Effect of tryptophan on extracellular concentrations of tryptophan and 5-hydroxyindoleacetic acid in the striatum and cerebellum

Measuring tryptophan dynamics using fast scan cyclic voltammetry at carbon fiber microelectrodes with improved sensitivity and selectivity

Despite the fact that tryptophan (Trp) is an essential amino acid that humans typically obtain through diet, there are several interesting tryptophan dynamics at play in the body. Quantifying and understanding these dynamics are crucial in studies of depression, autism spectrum disorder, and other disorders that involve neurotransmitters directly synthesized from tryptophan. Here we detail the optimization of waveform parameters in fast scan cyclic voltammetry at carbon fiber microelectrodes to yield four-fold higher sensitivity and six-fold higher selectivity compared to previously reported methods. We demonstrate the utility of our method in measuring (1) exogenous Trp dynamics from administration of Trp to PC-12 cells with and without overexpression of tryptophan hydroxylase-2 and (2) endogenous Trp dynamics in pinealocyte cultures with and without stimulation via norepinephrine. We observed interesting differences in Trp dynamics in both model systems, which demonstrate that our method is indeed sensitive to Trp dynamics in different applications.

Isoliquiritigenin Alleviates Semen Strychni-Induced Neurotoxicity by Restoring the Metabolic Pathway of Neurotransmitters in Rats

Acute neurotoxicity of Semen Strychni can result in sudden death in epilepsy. The detoxification method and mechanism of Semen Strychni acute poisoning have not been clarified. This experiment focused on the mechanism of Semen Strychni neurotoxicity and the alleviation effects of isoliquiritigenin. The rats were intraperitoneally injected with Semen Strychni extract (125 mg/kg), followed by oral administration of isoliquiritigenin (50 mg/kg) for 7 days. FJ-B staining was used to evaluate the degree of injury on hippocampus neurons. The concentration of monoamines, amino acids, and choline neurotransmitters, the Dopamine (DA) and 5-hydroxytryptamine (5-HT) metabolic pathway in the hippocampus, cerebellum, striatum, prefrontal cortex, hypothalamus, serum, and plasma were detected by LC-MS/MS. The expression of neurotransmitter metabolic enzymes [catechol-O-methyl transferase (COMT) and monoamine oxidase (MAO)] and neurotransmitter receptors [glutamate N-methyl-D-aspartic acid receptors (NMDARs) and gamma-aminobutyric acid type A receptor (GABRs)] were, respectively determined using ELISA and qRT-PCR. The results indicated that Semen Strychni induced neuronal degeneration in the hippocampal CA1 region. Meanwhile, Semen Strychni inhibited the mRNA expression of NMDAR1, NMDAR2A, NMDAR2B, GABRa1, GABRb2 and reduced the level of MAO, which disrupted the DA and 5-HT metabolic pathway. However, isoliquiritigenin reversed these effects. In summary, isoliquiritigenin showed alleviation effects on Semen Strychni-induced neurotoxicity, which could be attributed to restoring neurotransmitters metabolic pathway, most likely through the activation of NMDA receptors.

Free tryptophan/large neutral amino acids ratios in blood plasma do not predict cerebral spinal fluid tryptophan concentrations in interleukin-1-induced anorexia

Peripheral administration of interleukin-1 (IL-1) reduces food intake and affects brain serotonergic activity, suggesting a causal relationship. Furthermore, IL-1 increases the brain concentrations of the serotonin precursor, tryptophan (TRP), by unclear mechanism(s). We aimed at confirming the link between IL-1 administration, raised brain TRP concentrations and the development of anorexia, and at investigating the mechanisms of TRP entry into the brain. Thirty adult, overnight fasted Sprague-Dawley rats were randomly assigned to i.p. injections of 1 mug/kg BW of IL-1 alpha (n=10) or vehicle (n=10), or to pair-feeding with IL-1 animals (n=10). After 2 h, food intake, blood plasma concentrations of total TRP, free TRP, large neutral amino acids (LNAA; competing with TRP for brain entry) were measured. Cerebral spinal fluid (CSF) TRP concentrations were also measured. TRP brain availability was assessed by calculating the plasma ratio free TRP/LNAA. Following IL-1 injection, food intake significantly declined in IL-1 rats, which was paralleled by decreased plasma free TRP and increased plasma LNAA. Despite a decrease in the free TRP/LNAA ratios in plasma, IL-1 significantly increased concentrations of TRP in CSF. These data show that the acute peripheral administration of IL-1 induces anorexia and raises CSF TRP levels. Considering the possible role of the raised CSF TRP in influencing brain serotonin activity, it is postulated that increased serotonergic neurotransmission could be involved in IL-1 induced anorexia.

