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Variations of monoamines and their metabolites in the human brain putamen
Brain Research, 579 (1992) 285-290 © 1992 Elsevier Science Publishers B.V. All fights reserved. 0006-8993/92/$05.00
285
BRES 17691
Variations of monoamines and their metabolites in the human brain putamen Christine Konradi a'b, Johannes Kornhuber a, Emin Sot]c a'b, Stephan Heckers c, Peter Riederer a'b and Helmut Beckmann a University of Wiirzburg, "Department of Psychiatry and bClinical Neurochemistry Group, Warzburg (FRG) and CBeth Israel Hospital, Harvard Medical School, Department of Neurology, Boston, MA 02215 (USA) (Accepted 17 December 1991)
Key words: Catechol-O-methyltransferase; Monoamine; Dopamine; Human brain; Monoamine oxidase; Norepinephrine; Serotonin
The levels of the monoamines dopamine (DA), serotonin (5-HT) and norepinephrine (NE) and the monoaminergic metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanilhc acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) were measured with HPLC-ECD in 42 samples from human brain putamen. The influence of gender and of age was investigated and correlations between the monoamines were established. The DAergic system shows a significant difference between males and females, with females having lower DA and higher DOPAC levels and a higher DOPAC/DA ratio than males. No gender-related differences of 5-HT and its metabolites were observed, nor of NE. Three different age groups (group 1:0-9.9 years; group 2:10-59.9 years; group 3:60 years and older) were defined according to previous studies on ontogenesis and senescence in human brain. An increase in 5-HT levels, decrease in 5-HIAA levels and a decrease in the 5-HIAAJ5-HT ratio were observed after the first decade of life. Changes in the DAergic system were seen in senescence, with decreasing DA levels and an increase in the HVA/DA ratio. DOPAC, HVA and the DOPAC/DA ratio are unaffected. NE is similar in all age groups. The analysis of the relation of the levels of the three monoamines proved a strong correlation between the DAergic and 5-HTergic systems. The nature of this relationship might have an impact on neuro-psychiatric disorders and brain function. INTRODUCTION M o n o a m i n e s like d o p a m i n e ( D A ) , serotonin (5-HT) or n o r e p i n e p h r i n e (NE) serve as neurotransmitters in the brain and in p e r i p h e r a l organs. While N E and D A are synthesized in similar pathways, with tyrosine hydroxylase as the rate-limiting enzyme 4°, 5 - H T is the product of a different pathway with t r y p t o p h a n hydroxylase activity as the rate-limiting step 14. Metabolic inactivation of m o n o a m i n e s in the brain is p e r f o r m e d by m o n o a m i n e oxidase ( M A O ) and of D A and N E also by catechol-O-methyltransferase ( C O M T ) . M A O is an enzyme of the o u t e r mitochondrial m e m b r a n e , located in neurons and glial cells, with two i n d e p e n d e n t subtypes, M A O - A and -B. B o t h subtypes are characterized by substrate and inhibitor specificities 17 and transcribed from two separate genes 4. C O M T is a soluble enzyme and localized p r e d o m i nantly extraneuronally 3. A second, m e m b r a n e - b o u n d COMT, which is localized in neurons was described, with a hundred-fold lower K m than the soluble form 16'26'27. It is i m p o r t a n t to note that the D A - d e r i v e d product of M A O and an a l d e h y d e d e h y d r o g e n a s e , 3,4-dihydroxy-
phenylacetic acid ( D O P A C ) is therefore mostly of intraneuronal origin, while homovanillic acid ( H V A ) , a product of COMT, M A O and a l d e h y d e d e h y d r o g e n a s e , is extraneuronally synthesized. Variations of the m o n o a m i n e r g i c n e u r o t r a n s m i t t e r and respective metabolite levels in h u m a n C S F and brain are r e p o r t e d with respect to age 8'23'28'34, sex 1, b o d y size 5 and CNS disorders 1'31'32. U n f o r t u n a t e l y , the majority of these studies were done with h u m a n CSF, leaving questions about the true representation of brain metabolism. Therefore, the present study of non-pathological variations of m o n o a m i n e s and their metabolites was carried out in p o s t - m o r t e m h u m a n brain p u t a m e n . We m e a s u r e d D A , 5-HT and N E and the metabolites H V A , D O P A C and 5-hydroxyindoleacetic acid ( 5 - H I A A ) , with H P L C - E C D . A g e and genderrelated changes as well as the relationship of these substances to each o t h e r were analyzed. MATERIALS AND METHODS Samples from human brain putamen (24 males, 18 females), aged from 0.125 to 82 years, were used for the HPLC-ECD analysis. Tissue was taken at autopsy by a neuropathologist, and had a
Correspondence: C. Konradi. Present address: Massachusetts General Hospital, Harvard Medical School, Department of Molecular Neurobiology, Building 149 13th Street, Charlestown, MA 02129, USA. Fax: (1) (617) 726-5677.
