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Changes in brain serotonin metabolism associated with fasting and satiation in rats
Life Sciences Vol. 11, Part II, pp. 31-39, 1972. Printed in Great Britain
Pergamon Press
CHANGES IN BRAIN SEROTONINMETABOLISMASSOCIATEDWITH FASTINGAND SATIATION IN RATS J. P~rez.Cruet, A. Tagliamonte, P. Tagliamonte and G.L. Gessa Laboratory of Chemical Pharmacology, National Heart and Lung Institute, National Institutes of Health, Bethesda, Maryland 20014
(Received 21April 1971; in final f o r m 8 D e c e m b e r 1971) Summary Rats fasted for 24 hrs. were fed for two hours after which time food was removed. Food intake decreased brain 5-HIAA and tryptophan levels by about 22% and 27%, respectively, but did not modify brain serotonin concentration. These changes persisted for about six hours. The synthesis rate of brain serotonin was about 30% lower in rats fed for two hours than in rats fasted for 24 hours. Food intake produced changes in plasma tryptophan opposite to those produced in brain. Feeding also decreased brain serotonin turnover in hypophysectomized rats. Introduction A large number of studies have considered the possible role of brain catecholamines in the control of hunger and satiety mechanisms (for references see l ) .
However, there is indirect evidence that brain serotonin might also
be involved in appetite control.
The hypothalamus, where feeding and satiety
centers are located, is the brain area containing the highest concentrations of serotonin (2).
Fluorescence histochemical techniques have shown a large
number of serotonergic nerve endings in the superior salivatory nucleus in the pons (33. Anorexigenic agents, like fenfluramine (4,5) and E-chloroamphetamine (6,7), produce a rather selective depletion of brain serotonin; d-amphetamine stimulates the turnover not only of brain dopamine (8,9) but also that of serotonin (]O,ll).
E-Chlorophenylalanine, a selective inhibitor of serotonin
synthesis (12) produces voraciousness in cats (13) and anorexia in rabbits and in rats (14).
These considerations prompted us to study the possible relation-
ship between the turnover of brain serotonin and food intake. Methods Experiments were carried out with male Sprague-Dawley rats with an i n i t i a l 31
32
Brain 5 H T and Food Intake
weight of 180-200 g.
Vol. 11, No. I
The rats were housed, four to a cage, in wire mesh cages
15 X 24 inches at 20°C. They had free access to water, but were trained to eat their normal daily meal (Purina laboratory chow pellets, containing crude protein, not less than 23.0%; crude fat, not less than 4.5%; crude fiber, not more than 6.0%, and ash, not more than 9.0%; Purina chow was purchased from Ralston Purina Co., Checkerboard Square, St. Louis, Missouri)within two hours by presenting the food once a day for a 2-hr. period between IO:O0 A.M. and 12 Noon. The trained animals were used'after one week's training.
At this
time each rat ate an average of 12 ~0.3 (S.E.) g. of Purina chow per day. Animals were killed by decapitation and the brain quickly removed, frozen in dry ice and stored at -2O°C until analyzed. Blood, collected in centrifuge tubes containing heparin, was centrifuged and the plasma stored at -20°C until analyzed. Serotonin, 5-hydroxyindole acetic acid (5-HIAA) and tryptophan were assayed fluorometrically as previously described ( l l ) . Different groups of trained rats fasted 24 hours were allowed to eat for two hours after which time food was withdrawn. A group of animals was killed at the end of the feeding period (fed animals) and the other groups at different intervals thereafter (fasted animals). Results Table l shows that there were no statistically significant differences in the concentrations of brain serotonin between fed animals and the animals fasted for different periods.
On the other hand, brain 5-HIAA level declined
by about 22% at the end of the feeding period, remained for about six hours at this level and approached the fasting levels in about 12 hours. The levels of the serotonin precursor, tryptophan, showed changes that paralleled those of 5-HIAA, but after feeding, tryptophan levels declined to a greater extent than the levels of 5-HIAA. There were no significant differences in the levels of 5-HIAAand tryptophan between 24 and 48-hr. fasted rats.