Strain differences in basal and post-citalopram extracellular 5-HT in the mouse medial prefrontal cortex and dorsal hippocampus: relation with tryptophan hydroxylase-2 activity

We used the microdialysis technique to compare basal extracellular serotonin (5-HT) and the response to citalopram in different strains of mice with functionally different allelic forms of tryptophan hydroxylase-2 (TPH-2), the rate-limiting enzyme in brain 5-HT synthesis. DBA/2J, DBA/2N and BALB/c mice carrying the 1473G allele of TPH-2 had less dialysate 5-HT in the medial prefrontal cortex and dorsal hippocampus (DH) (20-40% reduction) than C57BL/6J and C57BL/6N mice carrying the 1473C allele. Extracellular 5-HT estimated by the zero-net flux method confirmed the result of conventional microdialysis. Citalopram, 1.25, 5 and 20 mg/kg, dose-dependently raised extracellular 5-HT in the medial prefrontal cortex of C57BL/6J mice, with maximum effect at 5 mg/kg, but had significantly less effect in DBA/2J and BALB/c mice and in the DH of DBA/2J mice. A tryptophan (TRP) load enhanced basal extracellular 5-HT in the medial prefrontal cortex of DBA/2J mice but did not affect citalopram's ability to raise cortical and hippocampal extracellular 5-HT. The impairment of 5-HT synthesis quite likely accounts for the reduction of basal 5-HT and the citalopram-induced rise in mice carrying the mutated enzyme. These findings might explain why DBA/2 and BALB/c mice do not respond to citalopram in the forced swimming test. Although TRP could be a useful strategy to improve the antidepressant effect of citalopram (Cervo et al. 2005), particularly in subjects with low 5-HT synthesis, the contribution of serotonergic and non-serotonergic mechanisms to TRP's effect remains to be elucidated.

Serotonin agonists and antagonists in obstructive sleep apnea: therapeutic potential

Obstructive sleep apnea hypopnea syndrome (OSAHS) is a prevalent disorder associated with substantial cardiovascular and neurobehavioral morbidity. Yet this is a disorder for which there are no widely effective pharmacotherapies. The pathophysiology of obstructive sleep apnea namely, normal respiration in waking with disordered breathing only in sleep, suggests that this disorder should be readily amenable to drug therapy. Over the past 10 years, we have gained tremendous insight into the neurochemical mechanisms involved in state-dependent control of respiration. It is apparent from this work that there are many potential avenues for pharmacotherapies, including several seemingly conflicting directions for serotonergic therapies. Serotonin delivery is reduced to upper airway dilator motor neurons in sleep, and this contributes, at least in part, to sleep-related reductions in dilator muscle activity and upper airway obstruction. The dilator motor neuron post-synaptic serotonin receptors are 5-HT(2A) and 5-HT(2C) subtypes, and in adults the presynaptic 5-HT receptor in motor nuclei is 5-HT(1B), an inhibitory receptor. Serotonin receptors are also found within central respiratory neuronal groups, and these receptor subtypes include 5-HT(1A) (inhibitory) and 5-HT(2) receptors. Peripherally, stimulation of 5-HT(2A), 5-HT(2C) and 5-HT(3) receptor subtypes have an inhibitory effect on respiration via action at the nodose ganglion. Many of these receptor subtypes and their signal transduction pathways may be affected by oxidative stress in obstructive sleep apnea. These alterations will make finding drug therapies for sleep apnea more challenging, but not insurmountable. Future directions are suggested for elucidating safe, well-tolerated serotonergic drugs for this disorder. Tryptophan was one of the first serotonergic drugs tested for OSAHS. This drug was withdrawn from the market as a result of reports linking tryptophan use with eosinophilic myalgia syndrome and life-threatening pulmonary hypertension. Newer drugs with serotonergic activity tested in persons with sleep-disordered breathing include buspirone, fluoxetine and paroxetine. Trials are presently being conducted to evaluate the effects of 5-HT(2A) and 5-HT(3) antagonists on OSAHS. Many of the drugs tested have not shown significant improvement in sleep apnea. However, with continued effort to elucidate the pharmacology of neurochemical control of state-dependent changes in respiratory control, the availability of pharmacological therapy for this disorder is not too far away.

