Rat Brain Norepinephrine Metabolism: Substantial Clearance Through 3,4-Dihydroxyphenylethyleneglycol Formation

To assess whether the metabolic clearance of rat brain norepinephrine (NE) through 3,4-dihydroxyphenylethyleneglycol (DHPG) formation is quantitatively comparable or greater than through 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) production, we studied the accumulation rates of conjugated DHPG and MHPG following probenecid administration in whole brain as well as in several brain regions. Administration of increasing doses of probenecid (100-500 mg/kg, i.p.) 1.5 h before sacrifice produced a dose-dependent increase of conjugated DHPG and MHPG levels. The maximum increment of these conjugated metabolites occurred at a dose of 300 mg/kg or higher. During the first hour following probenecid administration (300 mg/kg, i.p.), rat brain conjugated DHPG and MHPG levels accumulated linearly at a rate of 646 and 319 pmol/g/h, respectively. With the probenecid technique, the estimated appearance rates of conjugated DHPG significantly exceeded those of conjugated MHPG in hypothalamus, midbrain, brainstem, hippocampus, and cerebral cortex. These results clearly indicate that under resting conditions, formation and efflux of conjugated DHPG is the major route of metabolic clearance of rat brain NE.     

Brain noradrenergic neuronal activity affects 3,4-dihydroxyphenylethyleneglycol (DHPG) levels

Brain regional DHPG levels were determined following pharmacological manipulations that are known to alter brain noradrenergic neuronal activity. In rats given the α-adrenergic antagonist yohimbine (1, 5 and 10 mg/kg, i.p.) 2 h prior to sacrifice, there was a dose-dependent increase in cortical, midbrain, pons + medulla, hypothalamic and spinal total DHPG and MHPG concentrations. In contrast, cortical and spinal total DHPG and MHPG concentrations were markedly decreased 2 h following the α-adrenergic agonist, clonidine (10 and 250 μg/kg, i.p.). These findings indicate that rat brain DHPG formation is also sensitive to changes in brain noradrenergic neuronal impulse flow.     

DISTRIBUTION AND TURNOVER RATE OF VANILLYL-MANDELIC ACID AND 3-METHOXY- 4- HYDROXYPHENYLGLYCOL IN RAT BRAIN

Abstract— Vanillylmandelic acid (VMA) and 3-methoxy-4- hydroxyphenylglycol (MHPG) were measured in rat brain by a mass fragmentographic procedure. The concentration of VMA and MHPG in whole brain is 11 and 533 pmol/g, respectively. Both compounds were found in all areas of brain studied with VMA, as a percentage of both metabolites, ranging between about 1 and 8%. From the decline of the compounds after pargyline. 75 mg/kg i.p., we calculated that the rate of formation of VMA is 15 and for MHPG 202 pmol/g per h. The fractional rate of elimination of VMA and MHPG is 1.4 and 0.38 h⁻¹, respectively. The rapid rate of loss of VMA suggests that it is transported from brain. However, we were unable to block the elimination of VMA from brain by treatment with probenecid. In contrast, the elimination of MHPG could be blocked by treatment with probenecid. Our study adds support to the notion that MHPG is a major whereas VMA is a minor product of norepinephrine metabolism in brain.     

Postmortem Stability of Brain 3-Methoxy-4-Hydroxyphenylethyleneglycol and 3,4-Dihydroxyphenylethyleneglycol in the Rat and Mouse

Abstract: To assess the postmortem stability of brain 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) and 3,4-dihydroxyphenylethyleneglycol (DHPG) levels, groups of rats and mice were killed by cervical dislocation and left at either 21° or 4°C for intervals of up to 24 h until removal and freezing of whole brain. Whole brain free and total MHPG and DHPG levels were determined simultaneously by gas chromatography-mass fragmentography (GC-MF). By 2 h after death, statistically significant decrements occurred in rat brain free DHPG (20%), total MHPG (21%), and total DHPG (11%) at 4°C, but free MHPG increased significantly (50%) compared with controls. At 21°C, rat brain total MHPG increased compared with controls at 2 h (15%) but decreased at 4 h (15%) and 8 h (15%), whereas free MHPG levels were increased at these times. Although brain total and conjugated DHPG levels showed little change, free DHPG levels were reduced at all times. In mouse brain no significant changes occurred in free MHPG and DHPG by 24 h at 4°C. At 21°C, mouse brain DHPG levels decreased whereas MHPG concentrations increased over the 8-h period of study. These findings demonstrate the occurrence of significant postmortem time- and temperature-dependent changes in brain MHPG and DHPG concentrations and indicate caution in the interpretation of changes in these metabolites in studies employing human postmortem brain tissue.     

