THE NEURONAL CONNECTIO0NS OF CAUDATE NUCLEUS AND PUTAMEN WITH THE MEDIAL NUCLEI OF THALAMUS.AN EXPERIMENTAL STUDY BY MEANS OF AXONAL RETROGRADE TRANSPORT OF HORSERADISH PEROXIDASE

The afferent projections to the caudatoputamen of rat and the head of thecaudate nucleus and the putamen of cat,especially the projections from the medialnuclei of thalamus,were studied with the method of retrograde axonal transportof horseradish peroxidase(HRP).0.1-05μl of 50% solution of HRP was injected slowly into the caudatoputa-men of rat or the head of the caudate nucleus and the putamen of cat.After asurvival time of 1-3 days the animals were perfused with 2% paraformaldehydefixative.The frozen sections were incubated in a medium containing 3′,3-dia-minobenzidine tetrahydrochloride and hydrogen peroxide.The sections wereexamined microscopically under bright-field and particularly darkfield illumina-tion.In rats injected with HRP to the caudatoputamen,labelled neurons werefound scattered in the internal medullary lamina and concentrated mainly in theparafascicular nucleus of the thalamus.They were also found ipsilaterally in thesubstantia nigra and the dorsal nucleus of the midbrain raphe.In the case ofcats after injections of the enzyme in different depths of the head of caudate nu-cleus the labelled neurons were found in the medial nuclei of thalamus,includingthe centromedian nucleus,parafascicular nucleus,centrolateral nucleus,para-central nucleus,medial central nucleus and periventricular nucleus and their dis-tribution appeared to be roughly topographic.Fibers from the substantia nigra,the drosal nucleus of raphe and the dorsal tegmental nucleus in the midbrain werealso projected to this area.When HRP was injected in the putamen,the labelledceils were found located caudally in the centromedian nucleus and the posteriornuclear group of thalamus,and especially in the ventrolateral area of the centro-median nucleus and the suprageniculate nucleus.They were also found in thesubstantia nigra of the midbrain.The neuronal connections of the caudate nucleus with the intralaminar nucleias revealed in the present investigation may serve as an anatomical basis for thehypothesis in which a thalamo-forebrain circuit is supposed to be involved in themechanism of pain and pain control,