Cerebellar, medullary and spinal afferent connections of the paramedian reticular nucleus in the cat

The topographic organization of afferent projections from the deep cerebellar nuclei, medulla oblongata and spinal cord to the paramedian reticular nucleus (PRN) of the cat was studied using the horseradish peroxidase (HRP) method of retrograde labelling. Discrete placements of HRP within each of the dorsal (dPRN) and ventral (vPRN) regions of the PRN showed some segregation of input. The deep cerebellar nuclei project in a predominantly contralateral fashion upon the PRN. A small but significant ipsilateral fastigial afferent component is also present. The fastigial and dentate nuclei contribute the majority of fibers to the dPRN whereas the interposed nucleus provides very little. The vPRN receives a relatively uniform input from all 3 cerebellar nuclei. Both lateral vestibular nuclei contribute the majority of fibers from the vestibular nuclear complex largely from their dorsal division. Additional input arises from bilateral medial and inferior vestibular nuclei. The vPRN receives relatively more fibers from the inferior vestibular nuclei than does the dPRN while inputs from the medial vestibular nuclei are comparably sparse. The PRN receives bilateral projections from the nucleus intercalatus (of Staderini). A significant projection to the contralateral PRN occurs from the ventrolateral subnucleus of the solitary complex and its immediate vicinity. Additional sources of medullary afferent input include the lateral, gigantocellular and magnocellular tegmental fields, the contralateral PRN and the raphe nuclei. Sites of origin of spinal afferents to the dPRN are bilaterally distributed mainly within Rexed's laminae VII and VIII of the cervical cord whereas those to the vPRN are confined largely to the medial portion of the contralateral lamina VI in the C1 segment. A few labelled cells are found in the thoracolumbar cord with those to the vPRN being more caudal. These data provide the neuroanatomical substrate for a better understanding of the functional role of the PRN in mediating cardiovascular responses appropriate to postural changes.     

Afferent projections to the thalamic mediodorsal nucleus in the cat studied by retrograde and anterograde axonal transport of horseradish peroxidase

The afferent pathways to the thalamic mediodorsal nucleus (MD) in the cat were studied using the methods of anterograde and retrograde axonal transport of horseradish peroxidase (HRP) and wheat germ agglutinin conjugated to HRP (WGA-HRP). The MD receives fibers from the prefrontal cortex in a topically organized manner in accordance with the thalamocortical projections. The medial or ventral portion of the MD receives afferents from the islands of Calleja of the olfactory tubercle, the nucleus of the diagonal band, the amygdala and the claustrum. The lateral hypothalamic nucleus sends a moderate number of fibers to the medial MD, but other hypothalamic nuclei send only a few fibers to the MD. The lateral or dorsal portion of the MD receives fibers from the nucleus of the diagonal band, the ventral pallidum and the entopeduncular nucleus, but only few from the olfactory tubercle and the amygdala. The thalamic reticular nucleus sends many fibers to the MD without showing any topography. The MD, particularly its lateral part, receives afferents from brainstem structures, such as the substantia nigra, superior colliculus, reticular formation, raphe nuclei and nucleus loci coerulei. Only the interpeduncular nucleus sends fibers mainly to the medial part of the MD. The cerebellar nuclei send only a few fibers to the lateral part of the MD at posterior levels.     

Projections from brainstem nuclei to the nucleus paragigantocellularis lateralis in the cat

Afferent projections from the pons and medulla to the nucleus paragigantocellularis lateralis (PGL) have been mapped in the cat using retrograde transport of horseradish peroxidase (HRP). In the caudal medulla, the major sources of afferents were the medial and lateral divisions of the solitary nuclei complex and the contralateral trigeminal nucleus caudalis. Labelled cells were also present in the dorsal column nuclei, nucleus intercalatus and praepositus hypoglossi but this may have been due to uptake of HRP into fibres of passage. In the dorsolateral medulla and pons, neurones in the vestibular complex and in the parabrachial nucleus were labelled bilaterally. Nucleus raphe magnus and raphe obscurus were both found to send projections to the PGL and labelled cells were also present throughout the pontine and medullary reticular nuclei as well as in PGL on the side opposite to the injection of HRP. These findings are discussed in relation to the role of the PGL in cardiovascular regulation and in the control of pain.     

