Identification of axon terminals of the cerebello-olivary fibers in the cat: an electron microscope study using the anterograde horseradish peroxidase method

Cerebellar contributions to instrumental learning

There is emerging evidence that the cerebellum is involved in spatial and nonspatial instrumental learning tasks. Cerebellar-lesioned animals have deficits in water maze learning tasks that may be explained by two-way interactions with higher order brain regions. There is suggestive evidence that cerebellar modulation extends to shock avoidance and discrimination learning. Although this evidence needs to be confirmed by a wider range of lesion methods and choice of learning tasks, it is in line with the hypothesis that the cerebellum affects cognitive processes and is not strictly concerned with motor control and the acquisition and retention of conditioned reflexes.

Comparative study of glutamate decarboxylase immunoreactive boutons in the mammalian inferior olive

An antiserum raised against rat glutamate decarboxylase was used to map GABAergic boutons in the inferior olive of rabbit, cat, rhesus monkey, and human. A description of the human periolivary region is also included. The inferior olive of each species contained a dense GABAergic innervation, but immunostaining intensities varied among regions. These intensities were evaluated visually and photometrically, and the sizes and frequencies of occurrence of boutons in various olivary subnuclei were measured. The beta nucleus in all species was intensely immunostained and contained the largest boutons. The caudal subdivision of the dorsal accessory olive stained with a lower intensity than the beta nucleus, but contained similarly large GABAergic boutons. By visual analysis, the rostral subdivision and the subnucleus a of the medial accessory olive and the principal olive were stained with an intermediate intensity, and these regions contained small GABAergic boutons. Photometric analysis of focal regions of the neuropil, however, revealed species differences in teh staining intensity of the principal olive, which was lowest in rabbits and highest in primates. In all species, the lowest immunostaining intensity was observed in the subnucleus b of the medial accessory olive. Species variations in bouton sizes and regional staining intensities were observed in the dorsal cap and the dorsomedial cell column. The heterogeneous staining pattern and regional variation of bouton size argue for the existence of separate GABAergic projections to discrete regions of the inferior olive. Since glutamate decarboxylase immunostaining patterns in the olive are largely similar across species, the afferent projections producing these patterns may also be similar.

Ultrastructural study of the GABAergic, cerebellar, and mesodiencephalic innervation of the cat medial accessory olive: anterograde tracing combined with immunocytochemistry

The rostral medial accessory olive (MAO) of the cat was studied by using an ultrastructural technique combining wheat germ agglutinin-coupled horseradish peroxidase (WGA-HRP) anterograde tracing and postembedding GABA immunocytochemistry. One group of cats received a WGA-HRP injection in the posterior interposed nucleus of the cerebellum and another group received an injection in the nucleus of Darkschewitsch. Based on differences in their morphology three types of GABAergic and three types of nonGABAergic terminals were observed. One type of GABAergic terminal was often GABA/WGA-HRP double-labeled in the cerebellar experiments, and one type of nonGABAergic terminal was often WGA-HRP-labeled in the mesodiencephalic experiments. Following injections of WGA-HRP in the cerebellar nuclei virtually all WGA-HRP-labeled terminals were GABA positive. Quantification of these GABA/WGA-HRP-double-labeled terminals showed that 1) 30% of the GABAergic terminals randomly selected from the entire neuropil were double-labeled, 2) 13% of the GABAergic terminals adjacent to perikarya were double-labeled, and 3) 34% of the GABAergic terminals strategically located next to both of the dendritic elements linked by a gap junction were double-labeled. Statistical analysis of the above data showed that significantly fewer GABAergic terminals adjacent to perikarya were double-labeled (P less than .001) than would be expected from the double-labeled proportion of the randomly selected GABAergic terminals. Following injection of WGA-HRP in the nucleus of Darkschewitsch, all WGA-HRP-labeled terminals were GABA-negative. Quantification of these terminals showed that 1) 26% of the randomly selected nonGABAergic terminals were WGA-HRP labeled, 2) 20% of the nonGABAergic terminals adjacent to perikarya were WGA-HRP labeled, and 3) 23% of the nonGABAergic terminals strategically located next to a gap junction were WGA-HRP labeled. No significant differences were found among these populations. Quantification of terminals of both groups of experiments mentioned above showed that GABAergic terminals composed 1) 38% of the randomly selected terminals, 2) 64% of the terminals apposed to perikarya, and 3) 53% of the terminals strategically located next to gap junctions. Statistical analysis of the above data showed that significantly more GABAergic terminals were located adjacent to perikarya (P less than .001) and strategically next to a gap junction (P less than .05) than would be expected from the random GABAergic innervation. The above findings of the GABAergic, cerebellar, and mesodiencephalic input are discussed with regard to their functional role in the neuronal circuitry of the ros

