Ultrastructural immunocytochemistry for glycine in neurons of the dorsal cochlear nucleus of the guinea pig.

Neurons in the dorsal cochlear nucleus of the guinea pig were classified according to their positivity to the inhibitory neurotransmitter glycine, ultrastructure and projections to the inferior colliculus as indicated by tract-tracing and ultrastructural immunocytochemistry. Only some pyramidal and few giant cells, surrounded by glycinergic boutons containing flat and pleomorphic vesicles, projected to the inferior colliculus as glycine-negative excitatory cells. Smaller neurons in superficial layers of the dorsal cochlear nucleus did not project to the inferior colliculus, and were recognized as glycine-negative granule and unipolar brush cells. Few glycinergic, inhibitory neurons among granule cells were indicated as Golgi-stellate neurons. All small neurons associated to the granule cell areas received few, mainly glycinergic synapses, and their dendrites contacted large boutons (mossy fibers). Other medium-large glycine positive neurons in the superficial (cartwheel) and deep layers (tuberculo-ventral and large-giant) of the dorsal cochlear nucleus did not project to the inferior colliculus. Giant-large glycinergic neurons surrounded by sparse axo-somatic, mostly glycinergic synapses, probably represent commissural neurons projecting to the contralateral cochlear nucleus. Rare boutons, possibly descending from the inferior colliculus, were seen onto pyramidal cells or their dendrites, and these boutons mainly stored glycine positive pleomorphic vesicles or glycine negative round vesicles. No descending mossy fibers storing round vesicles were labelled from the central nucleus of the inferior colliculus. These observations suggest that very few terminals in the dorsal cochlear nucleus of the guinea pig are derived from the inferior colliculus.