Download Effects of tryptophan andror acute running on extracellular 5

Brain Research 740 Ž1996. 245–252
Research report
Effects of tryptophan andror acute running on extracellular 5-HT and
5-HIAA levels in the hippocampus of food-deprived rats
Romain Meeusen
a
a, )
, Katrien Thorre´ b, Francis Chaouloff d , Sophie Sarre b, Kenny De Meirleir a ,
Guy Ebinger c , Yvette Michotte b
Departments of Human Physiology and Sports Medicine, Vrije UniÕersiteit Brussel, Laarbeeklaan 101, B-1090 Brussels, Belgium
b
Departments of Pharmaceutical Chemistry and Drug Analysis, Vrije UniÕersiteit Brussel, Brussels, Belgium
c
Department of Neurology, Academic Hospital, Vrije UniÕersiteit Brussel, Brussels, Belgium
d
Laboratoire Genetique
du Stress, INSERM CJF 94-05, UniÕersite´ de Bordeaux, Bordeaux, France
´ ´
Accepted 9 July 1996
Abstract
The present microdialysis study has examined whether exercise-elicited increases in brain tryptophan availability Žand in turn 5-HT
synthesis. alter 5-HT release in the hippocampus of food-deprived rats. To this end, we compared the respective effects of acute exercise,
administration of tryptophan, and the combination of both treatments, upon extracellular 5-HT and 5-hydroxyindoleacetic acid Ž5-HIAA.
levels. All rats were trained to run on a treadmill before implantation of the microdialysis probe and 24 h of food deprivation. Acute
exercise Ž12 mrmin for 1 h. increased in a time-dependent manner extracellular 5-HT levels Žmaximal increase: 47%., these levels
returning to their baseline levels within the first hour of the recovery period. Besides, exercise-induced increases in extracellular 5-HIAA
levels did not reach significance. Acute administration of a tryptophan dose Ž50 mgrkg i.p.. that increased extracellular 5-HIAA Žbut not
5-HT. levels in fed rats, increased within 60 min extracellular 5-HT levels Žmaximal increase: 55%. in food-deprived rats. Whereas 5-HT
levels returned toward their baseline levels within the 160 min that followed tryptophan administration, extracellular 5-HIAA levels rose
throughout the experiment Žmaximal increase: 75%.. Lastly, treatment with tryptophan Ž60 min beforehand. before acute exercise led to
marked increases in extracellular 5-HT and 5-HIAA levels Žmaximal increases: 100% and 83%, respectively. throughout the 240 min that
followed tryptophan administration. This study indicates that exercise stimulates 5-HT release in the hippocampus of fasted rats, and that
a pretreatment with tryptophan Žat a dose increasing extracellular 5-HT levels. amplifies exercise-induced 5-HT release.
Keywords: Microdialysis; Hippocampus; Extracellular 5-HT; Extracellular 5-HIAA; Tryptophan administration; Exercise; Food deprivation
1. Introduction
Among animal models of psychiatric disorders, starvation-induced hyperactivity has received a great deal of
attention because it may be endowed with features observed in anorexia nervosa w2,3,19,38x andror obsessivecompulsive disorder w1x. This model is based on the observation that fed rats given access to running wheels progressively increase their activity; however, if food-restricted,
the rats develop high running wheel activity at the expand
of feeding Žthereby leading to marked weight losses and
possibly death.. A great number of studies has investigated
the transmitters putatively responsible for this ‘runners
high’, especially the opioidergic w19x, catecholaminergic
and serotonergic systems w38x. As far as serotonergic systems are concerned, increases in hypothalamus 5-hydroxy)
Corresponding author. Fax: q32 Ž2. 477-4607.
tryptamine Ž5-HT. turnover have been found in rats undergoing the starvation-induced hyperactivity paradigm w38x.
