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Catecholamine metabolites in human plasma as indices of brain function: Effects of debrisoquin
Life Sciences, Vol. 27, pp. 1857-1862 Printed in the U.S.A.
Pergamon Press
CATECHOLAMINE METABOLITES IN HUMAN PLASMA AS INDICES OF BRAIN FUNCTION: EFFECTS OF DEBRISOQUIN Alan C. Swann, James W. Maas, Susan E. Hattox 2, Harold Landis Department of Psychiatry and Pharmacology 2, Yale University School of Medicine, 333 Cedar Street, New Haven, Connecticut (Received in final form September 8, 1980) Summary Metabolites of catecholamine neurotransmitters in plasma are, potentially, an easily available indicator of brain function in man. The peripheral contribution to these metabolites was lowered by debrisoquin sulfate, a monoamine oxidase inhibitor that does not enter the brain. In the monkey, it had been shown that debrisoquin decreased peripheral production of the dopamine metabolite, homovanillic acid (HVA), without changing production by brain; production of the norepinephrine metabolite, 3-methoxy4-hydroxyphenethyleneglycol (MHPG) was decreased peripherally and in brain. Low-dose debrisoquin administration in man eliminated about 80% of the peripheral contribution to HVA and MHPG in plasma, resulting in a situation in which at least 75% of these metabolites in plasma were from the brain. Under these conditions, HVA and MHPG in plasma had a significant correlation. It could also be estimated that production of MHPG by brain was reduced 55%. Debrisoquin potentially provides a method for studying brain catecholamines through their metabolites in plasma and for treating conditions of brain noradrenergic excess. An easily available indicator of the function of the brain catecholaminergic systems in man would provide information about the regulation of these systems and their role in human pathophysiology. For this reason, a method was developed whereby the production by the awake brain of 3-methoxy-4-hydroxyphenethyleneglycol (MHPG) and homovanillic acid (HVA), the major metaboltites of norepinephrine and dopamine, respectively, could be determined by measuring their arteriovenous differences across the brain (1,2). These studies showed that, in human subjects, about 60% of MHPG and 33% of HVA excreted in the urine originated in brain (3-5). If the production of these metabolites outside the brain could be decreased without altering production in brain, plasma and urinary measurements of HVA and MHPG would provide safe and easily available indices of the functioning of brain norepinephrine and dopamine neuronal systems. Debrisoquin sulfate, an antihypertensive agent that inhibits monoamine oxidase (6-8) and does not enter brain C9J, has been used by Karoum et al to study catecholamine turnover in the rat (i0]. We gave debrisoquin to monkeys in the dose range used clinically, and measured brain production of MHPG and HVA before and during treatment. It ~as found that debrisoquin did not change the production of HVA by brain, consistent with reports that it does not cross the blood-brain barrier, but did produce a 55% decrease in plasma, consistent with its peripheral activity as a monoamine oxidase inhibitor. During debrisoquin treatment, plasma HVA correla~ ed strongly (r = .95) with brain production of HVA.
