Two modes of cerebellar input to the parietal cortex in the cat

Summary The characteristics of cerebellar input to the parietal cortex through the ventroanterior-ventrolateral (VA-VL) complex of the thalamus were investigated in the adult cat by using combined electrophysiological and anatomical methods. Two distinct parietal regions were activated by stimulation of the cerebellar nuclei (CN). In the first region located in the depth of the bank of the ansate sulcus, stimulation of the CN induced early surface positive-deep negative potentials and late surface negative-deep positive potentials. In this cortical area, potentials of similar shape and time course were evoked at a shorter latency by stimulation of the ventrolateral part of the VA-VL complex where large negative field potentials were evoked by stimulation of the CN. After injection of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) in this part of the VA-VL complex, axon terminals of thalamocortical (TC) fibers were found in layers I, III and IV in the depth of the bank of the ansate sulcus and layers I and III in the motor cortex. In the second region located in the suprasylvian gyrus, late surface negative-deep positive potentials were evoked by stimulation of the CN and similar potentials were evoked at a shorter latency from the dorsomedial part of the VA-VL complex where large cerebellar-evoked potentials could be recorded. PHA-L injection in this thalamic region stained TC fibers and their terminals in layer I of the suprasylvian gyrus, and in layers I and III of the motor cortex. The laminar distribution of TC axon terminals in two different regions of the parietal cortex could account for the depth profiles of the cerebellar- and the thalamic-evoked potentials in each region. These results show that cerebellar information is conveyed to two separate areas in the parietal cortex by two different TC pathways.     

Distribution of terminals of thalamocortical fibers originating from the ventrolateral nucleus of the cat thalamus

Anterograde labelling following focal injections of Phaseolus vulgaris leucoagglutinin was used to identify the threedimensional cortical distribution of thalamocortical (TC) fibers from the ventrolateral nucleus of the thalamus of the cat. The labelled TC fibers were distributed usually in layers I and III of the motor cortex and the terminals in layer III tended to aggregate into patches about 1-1.5 mm wide in a mediolateral direction. These patches were arranged in longitudinal strips about 2-5 mm long in a rostrocaudal direction and were separated by gaps of terminal free area.     

Morphological features of cat thalamo-parietal projection fibers investigated by PHA-L immunohistochemistry and electron microscopy.

Thalamo-parietal fibers originating from the ventroanterior-ventrolateral (VA-VL) complex in the cat were labeled with Phaseolus vulgaris leucoagglutinin (PHA-L) and examined by light and electron microscopy. PHA-L (2.5% aqueous solution) was injected iontophoretically through micropipets with anodal current pulses into the VA-VL complex. PHA-L-labeled terminals were distributed in the lateral and the suprasylvian gyri in the superficial and deep cortical layers. In layer I, horizontal varicose fibers and terminals were conspicuous in the upper one-third and were widely distributed. In the deeper cortical layers (layers III-V), varicose fibers and terminals were detected in moderate numbers. Electron microscopic examination revealed that the labeled terminals formed asymmetrical synapses on the dendritic spines of spiny neurons. These morphological findings appeared to be consistent with our previous intracellular recordings in this cortex.     

The postnatal development of thalamocortical projections from the pulvinar in the cat

Summary The postnatal development of thalamocortical projections from the pulvinar to an association cortex of the cat, the crown of the middle suprasylvian gyrus, was studied by using both HRP and evoked field potentials. From birth onward, the pulvinar sends dense fibres to this cortical area, but the cortical laminar distribution of the afferents was found to change markedly with aging. An orthograde HRP study showed that at birth and up to 2 weeks of age, the terminals are distributed mainly in layer I, whereas in adult cats and kittens older than 1 month, the terminals are found largely in and around layer IV and only sparsely in layer I. After HRP injections exclusively into layer I of the crown, numerous thalamic neurones were retrogradely labelled in both the ventroanterior-ventrolateral complex (VA-VL) and the pulvinar in 5-day-old kittens, but in the VAVL alone in 2-month-old kittens. In agreement with these anatomical findings, stimulation of the pulvinar elicited a surface-negative, depth-positive potential in 1-week-old kittens, indicating the existence of a large current sink in layer I; however, it induced a surface-positive, depth-negative potential in 1month-old kittens, reflecting the presence of a strong current sink in the deep cortical layers.     