Topographical organization in the origin of serotoninergic projections to different regions of the cat cerebellar cortex

The distribution of serotonin immunoreactivity in the cat cerebellum was studied by using the indirect antibody peroxidase-antiperoxidase (PAP) technique. Furthermore, the origin of these chemically defined afferents was determined by combining the retrograde transport of horseradish peroxidase (HRP) with the PAP technique. In the cerebellar cortex, serotonin immunoreactivity is present in a plexus of beaded fibers that is confined almost exclusively to the granule and Purkinje cell layers; a few fibers are present in the molecular layer. Serotoninergic axons and varicosities have a dense and uniform distribution throughout all lobules of the cerebellum with the exception of lobule X where the fiber density is sparse. Serotonin cell bodies were not found within the cerebellar cortex. However, following pretreatment with pargyline and L-tryptophan, serotonin positive cell bodies were found in all deep cerebellar nuclei as well as the raphe and reticular nuclei in the brainstem. The present study demonstrates that the serotoninergic projection to the cat's cerebellum has some degree of topographical organization. Serotoninergic fibers in the anterior vermis (lobules I-V) were shown to arise from neurons located within the paramedian reticular nucleus, the lateral reticular nucleus, and the lateral tegmental field. Injections of HRP into either the posterior vermis (lobule VI-IX) or the paramedian lobule, labeled serotoninergic neurons exclusively in the lateral reticular nucleus. Lobus simplex, crus I and crus II (the hemisphere) receive a serotoninergic input from cells located in the lateral tegmental field, the peri-olivary reticular formation and the paramedian reticular nucleus. In no cases were neurons in the raphe double-labeled, although there were cells positive for HRP or serotonin alone. The data indicate that there is a topographical organization in the serotoninergic projection from the caudal brainstem to specific regions of the cat's cerebellar cortex. In addition to climbing and mossy fibers, this projection represents a third major source of cerebellar afferents based on its dense and widespread distribution as well as its morphological and chemical characteristics.     

Afferent projections to the ventral tegmental area of Tsai and interfascicular nucleus: a horseradish peroxidase study in the rat.

Using the retrograde transport of horseradish peroxidase (HRP), a study has been made of projections to the ventral tegmental area of Tsai (VTA) and related dopaminergic cell groups (A 10). In order to minimise the possibility of damage to fibres of passage, a technique was evolved for the microiontophoresis of HRP such that minimal current strengths and durations were applied. In addition to a sham injection, control injections were also made to the medial lemnisuc, red nucleus, deep tegmental decussations, mesencephalic reticular formation and brachium conjunctivum. Following HRP injections confined to the areas of the VTA containing the dopamine cell groups, labelled neurons appeared in prefrontal cortex, dorsal bank of rhinal sulcus, nucleus accumbens, bed nucleus of stria terminalis, amygdala, diagonal band of Broca, substantis innominata, magnocellular preoptic area, medial and lateral preoptic areas, anterior, lateral and postero-dorsal hypothalamus, lateral habenular, nucleus parafascicular nucleus of thalamus, superior colliculus, nucleus raphe dorsalis, nucleus raphe nagnus and pontis, dorsal and ventral parabrachial nuclei, locus coeruleus and deep cerebellar nuclei. Regions containing catecholamine groups A 1, A 5, A 6, A 7, A 9, A 13 and the serotonin group B 7 corresponded to the topography of labeled cell groups. Injections of HRP to the interfascicular nucleus resulted in labeling predominantly confined to the medial habenular and median raphe nuclei. The results are discussed in relation to the known connections of these regions. Other regions of the brain labelled by VTA injections are assessed in relation to control injections and the limitations of the HRP technique. A review of the organisation of some of these afferents in relation to the known cortical-subcortical-mesencephalic projection systems, suggests that the VTA is in a position to recieve information from a massively convergent system derived ultimately from the entire archi-, paleo-, and neo-cerebral cortices. In addition A 10 dopaminergic neurons are known to project to restricted regions of both pre-frontal and entorhinal cortices, which themselves also recieve massively convergent association cortico-cortical connections. It would appear reasonable to propose that these neurons perform a correspondingly important integrative function.     

Afferent projections to the cholinergic pedunculopontine tegmental nucleus and adjacent midbrain extrapyramidal area in the albino rat. I. Retrograde tracing studies.