0024-3205/80/461857-06502.00/0 Copyright (c) 1980 Pergamon Press Ltd
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Plasma MHPG, HVA and Debrisoquin
Vol. 27, No. 20, 1980
MHPG in plasma was reduced by 65% with the decreases in MHPG consistently and significantly greater than the decreases in HVA (ii). Consistent with this greater reduction in MHPC, there was an unexpected significant decrease in the brain's production of MHPG and HVA, debrisoquin therefore decreases MHPG production by an indirect mechanism. From these data it was calculated that debrisoquin produced a reduction of 80% in the peripheral production of HVA, resulting in a situation in which about 70% of the total body production of HVA was by the brain. In the case of MHPG, peripheral production was reduced by 70% and central production by 60%. Since the reduction in peripheral production of HVA was slightly larger than that in MHPG, the difference between total reduction in MHPG and HVA provides a conservative estimate of the reduction in production of MHPG by brain during debrisoquin treatment. Given these data and the fact that debrisoquin can be used safely in man (6-8), administration of debrisoquin offered the possibility of producing a situation in human subjects in which plasma measures of HVA would provide a good index of the functioning of brain dopamine systems. In addition, if debrisoquin also selectively altered brain norepinephrine function in human subjects, it might be of value in the treatment of disorders possibly associated with hyperactivity of brain norepinephrine neurons, such as opiate withdrawal (12), Tourette's syndrome (13), obesity, and some aspects of acute psychotic states (14-17). To examine these possibilities, a study with human subjects was undertaken. Methods Healthy normotensive subjects ranging in age from 22 to 50 years participated in the study after giving informed consent. During the study they boarded at the General Medical Clinical Research Center, Yale-New Haven Hospital, and were allowed unrestricted physical activity. Their diet included restrictions for monoamine oxidase inhibitor treatment. Before the study, they underwent psychiatric and medical screening, which revealed no abnormalities (18,1~). Subjects were given placebo or debrisoquin four times daily. After one day at 20 mg and one day at 40 mg/day debrisoquin, 60 mg/day was given for three days. The dose range used clinically is 40-200 mg/day (6-8). Data for the last three days of debrisoquin were compared to three days of placebo. Metabolite levels were stable during the placebo period. The paired t test was used instead of analysis of variance in order to demonstrate that, at a given time, metabolite levels were consistently either larger or smaller than at another time. In each case where the paired t test was significant, the indicated change was uniformly present, for all subjects and all three days. This relationship is obscured in analysis of variance due to a) variance among baseline levels and b) presence of intermediate values between the times with the highest and lowest values. Plasma was separated promptly after blood collection and frozen at -70°C with appropriate internal standards until assay. Free MHPG and HVA in plasma were determined by mass spectrometry with selected ion monitoring as previously described ~1,2). Results Table I and II show levels of HVA and MHPG during administration of placebo and 60 mg/day debrisoquin. Each metabolite had a diurnal variation. NIIPG reached a peak at about noon during the baseline period; during dehrisoquin administration, MHPG appeared to remain constant throughout the day and was lowest at 8:00 p.m. The reduction in plasma HVA by debrisequin averaged 55% (Table I), comparable to the reduction in monkeys previously observed. Since debrisoquin does not affect production of HVA by brain (ii) and 33% of HVA in human plasma
Vol. 27, No. 20, 1980
Plasma MHPG, HVA and Debrisoquin
1859
TABLE I Effects of Debrisoquin Treatment on Plasma HVA Baseline
Debrisoquin
8:00 a.m.
10.46 ~ 1.332
4.92 ~ 0.73
12:00 noon
9.31 ~ 2.09
4.29 ± 1.05
4:00 p.m.
9.73 ~ 1.57
4.60 ~
.313
8:00 p.m.
