Differential Inhibition of Neuronal and Extraneuronal Monoamine Oxidase Graeme Eisenhofer, Ph.D., Jacques W. M. Lenders, M.D., Ph.D., Judith Harvey-White, B.S., Monique Ernst, M.D., Ph.D., Alan Zametkin, M.O., Dennis L. Murphy, M.O., and Irwin J. Kopin, M.D. Tiiis study examined wlzetlzcr tile neuronal and 1-dcprcnyl tlzan with dcbrisoquin (255'¼, compared to a cxtmneuronal sites of action of two 1110110a111i11e oxidase 27% increase). Tlze comparable decreases in plasma (MAO) inlzibitors, 1-deprenyl and debrisoq11i11, could be concentrations of DHPG indicate a similar inhibition of disti11g11islzcd by their effects 011 plasma concentrations of intmncuronal MAO by both drugs. Much larger increases cateclw/a111ine metabolites. Plas111a co11centratio11s of tlzc in 110m1ct11nephrine after 1-deprenyl than after debrisoquin i11tra11euro11al dea111inatcd metabolite of" 11orepi11eplzrine, arc consistent with a site of action of the latter drug directed diiiydroxypiienylglycol WHPG), were decreased by 77'½, at the neuronal rather than the extraneuronal compartment. after debrisoquin and by 64'½, after l-dcpm1yl ad111i11istmtio11. Thus, differential changes in deaminated and O-methylated Plasma conccntmtions of the extmneuronal O-111etlzyl11tcd 11111i11c metabolites allm:us identification of neuronal and 111etabolitc of 11orcpineplzri11e, normctaneplzrine, were extra neuronal sites of action of MAO inhibitors. incrrnsed s11bst11ntiall11 more during treatment zpitfz [Neuropsychopharmacology 15:296-301, 1996] KEY IVORDS: Monoa111inc oxidase; Monomnine oxidase a family with an X-linked point mutation of the MAO-A inhibitors; Catec/10/11mi11es; Met11bolis111; Norcpincplzrine; gene where afflicted males exhibit impaired impulse Non11et1111cpl1ri11c control provides the most compelling evidence for a The monoamine oxidases (MAO) A and B catalyze the role of MAO in the expression of behavior (Brunner et deamination of biogenic amines and represent targets al. 1993). for therapeutic intervention in se\·eral neuropsychiatric Recognizing that MAO is differentially distributed conditions (Murphy et al. 1980; Berry et al. 1994b ). Ab­ among and within tissues and cell types (Berry et al. normal MAO acti\·ity has been implicated in alcohol­ 1994a; Lai et al. 1994; Thorpe et al. 1987) raises the pos­ ism, depression, and other psychiatric disorders (Ger­ sibility that involvement of the enzyme in different neu­ shon et al. 1979; Devor et al. 1993), but the discovery of ropsychiatric conditions might depend on regional ab­ normalities in MAO activity. Targeting therapeutic interventions to specific tissues or cell types would From the Clinical l\ieuroscie>ncl' Branch, National Institute ot therefore provide a means for tailoring treatment to the Neurological Disorders and Stroh' (CE, JH-W, lJK), Child Psychia­ try Branch (VIE, AZ). and I.aborator\' of Clinical Science (DLM) . specific neuropsychiatric condition. ."..fational Institute of Mental He,1lth, National Institutes of Health, This study examined whether differences in MAO BPtlwsda, :vL1r\'land; and DPpartnwnt of 'Vledicin<' (JW\.1L), Di\'ision activity among neuronal and extraneuronal compart­ of General Internal Medicine, St. Radboud Uni\·ersit\' Hospital, Nijrnegen, The Netherlands. ments can be distinguished using plasma concentra­ Address corrl'spondencl' to: Craenll' Eist>nhofer, Ph.D., Building tions of catecholamine metabolites. Dihydroxyphenyl­ 10, Room 'iN21-I, National Institutl's ot Health, 10 Center Driw. glycol (DHPG)r produced largely by deamination of \.1SC 1-12-1, Betht'SL"Li, .\1D 20892-1-12-1. RecL•in•d August 17. l 99~; re\ isl'd October 17, 1LJ9'i; accepted norepinephrine within sympathetic neurons (Goldstein October 18, i 'J95. et al. 1988) proYided an index of intraneuronal MAO ac- N!:LRCll 1'.--,\l'ffl)['II \R\\.\(_l))t_)C) ]LJlJ()-\tl[. 1;, \.t).) :D 1LJ96 American Colll'ge of 1'europs\·chopharrnarnlog\' Published bv Else\·ier Science Inc. 0893-133X/96/$15 00 655 An'nut,-of the Americas, New York,;\, Y llJ()]() SSDI 0893-133X(95)00233-2 NEUROPSYCHOPHARMACOLOGY 19%-VOL. 15, 'JO.