Zonal organization of the climbing fiber projection to the flocculus and nodulus of the rabbit: a combined axonal tracing and acetylcholinesterase histochemical study

Zonal organization of the vestibulocerebellum in pigeons (Columba livia): I. Climbing fiber input to the flocculus

Previous studies in pigeons have shown that the neurons in the medial column of the inferior olive respond best to patterns of optic flow resulting from self-rotation. With respect to the axis of rotation, there are two functional groups: rVA neurons prefer rotation about the vertical axis, whereas rH45 neurons respond best to rotation about an horizontal axis oriented at 45 degrees ipsilateral azimuth. The rVA and rH45 neurons are located in the caudal and rostral margins of the medial column, respectively. These olivary neurons project as climbing fibers to the contralateral flocculus. In this study, injections of anterograde tracers into the medial column were used to investigate the zonal organization of the climbing fiber input to the flocculus of pigeons. Iontophoretic injections of either cholera toxin subunit-B or biotinylated dextrin amine were made into the medial column of the inferior olive at locations responsive to rVA or rH45 rotational optic flow. Anterogradely labeled climbing fibers in the flocculus showed a clear zonal organization. There were four parasagittal bands spanning both folia IXcd and X consisting of two rVA zones interdigitated with two rH45 zones. These findings are compared with the zonal organization of the flocculus in mammalian species.

Activity-dependent expression of calbindin in rabbit floccular Purkinje cells modulated by optokinetic stimulation

Optokinetic stimulation activates visual climbing fiber pathways that synapse upon contralateral floccular Purkinje cells. Long-term horizontal optokinetic stimulation causes a progressive decrease in gain of the optokinetic reflex and leads to the subsequent genesis of a prolonged negative optokinetic afternystagmus. Since the flocculus is involved in adaptation to optokinetic stimulation, we used the technique of differential display reverse transcription-polymerase chain reaction to explore transcriptional changes in the flocculus evoked by long-term optokinetically evoked climbing fiber discharge. Several differentially transcribed gene products were isolated and sequenced. One of these, calbindin mRNA, was expressed in relatively decreased abundance in the flocculus that received increased climbing fiber input. Decreased transcription of calbindin mRNA was confirmed by northern blots. Hybridization histochemistry was used to localize calbindin mRNA to Purkinje cells and confirmed decreased transcription of calbindin mRNA in Purkinje cells located in folium 1 of the flocculus. Western blots and immunohistochemistry localized the climbing fiber-evoked decreased expression of calbindin to Purkinje cells in folia 1 of the flocculus. The expression of four other calcium-binding proteins in the flocculus was not influenced by optokinetic stimulation. Changes in expression of calbindin could be evoked by decreases in intracellular calcium associated with climbing fiber-evoked decreases in Purkinje cell simple spike activity.The application of differential display reverse transcription-polymerase chain reaction has provided a positive screen for several molecules in addition to calbindin whose expression is affected by naturally evoked activity in a major synaptic pathway to the cerebellum. Further experiments will be required to specify the functional role of each of these molecules.

Compartmentation of the rabbit cerebellar cortex

The cytoarchitecture of the adult rabbit cerebellum is revealed by using zebrin II/aldolase c immunocytochemistry in both wholemount and sectioned material. Zebrin II is expressed by approximately half of the Purkinje cells of the cerebellar cortex. In most regions these form a symmetrical array of zebrin II positive and negative parasagittal bands. Four transverse expression domains are identified in the vermis: (1) an anterior zone, comprising four narrow bands, one at the midline and three laterally to either side, extending throughout the anterior lobe to the primary fissure; (2) a central zone with broad immunoreactive bands separated by narrow zebrin II negative bands that disappear caudally to leave no apparent compartmentation; (3) a posterior zone with prominent alternating zebrin II positive and negative bands; and (4) a nodular zone in which all Purkinje cells express zebrin II. In the hemispheres a striped topography is found in lobules HVI, HVII, and crus I, and all Purkinje cells are zebrin II+ in the flocculus and paraflocculus. Because of its importance for the classical conditioning of the eyeblink response, we made a detailed analysis of lobule HVI of the hemisphere. The immunocytochemical data show a complex substructure within HVI with three prominent zebrin II positive bands (probably homologous with P4a+, P4b+, and P5+ of rodents) separated by two zebrin II negative regions (P4- and P4b-). Thus, the organization of the rabbit cerebellum is consistent with the patterns described previously for rat, mouse, and opossum and suggests that there may be a common ground plan for the mammalian cerebellum.

