A correlative HRP, Golgi, and EM study of the intrinsic organization of the feline dorsal column nuclei

Transformation of neural coding for vibrotactile stimuli along the ascending somatosensory pathway

In mammals, action potentials fired by rapidly adapting mechanosensitive afferents are known to reliably time lock to the cycles of a vibration. How and where along the ascending neuraxis is the peripheral afferent temporal code transformed into a rate code are currently not clear. Here, we probed the encoding of vibrotactile stimuli with electrophysiological recordings along major stages of the ascending somatosensory pathway in mice. We discovered the main transformation step was identified at the level of the thalamus, and parvalbumin-positive interneurons in thalamic reticular nucleus participate in sharpening frequency selectivity and in disrupting the precise spike timing. When frequency-specific microstimulation was applied within the brainstem, it generated frequency selectivity reminiscent of real vibration responses in the somatosensory cortex and could provide informative and robust signals for learning in behaving mice. Taken together, these findings could guide biomimetic stimulus strategies to activate specific nuclei along the ascending somatosensory pathway for neural prostheses.

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.

Processing afferent proprioceptive information at the main cuneate nucleus of anesthetized cats

Medial lemniscal activity decreases before and during movement, suggesting prethalamic modulation, but the underlying mechanisms are largely unknown. Here we studied the mechanisms underlying proprioceptive transmission at the midventral cuneate nucleus (mvCN) of anesthetized cats using standard extracellular recordings combined with electrical stimulation and microiontophoresis. Dual simultaneous recordings from mvCN and rostroventral cuneate (rvCN) proprioceptive neurons demonstrated that microstimulation through the rvCN recording electrode induced dual effects on mvCN projection cells: potentiation when both neurons had excitatory receptive fields in muscles acting at the same joint, and inhibition when rvCN and mvCN cells had receptive fields located in different joints. GABA and/or glycine consistently abolished mvCN spontaneous and sensory-evoked activity, an effect reversed by bicuculline and strychnine, respectively; and immunohistochemistry data revealed that cells possessing strychnine-sensitive glycine receptors were uniformly distributed throughout the cuneate nucleus. It was also found that proprioceptive mvCN projection cells sent ipsilateral collaterals to the nucleus reticularis gigantocellularis and the mesencephalic locomotor region, and had slower antidromic conduction speeds than cutaneous fibers from the more dorsally located cluster region. The data suggest that (1) the rvCN-mvCM network is functionally related to joints rather than to single muscles producing an overall potentiation of proprioceptive feedback from a moving forelimb joint while inhibiting, through GABAergic and glycinergic interneurons, deep muscular feedback from other forelimb joints; and (2) mvCN projection cells collateralizing to or through the ipsilateral reticular formation allow for bilateral spreading of ascending proprioceptive feedback information.

Inhibition of the Rho/ROCK pathway prevents neuronal degeneration in vitro and in vivo following methylmercury exposure

Methylmercury (MeHg) is an environmental neurotoxicant which induces neuropathological changes in both the central nervous and peripheral sensory nervous systems. Our recent study demonstrated that down-regulation of Ras-related C3 botulinum toxin substrate 1 (Rac1), which is known to promote neuritic extension, preceded MeHg-induced damage in cultured cortical neurons, suggesting that MeHg-mediated axonal degeneration is due to the disturbance of neuritic extension. Therefore we hypothesized that MeHg-induced axonal degeneration might be caused by neuritic extension/retraction incoordination. This idea brought our attention to the Ras homolog gene (Rho)/Rho-associated coiled coil-forming protein kinase (ROCK) pathway because it has been known to be associated with the development of axon and apoptotic neuronal cell death. Here we show that inhibition of the Rho/ROCK pathway prevents MeHg-intoxication both in vitro and in vivo. A Rho inhibitor, C3 toxin, and 2 ROCK inhibitors, Fasudil and Y-27632, significantly protected against MeHg-induced axonal degeneration and apoptotic neuronal cell death in cultured cortical neuronal cells exposed to 100 nM MeHg for 3 days. Furthermore, Fasudil partially prevented the loss of large pale neurons in dorsal root ganglia, axonal degeneration in dorsal spinal root nerves, and vacuolar degeneration in the dorsal columns of the spinal cord in MeHg-intoxicated model rats (20 ppm MeHg in drinking water for 28 days). Hind limb crossing sign, a characteristic MeHg-intoxicated sign, was significantly suppressed in this model. The results suggest that inhibition of the Rho/ROCK pathway rescues MeHg-mediated neuritic extension/retraction incoordination and is effective for the prevention of MeHg-induced axonal degeneration and apoptotic neuronal cell death.

Differences in the Neurons That Project from the Dorsal Column Nuclei to the Diencephalon, Pretectum, and Tectum in the Cat

The dorsal column nuclei (DCN) project to a number of targets in the nervous system besides the ventroposterolateral nucleus (VPL) of the thalamus. Recent evidence obtained using double-labeling techniques indicates that DCN's diencephalic-projecting neurons differ in their location and morphology from those that project to some of its other targets, such as the cerebellum and tectum. The purpose of the present study was to characterize anatomically the DCN neurons that project another of DCN's targets, the pretectum, and to determine if any of these neurons have collateral projections to the tectum or diencephalon. The projections were studied using two double-labeling methods. One method made use of either tritiated inactivated horseradish peroxidase ([3H]apoHRP) or tritiated N-acetyl wheatgerm agglutinin ([3H]WGA) as a marker and HRP or WGA conjugated to HRP. The other method made use of the dyes Fast Blue and Nuclear Yellow. In each cat, one marker was injected into the DCN-recipient portions of the pretectum, tectum, or diencephalon, and the other marker was injected into another of these three targets. Neurons labeled by pretectal or tectal injections were of all sizes, fusiform and multipolar in shape, and similarly located. They were scattered through the rostral zone of DCN, but were distributed at the periphery of and at the junction between the gracile and cuneate nuclei in DCN's middle and caudal zones. In contrast to the pretectal- and tectal-labeled neurons, neurons labeled by diencephalic injections were round and large. They were found throughout the DCN complex, but were concentrated in DCN's middle and caudal zones. When both the pretectum and diencephalon were injected in the same cat, the two groups of neurons occupied similar locations in the rostral zone, but were distinct in the middle and caudal zones, with the pretectal-projecting neurons surrounding the clusters of diencephalic-projecting neurons. Very few neurons were double-labeled. These results demonstrate that the projections to the pretectum, tectum, and diencephalon originate from different populations of neurons within specific domains in DCN. When these results are compared with the results of electrophysiological and other anatomical studies, it appears that the pretectal- and tectal-projecting neurons may be part of a previously unrecognized system originating in DCN.(ABSTRACT TRUNCATED AT 400 WORDS)

Organization of primary afferent projections to the gracile nucleus of the dorsal column system of primates

In order to reveal the somatotopic organization of the gracile nucleus of the dorsal column-trigeminal complex, neuroanatomical tracers were injected subcutaneously into various parts of the hindlimb and tail of prosimian galagos, New World monkeys, and Old World monkeys. In most cases, tracers were injected bilaterally, and into more than one body part. In six cases, two different, distinguishable tracers were injected into the same hindlimb. Brainstem and spinal cord sections were processed for tracers transported by cutaneous afferents to terminations in the gracile nuclei. Foci of terminations were related to the cell-cluster architecture of the gracile nuclei in sections processed for cytochrome oxidase or stained for cell bodies (Nissl stain). In all taxa, terminations labeled by the injections were distributed in a patchy fashion along the rostrocaudal length of the ipsilateral gracile nucleus. Terminations were largely but not completely focused within the cytochrome oxidase dense cell clusters. Across taxa, afferents from the tail, foot, lower leg, and upper leg terminated in a mediolateral sequence within the gracile nucleus. Afferents from the glabrous skin of toes 1-5 terminated in a ventromedial to dorsolateral sequence in owl, squirrel, and macaque monkeys, but an altered arrangement was seen in the galagos, with a ventrolateral location for toe 1. The use of two tracers in squirrel monkeys indicated that terminations from adjacent toes formed adjacent and largely segregated patches. Terminations of afferents from the plantar pad (sole) of the foot tended to surround those from the glabrous toes.