7.64 ± 1.562
3.77 ±
.313
values are in ng/ml ± standard deviation b a c h corresponding baseline and debrisoquin value different, paired t = 15.38, p ~ .001 28:00 a.m. different from 8:00 p.m., t = 3.78, p = .03 34:00 p.m. different from 8:00 p.m., t = 2.94, p< .05
is from brain (3-5), it is estimated that, in human subjects, debrisoquin reduced the peripheral component of plasma HVA by 82%. Accordingly, during dehrisoquin treatment 75% of the HVA in plasma was derived from brain. Debrisequin reduced plasma MHPG by an average of 67% comparable to 65% previously observed in monkeys. As in the monkey, the reductions in human plasma MHPG were consistently and significantly larger than the reductions in HVA (paired t - 3.5, P <.01). Assuming a similar decrease in the peripheral contribution to plasma MHPG to the 80% observed for HVA, and taking into account the previous estimate that 60% plasma MHPG in man is from the brain C3-5) it can be estimated that, in the present study, debrisoquin reduced central nervous system production of MHPG by 57%, comparable to the reduction hy 55% obtained in monkeys (Ii). The proportion of plasma MHPG coming from the brain during debrisoquin treatment is estimated to be 80%. Plasma HVA correlated significantly with plasma MHPG during debrisoquin administration ( r = .64), as shown in Figure i. There was no correlation during the baseline ( r = .05). Since the correlation was only seen after inhibition by debrisoquin of HVA and MHPG production outside the brain, it arguably represents a correlation between MHPG and HVA output by human brain. A similar correlation between MHPG and HVA in human cerebrospinal has been reDorted (20), consistent with previously proposed relationships between norepinephrine and dopamine (21). Discussion The extent of which peripheral MHPG is derived from brain norepinephrine has been the subject of a number of experimental approaches. In the rat, most of the data, including studies using debrisoquin by Karoum et al (10) and Helmeste et al (22), have shown that only 10-30% of peripheral MHPG in that species is from brain. There are, however, several lines of evidence, indirect and direct, showing that in man at least 60% of peripheral MHPG is from brain. Indirect evidence has included studies my Maas and Landis (23) and Ebert and
1860
Plasma MHPG, HVA and Debrisoquin
Vol. 27, No. 20, 1980
TABLE II Effect of Debrisoquin Treatment on Plasma Free MHPG Baseline
Debrisoquin
8:00 a.m.
3.38 + .83
1.28 + .611
12:00 noon
3.92 ± .302,3
1.22 ± .81
4:00 p.m.
3.48 A .173
1.35 ± .68
8:00 p.m.
3.38
.57
.95 ~ .674
values are in ng/ml + standard deviation
IAII baseline
and debrisoquin times different, paired t = 3.8, p< .0025
212:00 noon different from 8:00 a.m., t = 3.8, p< .02 312:00 noon different from 4:00 p.m., t = 5.15, p< .007 48:00 p.m. debrisoquin different from 8:00 a.m. (p< .005), 12:00 noon (p< .005) and 4:00 p.m. (p< .05)
8
-
5
4
>
i
3
i .3
[ .6
l .9
I 1.2 M HPG
I 1.5
I 1.8
I 2.i
I 2~
I 2.7
(ng/ml)
FIG. I Relationship between plasma free HVA and MHPG. The dashed line is the least-squares regression line, with r = .64 (p < .01).
Vol. 27, No. 20, 1980
Plasma MHPG, HVA and Debrisoquin
1861
Kopin ~24) using radiolabeled precursors; further evidence is provided by the effect of clonidine on plasma MHPG in man recently reported by Leckman et al (25). More direct evidence can be derived from the MHPG content of human brain tissue (26) from which, combined with estimates of MHPG clearance rate obtained from monkeys (27), it can be calculated that about 60% of plasma MHPG is from brain (27). Finally, direct measurement of MHPG production by awake human brain has yielded estimates that about 65% of urinary MHPG was from brain (4). The effects of debrisoquin on MHPG and HVA in man and in monkeys (ll) appear similar to those of treatment with guanethidine, which also does not enter the brain, reported by Izzo et al (28). They interpreted the large decreases (163%) in urinary MHPG after guanethidine treatment as evidence that only a small proportion of human urinary MHPG was from brain. In view of the evidence cited above, an alternative explanation for their data is that guanethidine reduced the production of MHPG by brain in a