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.     

Effect of L-threo-3,4-dihydroxyphenylserine(L-threo-DOPS) on brain and serum MHPG levels in mice: evidence for NE formation in CNS

Effects of L-threo-DOPS on brain and serum concentrations of 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG), a major metabolite of 1-norepinephrine(NE) were studied in mice. An intraperitoneal(i.p.) injection of L-threo-DOPS markedly increased both serum and brain MHPG levels in mice. This increase in the brain was dose-dependent at doses up to 800 mg/kg, and lasted for 4 h or more. Though the increase in serum total-MHPG was 3-4 times greater than that in brain MHPG, the decline was rapid as compared with the case of brain MHPG. The L-threo-DOPS-induced increase in MHPG was inhibited by i.p. pretreatment with benserazide, a peripheral decarboxylase inhibitor, in both serum and brain. This inhibition in the brain, however, was observed at about 20 times higher doses of benserazide than that in serum. On the contrary, an intracerebroventricular(i.c.v.) injection of benserazide inhibited the increase in brain MHPG to about the same degree as that in serum MHPG. These results suggest that the L-threo-DOPS-induced increase in brain MHPG is not likely to originate in peripheral organs including the brain capillary, and that L-threo-DOPS can be converted to NE by aromatic L-amino acid decarboxylase(AADC) in the brain parenchyma.     

Repeated Clorgyline Treatment Inhibits Methamphetamine- induced Behavioral Sensitization in Mice

Following the expression of the behavioral sensitization by repeated administration of methamphetamine (METH) (1 mg/kg, intraperitoneal (i.p.), once per day for five consecutive days), male ICR mice were treated with clorgyline (1 mg/kg, subcutaneous, once per day for five consecutive days), a monoamine oxidase-A inhibitor. Two hours after the final treatment with clorgyline, the mice were challenged with METH (1 mg/kg, i.p.) and locomotor activity was measured for 1 h. The mice treated with clorgyline showed a significant decrease in both vertical locomotion and horizontal rearing, compared with those treated with saline. Clorgyline treatment altered the effect of single METH challenges on apparent dopamine turnover in the cerebral cortex of the mice sensitized to METH. These results suggested a possible association of the inhibition by clorgyline of METH-induced behavioral sensitization with the alteration of dopamine turnover in the cerebral cortex of the mouse.     

Presence of Kynurenic Acid in the Mammalian Brain

Kynurenic acid, a tryptophan metabolite able to antagonize the actions of the excitatory amino acids, has been identified and measured for the first time in the brain of mice, rats, guinea pigs, and humans by using an HPLC method. Its content was 5.8 +/- 0.9 in mouse brain, 17.8 +/- 2.0 in rat brain, 16.2 +/- 1.5 in guinea pig brain, 26.8 +/- 2.9 in rabbit brain, and 150 +/- 30 in human cortex (pmol/g wet wt. mean +/- SE). The regional distribution of this molecule was uneven. In rats, guinea pigs, and rabbits, the brainstem was the area richest in this compound. Tryptophan administration (100-300 mg/kg, i.p.) to rats resulted in a significant increase of the brain content of kynurenic acid. Similarly, 1 h after probenecid administration (200 mg/kg, i.p.), the brain content of kynurenate increased by fourfold, thus suggesting that its turnover rate is relatively fast.     

