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Fluoro-ruby injections in the rat locus coeruleus result in scattered chain-like arrays of varicose anterogradely labeled axons within the thalamic reticular nucleus of rats. An abundant meshwork of axons giving rise to en passant boutons is detected immunohistochemically within this thalamic nucleus by means of an antibody to dopamine-beta-hydroxylase (DBH). The density of DBH-positive axonal boutons within the reticular nucleus neuropil is greater than that found in the relay nuclei of the dorsal thalamus (with the exception of the anterior group nuclei). Single DBH-positive axons appear to contact both proximal and distal dendrites and occasionally the somata of reticular nucleus neurons. Labeled axons are seen closely juxtaposed not only to the swollen segments of the beaded reticular neuron dendrites, but to the constricted segments as well. Electron microscopic examination of DBH-positive axon terminals within the reticular nucleus neuropil indicates that many of the axonal boutons detected light microscopically participate in asymmetric synaptic contacts. The postsynaptic densities of these synapses are thicker than those of nearby symmetric synapses, but often subtend a shorter length of the postsynaptic membrane than the densities associated with other nearby asymmetric synapses. These observations indicate that the ascending noradrenergic system, in addition to influencing the dorsal thalamus and the cerebral cortex directly, is well situated to influence signal transmission through the nuclei of the dorsal thalamus indirectly via a moderately dense terminal projection upon the thalamic reticular nucleus.

A dense axonal plexus, arising in a portion of the magnocellular basal nucleus, was identified in the thalamic reticular nucleus in adult rats. The details of these axonal arbors as well as their relation to the neurons of the reticular nucleus were investigated by using Phaseolus vulgaris leucoagglutinin injections into the basal nucleus and intracellular injections of Lucifer yellow into reticular nucleus neurons. Axons arising in the caudal basal nucleus at the medial margin of the globus pallidus do not enter the dorsal thalamus but are confined to the reticular nucleus, where they arborize widely and densely. Neurons in the reticular nucleus are large, with sparsely spined and beaded dendrites, which radiate within the plane of the nucleus. Bouton-like swellings along basal nucleus axons are often found apposed to the somata of reticular nucleus neurons, although many are also apposed to dendrites. These morphological observations suggest a second potentially significant route, in addition to its well-known direct cortical projection, through which the magnocellular basal nucleus could influence cortical function: it may, by strategically modulating the excitability of reticular nucleus neurons, alter the general state of the thalamus and hence affect the initial transmission of information to the cortex.