Origin and trajectory of the cerebello-olivary projection: an experimental study with radioactive and fluorescent tracers in the cat

It was demonstrated with autoradiographic methods that a tract of thin fibers, which is for the most part distinct from the brachium conjunctivum and its crossed descending limb, can be followed from the cerebellar nuclei to the inferior olive. Neurons from all regions of the interposed and dentate nuclei contribute to this tract. With two fluorescent tracers, Fast Blue and Diamidino Yellow Dihydrochloride, the perikarya of neurons in the cerebellar nuclei that project to the inferior olive or thalamus were identified and the possibility of collateral projections from single neurons to both regions was investigated. Numerous retrogradely labeled neurons were seen in the contralateral interposed and dentate nuclei but no double-labeled cells were ever encountered. The mean diameters of the cells projecting to the inferior olive were between 15 and 20 microns while the mean diameters for those projecting to the nucleus ventrolateralis of thalamus were between 25 and 35 microns. Differences in the morphology of the two cell populations were noted. It was also found that the fastigio-olivary projection is small, terminating mainly contralaterally, in the caudal medial accessory olive and subnucleus beta. It is concluded that cerebello-olivary fibers are not collaterals of the cerebellothalamic projection and that there are two different populations of neurons in the cerebellar nuclei that give rise to these distinct efferent projections.

The cerebellar nucleo-olivary and olivocerebellar nuclear projections in the cat as studied with anterograde and retrograde transport in the same animal after implantation of crystalline WGA-HRP. III. The interposed nuclei

The bidirectional connections between the inferior olive and the cerebellar nuclei were investigated by means of anterograde and retrograde transport after implantation of crystalline wheat germ agglutinin-horseradish peroxidase complex in the interposed nuclei. The projections from the interposed nuclei to the inferior olive show a detailed topical arrangement. The main projection from the anterior interposed nucleus reaches the rostral two thirds of the dorsal accessory olive, while the main projection from the posterior interposed nucleus reaches the rostral half of the medial accessory olive. The projections from the inferior olive to the interposed nuclei show a more widespread distribution and appear to be less precisely organized. Both interposed nuclei receive afferents from the medial and dorsal accessory olives, the dorsomedial cell column, nucleus beta and the dorsal cap. Our findings give evidence that the olivo-interposed and interposito-olivary projections are in part reciprocally organized. Our observations are discussed and related to previous investigations on the cerebello-olivary and olivocerebellar pathways. Some methodological comments are made. It appears from our results that anterograde transport in some of our cases has occurred only from a restricted part of the stained area at the implantation site.