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Fig. 1. a: levels of DA, 5-HT and NE in the three age groups, b: levels of the metabolites of DA (DOPAC and HVA) and 5-HT (5-HIAA). Open bars are mean values. Group 1: under 10 years; group 2:10 until 60 years; group 3:60 years or older. *Indicates P < 0.05. **Indicates P < 0.005.
post-mortem delay between 6 and 82 h (mean 43.2 + 21.4 S.D.). The causes of death included accidents, sudden infant death and suicide. Brain tissue was frozen at -80°C, and stored for 84-898 days (mean 397 + 204 S.D.) until analysis. Attention to the even age distribution with regard to gender, post-mortem delay and storage time was paid. Statistical analysis included the Mann-Whitney U-test and Spearman's rank correlation. All values are given as mean + S.D. Concentrations are measured in/xg substance/g brain wet weight.
DOPAC/DA
ratio t h a n females. A g e and p o s t - m o r t e m
RESULTS
h i g h e r storage time.
Gender and monoamines
Monoamines in age
t i m e w e r e similar in e i t h e r g r o u p (Table I). T h e r e was a h i g h e r s t o r a g e t i m e of m a l e brains (Table I), which, since the m e t a b o l i z i n g e n z y m e M A O is n o t affected by freezing o r storage t i m e ~9'21, w o u l d r a t h e r a c c o u n t for d e c r e a s e d D A levels and an i n c r e a s e d D O P A C / D A
ra-
tio. T h e r e f o r e , t h e h i g h e r D A levels and l o w e r D O P A C / D A ratio in m a l e s c a n n o t be an artifact of the
L e v e l s o f m o n o a m i n e s and m o n o a m i n e r g i c m e t a b o -
W e arbitrarily established t h r e e age groups; g r o u p 1:
lites in m a l e s and f e m a l e s are s h o w n in Table I. M a l e s
p o s t n a t a l to u n d e r 10 years, g r o u p 2 : 1 0 years until 60 years and g r o u p 3 : 6 0 years and a b o v e . Since studies
had
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levels
and
a
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287 TABLE I
TABLE II
Gender and monoamines
Matching o f the three age groups
Values are mean + S.D.
n (M/F)
Group I 7/4
Age range (years) Mean age (years) Post-mortem delay (h)
0.13-6 14-55 50-82 0.99 + 1.71 35.7 + 12.0 74.5 + 6.93 52.8 + 21.3 42.0 + 22.1 36.1 + 18.1
Storage time (days)
432 + 169
Male n = 24
Female n = 18
5-HT ~mol/g) 5-HIAA (9mol/g) 5-HIAA/5-HT DA (,umol/g) HVA ~mol/g) DOPAC (~mol/g) HVA/DA DOPAC/DA NA ~mol/g)
1.06 1.19 1.34 3.33 7.53 0.61 2.58 0.23 0.03
0.97 1.00 1.17 2.51 6.33 1.05 3.04 0.50 0.03
age (years) post-mortem delay (h) storage time (days)
34.6 + 27.9 44.7 + 22.4 463 + 193
+ 0.45 + 0.44 + 0.92 + 1.85 + 3.39 + 0.35 + 1.05 + 0.19 ___0.03
+ + + + + + + + +
0.38 0.31 0.65 1.45" 2.24 0.63 1.59 0.43** 0.02
Group 2 12/9
393 + 238
Group 3 5/5
365 + 170
higher in group 1 when compared to group 2 (Fig. lb). The D A metabolites H V A and D O P A C are unchanged
37.4 + 28.3 41.3 + 20.5 308 + 188"
during life (Fig. lb). Investigation of the monoamine/respective metabolite ratios revealed a significantly higher 5 - H I A A / 5 - H T ratio in early childhood when compared to either one of the other groups, while the H V A / D A
* Indicates P < 0.05. ** Indicates P < 0.005 (Mann-Whitney U-Test).
