Membrane contact regulates transmitter phenotypic expression.

High cell density, with attendant aggregation, selectively increases expression of substance P (SP) and choline acetyltransferase (ChAT) in virtually pure neonatal sympathetic neuronal cultures. To investigate the specific role of cell contact in selective transmitter expression, SP content and ChAT activity were examined in such cultures under various conditions. At high neuronal density SP content, detectable 6 h after plating, doubled during the first two culture days and subsequently increased more than 10-fold. Similarly, ChAT activity appeared de novo after two days and rose rapidly thereafter. The increases closely paralleled perikaryal aggregation, suggesting that cell contact might be the critical factor. Moreover, interference with aggregation physically, using methylcellulose, or chemically, using tunicamycin, inhibited the increases in SP content and ChAT activity without affecting neuronal survival. Thus, cell contact appears to mediate the expression of ChAT and the rise of SP in high-density neuronal cultures. To determine whether interaction of membrane component(s) elicited the rises in ChAT activity and SP content, membranes extracted from the neonatal superior cervical ganglion (SCG) were added to cultures of varying densities. After 3 days in high-density cultures, membranes doubled the increases in ChAT and SP. Moreover, even in lower-density cultures, membranes elicited the appearance of ChAT activity. Specificity was defined by examining membranes extracted from a variety of neonatal rat tissues. Dorsal root ganglia membranes were most effective in stimulating ChAT, followed by membranes from the SCG, kidney and brain. Membranes derived from the adrenal gland, liver and spinal cord had no effect. Our findings suggest that interaction of cell membrane components regulates phenotypic expression in aggregating neurons.