Spinal cord cells innervating the bilateral parabrachial nuclei in the rat. A retrograde fluorescent double-labeling...

The internal lateral nucleus (IL) of the parabrachial nucleus receives information from the spinal cord. The IL perhaps relays nociceptive signals to the intralaminar nuclei of the thalamus, apparently being implicated in the motivational-affective component of pain reactions. However, cells of origin of spinal fibers to the IL have not been investigated enough. We intended to clarify these cells, as well as their shapes, by retrograde double-labeling techniques. Fast blue and diamidino yellow dyes were injected, respectively, into the left and right ILs. The distribution of double-labeled cells was almost the same as that of single-labeled cells on both sides of the spinal cord. The total number of bilateral double-labeled cells was highest in the dorsolateral part of the lateral funiculus (DL), followed, in order, by lamina I, the dorsomedial part of the lateral funiculus (DM), lamina V and lamina VII. A few double-labeled cells were seen in laminae II-IV, VI, VIII and X. The ratio of the total number of bilateral double-labeled cells to the total number of bilateral single-labeled cells through the spinal cord was 43% in the DL, 37% in the DM, 28% in lamina V and 24% in lamina I. The ratio was 10% or less in the other remaining laminae. No marked differences were observed between the shapes of double- and single-labeled cells.