Long-term potentiation and depression in the intermediate gray matter of rat spinal cord in vitro

Genes associated with gray matter volume alterations in schizophrenia

Although both schizophrenia and gray matter volume (GMV) show high heritability, however, genes accounting for GMV alterations in schizophrenia remain largely unknown. Based on risk genes identified in schizophrenia by the genome-wide association study of the Schizophrenia Working Group of the Psychiatric Genomics Consortium, we used transcription-neuroimaging association analysis to test that which of these genes are associated with GMV changes in schizophrenia. For each brain tissue sample, the expression profiles of 196 schizophrenia risk genes were extracted from six donated normal brains of the Allen Human Brain Atlas, and GMV differences between patients with schizophrenia and healthy controls were calculated based on five independent case-control structural MRI datasets (276 patients and 284 controls). Genes associated with GMV changes in schizophrenia were identified by performing cross-sample spatial correlations between expression levels of each gene and case-control GMV difference derived from the five MRI datasets integrated by harmonization and meta-analysis. We found that expression levels of 98 genes consistently showed significant cross-sample spatial correlations with GMV changes in schizophrenia. These genes were functionally enriched for chemical synaptic transmission, central nervous system development, and cell projection. Overall, this study provides a set of genes possibly associated with GMV changes in schizophrenia, which could be used as candidate genes to explore biological mechanisms underlying the structural impairments in schizophrenia.

Activity-dependent plasticity of spinal circuits in the developing and mature spinal cord

Part of the development and maturation of the central nervous system (CNS) occurs through interactions with the environment. Through physical activities and interactions with the world, an animal receives considerable sensory information from various sources. These sources can be internally (proprioceptive) or externally (such as touch and pressure) generated senses. Ample evidence exists to demonstrate that the sensory information originating from large diameter afferents (Ia fibers) have an important role in inducing essential functional and morphological changes for the maturation of both the brain and the spinal cord. The Ia fibers transmit sensory information generated by muscle activity and movement. Such use or activity-dependent plastic changes occur throughout life and are one reason for the ability to acquire new skills and learn new movements. However, the extent and particularly the mechanisms of activity-dependent changes are markedly different between a developing nervous system and a mature nervous system. Understanding these mechanisms is an important step to develop strategies for regaining motor function after different injuries to the CNS. Plastic changes induced by activity occur both in the brain and spinal cord. This paper reviews the activity-dependent changes in the spinal cord neural circuits during both the developmental stages of the CNS and in adulthood.

Tumor suppressor menin mediates peripheral nerve injury-induced neuropathic pain through potentiating synaptic plasticity

Synaptic plasticity is a crucial step in the development of central sensitization in the pathogenesis of neuropathic hyperalgesia. Menin, the product of the multiple endocrine neoplasia type 1 (MEN1) gene, possesses the property of synaptogenesis which plays an essential role in neuronal activity. We tested the contributing role of spinal menin in peripheral nerve injury-induced neuropathic hypersensitivity through modulating neuronal synaptic plasticity. After approval by the Institutional Animal Care and Use Committee, nociceptive responses were detected with von Frey filaments and thermal plate after spared nerve injury in C57BL/6 mice who were treated with either intrathecal antisense oligonucleotide of MEN1 (ASO) or vehicle. Extracellular spontaneous discharge frequency, field excitatory postsynaptic potential (fEPSP), and monosynaptic excitatory postsynaptic currents (EPSCs) were measured electrophysiologically. Intrathecal ASO alleviated nerve injury-induced mechanical and thermal hypersensitivity. Upregulated spinal menin after nerve injury colocalized with NeuN in the superficial laminae; genetic knockdown of spinal menin reduced nerve injury induced in vivo spontaneous activity and instantaneous frequency and in vitro field potentials; ASO decreased the frequency and amplitude of monosynaptic EPSCs, and reduced synaptic strength and total charge. Collectively, these findings highlight the role of upregulated neuronal menin in the spinal cord in potentiating spinal synaptic plasticity in peripheral nerve injury-induced neuropathic hypersensitivity.

Low dose ketamine: a therapeutic and research tool to explore N-methyl-D-aspartate (NMDA) receptor-mediated plasticity in pain pathways

Ketamine is a dissociative anaesthetic that has been used in the clinic for many years. At low, sub-anaesthetic doses, it is a relatively selective and potent antagonist of the N-methyl-D-aspartate (NMDA) receptor. It belongs to the class of uncompetitive antagonists and blocks the receptor by binding to a specific site within the NMDA receptor channel when it is open. Like other compounds of this class, ketamine can cause hallucinations or other untoward central effects which limit its use in the clinic. Nevertheless, because of the evidence on the importance of NMDA receptor-mediated pLasticity in chronic pain, low doses of ketamine have been explored in a wide range of pain conditions. The majority of studies with ketamine have shown efficacy; however, it has not been possible to separate safely the pain relief from the side effects of the drug. Hence, clinical use of ketamine as a pain treatment is very limited. Nevertheless, ketamine has served as a useful tool to provide a compelling rationale for developing other NMDA antagonists. Some of the new compounds of this class, particularly those acting at the NR2B subtype of the NMDA receptor, have shown promise in preclinical and clinical studies.