Organization of corticopontocerebellar connections to the paramedian lobule in the cat

Summary To reveal the organization and relative magnitude of connections from various parts of the cerebral cortex to the cerebellar paramedian lobule via the pontine nuclei, horseradish peroxidase conjugated to wheat germ agglutinin was injected in the paramedian lobule in conjunction with injection of the same tracer in various parts of the cerebral cortex in 14 cats. Termination areas of cortical fibres (anterogradely labelled) and pontine neurons projecting to the paramedian lobule (retrogradely labelled) were carefully plotted in serial sections through the pons. On the average 89% of all labelled cells were found in the pontine nuclei contralateral to the cerebellar injection, 11% in the ipsilateral pontine nuclei. The highest degree of overlap between anterograde and retrograde labelling was found after injections in the posterior sigmoid gyrus (SmI), while less overlap was found after injections of the anterior sigmoid gyrus (MsI). Injections of the second somatosensory area (SmII) and the parietal association cortex (areas 5 and 7) gave moderate degrees of overlap. Very little or no overlap was found after injections of the premotor cortex (area 6), the visual areas 17, 18 and 19 and the auditory cortex (AI and AII).It is concluded that a major cortical input to the paramedian lobule arises in the posterior sigmoid gyrus (SmI), but that additional contributions arise in the anterior sigmoid gyrus (MsI), the parietal areas 5 and 7 and the second somatosensory cortex (SmII). Among the latter regions probably the parietal areas contribute most. Overlap between terminal regions of cortical fibres and cells projecting to the paramedian lobule takes place at numerous discrete sites at virtually all rostrocaudal levels of the pons. Cerebrocortical afferents via the pontine nuclei to the intermediate zone of the posterior lobe are organized according to the same principles as described previously for cortical afferents to the hemispheral parts of the posterior lobe (crus I and II).     

Morphological evidence for axonal sprouting of cerebellothalamic neurons in kittens after neonatal hemicerebellectomy

Summary Changes in cerebellothalamic projections in kittens after neonatal hemicerebellectomy were studied by the retrograde and anterograde horseradish peroxidase (HRP)-tract-tracing methods. The number of cerebellar nuclear neurons labeled retrogradely with HRP injected into the ipsilateral VA-VL complex of the thalamus was much more numerous in neonatally hemicerebellectomized kittens than in intact kittens. Presumed terminals of ipsilateral cerebellothalamic fibers labeled anterogradely with HRP injected into the cerebellar nuclei were also distributed more densely and extensively in the thalamic areas, especially in the VA-VL complex, of hemicerebellectomized kittens than in the thalamic areas of the control kittens. These results are in good accordance with those obtained from the previous electrophysiological study (Kawaguchi et al., 1979) and offer corroborating evidence for axonal sprouting of cerebellothalamic neurons after neonatal hemicerebellectomy.     

The connections of cortical somatosensory areas I and II with separate nuclei in the ventroposterior thalamus in the raccoon

The thalamocortical afferents to cortical somatosensory areas I (SI) and II (SII) were investigated in the raccoon using the horseradish peroxidase technique. The purpose of this study was to determine if the cell bodies of origin for thalamocortical afferents to these cortical regions were localized in the same or different nuclei in the ventroposterior region of the thalamus. Horseradish peroxidase was injected into subdivisions of SI or SII and after post-injection survival periods of 12-72 hours the horseradish peroxidase in the tissue was reacted with the chromogens dihydrochlorobenzidine or tetramethylbenzidine in the presence of hydrogen peroxide. The results show that SI and SII receive projections from neurons in separate and distinct nuclei in the ventroposterior thalamus. Following injections into subdivisions of area I, a topographical distribution of retrogradely-labelled cell bodies was observed in the ventrobasal complex. Following injections of horseradish peroxidase into subdivisions of area II, a topographical distribution of labelled cell bodies was observed in the ventroposterior inferior nucleus. No labelled cell bodies were observed in the ventrobasal complex. The thalamocortical connections of somatosensory cortices I and II in raccoon are compared with those in other animals and it is suggested that these two cortical areas may be involved in differential processing of tactile information.     