Effects of pure macronutrient ingestion on plasma tryptophan and large neutral amino acids

The role of tryptophan and its competitor large neutral amino acids, proposed earlier for serotonin synthesis following carbohydrate or protein ingestion, was reassessed in relation to a recent study investigating serotonin release, including the so far unknown effects of fats. In the present study, meals of either carbohydrates, casein, or lard, were supplied to rats for 30 min and blood samples collected every 15 min to follow the changes in plasma large neutral amino acids. In response to carbohydrates, amino acid levels fell and the ratio tryptophan over sum of other amino acids increased. Following casein ingestion, all amino acids were enhanced, tryptophan somewhat less, leading to a decreased ratio. The lard meal induced a slight decrease in some amino acids while the ratio remained constant. Only in response to casein, and partly to carbohydrates, did a consistent relation appear between the previously observed serotonin changes and the ratio. These data suggest that a relationship between the ratio and the previously observed serotonin changes is not always encountered because the release is not obligatorily coupled to synthesis and is subject to behavioral influences. It remains that serotonin release is affected by the composition of the meal through peripheral metabolic mechanisms.

Acute effects of L-tryptophan on brain extracellular 5-HT and 5-HIAA levels in chronic experimental portal-systemic encephalopathy

Portal-systemic encephalopathy (PSE) is associated with increased brain turnover of serotonin (5-hydroxytryptamine; 5-HT). Despite this metabolic increase, neuronal release of 5-HT is unaltered in neocortex of portacaval shunted (PCS) rats. In the present study, frontal neocortical extracellular 5-HT and 5-hydroxyindole-3-acetic acid (5-HIAA) levels were determined in PCS rats and sham-operated controls prior to, as well as, after acute challenge with L-tryptophan (L-TRP; a bolus dose of 280 mg/kg i.p. followed by 5 consecutive hourly doses of 50 mg/kg). Neither basal 5-HT nor 5-HIAA extracellular levels were significantly altered in PCS rats compared to controls. L-TRP administration resulted in unaltered extracellular 5-HT but elevated 5-HIAA levels in PCS and sham rats. These findings do not suggest that changes in brain neuronal 5-HT release play any major functional role in the pathogenesis of chronic PSE. The present data also emphasize the importance of distinguishing between brain 5-HT metabolism and brain 5-HT release.

The effects of an inhibitor of tryptophan 2,3-dioxygenase and a combined inhibitor of tryptophan 2,3-dioxygenase and 5-HT reuptake in the rat

The effects of a novel inhibitor 680C91 ((E)-6-fluoro-3-[2-(3-pyridyl)vinyl]-1H-indole) of the key enzyme of tryptophan catabolism tryptophan 2,3-dioxygenase (TDO), and a novel inhibitor 709W92 ((E)-6-fluoro-3-[2-(4-pyridyl)vinyl]-1H-indole), of both TDO and 5-hydroxytryptamine (5-HT) reuptake, were examined on tryptophan catabolism, cerebrospinal fluid (CSF) concentrations of tryptophan and 5-HT and serotonergic-mediated physiology and behaviour in the rat. The catabolism of L-[ring-2-14C]tryptophan in vivo was completely inhibited by prior administration of 709W92. 709W92, but not 680C91, potentiated head-twitch produced by 5-hydroxytryptophan, prevented head-twitch and whole brain 5-HT depletion produced by p-chloroamphetamine and rapidly decreased dorsal raphe firing. Both 709W92 and 680C91 elevated CSF tryptophan by up to 260% of basal concentration. A maximally effective dose of 680C91 elevated a global measure of brain extracellular 5-HT (CSF 5-HT) to concentrations similar to those seen maximally after exogenous tryptophan administration (approx 170% of basal). Maximally effective doses of 709W92 increased CSF 5-HT to concentrations comparable to those seen after tryptophan and 5-HT reuptake inhibitor coadministration (approx 900% of basal) and to concentrations greater than those achieved maximally with serotonergically active antidepressant monotherapy (approx 500% of basal). 709W92 did not elevate CSF 5-HT to concentrations associated with the serotonin syndrome (approx 3000% of basal). The combined TDO inhibitor/5-HT reuptake inhibitor, 709W92, showed anxiolytic activity in the rat-pup vocalization model of anxiety. These results show that 709W92 (a novel inhibitor of both TDO and 5-HT reuptake), can produce an elevation of CSF 5-HT similar to that achieved with a serotonin reuptake inhibitor/tryptophan combination therapy but with a more sustained timecourse; such compounds may therefore have superior antidepressant efficacy in the clinic.