Quantitative stereological evaluation of the gracile and cuneate nuclei and their projection neurons in the rat

Stereological methods were employed to estimate the volume and neuron numbers of the rat dorsal column nuclei (DCN). These methods were applied to Nissl-stained sections from control animals and cases that received injections of horseradish peroxidase in the thalamus, the cerebellum, or the spinal cord. Additional cases received combinations of fluorescent tracers in the same structures, to examine whether some of the retrogradely labeled neurons sent collaterals to different targets. The mean volume of the DCN is 0.81 mm(3) (range 0.65-1.10 mm(3)), of which 3%, 39%, and 59% correspond, respectively, to the nucleus of Bischoff (Bi), the gracile (Gr), and the cuneate (Cu) nuclei. Within Cu, the middle division (CuM) is the largest (42%), followed by the rostral (CuR; 36%) and caudal (CuC; 22%) divisions. The mean total number of neurons in the DCN is 16,000 (range 12,400-19,500), of which 2.4%, 34.0% and 63.6% correspond, respectively, to Bi, Gr, and Cu. Within Cu, CuM contains 48% of all neurons, and 27% correspond to CuR and 25% to CuC. Interanimal variability is moderate for the whole DCN and Cu but increases when individual nuclei are considered. About 80% of DCN neurons project to the thalamus, 3% to the spinal cord, and 7% to the cerebellum. Thalamic-projecting cells are more numerous in CuM and Gr (83%), and relatively less common in Bi and CuC (72-74%). Most of the DCN neurons projecting to the spinal cord appear in CuC and CuM. Two-thirds of the neurons projecting to the cerebellum are located in CuR, 20% in CuM, and 15% in Gr. A small fraction of neurons projects simultaneously to spinal cord and thalamus.

Transmission security for single kinesthetic afferent fibers of joint origin and their target cuneate neurons in the cat

Transmission between single identified, kinesthetic afferent fibers of joint origin and their central target neurons of the cuneate nucleus was examined in anesthetized cats by means of paired electrophysiological recording. Fifty-three wrist joint afferent-cuneate neuron pairs were isolated in which the single joint afferent fiber exerted suprathreshold excitatory actions on the target cuneate neuron. For each pair, the minimum kinesthetic input, a single spike, was sufficient to generate cuneate spike output, often amplified as a pair or burst of spikes, particularly at input rates up to 50-100 impulses per second. The high security was confirmed quantitatively by construction of stimulus-response relationships and calculation of transmission security measures in response to both static and dynamic vibrokinesthetic disturbances applied to the joint capsule. Graded stimulus-response relationships demonstrated that the output for this synaptic connection between single joint afferents and cuneate neurons could provide a sensitive indicator of the strength of joint capsule stimuli. The transmission security measures, calculated as the proportion of joint afferent spikes that generated cuneate spike output, were high (>85-90%) even at afferent fiber discharge rates up to 100-200 impulses per second. Furthermore, tight phase locking in the cuneate responses to vibratory stimulation of the joint capsule demonstrated that the synaptic linkage preserved, with a high level of fidelity, the temporal information about dynamic kinesthetic perturbations that affected the joint. The present study establishes that single kinesthetic afferents of joint origin display a capacity similar to that of tactile afferent fibers for exerting potent synaptic actions on central target neurons of the major ascending kinesthetic sensory pathway.

The distribution and characterization of NADPH-d/NOS-IR neurons in the rat cuneate nucleus

Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry and nitric oxide synthase (NOS) immunohistochemistry have been used to characterize the nitric oxide (NO)-containing neurons in the rat cuneate nucleus. The present results showed that NADPH-d-positive/NOS-immunoreactive (-IR) neurons were distributed in the entire rostrocaudal extent of the nucleus. In the caudal region (approximately 1-2 mm caudal to the obex), NADPH-d/NOS-IR neurons were aggregated along the dorsal area of the nucleus notably in the lateral aspect. When traced rostrally, labeled neurons were progressively reduced and the cells were randomly distributed. The labeled neurons varied from round, ovoid to spindle-shaped with a mean profile area of about 140.1+/-1.7 microm(2) (n=720). They made up 7-10% of the neuronal population in the cuneate nucleus. By immunoelectron microscopy, the immunoreaction product was deposited throughout the cytoplasm extending from the soma to the proximal and distal dendrites. Results of NADPH-d staining paralleled that of NOS immunohistochemistry. Furthermore, NADPH-d reactivity and NOS-IR were colocalized in the same neurons following double labeling. Using NADPH-d histochemistry along with anti-gamma-aminobutyric acid (GABA) and -glycine postembedding immunolabeling for identification of GABA- and glycine-IR neurons, respectively, about 33% of the NADPH-d-positive neurons contained both GABA and glycine, 26% of them contained only glycine, while 41% of them showed neither GABA nor glycine labeling. Cuneothalamic neurons (CTNs) were identified by injecting the retrograde tracer Fluorogold (FG) into the ventrobasal complex of the thalamus. Numerous FG-labeled neurons were present in the contralateral cuneate nucleus, but none were reactive for NADPH-d. The present results suggest that approximately 60% of the NADPH-d/NOS-IR neurons in the cuneate nucleus are interneurons containing GABA and/or glycine.

Rhythmic neuronal interactions and synchronization in the rat dorsal column nuclei

Single-unit and multiunit activities were recorded from dorsal column nuclei of anesthetized rats in order to study the characteristics of the oscillatory activity expressed by these cells and their neuronal interactions. On the basis of their firing rate characteristics in spontaneous conditions, two types of dorsal column nuclei cell have been identified. Low-frequency cells (74%) were silent or displayed a low firing rate (1.9+/-0.48 spikes/s), and were identified as thalamic-projecting neurons because they were activated antidromically by medial lemniscus stimulation. High-frequency cells (26%) were characterized by higher discharge rates (27.2+/-5.1 spikes/s). None of them was antidromically activated by medial lemniscus stimulation. Low-frequency neurons showed a non-rhythmic discharge pattern spontaneously which became rhythmic under sensory stimulation of their receptive fields (48% of cases; 4.8+/-0.23Hz). All high-frequency neurons showed a rhythmic discharge pattern at 13.8+/- 0.68Hz either spontaneously or during sensory stimulation of their receptive fields. The shift predictor analysis indicated that oscillatory activity is not phase-locked to the stimulus onset in either type of cell, although the stimulus can reset the phase of the rhythmic activity of high-frequency cells. Cross-correlograms between pairs of low-frequency neurons typically revealed synchronized rhythmic activity when the overlapping receptive fields were stimulated. Rhythmic synchronization of high-frequency discharges was rarely observed spontaneously or under sensory stimulation. High-frequency neuronal firing could be correlated with the low-frequency neuronal activity or more often with the multiunit activity during sensory stimulation. Moreover, the presence of oscillatory activity modulated the sensory responses of dorsal column nuclei cells, favoring their responses. These findings indicate that thalamic-projecting and non-projecting neurons in dorsal column nuclei exhibited distinct oscillatory characteristics. However, both types of neuron may be entrained into an oscillatory rhythmic pattern when their overlapping receptive fields are stimulated, suggesting that in those conditions the dorsal column nuclei generate a populational oscillatory output to the somatosensory thalamus which could modulate and amplify the effectiveness of the somatosensory transmission.