In keeping with the observation that chronic inhibition of
5-HT synthesis by para-chlorophenylalanine ŽPCPA. w1x
and repeated administration of tryptophan Žthe precursor of
5-HT. w2x respectively exacerbates and diminuates the
development of starvation-induced hyperactivity, it has
been proposed that serotonergic systems may play a key
role in this model w1,2,38x. This suggestion is reinforced by
the early findings that food-deprivation w11,30x and acute
running in trained animals w11,12x both increase in a
time-dependent manner brain tryptophan availability
Žthrough lipolysis-mediated changes in the blood concentration of the so-called free tryptophan portion.. Moreover,
the combination of both treatments Ži.e. acute running
following food deprivation. leads to marked increases in
brain tryptophan that suggest synergistic influences of
these two lipolytic situations w11x.
0006-8993r96r$15.00 Copyright q 1996 Elsevier Science B.V. All rights reserved.
PII S 0 0 0 6 - 8 9 9 3 Ž 9 6 . 0 0 8 7 2 - 4
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R. Meeusen et al.r Brain Research 740 (1996) 245–252
In keeping with the early observation that the activity of
tryptophan hydroxylase, the rate-limiting enzyme in 5-HT
biosynthesis, is only partly saturated under basal conditions w7x, it is logical to observe that food deprivation,
acute exercise, or the combination of both increase 5-HT
synthesis Žand metabolism. w11x. However, the extent to
which precursor-induced changes in synthesis have functional consequences on 5-HT release has been the matter
of debate w26,31x, including in exercise models w8,36x. On
the one hand, treatments that decrease brain tryptophan
availability andror 5-HT synthesis Žsuch as PCPA. decrease evoked release of 5-HT w22,34,40x. On the other
hand, in vivo experiments using push-pull, voltammetry,
and more recently microdialysis techniques have found
that tryptophan administration increases w6,44,49x or does
not affect w16,34,42x extracellular 5-HT levels. The use of
superfused brain slices has also led to contradictory results
w18,40x, whereas behavioural experiments have suggested
that tryptophan, at the opposite of 5-hydroxytryptophan,
does not promote the so-called 5-HT syndrome w25x. At the
opposite to what can be observed under basal conditions,
there is a general agreement that tryptophan administration
increases release of 5-HT under depolarizing conditions
Žw18,42x; but see w40x. or when 5-HT metabolism into
5-HIAA is prevented w25,18,34x. Taken together, these
results have suggested that under basal conditions Ži.e.
without any electrical stimulation of serotonergic neurones: see below., increases in tryptophan lead to increases
in 5-HT synthesis that are immediately buffered by accelerated metabolism into 5-hydroxyindoleacetic acid Ž5HIAA. andror intraneuronal storage of 5-HT w31x.
In addition to the aforementioned situations, there are
Žless specific. events during which administration of tryptophan also leads to increases in 5-HT release. Hence,
stressful situations such as immobilisation w29x, streptozotocin-diabetes w5x, or food deprivation w41x allow the recognition of increases in extracellular 5-HT levels upon exogenous tryptophan administration. In keeping with this
last observation, it can thus be suggested that exercise-induced increases in brain tryptophan availability Žand in
turn 5-HT synthesis. lead to a marked release of 5-HT in
food-deprived rats. If true, this could suggest that starvation-induced hyperactivity leads to profound changes in
5-HT release, an event which would underlie the reciprocal
relationship that has been observed, through pharmacological approaches, between central serotonergic activity and
vulnerability to starvation-induced hyperactivity.
Taking into account this uncertainty, we have measured,
by means of microdialysis, the influence of one acute
running session upon extracellular 5-HT and 5-HIAA levels in the hippocampus of food-deprived rats. To get an
insight into the metabolic effects of running Žrather than
novelty stress-related influences., rats were trained to run
on the treadmill before acute exercise, and only those
which were not reluctant to run during the training sessions were kept for the final experiments. Moreover, be-
cause previous studies have suggested that tryptophan
administration may diminish through feedback mechanisms cell firing of serotonergic neurones w45x, we have
also compared the effects of tryptophan and tryptophan
plus exercise upon extracellular 5-HT and 5-HIAA levels.
2. Materials and methods
2.1. Animals
Male albino Wistar rats Ž200–300 g. were used housed
two per cage. When treadmill adaptation started Žsee further., the animals were adapted to individualised housing.
