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

Changes induced by sodium cromoglycate in brain catecholamine turnover in morphine dependent and abstinent mice

The effects of sodium cromoglycate (CRO) were studied in relation to the metabolism of brain catecholamines: dopamine (DA) and noradrenaline (NA), and their metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 4-hydroxy-3-methoxyphenylethyleneglycol (MHPG). CRO was injected SC in control mice, morphine-tolerant mice (tolerance was induced by SC implantation of a 75 mg morphine pellet; CRO was administered on day 4 of addiction) and 30 min before abstinence (withdrawal was induced by SC injection of naloxone (1 mg/kg) on day 4 of addiction). Brain catecholamines and their metabolites were measured using high performance liquid chromatography coupled with electrochemical detection (HPLC-ECD), for DA, NA, DOPAC and HVA, and coupled with fluorescence detection for MHPG. The ratios of DOPAC + HVA/DA and MHPG/NA were kept as an index of DA and NA turnovers, respectively. CRO administered 30 min before naloxone-precipitated withdrawal diminished significantly NA levels in frontal cortex. CRO increased DA turnover in striatum and frontal cortex in naive animals and significantly diminished DA levels in frontal cortex and DOPAC levels in frontal cortex and midbrain in morphine-dependent mice. These findings are discussed in relation to the protective effects of CRO on opiate withdrawal and the effects of CRO on locomotor activity.

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.