The entire trajectories of single olivocerebellar axons in the cerebellar cortex and their contribution to Cerebellar compartmentalization

The functional partitioning of the cerebellar cortex depends on the projection patterns of its afferent and efferent neurons. However, the entire morphology of individual projection neurons has been demonstrated in only a few classes of neurons in the vertebrate CNS. To investigate the contribution of the projection pattern of individual olivocerebellar axons to the cerebellar functional compartmentalization, we labeled individual olivocerebellar axons, which terminate in the cerebellar cortex as climbing fibers, with biotinylated dextran amine injected into the inferior olive in the rat, and completely reconstructed the entire trajectories of 34 olivocerebellar axons from serial sections of the cerebellum and medulla. Single axons had seven climbing fibers on average, which terminated at similar distances from the midline in a single or in multiple lobules. Cortical projection areas of adjacent olivary neurons were clustered as narrow but separate longitudinal segments and often innervated by collaterals of single neurons. Comparison of the cerebellar distribution of olivocerebellar axons arising from different sites within a single olivary subnucleus indicated that slightly distant neurons projected to complementary sets of such segments in a single longitudinal band. Several of these longitudinal bands formed a so-called parasagittal zone innervated by a subnucleus of the inferior olive. Single olivocerebellar axons projected rostrocaudally to segments within a single band but did not project mediolaterally to multiple bands. These results suggest fine substructural organization in the cerebellar compartmentalization that may represent functional units.

Projections from the nucleus of the basal optic root and nucleus lentiformis mesencephali to the inferior olive in pigeons (Columba livia)

The nucleus of the basal optic root (nBOR) of the accessory optic system (AOS) and the pretectal nucleus lentiformis mesencephali (LM) are involved in the analysis of optic flow and the generation of the optokinetic response. Previous studies have shown that the nBOR projects bilaterally to the medial column (mc) of the inferior olive (IO) and the LM projects to the ipsilateral mc. In the present study the retrograde tracer cholera toxin subunit B was injected into either the caudal or rostral mc. From all injections, retrogradely labeled cells were seen in the ipsilateral pretectum along the border of the medial and lateral subnuclei of the LM. Cells were also seen in bilaterally in the nBOR. On the contralateral side, a discrete group of cells was labeled in the rostral margin of the nBOR. These cells were localized in the dorsal portion of the nBOR proper and some were found in the adjacent nBOR dorsalis. On the ipsilateral side, a diffuse group of cells was seen in the caudal nBOR. Most of these cells were in the nBOR dorsalis and outside the nBOR complex in the area ventralis of Tsai and the reticular formation. From the injections into the caudal mc, a greater proportion of labeled cells was found in the LM, whereas a greater proportion of cells was found in the nBOR from the injections into the rostral mc. This differential projection from LM and nBOR to the caudal and rostral mc is consistent with the optic flow preferences of neurons in the mc, and a similar pattern of connectivity has been found in mammalian species.

Expression of heat-shock protein Hsp25 in mouse Purkinje cells during development reveals novel features of cerebellar compartmentation

The small heat shock protein Hsp25 is constitutively expressed in the adult mouse cerebellum by parasagittal stripes of Purkinje cells confined to the caudal central zone ( approximately lobules VI and VII), the nodular zone ( approximately ventral lobule IX and lobule X), and the paraflocculi/flocculi. During development several distinct phases in Hsp25 expression can be distinguished. Hsp25-immunopositive Purkinje cells are first seen at birth, when four clusters are visible in the vermis of lobules IV/V, and scattered Hsp25-immunoreactive Purkinje cells are seen in lobule VIII. By postnatal day 2/3, six narrow parasagittal stripes of Hsp25-immunopositive Purkinje cells are seen in the vermis of the anterior lobe. In the posterior lobules, most Purkinje cells in the vermis of lobules VIII and IX express Hsp25. This initial limited expression is followed by a phase of widespread expression (postnatal days 6-9) in which Hsp25 immunoreactivity is detected in virtually all Purkinje cells. This global cerebellar expression of Hsp25 then gradually disappears, first in the anterior zone and the hemispheres and subsequently in the posterior zone, to leave the restricted adult expression pattern. Western blotting analysis and immunoprecipitation with anti-Hsp25 suggest that all immunocytochemistry can be attributed the expression of Hsp25. Furthermore, visual deprivation had no effect on the development of Hsp25 expression in Purkinje cells, suggesting that visuomotor input is not responsible for the establishment of constitutive Hsp25 expression in the cerebellar cortex.

