Evidence for coexistence of enkephalin and glutamate in axon terminals andcellular sites for functional interactions of their receptors in the rat locus coeruleus

The authors previously showed that a subset of axon terminals in the locus coeruleus (LC) contains methionine(5)-enkephalin (ENK) and gamma-aminobutyr ic acid (GABA) immunoreactivities. However, numerous ENK-labeled terminals lacked GABA and exhibited synaptic specializations that were characteristic of excitatory-type transmitters. To determine whether ENK coexists with gl utamate in the LC, preembedding immunoperoxidase detection of ENK or immuno gold-silver was combined with postembedding identification of glutamate usi ng a gold marker. Indeed, 28% df the ENK-labeled axon terminals examined (n = 250 axon terminals) also contained glutamate. To define further sites fo r functional interactions between opiate ligands and excitatory amino acid receptors, the ultrastructural localization of the mu-opioid receptor (MOR) was examined with respect to either the kainate receptor (KAR) or the R1 s ubunit of the N-methyl-D-aspartate (NR1)-type glutamate receptor in the LC. Gold-silver labeling for MOR and peroxidase labeling for either KAR or NR1 indicated that the MOR often was localized to the plasma membrane of dendr ites that also exhibited immunolabeling for either glutamate receptor subty pe. In contrast to the KAR, which was identified primarily in somata and de ndrites, NR1 immunoreactivity also was found frequently in axon terminals a s well as in glial processes. Glial processes containing NR1 occasionally e xhibited immunolabeling for MOR and sometimes were directly apposed to MOR- containing dendrites in the LC. Furthermore, NR1-labeled receptors in axon terminals sometimes were presynaptic to MOR-labeled dendrites. The authors concluded that ENK and glutamate may be cotransmitters in LC afferents. Mor eover, Ligands at the KAR may modulate directly MOR-containing neurons in t he LC, whereas actions at NR1 receptors may affect opioid-sensitive neurons through multiple cellular mechanisms, i.e., through presynaptic, postsynap tic, or glial actions. J. Comp. Neurol. 417:103-114, 2000. (C) 2000 Wiley-L iss, Inc.