Descending influences on the cutaneous receptive fields of postsynaptic dorsal column neurones in the cat

The encoding of touch by somatotopically aligned dorsal column subdivisions

The somatosensory system decodes a range of tactile stimuli to generate a coherent sense of touch. Discriminative touch of the body depends on signals conveyed from peripheral mechanoreceptors to the brain through the spinal cord dorsal column and its brainstem target, the dorsal column nuclei (DCN)^1,2. Models of somatosensation emphasize that fast-conducting low-threshold mechanoreceptors (LTMRs) innervating the skin drive the DCN^3,4. However, postsynaptic dorsal column (PSDC) neurons within the spinal cord dorsal horn also collect mechanoreceptor signals and form a second major input to the DCN^5-7. The significance of PSDC neurons and their contributions to the coding of touch have remained unclear since their discovery. Here we show that direct LTMR input to the DCN conveys vibrotactile stimuli with high temporal precision. Conversely, PSDC neurons primarily encode touch onset and the intensity of sustained contact into the high-force range. LTMR and PSDC signals topographically realign in the DCN to preserve precise spatial detail. Different DCN neuron subtypes have specialized responses that are generated by distinct combinations of LTMR and PSDC inputs. Thus, LTMR and PSDC subdivisions of the dorsal column encode different tactile features and differentially converge in the DCN to generate specific ascending sensory processing streams.

Mechanoreceptor signal convergence and transformation in the dorsal horn flexibly shape a diversity of outputs to the brain

The encoding of touch in the spinal cord dorsal horn (DH) and its influence on tactile representations in the brain are poorly understood. Using a range of mechanical stimuli applied to the skin, large-scale in vivo electrophysiological recordings, and genetic manipulations, here we show that neurons in the mouse spinal cord DH receive convergent inputs from both low- and high-threshold mechanoreceptor subtypes and exhibit one of six functionally distinct mechanical response profiles. Genetic disruption of DH feedforward or feedback inhibitory motifs, comprised of interneurons with distinct mechanical response profiles, revealed an extensively interconnected DH network that enables dynamic, flexible tuning of postsynaptic dorsal column (PSDC) output neurons and dictates how neurons in the primary somatosensory cortex respond to touch. Thus, mechanoreceptor subtype convergence and non-linear transformations at the earliest stage of the somatosensory hierarchy shape how touch of the skin is represented in the brain.

Spino-olivary projections in the rat are anatomically separate from postsynaptic dorsal column projections

The gracile nucleus (GN) and lateral part of rostral dorsal accessory olive (rDAO) are important relays for indirect, postsynaptic dorsal column, and direct ascending pathways, respectively, that terminate as climbing fibers in the “hindlimb-receiving” parts of the C1 and C3 zones in the cerebellar cortex. While the spinal cells of origin of that project to GN and rDAO are from largely separate territories in the spinal cord, previous studies have indicated that there could be an area of overlap between these two populations in the medial dorsal horn. Given the access of these two ascending tracts to sensory (thalamic) versus sensorimotor (precerebellar) pathways, the present study therefore addresses the important question of whether or not individual neurons have the potential to contribute axons to both ascending pathways. A double-fluorescent tracer strategy was used in rats (red Retrobeads and Fluoro-Ruby or green Retrobeads and Fluoro-Emerald) to map the spatial distribution of cells of origin of the two projections in the lumbar spinal cord. The two pathways were found to receive input from almost entirely separate territories within the lumbar cord (levels L3–L5). GN predominantly receives input from lamina IV, while rDAO receives its input from three cell populations: medial laminae V–VI, lateral lamina V, and medial laminae VII–VIII. Cells that had axons that branched to supply both GN and rDAO represented only about 1% of either single-labeled cell population. Overall, the findings therefore suggest functional independence of the two ascending pathways. J. Comp. Neurol. 522:2179–2190, 2014. © 2013 Wiley Periodicals, Inc.

Serotonin alters multi-segmental convergence patterns in spinal cord deep dorsal horn and intermediate laminae neurons in an in vitro young rat preparation

Each spinal neuron has a receptive field that corresponds to stimulation of a specific area of skin or subcutaneous tissue. Receptive fields are plastic and can be altered during development and injury but the actions of neuromodulators, such as serotonin (5-hydroxytryptamine, 5-HT) on receptive field properties are not well known. We used stimulation of multiple adjacent dorsal root spinal segments as a measure of "receptive field size" to determine the effects of 5-HT on multi-segmental convergent input onto neurons in laminae IV-VII. Whole-cell patch-clamp recordings were undertaken in the in vitro hemisected thoracolumbar spinal cord of rats aged 8-10 days old. Based on synaptic responses, neurons could be divided into two predominant groups and 5-HT exerted different effects on these groups. The first group received excitatory post-synaptic potentials (EPSPs) from the homonymous dorsal root but inhibitory post-synaptic potentials (IPSPs) with increasing amplitude from more distant dorsal roots. In this group, 5-HT preferentially depressed the IPSPs from adjacent nerve roots while leaving the EPSP intact. The second group received short-latency EPSPs from all segments stimulated and 5-HT potently depressed all synaptic input. In both populations the depressant actions of 5-HT increased with dose (0.1-10.0 microM). Bicuculline and strychnine did not affect the 5-HT induced short-latency synaptic depression. These results suggest that descending serotonergic systems depress spinal sensory convergence in a graded and differentiated manner. The findings are discussed in relation to the modulation of nociceptive signaling.