Thalamic afferents to layer I of anterior sigmoid cortex originating from the VA-VL neurons with entopeduncular input

Summary Seventy seven thalamic neurons in the VA-VL nuclear complex of the cat which projected to the anterior sigmoid gyrus (ASG) were studied extracellularly, and their responses to stimulation of both the cerebellar nuclei (CN) and the entopeduncular nucleus (ENT) were examined. Forty two neurons were inhibited by ENT-stimulation but were not excited by CN-stimulation, while 27 neurons were excited by CN-stimulation but were not inhibited by ENT-stimulation. Eight neurons were inhibited by ENT-stimulation and also excited by CN-stimulation. Distribution of axons in each layer of ASG was examined by means of antidromic activation of the thalamic neurons by microstimulation at various depths of ASG. Almost all (13/14) of the ENT-inhibited neurons examined were antidromically activated by microstimulation in layer I with currents less than 35 A. On the other hand, most (11/13) of CN-excited neurons were antidromically activated by weak microstimulation in layers deeper than the second, but were not activated by microstimulation in layer I with currents less than 40 A. VA-VL neurons with inhibitory input from ENT were considered to project mainly to layer I of ASG.     

Evidence for collateral projections to the retrosplenial granular cortex and thalamic reticular nucleus from glutamate and/or aspartate-containing neurons of the anterior thalamic nuclei in the rat

 Small, stereotaxically guided injections of true blue (TB) were made into the retrosplenial granular cortex (RSg) and of diamidino yellow (DY) into the dorsal portion of the rostral pole of the thalamic reticular nucleus (TRN) in 16 adult rats to determine whether axons projecting from the anterior thalamic nuclear complex (ATN) to the TRN are branches of axons also projecting to the RSg. Following injections of the fluorescent dyes, serial coronal sections of the brain revealed single retrogradely labelled, and large numbers of double retrogradely labelled neuronal cell bodies in the ipsilateral anteroventral and anterodorsal nuclei and smaller numbers in the anteromedial nucleus of the ATN complex. In a se- cond series of six adult rats with similar double injections of TB and DY, two sections in three were immunoreacted, one with antiserum against glutamate and one with antiserum against aspartate, using indirect immunofluorescence with rhodamine to detect reactive cells. The great majority of both single and double retrogradely labelled cell bodies were also immunoreactive for aspartate or glutamate. In addition, a moderate to small number of non-immunolabelled neurons projecting to the TRN and/or to the RSg were also found in all three nuclei of the ATN complex. These results are compatible with the possibility that large numbers of neurons in the ATN send axonal branches to both the RSg and the TRN, and that many such neurons use glutamate and/or aspartate as transmitters. The findings also suggest that the projections from the ATN might be heterogeneous with respect to transmitter phenotype. Received: 27 June 1996 / Accepted: 5 February 1997     

The terminal distribution pattern of spinocerebellar fibers

Summary The terminal fields of spinocerebellar fibers from different levels (cervical, thoracic and lumbar) of the spinal cord in the chick were determined by using wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP), an anterograde labelling technique. More terminals were found in the anterior lobe than in the posterior lobe. Following injections in the lumbar spinal cord, mossy fiber terminals were found mostly in the anterior lobe (lobules I–V). The highest density of labelled terminals was found in lobule I, and labelled terminals were found in all parts of the lobule. However in lobules II–IV, labelled fibers were mediolaterally arranged in three longitudinal strips on both sides of the midline, and formed a distinct zonal distribution pattern. Labelled terminals were distributed evenly from the apical to basal regions of lobules I–V. A large number of mossy fiber terminals originating from the thoracic spinal cord were located in the anterior lobe and lobule VI; some labelled terminals were found in lobule IX. Three less distinct longitudinal strips, compared to those following WGA-HRP injections in the lumbar spinal cord, were recognized on each side of the midline. Labelled mossy terminals were observed in lobules II–IX following WGA-HRP injections in the cervical spinal cord. In transverse sections two longitudinal strips were found at the apical parts of lobules II and III, whereas four thin longitudinal strips were located in lobules IV and V.The present study showed that the terminal fields of spinocerebellar fibers from each level of the spinal cord have different distribution patterns.     

