Kimika Yoshino-Saito

Motor command for precision grip in the macaque monkey can be mediated by spinal interneurons

In motor control, the general view is still that spinal interneurons mainly contribute to reflexes and automatic movements. The question raised here is whether spinal interneurons can mediate the cortical command for independent finger movements, like a precision grip between the thumb and index finger in the macaque monkey, or if this function depends exclusively on a direct corticomotoneuronal pathway. This study is a followup of a previous report (Sasaki et al. J Neurophysiol 92: 3142-3147, 2004) in which we trained macaque monkeys to pick a small piece of sweet potato from a cylinder by a precision grip between the index finger and thumb. We have now isolated one spinal interneuronal system, the C3-C4 propriospinal interneurons with projection to hand and arm motoneurons. In the previous study, the lateral corticospinal tract (CST) was interrupted in C4/C5 (input intact to the C3-C4 propriospinal interneurons), and in this study, the CST was interrupted in C2 (input abolished). The precision grip could be performed within the first 15 days after a CST lesion in C4/C5 but not in C2. We conclude that C3-C4 propriospinal interneurons also can carry the command for precision grip.

Increased expression of the growth-associated protein 43 gene in the sensorimotor cortex of the macaque monkey after lesioning the lateral corticospinal tract.

To investigate the neural basis for functional recovery of the cerebral cortex following spinal cord injury, we measured the expression of growth‐associated protein 43 (GAP‐43), which is involved in the process of synaptic sprouting. We determined the GAP‐43 mRNA expression levels in the sensorimotor cortical areas of macaque monkeys with a unilateral lesion of the lateral corticospinal tract (l‐CST) at the C4/C5 level of the cervical cord and compared them with the levels in the corresponding regions of intact monkeys. Lesioned monkeys recovered finger dexterity during the first months after surgery, and the GAP‐43 mRNA levels increased in layers II–III in primary motor areas (M1), bilaterally. Double‐labeling analysis of the lesioned monkeys showed that GAP‐43 mRNA was expressed strongly in excitatory neurons but only rarely in inhibitory interneurons. Expression also increased in the medium‐sized (area, 500–1,000 μm2) and large pyramidal cells (area, >1,000 μm2) in layer V of the bilateral M1. The increased expression of GAP‐43 mRNA in the M1 contralateral to the lesion was more prominent during the early recovery stage than during the late recovery stage. In addition, GAP‐43 mRNA increased in layers II–III of both the contralesional ventral premotor area and the primary somatosensory area. These results suggest that GAP‐43 is involved in time‐dependent and brain region‐specific plastic changes after l‐CST lesioning. The expression patterns imply that plastic changes occur not only in M1 but also in the broad associative cortical network, including the ventral premotor and primary sensory areas. J. Comp. Neurol. 516:493–506, 2009.

Quantitative inter-segmental and inter-laminar comparison of corticospinal projections from the forelimb area of the primary motor cortex of macaque monkeys.

Corticospinal projections from the forelimb area of the primary motor cortex to the C2-Th2 spinal cord segments were quantitatively analyzed using the high resolution anterograde tracer, biotinylated dextran amine (BDA), in rhesus monkeys (n=5). The majority of descending axons were located in the contralateral dorsolateral funiculus (DLF) (85-98%), but a minor portion was observed in the ipsilateral DLF (1-12%) and ventromedial funiculus (VMF) (1-7%). In the gray matter, axon collaterals and terminal buttons were found mainly in the contralateral laminae VI-VII and IX and ipsilateral lamina VIII. The majority of projections to the contralateral gray matter originated from the contralateral DLF, but a minority originated from the ipsilateral DLF. Axons from the ipsilateral DLF were not found to project collaterals on the ipsilateral side, but directly entered the contralateral side after crossing the midline. On the other hand, projections to the ipsilateral lamina VIII were from the ipsilateral VMF, and commissural axons were from the contralateral DLF. Terminal buttons in the motoneuron pool in the contralateral lamina IX were found mainly at the C7-Th1 spinal cord segments, whereas the projections to the contralateral laminae VI-VII, ipsilateral lamina VIII, and commissural axons were also found in more rostral segments, abundantly at the C4-C8 segments, 1-3 segments rostral to the motoneuronal projections. These results suggest that cortical control of contralateral forelimb motoneurons accompanies regulation of interneuronal systems in the contralateral laminae VI-VII and the ipsilateral lamina VIII located a few segments rostral to the motoneurons.

