Presynaptic endosomes are intracellular organelles involved in the recycling and degradation of receptors and membrane proteins. They also play a role in maintaining the pool of synaptic vesicles and in the recycling of dense-core vesicle membranes during neurotransmitter release. Although the molecular mechanisms involving presynaptic endosomes are not yet well characterized, the small GTPase Rab5, the SNARE protein Vti1ab, and the adaptor protein complex AP-3 have been identified as components of the endosomal trafficking. In addition, a 'signaling endosome' functioning parallel to the degradation pathway has been implicated in the transport of neurotrophic signals and toxins from the synapse.