Functional plasticity following spinal cord lesions

Spinal cord injury results in marked modification and reorganization of several reflex pathways caudal to the injury. The sudden loss or disruption of descending input engenders substantial changes at the level of primary afferents, interneurons, and motoneurons thus dramatically influencing sensorimotor interactions in the spinal cord. As a general rule reflexes are initially depressed following spinal cord injury due to severe reductions in motoneuron excitability but recover and in some instances become exaggerated. It is thought that modified inhibitory connections and/or altered transmission in some of these reflex pathways after spinal injury as well as the recovery and enhancement of membrane properties in motoneurons underlie several symptoms such as spasticity and may explain some characteristics of spinal locomotion observed in spinally transected animals. Indeed, after partial or complete spinal lesions at the last thoracic vertebra cats recover locomotion when the hindlimbs are placed on a treadmill. Although some deficits in spinal locomotion are related to lesion of specific descending motor pathways, other characteristics can also be explained by changes in the excitability of reflex pathways mentioned above. Consequently it may be the case that to reestablish a stable walking pattern that modified afferent inflow to the spinal cord incurred after injury must be normalized to enable a more normal re-expression of locomotor rhythm generating networks. Indeed, recent evidence demonstrates that step training, which has extensively been shown to facilitate and ameliorate locomotor recovery in spinal animals, directly influences transmission in simple reflex pathways after complete spinal lesions.

Long-term potentiation in spinal nociceptive systems--how acute pain may become chronic

Chronic pain is a major problem since it is difficult to treat and the understanding of the underlying neurobiology is sparse. The mechanisms underpinning the transition of acute into chronic pain remain unclear. However, long-term potentiation (LTP) in spinal nociceptive systems may be one such mechanism. Here, we briefly review the literature regarding LTP in spinal nociceptive systems including our own data on LTP in deep convergent nociceptive neurons. Furthermore, we discuss the role of this phenomenon in understanding the neurobiology of chronic pain and the possible therapeutic implications.

Activation of spinal d1/d5 receptors induces late-phase LTP of C-fiber-evoked field potentials in rat spinal dorsal horn

Long-term potentiation (LTP) of C-fiber-evoked field potentials in spinal dorsal horn may be relevant to pathological pain. Our previous work has shown that the late phase of the spinal LTP is protein synthesis-dependent. Considerable evidence has accumulated that dopamine D1/D5 receptors are important for late-phase LTP in hippocampus. In this study, the role of D1/D5 receptors in LTP of C-fiber-evoked field potentials in spinal dorsal horn was evaluated in urethan-anesthetized Sprague-Dawley rats. We found the following. 1) Spinal application of SKF 38393, a D1/D5 receptor agonist, induced a slowly developed LTP of C-fiber-evoked field potentials, lasting for >10 h, and the effect was blocked by the D1/D5 antagonist SCH 23390, whereas a D2 receptor agonist (quinpirole) induced depression of C-fiber responses, lasting for 2 h. 2) The potentiation produced by D1/D5 receptor agonist occluded the late phase but not the early phase of the spinal LTP produced by tetanic stimulation. 3) SCH 23390 selectively depressed the late-phase LTP, when applied 40 min before tetanic stimulation. 4) The D1/D5 agonist-induced potentiation is blocked by the protein synthesis inhibitor anisomycin. 5) Activation of protein kinase A by spinal application of 8-Br-cAMP also induced spinal LTP, and the action occluded the potentiation induced by the D1/D5 receptor agonist. These results suggest that the spinal D1/D5 receptors participate in the protein synthesis-dependent late-phase LTP of C-fiber-evoked field potentials in spinal dorsal horn through the cAMP signaling pathway.

Central neuronal plasticity, low back pain and spinal manipulative therapy

OBJECTIVE

Recent experimental evidence demonstrating neuronal/synaptic plasticity and, in particular, long-term potentiation (LTP) and long-term depression (LTD) in spinal neurons is reviewed. The implications of these studies for possible mechanistic explanations of low back pain and its remediation by spinal manipulative therapy (SMT) are explored. Brief descriptions of LTP and LTD and elaboration of the key roles of calcium, glutamate, and glutamate receptors in LTP/LTD are provided as separate appendices.

DATA SOURCES

The referenced articles regarding LTP/LTD in spinal cord neurons and neuronal plasticity, in general, were identified from accumulated review of the neuroscience literature. Publications cited from chiropractic sources relevant to central neuronal plasticity and LTP/LTD were identified using the Index to Chiropractic Literature and informal review.

STUDY SELECTION

Experimental studies examining LTP/LTD mechanisms in spinal neurons and more general references useful as an introduction to central neuronal plasticity and LTP/LTD are included.Data Extraction Experimental evidence presented in this review has been previously published and illustrates neuronal plasticity from an animal model for low back pain.

