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.     

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.     

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.     

Systemic interleukin-1 administration stimulates hypothalamic norepinephrine metabolism parallelling the increased plasma corticosterone

Intraperitoneal injection of purified recombinant interleukin-1 (IL-1) into mice increased the cerebral concentration of the norepinephrine (NE) catabolite, 3-methoxy,4-hydroxyphenylethyleneglycol (MHPG), probably reflecting increased activity of noradrenergic neurons. This effect was dose-dependent and was largest in the hypothalamus, especially the medial division. Tryptophan concentrations were also increased throughout the brain. The increase of MHPG peaked around 4 hours after IL-1 administration, parallelling the increase of plasma corticosterone. Both the alpha- and beta-forms of IL-1 were effective, but the activity was lost after heat treatment of the IL-1. Noradrenergic neurons with terminals in the hypothalamus are known to regulate the secretion of corticotropin-releasing factor, thus our results suggest that IL-1 activates the hypothalamic-pituitary-adrenal axis by activating these neurons. Because initiation of an immune response is known to cause systemic release of IL-1, IL-1 may be an immunotransmitter communicating the immunologic activation to the brain. The IL-1-induced changes in hypothalamic MHPG may explain the increases of electrophysiological activity, the changes of hypothalamic NE metabolism, and the increases in circulating glucocorticoids previously reported to be associated with immunologic activation and frequently observed in infected animals.     

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.     

Nicotinic acid or N-methyl nicotinamide prolongs elevated brain dopa and dopamine in L-dopa treatment

A peripheral dopa decarboxylase inhibitor, benserazide, was given ip, followed by intubation with L-dopa. Brain dopa and DA levels were elevated maximally between 0.5-2.5 hr and 1.0-2.5 hr, respectively. Dopa in serum, liver, and brain were at control values after 4 hr. Supplementation of dopa with NAM or NAC, as possible methyl group acceptors to lower catabolism of DA, showed that NAM had no effect on DA levels or on SAM. However, with both NAC and N-methyl NAM (a methylated compound intended as a control) at time periods where dopa and DA were normally decreasing, the brain levels were increased over control values with benserazide and dopa alone. NAC or N-methyl NAM appeared to extend the period of elevated brain DA levels with L-dopa treatment. The mechanism responsible for these results is uncertain.     

The role of dopamine and serotonin in the prolongation of post-decapitation convulsions in mice.

L-DOPA (320 mg/kg, i.p.) increased the duration of the clonic phase of post-decapitation convulsions (PDC) by 60% in mice pretreated with the peripheral decarboxylase inhibitor, Ro 4-4602 (50 mg/kg, i.p.). Assays of brains at the time of decapitation showed a 300% increase in dopamine (DM), an 80% reduction in serotonin (5-HT) and no change in norepinephrine (NE) levels. The effect of L-DOPA on PDC was not blocked by haloperidol (0.5 – 5.0 mg/kg), a blocker of DM receptors, nor by diethyldithiocarbamate (400 mg/kg) an inhibitor of NE synthesis. Parachlorophenylalanine (300 mg/kg × 3 days) produced an 80% reduction in 5-HT and a prolongation of PDC similar to that observed after L-DOPA. Prolongation of PDC was also seen after the 5-HT antagonists methysergide (5 mg/kg) and cinanserin (10 mg/kg), but not after cyproheptadine (10 mg/kg). The 5-HT precursor, 5-hydroxytryptophan (100 mg/kg), produced no change in PDC when used alone but inhibited L-DOPA's prolongation of PDC. The results suggest that L-DOPA acts by depleting 5-HT in bulbospinal pathways and thus enhancing reflex activity in the spinal cord.     

