Brevetoxins (PbTx-1 to PbTx-10) are potent lipid-soluble polyether neurotoxins produced by the marine dinoflagellate Karina brevis, an organism associated with 'red tide' blooms in the Gulf of Mexico. Ingestion of shellfish contaminated with K. brevis produces neurotoxic shellfish poisoning (NSP) in humans. NSP symptoms emanate from brevetoxin activation of neurotoxin site 5 on voltage-gated sodium channels (VGSC) [Toxicon 20 (1982) 457]. In primary cultures of rat cerebellar granule neurons (CGN), brevetoxins produce acute neuronal injury and death. The ability of a series of naturally occurring and synthetic brevetoxins to trigger Ca(2+) influx in CGN was explored in the present study. Intracellular Ca(2+) concentration was monitored in fluo-3-loaded CGN using a fluorescent laser imaging plate reader. The naturally occurring derivatives PbTx-1, PbTx-2 and PbTx-3 all produced a rapid and concentration-dependent increase in cytosolic [Ca(2+)]. The maximum response to PbTx-1 was approximately two-fold greater than that of either PbTx-2 or PbTx-3. Two synthetic derivatives of PbTx-3, alpha-naphthoyl-PbTx-3 and beta-naphthoyl-PbTx-3, were also tested. Both alpha- and beta-naphthoyl-PbTx-3 stimulated a rapid and concentration-dependent Ca(2+) influx that was, however, less efficacious than that of PbTx-3. These data indicate that, analogous to neurotoxin site 2 ligands, activators of neurotoxin site 5 display a range of efficacies, with PbTx-1 being a full agonist and other derivatives acting as partial agonists.