DATA SYNTHESIS

Both in vitro and in vivo evidence identifying LTP and LTD in dorsal horn nociceptive neurons is reviewed. Of special interest are studies showing LTP in response to intense noxious stimulation and reports that Adelta-mechanosensitive afferent activation can reverse an existing LTP condition in dorsal horn neurons.

CONCLUSIONS

The potential involvement of LTP in low back pain is discussed and a role for LTD in spinal manipulative therapy is proposed. The need for future studies is identified in the areas of spatial and temporal changes in symptomatology post-SMT of the low back; combining, sequencing, and comparing several therapeutic approaches; and demonstrating LTD in spinal cord neurons post-SMT-like stimulation.

Instrumental learning within the spinal cord: VI. The NMDA receptor antagonist, AP5, disrupts the acquisition and maintenance of an acquired flexion response

Prior studies have shown that circuits within the spinal cord can support a simple form of instrumental learning. Spinally transected rats are given shock to one hind leg whenever the leg is extended. This response-outcome contingency causes an increase in flexion duration. The present experiments examine whether the NMDA receptor is involved in the acquisition and maintenance of this instrumental response. Experiment 1 showed that the NMDA receptor antagonist 2-amino-5-phosphonovalerate acid (AP5) reduces instrumental responding in a dose-dependent fashion. Experiment 2 showed that AP5 given after training eliminates the increase in flexion duration. The results implicate the NMDA receptor in the acquisition and maintenance of spinally mediated instrumental behavior.

Effects of corticotropin-releasing factor on plasticity of optically recorded neuronal activity in the substantia gelatinosa of rat spinal cord slices

We examined the effects of corticotropin-releasing factor (CRF) on plasticity of optically recorded neuronal activity in the substantia gelatinosa (lamina II) of 12-18-day-old rat spinal cord slices stained with a voltage-sensitive dye. Single-pulse test stimulation to the dorsal root that activated A and C fibres evoked prolonged (>100 ms) light-absorption change in the lamina II. This response represents the gross membrane potential change of all elements along the slice depth. After conditioning high-frequency stimulation of A-fibre-activating strength, test stimulus elicited less neuronal activity [-27+/-1% (7), (average+/-SE (n)), P<0.01 (*) at 45-60 min after conditioning]. When CRF (1 microM, 10 min) was applied during conditioning, the neuronal activity was facilitated rather than suppressed [+20+/-3% (5), P<0.05]. CRF alone exhibited insignificant effect [-5+/-1% (4), P=0.2]. In the presence of the inhibitory amino acid antagonists bicuculline (1 microM) and strychnine (0.3 microM) in the perfusate, in contrast, the conditioning facilitated it [+27+/-1% (12)*], and CRF treatment during conditioning inhibited the facilitation dose-dependently [0.1 microM: +18+/-2% (5)*, 1 microM: +13+/-1% (7)*]. Although interneuronal actions might contribute, these results suggest that CRF may have dual effects on excitatory synaptic transmission within the lamina II depending upon cellular conditions: a conversion from the induction of long-term depression to long-term potentiation (LTP), and inhibition of LTP induction. Since the LTP is thought to be responsible at least in part for the persistent pain, CRF could regulate the induction.

Roles of CaMKII, PKA, and PKC in the induction and maintenance of LTP of C-fiber-evoked field potentials in rat spinal dorsal horn

Long-term potentiation (LTP) of C-fiber-evoked field potentials in spinal dorsal horn may be relevant to hyperalgesia, an increased response to noxious stimulation. The mechanism underlying this form of synaptic plasticity is, however, still unclear. Considerable evidence has shown that calcium/calmodulin-dependent protein kinase II (CaMKII), protein kinase A (PKA), and protein kinase C (PKC) are important for LTP in hippocampus. In this study, the roles of these three protein kinases in the induction and maintenance of LTP of C-fiber-evoked field potentials were evaluated by application of specific inhibitors of CaMKII (KN-93 and AIP), PKA (Rp-CPT-cAMPS), and PKC (chelerythrine and Gö 6983) at the recording segments before and after LTP induction in urethane-anesthetized Sprague-Dawley rats. We found both KN-93 and AIP, when applied at 30 min prior to tetanic stimulation, completely blocked LTP induction. At 30 min after LTP induction, KN-93 and AIP reversed LTP completely, and at 60 min after LTP induction, they depressed spinal LTP in most rats tested. Three hours after LTP induction, however, KN-93 or AIP did not affect the spinal LTP. Rp-CPT-cAMPS, chelerythrine, and Gö 6983 blocked the spinal LTP when applied at 30 min before tetanic stimulation and reversed LTP completely at 15 min after LTP induction. In contrast, at 30 min after LTP induction, the drugs never affected the spinal LTP. These results suggest that activation of CaMKII, PKA, and PKC may be crucial for the induction and the early-phase but not for the late-phase maintenance of the spinal LTP.

