Patterns of glutamate, glycine, and GABA immunolabeling in four synaptic terminal classes in the lateral superior olive of the guinea pig

Abstract

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.

Ultrastructural localization of NADPH diaphorase and nitric oxide synthase in the neuropils of the snail CNS

“…Similarly to our results obtained in the snail CNS, NOS was observed in axon terminals containing small size ovoid or pleomorph agranular vesicles in the brain of the Atlantic salmon (Holmqvist and Ekström, 1997), the rat spinal trigeminal nucleus (Yeo et al, 1997), the monkey visual cortex (Aoki et al, 1993), as well as in neural components of the guinea-pig intestine (Wang et al, 1995). According to the vesicle morphology in insect and vertebrate neurons, these vesicles contain presumably an amino acid or acetylcholine (Watson, 1988;Holmqvist and Ekström, 1997;Helfert et al, 1992). Evidences for GABA or acetylcholine as a primary transmitter in NADPH-d reactive/NOS-IR terminals have been presented in insect sensory interneurons (Seidel and Bicker, 1997) and the identified B2 giant cell of the buccal ganglion of the snail, Lymnaea (Moroz 2000), as well as in the mammalian aspiny striatal interneurons and peduncolopontine tegmental neurons projecting to the thalamus (seen in: Vincent, 2010).…”

Localization of AMPA-selective glutamate receptors in the auditory brainstem of the barn owl

“…The overall pattern of AMPA receptor subunit distribution in the barn owl resembles that seen in mammals (Table 2; Petralia and Wenthold, 1992;Helfert et al, 1992;Hunter et al, 1993;Sato et al, 1993). In particular, the dense immunolabeling of the brainstem auditory nuclei with GluR4 antibodies is not surprising, since the flop splice variants of GluR4 subunits exhibit the temporally precise responses that characterize the auditory brainstem (Mosbacher et al, 1994;Geiger et al, 1995).…”

“…Glutamate receptors with distinct functional properties are expressed in neurons of the chick auditory brainstem (Raman and Trussell, 1992;Ravindranathan et al, 1996) and the barn owl midbrain (Feldman and Knudsen, 1994). Glutamate receptors are also widely distributed throughout the mammalian auditory system (Petralia and Wenthold, 1992;Helfert et al, 1992;Sato et al, 1993).…”

Afferent regulation of glycine receptor distribution in the gerbil LSO

“…Since CN afferents to the LSO are glutamatergic (Caspary and Faingold, 1989;Godfrey et al, 1978;Helfert et al, 1992;Wu and Kelly, 1992), it is possible that glutamate release is decreased following bilateral ablation, resulting in higher than normal glycine receptor aggregation. Another possibility is that degeneration of CN afferents reduces competition for space on LSO cells, allowing glycinergic synapses to occupy a greater fraction of postsynaptic space.…”