The Regional Rate of Serotonin Synthesis Estimated by the α-Methyl-Tryptophan Method in Rat Brain from a Single Time Point

Abstract

Summary: A simplified approach to the measurement of the rate of serotonin (5-HT) synthesis in laboratory ani mals is presented using autoradiography and labeled u-methyl-L-tryptophan, The method is based on the as sumption 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 es timated 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 Serotonin (5-hydroxytryptamine; 5-HT) is a neu rotransmitter widely distributed throughout the brain. Because of this, 5-HT has been implicated in many diseases (Asberg et aI., 1976; Ohuoha et aI., 1993; Wessel et aI., 1993) and in diverse behaviors such as locomotion and learning (Klein et aI., 1982 where both Vappcand 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 I. 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.

Serotonin synthesis and its light–dark variation in the rat retina

“…AM-5-HT was undetectable under our experimental conditions, although AM-Trp doses were similar to those used to investigate 5-HT synthesis in brain areas (Diksic et al 1995). Possible explanations could be low TPH amounts, a lower TPH affinity in the rat retina for AM-Trp than for endogenous Trp, and/or lower AADC affinity for AM-5-HTP than for 5-HTP.…”

“…injections (2 mL/kg) of either vehicle or tryptophan (Trp; Sigma, Saint-Quentin Fallavier, France) 100 mg/kg, or a-methyl-DL-tryptophan (AM-Trp; Sigma) 100 mg/kg, and/or m-hydroxybenzylhydrazine (NSD 1015; Sigma) 100 mg/kg, under the conditions reported in Table 1. Trp was tested for its influence on the retinal indole compound content, NSD 1015 (a decarboxylase inhibitor) was used to demonstrate possible increases in 5-HTP content, and AM-Trp was used as a competitive substrate for TPH, giving rise to artificial compounds a-methyl-5-HTP (AM-5-HTP) and a-methyl-5-HT (AM-5-HT) (Diksic et al 1995).…”

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

“…The tissue radioactivity concentrations were converted into volume of distribution (DV*; ml g −1 ) by dividing tissue concentrations [C* t (T); nCi g −1 ] by the plasma tracer concentration [C* p (T); nCi ml −1 ] at the end of the experiment. There is a linear relationship between DV* and ( min) [Θ=∫ T 0 C* p (t)dt/C* p (T)] with a slope K* (ml g −1 min −1 ; trapping constant for α-[ 14 C]MTrp as described by Nagahiro et al 1990 andDiksic et al 1995). The 5-HT synthesis rate (R; pmol g −1 min −1 ) is calculated by converting K* to K T (K T =K*/LC; where LC is lumped constant) and multiplying the calculated K T by the plasma free (non-protein-bound) Trp (C p ; pmol ml −1 ): R=C p K*/ LC=C p K T .…”

Serotonin in Aging, Late-Life Depression, and Alzheimer's Disease: The Emerging Role of Functional Imaging

“…Although there are no PET ligands currently available for the 5-HT 1B/D receptors, recent focus on characterizing this aspect of the 5-HT system may provide the basis for the future development of specific receptor ligands (Pauwels 1997). PET techniques have also been applied to the measurement of 5-HT synthetic rates in vivo (Diksic et al 1991(Diksic et al , 1995.…”