Sharleen T. Sakai

Comparison of cerebellothalamic and pallidothalamic projections in the monkey (Macaca fuscata): A double anterograde labeling study

To address the question of segregated projections from the internal segment of the globus pallidus (GPi) and the cerebellar nuclei (Cb) to the thalamus in the monkey, we employed a double anterograde labeling strategy combining the anterograde transport of horseradish peroxidase conjugated to wheat germ agglutinin (WGA‐HRP) with biotinylated dextran amine (BDA) transport. The tissue was processed sequentially for WGA‐HRP, and then BDA immunohistochemistry using two different chromogens. Since the two labels were easily distinguishable on the same histological section, the interrelationship between the cerebellar and pallidal projection systems could be directly evaluated. We found that both the cerebellothalamic and pallidothalamic label consisted of dense plexuses of labeled fibers and swellings in a patch‐like configuration. The patches or foci of labeling were distributed either as dense single label or as interdigitating patches of double label. We found dense single label in the central portion of the ventral anterior nucleus pars principalis (VApc) and the ventral lateral nucleus pars oralis (VLo) following the GPi injections or in the central portion of the ventral posterior lateral nucleus pars oralis (VPLo) and nucleus X (X) following the cerebellar nuclei injections. Complementary interdigitating patches of WGA‐HRP and BDA labeling were found primarily in transitional border regions between thalamic nuclei. On occasion, we found overlap of both labels. We observed a gradient pattern in the density of the pallidothalamic and cerebellothalamic projections. The pallidothalamic territory included VApc, VLo, and the ventral lateral nucleus pars caudalis (VLc), with the density of these projections decreasing along an anterior to posterior gradient in the thalamus. Occasional patches of pallidal label were found in VPLo and nucleus X. Conversely, the density of cerebellothalamic projections increased along the same gradient, with the cerebellothalamic territory extending anteriorly beyond the cell‐sparse zones of VPLo, X, and VLc to include VLo and VApc also. These data suggest that although the cerebellar and pallidal projections primarily occupy separate thalamic territories, individual thalamic nuclei receive differentially weighted inputs from these sources.

Social intelligence in the spotted hyena (Crocuta crocuta)

If the large brains and great intelligence characteristic of primates were favoured by selection pressures associated with life in complex societies, then cognitive abilities and nervous systems with primate-like attributes should have evolved convergently in non-primate mammals living in large, elaborate societies in which social dexterity enhances individual fitness. The societies of spotted hyenas are remarkably like those of cercopithecine primates with respect to size, structure and patterns of competition and cooperation. These similarities set an ideal stage for comparative analysis of social intelligence and nervous system organization. As in cercopithecine primates, spotted hyenas use multiple sensory modalities to recognize their kin and other conspecifics as individuals, they recognize third-party kin and rank relationships among their clan mates, and they use this knowledge adaptively during social decision making. However, hyenas appear to rely more intensively than primates on social facilitation and simple rules of thumb in social decision making. No evidence to date suggests that hyenas are capable of true imitation. Finally, it appears that the gross anatomy of the brain in spotted hyenas might resemble that in primates with respect to expansion of frontal cortex, presumed to be involved in the mediation of social behaviour.

Pallidal and cerebellar afferents to pre‐supplementary motor area thalamocortical neurons in the owl monkey: A multiple labeling study

