Retrograde axonal transport and the demonstration of non‐specific projections to the cerebral cortex and striatum from thalamic intralaminar nuclei in the rat, cat and monkey

Abstract The intralaminar nuclei of the thalamus have been examined particularly in the rat but with additional observations in the cat and squirrel monkey, with a view to determining the extent of their connections with the cerebral cortex and/or striatum. Cells in the intralaminar and other thalamic nuclei were labeled by retrograde axonal transport of the enzyme, horseradish peroxidase, from injection sites of varying size in the cerebral cortex and striatum. This system provides a useful means of determining certain parameters of the horseradish peroxidase technique. It is concluded that the degree of retrograde labeling of a cell is primarily dependent upon the number and concentration of its axon terminals in the vicinity of the injection site. Injections of the striatum in the rat and monkey cause intense labeling of many cells in the intralaminar nuclei. Conversely, injections in the medial, frontal and parietal cortex of all three species, though leading to heavy labeling of many cells in the appropriate thalamic relay nuclei, result in only light labeling of relatively few cells in the intralaminar nuclei. Cells in a single intralaminar nucleus, however, though showing a broad topographical relationship, can be labeled from quite wide areas of the cerebral cortex. These results are consistent with the view that the intralaminar nuclei (including the parafascicular and centre médian) project densely to the striatum and sparsely and diffusely upon the cerebral cortex.