Somatostatin-IR neurons are a major subpopulation of the cuneothalamic neurons in the rat cuneate nucleus

This study was aimed to localize and characterize the somatostatin-immunoreactive (SOM-IR) neurons in the rat cuneate nucleus (CN). By immuno-histochemistry, the SOM-IR neurons, which were widely distributed in the nucleus, were round, spindle or multiangular in shape (mean area = 226.1 +/ -3.1 microm(2), n = 1016). By electron microscopy, the neurons shared all the ultrastructural features of the cuneothalamic neurons (CTNs) which showed a slightly indented nucleus and a fairly rich cytoplasm containing well-developed Golgi apparatuses and rough endoplasmic reticulum (rER). The SOM immunoreaction product filled the cytoplasm of the neurons extending from the soma to the proximal and distal dendrites, which were postsynaptic to unlabeled boutons. In addition to soma and dendrites, SOM-IR boutons were also identified which made axodendritic synaptic contacts with SOM-IR dendrites. The SOM-IR neurons were characterized by using anti-SOM pre-embedding immunolabeling coupled with horseradish peroxidase (HRP) retrograde method, or SOM immunolabeling along with anti-glutamate, gamma-aminobutyric acid (GABA) or glycine post-embedding immunolabeling for identification of CTNs, glutamate-IR, GABA-IR and glycine-IR neurons, respectively. It was shown that more then 80% of the CTNs contained SOM and, furthermore, they contained glutamate but not GABA or glycine. On the basis of present findings, it is suggested the majority of the SOM-IR neurons in the rat CN are CTNs and that they may be involved in modulation of somatosensory synaptic transmission.

Cuneothalamic relay neurons are postsynaptic to glycine-immunoreactive terminals in the rat cuneate nucleus

This study was aimed to clarify whether the cuneothalamic relay neurons (CTNs) in the rat cuneate nucleus contained glycine or whether the neurons were modulated directly by presynaptic glycine-IR terminals. For this purpose, retrograde transport of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) and immunoperoxidase labelling for glycine have been used to ascertain if the CTNs in the rat are glycine-immunoreactive (glycine-IR). Our results have shown that the WGA-HRP-labelled CTNs (mean area = 318 +/- 6.5 microm(2)) were not reactive for glycine. Glycine immunoreactivity, however, was localized in some small-sized neurons (mean area = 210 +/- 6.2 microm(2)) and axon terminals associated with the CTNs. The synaptic organization between the glycine-IR terminals and CTNs was further analyzed using anti-glycine postembedding immunogold labelling. By electron microscopy, the immunogold-labelled glycine-IR terminals containing pleomorphic synaptic vesicles formed symmetrical synaptic contacts with the dendrites, dendritic spines, and somata of CTNs. Quantitative estimation showed that the mean ratios of glycine-IR terminals to total terminals associated with the soma, proximal dendrites and distal dendrites of the CTN were 49.5, 45.2, and 45.8%, respectively. The higher incidence of glycine-IR terminals on the soma, however, was not significantly different from that of the proximal and distal dendrites. Notwithstanding the above, this study has shown a large number of glycine-IR terminals making direct synaptic contacts with CTNs, suggesting that glycine is one of the important neurotransmitters involved in postsynaptic inhibition on the cuneothalamic relay neurons to modulate incoming somatosensory information from forelimb areas in the rat.

In vitro electrophysiological properties of rat dorsal column nuclei neurons

The dorsal column nuclei include the gracile and cuneate nuclei, which receive somatosensory information from the periphery and project to the ventroposterior nucleus of the contralateral thalamus. The aim of this study was to determine the electrophysiological and morphological characteristics of the neurons of the dorsal column nuclei and to identify synaptic events evoked by electrical stimulation of the dorsal column, using an in vitro slice preparation. The results show two types of neurons, termed type I and II. A repolarizing sag distinguished type I cells during hyperpolarizing current injection, suggesting the activation of a Q-current. Moreover, type I cells, but not type II cells, were capable of maintaining spontaneous rhythmic activity at 9-15 Hz. Both types of cells displayed a delay in their return to the resting membrane potential following hyperpolarizing current pulses, indicating the existence of an A-current. Electrical stimuli applied to the dorsal column elicited brief EPSPs and IPSPs in both cell types. EPSPs were abolished by 6-cyano-7-nitroquinoxaline-2,3-dione, indicating that they were mediated through non-NMDA receptors. IPSPs were blocked by picrotoxin, implying the activation of GABAA receptors. Intracellular staining with carboxyfluoresceine revealed that type I neurons had elongated somas and primary dendrites that extended radially. Type II cells were smaller and had round somas with few primary dendrites, most of them emerging from one pole of the soma. The axon of many type I neurons was stained and could be followed running ventrally and in rostral direction.

Reorganization of the raccoon cuneate nucleus after peripheral denervation

The effects of peripheral nerve transection on the cuneate nucleus were studied in anesthetized raccoons using extracellular, single-unit recordings. The somatotopic organization of the cuneate nucleus first was examined in intact, control animals. The cuneate nucleus in the raccoon is organized with the digits represented in separate cell clusters. The dorsal cap region of the cuneate nucleus contains a representation of the claws and hairy skin of the digits. Within the representation of the glabrous skin, neurons with rapidly adapting properties tended to be segregated from those with slowly adapting properties. The representations of the distal and proximal pads on a digit also were segregated. Electrical stimulation of two adjacent digits provided a detailed description of the responses originating from the digit that contains the tactile receptive field (the on-focus digit) and from the adjacent (off-focus) digit. Stimulation of the on-focus digit produced a short latency excitation in all 99 neurons tested, with a mean of 10.5 ms. These responses had a low threshold (426 microA). Stimulation of an off-focus digit activated 65% of these neurons. These responses had a significantly longer latency (15.3 ms) than on-focus responses and the threshold was more than twice as large. Two to five months after amputation of digit 4, 97 cells were tested with stimulation of digits 3 and 5. A total of 44 were in the intact regions of the cuneate nucleus. They had small receptive fields on intact digits and their responses to electrical stimulation did not differ from the control neurons. The remaining 53 neurons were judged to be deafferented and in the fourth digit region on the basis of their location with respect to intact neurons. All but two of these cells had receptive fields that were much larger than normal, often including more than one digit and part of the palm. When compared with the off-focus control neurons, their responses to electrical stimulation had lower thresholds and an increased response probability and magnitude. The latencies of these cells did not decrease, however, and were the same as the off-focus control values. The enhanced responses of the deafferented neurons to adjacent digit stimulation indicate that there is a strengthening of synapses that were previously ineffective. The increased proportion of neurons that could be activated after amputation suggests that there is also a growth of new connections. This experiment demonstrates that reorganization in the adult somatotopic system does occur at the level of the dorsal column nuclei. As a consequence, many of the changes reported at the cortex and thalamus may be due to the changes occurring at this first synapse in the somatosensory pathway.

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.

Primary motor cortex influences on the descending and ascending systems

The motor cortex plays a crucial role in the co-ordination of movement and posture. This is possible because the pyramidal tract fibres have access both directly and through collateral branches to structures governing eye, head, neck trunk and limb musculature. Pyramidal tract axons also directly reach the dorsal laminae of the spinal cord and the dorsal column nuclei, thus aiding in the selection of the sensory ascendant transmission. No other neurones in the brain besides pyramidal tract cells have such a wide access to different structures within the central nervous system. The majority of the pyramidal tract fibres that originate in the motor cortex and that send collateral branches to multiple supraspinal structures do not reach the spinal cord. Also, the great majority of the corticospinal neurones that emit multiple intracraneal collateral branches terminate at the cervical spinal cord level. The pyramidal tract fibres directed to the dorsal column nuclei that send collateral branches to supraspinal structures also show a clear tendency to terminate at supraspinal and cervical cord levels. These facts suggest that a substantial co-ordination between descending and ascending pathways might be produced by the same motor cortex axons at both supraspinal and cervical spinal cord sites. This may imply that the motor cortex co-ordination will be mostly directed to motor responses involving eye-neck-forelimb muscle synergies. The review makes special emphasis in the available evidence pointing to the role of the motor cortex in co-ordinating the activities of both descending and ascending pathways related to somatomotor integration and control. The motor cortex may function to co-operatively select a unique motor command by selectively filter sensory information and by co-ordinating the activities of the descending systems related to the control of distal and proximal muscles.

