The dominant class of somatosensory neurone recorded in the spinal dorsal horn of awake sheep has wide dynamic range properties

Understanding of Spinal Wide Dynamic Range Neurons and Their Modulation on Pathological Pain

The spinal dorsal horn (SDH) transmits sensory information from the periphery to the brain. Wide dynamic range (WDR) neurons within this relay site play a critical role in modulating and integrating peripheral sensory inputs, as well as the process of central sensitization during pathological pain. This group of spinal multi-receptive neurons has attracted considerable attention in pain research due to their capabilities for encoding the location and intensity of nociception. Meanwhile, transmission, processing, and modulation of incoming afferent information in WDR neurons also establish the underlying basis for investigating the integration of acupuncture and pain signals. This review aims to provide a comprehensive examination of the distinctive features of WDR neurons and their involvement in pain. Specifically, we will examine the regulation of diverse supraspinal nuclei on these neurons and analyze their potential in elucidating the mechanisms of acupuncture analgesia.

A new device concept for directly modulating spinal cord pathways: initial in vivo experimental results

We describe a novel spinal cord (SC) stimulator that is designed to overcome a major shortcoming of existing stimulator devices: their restricted capacity to selectively activate targeted axons within the dorsal columns. This device overcomes that limitation by delivering electrical stimuli directly to the pial surface of the SC. Our goal in testing this device was to measure its ability to physiologically activate the SC and examine its capacity to modulate somatosensory evoked potentials (SSEPs) triggered by peripheral stimulation. In this acute study on adult sheep (n = 7), local field potentials were recorded from a grid placed in the subdural space of the right hemisphere during electrical stimulation of the left tibial nerve and the spinal cord. Large amplitude SSEPs (>200 µV) in response to SC stimulation were consistently obtained at stimulation strengths well below the thresholds inducing neural injury. Moreover, stimulation of the dorsal columns with signals employed routinely by devices in standard clinical use, e.g., 50 Hz, 0.2 ms pulse width, produced long-lasting changes (>4.5 h) in the SSEP patterns produced by subsequent tibial nerve stimulation. The results of these acute experiments demonstrate that this device can be safely secured to the SC surface and effectively activate somatosensory pathways.

Neuronal hyperexcitability in the dorsal horn after painful facet joint injury

Excessive cervical facet capsular ligament stretch has been implicated as a cause of whiplash-associated disorders following rear-end impacts, but the pathophysiological mechanisms that produce chronic pain in these cases remain unclear. Using a rat model of C6-C7 cervical facet joint capsule stretch that produces sustained mechanical hyperalgesia, the presence of neuronal hyperexcitability was characterized 7 days after joint loading. Extracellular recordings of spinal dorsal horn neuronal activity between C6 and C8 (117 neurons) were obtained from anesthetized rats, with both painful and non-painful behavioral outcomes established by the magnitude of capsule stretch. The frequency of neuronal firing during noxious pinch (p<0.0182) and von Frey filaments applications (4-26g) to the forepaw was increased (p<0.0156) in the painful group compared to the non-painful and sham groups. In addition, the incidence and frequency of spontaneous and after discharge firing were greater in the painful group (p<0.0307) relative to sham. The proportion of cells in the deep laminae that responded as wide dynamic range neurons also was increased in the painful group relative to non-painful or sham groups (p<0.0348). These findings suggest that excessive facet capsule stretch, while not producing visible tearing, can produce functional plasticity of dorsal horn neuronal activity. The increase in neuronal firing across a range of stimulus magnitudes observed at day 7 post-injury provides the first direct evidence of neuronal modulation in the spinal cord following facet joint loading, and suggests that facet-mediated chronic pain following whiplash injury is driven, at least in part, by central sensitization.