Pallidofugal projections to thalamus and midbrain: A quantitative antidromic activation study in monkeys and cats

Summary The projections of monkey medial globus pallidus (and of cat entopeduncular nucleus) to thalamus and midbrain were studied with antidromic activation in order to determine the number of pallidal neurons sending axonal branches to the two sites. The animals were anesthetized with pentobarbital and several movable electrodes were used to stimulate the thalamic nuclear complex ventralis anterior — ventralis lateralis (VA-VL), the nucleus centre médian (CM), and the midbrain nucleus tegmenti pedunculopontinus (TPP). The responses of pallidal neurons were recorded with extracellular microelectrodes. In 3 monkeys 99% and 87% of 145 medial pallidal neurons responded antidromically to stimulation of VA-VL and TPP respectively. Reciprocal collision tests demonstrated that 86% of the 145 neurons sent axonal branches to the two sites. By comparison in 2 cats the tests demonstrated that 72% of 46 entopeduncular neurons branched to VA-VL and TPP. In 2 monkeys 68% of 53 medial pallidal neurons were shown to branch to VA-VL and CM thalamic nuclei. In the monkeys, the latencies of responses indicate that all pallidofugal fibers have the same mean conduction rate: 6 m/s. The fibers appear to branch profusely in VA-VL where less current was required to activate neurons antidromically than in TPP. The location of neurons in the medial pallidum is weakly correlated with the location of stimulation points in VA-VL activating the neurons antidromically at low threshold, suggesting some topography in the pallidothalamic projection. However there is no particular localization of medial pallidal neurons with and without branching projections. Apart from one exception, the 162 neurons recorded in the lateral pallidum failed to respond antidromically to the stimulation sites. We conclude that the great majority of medial pallidal neurons can send signals to both the thalamus and the midbrain in the cat and in the monkey.Supported by the Medical Research Council of CanadaPart of doctoral dissertationStudentship award from the Conseil de la recherche en sante du QuébecAssociate professor at the Dept. of Physiology of Laval University in Québec     

Non-cholinergic basal forebrain neurons project to the contralateral basal forebrain in the rat

Following injections of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) or the fluorescent tracer fluoro-gold into the magnocellular preoptic area and the horizontal limb of the diagonal band, retrogradely labelled neurons were found in the homotopic region of the contralateral basal forebrain. Labelled fibers apparently arising from these neurons travelled in the stria medullaris and the habenular commissure to terminate in the contralateral basal forebrain. Although the neurons retrogradely labelled with fluoro-gold in the contralateral basal forebrain were similar in size to choline acetyltransferase (ChAT)-immunoreactive neurons, and were intermingled with them, none was ChAT-positive. WGA-HRP injections into the nucleus basalis magnocellularis did not result in retrograde labelling in the contralateral basal forebrain. These findings suggest that non-cholinergic neurons may serve as a direct link between the two sides of selective magnocellular basal forebrain regions.     

Afferent connections of the rat substantia nigra pars lateralis with special reference to peptide-containing neurons of the amygdalo-nigral pathway

The afferent connections of the rat substantia nigra pars lateralis have been studied using the retrograde axonal transport of fluorescent latex microspheres. The most numerous groups of retrogradely labelled nerve cell bodies were observed bilaterally in the parabrachial complex and several hypothalamic nuclei, whereas the parietal neocortex, the fundus striati, the central nucleus of the amygdala and the bed nucleus of the stria terminalis were labelled on the injected side only. The neuronal projections from the central amygdaloid nucleus to the substantia nigra pars lateralis and lateral part of the rostral pars compacta have additionally been confirmed by anterograde tracing using wheat-germ agglutinin coupled to horseradish peroxidase. The presence of some peptides in this pathway was studied by combining the use of the same retrograde tracer with immunofluorescence after intra-amygdaloid injections of colchicine. With this method, we have demonstrated that Met-enkephalin, dynorphin and neurotensin are probably utilized as neurotransmitters or co-transmitters in the neurons of the amygdalo-nigral pathway.     

