NMDA receptor-independent mechanisms responsible for the rate of rise of cumulative depolarization evoked by trains of dorsal root stimuli on rat spinal motoneurones

Windup of Nociceptive Flexion Reflex Depends on Synaptic and Intrinsic Properties of Dorsal Horn Neurons in Adult Rats

Windup, a progressive increase in spinal response to repetitive stimulations of nociceptive peripheral fibers, is a useful model to study central sensitization to pain. Windup is expressed by neurons in both the dorsal and ventral horn of the spinal cord. In juvenile rats, it has been demonstrated both in vivo and in vitro that windup depends on calcium-dependent intrinsic properties and their modulation by synaptic components. However, the involvement of these two components in the adults remains controversial. In the present study, by means of electromyographic and extracellular recordings, we show that windup in adults, in vivo, depends on a synaptic balance between excitatory N-methyl-D-aspartate (NMDA) receptors and inhibitory glycinergic receptors. We also demonstrate the involvement of L-type calcium channels in both the dorsal and ventral horn of the spinal cord. These results indicate that windup in adults is similar to juvenile rats and that windup properties are the same regardless of the spinal network, i.e., sensory or motor.

ASIC channel inhibition enhances excitotoxic neuronal death in an in vitro model of spinal cord injury

In the spinal cord high extracellular glutamate evokes excitotoxic damage with neuronal loss and severe locomotor impairment. During the cell dysfunction process, extracellular pH becomes acid and may activate acid-sensing ion channels (ASICs) which could be important contributors to neurodegenerative pathologies. Our previous studies have shown that transient application of the glutamate analog kainate (KA) evokes delayed excitotoxic death of spinal neurons, while white matter is mainly spared. The present goal was to enquire if ASIC channels modulated KA damage in relation to locomotor network function and cell death. Mouse spinal cord slices were treated with KA (0.01 or 0.1mM) for 1h, and then washed out for 24h prior to analysis. RT-PCR results showed that KA (at 0.01mM concentration that is near-threshold for damage) increased mRNA expression of ASIC1a, ASIC1b, ASIC2 and ASIC3, an effect reversed by the ASIC inhibitor 4',6-diamidino-2-phenylindole (DAPI). A KA neurotoxic dose (0.1mM) reduced ASIC1a and ASIC2 expression. Cell viability assays demonstrated KA-induced large damage in spinal slices from mice with ASIC1a gene ablation. Likewise, immunohistochemistry indicated significant neuronal loss when KA was followed by the ASIC inhibitors DAPI or amiloride. Electrophysiological recording from ventral roots of isolated spinal cords showed that alternating oscillatory cycles were slowed down by 0.01mMKA, and intensely inhibited by subsequently applied DAPI or amiloride. Our data suggest that early rise in ASIC expression and function counteracted deleterious effects on spinal networks by raising the excitotoxicity threshold, a result with potential implications for improving neuroprotection.

Effect of high-frequency alternating current on spinal afferent nociceptive transmission

OBJECTIVE

The study was performed to test the hypothesis that high-frequency alternating current (HFAC) ranging from 2 to 100 kHz delivered to the spinal dorsal roots reduces activity of spinal wide dynamic range (WDR) dorsal horn neurons (DHNs) during noxious peripheral stimulation.

MATERIALS AND METHODS

This hypothesis was tested in both small and large animal in vivo preparations. Single-unit extracellular spinal DHN recordings were performed in seven adult rats and four adult goats while testing various parameters of HFAC delivered to the nerve roots or dorsal root entry zone using various electrode types. Frequencies tested ranged from 2 to 100 kHz but focused on the 3 to 50 kHz range. This study investigated the ability of HFAC to inhibit WDR neuronal activity evoked by noxious mechanical (pinch), and electrical stimuli was tested but was primarily focused on electrical stimulation.

RESULTS

Rat Study: Effects of HFAC were successfully tested on 11 WDR neurons. Suppression or complete blockade of evoked activity was observed in all 11 of these neurons. Complete data sets for neurons systematically tested with 15 baseline and post-HFAC stimulus sweeps were obtained in five neurons, the nociceptive activity of which was suppressed by an average of 69 ± 9.7% (p < 0.0001). Goat Study: HFAC was successfully tested on 15 WDR neurons. Conclusive suppression or complete nociceptive blockade was observed for 12/15 and complete data sets with at least 20 baseline and post-HFAC stimulus sweeps were obtained from eight DHNs. For these neurons the mean activity suppression was 70 ± 10% (p < 0.005).

