Horseradish peroxidase(HRP)was injected in 41 rats to investigate thevestibulothalamic projections.Labelled neurons were observed in various subdivisionsof the vestibular nuclear complex following injections of the ventral anterior nucleus(VA),the ventral medial nucleus(VM),the central lateral nucleus(CL),the ventralposterior nucleus(VP),the nucleus posterior thalami(PO),the magnocellular partof the medial geniculate body(MGBmc),and the ventral nucleus of the lateralgeniculate body(LGV).From the present investigation it appears that generallyspeaking,there exist two distinct categories of vestibulothalamic projections withregard to origin as well as site of termination of the fibers.The first Category ofprojections appears to originate from all four vestibular nuclei(superior,lateral,medial,and descending)and terminate in the nonspecific thalamic nuclei includingVA,VM,and CL.It may be concerned with regulation of the general excitabilityof neurons in cortex and participation in motor performance.The second categoryof projections appears to originate only from the medial and descending vestibularnuclei and terminate in the ventral posterior nucleus and posterior group of thethalamus(including PO,MGBmc,and LGV).It may be associated with consciousvestibular perception and convergence of vestibular and somatic impulses.

The nigrothalamic projections were studied with HRP technique in 40 rats.Theanimals were divided into two groups.In one group HRP was injected into varioussubdivisions of the thalamus to observe the retrograde labelled cells in the reticularzone of substantia nigra(SNR)and in the other group HRP was injected into SNRto observe the anterograde labelled fibers and terminals in the thalamus.The nigrothalamic projections were found to originate mainly from SNR,but afew from the compact zone of substantia nigra(SNC)were also observed.Thefibers originated from all parts of the SNR,but the fibers originating from thecaudal part out number those from the rostral part and the ventral part was foundto predominate over the dorsal part.The nigrothalamic projections mainly terminated in ventral medial nucleus ofthe thalamus,but a few fibers terminating in dorsal medial nucleus,parafascicularisnucleus,ventral anterior nucleus and central lateral nucleus of the thalamus were alsofound.It is essentially similar with the site of termination and distribution ofnigrothalamic fibers in the cat and monkey,but there are some quantitative differ-ences.

WGA-HRP was injected into the dorsal part of anterior cingulate area,prelim- bic area,and dorsal agranular insular area of the prefrontal cortex in 25 rats to observe the afferent connections from the diencephalon and subcortical telencephalon. The afferent projections from the diencephalon mainly originate from the dorsome- dial nucleus of the thalamus,and some topographic localization may exist.Further- more,the afferent projections to the prefrontal cortex arise from the thalamic intralaminar nuclei(centrolateral nucleus,paracentral nucleus,centromedial nucleus and parafascicular nucleus),the ventral thalamic nuclei(ventrolateral nucleus,ven- tromedial nucleus,ventroanterior nucleus and ventroposterior nucleus),the midline nuclei(rhomboid nucleus,reuniens nucleus,paratenial nucleus,and paraventricular nucleus of the thalamus),the anteromedial nucleus,the lateral habenular nucleus, the posterior nucleus of the thalamus,and the lateral nucleus of the thalamus.The numbers projecting to the prefrontal cortex were different in different thalamic nuclei.The afferent projections from the hypothalamus mainly originate from the lateral hypothalamic area,lateral preoptic area,caudal magnocellular nucleus,and supramamillary nucleus.A few of afferent projections from the posterior hypothala- mic area,dorsomedial and ventromedial nucleus of the hypothalamus and zona inserta were also observed.The afferent projections from the subcortical telence- phalon mainly originate from the globus pallidus,the diagonal band nucleus,the septal nuclei(medial septal nucleus,lateral septal nucleus,septofimbrial nucleus and nucleus triangularis septi),the amygdaloid nuclei(lateral amygdaloid nucleus,lateral and medial part of basal amygdaloid nucleus,medial amygdaloid nucleus and cortical amygdaloid nucleus)and the claustrum.

0.02～0.3?l of 33％ HRP (Sigma Ⅵ) solution in sterile water were injected with a stereotaxic instrument into various sites of the caudate-putamen in 40 rats. The animals were perfused transaortically with 3％ paraformatdehyde and 1％ glutaradehyde in 0.1 M phosphate buffer solution (pH7.4). Frozen sections were dried in a container with P_2O_5 for 2～4 hours and examined for fluorescence in the nucleus nigra. After which, the sections were processed for HRP reaction according to Mesulam's method. A number of cells of substantia nigra were found to show both fluorescence and HRP-positive product, thus confirming the dopaminergic nature of the nigroneostiatum projection.Furthermore, the nigroneostriatum projection was found to be topographically organized, the ventro-medial part of compact zone of the substantia nigra projecting to the dorso-medial caudate-putamen, the dorso-medial part of compact zone to the ventral caudate-putamen, the ventro-middle of compact zone to 1be dorso-lateral and central caudate-putamen, the caudo-lateral part to the central caudate-putamen, the middle part of reticular zone to the lateral and central caudate-putamen, the lateral zone of substantia nigra to the caudal caudate-putamen. Some overlap existed.Strionigral fibers were also topographically organized, which terminated predominantly about the cells of reticular zone. These may form a feedback loop concerning with the modulation of the activity of the dopaminergic neurons in the substantia nigra.