Suppressive effect of nantenine,isolated from Nandina domestica Thunberg,on the 5-hydroxy-L-tryptophan plus clorgyline-induced head-twitch response in mice

We investigated the effects of nantenine (9,10-Methylenedioxy-1,2 dimethoxyaporphine), a major alkaloid isolated from the fruit of Nandina domestica Thunb (Berberidaceae), on the 5-HT2A receptor-mediated head-twitch response (HTR) in mice. Intraperitoneal (i.p.) injection of nantenine (13.3, 20 and 30 mg/kg) as well as the 5-HT2A receptor antagonist ketanserin (0.0625, 0.25 and 1 mg/kg) inhibited the 5-hydroxy-L-tryptophan (l-5-HTP; 75 mg/kg, i.p.) plus monoamine oxidase inhibitor, clorgyline (1 mg/kg, i.p.)-induced HTR in a dose-dependent manner. In contrast, neither l-5-HTP plus clorgyline nor 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT; 5 microg/mouse, i.c.v.)-induced head weaving was affected by nantenine or ketanserin. Furthermore, neither nantenine (up to 30 mg/kg) nor ketanserin (up to 1 mg/kg) affect on the locomotor activity. In the receptor binding studies, nantenine showed affinity to the 5-HT2A receptors (Ki = 0.4 microM), while it had less affinity toward alpha1-adrenergic (Ki = 2.1 microM) and D2-dopaminergic (Ki = 1.7 microM) receptors of the mouse brain. These results suggest that nantenine inhibits l-5-HTP plus clorgyline-induced head- twitch response by blocking 5-HT2A receptors in the central nervous system.     

Effects of the Histamine H3-Agonist (R)-α-Methylhistamine and the Antagonist Thioperamide on Histamine Metabolism in the Mouse and Rat Brain

To study the feedback control by histamine (HA) H3-receptors on the synthesis and release of HA at nerve endings in the brain, the effects of a potent and selective H3-agonist, (R)-alpha-methylhistamine, and an H3-antagonist, thioperamide, on the pargyline-induced accumulation of tele-methylhistamine (t-MH) in the brain of mice and rats were examined in vivo. (R)-alpha-Methylhistamine dihydrochloride (6.3 mg free base/kg, i.p.) and thioperamide (2 mg/kg, i.p.), respectively, significantly decreased and increased the steady-state t-MH level in the mouse brain, whereas these compounds produced no significant changes in the HA level. When administered to mice immediately after pargyline (65 mg/kg, i.p.), (R)-alpha-methylhistamine (3.2 mg/kg, i.p.) inhibited the pargyline-induced increase in the t-MH level almost completely during the first 2 h after treatment. Thioperamide (2 mg/kg, i.p.) enhanced the pargyline-induced t-MH accumulation by approximately 70% 1 and 2 h after treatment. Lower doses of (R)-alpha-methylhistamine (1.3 mg/kg) and thioperamide (1 mg/kg) induced significant changes in the pargyline-induced t-MH accumulation in the mouse brain. In the rat, (R)-alpha-methylhistamine (3.2 mg/kg, i.p.) and thioperamide (2 mg/kg, i.p.) also affected the pargyline-induced t-MH accumulation in eight brain regions and the effects were especially marked in the cerebral cortex and amygdala. These results indicate that these compounds have potent effects on HA turnover in vivo in the brain.     

Conversion of 3,4-Dihydroxyphenylalanine and Deuterated 3,4-Dihydroxyphenylalanine to Alcoholic Metabolites of Catecholamines in Rat Brain

Abstract: We have investigated the effects of 3,4-dihydroxyphenylalanine l -DOPA) and its deuterated analogue on the concentrations of alcoholic metabolites of catecholamines in rat brain by means of gas chromatography/mass spectrometry with selected-ion monitoring. Whole brain concentrations of the two neutral norepinephrine metabolites, 3-methoxy-4-hydroxyphenylethylene-glycol (MHPG) and 3,4-dihydroxyphenylethyleneglycol (DHPG), were significantly increased in a dose-dependent manner by a single intraperitoneal injection of l -DOPA. Both MHPG and DHPG, as well as the corresponding dopamine metabolites, reached a maximum 1 h after injection. Brain MHPG and DHPG concentrations were elevated by 78 and 134%, respectively, 1 h after injection of 150 mg/kg l -DOPA. Analyses of discrete brain regions revealed that concentrations of the norepinephrine metabolites were elevated uniformly in all regions, except that MHPG showed a greater increase in the cerebellum than in other regions. The latter result appeared to be explained by the finding that 52% of the total MHPG in the cerebellum was unconjugated (compared to 15% in the whole brain). l -DOPA caused a proportionately greater increase in free MHPG than in total MHPG in the cerebellum and brain stem. By using deuterated l -DOPA in place of l -DOPA and measuring both the deuterated and nondeuterated norepinephrine metabolites, we demonstrated that virtually all of the increases in MHPG and DHPG were due to the conversion of the exogenous l -DOPA to norepinephrine. Thus, the effects of norepinephrine metabolism need to be considered in attempts to understand clinical and behavioral effects of l -DOPA.     