Three-dimensional morphology of cerebellar climbing fibers. A study by means of confocal laser scanning microscopy and scanning electron microscopy

The intracortical pathway of cerebellar climbing fibers have been traced by means of scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) to study the degree of lateral collateralization of these fibers in the granular Purkinje cell and molecular layers. Samples of teleost fish were processed for conventional and freeze-fracture SEM. Samples of hamster cerebellum were examined by means of CLSM using FM4-64 as an intracellular stain. High resolution in lens SEM of primate cerebellar cortex was carried out using chromium coating. At scanning electron and confocal laser microscopy levels, the climbing fibers appeared at the white matter and granular layer as fine fibers with a typical arborescence or crossing-over branching pattern, whereas the mossy fibers exhibited a characteristic dichotomous bifurcation. At the granular layer, the parent climbing fibers and their tendrils collaterals appeared to be surrounding granule and Golgi cells. At the interface between granule and Purkinje cell layers, the climbing fibers were observed giving off three types of collateral processes: those remaining in the granular layer, others approaching the Purkinje cell bodies, and a third type ascending directly to the molecular layer. At this layer, retrograde collaterals were seen descending to the granular layer. By field emission high-resolution SEM of primate cerebellar cortex, the climbing fiber terminal collaterals were appreciated ending by means of round synaptic knobs upon the spines of secondary and tertiary Purkinje cell dendrites.

Constitutive expression of the 25-kDa heat shock protein Hsp25 reveals novel parasagittal bands of purkinje cells in the adult mouse cerebellar cortex

Despite the reported absence of the 25-kDa heat shock protein Hsp25 in the rodent cerebellum, we have determined that Hsp25 is constitutively expressed in a subset of Purkinje cells in the adult cerebellum of the mouse. No other cerebellar neurons are Hsp25 immunoreactive, but there is weak staining associated with blood vessels. In the vermis, Hsp25-immunoreactive Purkinje cells are confined to two regions: one in lobules VI/VII, the other in lobules IX/X. In each region, only a subset of the Purkinje cells is immunoreactive. These cells are grouped in five parasagittal bands arranged symmetrically about the midline. The boundaries of these expression domains correspond to transverse zones previously inferred from other expression patterns. A third Hsp25-immunopositive domain is seen in the paraflocculus and flocculus. Again, only a subset of Purkinje cells within the paraflocculus and flocculus express Hsp25, revealing three distinct bands. Previous descriptions of compartmentation antigens have not differentiated between adult populations of Purkinje cells in these regions, suggesting that Hsp25 is a novel compartmentation antigen in the adult cerebellum.

Projections from the medial column of the inferior olive to different classes of rotation-sensitive Purkinje cells in the flocculus of pigeons

In the flocculus of pigeons, as in other species, there are two major types of Purkinje cell responses to rotational optokinetic stimuli. One type prefers rotation about the vertical axis (VA neurons) whereas the other prefers rotation about an horizontal axis oriented at 135 degrees ipsilateral azimuth (H-135 neurons). In this study, we injected the retrograde tracer cholera toxin subunit B into the VA and H-135 zones in attempt to determine the origin of inferior olive inputs. We found that VA and H-135 zones received input from the caudal and rostral margins of the medial column of the inferior olive, respectively. There is a similar pattern of connectivity in mammalian species.

A role of climbing fibers in regulation of flocculonodular lobe protein kinase C expression during vestibular compensation

The behavioral recovery from unilateral labyrinthectomy (UL) in rats is accompanied by asymmetric expression of Protein kinase C (PKC) in parasagittal regions of the flocculonodular lobe within 6 h after UL, which resolves to the control, symmetric pattern within 24 h. These changes consist of a regionally selective increase in the number of PKC-immunopositive Purkinje cells contralateral to the lesion. This study tested the hypotheses (1) that climbing fiber innervation inhibits PKC expression and (2) that climbing fibers are essential for the observed changes in PKC expression within 6 h after UL. The patterns of flocculonodular lobe Purkinje cell PKCdelta expression were analyzed 6 h post-operatively in both UL and sham-operated that had been treated previously with 3-acetylpyridine to destroy the inferior olive. These data were compared with previous results from rats with an intact olive. The results suggest that at least two signals regulate the zonal distribution of Purkinje cell PKCdelta expression in the flocculonodular lobe during the early period of compensation from UL. Climbing fiber activation appears to reduce PKC expression, while extraolivary mechanisms appear to up-regulate PKC expression. It is suggested that the climbing fiber signals may act as a molecular 'filter' or 'automatic gain control' which adjusts the contributions of these kinases to synaptic plasticity within the context of the background activity of climbing fibers.