Tryptophan levels
in plasma underwent changes in the opposite direction to those of brain
II
7
27
19
6
12
24
48
NS
NS
NS
NS
--
p*
0.70 + 0.15
0.68 + 0.02
0.62 + O.Ol
0.54 + 0.06
0.53 + 0.03
(~g/g ~ S.E.)
Brain 5-HIAA
<0.001
NS
--
p*
5.70 + 0.I0
6.17 + 0.80
5.20 + 0.90
4.23 + 0.78
4.32 + 0.60
(~g/g ~ S.E.)
Brain Tryptophan
NS
NS
--
p*
17.41 + 0.50
19.11 + 0.30
19.30 + 0.60
25.16 + 0.78
26.41 + 0.90
(,g/g ~ S.E.)
Plasma Tryptophan
<0.001
NS
--
p*
* Compared to the value for 0 hr. of fasting.
Values were treated s t a t i s t i c a l l y by the Student's t test (22).
Each value is the average + S.E. of the number of experiments reported. Animals fasted 24 hrs. were allowed to eat for 2 hrs. after which time food was removed. The duration of fasting (hrs.) reported indicates the intervals between food removal and sacrifice.
0.51 + 0.05
0.54 + 0.02
0.47 + 0.03
0.53 + 0.04
0.49 + 0.02
7
0
Brain Serotonin (~g/g ~ S.E.)
Number of Experiments
(hr)
Duration of Fasting
Fasted and Fed Rats
Serotonin, 5-hydroxyindoleacetic acid (5-HIAA) and Tryptophan Levels in Brain and Tryptophan Level in Plasma of
TABLE 1
¢Ai
R
0
~0
¢31
p.L
o
34
Brain 5HT and Food Intake
Vol. 11, No. 1
tryptophan; they rose by 38% at the end of the two-hour feeding period, remained high for about six hours and then returned to the fasting level in about 12 hours. The synthesis rates of brain serotonin of fasted and fed trained rats were compared. The rate of serotonin synthesis was measured by multiplying the rate constant of 5-HIAA decline, after inhibition of monoamine oxidase, by the steady-state level of 5-HIAA, according to the method of Tozer et al. (15).
Accordingly, two groups of rats, one fasted for 24 hours and another
fed for two hours after 24-hour fasting, were injected with pargyline (80 mg/kg, i.p.) and sacrificed 30, 60 and go minutes thereafter.
Table 2 shows
that the rate of synthesis of brain serotonin in fed animals was about 30% lower than in fasted rats.
Comparable values were obtained by calculating
the rate of synthesis of serotonin by the rate of accumulation of 5-HIAA in brain after the transport of this metabolite from brain was blocked with probenecid (16). The differences in serotonin metabolism between the 24-hour fasted trained rats and the two-hour fed trained rats cannot be due to circadian fluctuations because these animals were killed at the same time of day (12 Noon). Moreover, results similar to those reported above were obtained with animals killed at Midnight (Table 3). Since the observed variations in serotonin turnover could be secondary to stress-induced variations in plasma corticosterone, similar experiments were carried out with hypophysectomized rats.
Sprague-Dawleyrats, weighing 210-
230 g. (about 120 days old), hypophysectomized one month previously by the Zivic-Miller Laboratory, were used. These animals were trained to eat their daily meal (Purina chow pellets) within two hours, as in the experiments carried out with intact animals. These animals were used after ten day's training. Table 4 shows that hypophysectomized rats, fasted for 24 hours, have tryptophan and 5-HIAA levels and a rate of serotonin synthesis significantly
Vol. 11, No. 1
B r a i n 5HT and Food Intake
35
TABLE 2 Synthesis Rate of Brain Serotonin in Fasted and Fed Rats
Condition
Steady state level of 5-HIAA Rate constant of [5-HIAA]o 5-HIAA efflux (k)
Synthesis Rate of Serotonin Calculated Calculated According According to to Tozer et al. Neff et al. (15}-- --
(~glg ~ S.E,)
(hr. -l )
(16T
(uglg/hr.)
(uglg/hr.)
Fasted 24 hrs.
0.75 + 0.03
0.68 + 0.02
0.51
0.43
Fed
0.58 + 0.06
0.60 + O.lO
0.35 (31%+)
0.31 (28%+)
2 hrs.
