Effects of oral clonidine on plasma 3-methoxy-4-hydroxyphenethyleneglycol (MHPG) in man: Preliminary report

Repeated (N=15) administration of clonidine (0,1,5 μg/kg,p.o.) to three normotensive male subjects resulted in significant decreases in plasma free 3-methoxy-4-hydroxyphenethyleneglycol (MHPG) at three hours for both the 1 μg/kg dose (p < .05) and the 5 μg/kg dose (p < .01) when compared to concentrations following placebo. The mean decrement in plasma free MHPG following a 5 μg/kg dose was 36%. Systolic blood pressure fell a mean of 17 mmHg after 1 μg/kg and 37 mmHg after 5 μg/kg of clonidine. The application of a clonidine challenge test to assess noradrenergic receptor sensitivity $\text{in}$$\text{vivo}$ is discussed.     

Cerebrospinal fluid,plasma, and urinary MHPG in children

Cerebrospinal fluid (CSF) free MHPG levels for six autistic and four medicated Tourette Syndrome (TS) patients are in the usual adult range, but levels are elevated in two recently medicated TS patients. Plasma free MHPG levels in unmedicated autistic and TS patients are similar to those in normal boys, but are increased in two of seven medicated TS patients. There is a strong association between plasma free MHPG and urinary total MHPG levels in normal boys. Age and total urinary MHPG are positively correlated in autistic children. MHPG appears to be a useful index of noradrenergic function in childhood neuropsychiatric disorders.     

Yohimbine induced anxiety and increased noradrenergic function in humans: effects of diazepam and clonidine

Yohimbine (30 mg) produced significant increases in subjective anxiety, autonomic symptoms, blood pressure, and plasma 3-methoxy-4-hydroxy-phenylethyleneglycol (MHPG) in ten healthy subjects. The effects of pretreatment with diazepam (10 mg) or clonidine (5 micrograms/kg) on these yohimbine induced changes was examined. Both diazepam and clonidine significantly antagonized yohimbine-induced anxiety, but only clonidine significantly attenuated the yohimbine induced increases in plasma MHPG, blood pressure, and autonomic symptoms. When given alone, clonidine significantly decreased plasma MHPG and blood pressure, whereas diazepam did not. These findings indicate that: (1) noradrenergic hyperactivity may be a factor in the production of some anxiety states; (2) the anti-anxiety effects of clonidine appear to result from its actions on receptors which decrease noradrenergic activity; (3) diazepam reverses yohimbine-induced anxiety without effects on several physiological or biochemical indicators of noradrenergic activity in humans.     

Possible change in noradrenergic receptor sensitivity following methylphenidate treatment: Growth hormone and MHPG response to clonidine challenge in children with attention deficit disorder and hyperactivity

Growth hormone (GH) and 3-methoxy-4-hydroxyphenelethylene glycol (MHPG) response was measured hourly for 4 hours in 8 children with Attention Deficit Disorder with Hyperactivity (ADD+H) following an acute single-dose of clonidine. The clonidine challenge was repeated before, during, and one day after 12 weeks of treatment with methylphenidate (MPH). Before MPH treatment, the plasma growth hormone (GH) rose to 31.3 ± 4.6 (Mean ± SE) ng/ml; during MPH treatment, the GH peak was only 14.8 ± 3.2 ng/ml; one day after discontinuation of MPH, GH rose to only 20.8 ± 3.9 ng/ml. MHPG release was inhibited by clonidine in all treatment conditions but tended to be more decreased during MPH treatment. Some children with ADD+H may have hypersensitivity of the post-synaptic alpha-1 noradrenergic receptor which is diminished by MPH treatment. The extent to which these effects are pharmacological or represent a change in receptor sensitivity requires further study.     

Clonidine Prevents Methylxanthine Stimulation of Norepinephrine Metabolism in Rat Brain

Methylxanthines can produce behavior resembling opiate withdrawal in rats. Since previous studies have demonstrated the involvement of central noradrenergic systems during naloxone-precipitated withdrawal, the effects of 3-isobutyl-1-methylxanthine (IBMX) on norepinephrine metabolism in rat brain were studied. It was found that administration of IBMX elevated levels of the major norepinephrine metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG) in areas innervated by the locus coeruleus. The increases in MHPG was noted 1 h after administration and was maximal (270% of control) after 3 h. Levels of another norepinephrine metabolite, 3,4-dihydroxyphenylglycol, followed a similar pattern and time course. Coadministration of naloxone with IBMX did not affect the IBMX-induced elevation in MHPG. Administration of the alpha-agonist clonidine, however, antagonized the effects of IBMX on MHPG levels. The effects of IBMX and clonidine were dose dependent; the lowest dose of IBMX needed to elevate MHPG was 30 mumol/kg (i.p.), and clonidine (180 nmol/kg) reduced the effect of IBMX (100 mumol/kg) by 50%. The data, discussed in terms of a methylxanthine-noradrenergic interaction, suggest that withdrawal behaviors in general may be subserved by hyperactive noradrenergic neurons.     