SPP1 is expressed in corticospinal neurons of the macaque sensorimotor cortex

The cellular distribution of SPP1, which we recently identified as a gene with greater expression in the macaque primary motor cortex than in the premotor or prefrontal cortices, was examined in rhesus macaque, common marmoset, and rat brains. In situ hybridization histochemistry revealed that SPP1 mRNA was expressed specifically in pyramidal neurons in layer V of the sensorimotor cortex of the rhesus macaque. These SPP1 mRNA‐positive neurons were most abundant in the primary motor area, followed by Brodmann area 5 and the supplementary motor area, in accordance with the distribution of corticospinal neurons. In addition, injection of a retrograde neuroanatomical tracer into the lateral corticospinal tract (CST) of the spinal cord caused labeling of SPP1 positive neurons, indicating the expression of SPP1 in corticospinal neurons. SPP1 was also expressed in the thalamus, brainstem, and spinal ventral horn of the rhesus macaque. Although SPP1 was also detected in the brainstem and spinal cord of the marmoset and the rat, it was not detected in their cerebral cortices. Selective expression in the corticospinal neurons of the sensorimotor cortex of the rhesus macaque suggests that SPP1 plays a critical role in the functional or structural specialization of highly developed corticospinal systems in certain primate species. J. Comp. Neurol. 518:2633–2644, 2010.

Effects of early versus late rehabilitative training on manual dexterity after corticospinal tract lesion in macaque monkeys

Dexterous hand movements can be restored with motor rehabilitative training after a lesion of the lateral corticospinal tract (l-CST) in macaque monkeys. To maximize effectiveness, the optimal time to commence such rehabilitative training must be determined. We conducted behavioral analyses and compared the recovery of dexterous hand movements between monkeys in which hand motor training was initiated immediately after the l-CST lesion (early-trained monkeys) and those in which training was initiated 1 mo after the lesion (late-trained monkeys). The performance of dexterous hand movements was evaluated by food retrieval tasks. In early-trained monkeys, performance evaluated by the success rate in a vertical slit task (retrieval of a small piece of food through a narrow vertical slit) recovered to the level of intact monkeys during the first 1-2 mo after the lesion. In late-trained monkeys, the task success rate averaged ∼30% even after 3 mo of rehabilitative training. We also evaluated hand performance with the Klüver board task, in which monkeys retrieved small spherical food pellets from cylindrical wells. Although the success rate of the Klüver board task did not differ between early- and late-trained monkeys, kinematic movement analysis showed that there was a difference between the groups: late-trained monkeys with an improved success rate frequently used alternate movement strategies that were different from those used before the lesion. These results suggest that early rehabilitative training after a spinal cord lesion positively influences subsequent functional recovery.

Differential Expression of Secreted Phosphoprotein 1 in the Motor Cortex among Primate Species and during Postnatal Development and Functional Recovery

We previously reported that secreted phosphoprotein 1 (SPP1) mRNA is expressed in neurons whose axons form the corticospinal tract (CST) of the rhesus macaque, but not in the corresponding neurons of the marmoset and rat. This suggests that SPP1 expression is involved in the functional or structural specialization of highly developed corticospinal systems in certain primate species. To further examine this hypothesis, we evaluated the expression of SPP1 mRNA in the motor cortex from three viewpoints: species differences, postnatal development, and functional/structural changes of the CST after a lesion of the lateral CST (l-CST) at the mid-cervical level. The density of SPP1-positive neurons in layer V of the primary motor cortex (M1) was much greater in species with highly developed corticospinal systems (i.e., rhesus macaque, capuchin monkey, and humans) than in those with less developed corticospinal systems (i.e., squirrel monkey, marmoset, and rat). SPP1-positive neurons in the macaque monkey M1 increased logarithmically in layer V during postnatal development, following a time course consistent with the increase in conduction velocity of the CST. After an l-CST lesion, SPP1-positive neurons increased in layer V of the ventral premotor cortex, in which compensatory changes in CST function/structure may occur, which positively correlated with the extent of finger dexterity recovery. These results further support the concept that the expression of SPP1 may reflect functional or structural specialization of highly developed corticospinal systems in certain primate species.