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.

Synaptophysin immunoreactivity and distributions of calcium-binding proteins highlight the functional organization of the rat's dorsal column nuclei

The mammalian dorsal column nuclei (DCN) are principally composed of the cuneate (CN) and gracile (GN) nuclei. Data presented here support previously published anatomical and functional evidence that the longitudinal organization of the CN and GN reflect the complex role of the DCN in somatosensory processing. The CN is organized longitudinally into three parts. Within the middle portion of this nucleus, primary afferent projections and cuneothalamic cells are concentrated. Although traditional cytoarchitectonic analyses had failed to detect this tripartite organization in rats, we found evidence for it, with a functional middle region, extending approximately 0.2-0.9 mm caudal to the obex, characterized by precise somatotopy of primary afferent terminations and corresponding somatotopy of cytochrome oxidase (CO) blotches. Additional evidence is presented here consistent with a functionally distinct middle region within the rat's CN: (1) patches of dense synaptophysin (a synaptic-vesical-associated protein)-immunoreactivity (SYN-IR) are limited to the middle CN region, coincident with the dense CO blotches; (2) neurons immunoreactive for the calcium-binding proteins calbindin-D28 (CB), calretinin (CR) and parvalbumin (PV) are concentrated in the middle CN region. Furthermore, in adult rats subjected to perinatal forepaw removal, (1) the patterns of SYN-IR in the middle region of the CN are disrupted, as had previously been shown for the patterns of CO blotches; (2) in contrast, however, distributions of CN cells with PV-, CB- and CR-IR are unaffected. Evidence for a tripartite division in the GN is also presented, based on the distributions of cells with PV-, CB- and CR-IR.

Effects of target deprivation on the morphology and survival of adult dorsal column nuclei neurons

During development, interaction with target cells plays a critical role in the regulation of survival of afferent neurons. In an attempt to define the role of target cells in the adult central nervous system, the somatodendritic morphology and survival of adult cuneate neurons deprived of their targets by in situ injection of kainic acid in the rat thalamus were studied. In neuron-specific, enolase-immunostained sections, a 20% decrease in the mean longest diameter of the labeled cells was detected at 4 months postlesion. This somatic atrophy was accompanied by a loss of distal dendritic arborizations as observed after labeling by intracellular diffusion of horseradish peroxidase. Cytochrome oxidase staining did not reveal detectable alterations of the metabolic activity of these neurons, and an ultrastructural study also failed to demonstrate major changes in the neuronal somata. Cell counts indicated a much delayed death of 25% of the neurons at 10 months postlesion, whereas no neuronal death was detected at 7 months. The glial cells appeared unaltered both in number and in immunolabeling when using OX-42 antibodies or antiglial fibrillary acidic protein (anti-GFAP) antibodies. Results obtained in this time-course study indicate that neuronal death and alteration of the somatodendritic morphology are much delayed events after excitotoxic loss of targets. Somatodendritic atrophy occurs several months postlesion, and neuronal death occurs close to 1 year after lesion. These results suggest that the hypothesis of a necessary continuous trophic support by target cells does not hold as firmly for the adult central nervous system as during development.

Synaptic organization of the interstitial subdivision of the nucleus tractus solitarii and of its laryngeal afferents in the rat

The nucleus tractus solitarii, the first central relay for gustatory and a variety of visceral afferents, is also an integrative center for numerous functions. Its interstitial subdivision is involved in swallowing and respiratory reflexes. The ultrastructural characteristics of this subdivision and of its laryngeal afferents were investigated in adult rat by a serial-section study and by application of wheat germ agglutinin-horseradish peroxidase conjugate to the peripheral afferent fibers. The interstitial subnucleus contained scattered small neuronal cell bodies with such ultrastructural features as a large nucleus with deep indentations and an organelle-poor cytoplasm. On the basis of their size and vesicular content, the axon terminals were classified into three categories. Group I and group II terminals were small or large, respectively, and contained mainly small, round, and clear synaptic vesicles. Group III terminals were also small but contained small, pleomorphic, and clear vesicles. Axodendritic synapses were the most numerous. They were either asymmetrical, comprised of group I and II terminals, or symmetrical, comprised of group III terminals. More than 50% were part of complex synaptic arrangements in the form of rosettes or glomeruli. Axosomatic contacts involved both group I and group III terminals and were always symmetrical. A high frequency of axoaxonic synapses was found. They were symmetrical, comprised of group III terminals on group I or II terminals. Different types of symmetrical synaptic contacts made by dendrites were also found. This study indicates also that the ipsilateral interstitial subdivision constitutes the preferential site of termination for superior laryngeal afferents. The labeled axon terminals belonged exclusively to groups I and II and were involved in both axodendritic and axoaxonic synapses. Some of the axodendritic synapses were part of rosettes or glomeruli. All these synaptic arrangements may be considered a morphological substrate for important processing of afferent information in the nucleus tractus solitarii. They may account for some of the integrative functions of the interstitial subnucleus such as physiological processes triggered from the superior laryngeal nerve.

Colocalization of neuronal nitric oxide synthase with GABA in rat cuneate nucleus

Pre- and post-embedding immunocytochemistry were employed in this electron microscopic investigation of cuneate neurons that are enriched in GABA and in nitric oxide synthase, the enzyme responsible for the synthesis of nitric oxide. GABAergic neurons are local circuit interneurons; 10-20% of them also contain nitric oxide synthase. These are among the smallest GABA-positive perikarya. We describe a network of processes in the rat cuneate nucleus that are immunopositive for nitric oxide synthase. Axon terminals positive for nitric oxide synthase are small and make synapses mainly onto dendrites; they make only occasional axo-axonic contacts. Double-labelling immunocytochemistry verified that the large majority of terminals positive for nitric oxide synthase also contained GABA. However, most GABA-positive profiles were negative for nitric oxide synthase and GABA-positive terminals that are negative for nitric oxide synthase frequently made axo-axonic contacts. These results suggest that nitric oxide synthase is within a specialized subpopulation of interneurons in the cuneate nucleus.

Ultrastructural and immunocytochemical characterization of primary afferent terminals in the rat cuneate nucleus

The cuneate nucleus is a relay center for somatosensory information by receiving tactile and proprioceptive inputs from primary afferent fibers that ascend in the dorsal funiculus. The morphology, synaptic contacts, and neurochemical content of primary afferent terminals in the cuneate nucleus of rats were investigated by combining anterograde transport of horseradish peroxidase conjugated to wheat-germ agglutinin or to cholera toxin (injected in cervical dorsal root ganglia) with postembedding immunogold labeling for glutamate and GABA. Both tracers gave similar results. Two types of terminals were labeled: type I terminals were irregularly shaped, had a mean area of 4.0 microns 2, synapsed on several dendrites, and were contacted by other terminals, some of which were GABA positive. Type II terminals were dome-shaped, had a mean area of 2.18 microns 2, and made synaptic contact on a single dendrite. All the anterogradely labeled terminals (interpreted as endings of primary afferents) were enriched in glutamate but not in GABA. The finding that identified primary afferent terminals are enriched in glutamate with respect to other tissue profiles strongly suggests a neurotransmitter role for glutamate in this afferent pathway to the rat cuneate nucleus.

