Topographic organization of the projections from the interstitial system of the spinal trigeminal tract to the parabrachial nucleus in the rat.

Neurons in the paratrigeminal nucleus are known to project to the parabrachial region, but both these areas are heterogeneous, and the subnuclei that account for these connections are not known. To characterize better these projections, we injected small amounts of fluorogold or latex beads labeled with rhodamine or fluorescein into the parabrachial area in the rat and evaluated the retrograde transport of tracer to the paratrigeminal nucleus and neighboring regions. The results show that the rostral part of the paratrigeminal nucleus projects to the medial subnucleus of the parabrachial nucleus. The intermediary part of the paratrigeminal nucleus projects to both the external lateral and to the external medial subnuclei of the parabrachial nucleus. The caudal part of the paratrigeminal nucleus projects to the ventral lateral subnucleus of the parabrachial nucleus. The dorsal paramarginal nucleus projects to the external lateral and the extreme lateral subnuclei of the parabrachial nucleus. Lamina I and II of the spinal trigeminal nucleus also project to the external lateral and the extreme lateral subnuclei of the parabrachial nucleus. In conclusion, the rostral, intermediate, and caudal parts of the paratrigeminal nucleus and the dorsal paramarginal nucleus each have clearly different projection patterns and presumably have different functions.