In the present study, we determined where thalamic neurons projecting to the pre‐supplementary motor area (pre‐SMA) are located relative to pallidothalamic and cerebellothalamic inputs and nuclear boundaries. We employed a triple‐labeling technique in the same owl monkey (Aotus trivirgatus). The cerebellothalamic projections were labeled with injections of wheat germ agglutinin conjugated to horseradish peroxidase, and the pallidothalamic projections were labeled with biotinylated dextran amine. The pre‐SMA was identified by location and movement patterns evoked by intracortical microstimulation and injected with the retrograde tracer cholera toxin subunit B. Brain sections were processed sequentially using different chromogens to visualize all three tracers in the same section. Alternate sections were processed for Nissl cytoarchitecture or acetylcholinesterase chemoarchitecture for nuclear boundaries. The cerebellar nuclei primarily projected to posterior (VLp), medial (VLx), and dorsal (VLd) divisions of the ventral lateral nucleus; the pallidum largely projected to the anterior division (VLa) of the ventral lateral nucleus and the parvocellular part of the ventral anterior nucleus (VApc). However, we also found zones of overlapping projections, as well as interdigitating foci of pallidal and cerebellar label, particularly in border regions of the VLa and VApc. Thalamic neurons labeled by pre‐SMA injections occupied a wide band and were especially concentrated in the VLx and VApc, cerebellar and pallidal territories, respectively. Labeled thalamocortical neurons overlapped cerebellar inputs in the VLd and VApc and overlapped pallidal inputs in the VLa and the ventral medial nucleus. The results demonstrate that inputs from both the cerebellum and globus pallidus are relayed to the pre‐SMA. J. Comp. Neurol. 417:164–180, 2000. ©2000 Wiley‐Liss, Inc.

Overlapping corticostriatal projections from the supplementary motor area and the primary motor cortex in the macaque monkey: An anterograde double labeling study

The purpose of the present study was to determine if the cortical efferents from homologous body regions of the supplementary motor area (SMA) and the primary motor cortex (MI) project to separate or to overlapping regions in the striatum. In order to investigate the dual corticostriatal projections, we employed an anterograde double labeling paradigm in which two tracers could be simultaneously detected in the same histological section. Prior to the injections, the forelimb representation in the two cortical motor areas was identified by using intracortical microstimulation in four Japanese monkeys (Macaca fuscata). Multiple injections of biotinylated dextran amine (BDA) were made into the forelimb regions of MI and wheat germ agglutinin conjugated horseradish peroxidase (WGA‐HRP) was injected into the arm region of the SMA. In additional animals, the tracers were reversed such that BDA was injected into the SMA and WGA‐HRP was injected into the MI. The tissue was processed sequentially using different chromogens in order to visualize both tracers in a single section. We analyzed the distribution of the ipsilateral anterograde label. The striatal labeling from each cortical area basically consisted of a wide band of patchy dense labeling interrupted by lighter labeling. The SMA striatal projections were located mainly within the putamen, distributing from the level of the anterior commissure to the most posterior extent of the putamen. At an intermediate level, the label spread obliquely from the ventrolateral edge of the putamen dorsomedially as far as the lateral edge of the caudate nucleus. The label from the MI was observed in comparable portions of the putamen, although the SMA projections were shifted more anterior and dorsomedial to the MI projections and the heaviest projections from the SMA and the MI were separately located. On the basis of the double anterograde labeling technique, we found considerable overlap mainly in the central portion of the putamen from the SMA and MI forelimb representation. These results suggest that the homologous body regions of the SMA and MI send widespread, and substantially overlapping projections, to portions of the striatum.

Somatosensory input to the ventrolateral thalamic region in the macaque monkey: A potential substrate for parkinsonian tremor

In the present study, we determined the anatomic relationships between somatosensory and motor pathways within ventrolateral (VL) thalamic nuclei of the motor thalamus of macaque monkeys. In labeling experiments, four macaque monkeys (Macaca mulatta) received injections of biotinylated dextran amine and wheat germ agglutinin conjugated to horseradish peroxidase into the cerebellar nuclei or internal segment of the globus pallidus and cervical segments of the spinal cord, respectively. Each tracer was visualized in brain sections by sequentially using a different chromogen. Labeled terminals were plotted and superimposed on adjacent brain sections processed for Nissl substance, acetylcholinesterase, and the antigens for calbindin and Cat‐301 to reveal thalamic nuclei. The labeled cerebellar terminals were distributed throughout the posterior VL (VLp), whereas the labeled pallidothalamic terminals were concentrated in the anterior VL and the ventral anterior nucleus. The spinothalamic input was directed mostly to the ventral posterior complex and cells just caudal to it. In addition, the patches of spinothalamic terminations intermingled and partly overlapped with the cerebellothalamic, but not with the pallidothalamic terminations within VLp. The regions of overlap of somatosensory and cerebellar inputs within the VLp of the present study appear to correspond to the reported locations of the tremor‐related cells in parkinsonian patients. Thus, the overlapping spinothalamic and cerebellar inputs may provide a substrate for the altered activity of motor thalamic neurons in such patients. J. Comp. Neurol. 455:378–395, 2003.