The projection of different retinal ganglion cell classes to the dorsal lateral geniculate nucleus in the hooded rat

Summary The morphology of retinal ganglion cells which project to different parts of the dorsal lateral geniculate nucleus (DLG) in the hooded rat has been investigated. Small amounts of a retrograde tracer (horseradish peroxidase) were injected into the DLG, then labelled retinal ganglion cells were examined in retinal wholemounts. After injections into different parts of the DLG, differences were noted in the size, morphology and retinal distribution of labelled retinal ganglion cells. Specifically, after injections into the antero-ventral part of the DLG labelled retinal ganglion cells were spread sparsely across the retina, had large cell somas, and many were identified with Class I or Class III morphology. After injections into the postero-dorsal part of the DLG, labelled cells were more densely packed, had smaller somas, and more were identified with Class IIa and Class III morphology. The density of labelled cells was estimated to be no more than 37% of the total retinal ganglion cell density at any retinal position examined. These results show that in the rat, as in other species such as the cat or monkey, the terminals of different classes of retinal ganglion cells are segregated within different subdivisions of the DLG. However, unlike these other species, only a minority of the total retinal ganglion cell population projects to the DLG.     

Afferents to the cerebellar cortex of turtles studied by means of the horseradish peroxidase technique

Summary Origins of afferents to the cerebellar cortex from the brainstem were explored in turtles by means of the horseradish peroxidase (HRP) technique. Following relatively large injections involving all cortical layers, HRP label was observed in neural perikarya of the following structures: 1) contralateral reticular formation just lateral and ventral to the hypoglossal nucleus; 2) a few cells in the central gray of the cervical spinal cord; 3) neurons scattered in the dorsolateral, ventromedial and descending vestibular nuclei, mainly ipsilaterally; 4) a few solitary cells in the mesencephalic and medulalry tegmentum; 5) the nucleus isthmi magnocellularis caudalis on the ipsilateral side; 6) a group of small cells in the isthmic tectum; 7) the ipsilateral nucleus of the optic tract; 8) a prominent group of small cells in the isthmic region just rostral to the vestibular complex ipsilaterally. Most of these cells were localized within the so called nuclei gustatorius secundarius,-lemnisci lateralis and-isthmi parvocellularis. This parvocellular isthmic complex (PIC) was the only region containing labelled cells when small injections restricted to the molecular layer were achieved. We interpret the PIC as a source of climbing fibers, possibly corresponding to the mammalian inferior olive which migrates from the alar plate to its' ventral destination during ontogenesis. Connecting axons were sometimes homogeneously stained which permitted the tracing of connecting pathways. Contorted axon branches stained by anterograde HRP transport were found concentrated in cerebellar and superior vestibular nuclei and sparsely distributed in other vestibular nuclei.     

Non-motoneurons in the facial and motor trigeminal nuclei projecting to the cerebellar flocculus in the cat

Summary The cerebellar projection from the facial and motor trigeminal nuclei was studied in the cat by means of retrograde axonal transport of wheat germ agglutinin-horseradish peroxidase and fluorescent tracers. The feline facial nucleus was cytoarchitectonically subdivided into ventromedial, ventrolateral, lateral, dorsal, intermediate and medial divisions (see Papez 1927), and the motor trigeminal nucleus into medial, ventral, intermediate, lateral and dorsal divisions. The neurons in the facial and motor trigeminal nuclei were classified as small (ovoid to round cells with a maximum diameter of the cell body of about 20 m) or large (polygonal to round cells with maximum diameter of about 40 m). After floccular injections of the wheat germ agglutininhorseradish peroxidase complex, retrogradely labelled cells were found throughout the facial nucleus, but especially in its medial and dorsal divisions. In the motor trigeminal nucleus, labelled neurons were found only in the ventral, intermediate and lateral divisions. Cases with tracer deposition (implants or injections) in other parts of the cerebellar cortex or nuclei were all negative. All facial and motor trigeminal neurons labelled after floccular injections were smaller than the neurons labelled after injections in the facial mimic and masticatory muscles, and only single-labelled neurons were found following floccular injections of Fluoro-Gold and muscular injections of rhodamine-B-isothiocyanate in the same animals. These observations strongly suggest that the neurons in the facial and motor trigeminal nuclei which project to flocculus are of the non-motoneuron type.     

