Organization of catecholamine neurons projecting to the frontal cortex in the rat

The organization of the catecholamine innervation in the frontal lobe of the rat neocortex has been studied by means of the glyoxylic acid fluorescence method in combination with lesions and retrograde tracing of horseradish peroxidase to distinguished three dopaminergic terminal systems.

Abstract

The organization of the catecholamine innervation in the frontal lobe of the rat neocortex has been studied by means of the glyoxylic acid fluorescence method in combination with lesions and retrograde tracing of horseradish peroxidase. On the basis of fibre morphology and distribution of fibres and cell bodies of origin three dopaminergic terminal systems can be distinguished. The anteromedial system is formed by smooth axons originating in the medial part of the A10 cell group in the mesencephalon. The axons are distributed mainly in the pregenual part of the anteromedial cortex and the highest fibre density is found in the basal cortical layers. A much more sparse, caudal extension of this system is present in the supragenual part of the anteromedial cortex. The suprarhinal system forms the dorsal part of the dopaminergic innervation of the perirhinal cortex, surrounding the rhinal sulcus. It can be regarded as a direct lateral continuation of the anteromedial system, and can be followed from a coronal level just rostral to the nucleus accumbens to the level of the most rostral part of the caudate-puttamen. The axons are distributed mainly in the basal cortical layers and have their cell bodies of origin in the dorsolateral part of the A10 cell group. The supragenual system is formed by very fine, varicose axons distributed in a restricted area of the supragenual anteromedial cortex. The axons are localized in the superficial cortical layers (I–III) and the cell bodies of origin are distributed in the ventrolateral A10 and along the mediolateral extent of the substantia nigra (A9). Only one noradrenergic fibre system, originating in the locus coeruleus, was found. The locus coeruleus system projects to all parts of the frontal cortex. The medial cortex has the highest fibre density, the perirhinal somewhat lower, and the dorsal and lateral cortices are more sparsely innervated. In all cortical areas, the locus coeruleus innervation has the same general appearance with the highest fibre density in the molecular layer.