Involvement of intracellular calcium and protein phosphatases in long-term depression of A-fiber-mediated primary afferent neurotransmission

Long-term depression (LTD) of monosynaptic and polysynaptic excitatory postsynaptic potentials (EPSPs) in substantia gelatinosa (SG) neurons can be induced by brief high-frequency electrical stimulation (HFS, 300 pulses at 100 Hz) of primary afferent fibers in dorsal roots. Here we examined the possible cellular mechanism underlying spinal LTD. Conventional intracellular recordings were made from SG neurons in a transverse slice-dorsal root preparation of the young rat lumbar spinal cord. LTD of both monosynaptic and polysynaptic EPSPs was induced in 16 of 24 SG neurons by HFS of dorsal root in either the presence or absence of the GABA(A) receptor antagonist bicuculline and the glycine receptor antagonist strychnine. Loading the postsynaptic cell with BAPTA, an intracellular Ca(2+) chelator, almost completely blocked the induction of LTD. Induction of LTD was abolished by bath application of calyculin A (100 nM), a potent inhibitor of protein phosphatases 1 and 2A. These results indicate that: (i) a rise in postsynaptic Ca(2+) is necessary for LTD induction, (ii) synaptic activation of protein phosphatases 1 and 2A plays an important role in the induction of LTD of primary afferent A-fiber neurotransmission in the young rat spinal cord, and (iii) the effect of LTD may be physiologically relevant for transmission and integration of sensory information, including nociception.

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.

Changes in post-tetanic potentiation of A-fiber dorsal horn field potentials parallel the development and disappearance of neuropathic pain after sciatic nerve ligation in rats

Significant plastic changes in spinal nociceptive processing appear to accompany peripheral nerve injury or inflammation. Using a well-established model of neuropathic pain, we have recently reported that loose ligation of the sciatic nerve was accompanied by a long-lasting post-tetanic potentiation of sciatic-evoked A-fiber superficial dorsal horn field potentials. In the present study we demonstrate that the typical disappearance of thermal hyperalgesia as a behavioral sign of neuropathic pain several weeks after loose sciatic nerve ligation is accompanied by the loss of the long-lasting potentiation. These data suggest that a significant but reversible shift in the processing of sensory information in the spinal dorsal horn follows peripheral nerve injury, and lend further support to the notion that long-lasting synaptic plasticity may contribute to the development of neuropathic pain.

Mu opiates inhibit long-term potentiation induction in the spinal cord slice

Long-term potentiation (LTP) involves a prolonged increase in neuronal excitability following repeated afferent input. This phenomenon has been extensively studied in the hippocampus as a model of learning and memory. Similar long-term increases in neuronal responses have been reported in the dorsal horn of the spinal cord following intense primary afferent stimulation. In these studies, we utilized the spinal cord slice preparation to examine effects of the potently antinociceptive mu opioids in modulating primary afferent/dorsal horn neurotransmission as well as LTP of such transmission. Transverse slices were made from the lumbar spinal cord of 10- to 17-day-old rats, placed in a recording chamber, and perfused with artificial cerebrospinal fluid also containing bicuculline (10 microM) and strychnine (1 microM). Primary afferent activation was achieved in the spinal slice by electrical stimulation of the dorsal root (DR) or the tract of Lissauer (LT) which is known to contain a high percentage of small diameter fibers likely to transmit nociception. Consistent with this anatomy, response latencies of LT-evoked field potentials in the dorsal horn were considerably slower than the response latencies of DR-evoked potentials. Only LT-evoked field potentials were found to be reliably inhibited by the mu opioid receptor agonist [D-Ala(2), N-Me-Phe(4), Gly(5)] enkephalin-ol (DAMGO, 1 microM), although evoked potentials from both DR and LT were blocked by the AMPA/kainate glutamate receptor antagonist 6-cyano-7-nitroquinoxalene-2,3-dione. Moreover repeated stimulation of LT produced LTP of LT- but not DR-evoked potentials. In contrast, repeated stimulation of DR showed no reliable LTP. LTP of LT-evoked potentials depended on N-methyl-D-aspartate (NMDA) receptor activity, in that it was attenuated by the NMDA antagonist APV. Moreover, such LTP was inhibited by DAMGO interfering with LTP induction mechanisms. Finally, in whole cell voltage-clamp studies of Lamina I neurons, DAMGO inhibited excitatory postsynaptic current (EPSC) response amplitudes from LT stimulation-evoked excitatory amino acid release but not from glutamate puffed onto the cell and increased paired-pulse facilitation of EPSCs evoked by LT stimulation. These studies suggest that mu opioids exert their inhibitory effects presynaptically, likely through the inhibition of glutamate release from primary afferent terminals, and thereby inhibit the induction of LTP in the spinal dorsal horn.