Effect of 6R-L-erythro-5,6,7,8-tetrahydrobiopterin on the extracellular levels of dopamine and serotonin in the rat striatum: a microdialysis study with tyrosine or tryptophan infusion

The present study demonstrates the effects of 6R-L-erythro-5,6,7,8-tetrahydrobiopterin (6R-BH4) on turnover of dopamine and serotonin (5-HT) in rat striatum during continuous infusion of the amino acids tyrosine and tryptophan. By monitoring with microdialysis, it was found that the increase in dopamine and homovanillic acid (HVA) concentrations in rat striatal extracellular fluid (ECF) induced by 6R-BH4 was further enhanced by the continuous infusion of tyrosine at a relatively low dose (1 mumol/min/kg) as compared with the concentration which saturates tyrosine hydroxylation. This dose of tyrosine alone did not induce the elevation of dopamine and HVA concentrations in ECF. In contrast, though the concentration of 5-HT and 5-HIAA in striatal ECF was gradually increased by tryptophan infusion, 6R-BH4 had no further effect. Although the higher output of dopamine into ECF was induced by the dialytic perfusion of 6R-BH4 via the microdialysis probe into striatum, tyrosine infusion had no further effect on dopamine concentration in the dialysates. The in vivo measurement of DOPA accumulation during NSD 1015 perfusion suggests that the enhancement of dopamine concentration in ECF induced by tyrosine infusion and 6R-BH4 might be attributable to an increase in tyrosine hydroxylase activity in striatum. Tryptophan hydroxylase was also activated by tryptophan infusion and/or 6R-BH4, however, it did not induce an increase in 5-HT concentration in striatal ECF.

Inhibitory effects of clonidine on serotonergic neuronal activity as measured by cerebrospinal fluid serotonin and its metabolite in anesthetized rats

Clonidine-induced changes in the serotonergic neuronal activity of the central nervous system were estimated by measuring the concentrations of serotonin (5-HT) and its major metabolite, 5-hydroxyindole-3-acetic acid (5-HIAA), in the cerebrospinal fluid (CSF) of anesthetized rats. Clonidine (30 and 300 micrograms/kg, i.v.) led to 74% and 60% reductions in the concentration of 5-HT in the CSF 60 min after administration. CSF 5-HIAA concentrations were also decreased to 77% and 66%, respectively. Clonidine-induced (30 micrograms/kg, i.v.) decreases in CSF 5-HT and 5-HIAA concentrations were attenuated by pretreatment with idazoxan (5 mg/kg, i.p.). Idazoxan by itself did not alter the CSF 5-HT and 5-HIAA concentrations. Decreased CSF 5-HT and 5-HIAA concentrations after i.v. administration of clonidine (30 micrograms/kg) were abolished by noradrenergic denervation after pretreatment with 6-hydroxydopamine (200 micrograms/rat, i.c.v.). These results suggest the possibility that clonidine acts to inhibit the serotonergic neuronal activity, which is mediated via the alpha 2-adrenoceptors. It indicates, moreover, that noradrenergic nervous systems are involved in the clonidine-induced inhibition of serotonergic neuronal activity. Therefore, noradrenergic neurons play a significant role in mediating the actions of clonidine on serotonergic neuronal activity in the rat brain.

Spontaneous feeding-related monoaminergic changes in the rostromedial hypothalamus revealed by microdialysis

The activity of hypothalamic monoamines in response spontaneous feeding was investigated using the in vivo technique of brain microdialysis together with the instrumental recording of feeding pattern. The simultaneous variations of dopamine (DA), serotonin (5-HT), and their respective metabolites, DOPAC and 5-HIAA, were measured in the rostromedian hypothalamus, where the probe was located between the PVN and VMH. Throughout the experiment, the changes in DOPAC followed a mirror image of those in DA: DA regularly increased, reaching its zenith within the 15-min sample collected during the meal before returning to the same level as just before the meal. Following a premeal plateau, both 5-HT and 5-HIAA increased as soon as the beginning of feeding; 5-HT reached its zenith during the meal while 5-HIAA showed a more delayed and prolonged increase. When a new meal was initiated, 60 to 70 min later, a similar monoaminergic pattern was observed again. These data suggest that building up hunger is announced by an ascending slope of DA and setting up of satiation is concomitant with a descending slope of DA. Concerning serotonergic changes, the sharp 5-HT release during the meal would be a signal of satiation (transient preabsorptive fullness) while the longer-lasting increase in 5-HIAA, reflecting 5-HT synthesis, would be associated with satiety (more persistent postabsorptive state substituting satiation). These data partially confirm and extend previous pharmacological studies as well as the findings on deprivation-induced, imposed meals. They suggest a possible causal relation between monoaminergic changes and behavioral initiatives.