Target organ regulation of substance P in sympathetic neurons in culture

Target organ regulation of the putative, peptide neurotransmitter, substance P (SP) was examined in explants and dissociated cell cultures of the neonatal rat sympathetic superior cervical ganglion (SCG). SP levels increased dramatically in explants, rising more than 30-fold after 72 hr in culture. By contrast, peptide levels did not increase in dissociated ganglion cultures. However, SP increased almost 10-fold in cell cultures grown on a monolayer of cells derived from the pineal or salivary gland, targets of the SCG. By contrast, SP content did not increase in cultures grown on a substrate of cells derived from heart or intestine. Peptide identity in the SCG-target cocultures was authenticated by means of combined high-pressure liquid chromatography (HPLC)-radioimmunoassay. Moreover, immunohistochemical examination localized the peptide virtually exclusively to sympathetic neurons and nerve processes. Mechanisms mediating the sympathetic-target interaction were examined in SCG-pineal cocultures. The increase in peptide required interactions with living tissue, since substrates of killed target cells did not elevate SP levels. The target influences were not mediated by nerve growth factor or indoleamines, potential secretory products of pineal in culture. Veratridine treatment prevented the increase in SP in the cocultures, and tetrodotoxin blocked the veratridine effect, suggesting that sodium influx and membrane depolarization prevent SP elevation. Our observations suggest that sympathetic neuron interactions with target organs influence peptidergic expression, and that this interaction may be restricted to certain appropriate target structures.