Quantitation of total MHPG in the rat brain using a non enzymatic hydrolysis procedure. Effects of drugs

An acid-catalyzed procedure has been used to hydrolyze MHPG-sulfate in homogenates of rat brain. The samples (in 0.4 mol/L perchloric acid) are treated for 3 min. at 100 degrees C in a water bath and aliquots are injected into a reversed phase HPLC system. Detection is achieved fluorimetrically. The absolute detection limit for MHPG is 150 pg, which allows the reliable determination of either free or total MHPG in rat brain in concentrations down to 15 ng/g, using the described procedure. The concentration of total MHPG found in the brains of saline-treated rats are 101 +/- 21 ng/g (mean +/- S.D.) which is in a good accordance with the concentration value for the same samples obtained using a GC-MS method (115 +/- 19 ng/g). Rats treated with clonidine (300 micrograms/Kg, i.p.) or yohimbine (10 mg/Kg, i.p.) showed brain concentrations of total MHPG of 68 +/- 22 ng/g and 299 +/- 85 ng/g, respectively. The utility of this method for the analysis of brain regions or brain nuclei (e.g. locus coeruleus) is also shown.     

TURNOVER OF FREE AND CONJUGATED (SULPHONYLOXY) DIHYDROXYPHENYLACETIC ACID AND HOMOVANILLIC ACID IN RAT STRIATUM

Abstract— Conjugated (sulphonyloxy) dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were synthesized from free DOPAC and HVA and used as reference compounds in their fluorimetric determination in rat brain (detection limit 0.2 nmol/g). The conjugated DOPAC and HVA form 29 and 36% of the total DOPAC and HVA found in rat striatum, respectively. Dopamine (DA) metabolism was studied in the rat striatum by following the decline of both free and conjugated DOPAC and HVA after treatment with pargyline (100mg/kg. i.p.) either alone or in combination with tropolone (100 mg/kg, i.p.). or from the accumulation of the free and conjugated acids after treatment with probenecid (100-500mg/kg. i.p.). The rates of decline were analysed by a non-linear curve fitting method using a simple model of DA metabolism that postulates the formation of the conjugates exclusively from the free acids, and HVA from DOPAC, with first order kinetics and single open compartments only. The curves computed all passed through the s.e.m. of every experimental point. The rate constants thus found indicate that DOPAC turnover is about 23nmol/g/h. Of this about 16 nmol/g/h are O -methylated to HVA, about 6 nmol/g/h are conjugated and less than 1 nmol/g/h is eliminated as free DOPAC. Of the HVA formed, about 8.5nmol/g/h are conjugated and about 7.5 nmol/g/h eliminated as free HVA. The conjugates accumulated after treatment with probenecid (1 h) faster than the free acids. The maximal accumulation of all four metabolites found (21 nmol/g/h) approximates the total turnover of DOPAC.     

Increase in norepinephrine turnover after tyrosine or DL-threo-3,4-dihydroxyphenylserine (DL-threo-DOPS)

The amino acids tyrosine and DL-threo-3,4-dihydroxyphenylserine (DL-threo-DOPS) were compared for their effectiveness in increasing central nervous system norepinephrine (NE) turnover in both saline and DSP-4 pretreated mice. NE was decreased significantly in cortex, hippocampus and cerebellum, and only slightly in hypothalamus and brainstem two weeks after a single intraperitoneal injection of the neurotoxin DSP-4. Levels of the major NE metabolite, 3-methoxyl-4-hydroxyphenylethylene glycol (MHPG), decreased in parallel in these five brain regions. Neither administration of tyrosine (250 mg/kg, as the ethyl ester, i.p.) nor DL-threo-DOPS (200 mg/kg, i.p.) affected regional NE concentration. However, after tyrosine administration, MHPG levels increased significantly in cortex in control mice and in cortex and hippocampus of DSP-4 pretreated mice. In all five brain noradrenergic regions MHPG level increased after DL-threo-DOPS administration and this increase was enhanced (approximately doubled) in DSP-4 pretreated mice. Thus, both amino acids may be useful as precursors of central NE when its level is depleted (e.g. following administration of DSP-4); DL-threo-DOPS producing a generalized increase in brain NE turnover, while increases following tyrosine are specific to those areas in which neuronal activity is increased i.e. cortex and hippocampus.     

