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

Differential release of neurotransmitters from superficial and deep layers of the dorsal horn in response to acute noxious stimulation and inflammation of the rat paw

Experimental evidence suggests that release of neurotransmitters in response to acute noxious stimulation and inflammation can differ in superficial and deeper dorsal horn (DH) laminae. Using two different microdialysis probes, we studied changes in levels of glutamate, aspartate, arginine and GABA in dialysates collected from the surface of the spinal cord and within the DH induced by pinching the paw or paw inflammation. In penthotal anaesthetized rats, a flexible microdialysis probe was placed on the dorsal surface of the L4-L5 or L6-S2 spinal segments. In other rats, a rigid microdialysis probe was implanted within the DH of the same segments. Samples were collected every minute before, during and after pinching the hind paw (acute pain), and every half an hour after injecting either carrageenan or saline into the same paw (inflammation-induced pain). Amino acids were measured by capillary zone electrophoresis with laser-induced fluorescence detection (CZE-LIFD). Pinching the paw induced a significant but short lasting increase in extracellular glutamate and aspartate in dialysates from the surface of the DH. Carrageenan, but not saline, injected into the paw significantly increased concentrations of glutamate, aspartate and arginine both on the surface and within the DH of L4-L5 and also within the DH of the L6-S2 segments. The GABA level was significantly increased following carrageenan only within the DH. The maximum increase on the surface was detected 60-120 min after the onset of inflammation whereas the response within the DH reached a maximum between 150 and 180 min after carrageenan. These results indicate that unlike acute mechanical noxious stimulation which enhances amino acid neurotransmitters in surface dialysate, inflammation induced neurotransmitter release in all layers of the DH suggesting sensitization of the DH.

Spinal cord injury, dendritic spine remodeling, and spinal memory mechanisms

Spinal cord injury (SCI) often results in the development of neuropathic pain, which can persist for months and years after injury. Although many aberrant changes to sensory processing contribute to the development of chronic pain, emerging evidence demonstrates that mechanisms similar to those underlying classical learning and memory can contribute to central sensitization, a phenomenon of amplified responsiveness to stimuli in nociceptive dorsal horn neurons. Notably, dendritic spines have emerged as major players in learning and memory, providing a structural substrate for how the nervous system modifies connections to form and store information. Until now, most information regarding dendritic spines has been obtained from studies in the brain. Recent experimental data in the spinal cord, however, demonstrate that Rac1-regulated dendritic spine remodeling occurs on second-order wide dynamic range neurons and accompanies neuropathic pain after SCI. Thus, SCI-induced synaptic potentiation engages a putative spinal memory mechanism. A compelling, novel possibility for pain research is that a synaptic model of long-term memory storage could explain the persistent nature of neuropathic pain. Such a conceptual bridge between pain and memory could guide the development of more effective strategies for treatment of chronic pain after injury to the nervous system.

Cellular prion protein protects from inflammatory and neuropathic pain

Cellular prion protein (PrP^C) inhibits N-Methyl-D-Aspartate (NMDA) receptors. Since NMDA receptors play an important role in the transmission of pain signals in the dorsal horn of spinal cord, we thus wanted to determine if PrP^C null mice show a reduced threshold for various pain behaviours.

We compared nociceptive thresholds between wild type and PrP^C null mice in models of inflammatory and neuropathic pain, in the presence and the absence of a NMDA receptor antagonist. 2-3 months old male PrP^C null mice exhibited an MK-801 sensitive decrease in the paw withdrawal threshold in response both mechanical and thermal stimuli. PrP^C null mice also exhibited significantly longer licking/biting time during both the first and second phases of formalin-induced inflammation of the paw, which was again prevented by treatment of the mice with MK-801, and responded more strongly to glutamate injection into the paw. Compared to wild type animals, PrP^C null mice also exhibited a significantly greater nociceptive response (licking/biting) after intrathecal injection of NMDA. Sciatic nerve ligation resulted in MK-801 sensitive neuropathic pain in wild-type mice, but did not further augment the basal increase in pain behaviour observed in the null mice, suggesting that mice lacking PrP^C may already be in a state of tonic central sensitization. Altogether, our data indicate that PrP^C exerts a critical role in modulating nociceptive transmission at the spinal cord level, and fit with the concept of NMDA receptor hyperfunction in the absence of PrP^C.

