On the turnover of acetylcholine in mouse brain: Influence of dose size of deuterium labelled choline given as precursor

Abstract

The influence of the dose size of precursor choline (Ch) on the turnover rate of acetylcholine (ACh) has been studied in whole brain and striatum of mice. Different doses of deuterium labelled Ch (d 6-Ch) were injected i.v. and concentrations of endogenous and d 6-labelled Ch and ACh were estimated at various times by mass fragmentography using deuterium labelled (d 9 − ) Ch and ACh as internal standards. At doses of 1.25–20 μmole/kg of d 6-Ch the endogenous concentration of Ch in blood wastransiently elevated for 5 min. However, the endogenous concentrations of Ch and ACh in whole brain and striatum were not affected. The fractional rate constant of ACh synthesis and efflux, K a a measure of the turnover rate of ACh, was influenced by the dose size of precursor d 6-Ch. In whole brain, K a was ca. 0.7 at doses of 5–20 μmole/kg of d 6-Ch. At lower doses K a increased to ca. 1.0. In striatum, K a was ca. 0.7 at doses of 20–40 μmole/kg of the precursor. At lower doses K a became smaller and decreased to 0.10 at a dose of 1.25 μmole/kg of d 6-Ch. When the calculations of K a were corrected for the part of d 6-Ch concentration in the brain contained in blood the figures were invariably increased by 20–40%. However, the increase was not correlated with the precursor dose size. It was found that the ratio between S ACh and S Ch decreased with increasing dose of d 6-Ch until it reached plateau values at doses of 2.5 μmole/kg and higher in whole brain and at 10–20 μmole/kg and higher in striatum. It is proposed that the decreased ratio at low concentrations of d 6-Ch depends on saturation of the high affinity (HA) uptake system for Ch coupled to ACh synthesis. At higher doses of d 6-Ch, ACh synthesis by the low affinity (LA) uptake system for Ch became increasingly important and was not yet saturated at the highest dose given (40, μmole/kg). It is suggested that the synthesis rate of ACh can be studied with either very low doses of labelled Ch that do not saturate the HA uptake system or with sufficiently high doses at which the influence of the LA uptake system on the synthesis rate of ACh is dominant.