Recent studies suggest that brain neurons require extracellular signals for continued survival during maturity as well as development. However, factors underlying the survival of specific populations of central neurons remain to be defined. To examine the regulation of neuronal survival, we have studied the substantia nigra (SN) dopaminergic (DA) system, in dissociated cell culture. DA neuron number was monitored immunocytochemically with antibody to tyrosine hydroxylase (TH), the DA biosynthetic enzyme. Initially, mixed cultures were grown at low, medium, and high densities in serum-containing media. After 7 days, the number of neuron-specific enolase (NSE)-positive cells, a measure of total neuron number, was proportional to cell plating density. In contrast, high density culture elicited a marked, disproportionate increase in TH-immunopositive cells, suggesting that high density conditions selectively enhanced the DA subpopulation. To define the role of cellular interactions in the selective increase in DA cells, virtually pure neuron cultures were compared to support cell-neuron cocultures, in fully defined medium. In support cell-neuron cocultures, SN support cells evoked a four-fold increase in TH cells, while NSE number did not differ from controls. Moreover, local support cells elicited a greater increase in TH cell number than support cells derived from other brain regions. To determine whether increased TH cell number reflected enhanced survival, or possibly expression of TH by new populations, we monitored the time course of this effect. TH cell number remained constant after 3 days in cocultures, while declining fourfold in controls. In parallel studies, support cells were added to SN dissociates at zero time or after 3 days.(ABSTRACT TRUNCATED AT 250 WORDS)