5,5-Diphenylhydantoin Antagonizes Neurochemical and Behavioral Effects of p, p'-DDT but Not of Chlordecone

Abstract: Rats were given 75 mg/kg of 5,5-diphenylhydantoin (phenytoin) or vehicle 30 min prior to 75 mg/kg of 1, 1, 1-trichloro-bis( p -chlorophenyl)ethane ( p, p' -DDT) (p.o.) or chlordecone (i.p.) and tremor was measured 12 h later. Rats were then killed, and regional brain levels of biogenie amines and their acid metabolites and amino acids were determined. Pretreatment with phenytoin significantly attenuated the tremor produced by p, p' -DDT but enhanced that produced by chlordecone. p, p' -DDT had significant effects on the levels of asparate, glutamate, 5-hydroxyindoleacetic acid (5-HIAA), and 3-methoxy-4-hydroxyphenylglycol (MHPG), whereas chlordecone increased glycine, 5-HIAA, and MHPG levels. Pretreatment with phenytoin blocked p.p' -DDT-induced increases of aspartate in the brainstem and spinal cord, 5-HIAA in the hippocampus, and MHPG in the brainstem and hypothalamus. Phenytoin significantly enhanced chlordecone-induced increases of MHPG in the brainstem. These data indicate that organo-chlorine-induced increases in noradrenergic activity in the brainstem and spinal cord may be directly related to the tremorigenic effects of these chemicals.     

Inhibition of locus coeruleus neuronal activity by beta-phenylethylamine

The effect of beta-phenylethylamine (PEA) on brain noradrenaline (NA) neurons in the rat locus coeruleus (LC) was analyzed using single unit recording techniques including microiontophoretic methodology. Systemic injection of low doses of PEA consistently produced an instantaneous and dose-dependent inhibition of firing rate of the LC neurons. The effect was strongly antagonized by administration of the alpha 2-receptor antagonist yohimbine (1 mg/kg, i.v.) or by depletion of endogenous stores of NA by pretreatment with reserpine (10 mg/kg, i.p., 6 h), but unaffected by inhibition of tyrosine hydroxylase (alpha-met-hyl-p-tyrosine (alpha-MT), 250 mg/kg, i.p., 30 min). In contrast, the inhibitory effect of PEA on the LC neurons was strongly potentiated by pretreatment with the selective monoamine oxidase (MAO) - B inhibitor pargyline (2 mg/kg, i.p., 1 h), but, unexpectedly, also by pretreatment with the MAO-A selective inhibitors clorgyline (2 mg/kg, i.p., 1 h) or FLA 336 (2 mg/kg, i.p., 1 h). When microiontophoretically applied directly onto the LC neurons, PEA produced inhibition of a majority of the NA neurons. This action was prevented by intravenous injection of yohimbine (2.5 mg/kg). The results suggests that the action of PEA on NA neurons in the LC is an indirect effect, requiring availability of a reserpine-sensitive storage pool of NA, and mediated via activation of central alpha 2-receptors within the LC.     

Interspecies differences in the metabolism of brain norepinephrine to glycol metabolites

Using a highly sensitive and specific gas chromatography-mass spectrometric assay, the glycol metabolites of norepinephrine (NE), 3,4-dihydroxyphenylethyleneglycol (DHPG) and 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) were determined simultaneously in brain and body fluids of several mammalian species, including humans. Highest molar ratios of DHPG to MHPG were found in rat brain (1.20), a species in which these glycol metabolites were primarily conjugated. In mouse, guinea pig, hamster, monkey, and human brain, DHPG and MHPG were mostly unconjugated, and DHPG concentrations were about 30–60% of the respective MHPG levels. In dog cortex, MHPG occurred predominantly as conjugates, whereas DHPG could only be detected in its unconjugated form. In all species studies, highest DHPG and MHPG concentrations occurred in hypothalamus followed, in general, by midbrain and brainstem whereas cerebral cortex, caudate and cerebellum had the lowest values. These results demonstrate substantial differences in the degree of conjugation and relative abundance of brain DHPG compared to MHPG between the rat and other animal species studied.     