Each value is the average ~S.E. of 20 determinations. TABLE 3 Serotonin, 5-hydroxyindoleacetic acid (5-HIAA) and Tryptophan Levels in Brain and Tryptophan Level in Plasma of Fasted and Fed Rats Sacrificed at Midnight
Condition*
Fasted 24 hrs.
Brain Serotonin pg/g ~ S.E.
Brain 5-HIAA pg/g ~ S.E.
Brain Tryptophan ug/g ~ S.E.
Plasma Tryptophan ~g/ml ~ S.E.
0.56 + 0.02
0.66 + 0.03
4.23 + 0.12
25.00 + 1.26
-
Fed
2 hrs.
(6)
0.58 + 0.06 + -
-
(6)
0 . 4 5 + O.O1 §
(6)
-
(6)
-
(6)
3.77 + 0.09 § -
(6)
-
(6)
31.33 + 0.56 § -
(6)
Each value is the average ~ S.E. of the number of determinations in parentheses * Animals were sacrificed at 24:00. + N,S. by the Student's t test. § p <
O.Ol . . . . . . . .
36
Brain 5HT and Food Intake
Vol. 11, No. 1
higher than hypophysectomized rats fed for two hours. Discussion The present study demonstrates that the metabolism of brain serotonin is influenced by food intake.
Fasted rats have higher levels of the serotonin
precursor, tryptophan, and of the serotonin main metabolite, 5-HIAA. The synthesis rate of brain serotonin is 30% higher in fasted than in fed rats. I t is likely that the changes in serotonin turnover originate from the observed changes in tryptophan level in brain.
In fact, tryptophan hydroxy-
lase, the rate-limiting step in serotonin synthesis, has a Km for its substrate much higher than the concentrations of tryptophan normally present in brain (17).
Thus, the rate of synthesis of brain serotonin depends upon the avail-
ability of the substrate (17, 18, Ig, I I ) . The mechanism by which tryptophan levels in brain are increased in fasted animals ~or decreased in fed animals) is unknown. The changes in tryptophan levels are not due to circadian rhythms; indeed, the reported circadian variations in serotonin levels (20) might be secondary to the diurnal cycles of feeding. The changes in brain tryptophan are not due to changes in plasma tryptophan because they were influenced in an opposite direction by food intake Thus, the levels of brain tryptophan and plasma tryptophan are apparently controlled by different mechanisms. Moreover, the results of our experiments with hypophysectomized rats rule out the possibility that the observed changes in the metabolism and turnover of brain serotonin are secondary to changes in the level of plasma corticosterone, or in other hormones of pituitary origin and pituitary control.
Finally, changes in brain tryptophan and 5-HIAA levels
induced by food intake were not temporally correlated with the hyperthermia produced by the specific dynamic action of food.
In fact, in our experimental
rats, colonic temperature rose by about O.5°C after 60 min. of feeding and returned to the fasting values in about four hours. Our findings raise several questions:
a) Is serotonin turnover affected
by some food constituents or by gastric distension (acting reflexly upon the
0.69 + 0.03 (g)§
3.09 + 0.17 ( 9 ) §
4.73 + 0.24 (6)
(wg/g ~ S.E.)
Brain Tryptophan
30.22 + 1.76 (g)§
25.50 + 2.05 (6)
(pg/ml ~ S.E.)
Plasma Tryptophan
+N.S.
§ P < O.Ol by the Student's t test.
0.22 (20)
0.30 (16)
(pg/g/hr)
Synthesis Rate of Serotonint+
i-t Calculated according to Tozer et al. (15): the rate constant of 5-HIAA efflux was 0.36 + O.Ol and 0.32 + 0.02 hr.-1 in fasted a-nd--ired" animals, respectively.
t Animals were sacrificed at 12:00.
Each value is the average + S.E. of number of determinations in parentheses.
0.77 + 0.03 (9) +
Hypophysectomized fed 2 hrs.
0.82 + 0.02 (6)
(pg/g ~ S.E.)
(pg/g ~ S.E.)
0.80 + 0.02 (6)
Brain 5-HIAA
Brain Serotonin
Hypophysectomized fasted 24 hrs.