Pallidal and cerebellar inputs to thalamocortical neurons projecting to the supplementary motor area in Macaca fuscata: a triple-labeling light microcopic study

 We investigated the interrelationship between the supplementary motor area (SMA) thalamocortical projection neurons and the pallidothalamic and cerebellothalamic territories in the monkey (Macaca fuscata) using a combination of three tracers in a triple labeling paradigm. Thalamic labeling was analyzed following injections of the anterograde tracers, biotinylated dextran amine (BDA) into the internal segment of the globus pallidus (GPi) and wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the contralateral cerebellar interpositus and dentate nuclei. In addition, the retrograde tracer cholera toxin subunit B (CTB) was injected into the physiologically identified hand/arm representation of SMA. The tissue was processed sequentially using different chromogens in order to visualize all three tracers in a single section. We found that the SMA thalamocortical neurons occupied a wide band extending from the ventral anterior nucleus pars principalis (VApc) through the ventral lateral nucleus pars oralis (VLo) and the ventral lateral nucleus pars medialis (VLm) and into to the ventral lateral nucleus pars caudalis (VLc) including a portion of ventral posterior lateral nucleus pars oralis (VPLo) and nucleus X. The heaviest CTB labeling was found in VLo with dense plexuses of BDA labeled pallidothalamic fibers and swellings often observed superimposed upon retrogradely labeled CTB cells. In addition, dense foci of cerebellothalamic WGA-HRP anterograde label were observed coinciding with the occasional retrogradely CTB labeled neurons in VLc and transitional zones between VApc, VLo and VPLo. Our light microscopic results suggest that the SMA receives thalamic inputs with afferents largely derived from GPi and minor inputs originating from the cerebellum.

Nigrothalamic projections and nigrothalamocortical pathway to the medial agranular cortex in the rat: Single‐ and double‐labeling light and electron microscopic studies

Although the rat medial agranular cortex (AGm) has been implicated in a variety of motor functions, the source of the afferents impinging upon thalamic neurons projecting to the AGm is not directly known. The main purpose of this study was to determine whether the AGm is a major recipient of the nigrothalamocortical pathway. This issue was addressed by two sets of experiments. First, the organization of the nigrothalamic projections was studied by light and electron microscopy following injections of the anterograde tracer Phaseolus vulgaris‐leucoagglutinin (PHA‐L) into the pars reticulata of the substantia nigra (SNR). The major finding of this study was the disclosure of a heretofore unknown projection to the rostromedial part of the ventral anterior‐ventral lateral complex (VAL). This projection originates exclusively from the ventral portion of the SNR and is comparable in strength to the well‐known nigrothalamic projection to the ventromedial nucleus (VM). Electron microscopic examination revealed differences in the synaptic organization of nigral terminals in the VAL and the VM. A large proportion of the labeled terminals in the VAL was involved in axosomatic synapses, whereas, in the VM, the axosomatic synapses were rare, and 67% of nigral terminals were found in contact with thin dendrites.

