Characterization and distribution of acetylcholine receptors and acetylcholinesterase during electric organ development in Torpedo marmorata

The changes that occur in the distribution and properties of the nicotinic acetylcholine receptor and acetylcholinesterase during the development of the electric organ of Torpedo marmorata have been investigated. At early stages of development, both proteins are distributed diffusely over the myotube surface and with differentiation of the myotubes into electrocytes, they become increasingly restricted to the ventral cell surface. This process occurs before axons contact the electrocytes. The concentrations of the acetylcholine receptor and acetylcholinesterase remain at rather low and stable levels during these developmental changes. The acetylcholine receptor concentration begins to increase rapidly as soon as electromotor axons begin to contact the electrocytes. No significant differences in the subunit composition, affinity for d-tubocurarine, isoelectric point or immunochemical properties of the embryonic acetylcholine receptors were detected when they were compared to the receptors of the adult electric organ. The onset of receptor accumulation occurs before the increase in the amount of 17 S acetylcholinesterase, suggesting that increases in acetylcholine receptor and in acetylcholinesterase are not regulated by the same mechanisms. The various molecular forms of acetylcholinesterase undergo characteristic changes during development. Sequential extraction of the esterase forms indicates that their interaction with cellular compartments changes during development. The solubility properties of the esterase forms suggest that most of the 17 S and 13 S acetylcholinesterase become strongly associated with cellular components via ionic interactions and that a hydrophobic 6 S form begins to accumulate at later embryonic stages when the number of mature presynaptic terminals is beginning to show its rapid phase of increase. The results show that events concerning the distribution and some properties of essential components of the electromotor synapse occur during early embryonic development before synaptogenesis, whereas the sequential accumulation of acetylcholine receptor and esterase begins after the axons start contacting the electrocytes. This suggests that the ingrowing nerves exert some regulative influence on the metabolic state of the electrocytes during development