Light microscopic and ultrastructural investigation of the dopaminergic innervation of the ventrolateral outgrowth of the rat inferior olive

The ventrolateral outgrowth of the inferior olive is involved in the control of compensatory eye movement responses to optokinetic stimuli about the horizontal axis that is perpendicular to the ipsilateral anterior semicircular canal. Combining immunocytochemistry with retrograde tracing of WGA-BSA-gold, we demonstrated in the present study that this olivary subnucleus receives a substantial dopaminergic input, and that the prerubral parafascicular area and its surrounding regions form the sole source of this input. In addition, we investigated the postsynaptic distribution of the dopaminergic terminals in the inferior olive at the ultrastructural level. About a third (32%) of the dopaminergic terminals was found to make synaptic contacts in the olivary neuropil. The majority (81%) of these boutons terminated on cell bodies or extraglomerular dendrites, while the remaining terminals contacted dendritic spines inside glomeruli. In contrast, GABAergic terminals in the inferior olive formed more frequently (66%) synaptic contacts and they terminated more frequently (38%) in glomeruli. Thus, the ventrolateral outgrowth receives a dopaminergic input from the mesodiencephalic junction, and the postsynaptic distribution of this input reveals a characteristic pattern.

Topographical organization of inferior olive cells projecting to translation and rotation zones in the vestibulocerebellum of pigeons

Previous electrophysiological studies in pigeons have shown that the vestibulocerebellum can be divided into two parasagittal zones based on responses to optic flow stimuli. The medial zone responds best to optic flow resulting from self-translation, whereas the lateral zone responds best to optic flow resulting from self-rotation. This information arrives from the retina via a projection from the accessory optic system to the medial column of the inferior olive. In this study we investigated inferior olive projections to translational and rotational zones of the vestibulocerebellum using the retrograde tracer cholera toxin subunit B. Extracellular recordings of Purkinje cell activity (complex spikes) in response to large-field visual stimuli were used to identify the injection sites. We found a distinct segregation of inferior olive cells projecting to translational and rotational zones of the vestibulocerebellum. Translation zone injections resulted in retrogradely labeled cells in the ventrolateral area of the medial column, whereas rotation zone injections resulted in retrogradely labeled cells in the dorsomedial region of the medial column. Motion of any object through space, including self-motion of organisms, can be described with reference to translation and rotation in three-dimensional space. Our results show that, in pigeons, the brainstem visual systems responsible for detecting optic flow are segregated into channels responsible for the analysis of translational and rotational optic flow in the inferior olive, which is only two synapses from the retina.

Control of spatial orientation of the angular vestibuloocular reflex by the nodulus and uvula

Spatial orientation of the angular vestibuloocular reflex (aVOR) was studied in rhesus monkeys after complete and partial ablation of the nodulus and ventral uvula. Horizontal, vertical, and torsional components of slow phases of nystagmus were analyzed to determine the axes of eye rotation, the time constants (Tcs) of velocity storage, and its orientation vectors. The gravito-inertial acceleration vector (GIA) was tilted relative to the head during optokinetic afternystagmus (OKAN), centrifugation, and reorientation of the head during postrotatory nystagmus. When the GIA was tilted relative to the head in normal animals, horizontal Tcs decreased, vertical and/or roll time constants (Tc(vert/roll)) lengthened according to the orientation of the GIA, and vertical and/or roll eye velocity components appeared (cross-coupling). This shifted the axis of eye rotation toward alignment with the tilted GIA. Horizontal and vertical/roll Tcs varied inversely, with T(chor) being longest and T(cvert/roll) shortest when monkeys were upright, and the reverse when stimuli were around the vertical or roll axes. Vertical or roll Tcs were longest when the axes of eye rotation were aligned with the spatial vertical, respectively. After complete nodulo-uvulectomy, T(chor) became longer, and periodic alternating nystagmus (PAN) developed in darkness. T(chor) could not be shortened in any of paradigms tested. In addition, yaw-to-vertical/roll cross-coupling was lost, and the axes of eye rotation remained fixed during nystagmus, regardless of the tilt of the GIA with respect to the head. After central portions of the nodulus and uvula were ablated, leaving lateral portions of the nodulus intact, yaw-to-vertical/roll cross-coupling and control of Tc(vert/roll) was lost or greatly reduced. However, control of Tchor was maintained, and T(chor) continued to vary as a function of the tilted GIA. Despite this, the eye velocity vector remained aligned with the head during yaw axis stimulation after partial nodulo-uvulectomy, regardless of GIA orientation to the head. The data were related to a three-dimensional model of the aVOR, which simulated the experimental results. The model provides a basis for understanding how the nodulus and uvula control processing within the vestibular nuclei responsible for spatial orientation of the aVOR. We conclude that the three-dimensional dynamics of the velocity storage system are determined in the nodulus and ventral uvula. We propose that the horizontal and vertical/roll Tcs are separately controlled in the nodulus and uvula with the dynamic characteristics of vertical/roll components modulated in central portions and the horizontal components laterally, presumably in a semicircular canal-based coordinate frame.

