Molecular substrates mediating lanthanide-evoked neurotransmitter release in central synapses

Noncanonical secretagogues such as hypertonicity or α-latrotoxin have been extremely informative in studying neurotransmission. Lanthanum and lanthanides can also trigger neurotransmitter release through an unknown mechanism. Here, we studied the effect of lanthanides on neurotransmission in hippocampal cultures. Application of 2 mM La³⁺ caused rapid and robust neurotransmitter release within seconds. In addition, transient application of La³⁺ uncovered a sustained facilitation of miniature neurotransmission. The response to La³⁺ was detectable at 2 μM and increased in a concentration-dependent manner ≤2 mM. Rapid effect of La ³⁺ was independent of extracellular and intracellular Ca²⁺ and did not require La³⁺ entry into cells or activation of phospholipaseCβ. Synapses deficient in synaptobrevin-2, the major synaptic vesicle soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein in the brain, did not display any rapid release in response to La³⁺, whereas the slow facilitation of release detected after La ³⁺ removal remained intact. In contrast, preincubation with intracellular Ca²⁺ chelators selectively attenuated the delayed release triggered by La³⁺. Moreover, synapses deficient in synaptotagmin-1 maintained a rapid response to La³⁺, suggesting that La³⁺-triggered neurotransmitter release does not require synaptotagmin-1 as a sensor. Therefore La³⁺ has two separate effects on synaptic transmission. For its rapid action, La³⁺ interacts with a target on the surface membrane, and unlike other forms of release, it triggers strictly synaptobrevin-2-dependent fusion, implying that in central synapses synaptobrevin-2 function is secretagogue specific. For the delayed action, La³⁺ may act intracellularly after its entry or through intracellular Ca²⁺ via a mechanism that does not require synaptobrevin-2.