In 15 rats anaesthetized with ketamine, microiontophoretically applied acetylcholine (ACh) excited all 58 cells studied in the subthalamic nucleus (STN). 2. The ACh-evoked excitation was slow in onset and outlasted the ACh application. There was no sign of desensitization when the ACh application was prolonged or repeated. The excitation was prolonged by a concomitant application of physostigmine. 3. Acetyl-beta-methyl choline and oxotremorine were effective cholinomimetics. Nicotine had no effect. 4. The ACh excitation was antagonized by stropine and scopolamine but not by mecamylamine. 5. It was condluded that STN ACh receptors are muscarinic in character. 6. Since large microiontophoretic applications of Mg2+ did not suppress ACh-evoked excitation, it is suggested that ACh acts postsynaptically. 7. The excitatory response of STN cells to striatal or pallidal stimulation was unaffected by atropine administered either microiontophoretically to single cells or intravenously (3 mg/kg) to the whole animal.