~ Intra- and Extraneuronal Monoamine Oxidase 297 tivity. Normetanephrine and metanephrine, O-methy­ Blood samples (10 ml) were collected by venipuncture lated metabolites of norepinephrine and epinephrine from a forearm vein with subjects in the semi-supine and substrates for extraneuronal MAO, were indices of position. The data presented here represent results for extraneuronal MAO activity. Other measurements of samples drawn before and during the last week during the catecholamine precursor, dihydroxyphenylalanine which subjects received 1-deprenyl. (DOPA), and of the dopamine metabolite, dihydroxy­ phenylacetic acid (DOPAC), provided indices of ty­ Processing of Blood Samples rosine hydroxylase activity and dopamine deamination. Intraneuronal MAO was inhibited with debrisoquin, a Blood samples were transferred into tubes containing drug that is concentrated in sympathetic nerves by neu­ heparin or EDTA and centrifuged to separate the ronal uptake (Giachetti and Shore 1967; Medina et al. plasma. The plasma was removed and stored at -80°C 1969; Pettinger et al. 1969). The effects of debrisoquin until assayed for concentrations of catechols and metab­ were compared with those of 1-deprenyl, administered olites. Plasma concentrations of norepinephrine, epi­ at a dose sufficient to cause inhibition of both MAO-A nephrine, DHPG, DOPA, and DOPAC were estimated and MAO-B. by liquid chromatography with electrochemical detec­ tion after alumina extraction (Eisenhofer et al. 1986). Plasma concentrations of normetanephrine and meta­ METHODS nephrine were also estimated using liquid chromatog­ Subjects raphy with electrochemical detection following extrac­ tion on solid-phase ion-exchange columns (Lenders et The study population included 26 healthy volunteers al. 1993). Plasma concentrations of sulfate-conjugated (mean ::+:: SD, age 45 ::+:: 24 years) and 14 patients with at­ normetanephrine and metanephrine were estimated tention-deficit hyperacti\·ity disorder (mean ::+:: SD, age similarly after incubation of 200-µl samples of plasma 36 ::+:: 7 years). Subjects included 34 males and 6 females. with sulfatase (Sigma Chemical Co., St Louis, MO) at Women of childbearing potential were excluded. Exper­ 37°C for 30 minutes. imental protocols were approved by the intramural re­ view boards of the two institutes involved in the study Statistical Methods and subjects gave written informed consent to participate. Differences in plasma concentrations of catecholamines, Procedures DOPA, and metabolites before and during treatment with MAO inhibitors were assessed using the Student's Debrisoquin was obtained as a gift from Hoffman-La paired t-test. Differences in changes in concentrations Roche (Nutley, I\J) and formulated for administration during treatment with 1-deprenyl versus debrisoquin as tablets in the National Institutes of Health pharmacy. were assessed by two-way analysis of variance (ANOVA). The drug was administered to 26 normal volunteers at Because of the nature of the within-subjects study de­ graded doses over a 9-day inpatient stay at the Clinical sign, variances in changes in concentrations before and Center, National Institutes of Health. During the first 3 during MAO inhibitor treatment are shown using stan­ days, subjects were administered placebo tablets. On dard errors of the differences. the fourth and fifth days, subjects were administered 10 mg of debrisoquin daily. The dose was increased to 20 mg a day lWer the next 2 days and to 40 mg a day dur­ RESULTS ing the last 2 days. Total daily doses were divided into four equal doses deli\·ered 6 hours apart. Blood samples Plasma concentrations of norepinephrine were de­ (10 ml) were collected by venipuncture from a forearm creased by 20'½, to 26% during treatment with debriso­ vein with subjects in the supine position. The data pre­ quin (p < .001) or 1-deprenyl (p < .05), whereas plasma sented here represent results for samples drawn on the concentrations of epinephrine were unaffected (Table third and the last day on which subjects received de­ 1 ). Plasma concentrations of the catecholamine precur­ brisoquin. sor DOPA were decreased by 47% during treatment L-deprenyl (Selegiline hydrochloride, Eldepryl) was with debrisoquin (p < .001) and by 21 °/4, with 1-deprenyl provided by Somerset Pharmaceuticals Inc (Tampa, FL). (p < .002). Plasma concentrations of the deaminated The drug was administered orally to 14 subjects at a dopamine metabolite DOPAC were decreased similarly dose of 60 mg per day for 6 weeks, a dose previously es­ by 53'\, to 55'¾, during treatment with debrisoquin (p < tablished to cause inhibition of both MAO-A and MAO-B .001) or 1-deprenyl (p < .001), whereas concentrations of (Sunderland et al. 1985). Subjects received 1-deprenyl on DHPG, the deaminated metabolite of norepinephrine an outpatient basis but returned to the clinical center for and epinephrine, were decreased by 77°/o with debriso­ periodic clinical evaluation, including sampling of blood. quin (p < .001) and by 63'1/c, with 1-deprenyl (p < .001). 298 G. Eisenhofer et al. NEUROPSYCHOPHARMACOLOCY 1996-VOL. 15, NO. 3 Table 1. Plasma Concentrations of Catecholamines, DOPA, u DHPG DOPAC and Catecholamine Metabolites before and during < 0 Treatment with Debrisoquin or L-deprenyl 0 Q.. 0 -20 11 Baseline Treatment SED i::. :i::: Norepinephrine Q -40 OS Debrisoquin 20 1.1() 0.81 0.08* 5 L-deprenvl H '-,L.:J.:.. 2.01 0.22* 'i5. -60 Epinephrine .5 Debrisoquin 10