NEUROTENSIN INDUCES CALCIUM OSCILLATIONS IN CULTURED AMACRINE CELLS

The peptide, neurotensin, is found in a class of amacrine cells synaps ing chiefly with other amacrine cells in the chicken retina (Li & Lam, 1990; Watt et al., 1991). To investigate the possible effects of neur otensin, we have used Ca2+ imaging to measure cytosolic Ca2+ concentra tions in cultured chick amacrine cells. Following a delay of about 2 m in, neurotensin (300 nM) induced oscillations in Ca2+ concentration th at typically had a period of 2 min and peak values of about 300 nM whe n averaged over the cell body. The phospholipase C inhibitors U-73,112 and 4'-bromophenacyl bromide terminated oscillations induced by neuro tensin but the protein kinase inhibitors H7 and staurosporine did not inhibit oscillations, increasing their frequency instead. In the absen ce of external Ca2+, neurotensin induced only a single Ca2+ transient, much briefer than when external Ca2+ was present. Together these resu lts suggest that neurotensin activates phospholipase C, thereby produc ing IP3 that triggers Ca2+ release from an internal store. Although th is released Ca2+ contributes to periodic Ca2+ peaks, the majority of c ytosolic Ca2+, even in the first peak, comes from Ca2+ influx across t he plasmalemma.