manner similar to that in which debrisoquin has been shown directly to reduce production of MHPG by monkey brain (ll). The smaller effect of guanethidine on HVA is also consistent with the results described in this paper and with the fact that debrisoquin did not affect HVA production by monkey brain (ll). The mechanism by which debrisoquin appears to reduce production of MHPG by the brain is unknown. Several lines of evidence indicate that neither debrisoquin nor any metabolites with monoamine oxidase activity enter the brain: 1] debrisoquin is not detectable in brain even after high doses (9); 2) debrisoquin does not affect brain MAO activity at doses that inhibit MAO in peripheral tissues (9), and 3) debrisoquin decreases plasma HVA in the monkey without changing production of HVA by brain (ll). The reduction of MHPG production in brain during debrisoquin treatment is therefore likely to be an indirect result of one of debrisoquin's peripheral effects. The dichotomy between peripheral and central actions of drugs while heuristically useful can overlook the possibility that, without itself entering the brain, a drug with effects on a system (such as blood pressure) that is partially regulated by the brain could produce changes in brain function via endogenous control mechanisms From the data reported here, it can be concluded that a) low-dose debrisoquin eliminated the bulk of the non-central nervous system contribution to HVA and MHPG in plasma, resulting in a situation in which at least 3/4 of the MHPG and HVA in plasma is derived from the brain. Since this drug does not alter production of HVA by brain, during debrisoquin treatment, measurements of plasma HVA should provide an easily available index of brain dopamine function; b) debrisoquin consistently reduced plasma MHPG more than it did plasma HVA, corresponding to its effects in monkeys where it was shown directly that production of MHPG by brain was selectively reduced (ii); and c] during debrisoquin administration, plasma MHPG and HVA had a positive correlation, as has been shown in human cerebrospinal fluid (20). These data indicate that debrisoquin sulfate provides a safe and effective way to study roles of dopamine and norepinephrine in human pathophysiology. They also suggest that, by selectively reducing noradrenergic activity in the brain, debrisoquin may be useful in the treatment of disorders characterized by noradrenergic excess Acknowledgements: Supported by USPHS Grants MH24393, MH 25642, and RRI25. Susan Sturgeon and David Jablons coordinated subject participation and data collection; Rita Santamauro performed assays; Deborah Casher prepared the manuscript; the staff of the Adult General Medical Clinical Research Center, Yale-New Haven Hospital (P. Felig, M.D., Director)provided clinical care. References i. 2.
J.W. MAAS, S.E. HATTOX, D.H. LANDIS, and R.H. ROTH. Brain Res. 118 167-173, (1976) J.W. MAAS, S.E. HATTOX, D.M. MARTIN, and D.H. LANDIS. J. Neurochem. 32 839-
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3. 4. 5. 6. 7. 8. 9. i0. ii. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.
Plasma MHPG, HVA and Debrisoquin
Vol. 27, No. 20, 1980
843 (1978) J.W. MAAS, N.M. GREENE, S.E. HATTOX, and D.H. LANDIS. in Catecholamines: Basic and Clinical Frontiers (E. Usdin, Ed.), 1878-1880 (1979) J.W. MAAS, S.E. HATTOX, N.M. GREENE, and D.H. LANDIS, Science 2051025-1027, (1979) J.W. MAAS, S.E. HATTOX, N.M. GREENE, and D.H. LANDIS, J. Neurochem., In press M.H. LURIA and E.D. FREIS, Curr. Ther. Res. 7289-296 (1965) A.H. KTICHIN and R.W.D. TUP~R, Brit. Med. J. 2728-731 (1966) H.M. SOLOMIN, C. ASHLEY, N. SPIRT, and W.B. AB--RAMS, Clin. Pharmacol. Ther. i0229-238 (1968) M.A. MEDINA, A. GIACHETTI, and P.A. SHORE, Biochem. Pharmacol. 18891-901 (1969) F. KAROUM, R.J. WYATT, and E. COSTA, Neuropharmacology 13165-176 (1974) J.W. MAAS, S.E. HATTOX, and D.H. LANDIS, Biochem. Pharmacol. 283153-3156 (1979 ) M.S. GOLD, D.E. REDMOND, JR., and H.W. KLEBER. The Lancet ii 599 (1978) D.J. COHEN, J.G. YOUNG, J.A. NATHANSON, and B.A. SHAYWiTZ, The Lancet. In press. J.E. THORNBURG and K.E. MOORE, Neuropharmacology i_22853-866 (1973) D.M. JACKSON, R. DUNSTAN, and A. PERRINGTON, J. Neural Trans. 