Condition+
and Tryptophan Level in Plasma of Fasted and Fed Hypophysectomized Rats
Serotonin, 5-hydroxyindoleacetic acid (5-HIAA) and Tryptophan Levels in Brain
TABLE 4
-3
~J
¢}1
).L
38
B r a i n 5HT and Food Intake
CNS)? b)
Vol. 11, No. 1
Do the sensations of hunger and satiety influence serotonin turnover?
c) Or, does serotonin mediate sensations of hunger? On the other hand, i t may be possible that the observed changes of brain tryptophan results from changes in plasma levels of other amino acids, which compete with the same transport system into the brain tissue (21).
Presently, i t is hazardous to propose a
causal correlation between the observed changes in serotonin metabolism and physiological functions.
However, although the changes in brain serotonin
turnover observed were relatively small, this is the f i r s t instance, to our knowledge, in which serotonin turnover has been shown to change under physiological conditions. References I.
D.A. BOOTH, J. Pharmacol. Exp. Ther.
160, 336; 1968.
2.
D.F. BOGDANSKI, H. WEISSBACHand S. UDENFRIEND,J. Neurochem. ~, 272; 1957.
3.
A. DAHLSTROMand K. FUXE, Acta Physiol. Scand. 6_44(suppl. 247); 1965.
4.
J. DUHAULTand C. VERDAVAINNE,Arch. Int. Pharmacodyn. 170, 276; 1967.
5.
K. OPITZ, Brit. Med. J.
6.
A. PLETSCHER, G. BARTHOLINI, H. BRUDERER,W.P. BURKARDand K.F. GEY, J. Pharmacol. Exp. Ther. 145, 344; 1964.
7°
E. SANDERS-BUSHand F. SULSER, J. Pharmacol. Exp. Ther. l!5, 419; 1970.
8.
F. JAVOY, A.M. THIERRY, S.S. KETY and J. GLOWINSKI, Communs. Behav. Biol., Part A, ~, 43; 1968.
9.
E. COSTAand A. GROPPETTI: Amphetamines and Related Compounds: Proceedings of the Mario Negri Institute for Pharmacological Research, Milan, Italy, Edited by E. Costa and S. Garattini. Raven Press, New York, 231; Ig70.
259, 56; 1967.
lO.
W.D. REID, Br. J. Pharmac. 40, 483; 1970.
II.
A. TAGLIAMONTE, P. TAGLIAMONTE, J. PEREZ-CRUETand G.L. GESSA, J. Pharmacol. Exp. Ther. 177, 475; 1971.
12.
B.K. KOE and A. WEISSMAN, J. Pharmacol. Exp. Ther.
13.
J. FERGUSON,S. HENRIKSEN, H. COHEN, G. HOYT, G. MITCHELL, K. MCGARR, D. RUBENSON,L. RYANand W. DEMENT, Psychophysiology 6, 22]; Ig6g.
14.
G.L. GESSA, J. PEREZ-CRUET,A. TAGLIAMONTEand P. TAGLIAMONTE, Nature, (In press).
154, 499; 1966.
Vol. 11, No. 1
Bratn 5HT s~i Food Intake
39
15.
T.N. TOZER, N.H. NEFF and B.B. BRODIE, J. Pharmacol. Exp. Ther. 1966.
153, 177;
16.
N.H. NEFF, T.N. TOZERand B.B. BRODIE, J. Pharmacol. Exp. Ther. 1967.
158, 214;
17.
E. JEQUIER, W. LOVENBERGand A. SJOERDSMA,Mol. Pharmacol. 3, 274; 1967;
18.
H. GREEN, S.M. GREENBERG,R.W. ERICKSON, J.L. SAWYERand R.J. ELLISON, J. Pharmacol. Exp. Ther. 136, 174; 1962.
19.
A. TAGLIAMONTE, P. TAGLIAMONTE,J. PEREZ-CRUETand G.L. GESSA, Nature New Biology 229, 125; 1971.
20.
D.J. REIS, A. CORVELLI and J. CONNERS,J. Pharmacol. Exp. Ther. 167, 328; 1969.
21.
M. CHIRIGOS, P. GREENGARDand S. UDENFRIEND,J. Biol. Chem. 235, 2075; 1960.
22.
