Comparison of the release of endogenous and of 3H-acetylcholine from slices of rat cerebral cortex.

A comparison of the release of endogenous and of tritiated acetylcholine (3H-ACh) was performed in eserinized slices of rat cerebral cortex labelled in vitro in the presence of 0.43 microM tityustoxin (TsTX), a scorpion venom known to increase the overflow of endogenous acetylcholine. The endogenous and the radioactive ACh were spontaneously released into the organ bath. At the moment when the spontaneous efflux had levelled off, the endogenous ACh collected during 1 min accounted for 1.03 +/- 0.14% of the tissue content and a similar value (0.80 +/- 0.26%) was found for 3H-ACh. The basal outflow as well as the tissue retention of ACh and of 3H-ACh were not modified by exposure to the scorpion venom. In the presence of 20 mM KCl for 1 min, the overflow of both radioactive and endogenous ACh was increased up to approximately 100% above basal levels and this K+-evoked release was almost doubled under the action of TsTX. The results presented are compatible with the view that no major functional differences exist in the rat cortex between responses of pools storing 3H-ACh synthesized from 3H-choline and those storing ACh formed from endogenous substrate.