Lattice Boltzmann study of chemically-driven self-propelled droplets.

We numerically study the behavior of self-propelled liquid droplets whose motion is triggered by a Marangoni-like flow. This latter is generated by variations of surfactant concentration which affect the droplet surface tension promoting its motion. In the present paper a model for droplets with a third amphiphilic component is adopted. The dynamics is described by Navier-Stokes and convection-diffusion equations, solved by the lattice Boltzmann method coupled with finite-difference schemes. We focus on two cases. First, the study of self-propulsion of an isolated droplet is carried on and, then, the interaction of two self-propelled droplets is investigated. In both cases, when the surfactant migrates towards the interface, a quadrupolar vortex of the velocity field forms inside the droplet and causes the motion. A weaker dipolar field emerges instead when the surfactant is mainly diluted in the bulk. The dynamics of two interacting droplets is more complex and strongly depends on their reciprocal distance. If, in a head-on collision, droplets are close enough, the velocity field initially attracts them until a motionless steady state is achieved. If the droplets are vertically shifted, the hydrodynamic field leads to an initial reciprocal attraction followed by a scattering along opposite directions. This hydrodynamic interaction acts on a separation of some droplet radii otherwise it becomes negligible and droplets motion is only driven by the Marangoni effect. Finally, if one of the droplets is passive, this latter is generally advected by the fluid flow generated by the active one.

Switching dynamics in cholesteric liquid crystal emulsions.

In this work we numerically study the switching dynamics of a 2D cholesteric emulsion droplet immersed in an isotropic fluid under an electric field, which is either uniform or rotating with constant speed. The overall dynamics depend strongly on the magnitude and on the direction (with respect to the cholesteric axis) of the applied field, on the anchoring of the director at the droplet surface and on the elasticity. If the surface anchoring is homeotropic and a uniform field is parallel to the cholesteric axis, the director undergoes deep elastic deformations and the droplet typically gets stuck into metastable states which are rich in topological defects. When the surface anchoring is tangential, the effects due to the electric field are overall less dramatic, as a small number of topological defects form at equilibrium. The application of the field perpendicular to the cholesteric axis usually has negligible effects on the defect dynamics. The presence of a rotating electric field of varying frequency fosters the rotation of the defects and of the droplet as well, typically at a lower speed than that of the field, due to the inertia of the liquid crystal. If the surface anchoring is homeotropic, a periodic motion is found. Our results represent a first step to understand the dynamical response of a cholesteric droplet under an electric field and its possible application in designing novel liquid crystal-based devices.

Augmented cocaine-induced accumbal dopamine efflux, motor activity and place preference in rats fed with a tryptophan-deficient diet.

In the present study we demonstrate that consumption of a tryptophan-deficient diet for a period of 14 days decreased the striatal serotonin and 5-hydroxyindolacetic acid tissue content in rats, whereas the level of dopamine remained unchanged. Under this condition of diminished serotonergic tone, a challenge dose of cocaine (10mg/kg, i.p.) significantly increased motor activity and dopamine extracellular content in the nucleus accumbens compared to rats fed with a balanced diet. We moreover found that pretreatment with cocaine (7 and 10mg/kg, i.p.) produced a significant increase in preference for a cocaine-associated environment in the tryptophan-deficient group compared to control rats. Our experiments show that a low tone of serotonergic system, augments the behavioural reinforcing effect of cocaine and that this effect may be due to a increased cocaine-induced accumbal dopamine release. These data indicate that a tryptophan-deficient diet alters the behavioural and neurochemical effect of psychostimulants, such as cocaine, and suggest an important role of serotonin in modulation of these effects.

Vitamin A deficiency induces motor impairments and striatal cholinergic dysfunction in rats.