CONCLUSIONS

Delivery of HFAC to the region of epidural nerve root or nerve root entry inhibited afferent nociceptive input and therefore may have potential to serve as an alternative to traditional spinal cord stimulation without sensory paresthesia as neuronal activation cannot occur at frequencies in this range.

L-type calcium channels and NMDA receptors: a determinant duo for short-term nociceptive plasticity

In the dorsal horn of the spinal cord, pain-transmitting neurons exhibit action potential windup, a form of short-term plasticity, which consists of a progressive increase in neuronal response during repetitive stimulation of nociceptive input fibers. Windup depends on N-methyl-D-aspartate (NMDA) receptor activation, but previous in vitro studies indicated that windup also relies on intrinsic plateau properties of spinal neurons. In the present study, we considered the possible involvement of these properties in windup in vivo. For this purpose, we first studied a nociceptive flexion reflex in the rat. We showed that windup of the reflex is actually suppressed by blockers of L-type calcium current and Ca(2+)-activated non-specific cationic current (Ican), the two main depolarizing conductances of plateau potentials. We further showed that, during windup, NMDA receptors provide a critical excitatory component in a dynamic balance of excitatory and inhibitory inputs which ultimately activates L-type calcium channels. The nociceptive reflex involves at least two neuronal groups, which may express intrinsic amplification properties, motor neurons and dorsal horn neurons. By means of extracellular recordings in the dorsal horn, we showed that windup of dorsal horn neuron discharge was sensitive to the modulators of L-type calcium current. Altogether, our results suggest that, in vivo, windup also depends on the amplification properties of spinal neurons, the triggering of which requires previous activation of NMDA receptors.

Role of group II and III metabotropic glutamate receptors in rhythmic patterns of the neonatal rat spinal cord in vitro

Electrophysiological recordings were used to explore the role of group II and III metabotropic glutamate receptors (mGluRs) in oscillatory patterns generated by the neonatal rat spinal cord in vitro. Neither the group II agonist DCG-IV (and the selective antagonist EGLU), nor the group III agonist L-AP4 (and its selective antagonist CPPG) had any effect on lumbar motoneuron membrane potential or input resistance. This observation suggests that motoneurons expressed no functional group II and III mGluRs and received no network-based, tonic influence mediated by them. DCG-IV or L-AP4 strongly depressed synaptic responses evoked by single dorsal root (DR) stimuli, an effect counteracted by their respective antagonist. EGLU or CPPG per se had no effect on synaptic responses, indicating no mGluR autoreceptor-dependent control of transmitter release. L-AP4 largely depressed cumulative depolarization, windup and associated oscillations, whereas synaptic depression induced by DCG-IV waned with repeated stimuli. L-AP4 slowed down fictive locomotor patterns and arrested disinhibited bursting, which could, however, be promptly restored by DR electrical stimulation. DCG-IV had no significant effect on fictive locomotion, but it blocked disinhibited bursting. EGLU facilitated bursting, suggesting that burst termination was partly controlled by group II mGluRs. All these effects were reversible on washout. It is concluded that activation of group II and III mGluRs differentially modulated rhythmic patterns recorded from motoneurons via network-dependent actions, which probably included decrease in the release of neurotransmitters at key circuit points.

Low micromolar concentrations of 4-aminopyridine facilitate fictive locomotion expressed by the rat spinal cord in vitro

Upregulating the operation of spinal locomotor networks is one mechanism to restore, at least partially, lesion-impaired locomotion. We investigated if the K+ channel blocker 4-aminopyridine (4-AP) could facilitate spinal locomotor networks in addition to its well-known effect on motor nerve conduction. Fictive locomotor patterns were recorded from ventral roots (VRs) of the isolated spinal cord of the neonatal rat. 4-AP (0.1-50 microM) produced synchronous VR oscillations which did not develop into fictive locomotion. These oscillations had network origin, required intact glutamatergic transmission and were probably amplified via electrotonic coupling because of their depression by the selective gap junction blocker carbenoxolone. 4-AP (5 microM) slightly increased input resistance of lumbar motoneurons without affecting their action or resting potentials. Dorsal root (DR) evoked synaptic responses were enhanced (217 +/- 65%) by 5 microM 4-AP without changes in axon conduction. 4-AP (5 microM) accelerated fictive locomotion induced by N-methyl-d-aspartate (NMDA) and serotonin (5-HT) without altering cycle amplitude and facilitated the onset of fictive locomotion in the presence of sub-threshold concentrations of NMDA and 5-HT. Furthermore, in the presence of 4-AP, weak DR stimuli, previously insufficient to activate locomotor patterns, generated alternating VR discharges. Thus, although 4-AP per se could not directly activate the locomotor network of the spinal cord, it could strongly facilitate the locomotor program initiated by neurochemicals or electrical stimuli. These data suggest that the reported improvement by 4-AP in locomotor activity of spinal-injury patients may include activation of locomotor networks when low concentrations of this drug are administered in coincidence with appropriate stimuli.

