ABSTRACT
0.05～0.2?l of HRP (Sigma Ⅵ) were introduced into the periaqueduetal gray (PAG) of 45 rats at the superior collicular level, each brain was sectioned and processed with DAB, BDHC, O-D and TMB reactions. 29 were chosen for analysis and according to the locations of the injection center and the extent of reaction, were divided into 3 groups:1. The injection center was found in the lateral part of PAG; the reaction only involved a small area within PAG (11 rats), labeled cells were seen in PAG, tectum, reticular formation (RF) and dorsal raphe nucleus.2. The injection center was in the dorsolateral part of PAG; the reaction area extended out of PAG to the nearby RF and tectum (9 rats); labeled cells were seen in the structures mentioned in group 1 as well as in habenular nucleus (hn), parafascicular nucleus (PFN), hypothalamic nuclei (anterior, posterior, ventromedial, dorsomedial, lateral), thalamic nuclei (lateral, reticular, medial part of ventral nucleus), paraventricular gray (PVG), nucleus of the optic tract, nucleus of posterior commissure (NPC), parabigeminal nucleus (PBN), locus ceruleus, ventral nucleus of lateral geniculate body, entopeduncular nucleus.3. The injection center was in the lateral or ventrolateral part of PAG, the reaction area extended to the nearby RF, tegmentum and part of tectum (9 rats), labeled cells were seen in the structures mentioned in group 1 as well as in substantia nigra, H_1, H_2, zona inserta, HN, PFN, PVG, NPC, PBN, locus ceruleus, pretectral area and nuclei of hypothalamus. In this group 3 rats were processed with TMB reaction. Labeled cells were seen not only in the above-mentioned structures but also in the deep layers of anterior cingulate cortex (areas 23 and 24), nucleus raphe magnus, nucleus reticularis gigantocellularis, periventricular gray of 4th ventricle, dentate nucleus of cerebellum, lamina Ⅴ～Ⅶ of cervical spinal cord (the other segments were left out), preoptic and supraoptic nuclei, suprachiasmatic nucleus, anterior nucleus of mammillary body, medial nucleus of amygdaloid nucleus, spinal nucleus of nerve Ⅴ. The labelled cells were seen chiefly in the ipsilateral side. Neuronal connections also were analysed according to the disparity in labelled structures.