The afferent connections of the pedunculopontine tegmental nucleus (PPT) and the adjacent midbrain extrapyramidal area (MEA) were examined by retrograde tracing with wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). Major afferents to the PPT originate in the periaqueductal gray, central tegmental field, lateral hypothalamic area, dorsal raphe nucleus, superior colliculus, and pontine and medullary reticular fields. Other putative inputs originate in the paraventricular and preoptic hypothalamic nuclei, the zona incerta, nucleus of the solitary tract, central superior raphe nucleus, substantia innominata, posterior hypothalamic area, and thalamic parafascicular nucleus. The major afferent to the medially adjacent MEA originates in the lateral habenula, while other putative afferents include the perifornical and lateral hypothalamic area, periaqueductal gray, superior colliculus, pontine reticular formation, and dorsal raphe nucleus. MEA inputs from basal ganglia nuclei include moderate projections from the substantia nigra pars reticulata, entopeduncular nucleus, and a small projection from the globus pallidus, but not the subthalamic nucleus. Dense anterograde labeling was observed in the substantia nigra pars compacta, entopeduncular nucleus, subthalamic nucleus, globus pallidus, and caudate-putamen only following WGA-HRP injections involving the MEA. The results of this study demonstrate that the PPT and MEA share many potential afferents. Remarkable differences were found that support distinguishing between these two nuclei in future studies regarding the functional organization of the midbrain and pons. The results, for example, confirm our previous observations that the largely reciprocal connections between the midbrain and basal ganglia distinguish the MEA from the PPT. Afferents from the lateral habenula and contralateral superior colliculus represent extensions of more traditional basal ganglion circuitry which further delineate the MEA from the PPT. The results are discussed with respect to the important role of the midbrain and pons in behavioral state control and locomotor mechanisms.     

Afferents to brain stem nuclei (brain stem raphe, nucleus reticularis pontis caudalis and nucleus gigantocellularis) in the rat as demonstrated by microiontophoretically applied horseradish peroxidase

Using a retrograde tracer technique with microiontophoretically applied horseradish peroxidase (HRP), afferent projections to the brain stem raphe nuclei (BR, raphe magnus, pallidus and obscurus) and to two adjacent reticular nuclei, nucleus reticularis pontis caudalis (nRPC) and nucleus gigantocellularis (nGC) were identified. The most striking difference between the afferent projections to the BR and the adjacent nuclei as determined by this method is that afferents to the BR originate primarily from structures rostral to the pons, especially the mesencephalic central gray and the dorsal and ventral tegmentum. In contrast, the two reticular nuclei studied (nGC and nRPC) received afferent projections within or caudal to the pons-medulla. For example, the nGC receives prominent afferent projections from the gray matter of the spinal cord. In addition, evidence for interconnections between all of the adjacent nuclei (BR, nGC and nRPC) was found. Such afferent projections are compatible with the notion that the brain stem raphe nuclei may serve as connections within the brain stem for a descending system, while the nGC may be a relay in a feedback loop between the spinal cord and the reticular formation.     

Afferent and efferent connections of the optic tectum in the carp (Cyprinus carpio L.)

The afferent and efferent connections of the tectum opticum in the carp (Cyprinus carpio L.) were studied with the HRP method. Following iontophoretic peroxidase injections in several parts of the tectum anterograde transport of the enzyme revealed tectal projections to the lateral geniculate nucleus, dorsal tegmentum, pretectal nuclei, nucleus rotundus, torus longitudinalis, torus semicircularis, nucleus isthmi, contralateral tectum and to the mesencephalic and bulbar reticular formations.Tectal afferents were demonstrated by retrograde HRP transport in the area dorsalis pars centralis of the telencephalon, torus longitudinalis, torus semicircularis, nucleus isthmi, nucleus profundus mesencephali, several pretectal nuclei, dorsomedial and dorsolateral thalamic nuclei, nucleus of the posterior commissure, mesencephalic and bulbar reticular nuclei and nucleus ruber. Visuo-cerebellar circuitry was investigated by means of peroxidase injections in the various parts of the cerebellum. These experiments revealed indirect retino- and tecto-cerebellar pathways via the pretectal nuclei and the nucleus isthmi.     

