary isatin excretion during stress were investigated. This is the first study to demonstrate by HPLC that urinary isatin excretion is increased by stress. Both types of stress induced a marked increase in urinary isatin excretion during the 24 hr following the initiation of stress. Dexamethasone administration prevented the increase in urinary isatin excretion induced by both of the different types of stress. Furthermore, administration of either the benzodiazepine receptor agonist diazepam or the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine prevented the increase in urinary isatin excretion induced by acute food deprivation, whereas the dopamine-beta-hydroxylase inhibitor diethyldithiocarbamate proved ineffective. These observations suggest that during stress, activated catecholamine-synthesizing cells and corticotropin-releasing factor cells, both of which play central roles in stress responses, may be involved in total isatin production. Isatin may serve as an endogenously generated marker for some types of stress.

Function of endogenous monoamine oxidase inhibitors (tribulin)

Recent research on tribulin [low molecular weight endogenous inhibitory activity of monoamine oxidase (MAO)] has confirmed that its level is increased in both human urine and rat tissues by stress or anxiety, and by anxiogenic drugs. However tribulin is now known to contain several different molecules. The raised inhibitory activity in rat tissues is selective for MAO-A. There is a parallel decrease in MAO-A activity ex vivo, suggesting a possible functional effect. Increase in endogenous MAO I may competitively inhibit the binding of irreversible MAO I drugs, and may also help to mediate some mood altering effects of other drugs, or procedures such as ECT. In human urine both MAO-A I and MAO-B I have been found to be increased in mild stress. Similar findings have been made with human saliva. Selective inhibitors of MAO-A have been identified from human urine, and pig brain, but it is not yet clear to what extent they account for the MAO-A I activity increased in stress. Isatin is an endogenous selective inhibitor of MAO-B (K1 approximately 3 microM). It has a distinct distribution in rat brain, with highest concentration in the hippocampus of 0.1 microgram/g. Its level is increased by pentylene tetrazole, and isatin is itself anxiogenic in rodent models. Its administration also increases monoamine levels in the brain. It is a potent antagonist of the ANP receptor, and it may act to link the control of monoamine function and natriuresis.

Determination of isatin, an endogenous monoamine oxidase inhibitor, in urine and tissues of rats by HPLC

1. We have previously identified isatin as one of the endogenous monoamine oxidase (MAO) inhibitors in the urine and the brain of stroke-prone spontaneously hypertensive rats (SHRSP), using gas chromatography-mass spectrometry (GC-MS). 2. In this study, we attempted to develop a convenient assay to determine isatin using high performance liquid chromatography with an ultraviolet detector (HPLC-UV). The standard curve for authentic isatin was linear at a range from 2 to 20 nmol per ml. The coefficient of variance was within 3% for both intra-assay and inter-assay. The sensitivity was 20 pmol per 10 microl of urine sample. 3. Isatin concentration correlated significantly and positively with endogenous MAO activity (tribulin-like activity) in both urine (r=0.924, P<0.001) and kidney extracts (r=0.862, P<0.01). There was a significant difference in urinary isatin between Wistar Kyoto rats (WKY) and SHRSP. Oral administration of isatin increased urinary isatin concentration and systolic blood pressure in WKY. 4. Determination of isatin using HPLC-UV may be useful for elucidating role of isatin in various conditions of stress and disease.

Regional and molecular separation of the four bioactivities of 'tribulin'

The endocoid tribulin has four known bioactivities: monoamine oxidase A and B inhibitory activities (MAO-AI and MAO-BI) and peripheral and central benzodiazepine receptor binding inhibitory activities (PBR-I and CBR-I). Analysis of the four bioactivities in rat tissue reveals regional dissociation. Most notably liver was particularly rich in PBR-I yet contained no detectable MAO-AI. In addition we have succeeded in separating the four activities from human urine. MAO-AI and MAO-BI have greater retention on charcoal than PBR-I and CBR-I when eluted step-wise with aqueous methanol. MAO-AI can be separated from MAO-BI and in addition PBR-I can be separated from CBR-I by step aqueous methanol elution from Amberlite XAD-4. Hence we present two lines of evidence that, contrary to the original view, tribulin is composed of a number of distinct molecular components.

Monoamine oxidase A-inhibiting components of urinary tribulin: purification and identification

The endogenous monoamine oxidase (MAO) inhibitory activity, termed tribulin, contains several components. We have previously identified one of them, isatin, which is a selective inhibitor of MAO B. In the present study we have purified several further components of human urinary tribulin which act as selective inhibitors of MAO A. They have been identified by gas chromatography-mass spectrometry (GC-MS) as ethyl indole-3-acetate (and/or methyl indole-3-propionate), methyl indole-3-acetate and ethyl 4-hydroxyphenylacetate. IC50 values for MAO A were found to be 44 microM (105 microM for methyl indole-3-propionate), 88 microM and 120 microM, respectively, whilst those for MAO B were each greater than 1 mM. The artificial formation of these esters was excluded by carrying the parent acids, from which they are presumably synthesized, through the purification procedure. As tribulin output is increased during stress or anxiety, these results point to a possible role for tryptamine and tyramine pathways in such disorders.

