Ultrastructural localization of β-adrenergic receptor-like immunoreactivity in the cortex and neostriatum of rat brain

We sought to quantitatively examine the processes containing β-adrenergic receptor-like immunoreactivity (β-AR-LI) in the cerebral cortex and neostriatum using a previously characterized rabbit antiserum to frog erythrocyte β-ARs under optimized immunolabeling conditions. Quantitative assessments of the laminar distribution of β-AR-LI in the cortex was achieved by computer-assisted image analysis of immunoautoradiographs and by quantitative electron microscopic analysis of peroxidase-antiperoxidase (PAP) labeling in aldehyde-fixed sections and unfixed synaptosomes. In the somatosensory and anterior cingulate cortical areas, light microcopy of aldehyde-fixed sections immunolabeled by the PAP method revealed small (0.5–1.0 μm) punctate processes in all layers. In the deeper layers, rims of immunoreactivity around the plasmalemma of a population of neuronal perikarya and processes were also observed. By immunoautoradiography, labeling was seen in distinct, laminar distributions resembling the reported autoradiographic patterns using radioligands^{34, 40, 46}. By electron microscopy, the immunoreactive profiles in all cortical layers were primarily thick and thin postsynaptic densities (PSDs), comprising 4% of all identifiable PSDs in fixed sections and 12% in unfixed synaptosomal preparations. Also labeled were saccules of smooth endoplasmic reticulum and pinocytotic vesicles in dendrites, glial processes and lightly myelinated axons. In the neostriatum, the density of autoradiographic immunoreactivity was equivalent to the heavily labeled laminae of the cerebral cortex. Immunoreactivity detectable by light microscopy included punctate processes and rims of perikarya, as was seen in the cerebral cortex. The PAP reaction was shown by electron microscopy to be localized to the cytoplasmic surface of plasmalemma of a few proximal dendrites, but was most prominently associated with PSDs of dendritic spines. Preadsorption of the antiserum with a partially purified β-AR preparation abolished all detectable immunoreactivity. These results provide further support for the specificity of the antiserum for β-ARs, and are the first quantitative ultrastructural evidence for association of β-AR-LI with PSDs in the cerebral cortex. The neostriatum, whose major catecholaminergic innervation is dopaminergic, and not noradrenergic, is also confirmed to exhibit high levels of β-AR-LI within subcellular structures analogous to those seen in the cerebral cortex. The ultrastructural distribution of β-AR-LI in the two regions provides morphological support for the receptor's membrane-associated recycling, axonal transport, function at both excitatory and inhibitory synapses, and role in interactions between specific neuorons and glia.