The whole body receptive field of dorsal horn multireceptive neurones

Multireceptive neurones are found in the spinal dorsal horn and may be projection neurones and/or interneurones for polysynaptic reflexes. The cutaneous receptive field of a multireceptive neurone exhibits a gradient of sensitivity with the centre responding to any mechanical stimulus, including hair movements and light touch, while the periphery responds only to noxious stimuli. These neurones also receive signals from viscera, muscles and joints. This convergence of inputs means that multireceptive neurones are continuously capturing all the information from both the interface with the external environment (the skin) and the internal milieu (the viscera, muscles, etc.). This information constitutes a 'basic somaesthetic activity' that could help the somatosensory system build a 'global representation of the body'. In addition to be seen as a global entity, the output of multireceptive neurones should be understood in dynamic terms since the size of the peripheral fields of the individual neurones may change, as a result of the plasticity of both excitatory and inhibitory segmental processes. Furthermore, the activity of these neurones can be inhibited from most of the remaining parts of the body via supraspinal mechanisms. These diffuse noxious inhibitory controls (DNIC) are triggered by peripheral A delta- and C-fibres, involve brain structures confined to the caudal-most part of the medulla including the subnucleus reticularis dorsalis (SRD) and are mediated by descending pathways in the dorsolateral funiculi. A painful focus that both activates a segmental subset of neurones and inhibits the remaining population can seriously disrupt this basic activity, resulting in the distortion of the body representation in favour of the painful focus, which becomes pre-eminent and (relatively) oversized.

Modulatory actions of serotonin, norepinephrine, dopamine, and acetylcholine in spinal cord deep dorsal horn neurons

The deep dorsal horn represents a major site for the integration of spinal sensory information. The bulbospinal monoamine transmitters, released from serotonergic, noradrenergic, and dopaminergic systems, exert modulatory control over spinal sensory systems as does acetylcholine, an intrinsic spinal cord biogenic amine transmitter. Whole cell recordings of deep dorsal horn neurons in the rat spinal cord slice preparation were used to compare the cellular actions of serotonin, norepinephrine, dopamine, and acetylcholine on dorsal root stimulation-evoked afferent input and membrane cellular properties. In the majority of neurons, evoked excitatory postsynaptic potentials were depressed by the bulbospinal transmitters serotonin, norepinephrine, and dopamine. Although, the three descending transmitters could evoke common actions, in some neurons, individual transmitters evoked opposing actions. In comparison, acetylcholine generally facilitated the evoked responses, particularly the late, presumably N-methyl-D-aspartate receptor-mediated component. None of the transmitters modified neuronal passive membrane properties. In contrast, in response to depolarizing current steps, the biogenic amines significantly increased the number of spikes in 14/19 neurons that originally fired phasically (P < 0.01). Together, these results demonstrate that even though the deep dorsal horn contains many functionally distinct subpopulations of neurons, the bulbospinal monoamine transmitters can act at both synaptic and cellular sites to alter neuronal sensory integrative properties in a rather predictable manner, and clearly distinct from the actions of acetylcholine.

Serotonin Increases the Incidence of Primary Afferent-Evoked Long-Term Depression in Rat Deep Dorsal Horn Neurons

5-hydroxytryptamine (5-HT) is released in spinal cord by descending systems that modulate somatosensory transmission and can potently depress primary afferent-evoked synaptic responses in dorsal horn neurons. Since primary afferent activity-induced long-term potentiation (LTP) may contribute to central sensitization of nociception, we studied the effects of 5-HT on the expression of sensory-evoked LTP and long-term depression (LTD) in deep dorsal horn (DDH) neurons. Whole cell, predominantly current clamp, recordings were obtained from DDH neurons in transverse slices of neonatal rat lumbar spinal cord. The effect of 5-HT on dorsal-root stimulation-evoked synaptic responses was tested before, during, or after high-frequency conditioning stimulation (CS). In most cells (80%), 5-HT caused a depression of the naı̈ve synaptic response. Even though 5-HT depressed evoked responses, CS in the presence of 5-HT was not only still capable of inducing LTD but also increased its incidence from 54% in controls to 88% ( P < 0.001). Activation of ligands selective for 5-HT1A/1B and 5-HT1B, but not 5-HT2A/2C or 5-HT3receptors, best reproduced these actions. 5-HT also potently depressed postconditioning synaptic responses regardless of whether the induced plasticity was LTP or LTD. Our results demonstrate that in addition to depressing the amplitude of evoked sensory input, 5-HT can also control the direction of its long-term modifiability, favoring the expression of LTD. These findings demonstrate cellular mechanisms that may contribute to the descending serotonergic control of nociception.

