Morphological identification of an interneuron in the hypoglossal nucleus of the rat: a combined Golgi-electron microscopic study.

Hypoglossal small neurons of adult and juvenile (10-15-day-old) rats were examined by a combined Golgi-electron microscopic technique. In adult rats, Golgi-impregnated neurons were fusiform or ovoid (17 X 12 micron) and emitted a few primary dendrites with few branches and few spines and an axon mainly from the proximal portion of the primary dendrite. At the ultrastructural level, the soma displayed an invaginated nucleus with a conspicuous nucleolus and a relatively scanty cytoplasm in which cisternae of rough endoplasmic reticulum were not organized into extensive lamellar arrays. A moderate number of axon terminals, which contained spherical clear or pleomorphic vesicles approximately 25-40 nm in diameter, formed symmetric or asymmetric synapses on the soma, the dendrites, the axon hillock, and the initial segment. In a preparation from the juvenile rat, we could trace a full axon trajectory of the small neuron. In this sample, in which a small neuron and a motoneuron were simultaneously impregnated, the axon of the small neuron was found to receive an axoaxonic symmetric synapse with pleomorphic vesicles on a varicosity and to contact the motoneuron dendrites by means of another varicosity of the main axon and of two boutons from an axon collateral. The varicosity and the boutons contained pleomorphic synaptic vesicles and synapsed symmetrically with the motoneuron dendrites. We identified the small neuron in the rat hypoglossal nucleus morphologicaly as a Golgi type II interneuron. We discuss its function in relation to the GABAergic nature of small neurons (Takasu et al.: J. Comp. Neurol. 263:42-53, '87).