The afferent connections of the preoptic region in the rat were studied with retrograde horseradish peroxidase (HRP) method.The mico-injection syringe needle or microelectrophoretic pipette was inserted vertically into the brain. In some cases, the injection was made with the needle inclined and directed contralaterally or in inclined direction through the cerebellum.The results were as follows:Following HRP injection into the medial preoptic area, labeled neurons were found in periventricular hypothalamic gray, paraventricular nucleus, anterior hypothalamic nucleus, supraoptic nucleus, ventromedial hypothalamic nucleus, posterior hypothalamic nucleus and locus coeruleus in all cases. In some cases, labeled cells were also observed within bed nucleus of stria terminalis, dorsal raphe nucleus. In addition, in a few cases, a small number of labeled cells could be found in lateral septal nucleus, medial septal nucleus, periaqueductal gray matter and interpeduncular nucleus.After injection into the region between the medial and lateral preoptic area, besides the above mentioned labeled nuclei, there were still a number of labeled cells found in paratenial nucleus, nucleus reuniens and lateral hypothalamic nucleus, in some cases also found in the supramammillar nucleus, and in one case even in the cortical and medial amygdalar nuclei.Following the injection of HRP into the anterior hypothalamic nucleus, retrograde labeling of neurons was observed within the lateral septal nucleus, preoptic periventricular nucleus, medial preoptic nucleus, supraoptic nucleus, periventricular hypothalamic nucleus, paraventricular nucleus, ventromedial hypothalamic nucleus, periaqueductal gray matter and locus coeruleus in all cases. Among them, supraoptic and paraventricular nuclei were labeled bilaterally in some cases.

The afferent connections to the periaqueductal gray in the rat were examined by means of retrograde axonal transport method using horseradish peroxidase (HRP).The microinjection syringe needle or micro-electrophoretic pipette was inserted stereotaxically by an oblique approach through the cerebellum. HRP was introduced into the dorsal, lateral, ventrolateral areas of periaqueductal gray or periventricular gray of ventriculus Ⅲ at posterior commissure level respectively.The results were as follows:1. Injections of HRP in the dorsal area of periaqueductal gray(PAG).Numerous labeled cells were consistently found in the paraventricular nucleus, dorsomedial and ventromedial hypothalamic nuclei, posterior hypothalamic nucleus, medial mamillary nucleus, zona incerta, lateral and ventral nuclei of periaqueductal gray, superior colliculus, inferior colliculus, dorsal raphe nucleus and periventricular gray of ventriculus Ⅳ. In some cases, a smaller number of cells was found in the zona reticulata of the substantia nigra, and lateral hypothalamic nucleus. In one case, the enzyme was injected into the dorsal area of PAG, which diffused to the superior and inferior colliculi, resulted in labeling of Forel's field and reticular nucleus of thalamus.2. Injection of HRP in the lateral area of PAG.HRP-positive cells were observed within the bed nucleus of the stria terminalis, paraventricular nucleus, anterior hypothalamic nucleus, ventro-medial hypothalamic nucleus, medial thalamic nucleus, nucleus interstitialis magnocellularis, commissural posteriors, zona incerta, ventral nucleus of PAG, zona reticulata of substantia nigra, formatio reticularis of mesencephalon, dorsal raphe nucleus, and periventricular gray of ventriculus Ⅲ and Ⅳ. In some cases, a small number of cells was found in the supraoptic nucleus, superior and inferior colliculi.3. Injection of HRP in the ventrolateral area of PAG.In all cases the greatest number of HRP-positive neurons was shown in the supraoptic nucleus, paraventricular nucleus, periventricular gray of ventriculus Ⅲ, lateral nucleus of PAG, zona reticulata of substantia nigra, dorsal raphe nucleus and locus coeruleus. The labeled cells were mainly located ipsilaterally. But the labeling of supraoptic nucleus was observed bilaterally. In one case the enzyme diffused to the reticular formation, a smaller number of cells was found in nucleus cuneifomis and mesencephalic reticular formation of more caudal position.4. Injection of HRP in the periventricular gray of ventriculus Ⅲ.Following the injections of HRP in the periventricular gray of ventriculus Ⅲ, numerous HRP-positive cells were consistently observed in the bed nucleus of the stria terminalis, supraoptic nucleus, paraventricular nucleus, ventromedial nucleus of hypothalamus, medial nucleus pars medialis of thalamus, zona incerta, ventral nucleus of PAG, zona reticulata of substantia nigra, dorsal raphe nucleus and locus coeruleus.