The animals were kept on a 12:12 h lightrdark cycle
Žlights on at 06.00 h. and were fed a standard diet with
free access to food and water. All experiments started in
the morning between 09.00 and 09.30 h. The protocols
were carried out according to the national rules on animal
experiments and were approved by the Ethics Committee
of the Faculty of Medicine and Pharmacy of the Vrije
Universiteit Brussel.
2.2. Exercise training
Animals used for the exercise experiments Žsee below.
were placed on the treadmill four to five times in order to
adapt them to the experimental situation. Each time they
exercised for 15 to 30 min in order to familiarize them
with running. Animals reluctant to run during this testing
period, were not used in the experiment.
2.3. Surgery and intrahippocampal dialysis
Animals were anaesthetised with a mixture of ketamine
and diazepam Ž50 mgrkg: 5 mgrkg i.p.. and placed on a
stereotaxic frame. The skull was exposed and a guide
cannula ŽCMA Microdialysis, Stockholm, Sweden. was
implanted through a bore hole in the left ventral hippocampus Ž x: y4.6, y: y5.6, z: q4.6. according to the coordinates described by Paxinos and Watson w37x. The animals
were allowed to recover from surgery for two days, then
the microdialysis probe with a membrane length of 3 mm
ŽCMA Microdialysis. was inserted and the animals were
placed on the treadmill. The microdialysis probes were
connected to a microinfusion pump ŽBAS, USA. and
perfused with a modified Ringer’s solution Ž147.5 mmolrl
Naq, 4 mmolrl Kq, 2.2 mmolrl Ca2q, 153.7 mmolrl
Cly. at a constant flow rate of 1 m lrmin. From that
moment the rats were food deprived, and stayed on the
treadmill until the end of the experiment.
The sampling of the dialysates began 24 h after probe
implantation. The microdialysis samples were collected
every 20 min. The day of the experiment samples were
collected for at least 2 h Žsix samples. to verify stable
basal conditions before one of the following experimental
procedures was started.
R. Meeusen et al.r Brain Research 740 (1996) 245–252
2.4. Experimental procedures
For the exercise experiments, after at least 2 h of
baseline collections, the animals ran for 60 min at a
moderate speed Ž12 mrmin. on a motor driven horizontal
treadmill ŽOmnitech electronics, Columbus, Ohio, USA..
An adjustment was made to the treadmill in order to attach
the counter-balance arm of the microdialysis system. Sampling continued during recovery from exercise Ž120 min.,
total collection time being at least 300 min.
In another series of experiments, after at least 2 h of
baseline collections, Žresting. animals received either saline
or L-tryptophan Ž50 mgrkg.. Sampling continued during
140 min Ž7 collection samples..
Lastly, one experiment was aimed at measuring the
effects of tryptophan in exercising animals. To this end,
again after at least 2 h of baseline collections, rats were
pretreated Ži.p.. either with saline or L-tryptophan Ž50
mgrkg.. Sixty min after this injection, the animals started
running during 60 min at a moderate speed Ž12 mrmin..
Sampling continued for 2 h following exercise, total collection time being at least 360 min.
2.5. Analytical procedures
The microbore liquid chromatography with dual electrochemical detection assay that was used for the determination of 5-HT and 5-HIAA has previously been described
247
by Sarre et al. w39x. In summary, this system consists of a
LC-pump connected to a Sepstik w splitter kit ŽB.A.S.,
USA., to obtain a microflow over a microbore column
Ž100 = 1 mm i.d., C 18 , 3 m m.. This column is coupled
directly to a low dispersion injection valve with a 10 m l
loop. The connection with the electrochemical cell is made
by fused silica Ž50 m m i.d... The electrochemical cell of
the detector ŽLC 4B, BAS, USA. consists of two working
electrodes positioned in parallel in such a way that separate potentials Ž625 and 500 mV. can be set versus the
AgrAgCl reference electrode. The cell volume is reduced,
using a 16 m m gasket. A dual channel integration computer program is used to integrate the chromatograms
ŽIntegration Pack w Kontron, Milan, Italy..