Response properties of hind limb single motor units in normal rats and after carrageenan-induced inflammation

The properties of single motor units from hind limb muscles and the changes in situations of hyperalgesia are not known in detail. We have therefore characterized the properties of single motor units in normal Wistar male rats and in rats with carrageenan-induced inflammation, under alpha-chloralose anaesthesia. Units were studied from three different muscles: peroneus longus, tibialis anterior and extensor digitorum longus. The properties of single motor units were not homogeneous in the three muscles studied in normal animals, showing different sizes of cutaneous receptive fields, thresholds for natural and electrical stimulation, and encoding of responses at different intensities of stimulation. Intraplantar injections of carrageenan induced a significant inflammation of the paw and a change in spontaneous behaviour observed in open field experiments. After inflammation, the responses to cutaneous stimulation of the single motor units became more homogeneous. The threshold for mechanical stimulation was lower for peroneus longus and tibialis anterior but not for extensor digitorum longus units when compared to normal animals. The receptive fields were larger when mapped with a 500 mN von Frey hair but not when mapped using a threshold intensity hair. The threshold for thermal stimulation was lower after inflammation than in normal conditions in all cases, whereas the threshold for electrical stimulation was lower in tibialis anterior and extensor digitorum longus units. An enhancement of responses related to the increase of stimulus intensity was seen in normal animals in all muscles for mechanical and electrical stimuli (but not for thermal). After inflammation, a relationship between firing rate and intensity of stimulation was seen in all cases studied. The firing of single motor units showed over 50% adaptation in the normal condition and over 75% after inflammation when stimulated for 10 s at mechanical threshold intensity. After inflammation, the rate of adaptation was significantly lower when suprathreshold intensity was used for mechanical stimulation. No differences were seen in the adaptation of units to thermal stimulation. We conclude that, in situations of hyperalgesia due to inflammation, the threshold, encoding of stimulus intensity and adaptation of single motor units from different muscles changed, resulting in a narrower range of responses and a more homogeneous population of units.

A sheep model for continuous intrathecal infusion of test substances

Pharmaceutical research and new drug development rely extensively on animal research. The development of novel agents for intrathecal administration requires preclinical studies of toxic effects in an animal model. We have developed a nonrodent animal model for this purpose. Our sheep model: 1 Is an animal whose neural axis is similar to the human 2 Allows for the percutaneous placement of intrathecal catheters 3 Has minimal possibilities of infection because the infusion system is totally implanted 4 Provides continuous infusion of the test agent 5 Generates behavioral, motor, neurological and histopathological information so that safety guidelines can be established prior to preclinical studies.

Supraspinal inhibition of nociceptive dorsal horn neurones in the anaesthetized rat: tonic or dynamic?

1. Tonic inhibition of sensory spinal neurones is well known to descend from the rostroventral medulla. It is not clear if this inhibition is dynamically activated by peripheral noxious stimuli. 2. Transection of the ipsilateral dorsolateral funiculus (DLF) removed a descending inhibition of multireceptive spinal neurones and disproportionally prolonged the after-discharge component of their response to a noxious cutaneous stimulus. 3. Microinjection of GABA or tetracaine into the medullary nucleus gigantocellularis pars alpha (GiA) similarly prolonged the after-discharge in response to noxious stimuli. 4. Recordings of GiA cells, initially using minimal surgery, revealed that many had low levels of spontaneous activity and responded vigorously to noxious stimuli applied to any part of the body surface. One hour after the surgery necessary to expose the spinal cord, GiA cells had a high firing rate but responded weakly to noxious stimuli. 5. The response of GiA cells to noxious stimuli was abolished by transection of only the DLF contralateral to the stimulus. 6. It is concluded that the inhibition of multireceptive dorsal horn neurones from GiA is dynamically activated by noxious cutaneous stimuli via a projection in the contralateral DLF. Surgical exposure of the spinal cord tonically activates this inhibition and masks the dynamic component.

Primary afferent-evoked synaptic plasticity in deep dorsal horn neurons from neonatal rat spinal cord in vitro

Whole-cell patch clamp recordings of deep dorsal horn neurons were undertaken in 'thick' transverse slices to demonstrate plasticity of primary afferent-evoked synaptic responses following conditioning stimulation. Synaptic plasticity was observed in neurons throughout the age range examined (postnatal days 3-6 and 9-16) but only long-term depression (LTD) was evocable in older animals (P9-16). Both short- and long-latency synaptic responses could undergo long-term potentiation (LTP) and LTD suggesting that AMPA/kainate and NMDA receptor-evoked responses are modifiable.