Three types of neurons in the medial cuneate nucleus of the cat

Functional organization and connectivity of the dorsal column nuclei complex reveals a sensorimotor integration and distribution hub

The dorsal column nuclei complex (DCN-complex) includes the dorsal column nuclei (DCN, referring to the gracile and cuneate nuclei collectively), external cuneate, X, and Z nuclei, and the median accessory nucleus. The DCN are organized by both somatotopy and modality, and have a diverse range of afferent inputs and projection targets. The functional organization and connectivity of the DCN implicate them in a variety of sensorimotor functions, beyond their commonly accepted role in processing and transmitting somatosensory information to the thalamus, yet this is largely underappreciated in the literature. To consolidate insights into their sensorimotor functions, this review examines the morphology, organization, and connectivity of the DCN and their associated nuclei. First, we briefly discuss the receptors, afferent fibers, and pathways involved in conveying tactile and proprioceptive information to the DCN. Next, we review the modality and somatotopic arrangements of the remaining constituents of the DCN-complex. Finally, we examine and discuss the functional implications of the myriad of DCN-complex projection targets throughout the diencephalon, midbrain, and hindbrain, in addition to their modulatory inputs from the cortex. The organization and connectivity of the DCN-complex suggest that these nuclei should be considered a complex integration and distribution hub for sensorimotor information.

Integration of sensory quanta in cuneate nucleus neurons in vivo

Discriminative touch relies on afferent information carried to the central nervous system by action potentials (spikes) in ensembles of primary afferents bundled in peripheral nerves. These sensory quanta are first processed by the cuneate nucleus before the afferent information is transmitted to brain networks serving specific perceptual and sensorimotor functions. Here we report data on the integration of primary afferent synaptic inputs obtained with in vivo whole cell patch clamp recordings from the neurons of this nucleus. We find that the synaptic integration in individual cuneate neurons is dominated by 4-8 primary afferent inputs with large synaptic weights. In a simulation we show that the arrangement with a low number of primary afferent inputs can maximize transfer over the cuneate nucleus of information encoded in the spatiotemporal patterns of spikes generated when a human fingertip contact objects. Hence, the observed distributions of synaptic weights support high fidelity transfer of signals from ensembles of tactile afferents. Various anatomical estimates suggest that a cuneate neuron may receive hundreds of primary afferents rather than 4-8. Therefore, we discuss the possibility that adaptation of synaptic weight distribution, possibly involving silent synapses, may function to maximize information transfer in somatosensory pathways.

Stimulation within the cuneate nucleus suppresses synaptic activation of climbing fibers

Several lines of research have shown that the excitability of the inferior olive is suppressed during different phases of movement. A number of different structures like the cerebral cortex, the red nucleus, and the cerebellum have been suggested as candidate structures for mediating this gating. The inhibition of the responses of the inferior olivary neurons from the red nucleus has been studied extensively and anatomical studies have found specific areas within the cuneate nucleus to be target areas for projections from the magnocellular red nucleus. In addition, GABA-ergic cells projecting from the cuneate nucleus to the inferior olive have been found. We therefore tested if direct stimulation of the cuneate nucleus had inhibitory effects on a climbing fiber field response, evoked by electrical stimulation of the pyramidal tract, recorded on the surface of the cerebellum. When the pyramidal tract stimulation was preceded by weak electrical stimulation (5–20 μA) within the cuneate nucleus, the amplitude of the climbing fiber field potential was strongly suppressed (approx. 90% reduction). The time course of this suppression was similar to that found after red nucleus stimulation, with a peak suppression occurring at 70 ms after the cuneate stimulation. Application of CNQX (6-cyano-7-nitroquinoxaline-2,3-dione, disodium salt) on the cuneate nucleus blocked the suppression almost completely. We conclude that a relay through the cuneate nucleus is a possible pathway for movement-related suppression of climbing fiber excitability.

