Problems associated with the use of tetraethylammonium to test for monosynaptic connexions.

Several investigators of the molluscan nervous system have used TEA, injected into presynptic neurones, to determine whether the connexions made by these neurones are monosynaptic. The increase in spike duration produced by the TEA causes an increase in transmitter release, and hence an increase in the amplitude of the postsynaptic potential if the connexion is direct. If the connexion is indirect, the spike in an intercalated neurone will not be affected by the TEA, and the postsynaptic response will remain constant. Experiments described here show that TEA can cross electrotonic junctions in the gastropod mollusc Planorbis corneus. They also show that each TEA-prolonged presynaptic impulse may produce more than one postsynaptic impulse. A larger postsynaptic potential could therefore be produced by presynaptic injection of TEA in the case of an indirect connexion. This indicates that care must be taken when interpreting the results of experiments using TEA to test for monosynaptic connexions.