Segmental distribution and central projectionsof renal afferent fibers in the cat studied by transganglionic transport of horseradish peroxidase

The segmental and central distributions of renal nerve afferents in adult cats and kittens were studied by using retrograde and transganglionic transport of horseradish peroxidase (HRP). Transport of HRP from the central cut ends of the left renal nerves labeled afferent axons in the ipsilateral minor splanchnic nerves and sensory perikarya in the dorsal root ganglia from T12 to L4. The majority of labeled cells (85%) were located between L1 and L3. A few neurons in the contralateral dorsal root ganglia were also labeled. Labeled cells were not confined to any particular region within a dorsal root ganglion. Some examples of bifurcation of the peripheral and central processes within the ganglion were noted. A small number of preganglionic neurons, concentrated in the intermediolateral nucleus, were also identified in some experiments. In addition, many sympathetic postganglionic neurons were labeled in the renal nerve ganglia, the superior mesenteric ganglion, and the ipsilateral paravertebral ganglia from T12 to L3 Transganglionic transport of HRP labeled renal afferent projections to the spinal cord of kittens from T1 1 to L6, with the greatest concentrations between Ll and L3. These afferents extended rostrocaudally in Lissauer's tract and sent collaterals into lamina I. In the transverse plane, a major lateral projection and a minor medial projection were observed along the outer and inner margins of the dorsal horn, respectively. From the lateral projection many fibers extended medially in laminae V and VI forming dorsal and ventral bundles around Clarke's nucleus. The dorsal bundle was joined by collaterals from the medial afferent projection and crossed to the contralateral side. The ventral bundle extended into lamina VII along the lateroventral border of Clarke's nucleus. Some afferents in the lateral projection could be followed ventrally into the dorsolateral portion of lamina VII in the vicinity of the intermediolateral nucleus. In the contralateral spinal cord, labeled afferent fibers were mainly seen in laminae V and VI These results provide the first anatomical evidence for sites of central termination of renal afferent axons. Renal inputs to regions (laminae I, V, and VI) containing spinoreticular and spinothajamic tract neurons may be important in the mediation of supraspinal cardiovascular reflexes as well as in the transmission of activity from nociceptors in the kidney. In addition, the identification of a bilateral renal afferent projection in close proximity to the thoracolumbar autonomic nuclei is consistent with the demonstration in physiological experiments of a spinal pathway for the renorenal sympathetic reflexes.     

Anatomy of respiratory rhythmic systems in brain stem and cerebellum of the carp

The afferent and efferent connections of two respiratory rhythmic loci in the dorsal mesencephalic tegmentum were studied by retrograde and anterograde transport of horseradish peroxidase. the injection areas were determined with extracellular activity recording using HRP filled glass micropipettes, and then followed by electrical stimulation and subsequent iontophoretic HRP delivery. One area in the nucleus of the posterior commissure was found to be optic related respiratory in nature and possessed afferents from optic tectum, pretectal nuclei, preoptic nucleus and the bulbar reticular formation. An extensive set of efferents is present to the torus longitudinalis, nucleus rotundus, corpus cerebelli and the various levels of the reticular formation. The second respiratory rhythmic area was localized in the vicinity of the oculomotor nuclei. This area receives afferent information from corpus cerebelli, vestibular nucleus and reticular formation, and has efferent connections to corpus cerebelli, preoptic nucleus and a major projection to the various parts of the reticular formation. Stimulation of both areas resulted in respiratory movements of the lower jaw and the opercula. Several injections in the corpus cerebelli resulted in retrograde labeling in the nucleus of the posterior commissure, which suggests the involvement of cerebellar circuits in optic related respiratory reflexes.     