Factors affecting the ultrastructural pattern of anterograde labeling in axon terminals with HRP

Comparisons of anterograde labeling of axon terminals originating from short and long projection neurons were made in the hypoglossal nucleus. Injections of dilute and concentrated horseradish peroxidase (HRP) or wheat germ-agglutinin-horseradish peroxidase (WGA-HRP) were made via a glass micropipette into the nucleus reticularis parvocellularis (RPc = short projection neurons) and the Spinal V trigeminal complex (Sp. V = long projection neurons). Axon terminals in the hypoglossal nucleus, a common projection site of the two efferent systems, were evaluated ultrastructurally using diaminobenzidine (DAB) as the chromogen for the cobalt-glucose oxidase (CO-GOD) method of HRP labeling. Labeled axon terminals from these two sources demonstrated different distribution patterns of the reaction product. For the short pathway, high concentrations of the tracers resulted in diffuse, agranular labeling in the majority of axon terminals. Dilute concentrations of the tracers were associated with membrane-bound, granular type of labeling. All anterograde labeling of terminals of long projection neurons (Sp. V) was membrane-bound and granular irrespective of the tracer concentration. The length of the pathway and the concentration of the enzyme tracers are factors that affect the pattern of anterograde label in axon terminals of hypoglossal afferents.

The cerebellar nucleo-olivary and olivo-cerebellar nuclear projections in the cat as studied with anterograde and retrograde transport in the same animal after implantation of crystalline WGA-HRP. II. The fastigial nucleus

The bidirectional connections between the inferior olive and the fastigial nucleus were studied by means of anterograde and retrograde transport after implantation of crystalline wheat germ agglutinin-horseradish peroxidase (WGA-HRP) complex into the fastigial nucleus. The fastigio-olivary fibres terminate in the caudal half of the medial accessory olive, nucleus beta and the dorsal cap, and the olivo-fastigial projection has its origin within the same olivary regions. A topical arrangement is indicated for both pathways. The lateral part of the medial accessory olive appears to be connected with the lateral part of the fastigial nucleus, while the medial part of the medial accessory olive appears to be connected with more medial fastigial regions. Retrogradely labelled olivo-fastigial neurons were often located within the terminal field of anterogradely labelled fastigio-olivary fibres, indicating that the olivo-fastigial and fastigio-olivary projections are at least in part reciprocally organized. The findings are discussed and related to previous studies on the olivo-cerebellar nuclear and cerebellar nucleo-olivary pathways. Some methodological considerations are made, and comments are made concerning the active area for uptake and transport from the stained area at the WGA-HRP injection site.

The cerebellar nucleo-olivary and olivo-cerebellar nuclear projections in the cat as studied with anterograde and retrograde transport in the same animal after implantation of crystalline WGA-HRP. I. The dentate nucleus

The implantation technique described by Mori et al. has been modified for the implantation of crystalline wheat germ agglutinin-horseradish peroxidase (WGA-HRP) complex. This method permits a detailed analysis of the afferent and efferent connections of the cerebellar nuclei without the complication of uptake and transport of the tracer into passing fibres. We have used this method for studies of the olivo-dentate and dentato-olivary projections in the cat. After implantation of WGA-HRP into the dentate nucleus in all our cases, both anterogradely labelled terminal dentato-olivary fibres and retrogradely labelled olivo-dentate neurons were found in the contralateral inferior olive. It appears from our findings that both projections are topically organized. The dorsal part of dentate nucleus is bidirectionally connected with the rostral part of the principal olive, the ventrolateral part of the dentate is connected with the intermediate portion of the principal olive, while its ventromedial part is connected with the caudal portion of the principal olive. The olivo-dentate and dentato-olivary connections appear to be largely reciprocally organized. The advantages and drawbacks encountered with implantation of crystalline WGA-HRP are discussed, and our observations are considered in relation to previous studies on the olivo-cerebellar and cerebello-olivary connections.

