title: Mu opioid receptors are in somatodendritic and axonal compartments of GABAergic neurons in rat hippocampal formation.
abstract: Activation of mu opioid receptors (MORs) has a net excitatory effect in the hippocampal formation through inhibition of gamma-amino butyric acid (GABA)-containing interneurons. To determine the precise subcellular targets of MOR agonists, immunoreactivity against MOR1 and GABA was examined in single sections of the hippocampal formation prepared for dual-labeling electron microscopy. In both the CA1 region of hippocampus and the dentate gyrus, MOR-like immunoreactivity (-li) was present in neuronal somata, dendrites, axons, and axon terminals, as well as a very few glial processes. Axon terminals with MOR-li formed symmetric synapses with principal cell dendrites and somata. Many MOR-labeled profiles of all types also contained GABA-li, and the vast majority possessed the ultrastructural characteristics of interneurons. Additionally, in the dentate gyrus a very small proportion of granule cell dendrites contained MOR-li. MOR-li, identified using immunogold-silver particles, was often affiliated with the extrasynaptic regions of neuronal plasma membranes, consistent with responsiveness to diffusing endogenous neuropeptide ligands. Semiquantitative analysis of the distribution of MOR-li revealed significantly more "presynaptic" (axons and terminals) than "postsynaptic" (somata and dendrites) labeled profiles in most laminae. We conclude that in addition to previously described somatodendritic MOR-li, a substantial amount of MOR-li in hippocampal formation is presynaptic. Furthermore, MORs are almost exclusively in GABAergic interneurons.
authors.affiliation: Department of Neurology, Division of Neurobiology, Weill Medical College of Cornell University, 411 East 69th Street, New York, NY, USA. ctdrake@mail.med.cornell.edu
authors.forename: C T
authors.initials: CT
authors.lastname: Drake
authors.forename: T A
authors.initials: TA
authors.lastname: Milner
mesh_terms: D000818:Animals; D001369:Axons / Q000648:ultrastructure*; D003712:Dendrites / Q000648:ultrastructure*; D018891:Dentate Gyrus / Q000166:cytology; D006624:Hippocampus / Q000166:cytology*; D007150:Immunohistochemistry; D007395:Interneurons / Q000166:cytology; D008297:Male; D016253:Microscopy, Immunoelectron; D009474:Neurons / Q000166:cytology*; D017966:Pyramidal Cells / Q000166:cytology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D017450:Receptors, Opioid, mu / Q000032:analysis*; D005680:gamma-Aminobutyric Acid / Q000032:analysis*
chemical_list: D017450:Receptors, Opioid, mu; D005680:gamma-Aminobutyric Acid