Methamphetamine Reward in Mice as Assessed by Conditioned Place Preference Test with Supermex® Sensors: Effect of Subchronic Clorgyline Pretreatment

Recent studies in our laboratory have shown that methamphetamine (METH)-induced hyperlocomotion and behavioral sensitization in mice were inhibited by clorgyline, an irreversible monoamine oxidase inhibitor. In this study, the effect of clorgyline pretreatment on METH-induced rewarding effect was assessed by a conditioned place preference (CPP) test, using an apparatus developed with Supermex sensors (infrared pyroelectric sensors). Although intact male ICR mice showed significant CPP for METH (0.5 mg/kg, i.p.), pretreatment with subchronic clorgyline (0.1 and 10 mg/kg, s.c.) did not affect the magnitude of CPP. At a dose of 1 mg/kg, pretreatment of the mice with clorgyline showed a similar CPP index in both saline/saline and METH/saline pairing groups. During the conditioning session, the mice did not express behavioral sensitization to METH. Pretreatment with clorgyline (0.1, 1, and 10 mg/kg) decreased striatal apparent monoamine turnover in a dose-dependent manner. These results indicated that clorgyline pretreatment (0.1 and 10 mg/kg) did not influence the METH-induced rewarding effect in mice, although pretreatment of the mice with clorgyline at a dose of 1 mg/kg appeared to influence the CPP for METH.     

Relative Importance of 3-Methoxy-4-Hydroxyphenylglycol and 3,4-Dihydroxyphenylglycol as Norepinephrine Metabolites in Rat, Monkey, and Humans

Abstract: A gas chromatographic-mass spectrometric assay, which allowed simultaneous measurement of 3-methoxy-4-hydroxyphenylglycol (MHPG) and 3,4-dihydroxyphenylglycol (DHPG), was used to show that the concentration of MHPG in primate CNS far exceeded that of DHPG and that both metabolites were mainly in the unconjugated form. In rat brain, DHPG concentration was generally higher than that of MHPG, and both existed predominantly as conjugates. Rat and primate plasma contained more MHPG than DHPG. In plasma of primates but not of rats, higher proportions of the metabolites were conjugated, compared to those in brain. Significant correlations existed between MHPG and DHPG in rat brain, monkey brain, human plasma, and both monkey CSF and plasma. In monkeys, a significant CSF-plasma correlation was found for MHPG, but not for DHPG. Acute administration of piperoxane raised rat brain MHPG and DHPG concentration; desipramine prevented this rise in DHPG, but not in MHPG. Desipramine alone decreased DHPG, but not MHPG, concentration. Piperoxane increased monkey brain MHPG, but not DHPG, concentration. These data suggest that DHPG is a valuable metabolite to measure when assessing norepinephrine metabolism in the rat. Under certain conditions, measurement of rat brain MHPG and DHPG may provide information concerning the site of norepinephrine metabolism. However, in primates the importance of monitoring DHPG, in addition to MHPG, is uncertain.     

Changes in Histamine Metabolism in the Mouse Hypothalamus Induced by Acute Administration of Ethanol

The effect of acute ethanol administration on histamine (HA) dynamics was examined in the mouse hypothalamus. The steady-state level of HA did not change after intraperitoneal administration of ethanol (0.5-5 g/kg), whereas the level of tele-methylhistamine (t-MH), a predominant metabolite of brain HA, increased when 3 and 5 g/kg of ethanol was given. Pargyline hydrochloride (80 mg/kg, i.p.) increased the level of t-MH by 72.2% 90 min after the treatment. Ethanol at any dose given did not significantly affect the t-MH level in the pargyline-pretreated mice. Decrease in the t-MH level induced by metoprine (10 mg/kg, i.p.), an inhibitor of HA-N-methyltransferase, was suppressed by ethanol (5 g/kg), thereby suggesting inhibition of the elimination of brain t-MH. Ethanol (5 g/kg) significantly delayed the depletion of HA induced by (S)-alpha-fluoromethylhistidine (50 mg/kg, i.v.), a specific inhibitor of histidine decarboxylase. Therefore, a large dose of ethanol apparently decreases HA turnover in the mouse hypothalamus.