Postsynaptic dorsal column and cuneate neurons in raccoon: comparison of response properties and cross-correlation analysis

The responses of 111 postsynaptic dorsal column (PSDC) neurons in the cervical spinal cord and 51 cuneate neurons with receptive fields on the glabrous skin of the forepaw were studied in anesthetized raccoons using extracellular recording techniques. The PSDC neurons had larger receptive fields than the cuneate neurons, but in both groups the fields never extended onto hairy skin. PSDC and cuneate neurons had approximately the same mean latency to electrical stimulation of the receptive field, but PSDC neurons had significantly lower thresholds. The majority of both PSDC and cuneate neurons also responded to electrical stimulation of an adjacent digit, even though they did not respond to mechanical stimulation of that digit. Cross-correlation analysis of the activity of 51 pairs of PSDC and cuneate neurons recorded simultaneously revealed a significant interaction in 26 pairs during spontaneous activity. In 20 of these neuron pairs, the probability that the cuneate neuron would fire was greater after the PSDC neuron had fired (suggesting a spinocuneate interaction), five pairs showed an interaction in the opposite (cuneospinal) direction, and one pair had a significant inhibitory interaction. These interactions occurred more often when the receptive fields of the two neurons were overlapping than when their fields were on adjacent digits. Frequency response analysis revealed greater coherence for those pairs showing a spinocuneate interaction than for those with a cuneospinal interaction. These results support the hypothesis that the PSDC system exerts a tonic facilitatory effect on cuneate neurons and that there may be some somatotopic organization to the interactions. However, the similar response latencies of the two groups of neurons makes it unlikely that PSDC neurons could contribute to the rapid initial processing of cutaneous information by the cuneate nucleus.

Movement-related gating of climbing fibre input to cerebellar cortical zones

The inferior olive climbing fibre projection and associated spino-olivocerebellar paths (SOCPs) have been studied intensively over the last quarter of a century yet precisely what information they signal to the cerebellar cortex during movements remains unclear. A different approach is to consider the times during a movement when afferent signals are likely to be conveyed via these paths. Central regulation (gating) of afferent transmission during active movements is well documented in sensory pathways leading to the cerebral cortex and the present review examines the possibility that a similar phenomenon also occurs in SOCPs during movements such as locomotion and reaching. Several lines of evidence are considered which suggest that SOCPs are not always open for transmission. Instead, flow of sensory information to the cerebellum via climbing fibre paths is powerfully modulated during active movements. The findings are discussed in relation to the parasagittal zonal organization of the cerebellar cortex and, in particular, evidence is presented that different cerebellar zones are subject to similar patterns of gating during reaching but can differ appreciably in the pattern of modulation their SOCPs exhibit during locomotion. Furthermore, differences in gating can occur at different rostrocaudal loci within the same zone, suggesting that in the awake behaving animal, individual cerebellar zones are not functionally homogeneous. Finally, the data are interpreted in relation to the error detector hypothesis of climbing fibre function and the possibility explored that the gating serves as a task-dependent mechanism that operates to prevent self-generated 'irrelevant' sensory inputs from being relayed via the SOCPs to the cerebellar cortex, while behaviourally 'relevant' signals are selected for transmission.

Control of size and excitability of mechanosensory receptive fields in dorsal column nuclei by homolateral dorsal horn neurons