Spinal cord transcriptional profile analysis reveals protein trafficking and RNA processing as prominent processes regulated by tactile allodynia

Since partial peripheral injury does not necessarily lead to the development of neuropathic pain it is possible that a set of genes is directly regulated by the development of neuropathic pain independent of the genes regulated by nerve injury. This study identifies the genes expressed within the spinal cord that are uniquely regulated by tactile allodynia in rats. Using subtractive methods, genes regulated by allodynia were differentiated from those of nerve injury. Gene ontology analysis identified that allodynic genes are involved in a variety of processes including myelination, actin cytoskeleton reorganization, dephosphorylation, phosphorylation, response to stress, as well as protein trafficking and RNA processing. The processes of protein trafficking and RNA processing were found to be as statistically significant as other processes that have been associated with neuropathic pain development such as response to stress, phosphorylation, and cell migration. Trafficking and transcription are linked and undergo activity dependent regulation which results in both rapid and gradual synaptic changes (plasticity). The data presented here greatly expand the list of genes regulated by the development of tactile allodynia and reveal protein trafficking and RNA processing as prominent biological processes that may be involved in synaptic plasticity changes within the spinal cord in response to allodynia.

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.

Coantagonism of glutamate receptors and nicotinic acetylcholinergic receptors disrupts fear conditioning and latent inhibition of fear conditioning

The present study investigated the hypothesis that both nicotinic acetylcholinergic receptors (nAChRs) and glutamate receptors (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors (AMPARs) and N-methyl-d-aspartate glutamate receptors (NMDARs)) are involved in fear conditioning, and may modulate similar processes. The effects of the nAChR antagonist mecamylamine administered alone, the AMPAR antagonist NBQX administered alone, and the NMDAR antagonist MK-801 administered alone on cued fear conditioning, contextual fear conditioning, and latent inhibition of cued fear conditioning were examined. In addition, the effects of coadministration of either mecamylamine and NBQX or mecamylamine and MK-801 on these behaviors were examined. Consistent with previous studies, neither mecamylamine nor NBQX administered alone disrupted any of the tasks. However, coadministration of mecamylamine and NBQX disrupted both contextual fear conditioning and latent inhibition of cued fear conditioning. In addition, coadministration of mecamylamine with a dose of MK-801 subthreshold for disrupting either task disrupted both contextual fear conditioning and latent inhibition of cued fear conditioning. Coadministration of mecamylamine and NBQX, and coadministration of mecamylamine with a dose of MK-801 subthreshold for disrupting fear conditioning had little effect on cued fear conditioning. These results suggest that nAChRs and glutamate receptors may support similar processes mediating acquisition of contextual fear conditioning and latent inhibition of fear conditioning.

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.

Increased ability to induce long-term potentiation of spinal dorsal horn neurones in monoarthritic rats

Long-term potentiation (LTP) of transmission of impulses in unmyelinated (C-fibre) primary afferents by prior tetanic conditioning stimulation has been demonstrated in the dorsal horn of the spinal cord. Since this potentiation has been proposed to be relevant to the increased responsiveness of spinal neurones associated with peripheral inflammation (central sensitisation), the present experiments compared the induction of LTP in normal rats and rats with monoarthritis. Monoarthritis was induced by injection of complete Freund's adjuvant (CFA) into the left ankle joint of 12 rats. All animals showed behavioural signs of thermal hyperalgesia and were used for electrophysiological experiments after 4-8 days. In each animal, extracellular recordings were obtained from a single, wide dynamic range (WDR) dorsal horn neurone. High frequency tetanic conditioning stimulation of the sciatic nerve gave varying effects on the C-fibre-evoked responses of neurones in the normal rats, with potentiation in two, no change in five and a depression in five. By contrast, conditioning stimulation in rats with inflammation produced a long-lasting potentiation of C-fibre-evoked responses in 11 out of 12 neurones, with no effect in one. The ease with which LTP was induced in animals with inflammation supports the proposal that the underlying mechanisms of LTP are similar to those of the central sensitisation associated with peripheral inflammation.

