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Nerve terminal probes are a series of cationic styryl fluorescent dyes. The probes typically have a lipophilic tail and a highly hydrophilic head with cations. The probes are non-toxic to cells and essentially non-fluorescent in aqueous solution. They emit strong fluorescence once inserted into the outer layer of the cell membrane. Simply wash to remove non-specific cell surface membrane staining before imaging.
Characteristics of nerve terminal probes
Nerve terminal probes function by activity-dependent staining of synaptic vesicles. When the probes are incubated with cells or tissues, the aqueous portion of the probes are non-fluorescent, while the lipophilic tail of the probes insert into the cell membrane and exhibit strong fluorescence. After nerve stimulation, the nerve terminal probes are encapsulated in vesicles during endocytosis, so after washing away the probes attached to the cell surface, the intensity of the fluorescent signal indicates the number of newly formed vesicles. Conversely, upon exocytosis, the probes are released from the vesicle along with the neurotransmitter, resulting in a decrease in fluorescent signal.
Thus, changes in fluorescence intensity reflect either endocytosis/exocytosis or synaptic activity. The rate at which fluorescence increases during endocytosis - the "on-rate" and the rate at which it decreases during exocytosis - the "dissociation rate" varies by probes species. In general, probes with longer lipophilic tails and more double bonds have higher affinity for membranes, and therefore they have higher on-rates and lower off-rates.
Application of nerve terminal probes
Nerve terminal probes can be used to address dye fixation and permeabilization of cells for immunohistochemistry experiments. Nerve terminal probes are also used to track synaptic activity at neuromuscular junctions or synapses.