Presynaptic fibres of spiral neurons and reciprocal synapses in the organ of Corti in culture

Isolated segments of the newborn mouse organ of Corti were explanted together with the spiral ganglion components. Within the innervation provided by the spiral neurons, we observed presynaptic vesiculated nerve endings that form reciprocal ribbon-afferent/efferent synapses with inner hair cells. These intracochlear presynaptic fibres are characteristically located between adjoining inner hair cells, on the modiolar side, low and close to the supporting cells. The presynaptic fibres display different modes of synaptic connectivity, forming repetitive reciprocal synapses on single inner hair cells or on adjoining hair cells, or connecting adjoining inner hair cells through simultaneous efferent synapses. Many presynaptic fibres exhibit a distinctive ultrastructure: defined clusters of synaptic vesicles, dense core vesicles, coated vesicles, and mitochondria. These organelles occur focally at the synaptic sites; beyond the efferent synaptic specializations, the endings appear quite nondescript and afferent-like. We believe that the reciprocal synapses, although observed in cultures of the organ of Corti, represent real intracochlear synaptic arrangements providing a feedback mechanism between the primary sensory receptors and a special class of spiral ganglion cells that have yet to be recognized in the organ in situ.

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.

Enhanced cytochrome-oxidase staining of the cuneate nucleus in the rat reveals a modifiable somatotopic map

Existing cytochrome oxidase (CO)-staining techniques were modified to enhance sensitivity and contrast in order to examine patterns of CO-activity in the dorsal column nuclei (DCN) of adult Long-Evans rats. Within a rostrocaudally limited region in the middle of the cuneate nucleus (CN) distinctive blotches of intense CO-activity were observed. The CO-staining was maximally differentiated approximately 0.3-0.7 mm caudal to the obex. No CO-blotches were observed anywhere else in the DCN. Transganglionic labelling (WGA-HRP) demonstrated that some of the CO-blotches in the rat CN are related to the terminal projection fields of primary afferents from the skin of the forepaws. The corresponding location of primary afferent termination fields and CO-staining patterns supports a tripartite rostrocaudal division in the rat CN, similar to that described by other investigators in cats, monkeys and raccoons. Comparing the patterns of CO-staining to (1) the cytoarchitecture (Nissl-stained sections), or to (2) the dendritoarchitecture (distribution of microtubule-associated protein 2 (MAP2) or to (3) the organization of retrogradely labelled (WGA-HRP/HRP) cuneothalamic cells, revealed no topographical organization corresponding to the CO-blotches. Postnatal (at least up to 11 days postpartum) forepaw deafferentation or removal disrupted the CO-staining pattern in the CN.

The cuneate nucleus in the rat does have an anatomically distinct middle region

Recently obtained anatomical evidence supports the division of the rat cuneate nucleus (CN) into three rostrocaudal regions, with the middle region receiving a disproportionately greater share of the primary sensory input. The CN in the rat conforms to the basic rostrocaudal CN pattern described in other mammals, including cat, monkey and raccoon.

Somatotopic organization of the dorsal column nuclei in the rat: transganglionic labelling with B-HRP and WGA-HRP

To analyze the patterns of cutaneous primary afferent fibers projecting to the dorsal column nuclei in the rat, horseradish peroxidase (HRP)-based tracers were injected intracutaneously into a number of discrete regions of the forelimbs and hindlimbs. Three-4 days following the HRP injections, the rats were perfused transcardially; 60 microns transverse sections were cut, and the HRP was reacted using the tetramethyl benzidine method. Comparisons were made of projections following injections with choleragenoid-conjugated horseradish peroxidase (B-HRP) or with wheat-germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). B-HRP and WGA-HRP produced similar patterns of labelling, but B-HRP produced greater intensity of labelling and slightly larger projection areas. In the cuneate nucleus (CN), HRP labelling of primary afferents from small, delimited regions, e.g., from a portion of the skin of a single digit, appeared to be precisely restricted in rostrocaudally oriented columns, with little or no overlap (in the mediolateral and dorsoventral plane) into adjacent regions. With respect to rostrocaudal organization, a region in the CN containing a dense population of cutaneous primary afferent fibers appeared to be similar to the middle, or cluster, region in cats and in raccoons and the pars rotunda in primates. Projection patterns were very consistent from rat to rat, but their somatotopic organization differed from that suggested by electrophysiological studies: cutaneous afferents from forelimb digit 1 projected near the ventral border of the CN; those from digit 5 projected dorsomedially to those from digit 1; the projections from the remaining digits formed a crescent between the projections from digits 1 and 5. In the gracile nucleus, the organization of cutaneous afferent projections from hindlimb digits was more variable and complex than that found in the CN.

Ultrastructural organization of the interstitial subnucleus of the nucleus of the tractus solitarius in the cat: identification of vagal afferents

This electron microscopic study, based on serial section analysis, describes the synaptic organization of the interstitial subnucleus of the nucleus of the solitary tract and identifies the terminals of the vagal primary afferents utilizing degeneration and HRP transport. The interstitial subnucleus contains sparsely scattered cell bodies, numerous dendrites and axon terminals, and bundles of unmyelinated and myelinated axons. The cell bodies which are small in diameter have an organelle poor cytoplasm and a large invaginated nucleus. Axon terminals can be classified into two main types according to their vesicular shape. The first type contains clear, round vesicles and can be further subdivided into two subgroups on the basis of their morphology and the size of their vesicles. In the first subgroup the terminals are small, contain a few mitochondria and their vesicles are densely packed with an homogeneous size. In the second subgroup the terminals which vary from small to large, contain many mitochondria and contain round vesicles which are heterogeneous in size. The second main terminal type consists of axon terminals containing pleomorphic vesicles which are associated with asymmetrical or symmetrical synaptic contacts on dendrites. Axo-axonic contacts are present in the interstitial subnucleus. In general, the presynaptic axon terminals contain pleomorphic vesicles and the postsynaptic elements contain round vesicles of varying size. In some dendrites, identified by the presence of ribosomes, groups of round and/or pleomorphic vesicles are found associated with synaptic contacts. These dendrites are presynaptic to conventional dendrites and postsynaptic to axon terminals. After removal of the nodose ganglion, degenerative alterations are seen only at the caudal and middle levels of the interstitial subnucleus. Degeneration occurs in a few myelinated axons and in axon terminals which usually contain a mixture of small and larger round, clear vesicles. After HRP injection into the vagus nerve, the HRP reaction product is visible in axon terminals filled with clear, round vesicles which are heterogeneous in size. The labelled axon terminals establish single or multiple synaptic contacts. This study demonstrates that terminals of vagal primary afferents consist principally of terminals of the second subgroup. The morphology of these terminals are compared to primary afferents in the brainstem and spinal cord.

Gracile nucleus of streptozotocin-induced diabetic rats

This study reports ultrastructural changes in the gracile nucleus of male Wistar rats after streptozotocin-induced diabetes. During the acute phase (3-7 days) degenerating electron-dense dendrites and axon terminals were dispersed in the neuropil. Degenerating dendrites were characterized by an electron-dense cytoplasm, swollen mitochondria, dilated endoplasmic reticulum and scattered ribosomes. Degenerating axon terminals were characterized by an electron-dense cytoplasm and clustering of small spherical agranular vesicles. Degenerating axon terminals may form part of a synaptic glomerulus with a central electron-dense dendrite, or they may form the central element of a synaptic glomerulus. These degenerating profiles were absent in the gracile nucleus of the 3 and 7 days insulin-treated post-streptozotocin rats. Macrophages were present in the neuropil and were in the process of engulfing neuronal elements. During the medium phase (1-6 months), most of the degenerating dendrites and axon terminals had been engulfed or removed by macrophages. During the late phase (9-12 months) a second wave of degeneration occurred in the gracile nucleus, similar to the acute phase. During the medium and late phases, dystrophic axonal profiles were also significantly increased in the rats after streptozotocin treatment. It is concluded that the ultrastructural changes observed in the gracile nucleus in the present study were the result of streptozotocin-induced diabetes rather than a toxic effect of streptozotocin, even in the acute phase.

