Patterns of colocalization of GABA, glutamate and glycine immunoreactivities in terminals that synapse on dendrites of noradrenergic neurons in rat locus coeruleus

Amino acid transmitters play a key role in regulating the activity of noradrenergic neurons in the locus coeruleus. We investigated the anatomical substrate for this regulation by quantifying immunoreactivity for GABA, glutamate and glycine in terminals that contacted the dendrites of tyrosine hydroxylase‐immunoreactive principal neurons in rat locus coeruleus. Pre‐embedding peroxidase immunocytochemistry was used to detect tyrosine hydroxylase‐immunoreactivity in Vibratome sections of tissue perfused with 2.5% glutaraldehyde. GABA, glutamate and glycine were localized with postembedding immunogold labelling. Gold particle densities over terminals were measured in three semiserial ultrathin sections, each reacted for a different amino acid. More than 90% (range among rats, 89%−95%) of the terminals analyzed (n = 288) were immunoreactive for at least one amino acid. A high proportion (39%−49%) were positive for two or three amino acids. About two‐thirds (60%−69%) of the boutons contained GABA, of which more than half (51%−55%) also contained glycine. More than one‐third (36%−38%) of the terminals were positive for glycine. Terminals immunoreactive for glycine alone were rare (0%−2%). About one‐third of the terminals showed glutamate‐immunoreactivity (32%−37%). GABA and/or glycine occurred in one‐fifth to one‐third of these. These results show that amino acid‐immunoreactivity is present in almost all of the terminals that synapse on tyrosine hydroxylase‐positive dendrites in locus coeruleus. Glutamate provides a major excitatory input. The almost complete colocalization of glycine with GABA suggests that the inhibitory input to locus coeruleus is predominantly GABAergic with a contribution from glycine in about half of the GABAergic boutons.