The relationship between MI and SMA afferents and cerebellar and pallidal efferents in the macaque monkey

The purpose of the present study was to determine the interrelationship between the thalamic afferents arising from the cerebellum (Cb) and the internal segment of the globus pallidus (GPi) with the neurons projecting to the primary motor cortex (MI) and to the supplementary motor area (SMA). We combined fluorescent retrograde tracers with a double anterograde labeling technique. Multiple injections of a combination of Diamidino Yellow and Fast Blue were made into either the MI or SMA hand/arm representation as determined by intracortical microstimulation. In the same animal, biotinylated dextran amine was injected into the GPi and horseradish peroxidase conjugated to wheat germ agglutinin was injected into the contralateral cerebellar nuclei. The results revealed that the cerebellar and pallidal thalamic territories are largely separate. The ventral anterior nucleus (VA) and the ventral lateral nucleus pars oralis (VLo) contained a greater density of pallidal labeling while a greater density of cerebellar label was observed more caudally in the ventral posterior lateral nucleus pars oralis (VPLo) as well as in nucleus X (X). Moreover, we observed that the greatest coincidence of retrograde cell labeling was within the pallidal thalamic territory following the SMA injections and within the cerebellar thalamic territory following the MI injections. However, interdigitating foci of pallidal and cerebellar label were also observed particularly in the ventral lateral nucleus pars oralis (VLo) and the ventral lateral nucleus pars caudalis (VLc). In both VLo and VLc, we additionally observed coincidence between the cerebellar labeling and SMA projection neurons as well as between pallidal labeling and MI projection neurons. These data suggest that while MI primarily receives inputs originating from Cb and SMA primarily receives inputs originating from GPi, it also appears that MI and SMA receive secondary afferents arising from GPi and Cb, respectively.

Reticulospinal neurons in the pontomedullary reticular formation of the monkey (Macaca fascicularis)

Recent neurophysiological studies indicate a role for reticulospinal neurons of the pontomedullary reticular formation (PMRF) in motor preparation and goal-directed reaching in the monkey. Although the macaque monkey is an important model for such investigations, little is known regarding the organization of the PMRF in the monkey. In the present study, we investigated the distribution of reticulospinal neurons in the macaque. Bilateral injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) were made into the cervical spinal cord. A wide band of retrogradely labeled cells was found in the gigantocellular reticular nucleus (Gi) and labeled cells continued rostrally into the caudal pontine reticular nucleus (PnC) and into the oral pontine reticular nucleus (PnO). Additional retrograde tracing studies following unilateral cervical spinal cord injections of cholera toxin subunit B revealed that there were more ipsilateral (60%) than contralateral (40%) projecting cells in Gi, while an approximately 50:50 ratio contralateral to ipsilateral split was found in PnC and more contralateral projections arose from PnO. Reticulospinal neurons in PMRF ranged widely in size from over 50 microm to under 25 microm across the major somatic axis. Labeled giant cells (soma diameters greater than 50 microm) comprised a small percentage of the neurons and were found in Gi, PnC and PnO. The present results define the origins of the reticulospinal system in the monkey and provide an important foundation for future investigations of the anatomy and physiology of this system in primates.

The Spotted Hyena (Crocuta crocuta) as a Model System for Study of the Evolution of Intelligence

Abstract Large brains and great intelligence are metabolically costly, but the social complexity hypothesis suggests that these traits were favored nonetheless in primates by selection pressures associated with life in complex societies. If so, then cognitive abilities and nervous systems with primatelike attributes should have evolved convergently in nonprimate mammals living in large, elaborate societies in which social dexterity enhances individual fitness. Societies of spotted hyenas (Crocuta crocuta) are remarkably similar to those of cercopithecine primates with respect to size, structure, and patterns of competition and cooperation. These similarities set an ideal stage for comparative analysis of social intelligence and nervous system organization. As in cercopithecine primates, spotted hyenas use multiple sensory modalities to recognize their kin and other conspecifics as individuals, they know that some group-mates are more valuable social partners than others, they recognize 3rd-party kin and rank relationships among their clan-mates, and they use this knowledge adaptively during social decision-making. Examination of the available data strongly suggests convergent evolution of intelligence between spotted hyenas and primates. Evidence that less gregarious members of the family Hyaenidae lack some of the cognitive abilities found in spotted hyenas would provide further support for the idea that social complexity favors enhancement of intelligence in mammals.