Depression of windup of spinal neurons in the neonatal rat spinal cord in vitro by an NK3 tachykinin receptor antagonist

The effects of the NK3 tachykinin receptor antagonist SR 142801 on synaptic transmission and spike windup induced by trains of stimuli applied to a dorsal root were investigated with intra- and extracellular recording from the neonatal rat spinal cord in vitro. SR 142801 (10 microM) reduced the depolarization (recorded from lumbar ventral roots) induced by senktide (an NK3 agonist) more strongly than the one evoked by substance P methyl ester (SPMeO; an NK1 agonist). Nevertheless, after a long (>2 h) application time, SR 142801 largely depressed the response to SPMeO as well. When NK1 or NK3 receptors were blocked by >50% in the presence of SR 142801, there was also a significant reduction in the cumulative depolarization induced by repeated stimuli to a single dorsal root. This blocking action by SR 142801 was also observed in the presence of the N-methyl-D-aspartate (NMDA) receptor antagonist D-aminophosphonovalerate (APV) and the calcium channel blocker nifedipine. Intracellular data from lumbar motoneurons showed that the spike windup was the first and most sensitive target for the SR 142801 blocking effect. Increasing stimulus strength to dorsal root fibers could partly surmount such a block. SR 142801 per se had no direct action on fast synaptic transmission, membrane potential, or input resistance. These findings indicate that SR 142801 could lead to an early, large reduction in the windup of action potential discharge by motoneurons, suggesting its ability to suppress the reflex component of central sensitization evoked by repeated dorsal root stimuli.

Plateau potential-dependent windup of the response to primary afferent stimuli in rat dorsal horn neurons

In the spinal cord, repetitive stimulation of nociceptive afferent fibres induces a progressive build-up of dorsal horn neuron (DHN) responses. This 'action potential windup' is used as a cellular model of central sensitization to pain. It partly relies on synaptic plasticity, being reduced after blocking NMDA and neurokinin receptors. Using intracellular recordings in a slice preparation of the rat spinal cord, we have analysed the implication of an additional non-synaptic component of windup. Primary afferent fibres were electrically stimulated in the dorsal root. Of 47 responding deep DHNs, 17 (36%) produced action potential windup and afterdischarge during consecutive periods of repeated stimuli (0.4-1 Hz) activating high- (n = 13 neurons) and low-threshold (n = 6 neurons) afferent fibres. When the NMDA receptors were blocked, the rate of windup did not change. In all neurons, there was an absolute correlation between expression of windup and the production of calcium-dependent plateau potentials. Sensitization of the DHN response, similar to the synaptically induced windup, was obtained by repetitive intracellular injection of depolarizing current pulses. This intracellularly induced windup had the same pharmacology as the plateau potential. Synaptically induced windup was also abolished by nifedipine, an L-type calcium-channel blocker. Expression of plateau properties in DHNs is therefore a critical component of windup, operating downstream of synaptic processes. Being associated with calcium influx, generation of plateau potentials could be a link between short-term plasticity and the long-term modification of DHN excitability associated with central sensitization.

Wind-up of spinal cord neurones and pain sensation: much ado about something?

Wind-up is a frequency-dependent increase in the excitability of spinal cord neurones, evoked by electrical stimulation of afferent C-fibres. Although it has been studied over the past thirty years, there are still uncertainties about its physiological meaning. Glutamate (NMDA) and tachykinin NK1 receptors are required to generate wind-up and therefore a positive modulation between these two receptor types has been suggested by some authors. However, most drugs capable of reducing the excitability of spinal cord neurones, including opioids and NSAIDs, can also reduce or even abolish wind-up. Thus, other theories involving synaptic efficacy, potassium channels, calcium channels, etc. have also been proposed for the generation of this phenomenon. Whatever the mechanisms involved in its generation, wind-up has been interpreted as a system for the amplification in the spinal cord of the nociceptive message that arrives from peripheral nociceptors connected to C-fibres. This probably reflects the physiological system activated in the spinal cord after an intense or persistent barrage of afferent nociceptive impulses. On the other hand, wind-up, central sensitisation and hyperalgesia are not the same phenomena, although they may share common properties. Wind-up can be an important tool to study the processing of nociceptive information in the spinal cord, and the central effects of drugs that modulate the nociceptive system. This paper reviews the physiological and pharmacological data on wind-up of spinal cord neurones, and the perceptual correlates of wind-up in human subjects, in the context of its possible relation to the triggering of hyperalgesic states, and also the multiple factors which contribute to the generation of wind-up.

