ABSTRACT
The afferant projections and their chemical nature from the midbrain raphe nuclei and periaqueductal gray to the thalamic ventral posterior nucleus were st- udied by HRP retrograde tracing combined with PAP immunocytochemical method Following injections of HRP into the thalamic ventral posterior nucleus, and reactions by HRP and PAP were performed subsequently. The double labelling cells containing HRP-positive granules and substance P-like immunoreactivity were found in various areas of brain, that is, the contralateral principal sensory nucleus of trigeminal nerve,nucleus gracilis,nucleus cuneatus, bilateral ventrola- teral divisions of the periaqueductal gray and midbrain raphe nuclei.A comparison of cell counts of double labeled cells with the total HRP-labeled cells indicated that the cells observed in the dorsal raphe nuclei were 21?7 in each rat, which constituted 48% of total HRP-labeling cells; The double labeling neurons in nuclei raphe centralis superior were found in two cases only, the cell bodies were very sparse and distributed mainly in the caudal area of these nuclei The double labeling cells were detected in bilateral ventrolateral divisions of periaqu- eductal gray, predominantly ipsilaterally. The double labeling cells observed in each rat were 26?9 ipsilaterally and 11?4 contralateraIly. The numbers of double labeling cells were 38% of the total counts of HRP-labeling cells in each side. These results above indicate that the thalamic ventral posterior nucleus receives afferent projections from midbrain raphe nuclei and ventrolateral division of periaqueductal gray and in the pathway, a part of substance P-positive neurons are exhibited. Therefore, this study provides evidence for existance ascending projection of substance P-like immunoreactivity.