Mini inhibition

During an action potential neurons release a huge bolus of neurotransmitter, which can alter protein synthesis in the receiving cell and reshape synapses during learning. But neurons also release single packets of neurotransmitter in a background dribble that has been largely ignored. Now, Michael Sutton, Erin Schuman, and colleagues (Caltech, Pasadena, CA) have found that these “minis” inhibit translation in dendrites. This may help neurons to start from a lower basal level, thus making the increase in translation after action potentials more dramatic.

The Caltech team “stumbled upon [the effect] by accident,” says Sutton. “The idea was to remove all synaptic activity and add it back,” while looking for changes in dendritic protein synthesis. Inhibition of action potentials reduced protein synthesis, but inhibition of both action potentials and minis led to an increase in protein synthesis.

The minis have been presumed to lack any function, so the “potential effects of minis on neuronal physiology have been largely ignored,” says Sutton. The frequency of minis does increase or decrease when synapses are strengthened or weakened, respectively, during long term potentiation (LTP) and long term depression (LTD). In theory, an increase in minis and thus decrease in protein synthesis during LTP could be a negative feedback mechanism. But in other experiments minis were necessary for the maintenance of dendritic spines during sustained blockage of action potentials. Resolving the minis' ultimate effect will require further work in cultured cells or tissue slices. ▪

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