A study of the formation and metabolic disposition of 3,4 dihydroxyphenylethyleneglycol in whole rat brain

Time course of the metabolite patterns of intraventricularly injected [3H]noradrenaline in rat brain regions

In the hypothalamus, septum, pons with medulla, and hippocampus regions of rat brain, the level of radioactivity of [3H]noradrenaline and of five of its metabolites were determined up to 6 h after intraventricular injection of the tritiated amine. The following main results were found: In anterior hypothalamus and septum, the [3H]noradrenaline level declined in two phases. Similar turnover curves were obtained for the primary deaminated metabolites, with almost the same final half-lives as for [3H]noradrenaline. The level of the initial methylation product, normetanephrine, also showed a biphasic decline, which did not correspond to that of [3H]noradrenaline but rather was faster throughout the experiment. The final metabolites (i.e., the glycol sulfates) reached maximal levels in hypothalamus and septum earlier than in other regions. Thereafter, their levels declined with almost similar rates in all areas tested, but always faster than the [3H]noradrenaline level. The following conclusions were drawn: In areas rich in catecholaminergic nerve terminals, there seems to be a site, in addition to the vesicular storage pool, that accumulates exogenous noradrenaline and then releases it with relatively short half-lives. The contents of primary deaminated metabolites followed the turnover of [3H]noradrenaline at both sites. Exogenous [3H]noradrenaline seems to be methylated at two extraneuronal sites, which are distinguished by the rates of subsequent deamination. The size of the pool of slowly deaminated [3H]normetanephrine that is formed immediately after [3H]noradrenaline injection determined the apparent turnover of this product throughout the experiment and, thus, like the final metabolites, reflects for several hours the initial degradation of the unstored [3H]noradrenaline, rather than the metabolism of the stored amine.(ABSTRACT TRUNCATED AT 250 WORDS)

Further characterization of brain 3,4-dihydroxyphenylethyleneglycol (DHPG) formation: dependence on noradrenergic activity and site of formation

The dependence of brain and plasma 3,4-dihydroxyphenylethyleneglycol (DHPG) formation upon CNS noradrenergic neutronal activity was evaluated following manipulations that are known to alter the firing rate of the locus coeruleus (LC) neurons and as a consequence, noradrenaline (NA) release and turnover. In addition, the relative degree of intraneuronal formation of brain DHPG was assessed by studying the metabolism of released NA during uptake inhibition. Electrical stimulation of the LC for 20 min induced an increase in rat cortical (40-42%), hypothalamic (22-29%) and plasma (68-79%) total DHPG and 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) levels. Two hours following administration of the alpha-2 adrenoceptor antagonist yohimbine (10 mg/kg, i.p.), rat brain cortical conjugated DHPG and MHPG as well as free MHPG concentrations were increased whereas cortical free DHPG levels remained unchanged. The same treatment also increased plasma total DHPG and MHPG levels. In mice given the NA uptake inhibitor desipramine (10 mg/kg, i.p.) 2 h prior to sacrifice, brain free DHPG and MHPG concentrations were significantly reduced by 30 and 40%, respectively, whereas yohimbine (1-20 mg/kg, i.p.) induced a dose-dependent increase in brain DHPG (60-80%) and MHPG (60-220%) concentrations. Pretreatment with desipramine (10 mg/kg, i.p.) 30 min prior to yohimbine reduced, in rat, or abolished, in mice, the yohimbine-induced elevation of brain DHPG levels. In contrast, desipramine augmented the effect of yohimbine on brain MHPG levels resulting in a shift to the left of the dose response curves. These findings indicate that brain and plasma DHPG levels are sensitive to changes in brain noradrenergic neuronal impulse flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Formation and clearance of norepinephrine glycol metabolites in mouse brain

To determine the degree of conversion of 3,4-dihydroxyphenylethyleneglycol (DHPG) to 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) and the amount of DHPG eliminated unchanged from the brain, we have examined the kinetics of formation and disappearance of mouse brain MHPG and DHPG following clorgyline (10 mg/kg, i.p.) and/or tropolone (75 mg/kg, i.p.) treatment. During the first 10 min after tropolone, brain DHPG levels accumulated linearly at a rate of 1,300 pmol/g/h, whereas MHPG disappeared exponentially at a rate of 411 pmol/g/h. Following clorgyline administration, brain DHPG declined exponentially at a rate of 1,240 pmol/g/h. In contrast, the elimination of MHPG became a first-order process only when catechol-O-methyltransferase (COMT) was also inhibited in addition to monoamine oxidase. Thus, combined clorgyline and tropolone treatment resulted in an exponential decline of MHPG levels at a rate of 524 pmol/g/h, whereas DHPG levels were slightly but significantly elevated compared to control values. When the animals were treated with pargyline (75 mg/kg, i.p.) in combination with clorgyline and tropolone, brain DHPG and MHPG disappeared at rates of 40 and 660 pmol/g/h, respectively. The above observations suggest that mouse brain DHPG is cleared primarily through O-methylation with minimal direct elimination from brain. Assuming the disposition and clearance of norepinephrine metabolites are similar in mouse and human brain, peripherally measured DHPG in humans is likely derived principally from extracerebral sources and reflects peripheral sympathetic function.