Effects of acutely and chronically administered antidepressants on the clonidine-induced decrease in rat brain 3-methoxy-4-hydroxyphenylethylene-glycol sulphate content.

A study was undertaken of the effects of acutely and chronically administered agents on rat brain MHPGSO₄ content and on the ability of a low dose of clonidine (25 μg/kg) to lower brain MHPGSO₄ levels. This effect of clonidine is attributed to an activation of presynaptic α-adrenoceptors. The agents studied were desipramine and nisoxetine, both inhibitors of NA uptake, Org 6582, a specific inhibitor of 5-HT uptake, and iprindole and mianserin, two atypical antidepressants. In all studies, a dose of 10 mg/kg was used. Acutely administered mianserin increased rat brain MHPGSO₄ levels and prevented the clonidine-induced reduction. The clonidine-induced fall in MHPGSO₄ levels was also absent following the acute administration of desipramine or nisoxetine. However, the observed effect cannot be unequivocally attributed to a blockade of presynaptic α-adrenoceptors since both drugs decreased basal levels of MHPGSO₄. For chronic studies drugs, other than Org 6582, were injected every 12 h for 14 days and experiments were undertaken 12 h after the last injection. Org 6582 was injected once daily for 14 days and experiments undertaken 24 h after cessation of administration. Chronic mianserin and nisoxetine increased MHPGSO₄ levels. Only chronic desipramine blocked the clonidine-induced fall, the phenomenon developing between 5 and 9 days of chronic desipramine administration. This study indicates that, under the experimental conditions employed, the ability of chronic desipramine to elicit subsensitivity of presynaptic α-adrenoceptors does not extend to the other four agents studied.     

Clonidine Prevents Transient Loss of Noradrenaline in Response to Cholinergic Hypofunction Induced by Ethylcholine Aziridinium (AF64A)

Intracerebroventricular injection of ethylcholine aziridinium (AF64A) (2 nmol/ventricle) induced a considerable decrease in the level of acetylcholine (ACh) in hippocampus (from 21.14 +/- 0.84 to 10.04 +/- 0.59 pmol/mg of tissue; p less than 0.001) 4 days after application. The reduction of cholinergic function was accompanied by a decrease in the level of noradrenaline (NA) (from 1.96 +/- 0.08 to 1.41 +/- 0.06 pmol/mg of tissue; p less than 0.001). Two days after administration of AF64A (1 or 2 nmol/ventricle), the dose-dependent decrease in NA level was associated with an increase in the level of its major metabolite, 3-methoxy-4-hydroxyphenylglycol (MHPG), resulting in a considerable increase in the MHPG/NA molar ratio (from 0.84 +/- 0.06 to 1.62 +/- 0.17; p less than 0.002). Chronic treatment of AF64A-injected rats with clonidine (0.02-0.2 mg/kg, i.p., every 8-12 h) had no significant effect on the loss of ACh content, whereas the decrease in NA content in hippocampus was completely prevented. Clonidine induced aggressive behavior in the AF64A-treated rats, in contrast to sedation in vehicle-injected rats. The response to clonidine under these experimental conditions and the increased MHPG/NA molar ratio in response to AF64A suggest that the transient loss of NA content following AF64A administration results from increased NA release. The increased noradrenergic activity in hippocampus may be linked to the reduction of tonic inhibitory cholinergic input. These results are discussed in relation to possible implications for senile dementia of the Alzheimer type.     

Trace amines and Tourette's syndrome

There has been considerable interest in recent years in possible neurochemical abnormalities in Tourette's Syndrome (TS). In studies combining neuropsychological and neurochemical measurements, we have investigated the possible roles of trace amines in this disorder. Urinary levels of free -phenylethylamine (PEA) and plasma levels of its precursor amino acid phenylalanine were decreased in TS patients when compared to values in normal children. These urinary PEA levels in TS patients were inversely related to several scores from the Tourette's Syndrome Global Scale (TSGS). Further investigation of the group of subjects with low urinary PEA indicated that they also had low levels of MHPG, normetanephrine, 5-HT andm- andp-tyramine. Patients with low PEA were also compared on an extensive battery of neuropsychological measures and observed to perform significantly worse than TS patients with normal urinary PEA levels. Biochemical measurements also suggest a possible abnormality in tryptamine turnover in TS since urinary levels of indole-3-acetic acid (IAA; the acid metabolite of tryptamine) are significantly lower in TS patients than in normal controls.     

