The maitotoxin-evoked Ca2+ entry into synaptosomes is enhanced by cholera toxin

Effects of the Gs protein-mediated adenylate cyclase facilitatory system on Ca2+ entry into synaptosomes were studied, using two specific toxins. A putative Ca2+-channel agonist, maitotoxin (MTX), increased the 45Ca2+ entry and [Ca2+]i, determined with Quin-II, into synaptosomes of the rat brainstem, which were not attenuated by nifedipine. However, another Ca2+-channel agonist, BAY K-8644 did not alter the 45Ca2+ entry nor [Ca2+]i. The MTX-induced increase of the 45Ca2+ entry was significantly enhanced by addition of dibutyryl cyclic adenosine monophosphate and by the pretreatment with cholera toxin. These findings support the view that stimulation of presynaptic receptors coupled to the Gs-adenylate cyclase system may lead to a facilitation of the release of neurotransmitters, through a cAMP dependent enhancement of the opening of the Ca2+ channels located on nerve terminals.