[The number and form of cerebellar fastigial neurons projecting to the thalamic suprageniculate nucleus in cats studied using WGA-HRP tracing method].

This study was designed to investigate the number and form of cerebellar fastigial neurons projecting to the suprageniculate nucleus(Sg) by using retrograde axonal transport of wheat germ agglutinin-horseradish peroxidase(WGA-HRP). Six adult cats(weighing 2.5-3.5 kg) were anesthetized with ketamine hydrochloride(30 mg/kg, i.m.) and sodium pentobarbital(15 mg/kg, i.p.). In experiments using injected WGA-HRP in the Sg, retrogradely labeled neurons by WGA-HRP were found only in the caudal part of the bilateral fastigial nucleus (Ft) with ipsilateral predominance, and the ratio of labeled neurons in the contralateral Ft to that in the ipsilateral Ft in all cats was 496:670(1:1.35). Five types of Ft-Sg neurons were distinguished morphologicaly. Of the 246 labeled neurons that could be characterized, 6.1% were large stellate neurons, 43.5% medium stellate neurons, 24.0% bipolar neurons, 20.3% triangular neurons, and 6.1% granular neurons. Thus, we concluded that a mixed population of Ft neurons projects to the Sg. In summary, as shown in the Figure 4 the Ft-Sg connection is an important pathway joining two closed circuits, and can be part of the extrageniculate visual system. We also speculate that the Ft-Sg connections may have a role in sending visual modulating information.

Bifurcating projections from the cerebellar fastigial neurons to the thalamic suprageniculate nucleus and to the superior colliculus.

A double-labeling fluorescence microscopic study was performed on the mesencephalic and thalamic distribution of fastigial efferents. Anesthetized cats were injected with 2% fast blue into the suprageniculate nucleus and with 0.5% nuclear yellow into the superior colliculus. Analysis of serial sections through the cerebellar fastigial nucleus revealed that 25% of the neurons projecting to the superior colliculus and 10% of those projecting to the thalamus were double labeled. The results suggest that bifurcating fastigial fibers to the mesencephalon and to the visual thalamus may play a role in cerebellar visual control.

Distribution of terminals from pedunculopontine tegmental nucleus and synaptic organization in lateralis medialis-suprageniculate nucleus of cat's thalamus: anterograde tracing, immunohistochemical studies, and quantitative analysis.

The cat's lateralis medialis-suprageniculate nuclear complex (LM-Sg) in the thalamus receives input from various brain regions such as the superior colliculus, brain stem, and spinal cord, as well as from visual association cortex. In a previous study, we demonstrated that LM-Sg receives cholinergic fibers from the pedunculopontine tegmental nucleus (PPT) and that cholinergic terminals make synaptic contacts with the dendrites of glutamatergic projection neurons and of GABAergic interneurons (Hoshino et al., 1997). In this study, we investigate the distribution and the organization of PPT terminals by means of a combined anterograde tracer (biotinylated dextran amine, BDA) and immunohistochemical methods. When stained by acetylcholinesterase (AChE), the LM-Sg is not uniformly immunoreactive, but rather is patchily labeled and shows a streaming type of reactivity. The tissue content appears high in enzyme activity in AChE-positive zones and is much lighter in activity in AChE-negative zones. We compared the synaptic organization between AChE-positive and AChE-negative portions of the LM-Sg in separate groups of electron-microscopic material: four types of vesicle containing profiles (RS, RL, F1, and PSD) as well as synaptic glomeruli were observed in this nucleus. Among these, the PSD profiles were observed more frequently in AChE-positive portions than in AChE-negative zones. Furthermore, the number of glomeruli was significantly higher in AChE-positive than in AChE-negative zones. Following the injection of BDA into PPT, labeled terminals within LM-Sg were rather more concentrated in the AChE-positive portion. Although the majority of PPT terminals made synaptic contacts with dendrites in the neuropil, a few terminals were involved in the synaptic glomeruli. The present results show that the synaptic organization is distinctly different between the AChE-positive and AChE-negative portions of LM-Sg. These results suggest that the AChE-positive portions of LM-Sg are relatively more involved in integrating information arising from a diverse set of inputs and processing that information within glomeruli in a complex manner than occurs in the AChE-negative portion of LM-Sg.

Visual, somatosensory, auditory and nociceptive modality properties in the feline suprageniculate nucleus.

Response properties of 252 single-units to visual, auditory, somatosensory and noxious stimulation were recorded by means of extracellular microelectrodes in the suprageniculate nucleus of anaesthetized, immobilized cats. Of the 141 units tested for modality properties the majority (n=113, 80.1%) was found unimodal in the sense that stimuli of exclusively one sensory modality were able to elicit an activation of the unit. Twenty-four (17.0%) cells were bimodal and four (2.8%) were trimodal (visual, somatosensory and auditory). The visual modality dominated the unimodal cells (n=74, 65.5%), while cells responsive to somatic stimulation (n=20, 17.6%), auditory stimulation (n=16, 14.1%) or noxious stimulation of the tooth pulp (n=3, 2.6%) were less frequently encountered. Visual sensitivity dominated the multisensory cells, too. The visually responsive units were characterized by having a sensitivity to stimuli moving in a rather large, uniform receptive field that covered the contralateral lower quadrant, and encompassed a flanking area of about 20 degree width in both the upper contralateral and lower ipsilateral visual fields. Many cells (n=52, 47%) were sensitive to the direction of the stimulation and reacted to stimuli moving at a high velocity (20-200 deg/s). Most cells responded differently to stimuli of a variety of sizes. Somatosensory units reacted to stimuli presented over a wide area on the contralateral side of the body, thus showing no sign of somatotopic organization. The auditory sensitivity fell within a wide range of acoustic stimuli in extremely large auditory receptive fields. The physiological properties of suprageniculate nucleus cells strongly resemble the sensory properties of cells found along the ventral bank of the anterior ectosylvian sulcus and the deeper layers of the superior colliculus. Our results provide further support for the notion of a separate tecto-suprageniculate-anterior ectosylvian sulcus/insular pathway that takes part in the processing of multimodal signals important for various types of sensory related behaviours.