ratio was higher in group 3 when compared to group 1 (Fig. 2).
about receptor binding22'33, synaptogenesis t5 and mono-
C o r r e l a t i o n s b e t w e e n the m o n o a m i n e s
amine oxidase 21 have shown significant differences at the beginning of the first decade of life, age 10 years was picked as upper limit of group 1. The group consists of ten patients u n d e r age 2 years and one of age 6 years. Age 60 as lower limit of group 3 was chosen from observations of accelerated decline of brain functions and D A levels after this age 7. Table II lists the data of the
A positive correlation between the DAergic and 5-HTergic system was observed (Fig. 3). Both m o n o a m ines as well as their metabolites showed an interdependency (Fig. 3, Table III). N E did not correlate with either m o n o a m i n e . DISCUSSION
case histories of the three age groups. Looking at the m o n o a m i n e s , 5-HT is lowest in group 1 and D A lowest in group 3 (Fig. la). Of the metabolites, the 5-HT metabolite 5 - H I A A was significantly DOPAC/DA
Role of monoamine
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Even though w o m e n had lower D A levels in the brain p u t a m e n than men, their D O P A C levels were higher
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Fig. 2. The metabolite/substrate levels of DA and 5-HT in the three age groups/Open bars are mean values. Group I: under 10 years; group 2:10 until 50 years; group 3:50 years or older. *Indicates P < 0.05. **Indicates P < 0.001.
288
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tyramine, respectively 35, and since D A is described as a M A O - B substrate in humans 11, the D O P A C / D A ratio is a measure of intraneuronal M A O - B activity. Thus, the higher D O P A C / D A ratio in females is most likely a reflection of their higher M A O - B activity 24'29. In concordance, despite their higher D A levels, males, who in general have a lower M A O - B activity than females 24'29, show a lower D O P A C / D A ratio. This indicates, that in vivo the intraneuronal M A O - B level in the nigrostriatal pathway is so low that it is actually saturated, since higher enzyme levels at lower D A concentrations, as shown in females, show a higher yield of D O P A C . In concordance, M A O - B levels in immunocytochemical 2°'41 and histochemical studies TM of the DAergic perikarya of the substantia nigra, were under the detection level. In contrast, the age-related increase of M A O - B , as published earlier 1°'13'21'25'3°, was not reflected in the
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D O P A C / D A ratio, which is the same in all age groups (Fig. 2). This finding supports the hypothesis that the M A O - B activity increases in aging extraneuronally and represents gliosis 25'36. Moreover, the extraneuronal turnover H V A / D A is higher in group 3 (Fig. 2), which fits into this model. However, unlike in the DAergic neurons, the M A O - B level is very high in glial cells T M . A n increased M A O - B activity in glial cells therefore might not solely explain an increased H V A / D A turnover rate. A higher leakage of DAergic neurons in senescence or a decreased D A reuptake z'9'as, is more likely the reason for the increased H V A / D A rate and might also be responsible for the decrease in the D A level in senescence (group 3, Fig. la). Thus, while the observed gender differences in the D O P A C / D A ratio can be attributed to different levels of intraneuronal M A O - B activity, the observed age differences of the H V A / D A ratio can best be explained by an increased extraneuronal availability of D A (e.g., leakage of neurons or reduced reuptake).
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Fig. 3. Correlations of the monoamines DA and 5-HT and their metabolites HVA and 5-HIAA. Correlation coefficients and P-values: see Table III.
