Blood-brain barrier transfer of L-Trp and alpha-MTrp in Li-treated rats

Effects of 4-hydroxyisoleucine from Fenugreek Seeds on Depression-like Behavior in Socially Isolated Olfactory Bulbectomized Rats

Context:

Antidepressant-like effects of (2S, 3R, 4S)-4-hydroxyisoleucine (4-HI), a major amino acid from fenugreek seeds, has been reported in the animal model of acute depression.

Aims:

To evaluate effects of subacute administration of 4-HI in animal model of stress-induced depression namely socially isolated olfactory bulbectomized rats.

Materials and Methods:

Bilateral olfactory bulbectomy (OBX) were induced in 30 Sprague-Dawley rats. After recovery period of 14 days, rats were randomized into five groups of 6 rats each and stressed with social isolation (individual housing). The rats were orally treated with either vehicle (OBX-Iso), positive control, fluoxetine (30 mg/kg) or 4-HI (10, 30, 100 mg/kg) once a day from day 14 onward. Separate group of rats with social isolation but without OBX (Sham-Iso) was also maintained. The behavioral depression and anxiety related parameters using open field test (OFT), sucrose intake test, novelty suppressed feeding (NSF) and forced swim test (FST), and neurochemical estimation (brain monoamines viz., serotonin and nor-adrenaline, serotonin turnover, and serum cortisol) were performed.

Statistical Analysis Used:

Data was analyzed by either two-way ANOVA (OFT and FST) or one-way ANOVA (sucrose intake test, NSF, and neurochemical estimation) followed by Dunnett's multiple comparisons test. Differences were considered significant at P < 0.05.

Results:

The significant and dose-dependent protection from behavioral and neurochemical changes were observed in 4-HI co-administrated OBX-Iso rats.

Conclusion:

4-HI demonstrated the antidepressant and antianxiety effects in socially isolated stress-induced OBX rats with possible involvement of multiple stress relieving mechanisms.

HIGHLIGHTS OF PAPER

In this study, the subacute pretreatment of 4-HI showed strong and dose-dependent prevention of isolation stress related behavioral and neurochemical responses in olfactory bulbectomized rats. The prevention of hyperactive HPA axis in OBX-Iso stress-induced rats can be envisaged as probable mechanism of antidepressant and antianxiety effects of 4-HI.

SUMMARY

Effect of 4-hydroxyisoleucine (4-HI) in olfactory bulbectomized and socially isolated (Iso) rats was evaluated

4-HI showed significant and dose-dependent antidepressant effects during novelty suppressed feeding (NSF) and forced swim test (FST)

4-HI showed significant and dose-dependent antianxiety effects during OFT (open field test) and sucrose intake test

4-HI showed protection from OBX-Iso stress-induced brain monoamines, serotonin turnover, and serum cortisol level elevation.

Abbreviations used: SSRI: Selective Serotonin Reuptake Inhibitor; 4-HI: (2S, 3R, 4S)-4-hydroxyisoleucine; OBX: Olfactory bulbectomy; CPCSEA: Committee for the Purpose of Control and Supervision of Experiments on Animals; OFT: Open Field Test; NSF: Novelty Suppressed Feeding; FST: Forced Swimming Test; 5HT: 5-Hydroxytryptamine; 5-HIAA: 5-Hydroxyindoleacetic Acid; NA: Nor-adrenaline; and HPA: Hypothalamic-Pituitary Adrenal.

Acute and chronic D-fenfluramine treatments have different effects on serotonin synthesis rates in the rat brain: an autoradiographic study

The effects of acute and chronic treatments with D-fenfluramine on the regional rates of serotonin (5-hydroxy-tryptamine; 5-HT) synthesis were investigated using the alpha-[14C]methyl-L-tryptophan (alpha-[14C]MTrp) autoradiographic method. In the first series of experiments, acute D-fenfluramine treatment (5 mg/kg; i.p.) given 20 min before the tracer injection significantly (p<0.05) decreased 5-HT synthesis in the dorsal raphe, and significantly (p<0.05) increased the rates in the cerebral cortices and caudate nucleus, when compared to the rates in the control rats (saline treated). In a second series of experiments, following a 7-day treatment with D-fenfluramine (5 mg/kg/day; i.p.), a significant (p<0.05) decrease of 5-HT synthesis, in the dorsal raphe was observed, and significant (p<0.05) increases were observed in the hypothalamus, the dorsal thalamus, the medial and lateral geniculate body and some brain stem regions (locus ceruleus, inferior and superior colliculus). No significant changes were observed in the cerebral cortices.

