[Distribution and metabolism of cholinergic receptor sites during synaptogenesis]

The effects of a reduction or an increase in muscular activity during embryonic life were investigated in the posterior latissimus dorsi muscle of the chick embryo. The biochemical differentiation, number and localization of the developing neuromuscular junctions were studied after curarization or chronic medullar stimulation. The degradation of junctional as well as extrajunctional PLD cholinergic receptor sites was constant (about 28 h) up to day 10 after hatching, and therefore the disappearance of extrajunctional ACh-R at the end of embryonic life was due to a shut-off of their synthesis. Curarization did not modify the focal innervation of PLD fibres, whereas both the density of extrajunctional ACh-R and neurites per fibre increased. On the other hand, chronic stimulation led to the formation of supernumerary ACh-R clusters of PLD fibres with a parallel increase in the total number of spots of acetylcholinesterase activity per muscle.