Visual, somatosensory and auditory modality properties along the feline suprageniculate-anterior ectosylvian sulcus/insular pathway.

Physiological properties of single units were investigated in the suprageniculate nucleus (SG) and in the cerebral cortex along the anterior ectosylvian sulcus (AES), including the insular cortex. The recording was performed with the aid of carbon-filled glass micropipetts in barbiturate-anesthetized cats. The main findings of the study can be summarized as follows. 1. The physiological properties of the cells in the suprageniculate nucleus and in the AES/insular cortex exhibited striking similarities in a series of aspects: (a) The frequencies of occurrence of uni-, bi- and trimodal cells were similar. (b) The majority of the unimodal cells (75% in the AES/insular region and 65% in the SG) has visual sensitivity in both structures. The bimodal and trimodal cells were also dominated by visual sensitivity. (c) The somatosensory and auditory modalities were similarly present in both structures, although less frequently than the visual one. (d) No systematic topological organization was found in either structure. (e) The visual, somatosensory and auditory receptive fields were uniform and covered a fairly large proportion of the personal space. 2. Statistical comparison of some physiological properties of cells situated deep in the AES with those of cells in the insular cortex revealed differences as follows: (a) The insular cortex contained significantly more bi- and trimodal cells than the sulcal areas. (b) Cells in the insular cortex preferred significantly lower stimulus velocities and larger stimuli than cells in the depths of the AES. These results seem to support the notion of a suprageniculate-AES/insular thalamo-cortical multisensory entity. Additionally, the physiological differences between the sulcal AES cortex and gyral insula are in agreement with the morphological differences found earlier in the afferentation of these areas (Norita et al., 1986, 1991).

Organization of the colliculo-suprageniculate pathway in the cat: a wheat germ agglutinin-horseradish peroxidase study.

A wheat germ-agglutinated horseradish peroxidase (WGA-HRP) tracing technique was used to label the cell bodies of neurons in the superior colliculus that send projections into the visually sensitive region of the suprageniculate nucleus (Sg) in the feline thalamus. After determination of the position of the Sg by detecting characteristic single-unit responses to moving visual stimuli, WGA-HRP was injected into the Sg in five pentobarbital-anesthetized cats. The animals were than sacrificed, and serial frozen sections of the midbrain were processed for the demonstration of peroxidase activity. A total of 2,736 WGA-HRP-stained neurons were located within the ipsilateral superior colliculus (SC), and a few labeled cells were consistently found bilaterally in the external nuclei of the inferior colliculus. In each cat, a small but significant fraction of the labeled cells were encountered contralateral to the injection. Medial SC neurons tended to project to the posterior Sg, and lateral SC neurons tended to project to more rostral Sg. However, labeled cells were distributed homogeneously along the rostrocaudal extent of the SC, indicating the absence of a well defined topographic relationship. Nor was the Sg injection site location related to the laminar distribution of SC projection neurons. In all cases, the majority of the labeled cells were found in layer IV (49.0%), with fewer cells in layers III (17.5%) layer V (20.0%), and layer VI (11.8%). No labeled cells were located in layer I, although a few were located in the deep part of layer II. Five types of SC projection cells were distinguished morphologically. Of the 258 labeled cells that could be characterized, 25% were stellate cells, 25% vertical cells, 20% granular cells, 17% triangular cells, and 12% horizontal cells. The average diameter of 226 cells ranged between 8 and 47 microns. We conclude that a mixed population of SC cells projects to the Sg; the morphological heterogeneity and the distribution of these cells suggests that several functionally different pathways may be involved in the colliculothalamic pathway and in the processing of visual input in the SC.

Bilateral projections from the superior colliculus to the suprageniculate nucleus in the cat: a WGA-HRP/double fluorescent tracing study.

Following WGA-HRP injection into the right suprageniculate nucleus of the cat brain, retrogradely-labeled neurons were found not only in the ipsilateral, but also in the contralateral superior colliculi. After WGA-HRP injection into the unilateral superior colliculus, anterogradely-labeled axon terminals were observed in the bilateral suprageniculate nuclei, and electron microscopic examination revealed that these were large terminals which made asymmetric synaptic contacts with dendrites. When a different kind of fluorescent tracer was injected into each suprageniculate nucleus (Fast blue and Nuclear yellow), double-labeled neurons were observed in the rostral and middle portions of both superior colliculi. These results suggest that there are direct bilateral projections from the superior colliculus to the suprageniculate nuclei, and that some of these projections originate from branching colliculo-suprageniculate axons.

Direct projections from the cerebellar fastigial nucleus to the thalamic suprageniculate nucleus in the cat studied with the anterograde and retrograde axonal transport of wheat germ agglutinin-horseradish peroxidase.

Axonal transport of WGA-HRP injected into (1) the suprageniculate nucleus or (2) the fastigial nucleus, was investigated. Retrogradely labeled neurons were found in the caudal part of the bilateral fastigial nucleus following injection 1, and anterograde labeled axon terminals were observed in the bilateral suprageniculate nucleus following injection 2. Electron microscopic observations of these terminals revealed that they were large terminals making asymmetric synaptic contacts with dendrites. These results suggest that some neurons in the fastigial nucleus send their axons to the suprageniculate nucleus.