The spinothalamic system of the rat: structural types of retrogradely labelled neurons in the marginal zone (lamina I).

Retrogradely labelled lamina I neurons were studied after intrathalamic injections of free horseradish peroxidase mixed with dimethylsulphoxide, wheat germ agglutinin conjugated with horseradish peroxidase, and subunit B of cholera toxin. The first two tracers revealed only the perikaryal shape and the orientation of primary dendrites, while cholera toxin subunit B produced Golgi-like stainings. The morphological and morphometric analysis of the labelled marginal neurons in different planes showed them to belong to the pyramidal and the flattened types of our Golgi-based classification. These cells were located predominantly in the intermediate lateromedial portion of lamina I at all spinal levels, and it is suggested that their structural duality is matched by different functional properties. Distributions of the remaining spinothalamic cells labelled with the two horseradish peroxidase tracers were rather similar to those previously reported in the literature, including the almost exclusive occurrence of labelled cells, at lumbar levels, in the internal basilar column group. Cholera toxin subunit B labelled many more spinal cells and revealed considerable numbers of labelled cells in all cell groups at the lumbar enlargement.