Dose-dependent effects of L-carnosine on the renal sympathetic nerve and blood pressure in urethane-anesthetized rats

The physiological function of L-carnosine (beta-alanyl-L-histidine) synthesized in mammalian muscles has been unclear. Previously, we observed that intravenous (i.v.) injection of L-carnosine suppressed renal sympathetic nerve activity (RSNA) in urethane-anesthetized rats, and L-carnosine administered via the diet inhibited the elevation of blood pressure (BP) in deoxycorticosterone acetate salt hypertensive rats. To identify the mechanism, we examined effects of i.v. or intralateral cerebral ventricular (l.c.v.) injection of various doses of L-carnosine on RSNA and BP in urethane-anesthetized rats. Lower doses (1 microg i.v.; 0.01 microg l.c.v.) of L-carnosine significantly suppressed RSNA and BP, whereas higher doses (100 microg i.v.; 10 microg l.c.v.) elevated RSNA and BP. Furthermore, we examined effects of antagonists of histaminergic (H1 and H3) receptors on L-carnosine-induced effects. When peripherally and centrally given, thioperamide, an H3 receptor antagonist, blocked RSNA and BP decreases induced by the lower doses of peripheral L-carnosine, whereas diphenhydramine, an H1 receptor antagonist, inhibited increases induced by the higher doses of peripheral L-carnosine. Moreover, bilateral lesions of the hypothalamic suprachiasmatic nucleus eliminated both effects on RSNA and BP induced by the lower (1 microg) and higher (100 microg) doses of peripheral L-carnosine. These findings suggest that low-dose L-carnosine suppresses and high-dose L-carnosine stimulates RSNA and BP, that the suprachiasmatic nucleus and histaminergic nerve are involved in the activities, and that L-carnosine acts in the brain and possibly other organs.     

Variance in the production of homovanillic acid and 3-methoxy-4-hydroxyphenethyleneglycol by the awake primate brain

Previous experimental results, using a new technique whereby the production rates of the neurotransmitter metabolites homovanillic acid (HVA) and 3-methoxy-4-hydroxyphenethyleneglycol (MHPG) by the awake primate brain are determined, have shown a wide variance in metabolite production among both animal and human subjects. These data suggested that either individual subjects differ in the activity of brain dopamine (DA) or norepinephrine (NE) neurons and/or that the activities of these neurons fluctuate over time. For these reasons a series of experiments were performed in which measures of HVA and MHPG production were obtained at three time points in the same animal (monkeys) over a three hour period. It was found that the group mean values for the production of HVA and MHPG by brain were similar for each of the three time points. However, it was also found that marked variations in HVA and MHPG production occur within a single animal over a three hour period. The coefficients of variation for individual animals for HVA ranged from 9.3 to 31.9% and for MHPG from 10.1 to 62.3%. These variations were not correlated with grossly observable changes in behavioral states. Using an analysis of variance it was found that the variance in MHPG production was significantly greater than that for HVA (F = 6.2, p < 0.05) suggesting that brain NE systems are more liable and/or show greater change than do brain DA systems. These data are interpreted as indicating that in the awake, resting primate brain fluctuations in the activities of DA and NE neurons occur, i.e. there is not a steady, invariant production of metabolites but rather they are produced in pulses of varying lengths. This interpretation of the data is generally consistent with electrophysiological studies which indicate that catecholamine neurons fire in bursts which are then followed by silent periods. Finally, in terms of practical application of the V-A difference technique, these data indicate that replicable group mean estimates of brain HVA and MHPG production can be obtained by averaging values from a single time point whereas accurate information about an individual animal will require multiple samplings.Recent reports from this laboratory have described a method whereby a direct measure of the rates of production of neurotransmitter metabolites such as homovanillic acid (HVA), 3-methoxy-4-hydroxyphenethyleneglycol (MHPG), and 5-hydroxyindoleacetic acid (5-HIAA) by the awake primate brain can be determined (1, 2, 3, 4). Since the quantities of HVA, MHPG, and probably 5-HIAA in the brain vary as a function of the activity of dopamine (DA), norepinephrine (NE), and serotonin (5-HT) neurons (1, 5, 6, 7, 8), it is likely that these measures of neurotransmitter metabolite production reflect the functional state of brain DA, NE, and 5-HT neuronal systems. The experimental results thus far obtained with this technique have shown a wide variance in the rates of neurotransmitter metabolite production across both animal and human subjects even though the subjects were not in clearly different behavioral or emotional states (1, 2, 4, 9). These data suggested that either individual subjects differ markedly in the activities of brain DA, NE, and 5-HT neurotransmitter systems and/or that the activity of these systems fluctuates markedly over time. For these reasons, experiments were undertaken in which repeated measures of HVA and MHPG production by brain within the same animal were determined over a three hour period. The results of these experiments, which are reported here, indicate that there are marked changes in brain metabolite production which occur within animals. The implications of these findings for our understanding of the functioning of brain neurotransmitter systems and for the practical applications of this technique are discussed.     

