Movements of cats on a rotating cylinder: role of the substantia nigra pars reticulata and the deeper layers of the superior colliculus

Motor imagery evokes increased somatosensory activity in Parkinson's disease patients with tremor

Parkinson's disease (PD) is surprisingly heterogeneous: some patients have a prominent resting tremor, while others never develop this symptom. Here we investigate whether the functional organization of the voluntary motor system differs between PD patients with and without resting tremor, and whether these differences relate to the cerebral circuit producing tremor. We compared 18 PD patients with marked tremor, 20 PD patients without tremor, and 19 healthy controls. Subjects performed a controlled motor imagery task during fMRI scanning. We quantified imagery-related cerebral activity by contrasting imagery of biomechanically difficult and easy movements. Tremor-related activity was identified by relating cerebral activity to fluctuations in tremor amplitude, using electromyography during scanning. PD patients with tremor had better behavioral performance than PD patients without tremor. Furthermore, tremulous PD patients showed increased imagery-related activity in somatosensory area 3a, as compared with both healthy controls and to nontremor PD patients. This effect was independent from tremor-related activity, which was localized to the motor cortex, cerebellum, and thalamic ventral intermediate nucleus (VIM). The VIM, with known projections to area 3a, was unique in showing both tremor- and imagery-related responses. We conclude that parkinsonian tremor influences motor imagery by modulating central somatosensory processing through the VIM. This mechanism may explain clinical differences between PD patients with and without tremor.

Differential progression of proprioceptive and visual information processing deficits in Parkinson's disease

Indirect evidence suggests that patients with Parkinson's disease (PD) have deficits not only in motor performance, but also in the processing of sensory information. We investigated the role of sensory information processing in PD patients with a broad range of disease severities and in a group of age-matched controls. Subjects were tested in two conditions: pointing to a remembered visual target in complete darkness (DARK) and in the presence of an illuminated frame with a light attached to the index finger (FRAME). Differences in pointing errors in these two conditions reflect the effect of visual feedback on pointing. PD patients showed significantly larger constant and variable errors than controls in the DARK and FRAME condition. The difference of the variable error in the FRAME and DARK condition decreased as a function of the severity of PD. This indicates that any deficits in the processing of proprioceptive information occur already at very mild symptoms of PD, and that deficits in the use of visual feedback develop progressively in later stages of the disease. These results provide a tool for early diagnosis of PD and shed new light on the functional role of the brain structures that are affected in PD.

6-hydroxydopamine lesion in the A8 cell group of cats produces a short-lasting decreased accuracy in goal-directed forepaw-movements

Recently, feline studies have shown that a lesion in the retrorubral area, which includes the dopaminergic A8 cell group, produces motor programming deficits inherent to a hypofunction of the A9 system. A hypofunction in the striatal terminal area of A9 fibers, in turn, is known to produce a hypofunction of its first-order output station, namely the substantia nigra pars reticulata (SNR). The integrity of the SNR allows animals to execute (1) 'postural adjustments that rely on proprioceptive stimuli that originate in body parts at rest' and (2) 'non-externally guided' targeting movements. In view of these considerations, the (dys)function of the SNR of cats with a bilaterally 6-hydroxydopamine lesion of A8 cells in the retrorubral area was tested in an experimental set-up that allows the assessment of changes in these functions. The A8 lesion produced: (a) a short-lasting increase in the number of accurate targeting movements as well as an increase in the time required for the collection of six pellets: these deficits disappeared 4-7 days after the lesion; (b) a long-lasting disappearance of (1) 'postural adjustments that rely on proprioceptive stimuli that originate in body parts at rest' and (2) 'non-externally guided targeting movements'; and (c) a long-lasting display of a new strategy that allowed the lesioned cat to collect its pellets despite of its other deficits. These data led to the conclusion that a lesion of A8 cells even disrupts the function of the SNR, being one of the outputstations of the A8 cell group.

Organization of the zona incerta in the macaque: an electron microscopic study

BACKGROUND

The zona incerta (ZI) receives projections from many telencephalic and brainstem structures. On the basis of its connectivity and physiology, this nucleus has been implicated in the control of saccadic eye movements. Because of the complexity of its afferent signals and its simple efferent signal, there must be much local interaction within the ZI to integrate these various afferents. The purpose of this study was to investigate, at the ultrastructural level, whether the ZI contains the anatomical substrata which could subserve the control of eye movements.

METHODS

Blocks of tissue from the ZI of macaque monkeys were prepared for electron microscopy using standard techniques. Some of these animals were taken specifically for electron microscopy. Others had received injections of tracer substances and were prepared for electron microscopy subsequent to tracer visualization.

RESULTS

Cell bodies of medium-large neurons were found in our preparations. They have large nucleoli and relatively small volumes of karyoplasm. Cell bodies and dendrites of all sizes have many synaptic contacts. Three types of synaptic profiles were found, designated Types 1, 2, and 3. Type 1 profiles are symmetrical and contact cell bodies and small dendrites. Type 2 profiles are thought to be presynaptic dendrites and may have symmetrical or asymmetrical contacts. Type 3 profiles are asymmetrical and primarily contact small dendrites. Many synapses contacted vesicle-containing profiles. In some cases, it was clear that these profiles participated in serial synapses on presumptive presynaptic dendrites. Other profiles appeared to be axoaxonic contacts.

CONCLUSIONS

Afferent and efferent signals are likely to be modulated extensively within the ZI. Therefore, there needs to be complex interactions between neuronal elements of the ZI and its afferents. This study demonstrates that this nucleus possesses the structural substrata to subserve diverse roles, such as the gating of saccadic eye movements.