44175-186 (1979) N.E. ANDEN, U. STROMBOM, and T.H. SVENSSON. Psychopharmacology 54 243-248 (1977) R.J. WYATT, L.B. BIGELOW, and J.C. GILLIN, in Catecholamines: Basic and Clinical Frontiers (E. Usdin, l.J. Kopin, J. Barchas, Eds.) 1720-1722 (1979) C. SPITZER and J. ENDICOTT. Schedule for Affective Disorders and Schizophrenia, 3rd Ed. (1978) J.P. FEIGHNER, E. ROBINS, S. GUZE, R. WOODRUFF, G. WINOKUR, and R. MUNOZ, Arch. Gen. Psychiat. 2657-72 (1972) C. BOWDEN, D.E. REDMOND JR., A.C. SWANN, and J.W. MAAS, Psychopharmacol. Bull., In press S.M. ANTEI~AN and A.R. CAGGIULA. Science 195646-648 (1977) D.M. HELMESTE, H.C. STANCER, D.V. COSCINA, S. TAKAHASHI, and J.J. WARSH. Life Sci. 25601-606 (1979) J.W. MAAS ~ d D.H. LANDIS. J. Pharmacol. Exp. Ther. 177600-607 (1971) M.H. EBERT and l.J. KOPIN. Trans. Am. Assoc. Phys. 8~--256 (1975) J.F. LECKMAN, J.W. MAAS, D.E. REDMOND, and G.R. RENINGER. Life Sci. 26 2179-2185 (1980) J.W. MAAS, D.H. LANDIS, and H. DEKIRMENJIAN, Psychopharmacol. Commun. 2 403-410 (1976) J.W. MAAS, S.E. HATTOX, D.H. LANDIS, and R.H. ROTH, Eur. J. Pharmacol. 46 221-228 (1977) J.L. IZZO, JR., D. HORWITZ, and H.R. KEISER. Life Sci. 241403-1406 (1979)
Pergamon Press
CATECHOLAMINE METABOLITES IN HUMAN PLASMA AS INDICES OF BRAIN FUNCTION: EFFECTS OF DEBRISOQUIN Alan C. Swann, James W. Maas, Susan E. Hattox 2, Harold Landis Department of Psychiatry and Pharmacology 2, Yale University School of Medicine, 333 Cedar Street, New Haven, Connecticut (Received in final form September 8, 1980) Summary Metabolites of catecholamine neurotransmitters in plasma are, potentially, an easily available indicator of brain function in man. The peripheral contribution to these metabolites was lowered by debrisoquin sulfate, a monoamine oxidase inhibitor that does not enter the brain. In the monkey, it had been shown that debrisoquin decreased peripheral production of the dopamine metabolite, homovanillic acid (HVA), without changing production by brain; production of the norepinephrine metabolite, 3-methoxy4-hydroxyphenethyleneglycol (MHPG) was decreased peripherally and in brain. Low-dose debrisoquin administration in man eliminated about 80% of the peripheral contribution to HVA and MHPG in plasma, resulting in a situation in which at least 75% of these metabolites in plasma were from the brain. Under these conditions, HVA and MHPG in plasma had a significant correlation. It could also be estimated that production of MHPG by brain was reduced 55%. Debrisoquin potentially provides a method for studying brain catecholamines through their metabolites in plasma and for treating conditions of brain noradrenergic excess. An easily available indicator of the function of the brain catecholaminergic systems in man would provide information about the regulation of these systems and their role in human pathophysiology. For this reason, a method was developed whereby the production by the awake brain of 3-methoxy-4-hydroxyphenethyleneglycol (MHPG) and homovanillic acid (HVA), the major metaboltites of norepinephrine and dopamine, respectively, could be determined by measuring their arteriovenous differences across the brain (1,2). These studies showed that, in human subjects, about 60% of MHPG and 33% of HVA excreted in the urine originated in brain (3-5). If the production of these metabolites outside the brain could be decreased without altering production in brain, plasma and urinary measurements of HVA and MHPG would provide safe and easily available indices of the functioning of brain norepinephrine and dopamine neuronal systems. Debrisoquin sulfate, an antihypertensive agent that inhibits monoamine oxidase (6-8) and does not enter brain C9J, has been used by Karoum et al to study catecholamine turnover in the rat (i0]. We gave debrisoquin to monkeys in the dose range used clinically, and measured brain production of MHPG and HVA before and during treatment. It ~as found that debrisoquin did not change the production of HVA by brain, consistent with reports that it does not cross the blood-brain barrier, but did produce a 55% decrease in plasma, consistent with its peripheral activity as a monoamine oxidase inhibitor. During debrisoquin treatment, plasma HVA correla~ ed strongly (r = .95) with brain production of HVA.
