Abstract
To the Editor:
Dr. Diksic's letter addresses a wide range of topics concerning the suitability of α-methyl-L-tryptophan (αMTP) to measure serotonin synthesis. In this response, we address what we consider to be the two primary issues: (1) the formation of α-methylserotonin (αM5HT) in rat and monkey, and (2) whether irreversible uptake of αMTP occurs during the time period of a C-11 study.
In our study (Shoaf et al., 2000), we found < 4% αM5HT after 1 hour in monkeys. This is much lower than the percentage found in the rat brain. However, based on allometric parameters and the longer analysis time in rats, these data are in fact not inconsistent. It appears that the conversion of αMTP to αM5HT is not complete in rats and is very low in monkeys. Thus, because of these interspecies rate differences, the sensitivity of αMTP to serotonergic drugs in the rat does not ensure that alterations in serotonin synthesis will have significant effects on [11C]αMTP uptake in primates, particularly because of the limited time frame of a C-11 study. Furthermore, extrapolation to primates of Dr. Diksic's unpublished experiments in rats raises similar concerns.
Although αM5HT is not produced in significant quantities, Dr. Diksic argues that there is still irreversible uptake of tracer and that this uptake is related to the precursor pool for 5HT synthesis even if it is not rapidly converted to αM5HT. In that case, [11C]αMTP would be measuring an index that would, presumably, be correlated with serotonin synthesis rate, as also suggested by Chugani and Muzik (2000). This argument depends upon the presence of irreversible tracer uptake. However, we find no evidence of irreversible trapping in the cortical regions of the anesthetized primate.
Dr. Diksic makes a number of kinetic arguments to prove the existence of irreversible uptake in our own data. Dr. Diksic is correct that we state in Shoaf et al. (2000) that fits to the 4-parameter model (K1, k2, k3, and Vb) showed a statistically significant improvement over the 3-parameter model (K1, k2, and Vb) based on the F test. We went on to say that the addition of the k3 term made only a small improvement and did not remove all the lack of fit in the data (Shoaf et al., 2000; Fig. 2). If one extends the analysis further and applies a 5-parameter model including a clearance term (k4) from the “irreversible” compartment to our 180-minute tissue time activity curves, one finds yet another statistically significant improvement in the fit over the 4-parameter model. However, these small improvements in the fit cannot be used as proof of a physiologic mechanism. The most robust and least model-dependent method to demonstrate irreversible uptake is the Patlak plot that can clearly separate reversible from irreversible uptake. Our extended Patlak data shows little or no irreversible uptake of [11C]αMTP (Shoaf et al., 2000, Fig. 3). We further note that Dr. Diksic and colleagues have in fact observed a progressive flattening of the slope of Patlak plots when they fitted data from later time intervals (Nishizawa et al., 1998).
Dr. Diksic also notes that apparent volume of distribution (tissue/plasma) reached 1.4 mL/mL in our data, suggesting that brain uptake exceeds that which would be achieved by simple bidirectional transport. However, this is not the case. Following a bolus injection, the tissue-to-plasma ratio often approaches a constant value that can be substantially larger than the true ratio at equilibrium due to plasma clearance (Carson et al., 1993). As noted in Shoaf et al. (2000), the estimated equilibrium distribution volume (K1/ k2) from the 3-parameter model was 0.73 ± 0.18 mL/mL, a value that does not suggest accumulation of the tracer in the brain.
αMTP may be a marker of serotonin synthesis in the rat, particularly in the raphe nucleus; however, we find very little metabolized tracer and no evidence of irreversible uptake in the anesthetized monkey in the 3 hours following αMTP administration. Therefore, we must conclude that, in the monkey, the kinetics of [11C]αMTP are driven predominantly by blood–brain barrier exchange and [11C]αMTP uptake is not a marker of serotonin synthesis.