Superior paraolivary nucleus of the rat is a GABAergic nucleus

The presence of the inhibitory neurotransmitters glycine and GABA (gamma-am ino butyric acid) and GAD (glutamic acid decarboxylase), the synthesizing e nzyme for GABA, was examined by immunocytochemistry in the superior paraoli vary nucleus (SPON) of the rat. Only rarely were SPON neurons observed to b e glycine-immunoreactive, but the majority were GABA-immunoreactive. Using unbiased stereological counting methods, we estimated that this nucleus con tains approximately 2500 neurons. Moreover, 90% of SPON neurons were immuno labeled by antisera directed against either the 65- or 67-kD isoform of GAD , or a third antiserum that recognizes both GAD isoforms. Morphometric anal ysis of GAD-immunolabeled neurons indicated that SPON neurons possess cell bodies and dendritic arbors that are elongated rostrocaudally and relativel y flattened parasagittally. Abundant glycine-, GABA-, and GAD-immunoreactiv e punctate profiles-presumed to represent, for the most part, presynaptic a xon terminals-were observed in apposition to SPON neurons. We conclude that the rat SPON contains a homogeneous population of multipolar GABAergic neu rons that receive abundant GABAergic and glycinergic innervation. The vast majority of glycinergic inputs to SPON are presumed to originate in the ips ilateral medial nucleus of the trapezoid body; but the source(s) of its GAB Aergic innervation remains to be determined.