Immunoperoxidase immunocytochemistry was employed to examine the distribution of gamma-aminobutyric acid (GABA)-and glycine (GLY)-immunoreactive cells, fibers, and terminals in the guinea pig superior olivary complex. The nuclei studied were the lateral superior olive (LSO), medial superior olive (MSO), superior paraolivary nucleus (SPN), and the medial, ventral, and lateral nuclei of the trapezoid body (MNTB, VNTB, and LNTB, respectively). The majority of LSO neurons exhibited GABA-immunoreactive (+) labeling. These same neurons were also lightly GLY+. Extensive perisomatic punctate GLY + labeling was observed on most LSO neurons; these puncta most likely correspond to synaptic terminals. A very small number of MSO fusiform neurons were GABA +, and none were GLY +. The GLY positive perisomatic punctate labeling around most MSO neurons, although abundant, was not as profuse as that observed in the LSO. The MNTB neurons corresponding to the principal and elongate types were intensely GLY + and were contacted by small numbers of GLY + puncta. There was extensive GLY + punctate labeling in the SPN that surrounded the cell bodies of most of its large, radiate neurons and many of the smaller, fusiform neurons. The few large, radiate neurons that were lightly GLY + possessed far fewer GLY + puncta on their perikarya. The distribution of GABA + puncta was generally diffuse and scattered throughout the nuclei described above. In the VNTB and LNTB, several large neurons of various shapes were GLY + as were the small, oval neurons. The extent of GLY + punctate labeling was quite variable in both nuclei. The majority of perikarya in the VNTB and LNTB were GABA +. A light distribution of GABA + puncta was observed on most cell bodies in both nuclei. Peridendritic GABA + punctate labeling was dense in the VNTB neuropil. Two small populations of GLY + neurons were observed outside of the named nuclei of the SOC; one was located dorsal to the LSO, near its dorsal hilus, and the other was identified near the medial pole of the LSO. The somata of both populations possessed extremely sparse GLY + punctate labeling. In general, these results agree with and expand on findings in rodents from previous studies. There appears, however, to be differences between the guinea pig and cat with regard to the proportions of GABA + neurons in the LSO and GLY + punctate labeling in the MSO.

The goal of this study was to correlate synaptic ultrastructure with transmitter specificity and function in the lateral superior olive (LSO), a nucleus that is thought to play a major role in sound localization. This was accomplished by means of postembedding immunogold immunocytochemistry. Four classes of synaptic terminals were identified in the LSO. They were distinguishable from one another both morphologically and on the basis of their different patterns of immunolabeling for glutamate, glycine, and gamma-aminobutyric acid (GABA). The highest level of glutamate immunoreactivity was found in terminals that contained round vesicles (R) and formed synaptic contacts with asymmetric synaptic junctions. Round-vesicle terminals predominated on small caliber dendrites by a ratio of at least 2:1 over the other classes combined. The thinnest dendrites were typically contacted by R terminals only. The ratio of R terminals to the other types decreased as the caliber of the dendritic profiles they apposed increased so that on the soma, R terminals were outnumbered by at least 2:1 by the other types. Terminals containing flattened vesicles (F) exhibited intense immunoreactivity for both glycine and glutamate, although the glutamate immunolabeling was not as high as that in the R terminals. Flattened-vesicle terminals formed symmetric synaptic contacts with their targets and their distribution was the reverse of that described for R terminals; i.e., they were most abundant on LSO perikarya and fewest on small caliber dendrites. Two terminal types, both containing pleomorphic vesicles and forming symmetric synaptic junctions, were found in far fewer numbers. One group contained large pleomorphic vesicles (LP) and was immunoreactive for both glycine and GABA. The other group contained small pleomorphic vesicles (SP) along with a few dense-core vesicles and labeled for GABA only. The LP terminals were preferentially distributed on somata and large-caliber dendrites, while the SP terminals most often contacted smaller dendrites. Previous work suggests that a large percentage of the R terminals arise from spherical cells in the ipsilateral cochlear nucleus and are excitatory in action. This pathway may use glutamate as a transmitter. Many of the F terminals are thought to originate from the ipsilateral medial nucleus of the trapezoid body and appear to be the inhibitory (glycinergic) terminals from a pathway that originates from the contralateral ear. The origins and functions of LP and SP terminals are unknown, but a few possibilities are discussed along with the significance of cocontainment of neuroactive substances in specific terminal types.

Electron microscopic postembedding immunocytochemistry was used to analyze and assess the synaptic distribution of glycine (GLY) and gamma-amino butyric acid (GABA) immunoreactivities in the guinea pig cochlear nucleus (CN). Three classes of endings were identified containing immunolabeling for glycine, GABA, or both glycine and GABA (GLY/GABA). All classes were similar in that the terminals contained pleomorphic vesicles and formed symmetric synapses with their postsynaptic targets. A fourth class, which labeled with neither antibody, contained round vesicles and formed asymmetric synapses. Glycine endings predominated in the ventral CN, while GLY/GABA endings were prevalent in the dorsal CN. GABA endings were the least common and smallest in size. Glycine, GLY/GABA, and GABA endings differed in their proportions and patterns of distribution on the different classes of projection neurons in the CN, including spherical bushy, type I stellate/multipolar, and octopus cells in the ventral CN and fusiform cells in the dorsal CN. The vast majority of anatomically-defined, putative inhibitory endings contain GLY, GABA, or both, suggesting that most of the inhibition in the cochlear nucleus is mediated by these three cytochemically and, probably, functionally distinct classes of endings. The results of this study also suggest that a large proportion of the GABA available for inhibition in the CN coexists in terminals with glycine.

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