In this study a combination of HRP retrograde tract tracing and immunocytochemistry was employed to investigate the afferent projections of neuropeptide Y(NPY) -and neurotensin (NT)-immunoreactive(ir) neurons in the cervical, thoracic, lumbar and sacral spinal segments to the parabrachial nucleus in the rat. It was demonstrated that the retrogradely labeled neurons were scattered in bilateral laminae I, II, IV, V, VII, gray matter commissure dorsal to the central canal, lateral cervical nucleus and lateral spinal nucleus. The NPY-ir neurons were situated in bilateral lamina I, superficial portion of the lamina II, lateral cervical nucleus and lateral spinal nucleus. The NT-ir cells were seen in bilateral lamina I and lamina II of dorsal horn. In lamina I, lateral cervical nucleus and lateral spinal nucleus a few of NPY-HRP double labelled neurons could be observed. Individuals of NT-HRP double labeled neurons were identified only in lamina I. The results of this study suggest that NPY and NT neurons probably involved in the nociceptive transmission of the spino-parabrachial pathway.

By introducing a mixture of WGA-HRP and HRP or fluorogold into the parabrachial nucleus, the cell origin of the spinoparabrachial projections in the rat have been carefully examined. The labelled neurons were found in bilateral spinal gray matter, lateral spinal nucleus and lateral cervical nucleus with contralateral predominance. They were mainly located in lamina I, lamina II, lamina IV, lamina V and lamina VII of the gray matter and also in the gray matter commissure posterior to the central canal. Comparing the distribution patterns of the projection neurons in the cervical, thoracic, lumbar and sacral segments, we did not find any distinct differences. The fact that the parabrachial nucleus receives a wide extensive projections form the spinal segments suggests that the spino-parabrachial pathways are possibly involved in the transmission of multiple sensory inputs.