Selective depression of synaptic transmission by tetanus toxin: a comparative study on hippocampal and neostriatal slices.

Tetanus toxin reduces the release of neurotransmitters in several brain areas. We have studied the effects of this toxin on the intrinsic and synaptic activity of CA1 hippocampal neurons and of neostriatal cells in the rat by utilizing intracellular and extracellular recordings from slice preparations. Tetanus toxin (10 micrograms/ml) applied by bath produced an increase of the field potentials evoked by the orthodromic stimulation of the CA1 region coupled with the disappearance of the inhibitory period following the first conditioning stimulus. Orthodromically and antidromically activated postsynaptic hyperpolarizing potentials were also decreased and spontaneous bursting activity was observed following the application of tetanus toxin. At this concentration the toxin did not alter excitability in neostriatal cells and in these neurons even 30-50 micrograms/ml of the toxin produced only a slight increase of excitability. Higher concentrations (100 micrograms/ml) of tetanus toxin reduced the excitatory synaptic transmission in the hippocampus as well as in the neostriatum. The toxin (10-100 micrograms/ml) did not alter membrane potential, input resistance or directly evoked firing in both these structures. We conclude that, although the toxin's mechanisms of action in the neostriatum are similar to those operating in the hippocampus (i.e. decrease of inhibitory and/or excitatory inputs), the local synaptic circuits produce differential electrophysiological effects in these two structures.