Metabolic fate of 3,4-dihydroxyphenylethyleneglycol (DHPG) in mouse brain

Mouse brain DHPG and MHPG turnover rates were estimated by determining their initial rates of disappearance or accumulation following MAO and/or COMT inhibition. Similar turnover estimates of brain DHPG were obtained following MAO or COMT inhibition, which were comparable to the estimated NE turnover obtained from its initial accumulation following MAO plus COMT inhibition. It was estimated that negligible amounts of DHPG were eliminated directly from brain, the majority being cleared through O-methylation. These findings indicate that mouse brain NE is primarily cleared through DHPG formation followed by O-methylation, and also suggest that brain DHPG turnover is more indicative of NE turnover.

Rat brain and plasma norepinephrine glycol metabolites determined by gas chromatography-mass fragmentography

A gas chromatographic-mass fragmentographic (GC-MF) procedure is described for the simultaneous quantitation of 3,4-dihydroxyphenylethyleneglycol (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 sensitives 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 (greater than 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 efflux may be of equal or greater importance than MHPG in the metabolic clearance of CNS norepinephrine in some species.

Origin of free 3-methoxy-4-hydroxyphenylethyleneglycol

The origin of free 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG, a major metabolite of norepinephrine) in rat cerebrospinal fluid (CSF) was investigated using brain and spinal cord perfusions with artificial CSF, electrical stimulation of the locus coeruleus (LC) and the technique of retrograde cell labelling with horseradish peroxidase (HRP). The rates of appearance of MHPG into ventricular-cisternal and lumbar-cisternal perfusates were about 12.4 and 19.2 ng/hour respectively. Probenecid (200 mg/kg i.p.) did not alter the MHPG output in either preparation. Intravenous administration of 3 micrograms MHPG did not substantially enhance the outflow of the metabolite in the ventricular-cisternal perfusate, indicating that MHPG found in the perfusate is of central origin. Maximal activation of central norepinephrine (NE)-containing neurons of the LC by electrical stimulation induced a 50% increase of MHPG levels in the ventricular-cisternal perfusate. In addition HRP was found to be retrogradely transported from the lateral ventricle to these LC-neurons. These findings indicate that MHPG in CSF originates, at least in part from NE of nerve terminals adjacent to the cerebral ventricles, which have their origin in the LC. We calculated that the fraction of MHPG formed in the central nervous system, that was released into the CSF was about 34%. It is concluded that MHPG in CSF is a measure for changes in central NE turnover and that a considerable portion is dependent upon the activity of LC neurons.

3-Methoxy-4-hydroxyphenethyleneglycol production by human brain in vivo

A direct method has been employed to estimate the rate of production by human brain of 3-methoxy-4-hydroxyphenethyleneglycol, the major metabolite of brain norepinephrine, a brain neurotransmitter. Venous specimens were obtained from the internal jugular vein from ten awake human subjects at a puncture site above the common facial vein, the first major source of extracranial inflow. Arterial specimens were simultaneously obtained from the radial artery. Plasma samples were assayed and a highly significant difference was found in the concentration of the metabolite in plasma coming out of the brain (venous blood) as compared to plasma entering the brain (arterial blood). This venous-arterial difference was calculated to be 0.7 +/- 0.1 nanogram per milliliter of blood. Assuming an adult brain weight of 1400 grams and normal cerebral blood flow, it is estimated that the rate of production of 3-methoxy-4-hydroxyphenethyleneglycol by the awake human brain is approximately 597 nanograms per minute or 35.8 micrograms per hour. Urine specimens were also collected from six of these subjects during a period of 1 to 3.5 hours, which bracketed the time the blood samples were obtained. For these six subjects the output of 3-methyoxy-4-hydroxyphenethyleneglycol by whole brain was estimated to be 40.9 micrograms per hour, whereas the rate of its excretion into urine was 64.5 micrograms per hour.