The mobile phase consisted of 95% acetate-citrate buffer
Ž0.1 mM sodium acetate, 20 mM citric acid, 0.1 mM
octanesulfonic acid, 0.1 mM Na 2 EDTA and 1 mM dibutylamine, pH 4.2. and 5% MeOH. The flow rate was set at
0.7 mlrmin, yielding 58 m lrmin over the microbore
column. The limit of detection of the assay was 0.6 fmol
on column.
2.6. Statistics
5-HT could be measured in the dialysates without the
use of a reuptake inhibitor added to the perfusion fluid.
The extracellular concentrations of 5-HT and 5-HIAA in
the dialysates, not corrected for in vivo recovery, are
expressed as percentages of the baseline value Ž8.5 " 1.5
Fig. 1. Respective effects of exercise Ž12 mrmin. on extracellular 5-HT Župper panel. and 5-HIAA Žlower panel. levels in the hippocampus of the
food-deprived rat. Values are the mean " S.E.M. of five animals. Before the onset of exercise at least six samples Ž120 min. were collected Ždata of three
collections are shown.. ) P - 0.05 ŽANOVA for repeated measures with Fischer PLSD. for the differences with the last pre-exercise sample.
248
R. Meeusen et al.r Brain Research 740 (1996) 245–252
fmolr20 ml and 9 " 2.9 pmolr20 ml for 5-HT and 5HIAA in 24 animals.. The latter was the average concentration of the dialysate 5-HT or 5-HIAA levels that were
obtained during the first 120 min of collection. Data, given
as means " S.E.M., were analysed either through repeated
measures ANOVA followed, if significant, by Fischer
PLSD-test Žexercise., or through two-way ANOVA for
repeated measures Žtreatment= time. Žtryptophan, tryptophan and exercise. followed, if significant, by Fischer
PLSD-tests.
3. Results
3.1. Effects of acute exercise on extracellular 5-HT and
5-HIAA leÕels in the hippocampus of 24-h food-depriÕed
rats
As shown in Fig. 1, a 60-min exercise session at 12
mrmin increased in a time-dependent manner extracellular
5-HT levels, an effect that could still be observed during
the first 20-min recovery sample. Besides, exercise-induced elevation in 5-HIAA levels, which yielded a maximal increase of 26% at the end of exercise, did not reach
statistical significance.
3.2. Effects of tryptophan on extracellular 5-HT and 5HIAA leÕels in the hippocampus of 24-h food-depriÕed rats
As shown in Fig. 2, a 50 mgrkg dose of tryptophan
increased extracellular 5-HT levels Žmaximal increase:
55%., this effect being significant in the second, third and
fourth dialysate samples that followed tryptophan administration. On the other hand, the latter treatment increased
5-HIAA levels throughout the 160 min of experiment
Žmaximal increase: 75% at the end of the experiment., this
change being already observed in the second dialysate
sample that followed tryptophan administration ŽFig. 2..
By comparison, administration of a 50 mgrkg dose of
tryptophan to fed rats did not affect extracellular 5-HT
levels, although it increased extracellular 5-HIAA levels
Žmaximal increase: 90%. in a time-dependent manner similar to that observed in food-deprived rats Ždata not shown..
3.3. Effects of tryptophan and exercise on extracellular
5-HT and 5-HIAA leÕels in the hippocampus of 24-h
food-depriÕed rats
Administration of tryptophan Ž50 mgrkg. increased
extracellular 5-HT and 5-HIAA levels, compared with
saline administration, including during the exercise session
Fig. 2. Respective effects of saline and tryptophan Ž50 mgrkg. on extracellular 5-HT Župper panel. and 5-HIAA Žlower panel. levels in the hippocampus
of the food-deprived rat. Values are the mean " S.E.M. of four animals. Before injection Žarrow. at least six samples Ž120 min. were collected Ždata of
three collections are shown.. Two-way ANOVA Žtreatment= time.: ) significant different from baseline, a significant difference between saline and
tryptophan Ž P - 0.05..