Parasolitary nucleus: a source of GABAergic vestibular information to the inferior olive of rat and rabbit

At least two subnuclei of the inferior olive, the beta-nucleus, and the dorsomedial cell column (dmcc), contain vestibularly responsive neurons that receive a dense descending projection that uses gamma-aminobutyric acid (GABA) as the transmitter. In contrast to the GABAergic innervation of other olivary subnuclei, the terminal boutons that terminate on neurons in the beta-nucleus and the dorsomedial cell column remain intact after cerebellectomy, ruling out both the cerebellum and the cerebellar nuclei as afferent sources. By using both immunohistochemical as well as orthograde and retrograde tracer methods, we have identified the source of the GABAergic pathway to the beta-nucleus and dmcc in both rat and rabbit. Under physiologic recording of single olivary neurons to guide electrode placement, we injected the bidirectional tracer, wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) into the beta-nucleus and dmcc of the inferior olive. These injections retrogradely labeled neurons in the parasolitary nucleus (Psol) near the vestibular complex. Psol neurons were identified as GABAergic with an antibody to glutamic acid decarboxylase (GAD). In the rat, Psol neurons are small (5-7 microm in diameter) and number approximately 1,800. In the rabbit, they are slightly larger (6-9 microm in diameter) and number approximately 2,200. WGA-HRP injections in conjunction with GAD immunohistochemistry double labeled a high percentage of neurons in both the rat and rabbit Psol. Injection of the orthograde tracer Phaseolus vulgaris-leucoagglutinin into the area of the Psol revealed a projection from this region to both the beta-nucleus and dmcc. Subtotal electrolytic lesions of this division of the Psol caused a substantial reduction in GAD-positive synaptic terminals in both the ipsilateral beta-nucleus and dmcc. The location of these GABAergic neurons, bordering both the nucleus solitarius and caudal vestibular complex, emphasizes the importance of the Psol in the processing of both vestibular and autonomic information pertinent to postural control.

Topography and reciprocal activity of cerebellar Purkinje cells in the uvula-nodulus modulated by vestibular stimulation

In the rabbit uvula-nodulus, vestibular and optokinetic information is mapped onto parasagittal zones by climbing fibers. These zones are related functionally to different pairs of vertical semicircular canals, otolithic inputs and horizontal optokinetic inputs. Vestibular stimulation restricted to one of these zones modulates climbing fiber responses (CFRs). Within each of these zones, simple spikes (SSs) are modulated reciprocally with CFRs. In rabbits anesthetized with chloralose-urethan, we have used vestibular and optokinetic stimulation to evoke CFRs within a parasagittal zone while recording from Purkinje cells in adjacent zones. We have examined whether the CFRs evoked by vestibular stimulation in one zone influence the SSs of an adjacent zone. CFRs and SSs were recorded during roll vestibular stimulation. The orientation of the head of the rabbit with respect to the axis of rotation was varied systematically so that a climbing fiber null plane could be determined. This null plane was the orientation of the head about the vertical axis at which no modulation of the CFR was observed during rotation about the longitudinal axis of the vestibular rate table. In the left uvula-nodulus, a medial sagittal strip extending through all the folia contained Purkinje cells with CFRs that had optimal planes of stimulation coplanar with the left posterior-right anterior semicircular canals (LPC-RAC). Lateral to this strip was a strip of Purkinje cells with CFRs that were characterized by optimal planes corresponding to stimulation of the left anterior-right posterior semicircular canals (LAC-RPC). SSs in Purkinje cells were modulated out of phase with CFRs from the same Purkinje cell. The depth of modulation of both CFRs and SSs was reduced during rotation in the climbing fiber "null plane". The depth of modulation of SSs was greatest when recorded from Purkinje cells located at the center of semicircular canal-related strip. We observed that 1) all folia of the uvula-nodulus receive vestibular climbing fiber inputs; 2) these climbing fiber inputs convey information from the vertical semicircular canals and otoliths but not the horizontal semicircular canals; 3) CFRs evoked in a particular sagittal zone do not influence SSs in adjacent zones; 4) modulation of a CFRs in a particular Purkinje cell can occur without modulation of SSs in the same Purkinje cell, although modulation of SSs was not observed in the absence of CFR modulation; and 5) modulation of SSs sometimes preceded that of CFRs in the same cell, implying that interneuronal pathways may contribute to SS modulation. Climbing fiber-driven Golgi cells, the inhibitory axon terminals of which end on granule cell dendrites in the classic glomerular synapse, may provide this interneuronal mechanism.