The afferent connections of the interpeduncular nucleus (IPN) were studied with retrograde transport method of using horseradish peroxidase (HRP).A microinjected syringe needle or microelectrophoretic pipette was inserted into the brain in the vertical, inclined through cerebellum or inclined through cerebral cortex medially.The results were as follows:Following HRP injection into the IPN, labeled neurons were found in medial habenular nuclei, lateral habenular nuclei, dorsal raphe nucleus, superior central nucleus, dorsal tegmental nucleus of Gudden, dorsolateral tegmental nucleus, ventro-lateral and lateral portions of periaqueductal grey matter, and locus coeruleus. In addition, labeled cells were also observed within caudal portion of accumbens nucleus and nucleus tractus diagonalis (Broca).Following HRP injection into the lateral, ventral (or ventrolateral) and dorsal portions of the IPN, labeled cells were found in ipslateral medial habenular nucleus,medial regions of the bilateral medial habenular nuclei, and lateral regions of the bilateral medial habenular nuclei, respectively.After injection into the rostral or caudal pole of the IPN, no labeled cells were observed in bilateral medial habenular nuclei.Following injection of HRP into the caudal pole of the IPN, labeled neurons were observed within both sides of lateral habehular nucleus, dorsolateral tegmental nucleus and ventromedial portion of dorsal tegmental nucleus.No labeled neurons were observed in the ventral (deep) tegmental nucleus,premamillary nucleus, supramamillary nucleus and parabigeminal nucleus except in the cases in which the HRP injection site extended beyond the IPN.

Afferent connections to the nucleus tegmenti dorsalis were studied by means of retrograde axonal transport method of horseradish peroxidase (HRP). HRP was depositted stereotaxically via three approaches: vertical, oblique through the cerebellum or oblique through the cerebrum. The serial sections were treated by 3,3'-diamino-benzidine, benzidine dihydrochloride, diaminobenzidine-cobalt dichloride or tetrame-thylbenzidine respectively.Labeled neurons were observed in following nuclei: nucleus lateralis habenulae,nucleus interpeduncularis, nucleus mammillaris lateralis, nucleus posterior hypothalami, nucleus lateralis hypothalami, substantia grisea centralis mesencephali, nucleus dorsalis raphes, nucleus tractus solitarii, nucleus prepositus hypoglossi, nucleus parafascicularis and nucleus of Darkschewitsch.Then HRP was introduced into nucleus parafascicularis or nucleus prepositus hypoglossi, labeled terminals were also observed within the nucleus tegmenti dorselis.

The afferent connections to the nucleus amygdaloideus centralis and nucleusamygdaloideus medialis in 42 albino rats were examined by means of retrogradetransport method of horseradish peroxidase(HRP).The microinjection syringe needle or microelectrophoretic pipette was insertedstereotaxically from vertical or oblipue approaches.The injections of HRP weregiven in the central and medial nuclei of the amygdaloid complex.The results were as follows:Following the injections of HRP into the nucleus amygdaloideus centralis,label-ed cells were always found in prepiriform cortex,periamygdaloid cortex,entorhinalcortex,ventral part the caudoputamen and globus pallidus,nucleus lateralis hy-pothalami,nucleus parafascicularis,corporis geniculati medialis,nucleus raphe dosalis,nucleus parabrachialis,locus coeruleus and zona reticulate substantia nigra.While HRP was injected into the nucleus amygdaloideus medialis,the labeledcells were always found in bed nucleus of stria terminalis,nucleus ventromedialis(hypothalami)and substantia nigra pars compacta.As the injections were given from lateral oblique approaches,labeled cells werenot found in the neo-cortex.There were no labeled cells demonstrated in the nuc-leus solitarius.In the above-mentioned areas nucleus ventromedialis(hypothalami),nucleusparabrachialis and locus coeruleus were often labeled bilaterally while others,ipsila-terally.

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.