Neurexin Iα is a major α-latrotoxin receptor that cooperates in α- latrotoxin action

α-Latrotoxin is a potent neurotoxin from black widow spider venom that binds to presynaptic receptors and causes massive neurotransmitter release. A surprising finding was the biochemical description of two distinct cell surface proteins that bind α-latrotoxin with nanomolar affinities; Neurexin Iα binds α-latrotoxin in a Ca²⁺-dependent manner, and CIRL/latrophilin binds in a Ca²⁺-independent manner. We have now generated and analyzed mice that lack neurexin Iα tot est its importance in α-latrotoxin action. α- Latrotoxin binding to brain membranes from mutant mice was decreased by almost 50% compared with wild type membranes; the decrease was almost entirely due to a loss of Ca²⁺-dependent α-latrotoxin binding sites. In cultured hippocampal neurons, α-latrotoxin was still capable of activating neurotransmission in the absence of neurexin Iα. Direct measurements of [³H] glutamate release from synaptosomes, however, showed a major decrease in the amount of release triggered by α-latrotoxin in the presence of Ca²⁺. Thus neurexin Iα is not essential for α-latrotoxin action but contributes to α-latrotoxin action when Ca²⁺ is present. Viewed as a whole, our results show that mice contain two distinct types of α-latrotoxin receptors with similar affinities and abundance but different properties and functions. The action of α-latrotoxin may therefore be mediated by independent parallel pathways, of which the CIRL/latrophilin pathway is sufficient for neurotransmitter release, whereas the neurexin Iα pathway contributes to the Ca²⁺-dependent action of α-latrotoxin.