The substantia nigra(SN)has been considered to be related to motor function.The recent physiological study has shown that the SN is not only one of theelements of extrapyramidal system,but is also related to the sensory system and takepart in the modulation of pain.In order to provide morphological evidence for itsfunctional aspect,42 rats have been employed for study by retrograde transportmethod of horseradish peroxidase(HRP),using microinjective and microiontophoreticdelivery system through vertical and oblique approaches.The results showed that the main labeled nuclei and neuro-structures were thecaudoputamen nucleus,dorsal raphe nucleus,central amygdaloid nucleus,globuspallidus,cerebral cortex,median raphe nucleus,locus coeruleus,periaqueductal grayand parabrachium nuclei.Topographical organization of the projections from these labeled nuclei andneuro-structures to SN was studied in detail.Among the structures studied the labeled cells were found most numerous incaudoputamen nucleus(CP).The projection fibers start in a order from the antero-dorso-medial to postero-ventro-lateral part of CP and end in the medial to lateralof SN in succession.Topographical projection from dorsal raphe nucleus to SN was also found.Thedorsal raphe nucleus only project to the medial two-thirds of SN.The fibers from central amygdaloid nucleus and locus coeruleus terminate in thelateral two-thirds of SN.Only the medial one half of SN receives the projection fibers from globuspallidus.In cerebral cortex,the main regions where labeled cells could be seen wereinsula,frontal and parietal regions.The projection fibers coming from median raphe nucleus terminate to the medialone-third of SN.When HRP was injected into lateral or median one-third of SN,numerousand clear cut labeled cells can be seen in the ventro-lateral region of periaqueductalgray.The projection from parabrachium nuclei terminates to the lateral one half ofSN.

The connections between the dorsal raphe nucleus and other nuclei in the CNS were studied with Nauta degeneration method and HRP retrograde transport method.After dorsal raphe nucleus had been lesioned the degenerating efferent fibers could be divided into dorsal, intermediate and ventral groups. Via the dorsal group the ascending fibers passed through the periaqueductal gray matter and terminated in the substantia grisea centralis, substantia grisea periventricularis, and substantia grisea paraventricularis of the ventricularis tertius. The intermediate group, lying in the raphe of tegmental and reticular formation, was projected to the mesencephalic reticular formation, substantia nigra bilaterally. The ventral group, passing through the area tegmentalis ventralis, medial forebrain bundle (MFB), zona inserta, was projected to the lateral hypothalamic region, thalamus and telencephalon.The descending projections mainly from the intermediate and ventral groups, could be traced in the lateral region of the periaqueductal gray. The degenerating fibers passed through the ventral aspect of tegmentum along both sides of the raphe, and finally continued to the lower brain stem, and the dorsal horn of spinal cord.In the lower brain stem, the descending degenerated fibers terminated in the nucleus raphe pontis, nucleus raphe magnus, nucleus raphe pallidus, nucleus raphe obscurus, nucleus reticularis tegmenti pontis, nucleus reticularis gigantocellulari, nucleus reticularis paramedianus, nucleus olivaris inferior accessoris medialis, and nucleus tractus spinalis nervi trigemini.After the injection of HRP into the dorsal raphe nucleus, labelled neurons were found in forebrain structures, consisting of nucleus periventricularis, nucleus anterior medialis thalami,nucleus habenularis lateralis,nucleus anterior hypothalami,nucleus dorsomedialis hypothalami, nucleus preopticus medialis, nucleus mamillaris lateralis, ventral tegmental area of Tasi, zona inserta, nucleus supraopticus, and nucleus interpeduncularis. Below the level of injection site, labeled neurons were found in substantia nigra (zona reticularis and compacta), periaqueductal gray, nucleus interpeduncularis, locus coeruleus, nucleus tegmenti dorsalis, nucleus centralis superior, nucleus raphe pontis, nucleus raphe magnus, and nucleus reticularis lateralis.

Studies of the afferent connections of the nucleus accumbens in 56 rats were carried out by retrograde transport method of horseradish peroxidase (HRP). The microinjection or microiontophoretic delivery was performed either through ipsilateral vertical or through contralateral oblique approach into the nucleus. Labelled neurons were found in nucleus caudatus and putamen, nucleus septi lateralis, nucleus amygdaloideus basalis lateralis, nucleus periventricularis thalami, nucleus paratenialis, periventricular gray hypothalami, nucleus medialis thalami, nucleus reuniens, substantia nigra and area ventralis tegmenti (Tsai) in all cases. In addition, the HRP labelled neurons were observed in the cinguIate gyrus, hippocampus, nucleus parafascicularis, nucleus anterior thalami, nucleus dorsal raphe and interpeduncular nucleus in most cases.