TOPOGRAPHIC ORGANIZATION OF SPINAL AND TRIGEMINAL SOMATOSENSORY PATHWAYS TO THE RAT PARABRACHIAL AND KOLLIKER-FUSE NUCLEI

We examined the organization of somatosensory projections to the parab rachial (PB) and Kolliker-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 ma de into different parts of the spinal trigeminal complex, including th e paratrigeminal nucleus, and into different segments and laminae of t he spinal dorsal horn. The subnuclear distribution of axonal labeling in the PB and KF was mapped with a camera lucida. Our results show tha t the somatosensory input to the PB and KF is highly organized. Neuron s in the spinal trigeminal nuclei project predominantly to the KF and to the ventral portion of the external lateral PB. Neurons in the para trigeminal nucleus project to the ventral lateral PB, the external med ial PB, and to caudal aspects of the medial PB. These findings were su pported by retrograde tracing experiments with Fluorogold. Spinal cord neurons located in the superficial dorsal horn (laminae I-II) of uppe r cervical segments project specifically to the ventral portion of the external lateral PB and, although more sparsely, to various other lat eral PB nuclei. In contrast, neurons in the superficial dorsal horn of thoracic and lumbar spinal segments project mainly to the dorsal late ral and the central lateral PB. Finally, neurons in the lateral reticu lated area and the lateral spinal nucleus of all spinal segments proje ct almost exclusively to the internal lateral PB, whereas neurons in t he 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 path ways, which run from trigeminal and spinal neurons through the PB and KF to various forebrain, medullary, and spinal nuclei, form functional ly different neural circuits that are involved in somatoautonomic proc essing. (C) 1995 Wiley-Liss, Inc.