Protein synthesis inhibition blocks the late-phase LTP of C-fiber evoked field potentials in rat spinal dorsal horn

Previous studies have demonstrated that in the hippocampus the maintenance of long-term potentiation (LTP) requires de novo protein synthesis. To investigate the role of protein synthesis in the maintenance of LTP of C-fiber evoked field potentials in spinal dorsal horn, which may be relevant to hyperalgesia, protein synthesis inhibitor (either cycloheximide or anisomycin) was applied locally to the recording segments of spinal cord in anesthetized rats, 30 min prior to tetanic stimulation to the sciatic nerve. We found that both cycloheximide and anisomycin selectively inhibited late-phase maintenance of the spinal LTP but affected neither LTP induction nor baseline responses of C-fiber evoked field potentials. In the presence of cycloheximide, LTP of C-fiber evoked field potentials was 281.5 +/- 16.5% (n = 6) of baseline 1 h after tetanic stimulation and the potentiation significantly decreased to 235.5 +/- 18.5% at 145 min after tetanic stimulation (P < 0.05). Afterward, LTP of C-fiber evoked field potentials decreased continuously and at 270 min after tetanic stimulation reached 130.8 +/- 18.0%, which was no longer different from baseline (P > 0.05). Spinal application of anisomycin at 30 min before tetanic stimulation yielded similar results (n = 6). These results suggest that protein synthesis may be crucial for the late-phase maintenance of LTP of C-fiber evoked field potentials in spinal dorsal horn.

The involvement of glia in long-term plasticity in the spinal dorsal horn of the rat

We have examined the potential role of spinal glial cells in the induction of C fiber-evoked long-term potentiation (LTP) in the spinal cord. Tetanic stimulation of the sciatic nerve induced longterm potentiation of C-fiber-evoked field potentials in the spinal dorsal horn in all rats. Following intrathecal fluorocitrate (1 nmol), a glial metabolic inhibitor, tetanic stimulation induced longterm depression (LTD) but not LTP. The effects of fluorocitrate were abolished by kynurenic acid or 2-amino-5-phosphonovaleric acid (AP-5), but not by 6,7-dinitroquinoxaline-2,3-dione (DNQX), picrotoxin or strychnine. These data suggest that spinal glial cells may modulate the central sensitization of nociceptive neurons via NMDA receptors.

Analgesic effects of dextromethorphan and morphine in patients with chronic pain

N-methyl-aspartate (NMDA) receptor antagonists have been shown to improve opioid analgesia in the animal model. The cough suppressant dextromethorphan is a clinically available NMDA-receptor antagonist. In this randomised, double-blind, placebo-controlled study 20 patients with chronic pain of several years duration were given 100 mg of oral dextromethorphan or matching placebo 4 h prior to an intravenous infusion of morphine 15 mg. Pain intensity and adverse effects were assessed at 0, 4, 5 and 7 h. Dextromethorphan had no effect on morphine analgesia: the mean (+/-SEM) visual analogue scores for pain relief (VAS, 0-100 mm) at the end of the morphine infusion were 38 (+/-6) for dextromethorphan+morphine and 38 (+/-7) for placebo+morphine. VAS scores for pain intensity were comparable both at rest and at movement at all time points. The most common adverse effects reported were dizziness, nausea and sedation. There were no significant differences in either the incidence or severity of adverse effects. In conclusion, oral dextromethorphan 100 mg had no effect on pain relief by intravenous morphine 15 mg in patients with chronic pain.