Evidence of reduced uptake of convulsant in brain following prostaglandin E2

The toxic and convulsant effects of the acetylcholinesterase (AChE) inhibitor Soman, were examined in mice pretreated with various doses of prostglandin E₂ (PGE₂), administered by either intraperitoneal injection (i.p.) or by intracerebroventricular (i.c.v.) infusion. PGE₂ (i.p.) reduced the lethal effects of Soman slightly. PGE₂ (i.p. and i.c.v.) delayed the onset and reduced the severity of cholinergically-induced convulsions, resulting from Soman. Whole brain AChE was measured at various times after Soman or Soman preceded by PGE₂. PGE₂ (i.p. or i.c.v.) reduced the rate at which Soman inhibited brain AChE, which appeared to be related to the increased time to onset of convulsive activity. Repeated injections of PGE₂ did not delay convulsions indefinitely nor were convulsions terminated once they had started. The results suggest that the anticonvulsant properties of PGE₂ may have been due, in part, to decreased cerebral circulation with subsequent reduction in the access of the convulsant to the brain and in part to direct neuronal effects.     

5-Hydroxytryptamine affects rat migrating myoelectric complexes through different receptor subtypes: evidence from 5-hydroxytryptophan administration

The effect of the serotonin precursor 5-hydroxytryptophan (5-HTP) on jejunal migrating myoelectric complexes (MMCs) was investigated in conscious rats. Subcutaneous administration of low doses of 5-HTP (1-2 mg/kg) shortened the period between migrating complexes, whereas high doses of the compound (4-8 mg/kg) disrupted the MMC pattern. The serotonin (5-HT2) antagonist methysergide (8 mg/kg s.c.) did not alter basal MMC, neither did it prevent the effect of a low dose of 5-HTP; conversely, it antagonized the disruption due to the high dose. The 5-HT3 antagonist ICS 205-930 (30 micrograms/kg s.c.) decreased MMC frequency; administration of 2 mg/kg 5-HTP following ICS 205-930 brought the frequency of myoelectric complexes back to basal values. Both effects of 5-HTP were prevented by the decarboxylase inhibitor benserazide (85 mg/kg i.p.), which per se caused a transient inhibition of spiking activity. The results suggest that rat MMCs can be influenced in a composite fashion by progressively increasing concentrations of 5-HT, which in turn activate different receptor subtypes. A peripheral neuronal receptor, probably belonging to the 5-HT3 subclass, mediates the increase in MMC frequency observed after low doses of 5-HTP; higher levels of serotonin activate 5-HT2 receptors, causing disruption of cycling activity. Additionally, 5-HT3 receptors, but not 5-HT2, appear to be relevant for the regulation of the MMC pattern by the endogenous amine.     

