α2A-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 directed against the A-subtype of alpha(2)-AR (alpha(2A)-AR) and the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). Of the profiles containing peroxidase labeling for alpha(2A)-AR (alpha(2A)-AR-IR) in the LC (n = 735), most were dendrites (similar to 50%), glial processes (similar to 30%), and axon terminals (similar to 15%). alpha(2A)-AR-IR was also observed in unmyelinated axons and perikarya. Within dendrites, alpha(2A)-AR-IR was associated with nonsynaptic regions of the plasma membrane and subsurface cisternae. Approximately 60% of dendrites with alpha(2A)-AR-DR were dually labeled for TH. Fifty percent of the axon terminals contacting alpha(2A)-AR-immunoreactive dendrites formed asymmetric (excitatory) synaptic contacts. Axon terminals with alpha(2A)-AR-IR were not dually labeled for TH and generally formed asymmetric synapses with TH-immunoreactive dendrites that contained or lacked alpha(2A)-AR-IR. Astrocytic processes exhibiting alpha(2A)-AR-LR were closely apposed to TH-labeled dendrites, These results extend previous 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 catecholaminergic dendrites and presynaptic alpha(2A)-ARs on excitatory inputs to catecholaminergic 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 catecholaminergic dendrites and neighboring astrocytes. (C) 1998 Elsevier Science B.V. All rights reserved.