0024-3205/80/461857-06502.00/0 Copyright (c) 1980 Pergamon Press Ltd
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Plasma MHPG, HVA and Debrisoquin
Vol. 27, No. 20, 1980
MHPG in plasma was reduced by 65% with the decreases in MHPG consistently and significantly greater than the decreases in HVA (ii). Consistent with this greater reduction in MHPC, there was an unexpected significant decrease in the brain's production of MHPG and HVA, debrisoquin therefore decreases MHPG production by an indirect mechanism. From these data it was calculated that debrisoquin produced a reduction of 80% in the peripheral production of HVA, resulting in a situation in which about 70% of the total body production of HVA was by the brain. In the case of MHPG, peripheral production was reduced by 70% and central production by 60%. Since the reduction in peripheral production of HVA was slightly larger than that in MHPG, the difference between total reduction in MHPG and HVA provides a conservative estimate of the reduction in production of MHPG by brain during debrisoquin treatment. Given these data and the fact that debrisoquin can be used safely in man (6-8), administration of debrisoquin offered the possibility of producing a situation in human subjects in which plasma measures of HVA would provide a good index of the functioning of brain dopamine systems. In addition, if debrisoquin also selectively altered brain norepinephrine function in human subjects, it might be of value in the treatment of disorders possibly associated with hyperactivity of brain norepinephrine neurons, such as opiate withdrawal (12), Tourette's syndrome (13), obesity, and some aspects of acute psychotic states (14-17). To examine these possibilities, a study with human subjects was undertaken. Methods Healthy normotensive subjects ranging in age from 22 to 50 years participated in the study after giving informed consent. During the study they boarded at the General Medical Clinical Research Center, Yale-New Haven Hospital, and were allowed unrestricted physical activity. Their diet included restrictions for monoamine oxidase inhibitor treatment. Before the study, they underwent psychiatric and medical screening, which revealed no abnormalities (18,1~). Subjects were given placebo or debrisoquin four times daily. After one day at 20 mg and one day at 40 mg/day debrisoquin, 60 mg/day was given for three days. The dose range used clinically is 40-200 mg/day (6-8). Data for the last three days of debrisoquin were compared to three days of placebo. Metabolite levels were stable during the placebo period. The paired t test was used instead of analysis of variance in order to demonstrate that, at a given time, metabolite levels were consistently either larger or smaller than at another time. In each case where the paired t test was significant, the indicated change was uniformly present, for all subjects and all three days. This relationship is obscured in analysis of variance due to a) variance among baseline levels and b) presence of intermediate values between the times with the highest and lowest values. Plasma was separated promptly after blood collection and frozen at -70°C with appropriate internal standards until assay. Free MHPG and HVA in plasma were determined by mass spectrometry with selected ion monitoring as previously described ~1,2). Results Table I and II show levels of HVA and MHPG during administration of placebo and 60 mg/day debrisoquin. Each metabolite had a diurnal variation. NIIPG reached a peak at about noon during the baseline period; during dehrisoquin administration, MHPG appeared to remain constant throughout the day and was lowest at 8:00 p.m. The reduction in plasma HVA by debrisequin averaged 55% (Table I), comparable to the reduction in monkeys previously observed. Since debrisoquin does not affect production of HVA by brain (ii) and 33% of HVA in human plasma
Vol. 27, No. 20, 1980
Plasma MHPG, HVA and Debrisoquin
1859
TABLE I Effects of Debrisoquin Treatment on Plasma HVA Baseline
Debrisoquin
8:00 a.m.