Long-term potentiation in the inferior colliculus studied in rat brain slice

The purpose of this study is to determine whether long-term potentiation (LTP) can be induced in the central nucleus of the inferior colliculus (ICC) by electrical stimulation of the lateral lemniscus. If LTP can be induced, is it mediated by N-methyl-D-aspartate (NMDA) and/or other receptors? Brain slices of the ICC were obtained from 14-35 day old Wistar rats. The field potentials were recorded from the ICC after GABAergic and glycinergic inhibition was suppressed. Following tetanic stimulation (50 Hz, 20 s), the amplitude of the response was increased to about 146% of control response for at least 30 min. LTP was observed in about 78% of the cases tested. Induction of LTP in the ICC required activation of both NMDA and gamma-aminobutyric acid (GABA)(B) receptors. GABAergic inhibitory postsynaptic potentials (IPSPs) were blocked by the GABA(A) receptor antagonist, but not by the GABA(B) receptor antagonist. The IPSPs were decreased by the GABA(B) receptor agonist, baclofen. The intrinsic postsynaptic membrane properties were not affected by baclofen. These results suggest that GABAergic inhibition in the ICC is mediated only by GABA(A) receptors, but that it is modulated by presynaptic GABA(B) receptors. The GABA(B) receptors in the ICC may suppress GABAergic inhibition and promote induction of LTP.

Robust changes of afferent-induced excitation in the rat spinal dorsal horn after conditioning high-frequency stimulation

We investigated the neuronal plasticity in the spinal dorsal horn and its relationship with spinal inhibitory networks using an optical-imaging method that detects neuronal excitation. High-intensity single-pulse stimulation of the dorsal root activating both A and C fibers evoked an optical response in the lamina II (the substantia gelatinosa) of the dorsal horn in transverse slices of 12- to 25-day-old rat spinal cords stained with a voltage-sensitive dye, RH-482. The optical response, reflecting the net neuronal excitation along the slice-depth, was depressed by 28% for more than 1 h after a high-frequency conditioning stimulation of A fibers in the dorsal root (3 tetani of 100 Hz for 1 s with an interval of 10 s). The depression was not induced in a perfusion solution containing an NMDA antagonist, DL-2-amino-5-phosphonovaleric acid (AP5; 30 microM). In a solution containing the inhibitory amino acid antagonists bicuculline (1 microM) and strychnine (3 microM), and also in a low Cl(-) solution, the excitation evoked by the single-pulse stimulation was enhanced after the high-frequency stimulation by 31 and 18%, respectively. The enhanced response after conditioning was depotentiated by a low-frequency stimulation of A fibers (0.2-1 Hz for 10 min). Furthermore, once the low-frequency stimulation was applied, the high-frequency conditioning could not potentiate the excitation. Inhibitory transmissions thus regulate the mode of synaptic plasticity in the lamina II most likely at afferent terminals. The high-frequency conditioning elicits a long-term depression (LTD) of synaptic efficacy under a greater activity of inhibitory amino acids, but it results in a long-term potentiation (LTP) when inhibition is reduced. The low-frequency preconditioning inhibits the potentiation induction and maintenance by the high-frequency conditioning. These mechanisms might underlie robust changes of nociception, such as hypersensitivity after injury or inflammation and pain relief after electrical or cutaneous stimulation.

Long-term changes in sciatic-evoked A-fiber dorsal horn field potentials accompany loose ligation of the sciatic nerve in rats

The goal of the present study was to examine whether loose ligation of the sciatic nerve was associated with long-term changes in neuronal excitability in the spinal dorsal horn in urethane-anesthetized rats. The sciatic nerve was stimulated with 0. 1 ms long pulses at 1 stimulus/5 min, and the evoked dorsal horn field potentials remained stable in the absence of tetanic stimulation. In one set of control and ligated animals, high-frequency tetanic stimulation was applied to the nerve at 50 Hz (one 400 ms train of twenty 0.1 ms pulses), and the field potentials were recorded again (1 stimulus/5 min) for up to 4 h post-tetanus. In control animals, this protocol produced significant increases in field potential amplitudes at 15, 30 and 60 min post-tetanus. Interestingly, after this time the evoked field potentials began to decrease, and attained less than 50% of their pre-tetanic values at 240 min post-tetanus. In contrast, in ligated rats the pattern of post-tetanic potentiation was significantly different as the increases in amplitude persisted, and at 240 min post-tetanus the field potentials were almost twice their baseline values. In another set of control and ligated animals, low-frequency tetanic stimulation was applied at 5 Hz (one 400 ms train of two 0.1 ms pulses). Again a differential pattern of post-tetanic responses between control and ligated rats was seen. In control animals, a significant decrease in amplitude was evident within 30 min, and the depression became progressively more pronounced as the field potentials attained about a quarter of their baseline values at 180 min, and remained at these low levels at 240 min post-tetanus. On the other hand, in ligated animals, the depression was not significant, and at 240 min post-tetanus the field potentials were still at about 80% of their baseline values. These data demonstrate that long-term changes in spinal dorsal horn neuronal excitability accompany sciatic ligation to perhaps contribute to the development of neuropathic pain. These changes may result from a lessening of normally strong inhibitory processes in the spinal dorsal horn to generate conditions which favor post-tetanic potentiation over depression of dorsal horn neuronal responses.