Stressor predictability influences open field behavior, pain sensitivity and brain MAO inhibitory activity (tribulin) in the rat

The effects of predictable or unpredictable shocks on ambulation, pain sensitivity, plasma catecholamines and heart and brain monoamine oxidase (MAO) inhibitory activity were investigated in rats. Animals showed plasma catecholamines and heart MAO inhibitory activity sensitization irrespective of type of treatment, while differences between groups were observed when open field and hot plate tests and brain MAO inhibitory activity were considered. These effects parallel those observed using the classic triadic design leading to the helpless state. Our results suggest that predictability per se is able to generate this phenomenon.

The stress-induced reduction in monoamine oxidase (MAO) A activity is reversed by benzodiazepines: role of peripheral benzodiazepine receptors

1. The effect of benzodiazepine pretreatment on the stress-induced decrease in MAO activity in rat tissues using footshock as stress model was investigated. 2. Animals were injected with vehicle, Lorazepam (1.25 mg/kg), or Clonazepam (0.5 mg/kg) 2 hr before or with PK 11195 (0.45 mg/kg) 2.5 hr before being subjected to one session of 10 inescapable footshocks or to a sham session. At the end of the session animals were sacrificed and MAO A and B activities in hearts and brains were determined. 3. Pretreatment of the animals with both Lorazepam and Clonazepam abolished the decrease induced by footshock in MAO A activity in brain. Pretreatment with Lorazepam but not with Clonazepam abolished the stress-induced decrease in MAO A in the heart. Pretreatment with PK 11195 before Lorazepam reversed its effects in the heart but not in the brain. Neither footshock nor any of the drugs used had any effect on heart and brain MAO B. 4. Our results suggest that in the heart but not in the brain, peripheral benzodiazepine receptors play a role in the regulation of MAO A activity under stress conditions.

Changes in sympathetic nerve terminals in the heart of cold-exposed rats

Changes in sympathetic nerve terminals of the heart after varying periods of exposure of rats to 4 degrees C were investigated. Two indices were used for changes in the number of noradrenaline storage vesicles, i.e., vesicular dopamine beta-hydroxylase (DBH) activity and noradrenaline storage capacity. The latter was obtained after uptake of [3H]noradrenaline; endogenous content, uptake of exogenous noradrenaline, and degree of saturation of the vesicles were calculated using the specific activity of the [3H]noradrenaline. As a measure of tyrosine hydroxylase activity, whole ventricular noradrenaline, dopamine, and dihydroxyphenylacetic acid content were used. After 4 h of cold exposure there was an increase in vesicular endogenous noradrenaline content, uptake, storage capacity, and DBH activity as well as a large increase in whole ventricular dopamine. After 6 h in the cold, vesicular endogenous noradrenaline content, storage capacity, and DBH activity were decreased. The results suggest that during cold exposure there is an initial increase followed by a decrease in the number of functional vesicles in the nerve terminal, which could explain the fluctuations in the rate of noradrenaline release.

Increase of brain endogenous monoamine oxidase inhibitory activity (tribulin) in experimental audiogenic seizures in rats: evidence for a monoamine oxidase A inhibiting component of tribulin

Brain tribulin activity in rats with an inherited predisposition to audiogenic epilepsy was studied after seizures of different intensity were induced by an electric bell. Weak seizures (from 0 to 2 arbitrary units) did not produce any changes in endogenous inhibitory activity towards either monoamine oxidase (MAO) A or B. Moderate seizures were characterized by increases in both MAO A and MAO B inhibitory activity (up to 1.9-fold). Complete tonic epileptiform seizures with total areflexia (4 arbitrary units) induced further augmentation (up to 2.5-fold) of MAO A but not of MAO B inhibitory activity. This dissociation between the two inhibitory activities points to the existence of a separate MAO A-inhibiting component of brain tribulin which is different from isatin.

Impact of psychological dynamics of stress on the peripheral benzodiazepine receptor

In an attempt to dissociate the relative impact of psychological vs. physiological concomitants of stress on the peripheral benzodiazepine receptor (PBR), the influence of stressor controllability and predictability was investigated in rats. In addition, the effect of a purely psychological stressor, contextually conditioned fear, was examined. The response of the PBR in rats confronted with a naturalistic threat, a cat, was also tested. Various peripheral and CNS tissues were analyzed. Specific binding of [3H]Ro 5-4864 was significantly reduced in the kidneys of subjects receiving either controllable or uncontrollable shock. Similar changes were seen in the kidneys of subjects receiving either predictable or unpredictable shock. Mean [3H]Ro 5-4864 binding in lung was reduced following both predictable and unpredictable shock, but only the reduction in the predictable shock group reached significance. Controllability appeared to protect against the stress-induced reduction in [3H]Ro 5-4864 binding in lung. Contextually conditioned fear only affected PBR in the olfactory bulb, and exposure to a cat was without effect. These data suggest that the PBR responds only to potent stressors, and psychological influences on the PBR are tissue specific.

