Multiple substrates for serotonergic modulation of rat locus coeruleus neurons and relationships with kainate...

The ultrastructural substrates of serotonin (5-hydroxytryptamine [5-HT]) immunoreactive terminals with respect to noradrenergic neurons of the locus coeruleus (LC) have only been suggested from immunocytochemical analysis in adjacent tissue sections using antisera directed against tryptophan and tyrosine hydroxylase (TH) enzymes. Here, we conducted dual immunoelectron microscopy in the same section of tissue using antisera directed against 5-HT and TH to determine cellular substrates for proposed interactions between these two transmitter systems. Axon terminals containing peroxidase labeling for 5-HT possessed small clear as well as large dense core vesicles. Of 176 5-HT-labeled axons and terminals, 19% contacted TH-labeled dendrites. When a synaptic specialization was detectable, it was more often of the asymmetric type. Electrophysiological studies have also shown that 5-HT selectively attenuates excitatory amino acid-induced activation of neurons in the LC. Thus, to further examine the cellular relationship between 5-HT-labeled axon terminals and excitatory amino acid receptors, we conducted immunogold-silver labeling of an antibody which recognized the three identified members of the kainate receptor (KAr) subunit class (GluR 5,6,7) and peroxidase localization of 5-HT. Similar proportions of 5-HT-labeled terminals (9%) were either apposed to KAr-labeled dendrites or exhibited KAr immunoreactivity. Twenty-four percent of the 5-HT axon terminals examined were apposed by glial processes that contained KAr. These data indicate that 5-HT axon terminals are in direct contact with LC neurons and also suggest pre- and postsynaptic sites for modulation of 5-HT terminals by excitatory amino acid ligands as well as indirect sites via glial processes.