B R I E F N O T E S

Cultured cells cloned from the C1300 neuroblastoma have been shown by several laboratories to extend neurites (1-6) and to accumulate nervespecific enzymatic activities (7) when grown under conditions in which cell division is inhibited. Study of this system has been stimulated by the hope that it might serve as an in vitro model of nerve cell differentiation. Many of the neurite-inducing agents, including serum deprivation (3), prostaglandins (4), theophylline (5), and analogues of c A M P 1 including dibutyryl c A M P (B~cAMP), are thought to act principally by raising the intracellular concentrations of cAMP. Division is inhibited in all these cases, as well as in non-neural cell cultures treated with c A M P analogues (8). The observed stimulation of neurite extension in cAMP-trea ted neuroblastoma cultures might be explained in either of two ways. The "di rec t" explanation is that c A M P itself stimulates neurite outgrowth, and inhibits division only coincidentally. The " indirect" hypotheses maintains that since neurites grow during interphase and are retracted during mitosis (3), c A M P permits neurite extension by stopping the cell cycle in interphase. The action of c A M P under either hypothesis might or might not require interaction with or alteration of nuclear activity. We report here that enucleated neuroblastoma