Analysis of [C-11]alpha-methyl-tryptophan kinetics for the estimation of serotonin synthesis rate in vivo

We describe the tracer kinetic analysis of [C-11]-labeled alpha-methyl-tryptophan (AMT), an analogue of tryptophan, which has been developed as a tracer for serotonin synthesis using positron emission tomography (PET) in human brain. Dynamic PET data were acquired from young healthy volunteers (n = 10) as a series of 22 scans covering a total of 60 minutes and analyzed by means of a three-compartment, four-parameter model. In addition, functional images of the K-complex were created using the Patlak-plot approach. The application of a three-compartment model resulted in low identifiability of individual k-values, especially that of k3. Model identifiability analysis using a singular value decomposition of the final sensitivity matrix showed parameter identifiability to increase by 50% when the Patlak-plot approach was used. K-complex values derived by the Patlak-plot approach overestimated the compartmental values by 10 to 20%, because of the violation of the dynamic equilibrium assumption. However, this bias was fairly constant in all structures of the brain. The rank order of K-complex values from different brain regions corresponded well to the regional concentrations of serotonin in human brain (P < 0.0001). These results indicate that the Patlak-plot method can be readily applied to [C-11]AMT data in order to create functional images of the K-complex, reflecting serotonin synthesis rate, within an acceptable error margin.

The regional rate of serotonin synthesis estimated by the alpha-methyl-tryptophan method in rat brain from a single time point

A simplified approach to the measurement of the rate of serotonin (5-HT) synthesis in laboratory animals is presented using autoradiography and labeled alpha-methyl-L-tryptophan. The method is based on the assumption that the volume of distribution of an apparent precursor pool is the same in all brain structures. The apparent distribution volume (Vapp) of the tracer was estimated from our previous data and found to be 0.45 +/- 0.1 ml g-1 (n = 147) as an average in all brain structures. When this value of Vapp was used, the rates of 5-HT synthesis in the rat brain structures, calculated from the single time point method, were not significantly different from those calculated by the dual time point method, where both Vapp and K* were estimated. The rates from the two approaches did not differ when compared by analysis of variance (p > 0.8), Mann-Whitney rank sum test (p > 0.11), Bonferroni-corrected two-tailed t test, or when the ratio between two groups was tested against the hypothesis of being 1. The agreement between rates of 5-HT synthesis estimated by the two methods was very good, suggesting that the simplified method proposed here is appropriate for these kind of measurements.

Asymmetric radiosynthesis of alpha-[11C]methyl-L-tryptophan for PET studies

Asymmetric radiosynthesis of alpha-[11C]methyl-L-tryptophan has been achieved using the enantioselective [11C]methylation of the enolate of either 8-(phenylsulfonyl) or 8-acetyl substituted derivatives of dimethyl (2S.3aR, 8aS)-(+)-hexahydropyrrolo[2,3-b]indole-1,2-dicarboxylate. Reaction of the enolates generated by treatment with LDA at -78 degrees C, with [11C]methyl iodide at -78 degrees C gave in 5 min incorporation of the radiolabel of 86% for 8-phenylsulfonyl derivative and 63% for 8-acetyl derivative. The hexahydropyrrolo[2,3-b]indoles were then decyclized to the fully protected alpha-[11]methyl-L-tryptophan by treatment with trifluoroacetic acid. Removal of all the protecting groups, including the phenylsulfonyl, was achieved by reaction with 10 N NaOH at 210 degrees C in a sealed vial. Neutralization of the alkali with 10 N H2SO4 followed by purification by HPLC gave alpha-[11C]methyl-L-tryptophan with an overall radiochemical yield of 20% (uncorrected for decay) relative to the amounts of [11C]CH3I from 8-phenylsulfonyl derivative, and 9% (relative to [11C]CH3I; uncorrected for decay) from 8-acetyl derivative in a preparation time of 40 min after [11C]methyl iodide was introduced into the reaction mixture.

