Axon terminals always form synapses rather than releasing neurotransmitters into the extracellular space

Charles Ducrot, Marie-Josée Bourque, Constantin V. L. Delmas, Anne-Sophie Racine, Dainelys Guadarrama Bello, Benoît Delignat-Lavaud, M. D. Lycas, Aurélie Fallon, Charlotte Michaud-Tardif, Samuel Burke, Freja Herborg, U. Gether, A. Nanci, Hideto Takahashi, M. Parent, L. Trudeau. Dopaminergic neurons establish a distinctive axonal arbor with a majority of non-synaptic terminals. 2020. https://doi.org/10.1096/fj.202100201RR

Shows that monoamine neurons, such as dopaminergic neurons, establish a majority of non-synaptic terminals that release neurotransmitters without forming classical synapses.

Gianni G, Pasqualetti M. Wiring and Volume Transmission: An Overview of the Dual Modality for Serotonin Neurotransmission.. 2023. https://doi.org/10.1021/acschemneuro.3c00648

Discusses volume transmission as a dual modality where neuroactive molecules undergo extrasynaptic diffusion rather than point-to-point wiring transmission.

Miklós Antal. Molecular Anatomy of Synaptic and Extrasynaptic Neurotransmission Between Nociceptive Primary Afferents and Spinal Dorsal Horn Neurons. 2025. https://doi.org/10.3390/ijms26052356

Reports that certain molecules like neuropeptides are released extrasynaptically to modulate neuronal function via volume transmission rather than forming direct synaptic contacts.

Paul Bach-y-Rita. Nonsynaptic Diffusion Neurotransmission and Brain Plasticity. 1996. https://doi.org/10.1177/107385849600200511

Explains nonsynaptic diffusion neurotransmission, where neurotransmitters diffuse through the extracellular fluid from remote release points to activate extrasynaptic receptors.