Dynamics of intrinsic electrophysiological properties in spinal cord neurones

The spinal cord is engaged in a wide variety of functions including generation of motor acts, coding of sensory information and autonomic control. The intrinsic electrophysiological properties of spinal neurones represent a fundamental building block of the spinal circuits executing these tasks. The intrinsic response properties of spinal neurones--determined by the particular set and distribution of voltage sensitive channels and their dynamic non-linear interactions--show a high degree of functional specialisation as reflected by the differences of intrinsic response patterns in different cell types. Specialised, cell specific electrophysiological phenotypes gradually differentiate during development and are continuously adjusted in the adult animal by metabotropic synaptic interactions and activity-dependent plasticity to meet a broad range of functional demands.

Dorsal horn NMDA receptor function is changed after peripheral inflammation

The N-methyl-D-aspartic acid (NMDA) receptor antagonist D, L-2-amino-5-phosphonopentanoic acid (AP5) caused a stronger inhibition of wind-up in single wide dynamic range (WDR) neurons after carrageenan inflammation compared with control neurons without inflammation in the receptive field. This indicates that even a short period (2.5 h) of inflammation induces changes in the function of NMDA receptors. The drug effect was also studied in separate control experiments with few wind-up inducing stimulus trains and little nociceptive input prior to baseline recordings. In these control experiments all evoked responses were reduced by the drug, but the wind-up was significantly increased. A wind-up increase after NMDA receptor antagonism has been reported in two previous studies. Thus, other mechanisms than NMDA receptor stimulation may be more important for the wind-up in not sensitized dorsal horn neurons. As for long-term potentiation, it seems that NMDA receptor antagonists have an increased effect after sensitization. Thus, sensitized and not sensitized dorsal horn neurons may respond differently to an NMDA receptor active drug. In rats nerve stimulation and halothane anaesthesia induced larger evoked responses to afferent stimulation than cutaneous stimulation and urethane anaesthesia, the AP5 effect was however similar.

The tachykinin NK1 receptor antagonist GR205171 abolishes the retching activity of neurons comprising the central pattern generator for vomiting in dogs

Tachykinin NK1 receptor antagonists are known to act centrally and to have broad-spectrum antiemetic effects, but their precise site of action has not yet been defined. To identify this site, the effects of the NK1 receptor antagonist GR205171 on the activities of neurons comprising the central pattern generator (CPG) for vomiting were observed in decerebrate paralyzed dogs. A non-respiratory neuron in each of nine dogs was considered to be a CPG neuron based on its response to abdominal vagal stimulation, its location in the CPG area in the reticular formation dorsomedial to the retrofacial nucleus, its firing patterns in prodromal and retching phases and its response to apomorphine. In response to vagal stimulation at 3-10 Hz, the firing of these neurons transiently increased at the onset of stimulation (fast component), gradually increased again (slow component), and finally developed into rhythmic bursts synchronous with retching bursts of the phrenic and abdominal muscle nerves. GR205171 (25-50 microg/kg, i.v.) abolished the slow component and retching bursts in the neurons, and the retching activities of both nerves, but did not change the fast component. The responses of these neurons to repetitive pulse-train vagal stimulation exhibited a vigorous 'wind-up' and finally developed into retching bursts. Both the 'wind-up' phenomenon and retching bursts disappeared after the application of GR205171. These results suggest that the site of the antiemetic action of NK1 receptor antagonists is located in the CPG or in the pathway connecting the solitary nucleus to the CPG.

