Ultrastructural relations between beta-adrenergic receptors and catecholaminergic neurons

We performed dual electron microscopic immunocytochemistry to determine the precise cellular relations between beta-adrenergic receptors (beta AR) and catecholaminergic terminals within adult rat brains. An antibody, beta AR404, against a peptide corresponding to the C-terminus of the hamster lung beta AR (beta 2 subtype) together with an anti-tyrosine hydroxylase (TH), a catecholaminergic marker, were used. Results show predominant labeling for beta AR404 within small astrocytic processes (beta-A). This is in sharp contrast to earlier results which showed neuronal labeling when using antibodies against the third intracellular loop of the receptor and of neurons-plus-astrocytes labeled using antibodies against the whole beta AR molecule. beta-A within visual cortex and nuclei of the solitary tracts frequently contacted blood vessel basement membrane and TH-immunoreactive terminals. TH-immunoreactive axons forming axo-axonic juxtapositions with non-TH terminals were also noted to be surrounded by beta-A. In the area postrema, a brain region lacking a blood-brain barrier, few beta-A occurred adjacent to TH-immunoreactive terminals or elsewhere. Thus, 1) catecholamines may act beyond morphologically identifiable synapses; 2) beta-A may mediate interactions between catecholamines and other transmitters; 3) there may be substantial heterogeneity in the structure or the conformation of the beta AR protein between neurons and glia or across CNS regions.