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Abstract
Immunoreactive structures visualised with antibodies to glycine were prominent in
areas of the nucleus of the solitary tract (NTS) surrounding the tractus solitarius,
but scarcer in medial and ventral areas of the nucleus. This contrasted with a higher
density, more homogenous distribution of structures labelled for gamma-aminobutyric
acid (GABA). Immunolabelling of adjacent semi-thin sections nonetheless indicated
a close correspondence between cells and puncta labelled by glycine and GABA antisera
in certain NTS areas. With post-embedding electron microscopic immunolabelling, synaptic
terminals with high, presumed transmitter levels of glycine were discriminated from
terminals containing low, metabolic levels by quantitative analysis of gold particle
labelling densities. In a random sample of terminals, 28.5% qualified on this basis
as glycinergic (compared to 44.4% GABAergic); these glycinergic terminals targeted
mainly dendritic structures and contained pleomorphic vesicles and symmetrical synapses.
Serial section analysis revealed few terminals (5.2%) immunoreactive for glycine alone,
with 82% of glycinergic terminals also containing high levels of GABA immunoreactivity.
No evidence for co-localisation of glycine and glutamate was found. Light, confocal
and electron microscopic labelling with antibodies to proteins specific for glycine
and GABA synthesis, release and uptake confirmed that glycinergic terminals also containing
GABA are found predominantly in more lateral areas of NTS, despite glycine receptors
and the 'glial' glycine transporter (GLYT1) being expressed throughout all areas of
the nucleus. The data suggest that synaptic terminals in certain functionally distinct
areas of NTS co-release both inhibitory amino acids, which may account for the previously
reported differential inhibitory effects of glycine and GABA on NTS neurones.
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