Vitamin A and its derivatives, retinoids, are involved in the regulation of gene expression by binding two nuclear receptor families, retinoic acid receptors and retinoid X receptors. Retinoid receptors are highly expressed in the striatum, revealing an involvement of this system in the control of movement as demonstrated by previous observations in knockout mice. To further assess the role of retinoids in adult striatal function, the present study investigated the effect of vitamin A deprivation on rat motor activity and coordination, the rate of synthesis and release of dopamine, the functioning of D1 and D2 receptors and their expression in the striatum. Moreover, the content of acetylcholine in the striatum was measured. Results show that 24 weeks of postnatal vitamin A deprivation induced severe locomotor deficits and impaired motor coordination. Vitamin A deprivation rats showed a significant hyperactivity following D1 receptor stimulation by R(+)-6-chloro-7,8-dihydroxy-1-phenyil-2,3,4,5-tetrahydro-1H-3-benzazepine or amphetamine and reduced catalepsy in response to haloperidol treatment. This different response to the above drugs is not due to a change in striatal DA release or synthesis between vitamin A deprivation and control animals. In situ hybridization experiments showed identical level of expression for the D1 and D2 receptor transcripts. On the other hand, the striatal tissue content of acetylcholine was reduced significantly by about 30% starting from the initial manifestation of motor deficits. We suggest that the locomotor impairment could be imputable to the dysfunction in striatal cholinergic interneurons. Our results stress the basic role of vitamin A in the maintenance of basal ganglia motor function in the adult rat brain.

Vitamin A deficiency produces spatial learning and memory impairment in rats.

Vitamin A and its derivatives (retinoids) play important roles in many physiological processes. The recent finding of high levels of cellular retinol-binding protein type 1 immunoreactivity, cellular retinoic acid-binding protein type 1 immunoreactivity and the presence of nuclear retinoid receptors in the central nervous system of adult rodents suggests that retinoids may carry out important roles in the adult brain. In consideration of the role of the hippocampus in spatial learning and memory we evaluated the effect of vitamin A deprivation in adult rats on these functions. Following 12 weeks of vitamin A-free diet, rats were trained to acquire a radial-arm maze task. Results show that this diet induced a severe deficit in the spatial learning and memory task. The cognitive impairment was fully restored when vitamin A was replaced in the diet. We also found a significant decrease in hippocampal acetylcholine release induced by scopolamine, assessed using microdialysis technique, and a reduction in the size of hippocampal nuclei of CA1 region in vitamin-deficient rats, compared to rats fed with a vitamin A-sufficient diet. These results demonstrate that vitamin A has a critical role in the learning and memory processes linked to a proper hippocampal functioning.

Long-term voluntary ethanol consumption induces impairment of the mechanical performance in the papillary muscle of Sardinian alcohol-preferring rats.

The effects of chronic (26 weeks) ethanol consumption on cardiac muscle contractility in Sardinian alcohol-preferring drinkers (sP-D) and Sardinian alcohol-preferring naive (sP-N) rats were investigated. Experiments were carried out 1 week after ethanol administration ceased. Length-tension and force-frequency responses in left ventricular papillary muscles from both sP-D and sP-N rats were recorded in vitro. Papillary muscles were gradually stretched in steps of 5% of the reference (initial) length (L(r)) from 100 to 130% L(r). In sP-D rats, length-tension relationships showed a significant reduction in active tension: at optimal length for maximum active tension (130% L(r)), the developed force value (1.38 +/- 0.36 mN/mg, dry tissue) was 54% lower than that found in sP-N rats (3.16 +/- 0.6 mN/mg, dry tissue). In sP-D papillary muscles, a decrease in contractile behaviour was also observed in force-frequency responses (0.03-120 pulse/min), when compared with sP-N rats; developed force was found to be reduced by about 2.5 times. These results indicate that long-term ethanol consumption impairs the mechanical performance of sP papillary muscle, inducing a negative inotropic effect.

Hippocampal acetylcholine release correlates with spatial learning performance in freely moving rats.

To assess the activity of septohippocampal cholinergic neurons during the learning of a radial-arm maze task we measured changes in extracellular acetylcholine levels in the hippocampus by means of the vertical microdialysis technique. During the 12 days spent learning the spatial task the extracellular concentration of acetylcholine in the hippocampus was monitored while rats performed the test. One week before radial-arm maze training a guide cannula was implanted unilaterally in the hippocampus. On each day of testing a removable microdialysis probe was inserted through the guide cannula and the dialysate was collected during the test performance. The concentration of acetylcholine in the dialysate was detected by means of a high-performance liquid chromatograph coupled to an electrochemical detector. We found that hippocampal acetylcholine release progressively increased from 139% to 245% during the 12 days of radial-maze learning and the magnitude of change in acetylcholine output was positively correlated with spatial memory performance, thus suggesting that changes in the functioning of these neurons are involved in learning.

A physiological method to selectively decrease brain serotonin release.