Cellular memory in spinal nociceptive circuitry

Besides transmitting and processing, neurons may also store information for prolonged periods of time (e.g. by use-dependent change in synaptic strength). In 1966 long-term potentiation (LTP) of synaptic transmission was discovered in the hippocampus, an area implicated in learning and memory. Recent studies show that similar mechanisms apply to pain pathways, at least in the spinal cord, and may account for some forms of clinical problems like hyperalgesia, allodynia, and deafferentation pain states, such as phantom pain. In this review, we briefly summarize key aspects of synaptic plasticity known from the brain and in the spinal cord. Then we describe and discuss related changes in spinal nociceptive neurons based on results from our own laboratory.

Spinal substance P release in vivo during the induction of long-term potentiation in dorsal horn neurons

Long-term potentiation (LTP) in wide dynamic range (WDR) neurons in the dorsal horn has been suggested to contribute to central sensitization and the development of chronic pain. Indirect experimental evidence indicates an involvement of substance P (SP), in this respect. The aim of the present study was to monitor the extracellular level of substance P-like immunoreactivity (SP-LI) in the dorsal horn of the rat during and after induction of LTP in WDR neurons in vivo. Electrophysiological recordings of single (WDR) neurons were performed in parallel with microdialysis in the dorsal horn under urethane-anaesthesia. The amount of SP-LI in the microdialysate was determined by radioimmunoassay. As previously shown, high frequency conditioning stimulation of the sciatic nerve induced an increased firing response of WDR neurons. An increased response to C-fibre stimulation, but not A-fibre stimulation, could be determined. A significant increase of the extracellular level of SP-LI in the dorsal horn was detected during, but not after, induction of LTP. These data suggest that SP may be involved in the induction of LTP by high frequency stimulation. However, the maintenance of spinal LTP following high frequency peripheral nerve stimulation does not seem to depend on an increased release of SP.

The effects of Zn2+ on long-term potentiation of C fiber-evoked potentials in the rat spinal dorsal horn

Tetanic stimuli of peripheral C fibers produces long-term potentiation (LTP) in the spinal cord, which may contribute to sensitization of spinal pain-sensitive neurons. Zn2+ is widely distributed in the central nervous system and has blocked (LTP) in the hippocampus. The present study examined the effects of Zn2+ on the induction and maintenance of C fiber-evoked LTP in the deep dorsal horn of spinalized rats in vivo. The sciatic nerve was stimulated by tetanic stimuli for inducing LTP. (1) Topical administration of Zinc chloride (15 microM) to the spinal cord 15 min before tetanic stimulation completely blocked the induction of LTP, but not the baseline C responses. When Zn2+ was given 2 h after induction of LTP, no significant effect occurred. (2) Chelation of Zn2+ by disodium calcium ethylene diaminetelraacetate (CaEDTA) (500 microM) resulted in no effect on LTP. (3) Coadministration of Zn2+ (15 microM) and N-methyl-D-aspartic acid (NMDA) (5 microM) significantly attenuated C fiber-evoked potentials, which was prevented by the NMDA receptor antagonist AP-5 (100 microM). The present results showed that Zn2+ may contribute to the modulation of the formation, but not the maintenance, of spinal LTP. NMDA receptors may be involved in Zn2+-induced modulation.

Induction of long-term potentiation of single wide dynamic range neurones in the dorsal horn is inhibited by descending pathways

Previous studies have shown that long-term potentiation (LTP) in the dorsal horn may be induced by noxious stimuli. In this study it is investigated whether induction of LTP in the dorsal horn may be affected by the descending pathways. Extracellular recordings of wide dynamic range (WDR) neurones in the lumbar dorsal horn in intact urethane-anaesthetized Sprague--Dawley rats were performed, and the electrically evoked neuronal responses in these neurones were defined as A-fibre and C-fibre responses according to latencies. Using a short-term cold block of the thoracic spinal cord, which produced a completely reversible increase of the A-fibre and C-fibre responses, the influence of the descending inhibitory system on the induction of LTP by electrical high-frequency conditioning applied to the sciatic nerve was examined. As previously shown the A-fibre responses were almost unchanged following the conditioning. In contrast, the C-fibre responses following the same conditioning were strongly increased. Thus, a clear LTP of the nociceptive transmission in the dorsal horn was observed following electrical high-frequency conditioning. Interestingly, we found that the LTP was more powerful when the effects of the descending pathways were temporarily eliminated during conditioning. It is concluded that induction of LTP by electrical high-frequency conditioning stimulation, which may be part of the wider term central sensitization, is inhibited by descending pathways.