Projections of the nucleus of the basal optic root in pigeons (Columba livia) revealed with biotinylated dextran amine

The nucleus of the basal optic root (nBOR) of the accessory optic system is known to be involved in the analysis of the visual consequences of self-motion. Previous studies have shown that the nBOR in pigeons projects bilaterally to the vestibulocerebellum, the inferior olive, the interstitial nucleus of Cajal, and the oculomotor complex and projects unilaterally to the ipsilateral pretectal nucleus lentiformis mesencephali and the contralateral nBOR. By using the anterograde tracer biotinylated dextran amine, we confirmed these projections and found (previously unreported) projections to the nucleus Darkshewitsch, the nucleus ruber, the mesencephalic reticular formation, and the area ventralis of Tsai as well as ipsilateral projections to the central gray, the pontine nuclei, the cerebellar nuclei, the vestibular nuclei, the processus cerebellovestibularis, and the dorsolateral thalamus. In addition to previous studies, which showed a projection to the dorsomedial subdivision of the contralateral oculomotor complex, we found terminal labelling in the ventral and dorsolateral subdivisions. Individual fibers were reconstructed from serial sections, and collaterals to various nuclei were demonstrated. For example, collaterals of fibers projecting to the vestibulocerebellum terminated in the vestibular or cerebellar nuclei; collaterals of fibers to the inferior olive terminated in the pontine nuclei; many individual neurons projected to the interstitial nucleus of Cajal, the nucleus Darkshewitsch, and the central gray and also projected to the nucleus ruber and the mesencephalic reticular formation; collaterals of fibers to the contralateral nucleus of the basal optic root terminated in the mesencephalic reticular formation and/or the area ventralis of Tsai; neurons projecting to the nucleus lentiformis mesencephali also terminated in the dorsolateral thalamus. The consequences of these data for understanding the visual control of eye movements, neck movements, posture, locomotion, and visual perception are discussed.

Differences of the primate flocculus and ventral paraflocculus in the mossy and climbing fiber input organization

Potential sources of cerebellar cortical afferent fibers were identified in the vestibular ganglion, medulla oblongata, pons, and cerebellar nucleus of seven anesthetized Macaca fuscata after local injections of wheat germ agglutinin-conjugated horseradish peroxidase or Fast Blue into the flocculus (FL) or ventral paraflocculus (VP). There were differences in the sources of mossy fibers to the FL and VP. Labeled neurons, after injections into the FL, were located mainly in the ipsilateral vestibular ganglion, bilaterally in the vestibular and prepositus hypoglossal nuclei, nucleus reticularis tegmenti pontis, and the central part of the mesencephalic reticular formation including the raphe nuclei. Labeled neurons were rarely seen in the pontine nuclei after injections into the FL. By contrast, after injections into the VP, numerous labeled neurons were located in the contralateral pontine nuclei, but relatively few in the vestibular nuclei bilaterally. Sources of climbing fibers to the FL and VP were completely contralateral to the injection side. After the injection into the FL and VP, labeled neurons were located in the dorsal cap, ventrolateral outgrowth, and ventral part of the medial accessory olivary nucleus. The projections from these three olivary areas were generally consistent with a zonal pattern of terminations in the FL and VP. The present results are consistent with a hypothesis that the FL is mainly involved in the control of vestibulo-ocular reflex and that the VP is mainly involved in the control of smooth pursuit eye movements.

Transient changes in flocculonodular lobe protein kinase C expression during vestibular compensation

Protein kinase C (PKC) is a family of intracellular signal transduction enzymes, comprising isoforms that vary in sensitivity to calcium, arachidonic acid, and diacylglycerol. PKC isoforms alpha, gamma, and delta are expressed by cerebellar Purkinje cells and neurons in the cerebellar nuclei and vestibular nuclei of the Long-Evans rat. In control rats, these PKCs are distributed symmetrically in the flocculonodular-lobe Purkinje cells. Behavioral recovery from vestibular dysfunction produced by unilateral labyrinthectomy (UL) is accompanied by asymmetric expression of PKC isoforms in these regions within 6 hr after UL. These expression changes were localized within parasagittal regions of the flocculus and nodulus. The distribution of PKCalpha, -gamma, and -delta were identical, suggesting that they are coregulated in cerebellar Purkinje cells during this early compensatory period. The pattern of Purkinje cell PKC expression returned to the control, symmetric distribution within 24 hr after UL. It is hypothesized that these regional changes in Purkinje cell PKC expression are an early intracellular signal contributing to vestibular compensation. In particular, regulation of PKC expression may contribute to changes in the efficacy of cerebellar synaptic plasticity during the acute post-UL period.