The effect of subchronic, intermittent L-DOPA treatment on neuronal nitric oxide synthase and soluble guanylyl cyclase expression and activity in the striatum and midbrain of normal and MPTP-treated mice

We have investigated the effects of low (10 mg/kg) and high (100 mg/kg) doses of L-DOPA on the expression and activity of neuronal nitric oxide synthase (nNOS) and guanylyl cyclase (GC) in the striatum and midbrain of mice. L-DOPA was administered subchronically for 11 days (beginning 3 days after last MPTP/NaCl injection) or for 14 days (with dosing started immediately following the last MPTP/NaCl injection). Adult mice received three intraperitoneal (i.p.) injections of physiological saline or MPTP at 2h intervals (total dose of 40 mg/kg). Normal and MPTP-injected mice were treated twice a day for 11 or 14 days with low (10/2.5 mg/kg bw) or high (100/25mg/kg bw) doses of L-DOPA/benserazide. The present study indicates that several days of treatment with L-DOPA does not affect MPTP-activation of the nNOS/sGC/cGMP pathway or the neurodegenerative processes that occur in the striatum and midbrain of mice. In normal mice, L-DOPA upregulates the expression and activity of nNOS and GC to levels found in MPTP-injected mice. Due to upregulation of nNOS and GC, cGMP levels in the mouse striatum and midbrain are also elevated, however, significantly lower in mice administrated with low dose of L-DOPA. In both investigated brain regions of normal mice cGMP-dependent PDEs activities were elevated after low dose administration of L-DOPA, but no change in PDEs activities has been detected in MPTP and high L-DOPA-injected mice as compared to control values. The enhancement of nNOS mRNA and GCbeta1 mRNA levels were generated by both doses of L-DOPA, given in a time-dependent fashion. L-DOPA-injected for 11 or 14 days caused a decrease in TH protein levels in the striatum and midbrain, respectively; this result was noted irrespective of dose. L-DOPA therapy did not prevent the MPTP-induced decrease in TH protein levels in either investigated brain region.     

Feeding increases 5-hydroxytryptamine and norepinephrine within the hypothalamus of chicks

It is thought that hypothalamic 5-hydroxytryptamine (5HT) and norepinephrine (NE) are involved in the regulation of feeding in chicks. The present study was conducted to elucidate changes in the levels of extracellular 5HT and NE in the hypothalamus during feeding of chicks. In order to measure 5HT, NE and 4-hydroxy-3-methoxyphenylglycol (MHPG), which is a major metabolite of NE, we used brain microdialysis and high-pressure liquid chromatography with an electrochemical detector. After collecting samples to determine the basal levels of 5HT, NE and MHPG, food-deprived birds were given access to food. 5HT levels in the medial hypothalamus (MH) and lateral hypothalamus (LH) increased during the first 30 min of feeding, and then returned to basal levels. NE and MHPG in the LH increased during feeding, and remained elevated throughout the experiment. This study supports an idea that hypothalamic monoamines in the chick brain are involved in the regulation of feeding.     

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.     

Central beta-amyloid peptide-induced peripheral interleukin-6 responses in mice

beta-Amyloid peptides (Abetas) share with lipopolysaccharide, a potent pro-inflammatory agent, the property of stimulating glial cells or macrophages to induce various inflammatory mediators. We recently reported that central administration of lipopolysaccharide induces peripheral interleukin-6 responses via both the central and peripheral norepinephrine system. In this study, the effect of intracerebroventricular injection of various synthetic Abetas on plasma interleukin-6 levels was examined in mice. Abeta(1-42) dose-dependently increased plasma interleukin-6 levels: 'aged' Abeta(1-42) was more effective than fresh, whereas Abeta(42-1) had no effect. 'Aged' Abeta(1-42) (205 pmol/mouse i.c.v.)-induced plasma interleukin-6 peaked at 2 h post injection, which is earlier than the peak time of the Abeta(1-42)-induced brain interleukin-6, tumor necrosis factor-alpha and interleukin-1beta levels, which was 4, 4 and 24 h, respectively. Among various peripheral organs, Abeta(1-42) (205 pmol/mouse i.c.v.) significantly increased interleukin-6 mRNA expression in lymph nodes and liver. Abeta(1-42) (205 pmol/mouse i.c.v.) significantly increased norepinephrine turnover in both hypothalamus and spleen. Either central or peripheral norepinephrine depletion effectively inhibited the Abeta(1-42)-induced peripheral interleukin-6 response. Pretreatment with prazosin (alpha(1)-adrenergic antagonist), yohimbine (alpha(2)-adrenergic antagonist), and ICI-118,551 (beta(2)-adrenergic antagonist), but not with betaxolol (beta(1)-adrenergic antagonist), inhibited Abeta(1-42)-induced plasma interleukin-6 levels. These results demonstrate that centrally administered Abeta(1-42) effectively induces the systemic interleukin-6 response which is mediated, in part, by central Abeta(1-42)-induced activation of the central and the peripheral norepinephrine systems.     