Robust suppression of afferent-induced excitation in the rat spinal dorsal horn after conditioning low-frequency stimulation

The neuronal plasticity in the spinal dorsal horn induced after conditioning low-frequency stimulation of afferent A fibers, and its relationship with spinal inhibitory networks, was investigated with an optical-imaging method that detects neuronal excitation. High-intensity single-pulse stimulation of the dorsal root activating both A and C fibers evoked an optical response in the dorsal horn in transverse slices of 12- to 25-day-old rat spinal cords stained with a voltage-sensitive dye, RH-482. The optical response, reflecting the net excitation of neuronal elements along the thickness of each slice, was suppressed after a conditioning low-frequency stimulation (0.2-1 Hz for 10 min) to A fibers in the dorsal root. The degree of suppression was largest in the lamina II of the dorsal horn (48% reduction), where the majority of C fibers terminate, and much less in the deeper dorsal horn (5% reduction in laminae III-IV). The onset of suppression was somewhat slow; after the low-frequency stimulation, the magnitude of excitation gradually decreased, reached the maximum effect 30 min after the conditioning, and remained at the suppressed level for >1 h. Suppression was not observed when the low-frequency stimulation was given during a 20-min perfusion with a solution containing an NMDA-receptor antagonist, DL-2-amino-5-phosphonovaleric acid (30 microM). A brief application of an opioid-receptor antagonist, naloxone (0.5 microM), inhibited the induction, but not the maintenance, of low-frequency stimulus-induced suppression. However, treatments with the GABA(A) receptor antagonist bicuculline (1 microM) and the glycine receptor antagonist strychnine (0.3 microM) did not affect suppression induction and maintenance. In conclusion, conditioning low-frequency stimulation to A fibers interferes with the afferent-induced excitation in the dorsal horn. The low-frequency stimulation-induced suppression is maintained by a reduction of glutamatergic excitatory transmissions in the dorsal horn, not by an enhanced inhibition. Activation of the spinal opioid-mediated system by low-frequency stimulation, but not the inhibitory amino acid-mediated system, is necessary to initiate robust suppression. The long-term depression of afferent synaptic efficacy onto excitatory interneurons likely takes the primary role in the robust suppression of neuronal excitation in the dorsal horn.

Locomotor plasticity after spinal injury in the chick

Functional recovery after spinal cord injury likely depends, in part, on the reorganization of undamaged spinal circuitry. Segmental afferent input from the limbs remains largely intact after spinal injury and may provide an important source of activation and regulation of the spinal circuits that have lost descending input as a result of the injury. This purpose of this study was to investigate the contribution of cutaneous afferent inputs to the recovery of motor function after spinal injury in the chick. After lateral thoracic spinal hemisection, the motion of the ipsilateral limb was impaired during both walking and swimming. By 2 weeks postoperatively, limb motion recovered to preoperative values for walking but not for swimming. It was hypothesized that phasic afferent inputs experienced during walking, but not swimming, contributed to recovery of limb motion during walking. When a source of phasic cutaneous input was provided during swim training sessions, limb motion gradually improved to preoperative values. After 2 weeks of training, this improved motion was retained even after the source of cutaneous stimulation was removed. The proposed mechanism is an experience-dependent strengthening of the circuits activated during the improved limb motion, leading to a permanent change in limb action during swimming. Thus, the afferent inputs experienced during movement repetition are important during the acquisition of learned movements after spinal injury. These results are discussed in terms of behavioral, physiological, and anatomical evidence for spinal plasticity in other species. It is concluded that the spinal cord has significant plastic capabilities, and efforts should be directed toward maximizing the contribution of this plasticity to functional recovery after spinal cord injury.

Long term potentiation of single WDR neurons in spinalized rats

We report long-term potentiation (LTP) in single spinal wide dynamic range (WDR) neurons in urethane-anaesthetized spinalized rats with a complete neuromuscular blockade. Peripheral influences were excluded by a complete lidocaine block distal to the stimulation site on the sciatic nerve. As previously shown A-beta fibre evoked responses were not increased by the tetanic stimulation when there was a neuromuscular blockade during the experiment. Spinalization, excluding influences from supraspinal structures, increased all firing responses, and the LTP of C-fibre evoked responses when calculated in number of action potentials compared to intact animals and to previous studies. Furthermore, an LTP of the post discharge was observed after spinalization. An LTP of the post discharge has previously not been reported. Therefore, we conclude that LTP in the dorsal horn normally seems to be inhibited by descending pathways.

Expression of long-term potentiation in single wide dynamic range neurons in the rat is sensitive to blockade of glutamate receptors

Plasticity in pain control systems may play an important role in clinical pain and some mechanisms of plasticity may be similar to those involved in learning. In this study we investigate the importance of alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptors for the maintenance of long-term potentiation (LTP) in wide dynamic range (WDR) neurons. Doses of 6-nitro-7-sulphomoylbenzoylquinoxaline-2,3-dione disodium (NBQX) and D-Q-amino-5-phosphonopentanoic acid (D-AP5) equipotent in reducing C-fiber mediated responses in controls, reduced the established LTP by about 50 and 80%, respectively. The drug effect lasted less than 1 h in controls. After induction of LTP, NBQX caused a reversible reduction of the potentiation. D-AP5, however, caused a stronger attenuation of the LTP, outlasting the effect of the drug in controls. We suggest that both pre-emptive analgesia preventing LTP induction and an early reduction of the excitation of neurons is important for the inhibition of LTP and central sensitization. Thus, it is possible that an early antinociceptive treatment preventing an excessive excitation of neurons in the dorsal horn may be of importance in preventing longlasting and pathological pain states.

