Noradrenergic axon terminals in the cerebral cortex of rat. I. Radioautographic visualization after topical application of dl-[ 3H]norepinephrine

Abstract

Summary Three types of radioautographic reactions may be detected in the frontal cortex of adult rats, perfused with glutaraldehyde and post-fixed in osmium tetroxide, 1 or 3 h after topical application of NA-3H. (1) Diffuse reaction. Underneath application sites, a ubiquitous labelling is always visible. It presumably corresponds to an artefactual retention of free NA-3H and or tritiated metabolites. (2) Nerve cell body reaction. One hour after application, some pyramidal and stellate nerve cell bodies of the supragranular layers exhibit more silver grains than their environment, provided that monoamine oxidase is inhibited. This preferential reaction does not seem characteristic of noradrenergic neurons, since it survives earlier destruction of catecholamine-containing nerve cells by intraventricular 6-OH-DA. (3) Axonal reaction. In the cortical neuropil, certain unmyelinated axons selectively accumulate NA-3H, which is mainly concentrated within presynaptic enlargements, giving rise to clear-cut radioautographic signals. The specificity of this reaction is evidenced by the complete disappearance of all labelled axons in the cortex of rats pretreated with 6-OH-DA. Three hours after application of NA-3H in the presence of IMAO, the repartition and configuration of the reactive axons bear a close resemblance to the distribution pattern of noradrenergic fibers, previously demonstrated in rat cortex by means of the fluorescence technique of Falck and Hillarp33. Within such axons, exogenous NA-3H may be bound in segments devoid of synaptic vesicles. In the labeled presynaptic enlargements, small agranular vesicles are often associated with large dense-core vesicles. These are also found, however, in numerous unlabeled nerve endings. Membrane differentiations typical of synaptic contacts may be seen between reactive axonal enlargements and cortical dendrites, demonstrating the existence of true synaptic terminals for noradrenergic axons of the rat cerebral cortex.