In this protocol the effect of both an acute and chronic tryptophan (TRP)-free diet on brain serotonin (5-HT) release was studied in rats. Extracellular levels of cortical 5-HT, assessed by in vivo microdialysis, revealed a decrease in the release of this monoamine. Indeed, 120 min after the acute administration of a TRP-free diet, cortical 5-HT release decreased significantly by about 40% with respect to a balanced diet and the decrease persisted for more than 6 h. The chronic intake of a TRP-free diet induced a gradual reduction in 5-HT release. Five days after the diet consumption, our HPLC system detected no 5-HT in cortical dialysate. On the contrary, the acute or chronic administration of the TRP-free diet made no significant change in extracellular noradrenaline content in the frontal cortex, suggesting a specific action of the diet on the serotonergic system. Therefore, the administration of a TRP-free amino acid diet offers a non-pharmacological means for effectively decreasing brain 5-HT release. This diet can be used to study the physiological and behavioral effects of reduced brain 5-HT function.

A tryptophan-free diet markedly reduces frontocortical 5-HT release, but fails to modify ethanol preference in alcohol-preferring (sP) and non-preferring (sNP) rats.

It has been hypothesised that rat lines genetically selected for their alcohol preference consume large amounts of ethanol because they have a low 5-HT content. Since brain tryptophan (TRP) availability controls the rate at which neurons synthesise and release serotonin (5-HT), we assessed whether the administration of a TRP-supplemented or TRP-free diet for 3 consecutive days influenced alcohol intake in alcohol-preferring and non-preferring sP and sNP rats, respectively. In the same animals extracellular 5-HT concentration was monitored by microdialysis in the frontal cortex. A TRP-free diet progressively and markedly decreased cortical extracellular 5-HT in sP and sNP rats during the treatment period with respect to a balanced diet. However, the TRP-free diet failed to modify alcohol consumption and preference in sP and sNP rats. The TRP-supplemented diet also failed to alter the intake of alcohol in either group of rats. Therefore, these results do not support a specific role of 5-HT transmission in ethanol intake and preference in sP and sNP rats.

Biphasic effects of NMDA on the motility of the rat portal vein.

The effect of NMDA on the motility of the rat portal vein was studied in an isolated preparation. NMDA induced a concentration-dependent (10(-7) - 10(-4) M) increase of the contraction frequency (maximum increase, 148+/-6% of control at NMDA 10(-4) M). The NMDA-induced excitatory response was prevented by the competitive NMDA receptor antagonists (+/-)-2-Amino-5-phosphonopentanoic acid (AP-5, 5x10(-4) M) or (RS)-3-(2-carboxypiperazine-4-yl) propyl-1-phosphonic acid (CPP, 10(-4) M). Tetrodotoxin (TTX, 10(-6) M) or atropine (10(-4) M) abolished the NMDA-induced increase of the portal vein motility and reversed the excitatory effect to a concentration-dependent inhibition (maximum inhibition, 52+/-8 and 29+/-7% of controls, respectively, at NMDA 10(-3) M). Removal of the endothelium abolished the NMDA-induced inhibitory response. Sodium nitroprusside concentration-dependently (10(-7) - 10(-5) M) inhibited the portal vein motility, while L-N(G)-nitro-arginine methyl ester (L-NAME, 10(-4) M) reversed the inhibitory effect of NMDA (in the presence of TTX), restoring the portal vein spontaneous activity to control values. These results show that NMDA modulates the portal vein motility in a biphasic manner: via indirect activation, through prejunctional NMDA receptors presumably located on intrinsic excitatory neuronal afferences, or via direct inhibition, through endothelial NMDA receptors activating the nitric oxide pathway. Overall these findings support the hypothesis of the existence of a peripheral glutamatergic innervation modulating the contractile activity of the rat portal vein. British Journal of Pharmacology (2000) 129, 156 - 162

Tryptophan-Free Diets: A Physiological Tool to Study Brain Serotonin Function.

Tryptophan-free diets produce a specific reduction of brain serotonin synthesis and release. This method for lowering neural serotonin function has been extensively used in both laboratory animals and humans to study the role of serotonin in a variety of behaviors, such as aggressiveness, sleep, sexual behavior, anxiety, mood, memory, and so forth.

Effects of nimodipine on extracellular dopamine levels in the rat nucleus accumbens in ethanol withdrawal.