Afferent synaptic contacts on glycine-immunoreactive neurons in the rat cuneate nucleus

This study was aimed to clarify whether the primary afferent terminals (PATs), GABAergic terminals, and glutamatergic terminals made direct synaptic contacts with glycine-IR neurons in the cuneate nucleus of rats. In this connection, injection of the anterograde tracer WGA-HRP into brachial plexus, antiglycine preembedding immunoperoxidase, and anti-GABA, along with antiglutamate postembedding immunogold labeling, were used to identify the PATs, glycine-IR neurons, GABA-IR terminals, and glutamate-IR terminals, respectively. The present results showed that HRP-labeled PATs, immunoperoxidase-labeled glycine-IR terminals, immunogold-labeled GABA-IR, and glutamate-IR terminals made axodendritic synaptic contacts with immunoperoxidase-labeled glycine-IR neurons. The latter three presynaptic elements also formed axosomatic synapses with glycine-IR neurons. Statistical analysis has shown that the minimum diameter of the glycine-IR dendrites postsynaptic to the above-mentioned four presynaptic elements did not differ significantly. In addition, the synaptic ratio of the glutamate-IR terminals on the glycine-IR dendrites was higher than that of GABA-IR terminals. The synaptic ratio of the GABA-IR terminals on glycine-IR dendrite was in turn higher than that of the PATs and glycine-IR terminals. It is suggested that the PATs and glutamate-IR terminals on the glycine-IR neurons may be involved in subsequent postsynaptic inhibition for spatial precision of lateral inhibition. On the other hand, the GABA-IR and glycine-IR terminals which make synaptic contacts with the dendrites of glycine-IR neurons may provide a putative means for disinhibition or facilitation to maintain the baseline neuronal activity in the rat cuneate nucleus. The results of quantitative analysis suggest that glutamate act as the primary excitatory neurotransmitter, while GABA, when compared with glycine, may serve as a more powerful inhibitory neurotransmitter on glycine-IR neurons in the rat cuneate nucleus.

Morphometric study of glycine-immunoreactive neurons and terminals in the rat cuneate nucleus

The distribution of glycine-immunoreactive (glycine-IR) neurons and their associated axon terminals in the rat cuneate nucleus was studied using antiglycine postembedding immunoperoxidase labelling and immunogold staining, respectively. The immunoperoxidase-labelled glycine-IR neurons were widely distributed in the entire rostrocaudal extent of the nucleus. They made up 30.8% (9671/31368) of the neurons surveyed. Quantitative evaluation showed that the percentage of glycine-IR neurons in the caudal level was significantly higher than that in the middle and rostral levels. The glycine-IR neurons were small cells (mean area = 198+/-1.9 microm2, n = 2862) with ovoid or spindle-shaped somata. Statistical analysis showed that the size of the glycine-IR neurons in the rostral level was significantly smaller than that in the middle and caudal levels. Immunogold labelled glycine-IR terminals which contained predominantly pleomorphic synaptic vesicles were mostly small (mean area = 1.24+/-0.03 microm2, n = 286) and they constituted 24.7% (286/1158) of the total terminals surveyed. They formed axodendritic, axosomatic and axoaxonic synapses with unlabelled elements. It is suggested from this study that glycine is one of the major neurotransmitters involved in the depression of synaptic transmission in the cuneate nucleus.

Multiple inputs of GABA-immunoreactive neurons in the cuneate nucleus of the rat

Using anterograde transport of WGA-HRP and the experimental degeneration method for identification of corticocuneate (CCT) and primary afferent (PAT) terminals in conjunction with gamma-amino butyric acid (GABA) and glutamate immunocytochemistry, this study has demonstrated that the GABA-immunoreactive (GABA-IR) neurons in the rat cuneate nucleus were post-synaptic to PATs (some of them being glutamate-IR), GABA-IR and GABA-negative terminals. The HRP-labelled CCTs did not make any synaptic contacts with GABA-IR neurons but with some GABA-negative dendrites. PATs labelled by HRP or showing degenerating features made direct synaptic contacts with the dendrites of GABA-IR neurons. Beside the above GABA-IR boutons also showed axosomatic and axodendritic synapses with the GABA-IR neurons. In 'triple labeling' method for GABA, PAT and glutamate, it was found that the PATs which were usually glutamate-positive were presynaptic to the dendrites of GABA-IR neurons. Furthermore, some glutamate-IR terminals which were of non-PAT's origin also synapsed with the dendrites and somata of GABA-IR neurons. It is concluded from this study that the major inputs of GABA-IR neurons were from glutamate immunopositive PATs and glutamate terminals of non-PATs origin; other GABA-IR terminals either intrinsic or extrinsic also contributed to the afferent sources of GABA-IR neurons. The CCTs contributed very little, if any, to this input. It is suggested that the PATs and glutamate-IR terminals on GABA-IR neurons may be involved in lateral inhibition for increase of spatial precision. The synaptic contacts between GABA-IR boutons and dendrites or somata of GABA-IR neurons may provide a possible means for disinhibition.

