Choline acetyltransferase activity and mass fragmentographic measurement of acetylcholine in specific nuclei and tracts of rat brain

Abstract

The mass fragmentographic assay of acetylcholine (ACh) is the only available method to measure the acetylcholine content of rat brain nuclei. The ACh concentration and the choline acetyltransferase activity (ChA) of specific rat brain nuclei and tracts are reported. The highest ACh concentration (1.2 nmol mg protein) and ChA activity (576 nmol mg protein per hr) was in the nucleus interpeduncularis, which possessed twice as much ACh and ChA as the nucleus accumbens and nucleus caudatus. The midbrain nuclei: dorsalis raphes and linearis pars caudalis contained as much ACh as the nuclei accumbens and caudatus but the ChA activity was only a fraction of that of the accumbens and caudatus. The three pontine nuclei: locus eoeruleus, tegmenti dorsalis and dorsalis vagi contained slightly less ACh than the nuclei accumbens and caudatus but the ChA varied. It was low in the locus coeruleus but 10- fold higher in the dorsalis vagi. It is proposed that the ratio of ACh content to ChA activity may have some predictive value to determine whether most of the ACh measured in various brain nuclei is located in cell bodies or axon terminals. The data presented are compared with histochemical data on the location of acetylcholinesterase ( Palkovits and Jacobowitz, 1974). This comparison suggests that when the ratio ACh/ChA × 100 is greater than 0.7 and the ACh content is 0.30 nmol mg protein or greater, the transmitter may be located in nerve terminals. When this ratio is smaller than 0.4 and the ChA activity is greater than 5.0 nmol mg protein per hr, the ACh measured may be located in cholinergic cell bodies or small cholinergic interneurones. This suggestion is supported by measurement of the ACh concentration and ChA activity in brain nuclei which are known to contain cholinergic cell bodies (e.g. motor nucleus of the vagus), small cholinergic interneurones (n. caudatus and n. accumbens) and cholinergic nerve terminals (n. locus coeruleus, dorsalis raphes).