Different fluorescent tracers, Fast Blue (FB), Nuclear Yellow(NY), Propidium iodide (PI), Bisbenzimide (Bb), Evans Blue(EB), DAPI-Primuline (Pr), were injected into the frontal cortex, hippocampus, thalamus, cerebellum and cervical cord respectively in 31 rats to observe the fluorescent retrograde double labeling cells in the nucleus raphe magnus. The results showed that the axon collaterals of the nucleus raphe magnus projected to the :thalamus-cerebellum, thalamus-hippocampus, frontal cortex-thalamus, frontal cortex-cerebellum, hippocampus-cerebellum, hippocampus-cervical cord, cerebellum-cervical cord, thalamus-cervical cord and bilateral hippocampus and thalamus. The topographical organizations of divergent projection neurons of the nucleus raphe magnus were also found.

WGA-HRP was injected into different segment of the dorsomedial nucleus (DM) of thalamus by means of microiontophoretical delivery in 19 rats to observe the afferent projections of DM. It is found that the DM received afferent projections widely from the frontal cortex, basal telecephalon, limbic system, monoaminergic neuron, reticular formation, central gray matter and superior colliculus. Furthermore, some topographical organization has also been observed. The medial segment of DM received afferent projections mainly from dorsal agranular insular cortex, infralimbic cortex, amygdaloid complex, septum, bed nucleus of the stria terminalis and raphe nuclei, related with the function of the limbic system. The central segment of DM received afferent projections from the dorsal agranular insular cortex and the prepiriform cortex, also related with the function of the limbic system. The lateral segment of DM received afferent projections mainly from the medial precentral cortex, dorsal division of anterior cingulate cortex, E-W nucleus and superior colliculus, related with the function of eye movement. In addition, all the three segments of DM received numerous afferents from the substantia innominata, nucleus tractus diagonalis (Broca), lateral preoptic area and reticular nucleus of the thalamus.

The horseradish peroxidase tracing method was used to investigate the thalamostriate projections in the rat. Following the caudoputamen injections, numerous labeled neurons were present in the intralaminar nuclei of the thalamus. HRP-positive cells were found mainly in the central lateral nucleus following the injections of the medial portion of the caudoputamen, and in the paracentral nucleus following the injections of the lateral portion of the caudoputamen. Besides the intralaminar nuclei, however, considerable amount of labeled neurons were found in other tbalamic nuclei as well, including the ventral anterior, ventral lateral and ventral medial nuclei of the thalamus. In addition, a few labeled cells were also found in the dorso medial, anterior dorsal, anterior ventral, and anterior medial thalamic nuclei.

Using the HRP retrograde tracing method, the projections from the mammillary nucleus to the anterior nuclear complex of the thalamus were studied in the rat. The mamillo- thalamic projections arised from either the medial mammillary nucleus or the lateral mammillary nucleus. The efferent fibers of the lateral part of the medial mammillary nucleus projected mainly to the ipsilateral anteroventral nucleus of the thalamus. The efferent fibers from the medial part as well as the posterior part of the medial mammillary nucleus projected to the ipsilateral anteroventral and anteromedial nucleus of the thalamus. In addition, the projections from the median part of the medial mammillary nucleus to the anteromedial nucleus of the thalamus were also observed. The efferent fibers of the lateral mammillary nucleus projected bilaterally to the anterodorsal nucleus of the thalamus, but more on the ipsilateral side.

The projections of the cerebellar nuclei to the spinal cord were studied in the rat following injections of HRP into the cervical, thoracic, lumbar and sacral cord. Numerous HRP-positive neurons were found in the fastigial nucleus (F) and the posterior interpositus nucleus (IP) on both sides, but more on the contralateral side. In the F labelled neurons were mainly distributed in the ventrolateral and medial part of the middle portion of the nucleus, and considerable amount of labelled cells were found in the bundles of the cerebellar corticovestibular fibers as well. In the IP labelled neurons were seen only in the medial part of the nucleus. In addition, a few labelled cells were also found in the dentate nucleus (D) bilaterally. The present study suggests that the neurons of the F, IP and D project their fibers to C_4, a few fibers may extend caudally to C_8.

WGA-HRP was injected into the dorsomedial nucleus of thalamus (DM)by means of microiontophoretical delivery in 19 rats, and into the dorsal division of anterior cingulate cortex (ACd), the prelimbic area (PL), the dorsal agranular insular area (AId) of prefrontal cortex by using microinjective system in 17 rats to observe the efferent projections of the DM.It was found that the DM projected to a large part of the prefrontal cortex including the medial precentral area (PCm), ACd, PL, AId, ventral agranular insular area (AIv), medial orbital area (MO), ventral orbital area (VO), ventrolateral orbital area (VLO), and some topographical organizations were also found. The lateral segment of DM projects to the ACd, PCm, PL, AId, AIv and VLO. The intermediate segment of DM projects to the AId and AIv. The medial segment of DM projects to the AId, PL, IL, AIv, MO and VO. Furthermore, the DM also projects to some subcortical nuclei, such as the reticular nucleus of thalamus (R), the lateral preoptic area (LPO), the lateral hypothalamic area (LHA), the caudoputamen, the nucleus accumbens and the parafascicular nucleus of thalamus. In addition, some reciprocal connections between the DM and the ACd, PCm, PL, IL, AId, R, LPO and LHA were observed.