Low-affinity kainate receptors and long-lasting depression of NMDA-receptor-mediated currents in rat superficial dorsal horn

In an in vitro spinal cord slice preparation whole cell electrophysiological recordings of rat superficial dorsal horn neurons responding differentially to glutamate (Glu) and N-methyl-D-aspartate (NMDA) were investigated systematically for the role of kainate (KA) receptors in modulating their activity. In these neurons, coapplication of Glu and NMDA, as well as application of Glu immediately before NMDA, induced long- and short-lasting depressions of NMDA-induced currents as well as depression of NMDA-receptor-mediated excitatory postsynaptic currents. KA applied before NMDA mimicked Glu-induced attenuating effects. Furthermore, the low-affinity KA receptor antagonist 5-nitro-6,7,8,9- tetrahydrobenzo[G]indole-2,3-dione-3-oxime potentiated Glu-induced NMDA-receptor-mediated currents in neurons responding differentially to Glu and NMDA. These results provide evidence for a novel mechanism, which may relate to classical long-term depression, involving low-affinity KA receptors in long-lasting modulation of NMDA-receptor-mediated currents. This implies a physiological role of KA receptors in long-term modulation of sensory transmission in the superficial dorsal horn of rat spinal cord.

Macromolecular synthesis contributes to nociceptive response to subcutaneous formalin injection in mice

Subcutaneous formalin injection into the hindpaw produces two phases of nociceptive response: phase 1 and phase 2. Activation of N-methyl-D-aspartate (NMDA) receptors in the spinal cord during phase 1 is important for phase 2. We report here that phase 2 but not phase 1 requires new RNA and protein synthesis in the spinal cord.

Induction of long-term potentiation at spinal synapses by noxious stimulation or nerve injury

Use-dependent long-term potentiation of synaptic strength (LTP) is an intensively studied model for learning and memory in vertebrates. Induction of LTP critically depends on the stimulation parameters of presynaptic fibres with synchronous high-frequency bursts being most effective at many central synapses. It is, however, not known whether naturally occurring discharge patterns may induce LTP and whether LTP has any biological function in sensory systems. Here we have investigated the LTP of excitatory synaptic transmission between primary afferent C-fibres, many of which are nociceptors, and neurons in rat superficial spinal dorsal horn. LTP that lasted for 4-6 h could not only be induced by electrical stimulation of sural nerve but also by natural stimulation of heat-, mechano- or chemosensitive nociceptors in the skin or by acute nerve injury. Maintenance of LTP was not affected when afferent nerves were cut 1 h or 5 min after noxious skin stimulation, indicating that an ongoing afferent barrage is not required. Natural noxious stimuli induced LTP in animals which were spinalized but were ineffective in intact animals. Thus, induction of LTP is suppressed by tonically active supraspinal descending systems. We conclude that the natural non-synchronized discharge patterns that are evoked by noxious stimulation may induce LTP and that this new form of LTP may be an underlying mechanism of afferent induced hyperalgesia.

Spontaneous epileptiform bursts and long-term potentiation in rat CA3 hippocampal slices induced by chaotic stimulation of mossy fibers

The relation between long-term potentiation (LTP) and spontaneous rhythm in CA3 was investigated using rat hippocampal slices. Field potential response of CA3 to mossy fiber stimulation consisted of a mono-synaptic positive potential and subsequent poly-synaptic negative potentials. LTP of both field potentials was induced by chaotic mossy fiber stimulation. Although CA3 did not show any spontaneous rhythm before LTP induction in a normal perfusing medium, CA3 spontaneously caused epileptiform bursts after LTP induction by chaotic mossy fiber stimulation. The amplitude of those epileptiform bursts and the inter-burst interval were not uniform. After LTP induction, the cross-correlation function of spontaneous field potentials simultaneously recorded at two sites approximately 300 micron apart in CA3 showed a large central peak. This indicates that neuronal activity at two sites is synchronized. These results suggest that epileptiform bursts in CA3 are caused by synchronization of spontaneous CA3 pyramidal cell activity due to LTP induced by chaotic burst stimulation.

