Vesicular glutamate transporter-1 colocalizes with endogenous opioid peptides in axon terminals of the rat locus coeruleus

Abstract

We have previously shown that a subset of axon terminals in the locus coeruleus (LC) containing methionine(5)-enkephalin (ENK) forms type I (asymmetric-type) synaptic specializations that are characteristic of excitatory-type transmitters. In addition, we previously provided ultrastructural evidence showing that ENK is colocalized with glutamate using a combination of pre- and postembedding immunohistochemistry. To examine cellular substrates for interactions between glutamate and other endogenous opioid peptides in the LC, we examined the localization of the vesicular glutamate transporter 1 (VGLUT1), a transporter protein involved in the accumulation of the transmitter glutamate into synaptic vesicles, with either ENK or preprodynorphin (ppDYN). Dual-immunofluorescence and electron microscopy showed prominent coexistence of VGLUT1 and ENK in varicose processes of the LC, confirming our previous report using postembedding immunolabeling for glutamate. Likewise, VGLUT1 and ppDYN were identified in common varicose processes in the LC using confocal fluorescence microscopy. Immunoelectron microscopy using gold-silver labeling for VGLUT1 and peroxidase labeling for ppDYN established that this endogenous opioid peptide also colocalizes with glutamate transporters. The majority of these formed asymmetric-type synapses. Taken together, these results demonstrate that excitatory LC afferents are enriched with endogenous opioid peptides and are positioned to modulate LC neuronal activity dually.