Challenging catecholamine exocytosis with pardaxin, an excitatory ionophore fish toxin

Pardaxin is an excitatory polypeptide toxin from the Moses sole fish which triggers neurotransmitter release from a variety of nerve terminals. We have used several pharmacologial manipulations for studying the mechanisms of pardaxin-induced neurotransmitter release using catecholaminergic neurons. At noncytotoxic concentrations (<10^-5M) pardaxin stimulated exocytosis as assessed by the concomitant release of catecholamines, ATP and dopamine-ßhydroxylase in the presence or absence of extracellular calcium. The role of intracellular calcium in pardaxin action was investigated by using the fluorescent calcium indicator Fura-2 and ⁴⁵Ca fluxes. In the presence, but not in the absence, of extracellular calcium, pardaxin produced a steady, concentration-dependent rise in intracellular calcium. The pardaxin-induced calcium influx is partially inhibited (15% by nifedipine, occurred in PC12 cells in which L-calcium channels were down-regulated by Bay K-8644, indicating a major role of pardaxin pores in the calcium influx. The fact that pardaxin also stimulated catecholamines secretion without a rise in intracellular calcium suggest that the toxin in addition to being a pore-forming compound might affect exocytosis by calcium independent pathway(s). A pharmacological working model is proposed for studying the cellular signals induced by pardaxin to trigger neurotransmitter exocytosis. We suggest pardaxin as a pharmacological ionophore tool to modulate and investigate neuronal calcium homeostasis and synaptic transmission.