ALPHA(2A)-ADRENERGIC RECEPTORS IN THE RAT NUCLEUS LOCUS-COERULEUS - SUBCELLULAR-LOCALIZATION IN CATECHOLAMINERGIC DENDRITES, ASTROCYTES, AND PRESYNAPTIC AXON TERMINALS

To define the anatomic substrates subserving the inhibitory actions of alpha(2)-adrenergic receptors (alpha(2)-ARs) in the locus coeruleus ( LC), we used dual-label immunoelectron microscopy with antibodies dire cted against the A-subtype of alpha(2)-AR (alpha(2A)-AR) and the catec holamine-synthesizing enzyme tyrosine hydroxylase (TH). Of the profile s containing peroxidase labeling for alpha(2A)-AR (alpha(2A)-AR-IR) in the LC (n = 735), most were dendrites (similar to 50%), glial process es (similar to 30%), and axon terminals (similar to 15%). alpha(2A)-AR -IR was also observed in unmyelinated axons and perikarya. Within dend rites, alpha(2A)-AR-IR was associated with nonsynaptic regions of the plasma membrane and subsurface cisternae. Approximately 60% of dendrit es with alpha(2A)-AR-DR were dually labeled for TH. Fifty percent of t he axon terminals contacting alpha(2A)-AR-immunoreactive dendrites for med asymmetric (excitatory) synaptic contacts. Axon terminals with alp ha(2A)-AR-IR were not dually labeled for TH and generally formed asymm etric synapses with TH-immunoreactive dendrites that contained or lack ed alpha(2A)-AR-IR. Astrocytic processes exhibiting alpha(2A)-AR-LR we re closely apposed to TH-labeled dendrites, These results extend previ ous ultrastructural observations of alpha(2)-ARs in the LC and suggest that the inhibitory actions of norepinephrine and epinephrine in this region may be mediated by postsynaptic alpha(2A)-ARs on catecholamine rgic dendrites and presynaptic alpha(2A)-ARs on excitatory inputs to c atecholaminergic dendrites. In addition, the localization of alpha(2A) -AR-IR in astrocytic processes apposed to TH-immunoreactive dendrites suggests a role for alpha(2A)-ARs in functional interactions between c atecholaminergic dendrites and neighboring astrocytes. (C) 1998 Elsevi er Science B.V. All rights reserved.