Neurotransmitter plasticity of cultured sympathetic neurones. Are the effects of muscle-conditioned medium reversible?

Muscle-conditioned medium (CM) induces choline acetyltransferase (CAT) activity in primary cultures of new-born rat sympathetic neurones and depresses the development of tyrosine hydroxylase (TOH). By following these two enzymes, we have determined whether (1) the effects of CM are reversible and (2) the neurones progressively lose their sensitivity to CM with time in culture. When neurones were cultured in the presence of 50% CM (CM+ medium), TOH activity developed slowly but CAT activity developed at a high rate. When the cultures were then switched to unconditioned medium (CM- medium), CAT activity remained elevated and continued to develop at higher rate than in cultures that were never exposed to CM. On the other hand, the switch to CM- medium was accompanied by a transition from a low to a high rate of TOH development. CAT induction by CM was thus essentially irreversible, whereas the impairment of TOH development was fully reversible. Conversely, we studied the effects of altering CM- to CM+ medium at progressively later culture days. CAT remained fully inducible for at least 2 to 3 weeks. On the other hand, TOH activity, which initially developed rapidly in CM- medium, first decreased to low levels after a switch to CM+ medium and then increased again, but at a slower rate. Neuronal depolarization by elevated K+ and exposure to CM have mirror-image, and antagonistic, effects on both CAT and TOH developments (Raynaud et al. 1987a). Walicke, Campenot & Patterson (1977) showed that a previous depolarization reduced the induction of cholinergic traits by a subsequent exposure to CM. We found that (1) such a depolarization only delayed the induction of CAT by several days and did not prevent the transition to a state of low TOH expression caused by CM and (2) an exposure of the cultures to elevated K+ after exposure to CM did not cause a decline in CAT activity. These data thus suggest that a state of high TOH expression can superimpose on a previously induced state of elevated CAT expression, but that the induction of CAT caused by a delayed exposure to CM is accompanied by a transition from a high to a lower state of TOH expression. In addition, neuronal depolarization does not stabilize the noradrenergic phenotype in a permanent manner and can not reverse cholinergic expression of sympathetic neurones to a purely noradrenergic phenotype.