Cytoarchitecture of the substantia nigra pars lateralis in the opossum (Didelphis virginiana): a correlated light and electron microscopic study

BACKGROUND

The substantia nigra has been divided into three subdivisions. However, the cytoarchitecture of one of these subdivisions, the pars lateralis (SNl), has not been previously examined in detail at the light and electron microscopic levels in any species. In the adult opossum, the three nigral subdivisions can be easily distinguished as distinct, rostrocaudally oriented cell groups separated by neuron-free zones. Thus it was possible to determine the boundaries of the SNl unambiguously. This report covers the results of an examination of the morphology and organization of the SNl in the opossum.

METHODS

Material from 13 opossums was used for this study. Eight of the animals had been previously stained for Nissl substance (n = 4) or impregnated by the Golgi technique (n = 4). The remaining five animals were prepared for electron microscopic studies using standard procedures.

RESULTS

Two cell types were identified on the basis of morphological differences, small and medium-large neurons. Small neurons (10-18 microns long axis) have large nuclei with moderate amounts of heterochromatin and a thin rim of cytoplasm. They have long (up to 500 microns), spine-free dendrites. Medium-large neurons (18-54 microns long axis) have rounded nuclei with electron-lucent nucleoplasm. Few indentations of the nuclear envelope were observed. The surrounding cytoplasm has dense arrays of organelles. Nissl bodies are particularly prominent in the form of pyramids with their bases at juxtanuclear positions and their apices directed toward emerging dendrites. Dendrites of medium-large neurons are long (some > 1 mm in length), are primarily oriented in the frontal plane, and extend along the dorsal surface of or into the cerebral peduncle. Some cells have dendrites that are moderately spinous, whereas other neurons possess sparsely spinous dendrites. Relatively few synaptic profiles are observed to contact somata and proximal dendrites.

CONCLUSION

This report provides added morphological support for the idea that the SNl is a distinct subdivision of the substantia nigra, a distinction previously made on the basis of the physiologically characterized relationship between the lateral substantia nigra and orienting behaviors and seizure-related function.

Behavioral correlates of a progressive dysfunctioning of the deeper layers of the colliculus superior: effects of picrotoxin

Intracaudate injections of relatively high doses of apomorphine produce a regression in motor behavior of cats collecting food pellets in a treadmill design (25). It has been hypothesized that this regression is partly due to functional disturbances in brain regions receiving (in) directly striatal output signals. In view of this hypothesis it was investigated whether experimentally induced changes in GABAergic activity within the deeper layers of the colliculus superior, which is a second order output station of the caudate nucleus, are also able to elicit a regression in motor behavior. Therefore, motor behavior of cats was tested in the treadmill paradigm before and after intracollicular injections of the GABA antagonist picrotoxin. Picrotoxin produced dose-dependently a regression in motor behavior which was comparable to that elicited by intrastriatally injected apomorphine. The noted effects were GABA-specific since muscimol attenuated the picrotoxin-induced regression. The present data are discussed in view of a model for a hierarchical organization of the brain.

Search after neurobiological profile of individual-specific features of Wistar rats

The first part of this study demonstrates that the bimodal shape of variation in "fleeing" and "nonfleeing" or "freezing" rats of an outbred strain of Wistar rats forms part of an overall biomodal variation in behavioural responses to injections of agents, which selectively alter, or reflect, the noradrenergic or dopaminergic activity in the ventral striatum, and dopaminergic activity in the dorsal striatum, the GABA-ergic activity in the substantia nigra, pars reticulata, and the GABA-ergic activity in the deeper layers of the superior colliculus. It is concluded that the "fleeing" and "nonfleeing" rats, each of them marked by their own trans-situational consistency in pharmacological and behavioural responses, represent the two fundamentally different types of individuals which normally exist in unselected populations of rodents. The second part of this study demonstrates that the pharmacogenetic selection of apomorphine-susceptible (APO-SUS) and apomorphine-unsusceptible (APO-UNSUS) rats, i.e., one individual-specific feature of the overall bimodal variation for pharmacological responses in our outbred strain of rats, is a valid tool to disperse the above-mentioned individual-specific features as far as possible. First, these lines allowed us to prove that the overall bimodal shape of variation in pharmacological and behavioral responses of individual outbred rats is in part genetically determined. Second, these lines allowed us to prove that a bimodal variation in neurochemical features of the circuitry, in which the ventral striatum is embedded, underlies the overall bimodal variation in pharmacological and behavioural responses. Third, these lines allowed us to demonstrate that a fundamental difference in organizing behaviour with the help of external and internal information has to be considered as a common factor giving rise to the individual differentiation found in the present study. Given the notion that this individual differentiation appears to be valid across lines, substrains and strains of rats, the present study lays the foundation for understanding at least a part of the physiological basis underlying differences between the two fundamentally different types of individuals existing in normal populations of rodents.

Identification of the substantia nigra pars lateralis in the macaque using cytochrome oxidase and fiber stains

Classically, the substantia nigra is divided into three subdivisions, the pars compacta, the pars reticulata, and the pars lateralis. The purpose of the present study was to develop methods for routine identification of the non-compacta subdivisions in the macaque monkey. First, the pars reticulata and pars lateralis were identified based on their Nissl features. Second, nearby cytochrome oxidase and Gallyas fiber stained sections were examined for these two subdivisions. In both stains, a dorsolateral region could be identified to contain large fiber bundles. This region is coextensive with the pars lateralis as identified in Nissl-stained preparations. Lastly, we examined the complete extent of the pars lateralis using the cytochrome oxidase procedure. We conclude that in the macaque monkey the pars lateralis can be identified as a distinct nigral subdivision by using cytochrome oxidase and fiber staining techniques.