Withdrawal from chronic ethanol intoxication is associated with a reduction of dopamine neurotransmission. However, the mechanisms of dopamine depletion, a putative neurochemical correlate of the dysphoric symptomatology, are not yet understood. To assess the role of L-type calcium channels in the inhibition of the dopaminergic system in the withdrawal state, the effects of the dihydropyridine calcium channel antagonist nimodipine on the extracellular levels of dopamine were studied in the nucleus accumbens shell of awake rats 10 h after withdrawal from chronic ethanol intoxication. In control, chronic sucrose-withdrawn rats, nimodipine did not change extracellular dopamine levels. However, in ethanol-withdrawn rats nimodipine (5 or 10 mg/kg s.c.) increased extracellular dopamine to 136 +/- 16 and 305 +/- 19% of pre-administration values, respectively, the latter dose elevating levels above those of controls. The elevations of extracellular DA by nimodipine (10 mg/kg) were associated with a significant reduction (-17%) of the overall behavioural score of the withdrawal symptomatology, as evaluated for 11 behavioural items. Significant reductions of the score for convulsions (-47%) and, to a lesser extent, for catatonia (-30%) and tremors (-15%) contributed to the overall effect. It is suggested that overactivity of L-type calcium channels is involved in the mechanisms of dopamine depletion as well as in certain behavioural/neurological signs associated with ethanol withdrawal. By restoring depleted dopamine levels, dihydropyridines might ameliorate the dysphoric symptoms of ethanol abstinence.

Glutamate-induced increase of extracellular glutamate through N-methyl-D-aspartate receptors in ethanol withdrawal.

Ethanol withdrawal is a physiopathological state associated with increased number and function of N-methyl-D-aspartate glutamate receptors. We assessed the effect of N-methyl-D-aspartate receptor stimulation on the extracellular levels of glutamate in vivo by the focal application of N-methyl-D-aspartate in the striatum of dependent rats following withdrawal from chronic treatment with ethanol. In control, chronic sucrose-treated rats, 800 microM N-methyl-D-aspartate increased glutamate levels to 268% of baseline values. In ethanol-withdrawn animals, 12 h after interruption of the chronic treatment, the application of N-methyl-D-aspartate increased glutamate levels to 598% of baseline values. In ethanol-intoxicated rats N-methyl-D-aspartate was ineffective. Concentration-response curves showed that in ethanol withdrawn animals N-methyl-D-aspartate was five-fold more potent than in controls. In withdrawn animals, the non-competitive N-methyl-D-aspartate receptor antagonist dizocilpine (1.0 mg/kg i.p.) or ethanol (5 g/kg i.g.) markedly reduced the N-methyl-D-aspartate-induced increase in glutamate levels. These results are consistent with the up-regulation of N-methyl-D-aspartate receptors by chronic ethanol and add biochemical evidence for the presence of N-methyl-D-aspartate receptors facilitating glutamate release through a positive feedback mechanism. The glutamate-induced, N-methyl-D-aspartate receptor-mediated elevations of extracellular glutamate may constitute a neurochemical substrate for the neuropathological alterations associated with alcoholism.

Long-term voluntary ethanol consumption affects neither spatial nor passive avoidance learning, nor hippocampal acetylcholine release in alcohol-preferring rats.

Long-term ethanol consumption in humans and laboratory animals is associated with morphological and functional alterations of brain structures involved in cognitive processes. In the present experiments, we assessed whether voluntary long-term consumption of ethanol by alcohol-preferring (sP) rats under free choice condition with water (also) caused alterations in memory performance and hippocampal acetylcholine (ACh) release in vivo. A group of sP rats were offered a 10% v/v ethanol solution in a free choice with water for 36 weeks; controls had only tap water available. After withdrawal of ethanol, rats were tested in one trial passive avoidance test and thereafter were trained in a food-reinforced radial arm maze task for 12 days. One day after the last session in the radial-arm maze, rats were implanted with a microdialysis probe in the dorsal hippocampus and dialysate concentrations of ACh were measured. No significant differences were observed between sP drinking and control rats in retention latencies in the passive avoidance test, in radial arm-maze performance or in basal levels of hippocampal ACh release. These results show that long-term ethanol consumption by sP rats is not associated with cognitive impairments or with alterations in the hippocampal cholinergic function. To the extent that chronic ethanol intoxication can be considered a causal factor in the development of memory and neurochemical alterations, these results suggest that sP rats self-regulate ethanol consumption so as to avoid intoxication. These findings may challenge the notion that sP rat lines can be considered a valid model of human alcoholism.