Morphological correlates of bilateral synchrony in the rat cerebellar cortex

Simultaneous recordings of the left and right crus IIA of the cerebellar cortex in the rat have demonstrated that Purkinje cells of both sides can be activated synchronously by their climbing fibers. Because climbing fibers arise exclusively from the contralateral inferior olive (IO), this physiological finding seems to contradict the anatomy. To define the structural basis responsible for the bilateral synchrony, we examined the possibilities that bilateral common afferent inputs to the IO and interolivary connections form the underlying mechanisms. The bilaterality of the major afferents of the olivary regions that project to crus IIA was studied using Phaseolus vulgaris leucoagglutinin as an anterograde tracer. We found that the excitatory and inhibitory projections from the spinal trigeminal nucleus and dorsolateral hump of the interposed cerebellar nucleus to the transition area between the principal olive and dorsal accessory olive were bilateral. A second possible mechanism for bilateral synchrony, which is the possibility that axons of olivary neurons provide collaterals to the contralateral side, was investigated using biotinylated dextran amine as an anterograde tracer. Labeled axons were traced and reconstructed from the principal olive and dorsal and medial accessory olive up to the entrance of the contralateral restiform body. None of these axons gave rise to collaterals. The possibility that neurons in the left and right IO are electronically coupled via dendrodendritic connections was investigated by examining the midline region of the IO. The neuropil of the left and right IO is continuous in the dorsomedial cell column. Examination of Golgi impregnations of this subdivision demonstrated that (1) many dendrites cross from one side to the other, (2) neurons close to the midline give rise to dendrites that extend into both olives, and (3) dendrites of neurons in the dorsomedial cell column frequently traverse into adjacent olivary subdivisions such as the medial accessory olive and the transition area between the principal olive and dorsal accessory olive. Sections immunostained for dendritic lamellar bodies or GABAergic terminals showed the same pattern: the neuropils of the dorsomedial cell columns on both sides form a continuum with each other as well as with the neuropil of other adjacent olivary subdivisions. Ultrastructural examination of the dorsomedial cell column demonstrated that the midline area includes many complex glomeruli that contain dendritic spines linked by gap junctions. To verify whether the complex spike synchrony observed between left and right crus IIA could indeed be mediated in part through coupled neurons in the dorsomedial cell column, we recorded simultaneously from crus IIA areas and from left and right vermal lobule IX, which receives climbing fibers from the dorsomedial cell column. In these experiments we demonstrated that the climbing fibers of all four areas, i.e., the left and right crus IIA as well as the left and right lobule IX, can fire synchronously. The present results indicate that synchronous climbing fiber activation of the left and right crus IIA in the rat can be explained by (1) bilateral inputs to the transition areas between the principal olive and dorsal accessory olive and (2) dendrodendritic electrotonic coupling between neurons of the left and right dorsomedial cell column and between neurons of the dorsomedial cell column and adjacent olivary subdivisions.

Light-microscopic distribution and parasagittal organisation of muscarinic receptors in rabbit cerebellar cortex