G.W. SNEDECOR,Statistical Methods, 5th Edition (The Iowa State College Press, Ames, 1956).
Pergamon Press
CHANGES IN BRAIN SEROTONINMETABOLISMASSOCIATEDWITH FASTINGAND SATIATION IN RATS J. P~rez.Cruet, A. Tagliamonte, P. Tagliamonte and G.L. Gessa Laboratory of Chemical Pharmacology, National Heart and Lung Institute, National Institutes of Health, Bethesda, Maryland 20014
(Received 21April 1971; in final f o r m 8 D e c e m b e r 1971) Summary Rats fasted for 24 hrs. were fed for two hours after which time food was removed. Food intake decreased brain 5-HIAA and tryptophan levels by about 22% and 27%, respectively, but did not modify brain serotonin concentration. These changes persisted for about six hours. The synthesis rate of brain serotonin was about 30% lower in rats fed for two hours than in rats fasted for 24 hours. Food intake produced changes in plasma tryptophan opposite to those produced in brain. Feeding also decreased brain serotonin turnover in hypophysectomized rats. Introduction A large number of studies have considered the possible role of brain catecholamines in the control of hunger and satiety mechanisms (for references see l ) .
However, there is indirect evidence that brain serotonin might also
be involved in appetite control.
The hypothalamus, where feeding and satiety
centers are located, is the brain area containing the highest concentrations of serotonin (2).
Fluorescence histochemical techniques have shown a large
number of serotonergic nerve endings in the superior salivatory nucleus in the pons (33. Anorexigenic agents, like fenfluramine (4,5) and E-chloroamphetamine (6,7), produce a rather selective depletion of brain serotonin; d-amphetamine stimulates the turnover not only of brain dopamine (8,9) but also that of serotonin (]O,ll).
E-Chlorophenylalanine, a selective inhibitor of serotonin
synthesis (12) produces voraciousness in cats (13) and anorexia in rabbits and in rats (14).
These considerations prompted us to study the possible relation-
ship between the turnover of brain serotonin and food intake. Methods Experiments were carried out with male Sprague-Dawley rats with an i n i t i a l 31
32
Brain 5 H T and Food Intake
weight of 180-200 g.
Vol. 11, No. I
The rats were housed, four to a cage, in wire mesh cages
15 X 24 inches at 20°C. They had free access to water, but were trained to eat their normal daily meal (Purina laboratory chow pellets, containing crude protein, not less than 23.0%; crude fat, not less than 4.5%; crude fiber, not more than 6.0%, and ash, not more than 9.0%; Purina chow was purchased from Ralston Purina Co., Checkerboard Square, St. Louis, Missouri)within two hours by presenting the food once a day for a 2-hr. period between IO:O0 A.M. and 12 Noon. The trained animals were used'after one week's training.
At this
time each rat ate an average of 12 ~0.3 (S.E.) g. of Purina chow per day. Animals were killed by decapitation and the brain quickly removed, frozen in dry ice and stored at -2O°C until analyzed. Blood, collected in centrifuge tubes containing heparin, was centrifuged and the plasma stored at -20°C until analyzed. Serotonin, 5-hydroxyindole acetic acid (5-HIAA) and tryptophan were assayed fluorometrically as previously described ( l l ) . Different groups of trained rats fasted 24 hours were allowed to eat for two hours after which time food was withdrawn. A group of animals was killed at the end of the feeding period (fed animals) and the other groups at different intervals thereafter (fasted animals). Results Table l shows that there were no statistically significant differences in the concentrations of brain serotonin between fed animals and the animals fasted for different periods.
On the other hand, brain 5-HIAA level declined
by about 22% at the end of the feeding period, remained for about six hours at this level and approached the fasting levels in about 12 hours. The levels of the serotonin precursor, tryptophan, showed changes that paralleled those of 5-HIAA, but after feeding, tryptophan levels declined to a greater extent than the levels of 5-HIAA. There were no significant differences in the levels of 5-HIAAand tryptophan between 24 and 48-hr. fasted rats.
Tryptophan levels
in plasma underwent changes in the opposite direction to those of brain
II
7
27
19
6
12
24
48
NS
NS
NS
NS
--
p*
0.70 + 0.15
0.68 + 0.02
0.62 + O.Ol
0.54 + 0.06
0.53 + 0.03
(~g/g ~ S.E.)