Both accidental and experimental lesions of the spinal cord suggest that neuronal processes occurring in the spinal cord modify the relay of information through the dorsal column-lemniscal pathway. How such interactions might occur has not been adequately explained. To address this issue, the receptive fields of mechanosensory neurons of the dorsal column nuclei were studied before and after manipulation of the spinal dorsal horn. After either a cervical or lumbar laminectomy and exposure of the dorsal column nuclei in anesthetized cats, the representation of the hindlimb or of the forelimb was defined by multiunit recordings in both the dorsal column nuclei and in the ipsilateral spinal cord. Next, a single cell was isolated in the dorsal column nuclei, and its receptive field carefully defined. Each cell could be activated by light mechanical stimuli from a well-defined cutaneous receptive field. Generally the adequate stimulus was movement of a few hairs or rapid skin indentation. Subsequently a pipette containing either lidocaine or cobalt chloride was lowered into the ipsilateral dorsal horn at the site in the somatosensory representation in the spinal cord corresponding to the receptive field of the neuron isolated in the dorsal column nuclei. Injection of several hundred nanoliters of either lidocaine or cobalt chloride into the dorsal horn produced an enlargement of the receptive field of the neuron being studied in the dorsal column nuclei. The experiment was repeated 16 times, and receptive field enlargements of 147-563% were observed in 15 cases. These data suggest that the dorsal horn exerts a tonic inhibitory control on the mechanosensory signals relayed through the dorsal column-lemniscal pathway. Because published data from other laboratories have shown that receptive field size is controlled by signals arising from the skin, we infer that the control of neuronal excitability, receptive field size and location for lemniscal neurons is determined by tonic afferent activity that is relayed through a synapse in the dorsal horn. This influence of dorsal horn neurons on the relay of mechanosensory information through the lemniscal pathways must modify our traditional views concerning the relative independence of these two systems.

Colocalization of glycine and GABA in synapses on spinomedullary neurons

Spinomedullary neurons of the postsynaptic dorsal column pathway in adult cats were retrogradely labelled with horseradish peroxidase. Postembedding immunogold reactions were performed with antisera which recognise GABA or glycine to determine if synaptic boutons in contact with these neurons contain both transmitters. Analysis of series of ultrathin sections revealed that synaptic profiles with strong immunogold reactions for GABA usually also displayed strong immunogold reactions for glycine. Pre-embedding immunocytochemistry was performed on sections containing labelled cells with a monoclonal antibody which recognises the glycine receptor-associated protein, gephyrin. Many synapses onto postsynaptic dorsal column neurons were associated with gephyrin-like immunoreactivity and these typically contained irregularly shaped vesicles. Immunogold reactions showed that synaptic profiles apposed to gephyrin-immunoreactive junctions contained GABA and glycine. The evidence suggests that glycine is a neurotransmitter at synapses on spinomedullary neurons and that it is colocalized with GABA.

Synaptic organization of excitatory and inhibitory boutons associated with spinal neurons which project through the dorsal columns of the cat

The cell bodies and proximal dendrites of postsynaptic dorsal column neurons were examined for synaptic boutons which displayed immunoreactivity for the principal excitatory and inhibitory neurotransmitters, glutamate and GABA. The neurons were labelled by retrograde transport of horseradish peroxidase and GABA or glutamate-containing boutons were revealed by performing postembedding immunogold reactions on electron microscope sections. Five neurons were examined and all of them were postsynaptic to boutons which contained either GABA or glutamate. Quantitative analysis of two of the cells revealed that more than 90% of the synaptic profiles associated with them displayed immunogold reactions for these transmitters. Analysis of series of alternate sections, which were reacted for either GABA or glutamate, showed that there was no overlap in the populations of immunoreactive boutons. Furthermore, GABA and glutamate immunoreactions were associated with boutons which had different morphological characteristics. In addition, some large glutamate-enriched boutons were postsynaptic to small boutons which displayed immunogold reactions for GABA. This study demonstrates morphological bases for direct excitation, postsynaptic inhibition and presynaptic inhibition of postsynaptic dorsal column cells.

Direct catecholaminergic innervation of spinal dorsal horn neurons with axons ascending the dorsal columns in cat

Previous ultrastructural studies have shown that catecholamine-containing nerve terminals in the spinal dorsal horn form synaptic junctions with dendrites and somata, but the identity of the neurons giving rise to these structures is largely unknown. In this study we have investigated the possibility that spinomedullary neurons, which project through the dorsal columns to the dorsal column nuclei, are synaptic targets for descending catecholaminergic axons. Neurons with axons ascending the dorsal columns were retrogradely labelled after uptake of horseradish peroxidase by their severed axons in the thoracic (T10-T12) or cervical (C2-C3) dorsal columns. After the retrogradely labelled neurons were visualized, the tissue was immunocytochemically stained with antisera raised against tyrosine hydroxylase or dopamine-beta-hydroxylase. Three hundred forty-three retrogradely labelled neurons within laminae III-V of the lumbosacral dorsal horn were examined under high power with the light microscope. In Triton X-100 treated material, over 60% of cells were found to have dopamine-beta-hydroxylase-immunoreactive varicosities closely apposed to their somata and proximal dendrites. The number of contacts per cell varied from 1 to 22, with a mean number of 4.5. Fewer cells (34%) received contacts from axons immunoreactive for tyrosine hydroxylase as a consequence of the weaker immunoreaction produced by this antiserum. Correlated light and electron microscopic analysis confirmed that many of these contacts were regions of synaptic specialization and that immunostained boutons contained pleomorphic (round to oval) agranular vesicles together with several dense core vesicles. These observations suggest that catecholamines regulate sensory transmission through this spinomedullary pathway by a direct postsynaptic action upon its cells of origin. Such an action would be predicted to suppress transmission generally through this pathway.