Social isolation does not alter brain regional benzodiazepine binding site numbers, affinity and coupling in the rat

Male Sprague-Dawley rats were differentially housed for 21 days immediately after weaning. Isolated animals showed a selective suppression of exploration of the light side of a two compartment box; spending significantly less time in the light, and making fewer transitions between the light and dark compartments compared to socially reared controls. However, both basal and GABA-stimulated [3H] flunitrazepam binding was unaltered in the frontal cortex, hippocampus, amygdala and cerebellum following social isolation. These results are discussed in relation to other studies on central benzodiazepine receptor changes following a variety of experimental stressors.

Urinary output of endogenous monoamine oxidase inhibitor and isatin during acute migraine attacks

Urinary output of endogenous monoamine oxidase (MAO) inhibitory activity, was significantly raised in serial samples collected across a migraine attack compared with collections during attack-free periods and in healthy controls, which did not differ from each other. There was a highly significant correlation in output between isatin, a major fraction of the MAO inhibitory activity, and output of the MAO inhibitory activity itself. However, although there was a tendency towards increased isatin excretion during migraine attacks, it failed to reach statistical significance.

Footshock affects heart and brain MAO and MAO inhibitory activity and open field behavior in rats

This study examined the effects, after 1 min or 2 hr, of one footshock session on the activity of MAO in rat heart and brain, the MAO inhibitory activity of these tissues, and the animal's behavior in an open field. Internal ambulation was reduced at both times; the lowest score was registered at 1 min. The number of boluses emitted during the test was higher in the group tested at 2 hr than in the other groups. One min after shocks MAO activity in heart and brain was decreased. In the heart MAO was still decreased 2 hr later, then reaching the lowest levels, while at that time, brain MAO was not different from controls. When assayed separately (MAO A and B), only the A form was found to change. MAO inhibitory activity in heart was increased at both times, the highest activity observed 2 hr after footshock. Brain MAO inhibitory activity was increased only in the 1-min group. Ex vivo competition experiments with clorgyline suggested presence in vivo of a reversible MAO inhibitor. The time-dependent response to stress of both MAO activity and MAO inhibitory activity in the tissues correlates with the responses observed in the open field test. These findings suggest that the observed biochemical changes might be related to increased autonomic activity and to the state of fear and anxiety evoked by the stressful procedure.

Isatin and tribulin concentrations are increased in rabbit brain but not liver following pentylenetetrazole administration

Isatin has recently been identified in rat tissues and normal human urine where it constitutes the major part of the endogenous monoamine oxidase inhibitor, tribulin. In this study, we have measured both isatin (by gas chromatography/mass spectrometry) and tribulin (inhibition of a standard rat liver monoamine oxidase preparation) in extracts of rabbit brains and livers after intravenous administration of the anxiogenic, proconvulsant agent pentylenetetrazole. There were increased levels of both isatin and tribulin in rabbit brains, but not in livers, after pentylenetetrazole, compared with saline treated controls. This finding supports the hypothesis that isatin is a component of tribulin activity and may have a role in anxiety or epilepsy.

Repeated (isolation) stress increases tribulin-like activity in the rat

1. The effect of repeated isolation stress on MAO inhibitory activity (tribulin) in rat tissues as well as on plasma catecholamine levels was investigated. 2. Animals were subjected to a daily period of isolation (9 min) and sacrificed on days 1, 2, 4, and 5. 3. In brain and cerebellum the levels of both inhibitory activities were found to be significantly higher in animals sacrificed on days 1-2 than in either controls or animals sacrificed on days 4-5. 4. In heart and kidney the highest levels of both activities were found in animals sacrificed on days 4-5. 5. Plasma levels of dopamine on day 4 were significantly higher than those in controls or in any of the experimental groups. Plasma levels of epinephrine showed step-by-step increments from day 1 up to day 5, reaching statistical significance only on day 5. Plasma levels of norepinephrine were significantly increased on days 2, 4, and 5. 6. Under the experimental conditions of this study, we have shown a rapid and short-lasting increment of tribulin in the central nervous system. Its disappearance on days 4-5 could be related to adaptation to the novel situation. Changes in the peripheral tissues appeared later, and a similar adaptation was absent during the period of observation. 7. Tribulin would be related to the stressful situation not only as an anxiety-promoting agent but also in contributing to the maintenance of high levels of circulating catecholamines.