(n.s.) and, as a result, the D O P A C / D A ratio was higher (Table I). Since D O P A C and D A outside the neuron are rapidly metabolized by C O M T to H V A and 3-methoxy-
TABLE III Spearman's rank correlation coefficients between monoamines and monoaminergic metabolites
5-HT NE DOPAC HVA 5-HIAA DOPAC/DA HVA/DA 5-HIAA/5-HT * Indicates P < 0.05. ** Indicates P < 0.005. *** Indicates P < 0.001.
DA
5-HT
NE
DO PA C
HVA
5-HIAA
0.382* 0.187 0.098 0.493*** 0.335* -0.491"** -0.539"** -0.062
-0.042 -0.186 0.539*** 0.389* -0.337* 0.163 -0.576"**
0.127 -0.170 0.110 -0.077 -0.385* 0.239
0.157 -0.037 0.755"** 0.010 0.153
0.508*** -0.167 0.407* -0.082
-0.254 0.073 0.479**
289
Role of monoamine oxidase A While decreased DA levels (Fig. la) and an increased HVA/DA ratio (Fig. 2) were found in group 3, the 5-HTergic system had the highest turnover rate early in life. 5-HT levels were lowest in group 1 (Fig. la), and 5-HIAA levels were higher than in group 2 (Fig. lb), resulting in a higher 5-HIAA/5-HT ratio. Since 5-HT is metabolized by MAO-A to 5-HIAA 37, the 5-HIAA/ 5-HT ratio reflects MAO-A activity. Thus, a previous report which showed a higher MAO-A activity in children under 10 years 21 is supported by the present finding. Interdependent variations of monoamines In all age groups, a strong correlation of DA with 5-HT and most of their metabolites was seen (Table III, Fig. 3). DOPAC was the exception which correlated with the DOPAC/DA ratio only (Table III). As discussed above, DOPAC levels in the nigrostriatal system seem to reflect predominantly the intraneuronal MAO-B activity and are independent of DA concentrations
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Acknowledgements. This study was supported by the Deutsche Hirnliga (C.K. and P.R.) and by the Deutsche Forschungsgemeinschaft (J.K.).
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285
BRES 17691
Variations of monoamines and their metabolites in the human brain putamen Christine Konradi a'b, Johannes Kornhuber a, Emin Sot]c a'b, Stephan Heckers c, Peter Riederer a'b and Helmut Beckmann a University of Wiirzburg, "Department of Psychiatry and bClinical Neurochemistry Group, Warzburg (FRG) and CBeth Israel Hospital, Harvard Medical School, Department of Neurology, Boston, MA 02215 (USA) (Accepted 17 December 1991)
Key words: Catechol-O-methyltransferase; Monoamine; Dopamine; Human brain; Monoamine oxidase; Norepinephrine; Serotonin
The levels of the monoamines dopamine (DA), serotonin (5-HT) and norepinephrine (NE) and the monoaminergic metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanilhc acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA) were measured with HPLC-ECD in 42 samples from human brain putamen. The influence of gender and of age was investigated and correlations between the monoamines were established. The DAergic system shows a significant difference between males and females, with females having lower DA and higher DOPAC levels and a higher DOPAC/DA ratio than males. No gender-related differences of 5-HT and its metabolites were observed, nor of NE. Three different age groups (group 1:0-9.9 years; group 2:10-59.9 years; group 3:60 years and older) were defined according to previous studies on ontogenesis and senescence in human brain. An increase in 5-HT levels, decrease in 5-HIAA levels and a decrease in the 5-HIAAJ5-HT ratio were observed after the first decade of life. Changes in the DAergic system were seen in senescence, with decreasing DA levels and an increase in the HVA/DA ratio. DOPAC, HVA and the DOPAC/DA ratio are unaffected. NE is similar in all age groups. The analysis of the relation of the levels of the three monoamines proved a strong correlation between the DAergic and 5-HTergic systems. The nature of this relationship