The effects of 4-aminopyridine and tetrodotoxin on the release of acetylcholine from rat striatal slices

Summary

• 1. Rat striatal slices were incubated in the presence of various concentrations of Ca²⁺ and K⁺ and of 4-aminopyridine and tetrodotoxin and the release of acetylcholine (ACh) into the medium was measured. In the presence of 2.5 mM Ca²⁺ and 5 mM K⁺, the release of ACh was increased by 230–410% when 0.1 mM 4-aminopyridine had been added to the medium. The effect of 4-aminopyridine was lower when the concentration of Ca²⁺ was diminished and became insignificant when Ca²⁺ had been omitted; it was also lower when the concentration of K⁺ was raised.

• 2. The release of ACh, measured at 5 mM K⁺ and 2.5 mM Ca²⁺, was inhibited by only 15% when 1 μM tetrodotoxin had been added to the medium, an observation indicating that there was little conducted impulse activity in the slices. In the presence of 4-aminopyridine, however, tetrodotoxin inhibited the release of ACh by more than 60%. When expressed in absolute values of nmoles of ACh released, the inhibitory effect of tetrodotoxin on the release of ACh was 12–19 times as high in the presence as in the absence of 4-aminopyridine. This indicates that the stimulation of the release of ACh produced by 4-aminopyridine was associated with a substantial increase in the impulse activity in the slices.

• 3. Most of the 4-aminopyridine-induced increase in the release of ACh from the slices incubated in 5 mM K⁺ can be explained by the increase in impulse activity and by a prolongation of nerve terminal depolarizations. However, 4-aminopyridine increased the release of ACh by more than 50% even in the presence of 1 μM tetrodotoxin. Available data do not permit to decide whether this part of the effect of 4-aminopyridine was due to its direct action on the Ca²⁺ channels in the nerve terminals or to its action on the K⁺ channels, only secondarily promoting the appearance of regenerative inward currents of Ca²⁺.