A syntaxin-related protein controls acetylcholine release by different mechanisms in Aplysia

Polyclonal antibodies raised against rat syntaxin-1B and an affinity-purified fraction have been used to study the functional role of this protein in transmitter release from Aplysia neurons. In a ganglionic protein extract, this fraction recognized a 37,000 molecular weight protein which therefore might be the Aplysia homologue of rat brain syntaxin-1B. Immunoglobulins were injected in the presynaptic cell of an identified cholinergic synapse of the buccal ganglion of Aplysia californica. This treatment decreased the postsynaptic response due to a reduction of the number of quanta released in relation to a decline of presynaptic Ca2+ current. When antibodies were applied extracellularly, transmitter release also decreased. In contrast to intracellular injection, this reduction was not accompanied by a decrease of the Ca2+ current 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 neurons express a syntaxin-like protein which is involved in the control of the presynaptic Ca2+ influx triggering acetylcholine release from terminals. This protein appears to have an extracellular segment which might interact with outward current.