10.46 ~ 1.332
4.92 ~ 0.73
12:00 noon
9.31 ~ 2.09
4.29 ± 1.05
4:00 p.m.
9.73 ~ 1.57
4.60 ~
.313
8:00 p.m.
7.64 ± 1.562
3.77 ±
.313
values are in ng/ml ± standard deviation b a c h corresponding baseline and debrisoquin value different, paired t = 15.38, p ~ .001 28:00 a.m. different from 8:00 p.m., t = 3.78, p = .03 34:00 p.m. different from 8:00 p.m., t = 2.94, p< .05
is from brain (3-5), it is estimated that, in human subjects, debrisoquin reduced the peripheral component of plasma HVA by 82%. Accordingly, during dehrisoquin treatment 75% of the HVA in plasma was derived from brain. Debrisequin reduced plasma MHPG by an average of 67% comparable to 65% previously observed in monkeys. As in the monkey, the reductions in human plasma MHPG were consistently and significantly larger than the reductions in HVA (paired t - 3.5, P <.01). Assuming a similar decrease in the peripheral contribution to plasma MHPG to the 80% observed for HVA, and taking into account the previous estimate that 60% plasma MHPG in man is from the brain C3-5) it can be estimated that, in the present study, debrisoquin reduced central nervous system production of MHPG by 57%, comparable to the reduction hy 55% obtained in monkeys (Ii). The proportion of plasma MHPG coming from the brain during debrisoquin treatment is estimated to be 80%. Plasma HVA correlated significantly with plasma MHPG during debrisoquin administration ( r = .64), as shown in Figure i. There was no correlation during the baseline ( r = .05). Since the correlation was only seen after inhibition by debrisoquin of HVA and MHPG production outside the brain, it arguably represents a correlation between MHPG and HVA output by human brain. A similar correlation between MHPG and HVA in human cerebrospinal has been reDorted (20), consistent with previously proposed relationships between norepinephrine and dopamine (21). Discussion The extent of which peripheral MHPG is derived from brain norepinephrine has been the subject of a number of experimental approaches. In the rat, most of the data, including studies using debrisoquin by Karoum et al (10) and Helmeste et al (22), have shown that only 10-30% of peripheral MHPG in that species is from brain. There are, however, several lines of evidence, indirect and direct, showing that in man at least 60% of peripheral MHPG is from brain. Indirect evidence has included studies my Maas and Landis (23) and Ebert and
1860
Plasma MHPG, HVA and Debrisoquin
Vol. 27, No. 20, 1980
TABLE II Effect of Debrisoquin Treatment on Plasma Free MHPG Baseline
Debrisoquin
8:00 a.m.
3.38 + .83
1.28 + .611
12:00 noon
3.92 ± .302,3
1.22 ± .81
4:00 p.m.
3.48 A .173
1.35 ± .68
8:00 p.m.
3.38
.57
.95 ~ .674
values are in ng/ml + standard deviation
IAII baseline
and debrisoquin times different, paired t = 3.8, p< .0025
212:00 noon different from 8:00 a.m., t = 3.8, p< .02 312:00 noon different from 4:00 p.m., t = 5.15, p< .007 48:00 p.m. debrisoquin different from 8:00 a.m. (p< .005), 12:00 noon (p< .005) and 4:00 p.m. (p< .05)
8
-
5
4
>
i
3
i .3
[ .6
l .9
I 1.2 M HPG
I 1.5
I 1.8
I 2.i
I 2~
I 2.7
(ng/ml)
FIG. I Relationship between plasma free HVA and MHPG. The dashed line is the least-squares regression line, with r = .64 (p < .01).