Serotonin and acetylcholine release response in the rat hippocampus during a spatial memory task.

By using in vivo microdialysis we monitored the extracellular levels of acetylcholine and serotonin in the hippocampus of rats performing a spatial memory task. After rats were trained for 10 consecutive days to master a food-reinforced radial-arm maze task, they were implanted with a microdialysis probe in the dorsal hippocampus. On day 12, rats were tested in the maze and acetylcholine and serotonin outputs were monitored before the test, during the waiting phase and while performing the trials. In trained, food-rewarded rats, hippocampal acetylcholine levels increased during the waiting period (181 +/- 90 of baseline) and further increased during the radial-maze performance to 236 +/- 13% of baseline values, while serotonin levels did not change during the waiting period but increased to 142 +/- 3% during the maze performance. To discriminate whether the increase of acetylcholine and serotonin levels during the testing was associated with memory performance or with food consumption, we monitored hippocampal acetylcholine and serotonin release in rats that were trained, but not food rewarded, or in rats that were not trained, but rewarded only on the test day. In the trained, non-rewarded group, acetylcholine release increased during the waiting phase to 168 +/- 6%, but did not increase further during the task performance. In contrast, no change in serotonin release was observed in this group in any phase of the test. In rats which were not trained, but food rewarded, acetylcholine increased only during the maze period (150 +/- 5%). Serotonin increased gradually and become significant at the end of the trials. (130 +/- 3%). While both neurotransmitters could be implicated in feeding behaviour, only activation of cholinergic neurotransmission appears to be associated with memory function. Our results support the following hypotheses: (i) hippocampal acetylcholine could be involved in attentional and cognitive functions underlying motivational processes; (ii) serotonin could be implicated in non-cognitive processes (i.e. in the control of motor and feeding behaviour). Since serotonin and acetylcholine neurotransmission is simultaneously activated during the spatial memory task, this suggests that these neurotransmitter systems regulate behavioural and cognitive functions.

Chronic ethanol consumption induces hypomotility in the portal vein of Sardinian alcohol-preferring rats.

In order to study the physiopathological effects of chronic ethanol intake on the smooth muscle of the vascular system, we have assessed the length-tension relationship in isolated portal veins of Sardinian alcohol-preferring (sP) rats. Significant differences in motor performance were found between sP naive and sP rats exposed to ethanol consumption (12% w/v) for 48 weeks. Isolated portal veins of sP rats which consumed ethanol chronically showed a marked decrease of spontaneous and KCl-induced contraction waves when compared to sP naive rats. At optimum length (140% Lr) for maximal contractile performance, the mean amplitude wave in the portal veins of sP drinker rats was about five times less than in sP naive veins. Furthermore, in the veins of sP drinkers, the active curve showed lower values of tension at each elongation of the vascular segment, the maximum value of active tension (7.32 +/- 0.54 mN) represented a reduction in amplitude of about 32% with respect to sP naive veins. These results indicate that long-term ethanol consumption impairs portal vein motility.

Chronic ethanol consumption: from neuroadaptation to neurodegeneration.

In this review first we evaluate evidence on the role of the neurobiological alterations induced by chronic ethanol consumption in the development of ethanol tolerance, dependence and withdrawal. Secondly, we describe the neuropathological consequences of chronic ethanol on cognitive functions and on brain structures. Chronic alcohol consumption can induce alterations in the function and morphology of most if not all brain systems and structures. While tolerance mechanisms are unlikely to contribute to the neuroadaptive changes associated with ethanol dependence, it is otherwise clear that repeated high, intoxicating doses of ethanol trigger those neuroadaptive processes that lead to dependence and contribute to the manifestation of the abstinence syndrome upon withdrawal. An unbalance between inhibitory and excitatory neurotransmission is the most prominent neuroadaptive process induced by chronic ethanol consumption. Due to the diffuse glutamatergic innervation to all brain structures, the neuroadaptive alterations in excitatory neurotransmission can affect the function of most if not all of neurotransmitter systems. The expression of the withdrawal syndrome is the major causal factor for the onset and development of the neuropathological alterations. This suggests a link between the neuroadaptive mechanisms underlying the development of ethanol dependence and those underlying the functional and structural alterations induced by chronic ethanol. In animals and humans, specific alterations occur in the function and morphology of the diencephalon, medial temporal lobe structures, basal forebrain, frontal cortex and cerebellum, while other subcortical structures, such as the caudate nucleus, seem to be relatively spared. The neuropathological alterations in the function of mesencephalic and cortical structures are correlated with impairments in cognitive processes. In the brain of alcoholics, the prefrontal cortex and its subterritories seem particularly vulnerable to chronic ethanol, whether Korsakoff's syndrome is present or not. Due to the role of these cortical structures in cognitive functions and in the control of motivated behavior, functional alterations in this brain area may play an important role in the onset and development of alcoholism.