Brain 5-HIAA
<0.001
NS
--
p*
5.70 + 0.I0
6.17 + 0.80
5.20 + 0.90
4.23 + 0.78
4.32 + 0.60
(~g/g ~ S.E.)
Brain Tryptophan
NS
NS
--
p*
17.41 + 0.50
19.11 + 0.30
19.30 + 0.60
25.16 + 0.78
26.41 + 0.90
(,g/g ~ S.E.)
Plasma Tryptophan
<0.001
NS
--
p*
* Compared to the value for 0 hr. of fasting.
Values were treated s t a t i s t i c a l l y by the Student's t test (22).
Each value is the average + S.E. of the number of experiments reported. Animals fasted 24 hrs. were allowed to eat for 2 hrs. after which time food was removed. The duration of fasting (hrs.) reported indicates the intervals between food removal and sacrifice.
0.51 + 0.05
0.54 + 0.02
0.47 + 0.03
0.53 + 0.04
0.49 + 0.02
7
0
Brain Serotonin (~g/g ~ S.E.)
Number of Experiments
(hr)
Duration of Fasting
Fasted and Fed Rats
Serotonin, 5-hydroxyindoleacetic acid (5-HIAA) and Tryptophan Levels in Brain and Tryptophan Level in Plasma of
TABLE 1
¢Ai
R
0
~0
¢31
p.L
o
34
Brain 5HT and Food Intake
Vol. 11, No. 1
tryptophan; they rose by 38% at the end of the two-hour feeding period, remained high for about six hours and then returned to the fasting level in about 12 hours. The synthesis rates of brain serotonin of fasted and fed trained rats were compared. The rate of serotonin synthesis was measured by multiplying the rate constant of 5-HIAA decline, after inhibition of monoamine oxidase, by the steady-state level of 5-HIAA, according to the method of Tozer et al. (15).
Accordingly, two groups of rats, one fasted for 24 hours and another
fed for two hours after 24-hour fasting, were injected with pargyline (80 mg/kg, i.p.) and sacrificed 30, 60 and go minutes thereafter.
Table 2 shows
that the rate of synthesis of brain serotonin in fed animals was about 30% lower than in fasted rats.
Comparable values were obtained by calculating
the rate of synthesis of serotonin by the rate of accumulation of 5-HIAA in brain after the transport of this metabolite from brain was blocked with probenecid (16). The differences in serotonin metabolism between the 24-hour fasted trained rats and the two-hour fed trained rats cannot be due to circadian fluctuations because these animals were killed at the same time of day (12 Noon). Moreover, results similar to those reported above were obtained with animals killed at Midnight (Table 3). Since the observed variations in serotonin turnover could be secondary to stress-induced variations in plasma corticosterone, similar experiments were carried out with hypophysectomized rats.
Sprague-Dawleyrats, weighing 210-
230 g. (about 120 days old), hypophysectomized one month previously by the Zivic-Miller Laboratory, were used. These animals were trained to eat their daily meal (Purina chow pellets) within two hours, as in the experiments carried out with intact animals. These animals were used after ten day's training. Table 4 shows that hypophysectomized rats, fasted for 24 hours, have tryptophan and 5-HIAA levels and a rate of serotonin synthesis significantly
Vol. 11, No. 1
B r a i n 5HT and Food Intake
35
TABLE 2 Synthesis Rate of Brain Serotonin in Fasted and Fed Rats
Condition
Steady state level of 5-HIAA Rate constant of [5-HIAA]o 5-HIAA efflux (k)
Synthesis Rate of Serotonin Calculated Calculated According According to to Tozer et al. Neff et al. (15}-- --
(~glg ~ S.E,)
(hr. -l )
(16T
(uglg/hr.)
(uglg/hr.)
Fasted 24 hrs.
0.75 + 0.03
0.68 + 0.02
0.51
0.43
Fed
0.58 + 0.06
0.60 + O.lO
0.35 (31%+)
0.31 (28%+)
2 hrs.