Light microscopic and ultrastructural analysis of GABA-immunoreactive profiles in the monkey spinal cord

It is hypothesized that terminals containing gamma-aminobutyric acid (GABA) participate in presynaptic inhibition of primary afferents. To date, few convincing GABA-immunoreactive (GABA-IR) axo-axonic synapses have been demonstrated in support of this theory. The goal of this study is to document the relationship between GABA-IR profiles and central terminals in glomerular complexes in lumbar cord of the monkey (Macaca fascicularis). In addition, the relationship between GABA-IR profiles and other neural elements are analyzed in order to better understand the processing of sensory input in the spinal cord. GABA-IR cell bodies were present in Lissauer's tract (LT) and in all laminae in the spinal gray matter except lamina IX. GABA-IR fibers and terminals were heavily concentrated in LT; laminae I, II, and III; and present in moderate concentration in the deeper laminae of the dorsal horn, ventral horn (especially in association with presumed motor neurons), and lamina X. Electron microscopic analysis confined to LT and laminae I, II, and III demonstrated GABA-IR cell bodies, dendrites, and myelinated and unmyelinated fibers. GABA-IR cell bodies received sparse synaptic input, some of which was immunoreactive for GABA. The majority of the synaptic input to GABA-IR neurons occurred at the dendritic level. Furthermore, the presence of numerous vesicle-containing GABA-IR dendrites making synaptic interactions indicated that GABA-IR dendrites also provided a major site of output. Two consistent arrangements were observed in laminae I-III concerning vesicle-containing GABA-IR dendrites: 1) they were often postsynaptic to central terminals and 2) they participated in reciprocal synapses. The majority of GABA-IR axon terminals observed contained round clear vesicles and varying numbers of dense core vesicles. Only on rare occasions were GABA-IR terminals with flattened vesicles observed. GABA-IR terminals were not observed as presynaptic elements in axo-axonic synapses; however, on some occasions, GABA-IR profiles presumed to be axon terminals were observed postsynaptic to large glomerular type terminals. Our findings suggest that a frequent synaptic arrangement exists in which primary afferent terminals relay sensory information into a GABAergic system for further processing. Furthermore, GABA-IR dendrites appear to be the major source of input and output for this inhibitory system. The implications of this GABAergic neurocircuitry are discussed in relation to the processing of sensory input in the superficial dorsal horn and in terms of mechanisms of primary afferent depolarization (PAD).

A quantitative study of the projections of the gracile, cuneate and trigeminal nuclei and of the medullary reticular formation to the thalamus in the rat

Following injection of horseradish peroxidase into the thalamus of one side, the numbers of labelled neurons in the nuclei of the dorsal funiculi and in the trigeminal sensory complex were counted. A comparative study was made of the pattern of labelling after a range of survival times, and animals surviving for 72 h after injection were used to provide detailed quantitative information about the patterns of distribution of labelled cells. The principal sensory nucleus of the trigeminal nerve (8683 labelled neurons) and the nucleus of the spinal trigeminal tract, pars interpolaris (1920) label heavily after thalamic injection. Pars oralis of the spinal nucleus labels more sparsely (524 labelled neurons), while the pars caudalis (260 labelled neurons) shows a laminar labelling pattern which continues across the spinomedullary junction into the upper cervical segments. The gracile (2152 labelled neurons) and cuneate (2339) nuclei also show rostrocaudal variation in labelling density: the middle one-third of each nucleus contains 66% of labelled gracile and cuneate cells. The findings are correlated with known features of the arrangement of the ascending sensory projections from these nuclei in various species, and are compared with previous findings on the distribution of thalamically-projecting cells in the upper cervical segments of the spinal cord.

Neurochemical transmission in the dorsal column nuclei

The transmitter chemistry of the dorsal column nuclei is reviewed, with special emphasis on the monosynaptic component of the dorsal column-medial lemniscal pathway. It is maintained that in this anatomically addressed system concerned mainly with fast, secure sensory transmission, amino acids represent the predominant mechanism used for chemical relay of primary afferent impulses. The major excitatory primary afferent transmitter is most likely glutamic acid, whereas gamma-aminobutyric acid (GABA) fulfills adequately the role of transmitter of recurrent, postsynaptic and presynaptic inhibition. Recent immunohistochemical and physiological evidence indicates that 5-hydroxytryptamine, originating mainly from neurons of the raphé nuclei, plays a modulatory role in dorsal column transmission of innocuous sensory information. The basic synaptic elements involved in transmission across this relay, along with their corresponding chemical identities, are presented in the form of a speculative model.

Distribution of cells projecting to thalamus vs. those projecting to cerebellum in subdivisions of the dorsal column nuclei in raccoons

To learn the distribution of cells projecting to the thalamus, as opposed to the cerebellum, in the mechanosensory nuclei of the dorsal medulla of raccoons, we analyzed the retrograde transport of horseradish peroxidase from the ventrobasal complex of the thalamus and from the cerebellum. We found six nuclear regions projecting heavily to the thalamus with very small projections to the cerebellum: Bischoff's, central cuneate, central gracile, rostral cuneate, rostral gracile nuclei, and cell group z. Two regions showed heavy projections to the cerebellum with no projections to the thalamus: the lateral portion of the external cuneate nucleus and the compact portion of cell group x. Four regions showed more equivalent projections to both target regions: basal cuneate, medial portion of the external cuneate nucleus, medial tongue extension of the external cuneate nucleus, and reticular portion of cell group x. Three more ventral regions were labeled: lateral cervical nucleus from thalamic injections but not from cerebellar injections; central cervical nucleus from cerebellar injections, which crossed the midline, but not from thalamic injections; and lateral reticular nucleus from both target regions. In most medullary regions, most cells project to one target and very few project to the other; we suggest that the cells projecting to the minor target convey samples of the information going to the major target.

Peripheral and spinal inputs to physiologically identified thalamic and nonthalamic relay neurons in cat cuneate nucleus

A single-unit population study of the feline cuneate nucleus was carried out to identify principal neuron types, their distribution within the nucleus, pattern of peripheral activation, and receptive field characteristics. Units were also tested for response to isolated dorsal column or dorsolateral funicular electrical stimulation. The nucleus was explored in a uniform pattern, and sample size was optimized by applying the search stimulus shocks to the dorsal spinal cord. Single units were defined as spinal afferents, cuneothalamic-relay (CTR) neurons, and non-cuneothalamic-relay (non-CTR) neurons. The following features were observed: The distribution within the nucleus of specific cell types agreed with cytoarchitectural studies: Spinal afferent fibers were superficial and caudal; 22% of neurons were CTR neurons; CTR neurons were most dense in the middle of the nucleus and were largely separate from non-CTR neurons. Of the 58 neurons tested for response to isolated dorsal column and dorsolateral funicular stimulation, 24% were activated from both tracts. Convergent input from the off-focus periphery (defined as other than the ipsilateral forelimb) was detected in both CTR and non-CTR neurons, most commonly from the contralateral forepaw. Several neurons were activated from three limbs. Thirty-seven percent of units were unresponsive to hair movement, touch, muscle palpation, or movement of joints. Compared to spinal fibers and non-CTR neurons, CTR neurons were most likely to have an identifiable input.