Afferent projections to the rat locus coeruleus as determined by a retrograde tracing technique

Afferent connections to the rat locus coeruleus (LC), which contains exclusively noradrenergic neurons, have been traced using the technique of retrograde transport of horseradish peroxidase (HRP). In order to ensure accurate placement of adequate amounts of HRP in the LC, a microiontophoretic delivery technique coupled with single cell recording was employed. The use of electro physiological "landmarks" as aids in placing the injections is described. Following HRP injections into the LC, forebrain structures containing labelled neurons included the insular cortex, the central nucleus of the amygdala, the medial, lateral and magnocellular preoptic areas, the bed nucleus of the stria terminalis, and the dorsomedial, paraventricular and lateral hypothalamic areas. In the brainstem reactive neurons were observed in the central grey substance, the reticular formation, the raphe, vestibular, solitary tract and lateral reticular nuclei. In particular, the areas of catecholamine cell groups A1, A2 and A5 appeared to contain many reactive cells. Labelled neurons were also observed in the fastigial nuclei and in the marginal zones of the dorsal horns of the spinal cord. This pattern of afferent innervation supports suggestions for a role for the LC in behavioral arousal mechanisms and autonomic regulation.     

Origins of cerebellar mossy and climbing fibers immunoreactive for corticotropin-releasing factor in the rabbit

Corticotropin-releasing factor (CRF) has been implicated by both anatomical and physiological techniques as a potential cerebellar transmitter or modulator. In the present experiment, with the aid of immunohistochemistry, we have described specific cerebellar afferent pathways in the rabbit in which CRF is located. CRF-immunoreactive climbing fibers were present in the molecular layer throughout the cerebellum, but especially in lobules 8–9a. All inferior olivary neurons were CRF-immunoreactive. In lobules 8–9a, CRF-immunoreactive mossy fibers were organized in sagittal bands. The highest density of CRF-immunoreactive mossy fiber terminals was observed in the granule cell layer of lobules 8–9a and the flocculus. No CRF-immunoreactive perikarya were located in rabbit cerebellum. The brainstem origin of CRF-immunoreactive mossy fiber terminals was suggested by numerous CRF-immunoreactive perikarya located in the medial, lateral and descending vestibular nuclei, nucleus prepositus hypoglossi, nucleus x, paramedian reticular nucleus, gigantocellular reticular nucleus, lateral reticular nucleus, and raphe nuclei. Using double label experiments, we investigated the specific CRF afferent projection to the flocculus and posterior vermis. Horseradish peroxidase (HRP) injections into the posterior vermis double labeled CRF-immunoreactive neurons in the caudal medial and descending vestibular nuclei and nucleus prepositus hypoglossi. HRP injections into the flocculus double labeled more CRF-immunoreactive neurons in the nucleus prepositus hypoglossi than in the vestibular nuclei. HRP injections into either the posterior vermis or flocculus double labeled CRF-immunoreactive neurons in the paramedian reticular nucleus, nucleus reticularis gigantocellularis, and raphe nuclei. These data suggest that CRF may play an important role in vestibularly related functions of the cerebellum. © 1993 Wiley-Liss, Inc.     

A horseradish peroxidase study of the rubral and cortical afferents to the lateral reticular nucleus in the rat

The origin and organization of supraspinal afferents to the lateral reticular nucleus (LRN) in the rat were studied by means of the retrograde axonal transport of horseradish peroxidase (HRP). HRP was deposited into the LRN via both dorsal (stereotaxic) and ventral (microsurgical) routes. The entire cerebrum, brainstem, and cerebellum were surveyed for retrogradely labelled neurons. Significant projections arose from the contralateral red nucleus and the contralateral frontoparietal cortex. The rubral projection arose from neurons in the caudal two-thirds of the red nucleus. Ventrally and ventrolaterally located neurons projected to rostrolateral LRN, while dorsal and dorsomedial neurons projected to rostromedial LRN. The projection from the cerebral cortex arose from neurons located in layer V of the frontoparietal region. Rubral and cerebrocortical projections overlap in the rostral LRN, making this region of the nucleus a site of integration of descending inputs with ascending spinal signals.     

Interpeduncular nucleus afferents in the rat

Afferents to the interpeduncular nucleus (IPN) of the rat were studied with the horseradish peroxidase (HRP) retrograde transport method. HRP was deposited microelectrophoretically in the IPN of adult rats. Major projections to the IPN originate in the medial habenular nucleus, the region surrounding the dorsal tegmental nucleus (accessory dorsal tegmental nucleus and the so-called dorsal tegmental nucleus pars lateralis), and the midbrain raphe (nucleus centralis superior and nucleus raphe dorsalis). Also, minor projections originate in the central gray and nucleus locus coeruleus. Our results indicate that the habenulointerpeduncular projection originates solely from the medial habenular nuclei and is topographically organized; medial regions of the medial habenular nuclei project to ventral portions of IPN and lateral regions project to the dorsal IPN.     

