Caudate afferents from the retrorubral nucleus and other midbrain areas in the cat

In order to clarify which midbrain areas project to the caudate nucleus (Cd), horseradish peroxidase (HRP) was deposited in the Cd of 13 cats. Midbrain sections were reacted with diaminobenzidine and sometimes with cobaltous chloride to intensify the reaction, and then examined for retrograde labelling. Afferents to the Cd were found arising from the following ipsilateral midbrain areas: pars compacta, pars reticulata, and pars lateralis of the substantia nigra (SN), ventral tegmental area of Tsai, interpeduncular nucleus, central tegmental field, and the retrorubral nucleus (RR) of Berman, which lies caudal to the red nucleus and dorsal to the medial lemniscus. HRP labelled cells were also found within the "cell bridges" which lie between SN and RR, composed of neurons among the fascicles of the medial lemniscus. Acute lesions placed in the area of the lateral hypothalamus and medial forebrain bundle prior to HRP injection blocked HRP uptake in the ipsilateral SN, RR, and cell bridges. Thus, the SN, RR and connecting cell bridges share, at least in part, a pathway to the Cd in the cat.