THE RELEASE IN VIVO OF [3H]ACETYLCHOLINE FROM CAT CAUDATE NUCLEUS AND CEREBRAL CORTEX BY ATROPINE, PENTYLENETETRAZOL, K+‐DEPOLARIZATION AND ELECTRICAL STIMULATION

— In the present experiments, the resting and stimulus evoked release of newly synthesized [3H]acetylcholine from the caudate nucleus, the cerebral cortex and the cerebellar cortex into the perfusate of the push-pull cannula was studied in the unanesthetized, midpontine, pretrigeminally transected cat following infusion at the push-pull site of [3H]choline. Separation of the metabolites in the perfusate revealed that after 20 min, approximately 20% of the recovered radioactivity in the sample was in a lipid fraction, about 10% was found to be phosphorylcholine and around 3% was observed to be incorporated into acetylcholine. The rest of the recovered radioactivity remained as choline. Electrical stimulation applied directly to the caudate nucleus, local potassium depolarization, atropine and pentylenetetrazol were all observed to result in a significant and stimulus dependent increase in the levels of [3H]acetyIchoIine, but not [3H]choline or [14C]urea in the effluent of the push-pull cannula located in the caudate nucleus. A similar release of newly synthesized [3H]acetylcholine was observed following atropine and potassium stimulation in the cerebral but not the cerebellar cortex. The specificity of this evoked increase in the levels of [3H]acetylchoiine is substantiated by obtaining the release with stimuli having different modes of action, by the absence of stimulus evoked changes in the levels of other water-soluble elements found in the perfusate and by the absence of an observable release of [3H]acetylcholine in perfusion experiments involving the cerebellum, a tissue not thought to have strong cholinergic innervation. The percentage increases in release of [3H] acetylcholine over baseline levels evoked by the various methods closely corresponded to those reported in the literature for authentic acetylcholine. This was taken to suggest that the neuronal pools containing endogenous acetylcholine and those containing newly synthesized acetylcholine, if not identical, were disposed to behave in the same manner following the activation of the synapse.