A SYNTAXIN-RELATED PROTEIN CONTROLS ACETYLCHOLINE-RELEASE BY DIFFERENT MECHANISMS IN APLYSIA

Polyclonal antibodies raised against rat syntaxin-1B and an affinity-p urified fraction have been used to study the functional role of this p rotein in transmitter release from Aplysia neurons. In a ganglionic pr otein extract, this fraction recognized a 37,000 molecular weight prot ein which therefore might be the Aplysia homologue of rat brain syntax in-1B. Immunoglobulins were injected in the presynaptic cell of an ide ntified cholinergic synapse of the buccal ganglion of Aplysia californ ica. This treatment decreased the postsynaptic response due to a reduc tion of the number of quanta released in relation to a decline of pres ynaptic Ca2+ current. When antibodies were applied extracellularly, tr ansmitter release also decreased. In contrast to intracellular injecti on, this reduction was not accompanied by a decrease of the Ca2+ curre nt but by an increase of presynaptic outward current. When injected in the presynaptic neuron, syntaxin complementary RNA also depressed Ca2 + current and transmission. This work provides evidence that Aplysia n eurons express a syntaxin-like protein which is involved in the contro l of the presynaptic Ca2+ influx triggering acetylcholine release from terminals. This protein appears to have an extracellular segment whic h might interact with outward current.