Subcortical projections to the centromedian and parafascicular thalamic nuclei in the cat

The primary objective of this study is to identify the totality of input to the centromedian and parafascicular (CM-Pf) thalamic nuclear complex. The subcortical projections upon the CM-Pf complex were studied in the cat with three different retrograde tracers. The tracers used were unconjugated horseradish peroxidase (HRP), horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP), and rhodamine-labeled fluorescent latex microspheres (RFM). Numerous subcortical structures or substructures contained labeled neurons with all three tracing techniques. These labeled structures included the central nucleus of the amygdala; the entopeduncular nucleus; the globus pallidus; the reticular and ventral lateral geniculate nuclei of the thalamus; parts of the hypothalamus including the dorsal, lateral, and posterior hypothalamic areas and the ventromedial and parvicellular nuclei; the zona incerta and fields of Forel; parts of the substantia nigra including the pars reticularis and pars lateralis, and the retrorubral area; the pretectum; the intermediate and deep layers of the superior colliculus; the periaqueductal gray; the dorsal nucleus of the raphe; portions of the reticular formation, including the mesencephalic, pontis oralis, pontis caudalis, gigantocellularis, ventralis, and lateralis reticular nuclei; the nucleus cuneiformis; the marginal nucleus of the brachium conjunctivum; the locus coeruleus; portions of the trigeminal complex, including the principal sensory and spinal nuclei; portions of the vestibular complex, including the lateral division of the superior nucleus and the medial nucleus; deep cerebellar nuclei, including the medial and lateral cerebellar nuclei; and lamina VII of the cervical spinal cord. Moreover, the WGA-HRP and rhodamine methods (known to be more sensitive than the HRP method) revealed several afferent sources not shown by HRP: the anterior hypothalamic area, ventral tegmental area, lateral division of the superior vestibular nucleus, nucleus interpositus, and the nucleus praepositus hypoglossi. Also, the rhodamine method revealed labeled neurons in laminae V and VI of the cervical spinal cord.     

Brainstem location of serotonin neurons projecting to the caudal neurosecretory complex

Serotonergic fibers in the caudal neurosecretory complex (CNc) of poeciliids originate from neurons within, and extrinsic to this spinal cord nucleus. In the present study, retrograde tracing and immunofluorescence techniques were combined to localize extrinsic serotonergic projection neurons. The entire spinal cord and brain were sectioned after Fast Blue (FB) or horseradish peroxidase (HRP) was implanted in the CNc. No HRP or FB filled neurons were found in the spinal cord. Retrogradely filled neurons were found bilaterally in dorsolateral and ventromedial reticular nuclei, and the dorsal midbrain tegmentum. Fusiform cells in the medullary fasciculus longitudinalis medialis filled with FB but not HRP. Serotonin immunopositive neurons were found surrounding the third ventricle, in the raphe and in medullary reticular nuclei. Double labelled neurons in the medial reticular nucleus were determined to be the source of serotonergic projections to the CNc. Reticular projection nuclei are strategically situated to receive visceral sensory input from rhombencephalic cranial nerves. These putative pathways may provide an anatomical substrate by which visceral sensory information is transmitted to the CNc.     

Interpeducular nucleus afferents in the rat

Afferents to the interpeduncular nucleus (IPN) of the rat were studied with the horseradish peroxide (HRP) retrograde transport method. HRP was deposited microelectrophoretically in the IPN of adult rats. Major projections to the IPN originate in the medial habenular nucleus, the region surrounding the dorsal tegmental nucleus (accessory dorsal tegmental nucleus and the so-called dorsal tegmental nucleus pars lateralis), and the midbrain raphe (nucleus centralis superior and nucleus raphe dorsalis). Also, minor projections originate in the central gray and nucleus locus coeruleus. Our results indicate that the habenulointerpeduncular projection originates solely from the medial habenular nuclei and is topographically organized; medial regions of the medial habenular nuclei project to ventral portions of IPN and lateral regions project to the dorsal IPN.     

Subcortical afferents to the interstitial nucleus of Cajal: an anatomical retrograde tracing study in the rabbit.