might have an impact on neuro-psychiatric disorders and brain function. INTRODUCTION M o n o a m i n e s like d o p a m i n e ( D A ) , serotonin (5-HT) or n o r e p i n e p h r i n e (NE) serve as neurotransmitters in the brain and in p e r i p h e r a l organs. While N E and D A are synthesized in similar pathways, with tyrosine hydroxylase as the rate-limiting enzyme 4°, 5 - H T is the product of a different pathway with t r y p t o p h a n hydroxylase activity as the rate-limiting step 14. Metabolic inactivation of m o n o a m i n e s in the brain is p e r f o r m e d by m o n o a m i n e oxidase ( M A O ) and of D A and N E also by catechol-O-methyltransferase ( C O M T ) . M A O is an enzyme of the o u t e r mitochondrial m e m b r a n e , located in neurons and glial cells, with two i n d e p e n d e n t subtypes, M A O - A and -B. B o t h subtypes are characterized by substrate and inhibitor specificities 17 and transcribed from two separate genes 4. C O M T is a soluble enzyme and localized p r e d o m i nantly extraneuronally 3. A second, m e m b r a n e - b o u n d COMT, which is localized in neurons was described, with a hundred-fold lower K m than the soluble form 16'26'27. It is i m p o r t a n t to note that the D A - d e r i v e d product of M A O and an a l d e h y d e d e h y d r o g e n a s e , 3,4-dihydroxy-
phenylacetic acid ( D O P A C ) is therefore mostly of intraneuronal origin, while homovanillic acid ( H V A ) , a product of COMT, M A O and a l d e h y d e d e h y d r o g e n a s e , is extraneuronally synthesized. Variations of the m o n o a m i n e r g i c n e u r o t r a n s m i t t e r and respective metabolite levels in h u m a n C S F and brain are r e p o r t e d with respect to age 8'23'28'34, sex 1, b o d y size 5 and CNS disorders 1'31'32. U n f o r t u n a t e l y , the majority of these studies were done with h u m a n CSF, leaving questions about the true representation of brain metabolism. Therefore, the present study of non-pathological variations of m o n o a m i n e s and their metabolites was carried out in p o s t - m o r t e m h u m a n brain p u t a m e n . We m e a s u r e d D A , 5-HT and N E and the metabolites H V A , D O P A C and 5-hydroxyindoleacetic acid ( 5 - H I A A ) , with H P L C - E C D . A g e and genderrelated changes as well as the relationship of these substances to each o t h e r were analyzed. MATERIALS AND METHODS Samples from human brain putamen (24 males, 18 females), aged from 0.125 to 82 years, were used for the HPLC-ECD analysis. Tissue was taken at autopsy by a neuropathologist, and had a
Correspondence: C. Konradi. Present address: Massachusetts General Hospital, Harvard Medical School, Department of Molecular Neurobiology, Building 149 13th Street, Charlestown, MA 02129, USA. Fax: (1) (617) 726-5677.
286
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Fig. 1. a: levels of DA, 5-HT and NE in the three age groups, b: levels of the metabolites of DA (DOPAC and HVA) and 5-HT (5-HIAA). Open bars are mean values. Group 1: under 10 years; group 2:10 until 60 years; group 3:60 years or older. *Indicates P < 0.05. **Indicates P < 0.005.
post-mortem delay between 6 and 82 h (mean 43.2 + 21.4 S.D.). The causes of death included accidents, sudden infant death and suicide. Brain tissue was frozen at -80°C, and stored for 84-898 days (mean 397 + 204 S.D.) until analysis. Attention to the even age distribution with regard to gender, post-mortem delay and storage time was paid. Statistical analysis included the Mann-Whitney U-test and Spearman's rank correlation. All values are given as mean + S.D. Concentrations are measured in/xg substance/g brain wet weight.
DOPAC/DA
ratio t h a n females. A g e and p o s t - m o r t e m
RESULTS
h i g h e r storage time.
