ALPHA-LATROTOXIN IS A POTENT INDUCER OF NEUROTRANSMITTER RELEASE IN TORPEDO ELECTRIC ORGAN FUNCTIONAL AND MORPHOLOGICAL CHARACTERIZATION

In this report we show that alpha-latrotoxin from black widow spider v enom is a potent activator of neurotransmitter release in synaptosomes from the Torpedo electric organ. Binding of the purified toxin (5 nM) to the synaptosomal fraction occurs already at 4 degrees C and is dep endent on the presence of divalent ions. However, neurotransmitter rel ease commences only after temperature elevation (22 degrees C) and is completed within 2 min. The effect of alpha-latrotoxin on release is a chieved at 1 nM and is already saturated at 5 nM. The release is stimu lated by the presence of Ca2+ ions. Activation of release by alpha-lat rotoxin is accompanied by morphological changes in electric organ syna ptosomes. The synaptosomes swell, resulting in a 55% increase in secti on area. Moreover, the number of synaptic vesicles per unit area decre ases about three-fold, and rows of docked synaptic vesicles are rarely detected as opposed to control synaptosomes. These morphological chan ges indicate that the massive release is mainly due to synaptic vesicl e fusion. alpha-Latrotoxin binding sites are highly concentrated in th e innervated face of the electrocytes. Immunoelectron microscopy on el ectric organ sections reveals alpha-latrotoxin binding sites over the entire plasma membrane at release sites and facing Schwann cells surro unding Torpedo nerve terminals. Surprisingly, a high concentration of binding sites is also found at structures surrounding branching unmyel inated axons. This staining is in close proximity to Schwann cell enve lopes and to the basal lamina around axonal tips. The mode of action o f alpha-latrotoxin in view of the localization of its binding sites is discussed.