Superior paraolivary nucleus of the rat is a GABAergic nucleus

The presence of the inhibitory neurotransmitters glycine and GABA (gamma-amino butyric acid) and GAD (glutamic acid decarboxylase), the synthesizing enzyme for GABA, was examined by immunocytochemistry in the superior paraolivary nucleus (SPON) of the rat. Only rarely were SPON neurons observed to be glycine-immunoreactive, but the majority were GABA-immunoreactive. Using unbiased stereological counting methods, we estimated that this nucleus contains approximately 2500 neurons. Moreover, 90% of SPON neurons were immunolabeled by antisera directed against either the 65- or 67-kD isoform of GAD, or a third antiserum that recognizes both GAD isoforms. Morphometric analysis of GAD-immunolabeled neurons indicated that SPON neurons possess cell bodies and dendritic arbors that are elongated rostrocaudally and relatively flattened parasagittally. Abundant glycine-, GABA-, and GAD-immunoreactive punctate profiles-presumed to represent, for the most part, presynaptic axon terminals-were observed in apposition to SPON neurons. We conclude that the rat SPON contains a homogeneous population of multipolar GABAergic neurons that receive abundant GABAergic and glycinergic innervation. The vast majority of glycinergic inputs to SPON are presumed to originate in the ipsilateral medial nucleus of the trapezoid body, but the source(s) of its GABAergic innervation remains to be determined.