Gender and monoamines
Monoamines in age
t i m e w e r e similar in e i t h e r g r o u p (Table I). T h e r e was a h i g h e r s t o r a g e t i m e of m a l e brains (Table I), which, since the m e t a b o l i z i n g e n z y m e M A O is n o t affected by freezing o r storage t i m e ~9'21, w o u l d r a t h e r a c c o u n t for d e c r e a s e d D A levels and an i n c r e a s e d D O P A C / D A
ra-
tio. T h e r e f o r e , t h e h i g h e r D A levels and l o w e r D O P A C / D A ratio in m a l e s c a n n o t be an artifact of the
L e v e l s o f m o n o a m i n e s and m o n o a m i n e r g i c m e t a b o -
W e arbitrarily established t h r e e age groups; g r o u p 1:
lites in m a l e s and f e m a l e s are s h o w n in Table I. M a l e s
p o s t n a t a l to u n d e r 10 years, g r o u p 2 : 1 0 years until 60 years and g r o u p 3 : 6 0 years and a b o v e . Since studies
had
significantly
higher
DA
levels
and
a
lower
287 TABLE I
TABLE II
Gender and monoamines
Matching o f the three age groups
Values are mean + S.D.
n (M/F)
Group I 7/4
Age range (years) Mean age (years) Post-mortem delay (h)
0.13-6 14-55 50-82 0.99 + 1.71 35.7 + 12.0 74.5 + 6.93 52.8 + 21.3 42.0 + 22.1 36.1 + 18.1
Storage time (days)
432 + 169
Male n = 24
Female n = 18
5-HT ~mol/g) 5-HIAA (9mol/g) 5-HIAA/5-HT DA (,umol/g) HVA ~mol/g) DOPAC (~mol/g) HVA/DA DOPAC/DA NA ~mol/g)
1.06 1.19 1.34 3.33 7.53 0.61 2.58 0.23 0.03
0.97 1.00 1.17 2.51 6.33 1.05 3.04 0.50 0.03
age (years) post-mortem delay (h) storage time (days)
34.6 + 27.9 44.7 + 22.4 463 + 193
+ 0.45 + 0.44 + 0.92 + 1.85 + 3.39 + 0.35 + 1.05 + 0.19 ___0.03
+ + + + + + + + +
0.38 0.31 0.65 1.45" 2.24 0.63 1.59 0.43** 0.02
Group 2 12/9
393 + 238
Group 3 5/5
365 + 170
higher in group 1 when compared to group 2 (Fig. lb). The D A metabolites H V A and D O P A C are unchanged
37.4 + 28.3 41.3 + 20.5 308 + 188"
during life (Fig. lb). Investigation of the monoamine/respective metabolite ratios revealed a significantly higher 5 - H I A A / 5 - H T ratio in early childhood when compared to either one of the other groups, while the H V A / D A
* Indicates P < 0.05. ** Indicates P < 0.005 (Mann-Whitney U-Test).
ratio was higher in group 3 when compared to group 1 (Fig. 2).
about receptor binding22'33, synaptogenesis t5 and mono-
C o r r e l a t i o n s b e t w e e n the m o n o a m i n e s
amine oxidase 21 have shown significant differences at the beginning of the first decade of life, age 10 years was picked as upper limit of group 1. The group consists of ten patients u n d e r age 2 years and one of age 6 years. Age 60 as lower limit of group 3 was chosen from observations of accelerated decline of brain functions and D A levels after this age 7. Table II lists the data of the
A positive correlation between the DAergic and 5-HTergic system was observed (Fig. 3). Both m o n o a m ines as well as their metabolites showed an interdependency (Fig. 3, Table III). N E did not correlate with either m o n o a m i n e . DISCUSSION
case histories of the three age groups. Looking at the m o n o a m i n e s , 5-HT is lowest in group 1 and D A lowest in group 3 (Fig. la). Of the metabolites, the 5-HT metabolite 5 - H I A A was significantly DOPAC/DA
Role of monoamine
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Even though w o m e n had lower D A levels in the brain p u t a m e n than men, their D O P A C levels were higher
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Fig. 2. The metabolite/substrate levels of DA and 5-HT in the three age groups/Open bars are mean values. Group I: under 10 years; group 2:10 until 50 years; group 3:50 years or older. *Indicates P < 0.05. **Indicates P < 0.001.