Vol. 27, No. 20, 1980
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Kopin ~24) using radiolabeled precursors; further evidence is provided by the effect of clonidine on plasma MHPG in man recently reported by Leckman et al (25). More direct evidence can be derived from the MHPG content of human brain tissue (26) from which, combined with estimates of MHPG clearance rate obtained from monkeys (27), it can be calculated that about 60% of plasma MHPG is from brain (27). Finally, direct measurement of MHPG production by awake human brain has yielded estimates that about 65% of urinary MHPG was from brain (4). The effects of debrisoquin on MHPG and HVA in man and in monkeys (ll) appear similar to those of treatment with guanethidine, which also does not enter the brain, reported by Izzo et al (28). They interpreted the large decreases (163%) in urinary MHPG after guanethidine treatment as evidence that only a small proportion of human urinary MHPG was from brain. In view of the evidence cited above, an alternative explanation for their data is that guanethidine reduced the production of MHPG by brain in a manner similar to that in which debrisoquin has been shown directly to reduce production of MHPG by monkey brain (ll). The smaller effect of guanethidine on HVA is also consistent with the results described in this paper and with the fact that debrisoquin did not affect HVA production by monkey brain (ll). The mechanism by which debrisoquin appears to reduce production of MHPG by the brain is unknown. Several lines of evidence indicate that neither debrisoquin nor any metabolites with monoamine oxidase activity enter the brain: 1] debrisoquin is not detectable in brain even after high doses (9); 2) debrisoquin does not affect brain MAO activity at doses that inhibit MAO in peripheral tissues (9), and 3) debrisoquin decreases plasma HVA in the monkey without changing production of HVA by brain (ll). The reduction of MHPG production in brain during debrisoquin treatment is therefore likely to be an indirect result of one of debrisoquin's peripheral effects. The dichotomy between peripheral and central actions of drugs while heuristically useful can overlook the possibility that, without itself entering the brain, a drug with effects on a system (such as blood pressure) that is partially regulated by the brain could produce changes in brain function via endogenous control mechanisms From the data reported here, it can be concluded that a) low-dose debrisoquin eliminated the bulk of the non-central nervous system contribution to HVA and MHPG in plasma, resulting in a situation in which at least 3/4 of the MHPG and HVA in plasma is derived from the brain. Since this drug does not alter production of HVA by brain, during debrisoquin treatment, measurements of plasma HVA should provide an easily available index of brain dopamine function; b) debrisoquin consistently reduced plasma MHPG more than it did plasma HVA, corresponding to its effects in monkeys where it was shown directly that production of MHPG by brain was selectively reduced (ii); and c] during debrisoquin administration, plasma MHPG and HVA had a positive correlation, as has been shown in human cerebrospinal fluid (20). These data indicate that debrisoquin sulfate provides a safe and effective way to study roles of dopamine and norepinephrine in human pathophysiology. They also suggest that, by selectively reducing noradrenergic activity in the brain, debrisoquin may be useful in the treatment of disorders characterized by noradrenergic excess Acknowledgements: Supported by USPHS Grants MH24393, MH 25642, and RRI25. Susan Sturgeon and David Jablons coordinated subject participation and data collection; Rita Santamauro performed assays; Deborah Casher prepared the manuscript; the staff of the Adult General Medical Clinical Research Center, Yale-New Haven Hospital (P. Felig, M.D., Director)provided clinical care. References i. 2.
J.W. MAAS, S.E. HATTOX, D.H. LANDIS, and R.H. ROTH. Brain Res. 118 167-173, (1976) J.W. MAAS, S.E. HATTOX, D.M. MARTIN, and D.H. LANDIS. J. Neurochem. 32 839-
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