Histological and electrophysiological analysis of the corticospinal pathway to forelimb motoneurons in common marmosets

Using histological and electrophysiological methods, we identified the neuroanatomical properties of the common marmoset corticospinal tract (CST), which underlies hand/arm motor control. Biotinylated dextran amine (BDA) was injected into the primary motor cortex to anterogradely label CST axons in the cervical segments, revealing that most CST axons descend in the contralateral dorsolateral funiculus (DLF; 85.0%), and some in the ipsilateral DLF (10.7%). Terminal buttons were mainly found in the contralateral lamina VII of the gray matter, but projection to lamina IX, where forelimb motoneurons are located, was rare. Bilateral projections were more abundant than found in the rat CST, resembling the CST organization of other primates. Intracellular recordings were made from 57 forelimb motoneurons on the contralateral side to stimulation, which revealed no monosynaptic excitatory postsynaptic potentials (EPSPs), but di- or polysynaptic EPSPs and inhibitory synaptic potentials were commonly found. Local field potentials showed monosynaptic excitation mainly in laminae VII, where abundant BDA-labeled CST terminals were observed. These results suggest that direct corticomotoneuronal projection is absent in common marmosets but di- or oligosynaptic effects would be mediated by spinal interneurons.

SPP1 expression in spinal motor neurons of the macaque monkey

In the macaque cerebral cortex, the SPP1 (secreted phosphoprotein 1) gene is mainly expressed in corticospinal neurons. In this study, we found that SPP1 was principally expressed in motor neurons in lamina IX of the macaque spinal cord. The expression level varied among different spinal segments and correlated positively with neuron size. The expression was weak in Errγ-positive neurons, presumably gamma motor neurons, and in neurons in sacral Onufs nucleus. These results suggest that SPP1 is a molecular characteristic of spinal motor neurons and is preferentially expressed in neurons with high conduction velocities.

Functional Annotation of Genes Differentially Expressed Between Primary Motor and Prefrontal Association Cortices of Macaque Brain

DNA microarray-based genome-wide transcriptional profiling and gene network analyses were used to characterize the molecular underpinnings of the neocortical organization in rhesus macaque, with particular focus on the differences in the functional annotation of genes in the primary motor cortex (M1) and the prefrontal association cortex (area 46 of Brodmann). Functional annotation of the differentially expressed genes showed that the list of genes selectively expressed in M1 was enriched with genes involved in oligodendrocyte function, and energy consumption. The annotation appears to have successfully extracted the characteristics of the molecular structure of M1.

Selective SPP1 expression in amyotrophic lateral sclerosis-vulnerable motor neurons of the macaque monkey

The aim of this study was to identify the origin of multisynaptic inputs from the GPi to two sectors of the PMd. We injected rabies virus into the rostral (F2r) or caudal part (F2c) of the PMd in macaque monkeys. The virus was transported across synapses from postsynaptic to presynaptic neurons. After a 3-day postinjection period that allows for the second-order neuron labeling, the GPi contained labeled neurons. There was a difference in the distribution area of neuronal labeling between the two injection cases. After the F2r injection, labeled GPi neurons were found in the associative territory. By contrast, the F2c injection led to labeling of the motor territory. The present results revealed that F2r and F2c participate in the associative or motor loop, respectively, suggesting a distinct involvement of these regions in motor planning versus execution.