R. Meeusen et al.r Brain Research 740 (1996) 245–252
249
Fig. 3. Respective effects of combined tryptophan Ž50 mgrkg. and exercise Ž12 mrmin. on extracellular 5-HT Župper panel. and 5-HIAA Žlower panel.
levels in the hippocampus of the food-deprived rat. Before injection Žarrow. at least six samples Ž120 min. were collected Ždata of three collections are
shown.. Values are the mean " S.E.M. of 5–6 animals. Two-way ANOVA Žtreatment= time.: ) significant different from baseline, a significant
difference between saline and tryptophan Ž P - 0.05..
and the recovery period ŽFig. 3.. Actually, 5-HT levels
reached a plateau Žmaximal increase: 100%. at the end of
exercise whereas 5-HIAA levels reached a plateau Žmaximal increase: 80%. during exercise recovery ŽFig. 3..
4. Discussion
On the basis of exercise-induced increases in tryptophan
and 5-HIAA levels in whole brain, brain regions, or cerebrospinal fluid Žfor reviews: w8,36x., it has been suggested
that such a paradigm increases brain 5-HT synthesis and
metabolism. Actually, experiments aimed at determining
the mechanisms underlying exercise-induced increases in
central tryptophan levels have shown that lipolysis plays a
key role w11,12x. Thus, because free fatty acids displace
tryptophan from its binding to albumin w35x, lipolysis-induced increases in circulating free fatty acids increase the
free Žas opposed to the albumin-bound. portion of tryptophan w11,12x. A similar picture may be observed in fooddeprived rats w11,30x. Exercise having little effects on
those circulating amino acids competing with tryptophan
for entry into the brain w4,12x, lipolysis-induced increases
in circulating free tryptophan lead to parallel increases in
brain tryptophan and in turn 5-HT synthesisrmetabolism
w11,12x. In keeping with the observation that Ži. the aforementioned changes take place in trained rats w11,12x, and
Žii. human volunteers performing exercise display also
increases in circulating free fatty acids and in free tryptophan w4,14,21x, this suggests that it is exercise per se,
rather than the psychological aspects of this stress-like
procedure, that underlies the aforementioned changes in
central serotonergic systems. However, whether exerciseinduced increases in 5-HT synthesis are associated with
effective changes in 5-HT release has been the matter of
debate w8,9x.
As indicated in Section 1, tryptophan-induced increases
in 5-HT synthesis in undisturbed animals may lead to
increased metabolism into 5-HIAA without functional consequences on transmission w16,18,34,42x. Whether this lack
of direct relationship is due to the opposite circadian
fluctuations between synthesis and release of 5-HT w26x
and the circadian-dependent relationship between 5-HT
release and arousal w50x is worth mentioning. On the other
hand, it has been suggested that synthesis of 5-HT, which
increases 5-HT release during stressful events, serves only
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R. Meeusen et al.r Brain Research 740 (1996) 245–252
to replenish serotonergic neurones following depolarisation-induced release of the amine w9x. The latter hypothesis
would thus suggests that synthesis of 5-HT would only
serve to avoid depletion of the amine in serotonergic
neurones.