Each value is the average ~S.E. of 20 determinations. TABLE 3 Serotonin, 5-hydroxyindoleacetic acid (5-HIAA) and Tryptophan Levels in Brain and Tryptophan Level in Plasma of Fasted and Fed Rats Sacrificed at Midnight
Condition*
Fasted 24 hrs.
Brain Serotonin pg/g ~ S.E.
Brain 5-HIAA pg/g ~ S.E.
Brain Tryptophan ug/g ~ S.E.
Plasma Tryptophan ~g/ml ~ S.E.
0.56 + 0.02
0.66 + 0.03
4.23 + 0.12
25.00 + 1.26
-
Fed
2 hrs.
(6)
0.58 + 0.06 + -
-
(6)
0 . 4 5 + O.O1 §
(6)
-
(6)
-
(6)
3.77 + 0.09 § -
(6)
-
(6)
31.33 + 0.56 § -
(6)
Each value is the average ~ S.E. of the number of determinations in parentheses * Animals were sacrificed at 24:00. + N,S. by the Student's t test. § p <
O.Ol . . . . . . . .
36
Brain 5HT and Food Intake
Vol. 11, No. 1
higher than hypophysectomized rats fed for two hours. Discussion The present study demonstrates that the metabolism of brain serotonin is influenced by food intake.
Fasted rats have higher levels of the serotonin
precursor, tryptophan, and of the serotonin main metabolite, 5-HIAA. The synthesis rate of brain serotonin is 30% higher in fasted than in fed rats. I t is likely that the changes in serotonin turnover originate from the observed changes in tryptophan level in brain.
In fact, tryptophan hydroxy-
lase, the rate-limiting step in serotonin synthesis, has a Km for its substrate much higher than the concentrations of tryptophan normally present in brain (17).
Thus, the rate of synthesis of brain serotonin depends upon the avail-
ability of the substrate (17, 18, Ig, I I ) . The mechanism by which tryptophan levels in brain are increased in fasted animals ~or decreased in fed animals) is unknown. The changes in tryptophan levels are not due to circadian rhythms; indeed, the reported circadian variations in serotonin levels (20) might be secondary to the diurnal cycles of feeding. The changes in brain tryptophan are not due to changes in plasma tryptophan because they were influenced in an opposite direction by food intake Thus, the levels of brain tryptophan and plasma tryptophan are apparently controlled by different mechanisms. Moreover, the results of our experiments with hypophysectomized rats rule out the possibility that the observed changes in the metabolism and turnover of brain serotonin are secondary to changes in the level of plasma corticosterone, or in other hormones of pituitary origin and pituitary control.
Finally, changes in brain tryptophan and 5-HIAA levels
induced by food intake were not temporally correlated with the hyperthermia produced by the specific dynamic action of food.
In fact, in our experimental
rats, colonic temperature rose by about O.5°C after 60 min. of feeding and returned to the fasting values in about four hours. Our findings raise several questions:
a) Is serotonin turnover affected
by some food constituents or by gastric distension (acting reflexly upon the
0.69 + 0.03 (g)§
3.09 + 0.17 ( 9 ) §
4.73 + 0.24 (6)
(wg/g ~ S.E.)
Brain Tryptophan
30.22 + 1.76 (g)§
25.50 + 2.05 (6)
(pg/ml ~ S.E.)
Plasma Tryptophan
+N.S.
§ P < O.Ol by the Student's t test.
0.22 (20)
0.30 (16)
(pg/g/hr)
Synthesis Rate of Serotonint+
i-t Calculated according to Tozer et al. (15): the rate constant of 5-HIAA efflux was 0.36 + O.Ol and 0.32 + 0.02 hr.-1 in fasted a-nd--ired" animals, respectively.
t Animals were sacrificed at 12:00.
Each value is the average + S.E. of number of determinations in parentheses.
0.77 + 0.03 (9) +
Hypophysectomized fed 2 hrs.
0.82 + 0.02 (6)
(pg/g ~ S.E.)
(pg/g ~ S.E.)
0.80 + 0.02 (6)
Brain 5-HIAA
Brain Serotonin
Hypophysectomized fasted 24 hrs.