Effects of visual deprivation on the postnatal development of the geniculocortical projection in kittens

In kittens reared with either monocular, binocular or reverse suture, beginning before the physiological eyelid opening (around one week of age) and lasting until after one month, the cortical laminar distribution of geniculocortical afferents to area 17 was examined by using orthograde transport of wheat germ agglutinin conjugated with horseradish peroxidase, and compared with that in normal kittens. In normal kittens, at birth, the afferents were distributed most densely in layer I and, to a lesser extent, widely from the upper part of layer II to layers V or VI. After one month, the afferents were found mainly in and around layer IV and very sparsely in layer I. Neither binocular nor monocular suture affected this normal development. In contrast, when the present procedure of monocular suture had been followed by opening the sutured lid and suturing the other lid (reverse suture) for one week, the distribution was altered. The density of the afferents in layer I was increased while the labelled terminals in deeper layers were as segregated in and around layer IV as observed in normal kittens. Such increase in density of the afferents resulted only when the injected tracer covered the medial or intermediate part of the C complex of the lateral geniculate nucleus. To confirm these findings, geniculate neurons retrogradely labelled by horseradish peroxidase injections into layer 1 of area 17 were examined in normal and reverse-sutured kittens. In both kinds of kittens, the labelled neurons were dense in the C complex, and absent or sparse in the A laminae. But, the number was higher in reverse-sutured kittens. These results suggest an involvement of geniculocortical layer I projections in reorganization of neuronal circuits in the visual cortex.     

The origin and survival of gastric parietal cells in the mouse

Using microradioautography, the origin of the gastric parietal (and zymogen) cells was deduced by observing the time of appearance of labelled parietal cells in relation to the other labelled cells in the gastric mucosa. In order to see whether the parietal cell divides in the adult mouse, time grain count curves of the labelled parietal cells were made from animals which were killed at 1–291 days after thymidine ³H injection DPT). Parietal cell survival was followed by observing the disappearance of the labelled parietal cell population. Parietal cells appear to be entirely derived from other cells since a significant number of labelled parietal cells does not appear unless the animal is allowed to survive for several days. Parietal cells do not appear unless the animal is allowed to survive for several days. Parietal cells do not seem to undergo mitosis in the adult mouse because the time grain count curves did not shift with time. Although a few labelled parietal cells persisted in the 291 DPT animals, a large majority of the labelled parietal cells disappeared by 90 DPT and had a half life of roughly 23 days. This suggests an element of renewal for the parietal cell population.     

The distribution of posterior parietal fibers in the corpus callosum of the rhesus monkey

Summary The distribution of posterior parietal fibers in the corpus callosum of the rhesus monkey was analyzed using autoradiographic techniques. Posterior parietal fibers are located in the posterior half of the body of the corpus callosum. There is some segregation of fibers with respect to their place of origin within the posterior parietal lobe. However, there is also overlap, particularly between fibers coming from the caudal inferior parietal lobule and the medial parietal lobe.Supported by the Veterans Administration, Edith Nourse Rogers Memorial Veterans Hospital, Bedford, Massachusetts and N.I.H. Grants NS 09211 and NS 16841     

Quantitative characteristics of the cortex area 4y associative projections to subfields of the sensomotor and parietal cerebral cortex in cat

The ipsilateral association connections of the motor cortex area 4y after its local electrolytic lesion in the cat were studied using Nauta-Gigax method. Relative quantitative distribution of the efferent association fibers projecting from area 4y to the somatosensory regions I and II, motor and parietal cortex, was determined. It was shown that the major projections from area 4y are directed to area 2pri (secondary somatosensory zone) and to area 5ab. Sparse degenerating fibers were found in the areas 1, 2, 3a and 3b of the primary somatosensory cortical zone. No efferent fibers were traced projecting from area 4y to the areas 4fu, 4sfu, 6aa, 6ab and 6ifu. It is suggested that the morphological basis of the motor reactions implemented by area 4y is formed not by fundal (4fu, 4sfu, 6ifu) or premotor (6aa, 6ab) subfields, but by the areas 2pri and 5ab which are involved to a maximum degree.