NONSYNAPTIC DIFFUSION NEUROTRANSMISSION (NDN) IN THE BRAIN

The synapse has dominated the conceptual model of neurotransmission; o ther mechanisms, such as neuromodulation, have been considered to supp ort and complement synaptic transmission. In this commentary, the conc eptual framework considers synaptic transmission as one of several mec hanisms of neurotransmission. One of these is nonsynaptic diffusion ne urotransmission (NDN), which includes both the diffusion of neurotrans mitters and other neuroactive substances through the extracellular flu id to reach extrasynaptic receptors, and the diffusion of substances s uch as nitric oxide through both the extracellular fluid and cellular membranes to act within the cell. The possible roles of NDN in mass, s ustained functions such as mood, sleep and brain ''tone'', as well as in various other functions, such as in long term potentiation, at the retinal, lateral geniculate nucleus and visual cortex levels of the vi sual system, in recovery from brain damage and in neuropharmacology, a re explored.