Determination of the Lumped Constant for the α‐Methyltryptophan Method of Estimating the Rate of Serotonin Synthesis

Abstract

The lumped constant (LC) for the α‐methyl‐l‐tryptophan method to convert the brain's uptake of labeled α‐methyl‐l‐tryptophan into the regional rate of serotonin synthesis was estimated. The method involved independently estimating the unidirectional uptake constant of the tracer (α‐[14C]methyl‐l‐tryptophan) to the tissue and the tracee (tryptophan) (with the addition of a radioactive compound) and calculating their ratio. The LC was estimated from logarithmically transformed data. Similar experiments were performed using rats treated with the drug probenecid, which blocks the efflux of 5‐hydroxyindoleacetic acid (a metabolite of serotonin) from the brain. The experiments using probenecid, corrected for the difference in the levels of plasma free tryptophan (increased in probenecid‐treated rats) relative to control experiments, gave an average LC for the rat brain of 0.46 ± 0.14 (mean ± SD). This value was not significantly different from the one obtained in controls (0.43 ± 0.13). In addition, the LC was also calculated using unidirectional uptake constants in the probenecid‐treated rats for α‐methyl‐l‐tryptophan and l‐tryptophan. This LC value was 0.39 ± 0.10. There was no significant difference between these three LC values. Thus, an average ± SD LC of 0.42 ± 0.07 for 28 brain structures investigated in this study was obtained. Statistically the LC obtained in different structures had a variability that could be accounted for by errors in measurements alone. In other words, dispersion in the LC values could be fully accounted for by chance alone. Data confirmed that the LC value did not change when the rate of serotonin synthesis was increased by probenecid treatment. We also showed that the rate of 5‐hydroxyindoleacetic acid accumulation in probenecid‐treated rats was 58 pmol g−1 min−1 (rat brain), which is about twice as much as reported by others for a normal rat. This difference could also be accounted for by the increase in the plasma level of free tryptophan in probenecid‐treated rats.

The opposite effect of a 5-HT1B receptor agonist on 5-HT synthesis, as well as its resistant counterpart, in an animal model of depression

“…The rate of 5-HT synthesis (R; pmol/g/min) was calculated by converting K (slopes obtained from volume of distribution fit as a function of exposure time ; here Cp*(t) and C*p(T) are plasma tracer concentration as a function of time (t) and at the end of experiment (T), respectively) to K T (K T = K/LC; division by the lumped constant (LC)) and then multiplying the calculated K T by the plasma free (non-protein-bound) Trp (Cp; pmol/mL); R = CpK/LC = CpK T . The LC for α-MTrp used in the present study was previously measured in vivo and found to be 0.42 ± 0.07 [52], and it is not influenced by drugs [41,52].…”

Chronic administration of citalopram in olfactory bulbectomy rats restores brain 5-HT synthesis rates: an autoradiographic study

“…We chose to use plasma free Trp in this calculation because of the reports indicating that this plasma fraction is in dynamic equilibrium with brain Trp (Bloxam and Curzon 1978;Salter et al 1989;Takada et al 1993). The LC was measured in vivo and found to be homogenous throughout the brain and has a value of 0.42±0.07 (Vanier et al 1995). In the present experiments, all of the linear relationships between Θ and DV* had a significant (p<0.05; F-statistics) and positive slope (Nagahiro et al 1990).…”

AGN-2979, an inhibitor of tryptophan hydroxylase activation, does not affect serotonin synthesis in Flinders Sensitive Line rats, a rat model of depression, but produces a significant effect in Flinders Resistant Line rats

“…After conversion into the volume of distribution (VD [ml/g]; division of tissue radioactivity by plasma tracer concentration (Cp; nCi/ml) at the time of euthanasia), the slope of the unidirectional net uptake of the tracer, required in the rate calculation, was calculated from the linear relationship between VD (ml/g) and exposure time Θ (Θ=∫Cp.dt/Cp(T) [min] (see details in Nagahiro et al 1990; Diksic et al 1995; Diksic and Young, 2001). The LC of 0.42 measured in the rats was used in the calculations (Vanier et al 1995). …”