The Evoked Potential in the Slice of Guinea Pig Hippocampus, and the Effects of Zinc on It

1) A thin slice was prepared from the hippocampus og guinea pig. It was activated by the direct electrical stimulus and the potential was recorded at the dissected surface nearby the stimulating point. 2) The recorded potential, called as evoked one in the present paper, was biphasic : i. e. a slow negative wave of 200～400 msec in duration was followed by a positive one of about the same duration and small amplitude. The potential reversed in polarity, when the slice was inverted and it was recorded from the surface of pia mater. 3) Acetylcholine (1×10-6 M), in the presence of physostigmine, depressed the evoked potential in amplitude, but the potential recovered in 10 minutes when the slice was washed with the Krebs-Ringer's solution discontaining the drug (normal solution). 4) When chloride ion in the incubating medium was completely replaced by propionate ion, the same stimulus provoked a train of discharges, which could not be observed after washing with the normal solution. 5) In the presence of zinc chloride, the decreaseof ampitude and the prolongation af the duration of the potential, especially the slowing of faling phase, were observed. The threshold concentration was 5×10-6 molar. The changes of the potential did not recover after washing. 6) A block of impilse-transmission at the presynaptic terminal, a change in excitability of postsynaptic membrane and/or of neuron-soma and an alteration of inhibitory synaptic action of afferent impulse were discussed as the possible affective mechanisms of zinc.