GABAergic neurons are present in the dorsal column nuclei but not in the ventroposterior complex of rats

Neurons containing glutamatic acid decarboxylase (GAD) are known to exist in the spinal dorsal horn, dorsal column nuclei (DCN), n. ventralis posterior (VP), and somatosensory cortex of cats. Recent work suggested that species differences exist concerning the presence and/or density of GAD-positive neurons in VP. The present experiments demonstrate that, in contrast with carnivores and primates, the rat's VP contains virtually no GAD-positive neurons and that virtually all neurons in it project to the cortex. This conclusion is supported by the failure to find, in Golgi-impregnated material, neurons with characteristics commonly attributed to Golgi type II neurons in VP of cats. The lack of GAD-positive neurons in VP of rats contrasts also with the presence of such neurons in the DCN in the same species. As in cats, about one third of the neurons in the cuneate n. are GAD-positive; these have mostly small perikarya and they are present throughout the nucleus. It is likely that these are intrinsic neurons, i.e. non-projecting beyond the limits of the DCN since a comparable percentage of neurons are unlabeled by simultaneous injections of horseradish peroxidase in multiple targets of the DCN. Like GAD-positive neurons, neurons unlabeled by the retrograde transport of HRP have, for the most part, small perikarya. It is possible that inhibitory mechanisms necessary for basic transfer functions in VP of rats are sustained through projections to this nucleus from the n. reticularis thalami. Extrinsic source of GABAergic input to the DCN seem to be absent or very weak. From this and previous evidence it may be proposed that intrinsic inhibitory interneurons have gradually developed in VP of rabbits, carnivores, and primates in parallel with more elaborate levels of thalamic integration of somatosensation.

The pattern of spinal and medullary projections from a cutaneous nerve and a muscle nerve of the forelimb of the cat: a study using the transganglionic transport of HRP

The transport of HRP into the spinal cord and medulla in the cat has been examined from a forelimb cutaneous nerve, the lateral superficial radial nerve (LSR), and from the muscle nerves supplying both heads of the forelimb muscle, extensor carpi radialis (ECR). HRP transported by the LSR was widely distributed in the spinal cord throughout laminae I-IV in the vicinity of the root entry zone and from spinal segments T1 to C5. HRP was also transported from the LSR to the medulla where there was intense patchy, discontinuous labelling in the main cuneate nucleus. The pattern of labelling in the cuneate nucleus did not follow any simple somatotopic plan. Exposure of the muscle nerve to HRP led to labelling in the spinal dorsal horn in lamina I, in the deep dorsal horn on the lamina V/VI border, and in lateral and medial lamina VI at sites that contain cells of origin of spinocerebellar tracts. The medial lamina VI label was contiguous with a deposit that extended medially to the central canal. The label in lateral lamina VI was patchy and formed a discontinuous column from T1 to C5. HRP transported by the muscle nerve also produced label in the more ventral regions of the cuneate nucleus where it had a lacy appearance, in part due to its extensive distribution around dendrites. A relatively dense, patchy, and discontinuous deposit of reaction product was also present in the external cuneate nucleus after muscle nerve exposure. This deposit was most intense on the dorsomedial surface of this nucleus, but another, less intense, deposit was also present ventrally.

Synaptic terminals in the ventroposterolateral nucleus of the thalamus from neurons in the dorsal column and lateral cervical nuclei: an electron microscopic study in the cat

The afferent fibres to the ventroposterolateral nucleus (VPL) of the contralateral thalamus from neurons in the dorsal column nuclei (DCN) and the lateral cervical nucleus (LCN) were labelled by anterograde transport of wheat germ agglutinin-horseradish peroxidase conjugate and subsequent histochemical processing with tetramethyl benzidine. In accordance with the results of previous light microscopical studies using the degeneration method or autoradiographic tracing technique, the distribution of the afferents from the DCN and LCN in the VPL differed considerably. Thus the DCN terminals, which were calculated to constitute about 7-8% of the total number of boutons in the VPL, were found throughout the entire VPL, whereas the LCN terminals were mainly located in its dorsal and dorsolateral parts, where they made up about 1% of the total number of boutons. However, the morphology and synaptic organization of the terminals from the DCN and LCN were virtually identical. Thus the synaptic terminals of the two afferent pathways seemed to be represented by large boutons of a similar type, which had large, slightly oval and loosely packed synaptic vesicles and contained numerous mitochondria. Both DCN and LCN terminals synapsed preferentially on medium-sized to large dendrites, but were also presynaptic to other vesicle-containing profiles, probably of internuncial origin, which in turn were in synaptic contact with the same dendrites as the labelled ones. It is suggested that the differences in physiological properties between the somatosensory information that is transmitted to the somatosensory cortex via the dorsal column-medial lemniscus pathway and the spino-cervico-thalamic tract do not seem to have a counterpart in differences in the synaptic organization of their relay in the VPL.

Glutamic acid decarboxylase-containing neurons in the dorsal column nuclei of the cat

The retrograde transport of horseradish peroxidase (HRP) and immunocytochemistry for glutamic acid decarboxylase (GAD) have been employed to examine whether local circuit neurons (LCNs) exist in the dorsal column nuclei (DCN) and whether these neurons may be GABA-ergic. Observations focused on the dorsal part of the middle cuneate nucleus (MCd), since this region has been previously shown to contain projecting neurons whose axons terminate almost exclusively in the contralateral thalamus. After large injections of HRP in the nucleus ventralis posterolateralis and surrounding structures of the feline thalamus, the majority of neurons in MCd are labeled. These represent about 89% of the neurons in MCd as counted in 40-microns frozen sections, and about 69% as counted in plastic-embedded, 2.5-microns-thick section. Unlabeled by the same injections are some medium to large neurons at the dorsal rim of MCd, and many characteristically small (mean = +/- 250 microns2) neurons at the periphery of the cell clusters formed by thalamic-projecting neurons. These small neurons represent 10-12% of the neuronal population of MCd, as counted in 40-microns-thick frozen sections, and about 30%, as counted in plastic-embedded, 2.5-microns-thick sections. Neurons in this size range are also unlabeled after injection of retrograde tracer in the pretectal area, inferior and superior colliculi, inferior olivary complex, and/or spinal cord. These injections, however, result in the labeling of neurons along the dorsal rim of MCd and/or in other regions of the cuneate nucleus. In adult, colchicine-treated cats, the use of anti-GAD serum reveals a population of labeled neurons uniformly distributed throughout the DCN. In MCd, these are small (mean = +/- 235 microns2) neurons mainly intercalated between cell clusters, and represent about 25% of the neuronal population of this nuclear subdivision as counted in plastic-embedded, 2.5-microns-thick sections. Labeled processes densely infiltrate the cell clusters, and labeled varicosities appear to cover the soma and dendrites of unlabeled neurons. At the electron-microscopic level, most labeled profiles contain vesicles and correspond to F boutons usually involved in "axoaxonic" contacts with terminals of dorsal root afferent and presynaptic to dendrites. Other vesicle-containing, GAD-positive endings seem to correspond to the P boutons described by Ellis and Rustioni (1981) and are believed to be, at least in part, of dendritic origin. It is suggested that GAD-positive neurons are GABA-ergic LCNs and that these can mediate both pre- and postsynaptic inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiology of raccoon cuneocerebellar neurons

Electrophysiological experiments were undertaken in order to locate and functionally characterize cells of the raccoon main cuneate nucleus (MCN) that can be activated by electrical stimulation of the cerebellum. A total of 98 such units were studied in pentobarbital sodium-anesthetized, methoxyflurane-anesthetized, or decerebrate preparations. Aside from a greater likelihood of resting discharge in the decerebrate preparations, no appreciable variability in physiological properties of the neurons could be attributed to differences in the type of preparation. Using constant latency of response and ability to be blocked by collision as principal criteria, both antidromically (n = 31) and synaptically (n = 67) activated neurons of the main cuneate nucleus could be identified. A small number of MCN neurons could be activated by both cerebellar and thalamic stimulation, but no unit was antidromically activated from both locations. MCN neurons projecting to the cerebellum are located primarily in the ventral polymorphic cell region of the nucleus at and rostral to the obex, corresponding to the "medial tongue" region of Johnson et al. (1968). In contrast, neurons synaptically activated from the cerebellum are found throughout the dorsoventral extent of the rostral MCN, including the "clusters" region. The majority of antidromically activated units responded to mechanical stimulation of deeper tissues, and most of these were activated by muscle stretch. Only a small portion (13-15%) of either antidromically or synaptically activated units were classed as light touch units with peripheral receptive fields (RFs) restricted to glabrous surfaces of the forepaw. Glabrous skin RFs located on the digital surfaces are smaller than those located on the palm pads. In both cases, RFs are larger than those associated with primary afferent fibers, but toward the low end of the distribution for MCN neurons not activated by cerebellar stimulation. All MCN units activated by cerebellar stimulation, regardless of modality, respond to mechanical stimulation with trains of irregularly spaced single spikes. Glabrous skin cutaneous mechanoreceptive MCN neurons, whether rapidly or slowly adapting, respond to ramp indentations with an instantaneous frequency which may be described as a power function of ramp velocity, with exponents less than one. These values are in the same range as those previously reported for primary afferents of the cuneate fasciculus (Pubols and Pubols, 1973).