288
2.0
O0
tyramine, respectively 35, and since D A is described as a M A O - B substrate in humans 11, the D O P A C / D A ratio is a measure of intraneuronal M A O - B activity. Thus, the higher D O P A C / D A ratio in females is most likely a reflection of their higher M A O - B activity 24'29. In concordance, despite their higher D A levels, males, who in general have a lower M A O - B activity than females 24'29, show a lower D O P A C / D A ratio. This indicates, that in vivo the intraneuronal M A O - B level in the nigrostriatal pathway is so low that it is actually saturated, since higher enzyme levels at lower D A concentrations, as shown in females, show a higher yield of D O P A C . In concordance, M A O - B levels in immunocytochemical 2°'41 and histochemical studies TM of the DAergic perikarya of the substantia nigra, were under the detection level. In contrast, the age-related increase of M A O - B , as published earlier 1°'13'21'25'3°, was not reflected in the
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D O P A C / D A ratio, which is the same in all age groups (Fig. 2). This finding supports the hypothesis that the M A O - B activity increases in aging extraneuronally and represents gliosis 25'36. Moreover, the extraneuronal turnover H V A / D A is higher in group 3 (Fig. 2), which fits into this model. However, unlike in the DAergic neurons, the M A O - B level is very high in glial cells T M . A n increased M A O - B activity in glial cells therefore might not solely explain an increased H V A / D A turnover rate. A higher leakage of DAergic neurons in senescence or a decreased D A reuptake z'9'as, is more likely the reason for the increased H V A / D A rate and might also be responsible for the decrease in the D A level in senescence (group 3, Fig. la). Thus, while the observed gender differences in the D O P A C / D A ratio can be attributed to different levels of intraneuronal M A O - B activity, the observed age differences of the H V A / D A ratio can best be explained by an increased extraneuronal availability of D A (e.g., leakage of neurons or reduced reuptake).
2.52.0-
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Fig. 3. Correlations of the monoamines DA and 5-HT and their metabolites HVA and 5-HIAA. Correlation coefficients and P-values: see Table III.
(n.s.) and, as a result, the D O P A C / D A ratio was higher (Table I). Since D O P A C and D A outside the neuron are rapidly metabolized by C O M T to H V A and 3-methoxy-
TABLE III Spearman's rank correlation coefficients between monoamines and monoaminergic metabolites
5-HT NE DOPAC HVA 5-HIAA DOPAC/DA HVA/DA 5-HIAA/5-HT * Indicates P < 0.05. ** Indicates P < 0.005. *** Indicates P < 0.001.
DA
5-HT
NE
DO PA C
HVA
5-HIAA
0.382* 0.187 0.098 0.493*** 0.335* -0.491"** -0.539"** -0.062
-0.042 -0.186 0.539*** 0.389* -0.337* 0.163 -0.576"**
0.127 -0.170 0.110 -0.077 -0.385* 0.239
0.157 -0.037 0.755"** 0.010 0.153
0.508*** -0.167 0.407* -0.082
-0.254 0.073 0.479**
289
Role of monoamine oxidase A While decreased DA levels (Fig. la) and an increased HVA/DA ratio (Fig. 2) were found in group 3, the 5-HTergic system had the highest turnover rate early in life. 5-HT levels were lowest in group 1 (Fig. la), and 5-HIAA levels were higher than in group 2 (Fig. lb), resulting in a higher 5-HIAA/5-HT ratio. Since 5-HT is metabolized by MAO-A to 5-HIAA 37, the 5-HIAA/ 5-HT ratio reflects MAO-A activity. Thus, a previous report which showed a higher MAO-A activity in children under 10 years 21 is supported by the present finding. Interdependent variations of monoamines In all age groups, a strong correlation of DA with 5-HT and most of their metabolites was seen (Table III, Fig. 3). DOPAC was the exception which correlated with the DOPAC/DA ratio only (Table III). As discussed above, DOPAC levels in the nigrostriatal system seem to reflect predominantly the intraneuronal MAO-B activity and are independent of DA concentrations
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