Turnover of Neurotransmitters in the Brain: An Introduction

Abstract

In order to function, all living organisms take up exogenous substances for use in the various metabolic processes required to maintain their integrity. Metabolites are interconverted and eventually degraded to yield products which can be excreted. The total quantity of a metabolite in a cell is often referred to as the metabolic pool of that substance and the amount that is transported or metabolised, the turnover (Zilversmit et al., 1943). Often the term turnover rate is used instead of turnover although both expressions have the same dimensions of mass. time ^-1. In some cases (e.g. glutamic acid in nervous tissue), it has been recognised that the metabolic pool is not homogeneous and that different turnover rates apply to different proportions of the metabolite. Under these circumstances the metabolite is said to exist in different pools or compartments. A compartment is defined as a quantity of metabolite having uniform and distinguishable kinetics of transformation or transport. The term (fractional) rate constant (with the dimension of time^-1 is also used in estimates of turnover (Robertson, 1957; Atkins, 1969) and refers to the ratio of the turnover of a compartment to the size of that compartment.