Mouse pancreatic polypeptide modulates food intake, while not influencing anxiety in mice

This study was designed to investigate the effects of synthetic mouse pancreatic polypeptide (mPP) on feeding and anxiety in mice. The intracerebroventricular (i.c.v.) injection of mPP (0.003-3 nmol) dose-dependently increased food intake. A significant increase was observed 20 min after i.c.v. injection and continued for 4 h. The intraperitoneal (i.p.) injection of mPP (0.03-30 nmol) dose-dependently decreased food intake. A significant decrease was observed 20 min after i.p. injection and continued for 4 h. In the elevated plus maze test, the i.c.v. injection of mPP (0.003-3 nmol) did not affect anxiety behavior. These results suggest that mPP modulates food intake and the Y4 receptor in the brain may contribute to the regulation of feeding, whereas appearing not to influence anxiety in mice.     

IgE production after four routes of injections of Japanese cedar pollen allergen without adjuvant: crucial role of resident cells at intraperitoneal or intranasal injection site in the production of specific IgE toward the allergen

The production of specific IgE antibodies directed toward cedar pollen correlates well with the onset of allergic rhinitis; but the mechanisms of allergen recognition as nonself and Ig class switch to IgE by the immune system are still not fully understood. In the present study, we injected cedar pollen into mice through 4 different routes (intranasal (i.n.), intraperitoneal (i.p.), intravenous (i.v.), and subcutaneous (s.c.)) without adjuvant 1 to 3 times, and determined time-dependent changes in the total and specific serum IgE levels compared with those in the serum levels of other isotype Igs. After an i.p. or i.n. injection of allergen into the mice, they produced a 1.5-to 1.7-fold increase in total IgE, but none in IgG, IgM, or IgA antibodies in their serum, whereas an i.v. or s.c. injection of allergen was inactive as an inducer of total IgE antibodies. Upon a 2nd (s.c.) injection of the allergen into the i.p. or i.n. sensitized mice, a large amount of allergen-specific IgE antibodies was found in the serum. In the case of i.v. or s.c. sensitized mice, however, they produced total, but not specific, IgE antibodies; and a 3rd (s.c.) injection of the allergen resulted in a large amount of specific IgE antibodies in the serum. These results imply that resident cells at the i.p. or i.n. injection site may play a crucial role in the efficient production of total and specific IgE antibodies toward the allergen.     

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.     

Regional brain monoamine levels and utilization in middle-aged rats.

Significant changes in monoamine levels and utilization were noted in certain brain regions of middle-aged Fisher 344 rats when compared with young adult controls. In the prefrontal cortex and septum, 3,4 dihydroxyphenylglycol (MHPG) and the MHPG/norepinephrine (NE) ratio were decreased. The septum also showed increases in dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) but there was a decrease in the DOPAC/DA ratio. The striatum showed an increase in the MPHG/NE ratio and an increase in DOPAC. The hippocampus and thalamus showed an increase in 5-hydroxyindoleacetic acid (5HIAA). This demonstrates that selected neurotransmitter systems in the brain are altered at an early stage of senescence. This could lead to ensuing neurological deficits.     

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.