Torpedo electromotor system development: Biochemical differentiation of Torpedo electrocytes in vitro

Abstract

The accumulation of 2 postsynaptic proteins—the acetylcholine receptor and acetylcholinesterase, total protein and lactate dehydrogenase levels, and the evolution of the multiple molecular forms of acetylcholinesterase (exhibiting apparent sedimentation coefficients of 17, 13, 11 and 6S) have been examined in aneural cultures of embryonic Torpedo electric organ explanted before, during or after electrocyte differentiation and the onset of synaptogenesis. During electrocyte differentiation in vitro, with explants taken before the 38 mm stage, the relative proportions of the 17, 13 and 11S forms change in vitro as in vivo but the 6S form remains abnormally dominant. In tissue explants taken from 38 to 47 mm stage embryos, the 4 major molecular forms of acetylcholinesterase differentiate in a manner identical to that observed in vivo. In explants taken after the onset of synaptogenesis (55–80 mm stages), the proportions of the acetylcholinesterase forms change as in vivo only during the first week in vitro whilst accumulation is occurring at the normal in vivo rate. The switch to the high acetylcholine receptor and acetylcholinesterase accumulation rate that occurs when synaptogenesis begins in vivo is not observed after any time lag in vitro with tissue explanted before the stage (55 mm) at which synaptogenesis begins. The effects on acetylcholinesterase and acetylcholine receptor accumulation of supplementing the medium with a neural tissue extract are described. The experiments were designed to elucidate the factors and mechanisms that regulate the differentiation and formation of chemical synapses using the electric organ of Torpedo marmorata as a model system. The results demonstrate that the complex changes occurring in the multiple molecular forms of acetylcholinesterase during electrocyte differentiation are not under direct neural control but that the switch to an increased acetylcholinesterase and acetylcholine receptor accumulation rate may be triggered by an external, possible neural factor.