Responses of red nucleus neurones to cutaneous afferent inputs

Inhibitory synaptic input to identified rubrospinal neurons in Macaca fascicularis: an electron microscopic study using a combined immuno-GABA-gold technique and the retrograde transport of WGA-HRP

Rubrospinal neurons of the magnocellular division of the red nucleus of Macaca fascicularis were retrogradely labeled following spinal cord microinjections of wheat germ agglutinin-horseradish peroxidase, as demonstrated by the chromagen tetramethylbenzidine, identifying the mesencephalic cells of origin of this descending motor pathway. The tissue was processed for electron microscopy and subsequently tested on the electron microscope grid for immunoreactivity of gamma aminobutyric acid (GABA) in presumed local circuit neuronal somata, in dendrites, and in axonal terminals. Results demonstrate the presence of retrogradely labeled rubrospinal neurons of medium and large diameters (30-90 microns) and immunoreactive neurons of small size (less than 20 microns in diameter) within the nucleus. In addition, there are substantial numbers of GABAergic, presumably inhibitory, synaptic structures contacting somata and primary, medium, and small sized dendrites, as well as spineheads of rubrospinal neurons. The immunoreactive presynaptic profiles exhibit two different morphological appearances: one axonal and the other dendritic. Axonal terminals contain densely packed pleomorphic to flattened vesicles and form primarily symmetrical synapses with somata and all regions of the dendritic arbor. GABAergic profiles resembling presynaptic dendrites (PSDs) are also present. These profiles possess scattered flattened to pleomorphic synaptic vesicles in a translucent cytoplasm and are often postsynaptic to axonal terminals of unknown origin, or to GABAergic profiles. GABAergic local circuit neurons (LCNs), the neurites of which remain within the confines of the nucleus, appear to be contacted primarily by cortical and cerebellar afferents. These LCNs may or may not possess axons and thus may represent both the source of the GABAergic axonal terminals as well as that of the PSDs. Inhibitory afferents from other sources, such as the mesencephalic reticular formation, may also account for GABAergic terminals involved in this inhibition. We propose that the level of excitability of rubrospinal neurons and their subsequent activation of spinal motor neurons and interneurons is significantly regulated by the local circuit GABAergic inhibitory interneuronal population of the nucleus proper and probably by axons entering the nucleus from an extranuclear source.

Inhibition of rubral neurons by noxious and non-noxious pressure

The role of the red nucleus (RN) in nociception was investigated in this study. Extracellular recordings from spontaneously active RN neurons were conducted in the rat while noxious pressure was delivered to the hindpaws or tail. Cells in the RN were predominantly inhibited by the stimuli. The units were most responsive when noxious pressure was applied to the contralateral hindpaw. Furthermore, more cells in the magnocellular division of the RN responded to the stimuli than cells in the parvocellular division. Delivery of a graded pressure stimulus to the contralateral hindpaw revealed 4 cell types in the RN: non-responsive cells; cells only responsive during the early, non-noxious portion of the stimulus; cells only responsive during the later, noxious portion of the stimulus; and cells that showed an initial response during the non-noxious part of the stimulus and a second, later response during the noxious portion of the stimulus. To further examine the putative role of the RN in nociception, oxotremorine, gamma-aminobutyric acid (GABA), serotonin, glutamate, and morphine were unilaterally microinjected into the RN and the responses of the animals in the tail flick test were assessed. Only morphine produced a significant antinociception in the animals following intrarubral microinjection. However, it is unclear whether this alteration was mediated through the RN because an antinociception of equal magnitude could be elicited from the reticular formation surrounding the RN and lesions of the RN did not alter the antinociception produced by systemic administration of morphine. Although other explanations cannot be ruled out, it appears that the RN may be involved in coordinating the motor response to pain rather than modulating sensory transmission.

Spatio-temporal organization of the somaesthetic projections in the red nucleus transmitted through the spino-rubral pathway in the cat

Although it has been known for a long time that in awake cats, natural stimulation of the skin induces short latency responses in rubrospinal cells, the pathway possibly involved has been identified only recently (Padel et al. 1988). This tract, which was described in acute, chloralose anaesthetized cats, ascends in the ventromedial spinal cord and is activated via collaterals of primary afferent fibres running in the dorsal columns of the spinal cord. The present study demonstrates that this newly described spino-rubral tract is able to send detailed somaesthetic information to the red nucleus. After lesions leaving intact only the spino-rubral pathway, excitatory and inhibitory responses to natural peripheral stimulations were recorded in identified rubral efferent cells. The most effective stimuli were touching the skin, passive joint rotation and hair displacement. Each cell was found to possess a particular receptive field. These fields which could be ipsi-, contra-, or bi-lateral were generally located on a single limb, although they could include two or more limbs, or even exceptionally the whole body with or without preferential zones. The topographic organization of receptive fields was arranged somatotopically in the red nucleus and overlapped the motor representation. The somaesthetic inputs transmitted through the spino-rubral pathway to the red nucleus are very similar to those previously observed in the intact cat, which supports the idea that this pathway may play a functional role in motor control. The spino-rubro-spinal loop may provide a fast adaptation of the descending motor command, thus producing a fine and harmonious tuning between the changing surroundings and the animal's movements.

Quantitative analysis of the neuronal population of the red nucleus of the cat

There is strong evidence in the literature for a correlation between the two parts of the red nucleus, magnocellular and parvocellular, and different functions. Unfortunately in the cat, the species most studied both physiologically and anatomically, there are no morphological criteria distinguishing the two portions. With quantitative techniques applied to Nissl preparations the neuronal population of the Red Nucleus has been studied in serial sections along the rostrocaudal axis of the mesencephalon of the cat. Statistical analysis of the data revealed a horizontal plane dividing the two portions of the nucleus with a high statistical significance level. This plane lies between the caudal two-thirds and the rostral third of the nucleus. Although in the model two portions can be distinguished, it is not possible to assign to either a single type of neuron, whether or considered in terms of shape or size.

Input-output relations of the red nucleus in the cat

In unanesthetized cats, microstimulation within the red nucleus produces contraction of single muscles of the contralateral limbs and face. Separate zones may activate different muscles. Forelimb muscles were primarily activated from areas in the dorsomedial quadrants of the red nucleus whereas hindlimb muscles were predominantly activated from the ventorlateral quadrants. With stimulus currents of 10 muA there was considerable overlap in the effective zones activating different muscles. In the majority of cases the minimal threshold was under 10 muA when stimilating with a 50-msec pulse train. Current thresholds for electromypgraphic changes in the muscles varied inversely with pulse frequency and train duration. When long stimulus trains were applied to the red nucleus, the resulting muscle contraction was sustained for the duration of the stimulus. These motor effects did not depend upon the motor cortex or pyramidal tract but were mediated by a tract in contralateral dorsal quadrants of the spinal cord which was likely to be the rubrospinal tract. Units within the red nucleus typically had wide cutaneous receptive fields and responded to deep pressure and joint rotation in one or more limbs. Usually the focus driving the cell most briskly was located in one of the contralateral limbs and corresponded to the limb where muscle contraction was elicited by microstimulation with the same electrode. It is concluded that the red nucleus includes overlapping efferent neuronal colonies controlling individual muscles irrespective of their functional class. This property is shared by the motor cortex and suggests that these two structures may complement each other in the control of movement. The more diffuse activation of rubral than cortical neurons by natural stimuli suggests that rubral activity may not be as tightly linked as that of the motor cortex to specific peripheral input.