Organization of brainstem projections to the interstitial nucleus of Cajal (INC) in the rabbit has been studied using the method of retrograde transport of horseradish peroxidase or wheat germ agglutinin-horseradish peroxidase conjugate. Injections of tracers into INC resulted in bilateral labelling in the medial terminal nucleus of the accessory optic tract, medial region of the zona incerta, vestibular nuclei (superior, medial, inferior), rostral portion of the prepositus nucleus and several nuclei of the pontine and medullary reticular formation. Retrograde labelling on the contralateral side was noted in all 4 deep cerebellar nuclei, the lateral vestibular nucleus, group Y, the rostral interstitial nucleus of the medial longitudinal fascicle, INC and mesencephalic reticular formation dorsal and lateral to the red nucleus. Cells of origin for the ipsilateral afferents of INC were found only in the nucleus of the posterior commissure. These data are discussed in relation to other morphological and physiological studies of afferent connectivity of INC in other species.     

Afferents to the zona incerta in the rat: a combined retrograde and anterograde study

In a first set of experiments, the retrograde transport of horseradish peroxidase (HRP) was utilized to investigate the afferent projections to the zona incerta (ZI) in the hooded rat. HRP was introduced in its crystalline form into various sectors of the ZI of seven subjects. The largest contingent of afferents arises from the following centers: the cingulate and somatosensory cortices, central amygdaloid nucleus, ventromedial hypothalamic nucleus, posterior thalamic nucleus, anterior pretectal nucleus, peripeduncular area, deep and intermediate layers of the superior colliculus, dorsal and ventral parabrachial nuclei, principal and interpolar trigeminal subnuclei, and cuneate nucleus. Other centers less systematically or more sparsely labeled were the lateral hypothalamic area, ventrobasal complex, lateral geniculate nucleus pars ventralis, medial geniculate nucleus, interstitial nucleus of Cajal, Darkschewitsch nucleus, perirubral fields, cuneiform, tegmental pedunculopontine, and deep mesencephalic reticular nuclei, pontine reticular nucleus pars oralis, lateral and interpositus cerebellar nuclei, and gracile nucleus. In a second set of experiments, an anterograde tracer (WGA-HRP) was injected in several centers projecting to the ZI in order to localize their terminal fields within this structure. It has been thus possible to distinguish a ventral zone (ventral sector of pars caudalis and pars ventralis) in which the somesthetic (somatosensory cortex, trigeminal complex, and dorsal column nuclei (DCN), collicular, and cerebellar projections terminate, from a dorsal zone (pars dorsalis) to which a limbic input (cingulate cortex and ventromedial hypothalamic nucleus) is directed. In most cases, the labeled terminal fields consisted of well-delimited, narrow bands disposed obliquely, parallel to the cerebral peduncle or the internal capsule. The contingent of somatosensory afferents is relatively large and there is a high degree of overlapping between the different somatosensory terminal fields within the ventral ZI. This suggests a participation of this structure in the treatment of somesthetic information and/or in the transmission of noxious stimuli.     

Topographic organization of the brainstem afferents to the mediodorsal thalamic nucleus

The afferent projections from the brainstem to the mediodorsal thalamic nucleus (MD) were studied in the cat, by means of retrograde transport of horseradish peroxidase. A topographical arrangement of these projections is described. The medial part of MD is the area of the nucleus which receives fewer afferents from the brainstem. After injections in this part, labeled neurons were observed mainly in the interpeduncular nucleus, the ventral tegmental area and the substantia nigra. After injections of HRP in the intermediate part of the MD, labeled cells were seen mainly in the interpeduncular nucleus, substantia nigra, dorsal and centralis superior raphe nuclei, dorsal tegmental nucleus, and coeruleus complex. Less conspicuous was the number of labeled cells in the central gray and the dorsolateral portion of the tegmentum of the mesencephalon and pons. After injections in the lateral part of MD, labeled neurons were observed mainly in the deep layers of the superior colliculus, central gray, the oral paramedian pontine reticular tegmentum, and the interpeduncular nucleus. Labeled cells were also observed in the substantia nigra, locus coeruleus, dorsal tegmental nucleus, cuneiform area, and the mesencephalic reticular formation. These findings show the MD as a thalamic link of three different groups of brainstem structures projecting to different cortical areas with different functional significance.