Central and peripheral noradrenergic tone in primary hypertension

The contents of norepinephrine (NE), epinephrine (E), dopamine (DA), normetanephrine (NMN), and 4-hydroxy-3-methoxyphenylethylene glycol (MHPG) were measured in the plasma and cerebrospinal fluid (CSF) of 66 patients with primary hypertension and 24 patients with normal blood pressure and minor neurological disorders. Plasma and CSF NE and NMN concentrations were raised in the hypertensive patients. The plasma and CSF NE levels and arterial blood pressure of a small subset of hypertensive patients were normalized after clonidine therapy. In hypertensive patients the content of DA was lower and the ratio of NE/DA was greater; CSF and plasma NE contents were related to the level of arterial blood pressure; and the content of MHPG in CSF was linked strongly with NE content in plasma and CSF and to the level of arterial blood pressure. Thus both central sympathetic nerve tone and peripheral sympathetic nerve tone were enhanced in young patients with uncomplicated hypertension. The elevated levels of neurohormones and their metabolites in some patients with primary hypertension may be related to increased synthesis and release of neural NE and may be pathogenic in the blood pressure elevation.     

The contribution of CNS MHPG to plasma MHPG in the rat

Two different experimental approaches were used to determine the central nervous system (CNS) contribution to plasma total 3-methoxy-4-hydroxyphenylglycol (MHPG) levels in the rat. One experiment, using intracisternal injection of 6-hydroxydopamine (6-OHDA), depleted total forebrain norepinephrine (NE) and MHPG to 26 and 34% of control values, respectively. In spite of the substantial reduction in CNS MHPG, plasma MHPG was not significantly different from control values. The second experiment used clonidine and debrisoquine to differentially impair central and peripheral NE metabolism. The results of this experiment confirm those of the 6-OHDA experiment in suggesting that the CNS contribution to plasma MHPG in the rat is negligeable.     

Central alpha-activation by clonidine reduces plasma level of beta-endorphin in patients with essential hypertension

Whether peripheral beta-endorphin contributes to the antihypertensive action of clonidine was examined by measuring plasma levels of beta-endorphin-like immunoreactivity (beta EpLI) after acute administration of clonidine in patients with essential hypertension. Administration of clonidine (0.225 mg) in one dose significantly lowered blood pressure, decreased heart rate and reduced the plasma level of beta EpLI and ACTH, while the placebo had no effect on blood pressure, heart rate or plasma level of beta EpLI suggesting that peripheral beta-endorphin does not play a major role in the antihypertensive action of acute clonidine administration.     

The effects of caffeine on plasma MHPG,subjective anxiety,autonomic symptoms and blood pressure in healthy humans

The effects of oral administration of caffeine (10 mg/kg) on plasma free 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) levels, behavioral ratings, blood pressure, and autonomic symptoms was determined in eleven healthy subjects. Caffeine produced robust increases in subject rated anxiety and nervousness and small elevations in blood pressure and a decrease in heart rate. Caffeine did not alter plasma MHPG in a consistent fashion and there was no correlation between changes in plasma MHPG and changes in anxiety or other ratings. Caffeine may produce symptoms of anxiety-nervousness without increasing central norepinephrine turnover.     

Diurnal rhythm of 3-methoxy-4-hydroxyphenylglycol (MHPG): relationship between plasma and urinary levels

Plasma and urinary levels of MHPG were determined in six normal volunteers. Samples were obtained at 3-hour intervals for plasma and at 12-hour intervals for urine. Acrophase, amplitude and period were determined for plasma MHPG levels. A sinusoidal pattern was obtained for diurnal plasma MHPG with a peak at 15:00 hrs. +/- 46 min. Urinary MHPG, corrected for creatinine levels, correlated with both 9 AM plasma MHPG and with baseline plasma MHPG. Furthermore, the relationship between plasma and urinary MHPG was linear when the rhythm of urinary levels was assumed to lag 6.2 hours behind the plasma rhythm. It was concluded that free MHPG is evenly distributed in the total body space and that conjugated MHPG is largely restricted to the blood.     

