Blocking action of intracellularly injected neuraminidase on central synapses in vivo

The effect of neuraminidase on synaptic transmission was studied at cholinergic and noncholinergic contacts in the buccal and cerebral ganglion of Ap]lysia. The amplitudes of monosynaptic unitary postsynaptic potentials generated by intracellular stimulation of identified presynaptic neurones were measured as indication for the efficacy of synaptic transmission. Neuraminidase was either intrasomatically injected into a presynaptic neurone, or the whole ganglion was incubated with the enzyme. Intrasomatic injection of the enzyme resulted in complete failure of synaptic transmission. This effect occurred independently of the transmitter used. The synaptic failure was presynaptic in origin. The biophysical characteristics of an injected neurone, particularly the amplitude and propagation of its action potential, did not appear to be affected by neuraminidase. Synaptic transmission and biophysical membrane properties were unaffected by extracellular neuraminidase. We conclude that the synaptic blockade is due to the enzyme's action inside the presynaptic nerve ending. It seems most likely that neuraminidase cleaves sialic-acid-containing-compounds associated with the nerve terminal surface membrane, probably thus causing failure of transmitter release.