Autoradiographic evaluation of the influence of hypothalamic 5,7-dihydroxytryptamine lesion on brain serotonin synthesis

The influence of a unilateral stereotaxically induced 5,7-dihydroxytryptamine (5,7-DHT) lesion in the dorsolateral hypothalamus on brain serotonin synthesis was evaluated by an autoradiographic method, using labelled alpha-methyl-L-tryptophan (alpha-MTrp). The hypothalamus was selected as the lesion site because it receives well defined and relatively large projections from the raphe nuclei. Data suggest that the unilateral lesion in the dorsolateral hypothalamus had a significant influence (an increase) on the rate of serotonin synthesis in the large majority of ipsilateral brain structures examined. It seems that the effect was the greatest in the hippocampal structures, the thalamus, and the parietal and sensory motor cortices. The average increase in the rate of serotonin synthesis on the lesion side when compared with the contralateral side was between 3% (amygdala) and 52% (dorsal hippocampus; CA3 layer of hippocampus). Since in the sham-injected rats (same volume of saline) there was no obvious injection-contralateral side asymmetry observed (except for two structures, probably affected by the injection needle, which showed a significant difference), we concluded that the effect observed in the present study was most likely related to the 5,7-DHT-induced lesion on the serotonergic terminals in the hypothalamus. Comparison of the rate of synthesis in the dorsal and medial raphe and the pineal body with the rates reported earlier for these structures led us to conclude that either the 5,7-DHT lesion in the hypothalamus did not influence the rates in these structures in their entirety, or the method used was not sensitive enough to reveal this influence. Data reported here also demonstrate how a highly specific tracer (alpha-MTrp), in conjunction with a specific and localized lesion, could aid our understanding of the brain serotonergic system.

Rapid steady-state analysis of blood-brain transfer of L-Trp in rat, with special reference to the plasma protein binding

We estimated constants for the binding of tryptophan (Trp) to plasma proteins, and for the transfer of Trp from plasma to brain in rat. The measurements were made under conditions in which the plasma and brain concentrations of Trp were raised to new steady-states for at least 10 min before being measured. The concentration of other competing amino acids were also at a steady-state. The plasma Trp concentration was elevated by i.p. injection of different doses of L-tryptophan methyl ester 60 min before the measurement of the plasma-brain transfer. We simultaneously measured blood flow with [14C]-butanol, and the brain tissue Trp uptake with [3H]Trp. The maximal velocity (Vmax), apparent half-saturation Michaelis-Menten constant (Km(app)), and diffusion constant (PdS) for Trp transport from plasma into brain were found to be 7.0 +/- 2.1 nmol g-1 min-1, 36 +/- 17 microM, and 0.065 +/- 0.006 ml g-1 min-1, respectively. The maximum plasma protein binding (Bmax) and dissociation constant (KD) for Trp were estimated at 360 +/- 16 nmol/ml-plasma and 81 +/- 10 microM, respectively. We conclude that the plasma protein binding of Trp inhibits the blood-brain transfer in inverse proportion to the plasma free Trp concentration.

Synthesis of enantiomerically pure alpha-[14C]methyl-L-tryptophan

A practical method for the preparation of large amounts of enantiomerically pure alpha-[14C]methyl-L-tryptophan using the enzymatic resolution of the corresponding D,L-methyl ester is reported. The radiolabelled alpha-methyl group was introduced using the alpha-methylation of the lithium enolate of the Schiff base of L-tryptophan methyl ester. Hydrolysis of the Schiff base with 1N HCl provided the D,L-methyl ester of alpha-[14C]methyl tryptophan. Enantioselective enzymatic hydrolysis of the L-methyl ester by alpha-chymotrypsin gave the enantiomerically pure alpha-[14C]methyl-L-tryptophan. The overall yield of this preparation was 33%.