Characterization of long-term potentiation of C-fiber-evoked potentials in spinal dorsal horn of adult rat: essential role of NK1 and NK2 receptors

Impulses in afferent C fibers, e.g., during peripheral trauma, may induce plastic changes in the spinal dorsal horn that are believed to contribute to some forms of hyperalgesia. The nature of lasting changes in spinal nociception are still not well understood. Here we characterized the long-term potentiation (LTP) of spinal field potentials with a negative focus in superficial spinal dorsal horn evoked by supramaximal electrical stimulation of the sciatic nerve in urethan-anesthetized adult rats. The field potentials studied in this work had high thresholds (>/=7 V, 0.5 ms), long latencies (90-130 ms), and long chronaxy (1.1 ms) and were not abolished by muscle relaxation and spinalization. Thus they were evoked by afferent C fibers. In response to 1-Hz stimulation of afferent C fibers, amplitudes of C-fiber-evoked field potentials remained constant, whereas number of action potentials of some dorsal horn neurons increased progressively (wind-up). In all 25 rats tested, high-frequency, high-intensity stimulation (100 Hz, 30-40 V, 0.5 ms, 400 pulses given in 4 trains of 1-s duration at 10-s intervals) always induced LTP (to approximately 200% of control), which consistently lasted until the end of recording periods (4-9 h). This tetanic stimulation also significantly decreased mean threshold of C-fiber-evoked field potentials. The C-fiber volley, which was recorded simultaneously in sural nerve, was, however, not affected by the same tetanic stimulation. High-frequency, low-intensity stimulation (100 Hz, 3 V, 0.5 ms) never induced LTP in six rats tested. At an intermediate frequency, high-intensity stimulation (20 Hz, 40 V, 0.5 ms, 400 pulses given in 4 trains of 5 s at 10-s intervals) induced LTP in four out of six rats, which lasted until end of recording periods (3-6 h). In the remaining two rats, no LTP was induced. Low-frequency, high-intensity stimulation (2 Hz, 30-40 V, 0.5 ms, 400 pulses) induced LTP that lasted for 2-8 h in four out of five rats. Intravenous application of neurokinin 1 (NK1) or neurokinin 2 (NK2) receptor antagonist RP 67580 (2 mg/kg, n = 5) or SR 48968 (0.3 mg/kg, n = 5) 30 min before high-frequency, high-intensity stimulation blocked the induction of LTP in all rats tested. In contrast, the same dose of their inactive enantiomers RP 68651 (n = 5) or SR 48965 (n = 5) did not affect the induction of LTP. Spinal superfusion with RP 67580 (1 microM) from 30 min before to 30 min after high-frequency, high-intensity stimulation blocked induction of LTP in all five rats tested. Spinal application of SR 48968 (10 nM) prevented LTP in five out of seven rats. However, when spinal superfusions with RP 67580 (1 microM, n = 3) or SR 48968 (10 nM, n = 3) were started 1 h after high-frequency, high-intensity stimulation, established LTP was not affected. Thus the activation of neurokinin receptors is necessary for the induction but not for the maintenance of LTP of C-fiber-evoked field potentials in spinal dorsal horn. This model may be useful to study plastic changes in spinal cord induced by peripheral C-fiber stimulation. The LTP of C-fiber-evoked field potentials may be a mechanism underlying some forms of hyperalgesia.

Low-frequency stimulation of afferent Adelta-fibers induces long-term depression at primary afferent synapses with substantia gelatinosa neurons in the rat

Impulses in primary afferent nerve fibers may produce short- or long-lasting modifications in spinal nociception. Here we have identified a robust long-term depression (LTD) of synaptic transmission in substantia gelatinosa neurons that can be induced by low-frequency stimulation of primary afferent Adelta-fibers. Synaptic transmission between dorsal root afferents and neurons in the substantia gelatinosa of the spinal cord dorsal horn was examined by intracellular recording in a transverse slice dorsal root preparation of rat spinal cord. Conditioning stimulation of dorsal roots with 900 pulses given at 1 Hz (10 V, 0.1 msec) produced LTD of EPSP amplitudes in substantia gelatinosa neurons to 41 +/- 10% of control that lasted for at least 2 hr. When A- and C-fibers were recruited, conditioning stimulation was as effective as A-fiber stimulation alone. After LTD, synaptic strength could be increased to its original level by applying a second, high-frequency tetanic stimulus to the dorsal root, indicating that LTD is reversible and not attributable to damage of individual synapses. Bath application of the GABAA receptor antagonist bicuculline and glycine receptor antagonist strychnine did not affect LTD. When NMDA receptors were blocked by bath application of D-2-amino-5-phosphonovaleric acid, LTD was abolished or strongly reduced. Loading substantia gelatinosa neurons with Ca2+ chelator BAPTA also blocked or reduced LTD. After incubation of slices with calyculin A, a selective and membrane permeable inhibitor of protein phosphatases 1 and 2A, LTD was not attenuated. We propose that this form of LTD may be relevant for long-lasting segmental antinociception after afferent stimulation.

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.

Sensorimotor stimulation to improve locomotor recovery after spinal cord injury

Functional recovery after CNS injury may depend, in part, upon reorganization of undamaged neural pathways. Spinal cord circuits are capable of significant reorganization, in the form of both activity-dependent and injury-induced plasticity. This plasticity is manifest behaviourally in the ability of spinal animals to learn new locomotor tasks. Recent work with spinal-injured humans demonstrates that training can improve functional locomotor abilities. New methodologies to enhance limb movement are designed to exploit further the plastic capabilities of the spinal cord by reinforcing appropriate connections in an activity-dependent manner. In the future, these methods might also prove useful in guiding and strengthening functional synaptogenesis of regenerating axons to maximize their contribution towards restoration of function.