Corticofugal actions on lemniscal neurons of the cuneate, gracile and lateral cervical nuclei of the cat

Extracellular records were made from single identified lemniscal neurons of the cell-cluster regions of the cuneate and gracile nuclei, and of the lateral cervical nucleus, in pentobarbitone-anaesthetized cats. Forepaw, hind paw or face regions of the contralateral Sm I cortex were identified by recording through an inserted microelectrode which was then used for stimulation. The effect of a double cortical shock or train of shocks was usually inhibition: occasionally facilitation was observed, or mixed effects with facilitation preceding inhibition. Effects were seen in about half the cells studied in all three nuclei. Some cells of the lateral cervical nucleus were strongly excited, an effect not seen in the other nuclei. No component of these responses depended on suprathreshold stimulus intensities. Some lateral cervical cells were studied after deafferentiation by section of the dorsolateral spinal white matter; the same pattern of effects was seen. With an upper stimulus limit of 200 microA, cuneate but not gracile cells were affected from the cortical forepaw region, and gracile but not cuneate cells from the hind paw region. With threshold stimuli in an identified part of the forepaw cortical representation it was clear that cuneate cells with cutaneous receptive fields in corresponding parts of the forepaw had the lowest thresholds (minimum 6 microA). Threshold rose steeply with distance across the paw, suggesting quite sharp focusing of corticofugal effects in this system. When using similar procedures with the lateral cervical nucleus, with an upper limit of 200 microA, stimulation of forelimb cortex, or of facial cortex, affected both neurons with forelimb and those with hind limb fields.(ABSTRACT TRUNCATED AT 250 WORDS)

Tonic descending modulation of spinal neurons with renal input

Experiments were conducted to determine the influence of tonically active descending pathways on thoracolumbar spinal neurons that respond to renal nerve stimulation in anesthetized cats. We examined the effect of reversible blockade of spinal conduction on spontaneous activity, responses to renal nerve stimulation and responses to somatic stimuli of 71 spinal neurons. Mid-thoracic cold block resulted in enhanced responses (tonically inhibited neurons), reduced responses (tonically excited neurons), or did not affect neuronal responses. The spontaneous activity of 47 of 69 neurons (68%) increased from 7.3 +/- 2.0 spikes/s before cooling to 23.3 +/- 4.5 spikes/s during cooling. Activity of 8 neurons (12%) decreased while 14 (20%) had no change in activity. Cooling increased the responses of 51 of 71 neurons (72%) to renal nerve stimulation. Renal nerve stimulation evoked a two-fold increase in both short latency (early) and long latency (late) responses. Four neurons had a late response which was revealed by cold block. Cooling decreased responses of 8 of 71 neurons (11%) and 9 neurons (13%) were not affected. Cooling increased the early responses but decreased the late responses of 3 of 71 neurons (4%). All neurons had somatic receptive fields and 33 of 56 exhibited increased responses to somatic stimulation during cooling. In addition, receptive field sizes of 26 neurons increased. Four neurons had a decrease and 25 neurons had no change in receptive field size during cooling. These data indicate that tonically active descending pathways modulate the activity of most spinal neurons with renal input and the major effect of these pathways is inhibitory. This influence may be important in the modulation of spinal circuits that participate in reflexes evoked by renal afferent fibers.

Response properties and descending control of rat dorsal horn neurons with deep receptive fields

The study was designed to obtain information on the spinal processing of input from receptors in deep somatic tissues (muscle, tendon, joint). In anaesthetized rats, the impulse activity of single dorsal horn cells was recorded extracellularly. In a pilot series, the proportion of neurons responding to mechanical stimulation of deep tissues was determined: 46.7% had receptive fields in the skin only, 35.5% could only be driven from deep tissues (deep cells), and 17.7% possessed a convergent input from both skin and deep tissues (cutaneous-deep cells). In each category, neurons with low and high mechanical thresholds were encountered. Experiments employing a reversible cold block of the spinal cord showed that deep cells with high threshold were subject to a stronger descending inhibition than low-threshold deep cells. In cutaneous-deep neurons the input combination high-threshold cutaneous and high-threshold deep was the most frequent one (48.7% of the cutaneous-deep cells). In these presumably nociceptive cells the descending inhibition had a differential action in that the input from deep tissues was more strongly affected than was the cutaneous input to the same neuron. The recording sites of the neurons with deep input were located in the superficial dorsal horn and in and around lamina V. The results suggest that in the rat a considerable proportion of dorsal horn cells receives input from deep nociceptors and that this input is controlled by descending pathways in a rather selective way.