SELECTIVE DISTRIBUTION OF THE NMDA-R1 GLUTAMATE-RECEPTOR IN ASTROCYTES AND PRESYNAPTIC AXON TERMINALS IN THE NUCLEUS LOCUS-COERULEUS OF THERAT-BRAIN - AN IMMUNOELECTRON MICROSCOPIC STUDY

The regional and cellular distribution of the different classes of exc itatory amino acid receptors with respect to the noradrenergic neurons of the nucleus locus coeruleus (LC) are unknown. We therefore combine d immunoperoxidase labeling for the R1 subunit of the N-methy-D-aspart ate (NMDA) receptor with immunogold-silver localization of the catecho lamine synthesizing enzyme, tyrosine hydroxylase (TH), in single secti ons through the rat LC to determine the subcellular localization of th is glutamate receptor subtype with respect to the noradrenergic neuron s. At the light microscopic level, there was light to moderate labelin g for the NMDA-R1-like (li) receptor in the caudal pole of the LC and dense labeling in the dorsolateral aspect of the LC adjacent to the su perior cerebellar peduncle. In the rostral pole of the LC which is enr iched with noradrenergic dendrites, significant overlap between both i mmunoreactivities could be observed. At the ultrastructural level, imm unoperoxidase labeling for NMDA-R1 was selectively distributed in astr ocytic processes and within presynaptic axon terminals but was rarely seen in catecholamine-containing somata or dendrites. Peroxidase label ing for NMDA-R1, however, was occasionally observed in dendrites in th e rostral pole of the LC. Most of these dendrites lacked detectable le vels of TH, although TH immunoreactivity was apparent in the neuropil. Dendrites containing NMDA-R1-li immunoreactivity often received asymm etric (excitatory-type) contacts from unlabeled terminals. NMDA-R1-li- immunoreactive axon terminals usually contained small clear, as well a s large dense-core vesicles and were often apposed to unlabeled dendri tes, axon terminals and/or glial processes. These results provide the first ultrastructural evidence that NMDA-R1-li immunoreactivity is sel ectively distributed within astrocytic processes and presynaptic axon terminals within the LC. (C) 1996 Wiley-Liss, Inc.