Subcortical afferents to the frontal lobe in the rhesus monkey studied by means of retrograde horseradish peroxidase transport

The present study tried to determine in the rhesus monkey the precise location of various subcortical cell populations which send their fibers to different parts of the precentral gyrus and of the rostrally adjoining cortical areas. For this purpose in 14 rhesus monkeys horseradish peroxidase (HRP) was injected in different parts of the frontal lobe followed by a detailed study of the location of the diencephalic neurons to which the enzyme was transported retrogradely. In 14 animals 6-30 closely spaced injections of 0.6 #1 of 10 ~ HRP (Sigma type VI) in distilled water were made in the different cortical areas indicated in Fig. la. In 5 monkeys, the injections were made unilaterally but in the other animals they were made bilaterally. In one control animal, 30 x 0.6 #1 of 0.0075 ~o saline was injected in the precentral gyrus on one side. This solution has approximately the same molarity as 10~ HRP. The animals survived the operations for 3 days. They were then deeply anesthetized with Nembutal and subsequently perfused with a solution of 6 ~ dextran in 0.9 ~ saline followed by a mixture of 2.5 ~ glutaraldehyde-0.5 9/00 paraformaldehyde in 0.1 M cacodylate buffer (pH 7.2). After the brains had been removed they were saturated with a solution of 3 0 ~ sucrose in cacodylate buffer and cut in 40-#m transverse sections on a Leitz freezing microtome. The sections were incubated in a medium containing Tris buffer, 3,3'-diaminobenzidine and hydrogen peroxide, and then dehydrated and covered. They were studied microscopically under dark-field and bright-field illumination. Some sections were lightly counter-stained with cresyl violet. The location of the neurons to which the enzyme was transported retrogradely was charted with the aid of an X-Y plotter. In the animals with HRP injected in the frontal lobe, retrogradely labeled neurons, containing HRP reaction granules in the cell body and proximal parts of the dendrites, were present in the thalamus as well as in the substantia innominata and the hypothalamus (Fig. 2a). The latter areas will be referred to as the basal forebrain areas. In some cases, labeled neurons were also present in the claustrum and in 7 animals in which the lower brain stem was studied, labeled neurons were also found in the locus coeruleus (Fig. 2b). In the 4 animals in which the enzyme was injected in one