Sympathetic neuron density differentially regulates transmitter phenotypic expression in culture.

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Abstract
The effects of cell density and aggregation on expression of transmitter traits were examined in dissociated, pure [rat] sympathetic neuron cultures, grown in fully defined, serum-free medium. After 1 wk at a density of 7-8 .times. 103 neurons per 35-mm dish, moderate levels of tyrosine hydroxylase (tyrosine 3-monooxygenase, EC 1.14.16.2) activity and substance P were detected. When neuron density was increased 4-fold, a 4-fold increase in tyrosine hydroxylase activity was observed; i.e., there was no change in tyrosine hydroxylase activity per neuron. Substance P increased 30-fold, corresponding to a 7-fold increase in substance P per neuron. Choline O-acetyltransferase (EC 2.3.1.6) activity, not detected at low cell densities, was first detectable at a concentration of 15,000 neurons/dish and increased 6-fold when this cell concentration was doubled. Medium conditioned by high-density cultures failed to reproduce these effects on low-density cultures, suggesting that diffusible factors are not involved in the density-dependent differential regulation. Time-lapse phase-contrast microscopy of high-density cultures showed neuronal migration and progressive aggregation, which did not occur in low-density cultures. Cell contact may mediate differential expression of transmitter traits.