The effect of desmethylimipramine on the metabolism of norepinephrine

Eleven normal volunteers were given an acute and two chronic doses of desipramine (DMI). The plasma norepinephrine (NE), 3-methoxy-4-hydroxyphenylglycol (MHPG), and dihydroxyphenylglycol (DHPG) concentrations were measured before and during drug administration. DMI reduced plasma concentrations of MHPG by 13% and DHPG by 17%. After two weeks of drug administration, the MHPG/NE ratio was reduced, and there was a significant negative correlation with the concurrent drug concentration. These results suggest that DMI: (1) reduces the turnover of NE; and (2) diminishes the oxidative deamination of NE. In addition, the drug concentration response relationship indicates that the effects of uptake inhibition may not be maximal until concentrations in the apparent therapeutic range are achieved.     

Plasma 3,4-dihydroxyphenylglycol (DHPG) and 3-methoxy-4-hydroxyphenylglycol (MHPG) are insensitive indicators of alpha 2-adrenoceptor mediated regulation of norepinephrine release in healthy human volunteers.

The usefulness of the plasma concentrations of two major metabolites of norepinephrine (NE), 3,4-dihydroxyphenylglycol (DHPG) and 3-methoxy-4-hydroxyphenylglycol (MHPG), as indicators of neuronal NE release was investigated. The potent alpha 2-adrenoceptor agonist, dexmedetomidine, induced only about 15% maximal reductions in the metabolite concentrations, in spite of almost total inhibition of neuronal NE release, as evidenced by 90% reductions in plasma NE concentrations. Similarly, administration of the alpha 2-adrenoceptor antagonist atipamezole was followed by only small increases in plasma DHPG and no change in MHPG levels, in spite of almost six-fold, albeit short-lasting, increases in plasma NE. In contrast, a single dose of the reversible monoamine oxidase type A (MAO-A) inhibitor moclobemide reduced plasma DHPG levels by 78% and MHPG levels by 51%. It is concluded that the plasma concentrations of DHPG and MHPG are largely determined by intraneuronal, MAO-A-dependent metabolism of NE, and do not accurately reflect acute alterations in neuronal NE release. The concentration of NE in venous plasma is clearly a more sensitive indicator of alpha 2-adrenoceptor-mediated regulation of NE release.     

Clonidine reduces elevated cerebrospinal fluid catecholamine levels in patients with essential hypertension

Cerebrospinal fluid (CSF) catecholamines were measured in normotensive patients and in patients with mild to moderate essential hypertension. CSF-norepinephrine (NE) concentrations were 50% lower in the normotensive individuals (127 ± 28 vs. 240 ± 23 pg/m1) (P<0.01). In hypertensive patients, CSF-NE was inversely related to age (r =-0.68; P<0.01) and directly related to plasma NE (r = 0.61; P<0.05). Clonidine (450 mcg/day for 2 weeks) significantly reduced CSF-NE (?40%) in hypertensive patients. In addition, it decreased blood pressure, plasma and urinary NE. Urinary VMA was not affected by clonidine. No correlation was observed between clonidine effects on BP and on plasma or CSF catecholamines. This study indicates that patients with essential hypertension have elevated levels of CSF-NE which are reduced after treatment with clonidine. The elevation of CSF-NE suggests that central (spinal?) noradrenergic activity may be increased in patients with mild to moderate essential hypertension, and that can be reduced by treatment with clonidine.     

Rapid non-enzymatic HPLC determination of total MHPG in human plasma

We have previously reported a method for the determination of total 3-methoxy-4-hydroxy phenylethylene glycol (MHPG) in brain, based on a simple acid-catalyzed hydrolysis. Now we extend this procedure to the determination of plasma total MHPG. The method involves the deproteinization of plasma with perchloric acid, followed by 3 minutes of an acid-catalyzed step. The hydrolysates are injected into the HPLC system, using a formic acid/methanol eluent with fluorimetric detection. Sample detection limit is below 1 ng MHPG/mL of plasma. This procedure has been used for the determination of plasma total MHPG from 109 healthy individuals of both sexes. Mean value was: 5.4 + 2.3 ng total MHPG/mL of plasma (means +/- S.D., N = 109). No sex differences were observed, and a slight correlation with age (r = 0.24, p less than 0.02) has been found. Plasma-free MHPG was also determined in a subgroup of 15 randomly chosen individuals (3.0 +/- 1.2 ng free MHPG/mL plasma, means +/- S.D.). A significant correlation was obtained with plasma total MHPG (r = 0.77, p less than 0.001, N = 15). The main advantage of the present method lays in its simplicity, since no enzymatic hydrolysis or extraction procedures are needed, being its reliability fully proven through 109 plasma total MHPG determinations.     