Tryptophan metabolism in male Sardinian alcohol-preferring (sP) and -non-preferring (sNP) rats.

Parameters of tryptophan (Trp) and related metabolism were compared in male Sardinian alcohol-preferring (sP) and -non-preferring (sNP) rats. Liver Trp pyrrolase activity was 38-58% higher in sP than in sNP rats, and this was associated with a greater expression of the enzyme mRNA as measured by multiprobe oligonucleotide solution hybridization. Moderately (about 10-19%), but significantly, lower concentrations of free serum, total serum, and brain Trp were also observed in sP compared with sNP rats. Concentrations of whole brain 5-hydroxytryptamine (5-HT) and its major metabolite 5-hydroxyindol-3-yl-acetic acid (5-HIAA) were, however, 14-21% higher in sP rats. Serum corticosterone concentration was 18% higher in sP rats. We conclude that alcohol preference in Sardinian rats is associated with increased liver Trp pyrrolase activity and mRNA expression leading to a decrease in Trp availability to the brain. Although a simple serotonin deficiency could not be demonstrated in the whole brain, the possibility could not be ruled out that a deficiency may be present in discrete areas of the brain of the sP rat.

The decrease of serotonin release induced by a tryptophan-free amino acid diet does not affect spatial and passive avoidance learning.

We assessed whether consumption of a diet lacking in tryptophan (TRP) resulted in alteration in learning and memory performance and hippocampal 5-HT release in rats. Two hours after the acute administration of TRP-free (T) and balanced (B) diet rats were trained in a one-trial passive avoidance task. The two groups of rats showed no significant difference in retention latencies. Two other groups of rats, fed with the above diets during the acquisition of a radial-arm maze task, showed no difference in baseline performance. The acute ingestion of the T diet produced a significant and long lasting decrease of hippocampal and cortical 5-HT release in rats when compared to the B diet, while the 12th day of the T diet, 5-HT was not detectable in the dialysate. These data indicate that the diminished brain release of 5-HT induced by a T diet is not sufficient to impair cognitive processes.

Acute administration of a tryptophan-free amino acid mixture decreases 5-HT release in rat hippocampus in vivo.

The effect of oral administration of a tryptophan-free amino acid mixture or the same mixture containing tryptophan (Trp) on hippocampal serotonin (5-HT) extracellular levels was studied using in vivo brain microdialysis of freely moving rats. During chloral hydrate anesthesia rats were implanted with dialysis probes in the dorsal hippocampus, and experiments were performed 24 h later. In vehicle-treated rats, the extracellular levels of 5-hydroxyindolacetic acid (5-HIAA) and 5-HT did not change during 240 min after ingestion. Oral administration of the Trp-free amino acid mixture significantly decreased basal 5-HT and 5-HIAA output 100 min after ingestion (65 and 81% of basal value, respectively) and remained at this level for another 140 min. The amino acid mixture containing Trp failed to significantly change basal extracellular levels of 5-HT, but enhanced that of 5-HIAA by approximately 134%. Moreover, in rats receiving the Trp-free amino acid mixture, the increase of hippocampal 5-HT release induced by d-fenfluramine (206%) was smaller than that released by the same drug in rats receiving the nutritionally balanced amino acid mixture (271%). Thus these results show that removal of Trp from the balanced amino acid mixture decreases spontaneous and d-fenfluramine-induced release of 5-HT in the hippocampus. In conclusion, our study supports the hypothesis that the mood-lowering effect observed in man after ingestion of a Trp-free amino acid mixture is associated with diminished 5-HT release in the brain.