The tachykinin NK1 receptor antagonist GR205171 prevents vagal stimulation-induced retching but not neuronal transmission from emetic vagal afferents to solitary nucleus neurons in dogs

Tachykinin NK1 receptor antagonists injected into the medulla oblongata are known to abolish vomiting induced by vagal afferent stimulation. Emetic vagal afferents have been shown to synapse with neurons in the medial solitary nucleus (mNTS), which suggests that substance P is a transmitter in the synapse. To examine this possibility, the effects of GR205171, an NK1 receptor antagonist, on retching and mNTS neuronal responses to the stimulation of abdominal vagal afferents were investigated in decerebrate dogs. GR205171 (0.05-0.7 mg kg-1, i.v.) abolished retching induced by either vagal or mNTS stimulation within 5 min. Firing of mNTS neurons in response to pulse-train and sustained vagal stimulation did not change even after the abolition of retching. Similarly, GR205171 did not have any effects on mNTS evoked potentials induced by pulse-train vagal stimulation. In about 20% of mNTS neurons, the peak firing frequency was facilitated to about 150% with repetitive pulse-train vagal stimulation. This facilitation remained even after the abolition of retching. Administration of GR205171 (1 mg ml-1, 30 microliters) into the 4th ventricle abolished retching, with latencies in excess of 120 min These results suggest that substance P does not participate in synaptic transmission between emetic vagal afferents and mNTS neurons in dogs.

Sensitization of pain pathways in the spinal cord: cellular mechanisms

Sensitization is manifested as an increased response of neurones to a variety of inputs following intense or noxious stimuli. It is one of the simplest forms of learning and synaptic plasticity and it represents an important feature of nociception. In the spinal cord, repeated stimulation (at constant strength) of dorsal root afferents including nociceptive C fibres can elicit a progressive increase in the number of action potentials generated by motoneurones and interneurones. This phenomenon is termed "action potential windup" and is used as a cellular model of pain sensitization developing at the level of the central nervous system. Understanding the mechanisms responsible for windup generation might allow clarification of the cellular mechanisms of pain signalling and development of new strategies for pain treatment. Action potential windup is observed in a minority of cells only, indicating that certain cell-specific mechanisms are responsible for its generation. The most reliable index to predict windup generation is the rate at which the membrane potential is depolarized during repetitive stimulation. This phenomenon has been proposed to be due to gradual recruitment of NMDA receptor activity, to summation of slow excitatory potentials mediated by substance P (and related peptides) or to facilitation of slow calcium channels by metabotropic glutamate receptors. Little is known about the role of synaptic inhibition in windup, although it should not be underestimated. Each theory per se is unable to account for all the experimental observations. Since NMDA receptors are involved in many forms of synaptic plasticity, additional mechanisms such as summation of slow peptidergic potentials, facilitation of slow Ca2+ currents and disinhibition are proposed as necessary to impart specificity to pain-induced sensitization. These additional mechanisms might be species specific and change during development or chronic pain states.

The tachykinin NK1 receptor. Part II: Distribution and pathophysiological roles

The tachykinin NK1 receptor is widely distributed in both the central and peripheral nervous system. In the CNS, NK1 receptors have been implicated in various behavioural responses and in regulating neuronal survival and degeneration. Moreover, central NK1 receptors regulate cardiovascular and respiratory function and are involved in activating the emetic reflex. At the spinal cord level, NK1 receptors are activated during the synaptic transmission, especially in response to noxious stimuli applied at the receptive field of primary afferent neurons. Both neurophysiological and behavioural evidences support a role of spinal NK1 receptors in pain transmission. Spinal NK1 receptors also modulate autonomic reflexes, including the micturition reflex. In the peripheral nervous system, tachykinin NK1 receptors are widely expressed in the respiratory, genitourinary and gastrointestinal tracts and are also expressed by several types of inflammatory and immune cells. In the cardiovascular system, NK1 receptors mediate endothelium-dependent vasodilation and plasma protein extravasation. At respiratory level, NK1 receptors mediate neurogenic inflammation which is especially evident upon exposure of the airways to irritants. In the carotid body, NK1 receptors mediate the ventilatory response to hypoxia. In the gastrointestinal system, NK1 receptors mediate smooth muscle contraction, regulate water and ion secretion and mediate neuro-neuronal communication. In the genitourinary tract, NK1 receptors are widely distributed in the renal pelvis, ureter, urinary bladder and urethra and mediate smooth muscle contraction and inflammation in response to noxious stimuli. Based on the knowledge of distribution and pathophysiological roles of NK1 receptors, it has been anticipated that NK1 receptor antagonists may have several therapeutic applications at central and peripheral level. At central level, it is speculated that NK1 receptor antagonists could be used to produce analgesia, as antiemetics and for treatment of certain forms of urinary incontinence due to detrusor hyperreflexia. In the peripheral nervous system, tachykinin NK1 receptor antagonists could be used in several inflammatory diseases including arthritis, inflammatory bowel diseases and cystitis. Several potent tachykinin NK1 receptor antagonists are now under evaluation in the clinical setting, and more information on their usefulness in treatment of human diseases will be available in the next few years.