Direct Stimulation of the Hypoglossal Nucleus by Acetylcholine in Extreme Dilutions

Acetylcholine (ACh.), when applied in minute amounts to the eserinized cerebral cortex, induces marked excitatory effects; 1 it was inferred that the ACh. stimulates or facilitates various cortical synapses. The experiments here reported indicate that ACh., on direct application in extreme dilutions, stimulates a motor nucleus, namely the hypoglossal nucleus; this nucleus is readily accessible, as the trigonum hypoglossi, immediately beneath the floor of the fourth ventricle. 2

Methods. Decerebration under ether was by the Sherrington decerebrator. Both common carotids were tied or, in other experiments, both internal carotids were tied and also the branches of external carotids, but leaving lingual arteries intact; essentially similar results were obtained in both preparations. The skull was held in a special clamp, so that the entire tongue could be observed. The floor of fourth ventricle was exposed by removing median parts of cerebellum. Acetylcholine chloride (ACh. Cl) (Merck) was dissolved in buffered Ringer-Locke solution and was applied in small rectangles 2 mm x 6 mm of spot test paper to the XII nuclei of both sides.

Results. Preliminary localization of effects from the XII nucleus was by unipolar faradization with a fine silver wire, ending in a minute bulb; the diffuse electrode was a saline-soaked pad on animal's back. Secondary distances varied from 35 to 50 cm, using a large Palmer inductorium (Fig. 1).

The following effects of ACh. were obtained in a decerebrate cat (3100 g), having lingual arteries intact. Preliminary eserinization was by 0.25 mg eserine sulphate (Merck) intravenously. A rectangle of spot test paper soaked in ACh. Cl. 1:50 millions was applied to both XII nuclei: in 4 sec. a slight median furrowing appeared at base of tongue between circumvallate papillae (Fig. 2); the furrowing, which coincided with inspiration, increased and was accompanied by simultaneous retraction of tongue base, elevation of soft palate and dilatation of remainder of mouth cavity.