Past studies have suggested, through indirect indices,
that exercise may increase 5-HT release. Thus, a nonselective 5-HT2A receptor antagonist blocked exercise-induced
prolactin release w15x, whereas the hypophagic effect of the
5-HT uptake blockerr5-HT releaser dextrofenfluramine
proved more consistent in exercising animals w10x. Since
the completion of these studies, more direct analyses have
been conducted with microdialysis w23,24,51x. In one study,
it was found that extracellular 5-HT levels in the ventral
horn of the spinal cord did not change during exercise, but
began to drop on cessation of the stimulus w24x. In keeping
with the findings that Ži. treadmill locomotion stimulates
Žin a speed-dependent manner. the discharge rate of serotonergic caudal raphe neurons w28,46x, and Žii. locomotion
may be associated with a rapid reuptake of 5-HT w47x, it
has been suggested that exercise increases both release and
reuptake of 5-HT w24x. Interestingly, one study reported
walking-induced increases in the extracellular 5-HT levels
in the frontal cortex of Žuntrained. rats w32x whereas another study reported running-induced increases in extracellular 5-HIAA levels in the latter region, but not in the
raphe dorsalis Ži.e. a serotonergic cell body-enriched region. w13x. Lastly, two recent studies Žpublished while this
manuscript was in preparation. related to extracellular
5-HT levels in the ventral funiculus of the spinal cord w23x
and the hippocampus w51x have shown that exercise increases extracellular 5-HT levels. The present study, which
was conducted in food-deprived rats, confirms those findings in fed rats. Hence, all these studies Žincluding the
present one. reveal that extracellular 5-HT levels rise in a
time-dependent manner during exercise, but return progressively toward baseline levels on immediate cessation
of the exercise. In keeping with these results, it is noteworthy that behavioural activity Žincluding locomotion. has
been reported to correlate positively with hippocampal
extracellular 5-HT levels w33x. Besides, the maximal increase in 5-HT levels differed markedly among studies, a
difference which is likely to be due to the training andror
exercise protocols and to the region analysed therein. It has
been claimed that extracellular 5-HT levels rise in poor
runners, but not in good runners w51x. The lack of clear
interindividual differences in the present study renders
however this hypothesis unlikely. On the other hand, it has
been reported that exercise either increases extracellular
5-HIAA levels, with a pattern parallel to that observed
with 5-HT levels w23x, or not w51x. The present study
reports intermediate results, i.e. extracellular 5-HIAA levels followed a pattern similar to those observed with 5-HT,
but nowhere were the changes in 5-HIAA levels significantly different from the pre-exercise period. On the basis
of the parallelism between extracellular 5-HT and 5-HIAA
levels, it has been claimed that extracellular 5-HIAA levels
reflect 5-HT release, at least during exercise w23x. As
underlined recently w48x, extracellular 5-HIAA levels reflect the combination of synthesis, intraneuronal
metabolism, metabolism following release of the parent
amine, and probenecid-sensitive efflux, thereby indicating
the difficulty to acknowledge the functional significance of
extraneuronal 5-HIAA levels. On the other hand, one
crucial question is the extent to which exercise-induced
increases in synthesis may participate in the aforementioned increases in extracellular 5-HT levels. Because parallel experiments aimed at measuring tryptophan, 5-HT
and 5-HIAA in hippocampal tissues were not performed, it
is impossible to provide a clearcut answer. It is our belief
that future experiments should investigate whether 5-HT
synthesis participates Žpartly or totally. in the exercise-induced changes in extracellular levels of 5-HT, or if at the
opposite 5-HT synthesis serves only to replenish depolarisation-induced release of 5-HT Žsee above..
As indicated in Section 1, the hypothesis that central
serotonergic systems may play a key role in the
starvation-induced hyperactivity model w1,2,38x has
prompted us to analyse the effects of running in food-deprived rats. Although it is clear that the combination of
food deprivation and exercise Žas used herein. is a paradigm
that does not strictly parallel hyperactivity-induced fasting,
we felt that at least in a first stage, assessing the question
of the functional effects of food deprivation and exercise
on extracellular 5-HT levels would help in our comprehension of the mechanisms underlying food restriction-induced hyperactivity. However, because activity on a running wheel, at the opposite of treadmill running, does not
affect central tryptophan levels but decreases hippocampal
5-HT and 5-HIAA levels w17,27x, it could be argued that
the pattern of extracellular 5-HT levels may vary between
treadmill exercise and spontaneous wheel running. Besides
the observation that spontaneous wheel running may actually increase 5-HT release from the hippocampus w27x, it is
important to state that both the speed and the duration of
the running wheel sessions in fed rats are far less important than the ones measured in the starvation-induced
hyperactivity paradigm. However, a clearcut answer will
only arise from studies using microdialysis in wheel running rats, i.e., a somewhat complex protocol to set up.
