Torpedo electromotor system development: Developmentally regulated neuronotrophic activities of electric organ tissue

Explant cultures of electric lobe from 45–60 mm stage Torpedo embryos and both ganglionic and dissociated cell cultures prepared from 8‐day chick ciliary ganglia have been used to determine whether the electric organs of Torpedo marmorata contain developmentally regulated neuronotrophic activity. Electric lobe explants were evaluated by measuring their neurone density choline acetyltransferase (CAT), and low salt, Triton X‐100‐soluble protein contents. Addition of soluble extracts prepared from the electric organs of late stage embryos (8–105 mm) to standard medium results in the maintenance of nearly theoretical neurone densities in electric lobe explants during a 7‐day culture period. Soluble electric organ extracts from early embryonic stages (42–59 mm) do not increase neurone density relative to control cultures but cause an elevation in the CAT content of the explants over control values. On the basis of this analysis it is concluded (1) that late embryonic stage and adult electric organs contain neuronotrophic activity that allows electromotor neurones to survive in vitro and (2) that activity increases rapidly in the electric organs between the 59 and 72 mm stages of development at a time when rapid increases in postsynaptic membrane markers in the electric organs occur and when peripheral synaptogenesis begins. The activity of late stage embryonic electric organs is heat stable and lost on dialysis. Using ciliary ganglion explants and evaluating both the initial fibre outgrowth and the CAT content after 4 days in vitro, trophic activity is found to be maximal at early embryonic stages (45–55 mm) and to decline thereafter. It is shown that the decline in activity is not due to an increase in toxicity. Using established dissociated ganglionic cell survival assays the specific activity of neuronotrophic factors allowing survival is constant between the 45 and 73 mm stages in the electric organs and then rapidly declines, but activity per electric organ increases rapidly between the 45 and 73 mm stages and then remains at a constant level. The use of poly‐dl‐ornithine substrates coated with heart‐conditioned medium for the cell survival assay results in up to tenfold increases in the trophic titre of the electric organ extracts. The neuronotrophic activity supporting survival of ciliary motorneurones present in embryonic electric organs is heat labile and retained on dialysis. It is concluded that developing electric organs contain at least two neuronotrophic factors that have different properties and are differently regulated. Both factors may contribute during development to bringing naturally occurring electromotor neurone cell death to an end.