Recent studies on the effects of intrafloccular injections of muscarinic agonists and antagonists on compensatory eye movements in rabbit, indicate that muscarinic receptors may play a modulatory role in the rabbit cerebellar circuitry. It was previously demonstrated by Neustadt et al. (1988), that muscarinic receptors in rabbit cerebellar cortex are distributed into alternating longitudinal zones of very high and very low receptor density. In the present study, the zonal and cellular distribution of muscarinic receptors in the rabbit cerebellar cortex is investigated in detail using in vitro ligand autoradiography with the non-selective high-affinity antagonist [3H]quinuclidinyl benzilate (QNB), and the M2-specific antagonist [3H]AF-DX384, and immunocytochemistry with a monoclonal antibody specific for the cloned m2 muscarinic receptor protein. [3H]QNB and [3H]AF-DX384 binding sites and m2-immunoreactivity had similar overall distributions: dense labeling occurred in the dendritic arbors of a subset of Purkinje cells that are organized into parasagittal bands. A high level of muscarinic receptor labeling was also observed in a thin substratum of the molecular layer immediately above the Purkinje cell layer of the vestibulo-cerebellar lobules, i.e. the nodulus, the ventral uvula and the flocculus. Labeling in this stratum was associated with densely packed fibres, which were putatively identified as parallel fibres. Also Golgi cells, which were localized in part in the molecular layer, and a subset of mossy fibre rosettes, primarily concentrated in lobule VI, were immunoreactive for the m2 receptor. The parasagittal band of labeled Purkinje cell dendrites were most prominent in the anterior lobe (lobules I-V), in crus 1 and 2, in the flocculus, the ventral paraflocculus and the rostral folium of the nodulus. In other lobules, only infrequent Purkinje cells contained muscarinic receptors. The parasagittal organisation of muscarinic receptors differed from that of zebrin I, a Purkinje cell-specific protein which is often used as a marker of parasagittal parcelation of the cerebellar cortex. In the anterior lobe, however, there was a partial correspondence between muscarinic receptor and zebrin I bands. In the flocculus the distribution of muscarinic-receptor-positive Purkinje cells was related to the distinct white matter compartments as revealed with acetylcholinesterase (AChE) histochemistry. Muscarinic receptor-containing Purkinje cells were located primarily in the floccular zone 1, which is implicated in the control of eye movements about a horizontal axis. In order to relate the distribution of muscarinic receptor labeling to that of cholinergic nerve terminals, [3H]QNB binding sites and sodium-dependent [3H]hemicholinium-3 binding were compared. Sodium-dependent [3H]hemicholinium-3 binding sites mainly occurred in the granule cell layer of the vestibulo-cerebellum, which corresponds well with the distribution of the acetylcholine synthesizing enzyme, choline acetyltransferase (ChAT). However, sodium-dependent [3H]hemicholinium binding complemented, rather than co-localized with, muscarinic receptors which were primarily distributed in the molecular layer of the lobules of the vestibulo-cerebellar lobules. Their functional significance is puzzling, since their distribution does not correspond to that of markers of cholinergic innervation.

Anatomical compartments in the white matter of the rabbit flocculus

The white matter of the rabbit flocculus is subdivided into five compartments by narrow sheets of densely staining acetylcholinesterase-positive fibers. The most lateral compartment is continuous with the C2 compartment of the paraflocculus and contains the posterior interposed nucleus. The other four compartments are numbered from lateral to medial as floccular compartments 1, 2, 3, and 4 (FC1-4). FC1-3 continue across the posterolateral fissure into the adjacent folium (folium p) of the ventral paraflocculus. FC4 is present only in the rostral flocculus. In the caudal flocculus FC1 and FC3 abut dorsal to FC2. Fibers of FC1-4 can be traced into the lateral cerebellar nucleus and the floccular peduncle. The presence of acetylcholinesterase in the deep stratum of the molecular layer of the flocculus and ventral paraflocculus distinguishes them from the dorsal paraflocculus. The topographical relations to the flocculus and the floccular peduncle with group y and the cerebellar nuclei are discussed.

Zonal organization of the flocculovestibular nucleus projection in the rabbit: a combined axonal tracing and acetylcholinesterase histochemical study

With the use of retrograde transport of horseradish peroxidase we confirmed the observation of Yamamoto and Shimoyama ([1977] Neurosci Lett. 5:279-283) that Purkinje cells of the rabbit flocculus projecting to the medial vestibular nucleus are located in two discrete zones, FZII and FZIV, that alternate with two other Purkinje cell zones, FZI and FZIII, projecting to the superior vestibular nucleus. The retrogradely labeled axons of these Purkinje cells collect in four bundles that occupy the corresponding floccular white matter compartments, FC1-4, that can be delineated with acetylcholinesterase histochemistry (Tan et al. [1995a] J. Comp. Neurol., this issue). Anterograde tracing from small injections of wheat germ agglutin-horseradish peroxidase in single Purkinje cell zones of the flocculus showed that Purkinje cell axons of FZII travel in FC2 to terminate in the medial vestibular nucleus. Purkinje cell axons from FZI and FZIII occupy the FC1 and FC3 compartments, respectively, and terminate in the superior vestibular nucleus. Purkinje cell axons from all three compartments pass through the floccular peduncle and dorsal group y. In addition, some fibers from FZI and FZII, but not from FZIII, arch through the cerebellar nuclei to join the floccular peduncle more medially. No anterograde tracing experiments were available to determine the projections of the FZIV and C2 zones. The functional implications of these results are discussed.

Olivary projecting neurons in the nucleus prepositus hypoglossi, group y and ventral dentate nucleus do not project to the oculomotor complex in the rabbit and the rat

The nucleus prepositus hypoglossi, dorsal group y, ventral dentate nucleus, and medial vestibular nucleus all project to both the oculomotor complex and inferior olive. In the present study, we demonstrate with the use of retrograde double labeling techniques in rabbits and rats, that the neurons in these pre-oculomotor nuclei that project to the inferior olive are intermingled with those that project to the oculomotor nucleus, but that virtually none project to both.