As stated in Section 1, numerous studies w16,18,34,42x
but not all w6,40,44,49x have found that tryptophan administration yields increases in 5-HT release only under stimulated conditions. The present study confirms this hypothesis because tryptophan administration did not increase
extracellular 5-HT levels in fed rats although a similar
treatment increased extracellular 5-HT levels in food-deprived rats, thus confirming data from Ref. w41x. Interestingly, extracellular 5-HIAA levels rose to similar extent
independently from the metabolic state of the animal,
thereby suggesting that under certain conditions Že.g. tryptophan administration to fed rats., extracellular 5-HIAA
R. Meeusen et al.r Brain Research 740 (1996) 245–252
levels are not indices of 5-HT release. It has been claimed
that the differential effect of tryptophan upon extracellular
5-HT levels Žaccording to the metabolic state of the animals. may lie into a better entry of tryptophan into the
brain w41x. However this is unlikely because Ži. the intrinsic effectiveness of tryptophan uptake was found to be
decreased in 24-h food deprived rats, compared with fed
rats w11x, and Žii. increasing the dose of tryptophan up to
100 mgrkg did not prove more effective than a 50 mgrkg
dose upon extracellular 5-HT levels w42x. Actually, another
explanation would lie into the suggestion that food deprivation actually increases the firing rate of serotonergic
neurones, thereby allowing de novo synthesis of 5-HT to
Žpartly or totally. increase 5-HT release. Hence, this effect
of tryptophan would thus resemble that observed in tryptophan-injected rats undergoing other stressors such as immobilisation w29x or streptozotocin-diabetes w5x. On the
other hand, the possibility that tryptophan is effective in
food-deprived rats Žcompared to fed rats. because the
former rats are more reactive than the latter ones to the
paradigm used Ži.e. handling, injection. cannot be discarded.
Past works have shown that tryptophan administration
may, under certain conditions, decrease serotonergic nerve
firing w45x, an effect thought to be mediated by 5-HT
release, and in turn, stimulation of Ž5-HT1A . autoreceptors
endowed with self-inhibitory functions upon serotonergic
nerve firing w43x. In keeping with this possibility, we
investigated whether a pretreatment with tryptophan Žat a
dose that increases 5-HT release. would diminish
exercise-induced increases in extracellular 5-HT levels.
Actually, tryptophan and exercise-induced increases in extracellular 5-HT levels were merely additive throughout
the experiment. This could suggest two complementary
mechanisms, Ži. tryptophan increasing synthesis Žand release. in neurons initially depolarised by food deprivation,
and in turn Žii. exercise amplifying serotonergic nerve
depolarisation. For some unknown reason, saline-pretreated runner rats displayed increased extracellular 5HIAA levels whereas these levels did not change significantly in our initial experiments in saline-treated rats or in
exercised rats. Whether this difference is due to a synergistic interaction between the stress of the injection procedure
and exercise is a possibility which merits consideration.
Taken together, the results from the present study show
that exercise in food-deprived rats is associated with increases in extracellular 5-HT levels in the hippocampus,
thus suggesting sustained 5-HT release. However, because
rats were exercised during the inactive phase of their
circadian cycle Ži.e. when serotonergic firing rate is the
lowest w28x., it remains to be discovered wether the amplitude of these increases would be more affected during the
circadian cycle. Actually, this experiment would also help
to solve the question regarding the origin Žexercise andror
arousal. of the aforementioned increases in 5-HT release.
This study also shows that tryptophan administration to
251
food-deprived rats, but not to fed rats, increases hippocampal 5-HT release, thereby confirming past results, whilst
tryptophan administration prior to running amplifies, rather
than lowers, exercise-induced increases in extracellular
5-HT levels. These results suggest that starvation-induced
hyperactivity may be associated with increased hippocampal release of 5-HT. In keeping with this possibility, the
observation that administration of valine, an amino acid
competing with tryptophan for entry into the brain, decreases evoked, but not basal release of 5-HT in the
hippocampus w22x and exercise-induced increases in 5-HT
synthesis w11,20x is worth mentioning. If release of 5-HT is
involved in the exacerbation of the starvation-induced
hyperactivity syndrome, these results suggest that the use
of valine Žor some other neutral amino acid. could prove
of most interest.
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