The ultrastructural identification of reticulo-hypoglossal axon terminals anterogradely labeled with horseradish peroxidase

Injections of horseradish peroxidase (HRP) or wheat-germ agglutinin-horseradish peroxidase (WGA-HRP) into the nucleus reticularis parvocellularis (RPc) produced anterograde labeling of axon terminals within the hypoglossal nucleus. Based on morphological parameters of vesicle population, membrane specializations, and postsynaptic articulations, two types of axon terminals derived from neurons in RPc end on hypoglossal neurons. More than half of the terminals contained spherical vesicles (S-type), established asymmetrical membrane specializations and contacted proximal and medium-sized dendrites. The remaining labeled terminals had flattened vesicles (F-type), symmetrical membrane densities and apposed medium and small dendrites. The morphological differences expressed in the two types of terminals may reflect physiological and/or pharmacological differences in the action of RPc neurons on motoneurons in the hypoglossal nucleus.

A light and electron microscopic study of premotor neurons for the trigeminal motor nucleus

Premotor neurons sending their axons to the trigeminal motor nucleus were observed in the cat by light and electron microscopy after labeling the neurons retrogradely or anterogradely with horseradish peroxidase (HRP). After HRP injection into the trigeminal motor nucleus, retrogradely labeled neurons were seen most frequently in the parvocellular reticular formation bilaterally. Many labeled neurons were also seen contralaterally in the intermediate zone at the rostralmost levels of the cervical cord and its rostral extension into the caudalmost levels of the medulla oblongata. Additionally, some neurons were labeled ipsilaterally in the mesencephalic trigeminal nucleus, contralaterally in the main sensory trigeminal nucleus and the trigeminal motor nucleus, and bilaterally in the oral and interpolar subnuclei of the spinal trigeminal nucleus. Only a few labeled neurons were seen in the confines of the gigantocellular reticular formation. All labeled neurons were small or of medium size; no large neurons were labeled. After HRP injection into the regions around the trigeminal motor nucleus or the parvocellular reticular formation, axodendritic terminals containing HRP granules were found contralaterally within the trigeminal motor nucleus. Some of these labeled terminals were filled with round synaptic vesicles and others contained pleomorphic synaptic vesicles. The varied morphology of labeled axon terminals was considered to reflect the functional heterogeneity of the premotor neurons for the trigeminal motor nucleus.

The ultrastructural identification of vagal terminals in the solitary nucleus of the cat after anterograde labelling with horseradish peroxidase

Horseradish peroxidase (HRP) was injected into the nodose ganglion of the cat in order to label the axon terminals of vagal afferent fibres in the solitary nucleus. HRP reaction product was seen in axons, the majority of which were unmyelinated. Axon terminals containing round clear vesicles and an occasional dense-core vesicle were also labelled. These terminals synapsed upon both large and small dendritic elements, the majority of which were less than 1.0 micrometers in diameter. The labeling method appears to affect adversely the normal ultrastructural characteristics of terminals or their postsynaptic relationships and in these respects is superior to electron microscopic degeneration methods.

Anterograde tracing of horseradish peroxidase (HRP) with the electron microscope using the tetramethylbenzidine reaction

A technique is described for anterograde horseradish peroxidase (HRP) tracing with the electron microscope using tetramethylbenzidine (TMB) histochemistry. Following HRP injection into the primary visual cortex or one eye of rabbits the superior colliculus was studied with the light and electron microscope in adjacent sections. In target preparations from the colliculus examined with the electron microscope labelled axons and axon terminals were found. The TMB reaction product was identified as crystal like electron-dense structures. In control sections from the opposite colliculus the neuropil was free from label.

The cerebellar nucleo-olivary projection in the cat

The crossed cerebello-olivary projection in the cat was studied by means of retrograde transport of HRP. The cerebello-olivary connection is organized according to a zonal pattern similar to that of the olivocerebellar projection. However, some labelled neurons are in addition found in cerebellar nuclear areas adjacent to a nuclear zone sending its fibres to the corresponding olivary region. This observation indicates that there is a certain degree of overlapping between the different nuclear zones. The cerebello-olivary fibres from the fastigial nucleus appear to be more widely distributed than those from the other cerebellar nuclei. Nuclear neurons of all sizes project to the inferior olive, but the majority of the cells are medium sized. The findings are discussed and related to previous studies on the cerebello-olivary connection.