Divergent Regulation of Muscarinic Binding Sites and Acetylcholinesterase in Discrete Regions of the Developing Human Fetal Brain
The expression of muscarinic acetylcholine binding sites and of cholinesterases was studied in extracts prepared from discrete regions of the human fetal brain, between the gestational ages of 14 and 24 weeks. The specific binding of [3H]N-methyl-4-piperidyl benzilate ([4H]-4NMPB) to muscarinic binding sites ranged between 0.05 and 1.30 pmol/mg protein in the different brain regions, withK d values of 1.2 ± 0.2 nM. Binding of the cholinergic agonist oxotremorine fitted, in most of the brain regions examined, with a two-site model for the muscarinic binding sites. The density of muscarinic binding sites increased with development in most regions, with different rates and onset times. It was higher by about sixfold in some areas destined to become cholinergic, such as the cortex and midbrain, than in noncholinergic areas such as the cerebellum. In other areas destined to become cholinergic, such as the hippocampus and the caudate putamen, the receptor density remained low. Average density values increased from 0.1 ± 0.1 at 14 weeks up to 0.7 ± 0.4 pmol/mg protein at 24 weeks.
The variability in the specific activities of cholinesterase was relatively low, and extracts from different brain regions hydrolyzed from 5 to 30 nmol of [3H]acetylcholine/min/mg protein. These were mostly “true” acetylcholinesterase (EC 188.8.131.52) activities, inhibited by 10−5 M BW284C51, with minor pseudocholinesterase (EC 184.108.40.206) activities, inhibited by 10−5 M iso-OMPA. The enzyme from different brain regions and developmental stages displayed similarK m values toward [3H]acetylcholine (ca. 4 × 10−4 M −1). The ontogenetic changes in cholinesterase specific activities had no unifying pattern and/or relationship to the cholinergic nature of the various brain areas. In most of the brain regions, the arbitrary ratio between the specific activity of cholinesterase and the density of muscarinic binding sites decreased with development, with average values and variability ranges of 83 ± 50 and 19 ± 19 at 14 and 24 weeks, respectively. Our findings suggest divergent regulation for cholinergic binding sites and cholinesterase in the fetal human brain and imply that the expression of muscarinic receptors is related to the development of cholinergic transmission, while acetylcholinesterase is also involved in other functions in the fetal human brain.
Key wordsacetylcholinesterase muscarinic receptors human brain cholinergic transmission
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- Chubb, I. (1984). Acetylcholinesterase-multiple functions. InCholinesterases—Fundamental and Applied Aspects (Brzin, M., Kiauta, T., and Barnard, E. A., Eds.), Walter de Gruyter, Berlin, pp. 345–349.Google Scholar
- Egozi, Y., Kloog, Y., and Sokolovsky, M. (1980). Studies of post-natal changes of muscarinic receptors in mouse brain. InNeurotransmitters and Their Receptors (Littauer, U. Z., Dudai, Y., Silman, I., Teichberg, V. I., and Vogel, Z., Eds.), John Wiley and Sons, New York, pp. 201–215.Google Scholar
- Karczmar, A. G. (1976). Central actions of acetylcholine, cholinomimetic and related drugs. InBiology of Cholinergic Function (Goldberg, A. M., and Hanin, I., Eds.), Raven Press, New York, pp. 395–449.Google Scholar
- Muller, F., Dumez, Y., and Massoulie, J. (1985). Molecular forms and solubility of acetylcholinesterase during the embryonic development of rat and human brain.Brain Res. (in press).Google Scholar
- Olivier, G., and Pineau, H. (1961). Horizons de Streeter et age embryonnaire.Bull. Assoc. Anat. (Nancy)47e573–576.Google Scholar
- Rakic, P. (1981). Developmental events leading to laminar and areal organization of the neocortex. InThe Cerebral Cortex (Schmidt, F. D., Ed.), MIT Press, Cambridge, Mass., pp. 7–28.Google Scholar