Improved visualization of neurons labeled with horseradish peroxidase: silver-intensification of the pyrocatechol/p-phenylenediamine reaction product

A silver intensification procedure suitable for use with pyrocatechol/p-phenylenediamine (PC-PPD) product of the horseradish peroxidase (HRP) reaction is described. Qualitative and quantitative results from retrogradely labeled neurons in the cat cortex after thalamic injection of HRP demonstrate an increase of the intensity of labeling and in the number of darkly labeled cells after the intensification procedure. In both the non-intensified and the intensified PC-PPD reacted tissue the sensitivity was comparable to that of TMB-treated material. The ratio of lightly to darkly labeled neurons was very similar in intensified PC-PPD and TMB material, suggesting that the lightly labeled cells may have fewer terminals present at the level of the injected target.

Receptive field organization and response properties of spinal neurones with axons ascending the dorsal columns in the cat

Micro-electrode recordings were made from single post-synaptic axons in the dorsal columns of cats anaesthetized with chloralose and paralysed with gallamine triethiodide. The recordings were made from the L5 segment and the axons were shown to project to the upper cervical level. Forty-eight units were recorded and the axons had conduction velocities of 22-61 ms-1, averaging 38.3 ms-1. Excitatory receptive fields were complex in many units, being made up of clearly defined, separate, low and high threshold areas. The receptive fields were often discontinuous. Only a few units behaved as if they received excitatory input from a single class of mechanoreceptors. A minority (13%) of units had labile, excitatory receptive fields that expanded in size during the recording period. About 40% of the units had inhibitory receptive fields. These were of two main types: either small and within or adjacent to the excitatory field, or large and separated from or adjacent to the excitatory field. The great majority of units had resting discharges upon isolation and these consisted of single impulses or bursts of impulses at short intervals separated by longer, irregular periods. The time course of inhibition produced by electrical stimulation of cutaneous nerves suggested presynaptic inhibitory components to the inhibition. Some inhibitory curves were very prolonged with maxima at about 100 ms and total durations of up to 400 ms. The complexity of the receptive field organization in these dorsal horn neurones is discussed, as is their possible significance as input neurones to the dorsal column nuclei.

Primary afferent projections to the spinal cord and the dorsal column nuclear complex in the turtle Pseudemys

Primary afferent projections from cervical and lumbar levels were studied in the turtle Pseudemys scripta elegans. Injections of radioactive amino acids, wheat germ agglutinin and horseradish peroxidase were made into the dorsal root ganglia or the spinal cord. Previous reports on the terminal distribution of primary afferents within the ipsilateral segment of entry were confirmed (Kusuma and ten Donkelaar 1979, 1980) and additional dorsal root projections were demonstrated to the contralateral side and to several neighboring spinal segments. The primary afferent projections to the brainstem were essentially restricted to a dorsolateral area that appears to be homologous to the main dorsal column nuclei (n. gracilis and n. cuneatus medialis) in mammals. While exhibiting a similarly extensive rostro-caudal span, the projections originating from lumbar injections terminated more medially, those from cervical injections more laterally. The labeling pattern suggested that terminations are mainly on dorsally extending dendrites.

Dendro-dendritic synapses in substantia nigra: descriptions based on analysis of serial sections

Dendro-dendritic synapses were studied in serial sections of rat substantia nigra in subjects pretreated with intraventricularly administered 5-hydroxydopamine, a monoaminergic synaptic label. These synapses had symmetrical membrane thickenings and small clusters of heavily labeled pleomorphic vesicles in the presynaptic dendrites, which were assumed to extend from the dopaminergic cells of pars compacta. Dendro-dendritic synapses were seen in the pars compacta of the substantia nigra, but were not found in blocks trimmed to include only the nondopaminergic pars reticulata. The presynaptic dendrites did not receive other synapses near the sites of dendro-dendritic synapses. These dendrites were frequently apposed to other dendrites for long distances with no intervening glial processes. In many cases, several adjacent dendro-dendritic contacts were made by a single presynaptic dendrite onto several different postsynaptic dendrites. Presynaptic dendrites did not participate in reciprocal synapses, serial synapses, or dendro-axonic synapses. Presynaptic and postsynaptic dendrites engaged in dendro-dendritic synaptic contact were of similar appearance and both had cross-sectional diameters of 0.23-1.9 microns. In several cases, label could also be detected in the postsynaptic dendrite in cisternae of smooth endoplasmic reticulum, providing evidence for dendro-dendritic synapses between dopaminergic neurons. Release of dopamine from dopaminergic dendrites and their role in the control of neuronal activity in substantial nigra are discussed.

The corticocuneate pathway in the cat: relations among terminal distribution patterns, cytoarchitecture, and single neuron functional properties

A combined anatomical and physiological strategy was used to investigate the organization of the corticocuneate pathway in the cat. The distribution of the corticocuneate projection was mapped by means of the anterograde horseradish peroxidase (HRP) labeling technique and correlated with the nuclear cytoarchitecture in Nissl and Golgi material, the distribution of retrogradely labeled relay cells after HRP injections in the ventrobasal complex of the thalamus, and the topographic organization derived from single- and multiunit recordings in the decerebrate, unanesthetized cat. This approach provided details about the arrangement of the corticocuneate pathway that were not available from previous studies with anterograde degeneration methods. On the basis of cytoarchitectonic and connectional features, a number of subdivisions are identified in the cuneate nucleus, each of which is associated with characteristic functional properties. In agreement with previous studies, it is found that a large portion of the cuneate nucleus, the middle dorsal part (MCd), is exclusively devoted to the representation of cutaneous receptive fields on the digits. This "core" region contains more thalamic projecting neurons than any other subdivision of the cuneate nucleus. A topographic arrangement also exists in the subdivisions of the rostral cuneate and of the nuclear region ventral to MCd, although in these, receptive fields are larger and predominantly, but not exclusively, related to deep receptors and involve the arm, shoulder, and trunk. Observations on corticocuneate projections were based on injections, mainly focused on functional subdivisions of the primary somatosensory cortex (SI) as described by McKenna et al. (1981). Although cortical projections are mainly to cuneate regions other than its core, a significant proportion of fibers from the region of SI where the digits are represented (particularly area 3b) do project to the MCd region of the cuneate nucleus. Similarly, nuclear areas associated with receptive fields on the arm and trunk are labeled after injection in SI arm and trunk regions, respectively. Thus, a close topographic relationship appears to exist between the somatosensory cortex and cuneate regions related to the same body representation, although nuclear regions in which receptive fields on the neck area are represented receive very sparse or no detectable cortical projections even when the injection of the tracer involves the entire sensorimotor cortex.(ABSTRACT TRUNCATED AT 400 WORDS)