Journal of Neuroscience, Vol 12, 781-792, Copyright © 1992 by Society for Neuroscience
Beta-adrenergic receptors: astrocytic localization in the adult visual cortex and their relation to catecholamine axon terminals as revealed by electron microscopic immunocytochemistry
C Aoki
Center for Neural Science, New York University, New York 10003.
It has long been recognized that noradrenaline, the most abundant
catecholamine within the visual cortex, plays important roles in modulating
the sensitivity of cortical neurons to visual stimuli. However, whether or
not these noradrenaline effects are confined to a discrete synaptic
specialization or mediated by diffuse modulation of a group of synapses has
remained an issue open for debate. The aim of this study was to examine the
cellular basis for noradrenaline action within the visual cortex of adult
rats and cats. To this end, I used electron microscopic immunocytochemistry
to examine the relationship between (1) catecholamine axon terminals and
beta-adrenergic receptors (beta AR), which, together, may define the
effective sphere of noradrenaline modulation; and then (2) these putative
sites for catecholamine modulation and axospinous asymmetric junctions
where excitatory neurotransmission is likely to dominate. Antibodies
against beta AR were used at light and electron microscopic levels on the
visual cortex of rat and cat. Rat visual cortex was also labeled
simultaneously for beta AR and the catecholamine-synthesizing enzyme,
tyrosine hydroxylase (TH), to determine the ultrastructural relationships
between catecholamine terminals and beta AR. Immunoperoxidase labeling
revealed that beta AR404, a polyclonal antibody directed against the
C-terminal tail of hamster lung beta AR (beta 2-type), recognized
astrocytic processes predominantly. In contrast, beta AR248, a polyclonal
antibody directed against the third cytoplasmic loop, recognized neuronal
perikarya as observed in previous studies. Dual labeling for beta AR404 and
TH revealed that catecholamine axon terminals that contained numerous
vesicles formed direct contacts with astrocytic processes exhibiting beta
AR404 immunoreactivity. However, some catecholamine axon terminals that
lacked dense clusters of vesicles were positioned away from beta AR404-
immunoreactive astrocytes. Frequently, beta AR-immunoreactive astrocytic
processes surrounded asymmetric axospinous junctions while also contacting
catecholamine axon terminals. These observations support the possibility
that, through activation of astrocytic beta AR, noradrenaline modulates
astrocytic uptake mechanism for excitatory amino acids, such as
L-glutamate. Astrocytic beta AR might also define the effective sphere of
catecholamine modulation through alterations in the morphology of distal
astrocytic processes and the permeability of gap junctions formed between
astrocytes.
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