Topographic organization of spinal and trigeminal somatosensory pathways to the rat parabrachial and Kölliker-Fuse nuclei.

We examined the organization of somatosensory projections to the parabrachial (PB) and Kölliker-Fuse (KF) nuclei by employing the retrograde and anterograde axonal transport of Fluorogold and Phaseolus vulgaris-leucoagglutinin (PHA-L), respectively. Small PHA-L injections were made into different parts of the spinal trigeminal complex, including the paratrigeminal nucleus, and into different segments and laminae of the spinal dorsal horn. The subnuclear distribution of axonal labeling in the PB and KF was mapped with a camera lucida. Our results show that the somatosensory input to the PB and KF is highly organized. Neurons in the spinal trigeminal nuclei project predominantly to the KF and to the ventral portion of the external lateral PB. Neurons in the paratrigeminal nucleus project to the ventral lateral PB, the external medial PB, and to caudal aspects of the medial PB. These findings were supported by retrograde tracing experiments with Fluorogold. Spinal cord neurons located in the superficial dorsal horn (laminae I-II) of upper cervical segments project specifically to the ventral portion of the external lateral PB and, although more sparsely, to various other lateral PB nuclei. In contrast, neurons in the superficial dorsal horn of thoracic and lumbar spinal segments project mainly to the dorsal lateral and the central lateral PB. Finally, neurons in the lateral reticulated area and the lateral spinal nucleus of all spinal segments project almost exclusively to the internal lateral PB, whereas neurons in the respective nuclei of upper cervical segments also project to the KF. From our data we conclude that the somatosensory projections to the PB and KF are topographically organized. It is assumed that these pathways, which run from trigeminal and spinal neurons through the PB and KF to various forebrain, medullary, and spinal nuclei, form functionally different neural circuits that are involved in somatoautonomic processing.