Rat Brain and Plasma Norepinephrine Glycol Metabolites Determined by Gas Chromatography-Mass Fragmentography

Abstract: A gas chromatographic-mass fragmentographic (GC-MF) procedure is described for the simultaneous quantitation of 3,4-dihydroxyphenyl-ethyleneglycol (DHPG) and 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) in brain tissue and plasma. DHPG and MHPG were assayed as their respective acetyl-trifluoroacyl esters, using [²H₂]DHPG and [²H₃]MHPG as internal standards. Assay sensitivities of at least 1 ng per sample were attainable for the quantitation of free glycols, whereas for determination of total DHPG, assay sensitivity was 2.5 ng. Whole rat brain total (99.2 ±4.11 ng/g) and free (13.0 ± 1.14 ng/g) DHPG concentrations were similar to respective total (86.0 ± 3.70 ng/g) and free (12.3 ± 0.41 ng/g) MHPG levels. Total DHPG concentrations exceeded total MHPG levels in hypothalamus (3.0:1), midbrain (1.4:1), pons plus medulla (1.3:1), and hippocampus (1.5:1), whereas in other brain regions the levels of these metabolites were similar. In plasma, however, total DHPG levels were only 20% as high as MHPG concentrations. In mouse brain, DHPG and MHPG occurred almost entirely in free form (>90%), but total DHPG levels were only 50% as high as respective MHPG concentrations. These results emphasize the substantial formation of DHPG compared with MHPG in rat and mouse brain and suggest that DHPG formation and eMux may be of equal or greater importance than MHPG in the metabolic clearance of CNS norepinephrine in some species.     

Factors influencing preoperative stress response in coronary artery bypass graft patients

BACKGROUND: In many studies investigating measures to attenuate the hemodynamic and humoral stress response during induction of anaesthesia, primary attention was paid to the period of endotracheal intubation since it has been shown that even short-lasting sympathetic cardiovascular stimulation may have detrimental effects on patients with coronary artery disease. The aim of this analysis was, however, to identify the influencing factors on high catecholamine levels before induction of anaesthesia. METHODS: Various potential risk factors that could impact the humoral stress response before induction of anaesthesia were recorded in 84 males undergoing coronary aortic bypass surgery, and were entered into a stepwise linear regression analysis. The plasma level of norepinephrine measured immediately after radial artery canulation was chosen as a surrogate marker for the humoral stress response, and it was used as the dependent variable in the regression model. Accordingly, the mean arterial blood pressure, heart rate and the calculated pressure-rate product were taken as parameters of the hemodynamic situation. RESULTS: Stepwise regression analysis revealed that the oral administration of low-dose clonidine (mean dose 1.75 μg.kg-1) on the morning of surgery was the only significant predictor (p = 0.004) of the high variation in preoperative norepinephrine plasma levels. This intervention decreased norepinephrine levels by more than 40% compared to no clonidine administration, from 1.26 to 0.75 nmol.l-1. There was no evidence for dose-responsiveness of clonidine. All other potential predictors were removed from the model as insignificant (p > 0.05). The use of beta-blocker, ace-inhibitors, ejection fraction, and body mass index were significant determinants for the hemodynamic situation (heart rate, mean arterial pressure, pressure rate product) of the patient during the pre-induction period. CONCLUSION: The oral administration of clonidine is the only significant predictor for the observed variation of norepinephrine levels during the preoperative period. Lack of significant dose responsiveness suggests that even a low dose of the drug can attenuate the preoperative stress response and thus is recommended in cardiovascular high risk patients.     

Clonidine and the hormonal responses to graded exercise in healthy subjects

The aim of this study was to assess the acute effects of clonidine, an alpha 2-adrenergic agonist, on hormonal responses to graded exercise in 8 healthy young men. After fasting overnight, each subject was tested on 2 mornings, 1 week apart. On one occasion he was given 200 micrograms clonidine orally and on the other identical placebo tablets, the order being randomized in a double-blind fashion over the 2 days. Thereafter each subject performed 2 successive treadmill runs, equivalent to 60 and 100%, respectively, of maximal aerobic power. Clonidine pretreatment blunted the maximal increase in plasma catecholamines by more than 60% of the control response (p less than 0.01), without significantly altering the rise of plasma cortisol or ACTH. Furthermore, clonidine significantly reduced the exercise-induced maximal rise in plasma glucose, without modifying the slight decline in mean plasma insulin or increase in pancreatic glucagon levels. The drug did not affect the maximal increments in plasma growth hormone or prolactin occurring after exercise. It was concluded that a single dose of clonidine suppressed peripheral sympathetic responses, without altering central (pituitary) alpha-adrenergic-mediated hormonal responses, to short-term exercise in healthy men.