The synaptic interrelationships between primary afferent terminals, cuneothalamic relay neurons and GABA-immunoreactive boutons in the rat cuneate nucleus

The present study described an ultrastructural synaptic configurations between primary afferent terminals (PATs), cuneothalamic relay neurons (CTNs) and GABA-immunoreactive (GABA-IR) boutons in the cuneate nucleus of rats using cervicothoracic dorsal rhizotomies, retrograde transport of wheat germ agglutinin conjugated with horseradish peroxidase complex (WGA-HRP) and anti-GABA immunogold labelling methods. With this procedure, direct synaptic relationships between the PATs, CTNs and GABA-IR boutons have been demonstrated. The most remarkable feature was the observation whereby an immunogold-labelled GABA-IR bouton was presynaptic to a WGA-HRP labelled dendrite of CTN and a degenerating PAT; the same PAT was in turn presynaptic to the HRP-labelled dendrite. This was evident in ten out of a total of 133 synaptic configurations that were closely scrutinized. Results of this study support the concept that GABA-IR boutons are not only involved in presynaptic inhibition on the primary afferent input to the cuneothalamic relay neurons, but also exert a simultaneous postsynaptic inhibition on these cells.

Synaptic relationships between GABA-immunoreactive boutons and primary afferent terminals in the rat cuneate nucleus

The present study examined the synaptic relation between the primary afferent terminals and intrinsic neuronal elements in the rat cuneate nucleus. For this purpose, experimental degeneration after multiple cervicothoracic dorsal rhizotomies or anterograde transport of wheatgerm agglutinin conjugated to horseradish peroxidase were used to identify the primary afferent terminals, while immunogold postembedding staining was employed to identify the GABA-immunoreactive boutons. The combined procedure allowed us to demonstrate a direct synaptic relationship between the primary afferent terminals and GABA-immunoreactive boutons. At least two types of synaptic relation were observed between the primary afferent terminals, identified by their degenerating features or labeled by wheatgerm agglutinin conjugated to horseradish peroxidase, and the immunogold-labeled GABA-immunoreactive boutons (i) a GABA-immunoreactive bouton making a simple presynaptic contact with the primary afferent terminal; and (ii) a synaptic glomerular complex in which the centrally located primary afferent terminal was postsynaptic to a GABA-immunoreactive bouton and presynaptic to dendrites closely associated with it; both terminals were sometimes presynaptic to a common dendrite. It is speculated from this study that the incoming impulses from the forelimb area are modulated by the GABA-immunoreactive boutons in the cuneate nucleus of the rat.

Spatial overlap of rubrospinal and corticospinal terminals with input to the inferior olive

Somatosensory responses of cells in the dorsal accessory olive are suppressed following stimulation of the magnocellular red nucleus. Since the magnocellular red nucleus of the cat does not project directly to the dorsal accessory olive, the present experiments were designed to identify indirect pathways that might mediate suppression of olivary responsiveness. Wheat germ agglutinin-horseradish peroxidase was used to compare the location of magnocellular red nucleus terminals with the locations of cells providing input to the rostral dorsal accessory olive. Cells projecting to forelimb rostral dorsal accessory olive can be divided into two main groups: one group comprises a column of large cells located in the ventral caudal cuneate nucleus extending into lamina VI of C1 and C2, and a second group comprises smaller cells located in the ventral rostral cuneate nucleus. Terminations of fibers originating in the magnocellular red nucleus were found to target both groups of cells projecting to the dorsal accessory olive. Therefore, it is possible that the responsiveness of olivary cells is influenced via these terminations. Stimulation of sensorimotor cortex has also been shown to inhibit olivary responsiveness. Terminations from sensorimotor cortex target the same regions of cells that project to the dorsal accessory olive as those of the magnocellular red nucleus, and a similar, perhaps identical, anatomical substrate may serve to modulate olivary sensitivity by the two descending systems.

Reptilian dorsal column nucleus lacks GAD immunoreactive neurons

Brains of reptiles, Caiman crocodilus, were processed by standard immunocytochemical methodology using a polyclonal antibody to glutamic acid decarboxylase (GAD) as well as several monoclonal antibodies to GAD. No neurons immunoreactive for GAD, GAD(+), were observed in the dorsal column nucleus, although GAD(+) puncta were seen. These findings suggest that in Caiman, the dorsal column nucleus, like the dorsal thalamus, lacks local circuit neurons.