Wind-up and the NMDA receptor complex from a clinical perspective

More rational treatment of chronic pain depends on increased understanding of the pathophysiological mechanisms underlying the various symptoms and characteristics of chronic pain. Central sensitization of pain represents an important pathophysiological mechanism that has received great attention in recent years. The experimental models used to explore mechanisms of central sensitization include the study of wind-up in animals and temporal summation of pain in humans. Wind-up was described more than 30 years ago as progressively increasing activity in dorsal horn cells following repetitive activation of primary afferent C-fibres. In humans, temporal summation of repeated painful stimuli has been regarded as a psychophysical correlate of wind-up. This review focuses on the relationship between wind-up, temporal summation and central sensitization. In particular, the role of NMDA receptor mechanisms in the modulation of wind-up/temporal summation is discussed. The data presented here indicate that the study of wind-up and temporal summation has given information about some of the complex mechanisms underlying central sensitization. Both wind-up and temporal summation appear to be dependent on NMDA receptor activation. The results of clinical trials in patients with chronic pain suggest that the NMDA receptor may represent a new target for modulation of abnormal temporal summation of pain, as well as other characteristics of chronic pain.

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.

Natural noxious stimulation can induce long-term increase of spinal nociceptive responses

It is conceivable that plasticity in pain control systems and chronic pain may be due to mechanisms similar to learning. Long-term potentiation (LTP) in the hippocampus is often studied as a model of learning and memory. It has recently been shown that long-term excitation may be induced in single wide dynamic range (WDR) neurones in the spinal dorsal horn of rats after tetanic stimulation to the sciatic nerve. The present study shows that similar long-term changes can also be induced by a severe natural stimulus. Single unit extracellular recordings were made in urethane anaesthetized rats and the firing responses of WDR neurones evoked by a single electrical stimulus to the peripheral nerve were recorded every 4 min. After repeated crushing of tissue (including bone) corresponding to the receptive field of the WDR neurones (the conditioning stimulus) followed by a proximal total peripheral nerve block, the C-fibre evoked responses were increased (P < 0.001) for a 3 h observation period compared with baseline responses and control animals. In control animals the nerve block was applied before the conditioning stimulus. We suggest that a long-term increase of the excitability of WDR neurones may be important for the development of long lasting and chronic pain disorders after an acute but severe noxious stimulus.

Recording of long-term potentiation in single dorsal horn neurons in vivo in the rat

We have published several reports on long-term potentiation (LTP) in single spinal wide dynamic range (WDR) neurons (responding to both innocuous and noxious stimuli) in urethane-anaesthetised rats. The protocol presented here, with single unit recordings of dorsal horn neurons before and after a nociceptive conditioning stimulation, may be useful in many electrophysiological studies of plastic changes in the spinal cord, such as LTP. We invite others to use this protocol for the study of spinal plasticity. Findings using this technique may be relevant for the understanding of changes in nociceptive transmission, induction of central sensitisation and maybe even in mechanisms of pathological pain and chronic pain states. We describe modified and alternative protocols for the study of LTP mechanisms under different conditions in intact and in spinalised animals, and after natural noxious stimuli. We present a novel method minimising peripheral influence of afferent input induced by antidromic neurogenic inflammation or inflammatory changes following a natural noxious stimulation. This is made possible by dissection of the sciatic nerve at two separate locations and local anaesthetic block distal to the stimulation site.