Hippocampal α(2A)-adrenergic receptors are located predominantly presynaptically but are also found postsynaptically and in selective astrocytes

Teresa A. Milner, Amy Lee, Sue A. Aicher, Diane L. Rosin

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Abstract

Alpha-adrenergic receptor, subtype 2A (α(2A)-AR), activation is one of the primary modes of action for norepinephrine (NE) in the rat hippocampal formation. In this study, α(2A)-AR immunoreactivity (α(2A)-AR-I) was localized by light and electron microscopy in the rat hippocampus and dentate gyrus by using a previously characterized antibody to the rat α(2A)-AR. By light microscopy, dense α(2A)-AR-I was observed in the pyramidal and granule cell layers. Diffuse and slightly granular α(2A)-AR-I was found in the neuropil in all other laminae, notably stratum lacunosum-moleculare. Ultrastructurally, α(2A)-AR-I was found in neuronal cytoplasm associated with large multivesicular-like organelles and with clusters adjacent to endoplasmic reticula and/or plasmalemma. The distribution of α(2A)-AR-I in the strata oriens, radiatum, and lacunosum-moleculare of hippocampal CA1 and CA3 regions and in the molecular layer of the dentate gyrus was remarkably similar (n > 2,000 profiles examined): α(2A)-AR-I was found in axons and terminals (~40%), glia (~30%), dendritic spines (~25%), and dendritic shafts (~5%). This mixed pre- and postsynaptic distribution was not seen in the stratum lucidum of the CA3 region and the dentate hilar region, where most α(2A)-AR-I was found in axons (~60%) and glia (~30%). Alpha-2A-AR- labeled axons were small and unmyelinated; labeled terminals usually formed asymmetric synapses on unlabeled spines; and labeled dendritic spines were morphologically similar to pyramidal or granule cells. Dual labeling studies demonstrated that some axons contained α(2A)-AR-I and tyrosine hydroxylase (TH), the catecholaminergic synthesizing enzyme, and that some TH-labeled terminals were in close proximity to α(2A)-AR-labeled spines and glia. These studies demonstrate that hippocampal α(2A)-AR-I is localized (1) presynaptically in both noncatecholaminergic and catecholaminergic terminals, (2) postsynaptically in the dendritic spines of pyramidal and granule cells near catecholaminergic terminals, and (3) in some glial processes. These results suggest several sites for NE to exert its effects on hippocampal α(2A)-ARs.

Original languageEnglish (US)
Pages (from-to)310-327
Number of pages18
JournalJournal of Comparative Neurology
Volume395
Issue number3
DOIs
StatePublished - Jun 8 1998

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Keywords

  • Catecholamine receptors
  • Catecholaminergic axons
  • Dendritic spines
  • Electron microscopy

ASJC Scopus subject areas

  • Neuroscience(all)

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