Condition+
and Tryptophan Level in Plasma of Fasted and Fed Hypophysectomized Rats
Serotonin, 5-hydroxyindoleacetic acid (5-HIAA) and Tryptophan Levels in Brain
TABLE 4
-3
~J
¢}1
).L
38
B r a i n 5HT and Food Intake
CNS)? b)
Vol. 11, No. 1
Do the sensations of hunger and satiety influence serotonin turnover?
c) Or, does serotonin mediate sensations of hunger? On the other hand, i t may be possible that the observed changes of brain tryptophan results from changes in plasma levels of other amino acids, which compete with the same transport system into the brain tissue (21).
Presently, i t is hazardous to propose a
causal correlation between the observed changes in serotonin metabolism and physiological functions.
However, although the changes in brain serotonin
turnover observed were relatively small, this is the f i r s t instance, to our knowledge, in which serotonin turnover has been shown to change under physiological conditions. References I.
D.A. BOOTH, J. Pharmacol. Exp. Ther.
160, 336; 1968.
2.
D.F. BOGDANSKI, H. WEISSBACHand S. UDENFRIEND,J. Neurochem. ~, 272; 1957.
3.
A. DAHLSTROMand K. FUXE, Acta Physiol. Scand. 6_44(suppl. 247); 1965.
4.
J. DUHAULTand C. VERDAVAINNE,Arch. Int. Pharmacodyn. 170, 276; 1967.
5.
K. OPITZ, Brit. Med. J.
6.
A. PLETSCHER, G. BARTHOLINI, H. BRUDERER,W.P. BURKARDand K.F. GEY, J. Pharmacol. Exp. Ther. 145, 344; 1964.
7°
E. SANDERS-BUSHand F. SULSER, J. Pharmacol. Exp. Ther. l!5, 419; 1970.
8.
F. JAVOY, A.M. THIERRY, S.S. KETY and J. GLOWINSKI, Communs. Behav. Biol., Part A, ~, 43; 1968.
9.
E. COSTAand A. GROPPETTI: Amphetamines and Related Compounds: Proceedings of the Mario Negri Institute for Pharmacological Research, Milan, Italy, Edited by E. Costa and S. Garattini. Raven Press, New York, 231; Ig70.
259, 56; 1967.
lO.
W.D. REID, Br. J. Pharmac. 40, 483; 1970.
II.
A. TAGLIAMONTE, P. TAGLIAMONTE, J. PEREZ-CRUETand G.L. GESSA, J. Pharmacol. Exp. Ther. 177, 475; 1971.
12.
B.K. KOE and A. WEISSMAN, J. Pharmacol. Exp. Ther.
13.
J. FERGUSON,S. HENRIKSEN, H. COHEN, G. HOYT, G. MITCHELL, K. MCGARR, D. RUBENSON,L. RYANand W. DEMENT, Psychophysiology 6, 22]; Ig6g.
14.
G.L. GESSA, J. PEREZ-CRUET,A. TAGLIAMONTEand P. TAGLIAMONTE, Nature, (In press).
154, 499; 1966.
Vol. 11, No. 1
Bratn 5HT s~i Food Intake
39
15.
T.N. TOZER, N.H. NEFF and B.B. BRODIE, J. Pharmacol. Exp. Ther. 1966.
153, 177;
16.
N.H. NEFF, T.N. TOZERand B.B. BRODIE, J. Pharmacol. Exp. Ther. 1967.
158, 214;
17.
E. JEQUIER, W. LOVENBERGand A. SJOERDSMA,Mol. Pharmacol. 3, 274; 1967;
18.
H. GREEN, S.M. GREENBERG,R.W. ERICKSON, J.L. SAWYERand R.J. ELLISON, J. Pharmacol. Exp. Ther. 136, 174; 1962.
19.
A. TAGLIAMONTE, P. TAGLIAMONTE,J. PEREZ-CRUETand G.L. GESSA, Nature New Biology 229, 125; 1971.
20.
D.J. REIS, A. CORVELLI and J. CONNERS,J. Pharmacol. Exp. Ther. 167, 328; 1969.
21.
M. CHIRIGOS, P. GREENGARDand S. UDENFRIEND,J. Biol. Chem. 235, 2075; 1960.
22.
G.W. SNEDECOR,Statistical Methods, 5th Edition (The Iowa State College Press, Ames, 1956).