Glutamate-mediated slow synaptic currents in neonatal rat deep dorsal horn neurons in vitro

Firing properties of Renshaw cells defined by Chrna2 are modulated by hyperpolarizing and small conductance ion currents Ih and ISK

Renshaw cells in the spinal cord ventral horn regulate motoneuron output through recurrent inhibition. Renshaw cells can be identified in vitro using anatomical and cellular criteria; however, their functional role in locomotion remains poorly defined because of the difficulty of functionally isolating Renshaw cells from surrounding motor circuits. Here we aimed to investigate whether the cholinergic nicotinic receptor alpha2 (Chrna2) can be used to identify Renshaw cells (RCs(α2)) in the mouse spinal cord. Immunohistochemistry and electrophysiological characterization of passive and active RCs(α2) properties confirmed that neurons genetically marked by the Chrna2-Cre mouse line together with a fluorescent reporter mouse line are Renshaw cells. Whole-cell patch-clamp recordings revealed that RCs(α2) constitute an electrophysiologically stereotyped population with a resting membrane potential of -50.5 ± 0.4 mV and an input resistance of 233.1 ± 11 MΩ. We identified a ZD7288-sensitive hyperpolarization-activated cation current (Ih) in all RCs(α2), contributing to membrane repolarization but not to the resting membrane potential in neonatal mice. Additionally, we found RCs(α2) to express small calcium-activated potassium currents (I(SK)) that, when blocked by apamin, resulted in a complete attenuation of the afterhyperpolarisation potential, increasing cellular firing frequency. We conclude that RCs(α2) can be genetically targeted through their selective Chrna2 expression and that they display currents known to modulate rebound excitation and firing frequency. The genetic identification of Renshaw cells and their electrophysiological profile is required for genetic and pharmacological manipulation as well as computational simulations with the aim to understand their functional role.

Serotonin, dopamine and noradrenaline adjust actions of myelinated afferents via modulation of presynaptic inhibition in the mouse spinal cord

Gain control of primary afferent neurotransmission at their intraspinal terminals occurs by several mechanisms including primary afferent depolarization (PAD). PAD produces presynaptic inhibition via a reduction in transmitter release. While it is known that descending monoaminergic pathways complexly regulate sensory processing, the extent these actions include modulation of afferent-evoked PAD remains uncertain. We investigated the effects of serotonin (5HT), dopamine (DA) and noradrenaline (NA) on afferent transmission and PAD. Responses were evoked by stimulation of myelinated hindlimb cutaneous and muscle afferents in the isolated neonatal mouse spinal cord. Monosynaptic responses were examined in the deep dorsal horn either as population excitatory synaptic responses (recorded as extracellular field potentials; EFPs) or intracellular excitatory postsynaptic currents (EPSCs). The magnitude of PAD generated intraspinally was estimated from electrotonically back-propagating dorsal root potentials (DRPs) recorded on lumbar dorsal roots. 5HT depressed the DRP by 76%. Monosynaptic actions were similarly depressed by 5HT (EFPs 54%; EPSCs 75%) but with a slower time course. This suggests that depression of monosynaptic EFPs and DRPs occurs by independent mechanisms. DA and NA had similar depressant actions on DRPs but weaker effects on EFPs. IC₅₀ values for DRP depression were 0.6, 0.8 and 1.0 µM for 5HT, DA and NA, respectively. Depression of DRPs by monoamines was nearly-identical in both muscle and cutaneous afferent-evoked responses, supporting a global modulation of the multimodal afferents stimulated. 5HT, DA and NA produced no change in the compound antidromic potentials evoked by intraspinal microstimulation indicating that depression of the DRP is unrelated to direct changes in the excitability of intraspinal afferent fibers, but due to metabotropic receptor activation. In summary, both myelinated afferent-evoked DRPs and monosynaptic transmission in the dorsal horn are broadly reduced by descending monoamine transmitters. These actions likely integrate with modulatory actions elsewhere to reconfigure spinal circuits during motor behaviors.

Abnormal muscle afferent function in a model of Taxol chemotherapy-induced painful neuropathy

Despite muscle pain being a well-described symptom in patients with diverse forms of peripheral neuropathy, the role of neuropathic mechanisms in muscle pain have received remarkably little attention. We have recently demonstrated in a well-established model of chemotherapy-induced painful neuropathy (CIPN) that the anti-tumor drug paclitaxel (Taxol) produces mechanical hyperalgesia in skeletal muscle, of similar time course to and with shared mechanism with cutaneous symptoms. In the present study, we evaluated muscle afferent neuron function in this rat model of CIPN. The mechanical threshold of muscle afferents in rats exposed to paclitaxel was not significantly different from the mechanical threshold of muscle afferents in control animals (P = 0.07). However, paclitaxel did produce a marked increase in the number of action potentials elicited by prolonged suprathreshold fixed intensity mechanical stimulation and a marked increase in the conduction velocity. In addition, the interspike interval (ISI) analysis (to evaluate the temporal characteristics of the response of afferents to sustained mechanical stimulation) showed a significant difference in rats treated with paclitaxel; there was a significantly greater ISI percentage of paclitaxel-treated muscle afferents with 0.01- and 0.02-s ISI. In contrast, an analysis of variability of neuronal firing over time (CV2 analysis) showed no effect of paclitaxel administration. These effects of paclitaxel on muscle afferent function contrast with the previously reported effects of paclitaxel on the function of cutaneous nociceptors.

Stress enhances muscle nociceptor activity in the rat

Chronic widespread pain, such as observed in irritable bowel (IBS) and fibromyalgia (FMS) syndrome, are markedly affected by stress. While such forms of stress-induced hyperalgesia are generally considered manifestations of "central sensitization," recent studies in patients with IBS and FMS suggest an additional, peripheral contribution. To examine the effect of stress on muscle nociceptor function, we evaluated activity in nociceptors innervating the gastrocnemius muscle in an animal model of chronic widespread pain, water avoidance stress, in the rat. This stressor, which produces mechanical hyperalgesia in skeletal muscle produced a significant decrease (∼34%) in mechanical threshold of muscle nociceptors and a marked, ∼two-fold increase in the number of action potentials produced by a prolonged (60 s) fixed intensity suprathreshold 10 g stimulus. Stress also induced an increase in conduction velocity from 1.25 m/s to 2.09 m/s, and increased variability in neuronal activity. Given that these changes, each of at least moderate magnitude, would be expected to enhance nociceptor activity, it is likely that, taken together, they contribute to the enhanced nociception observed in this model of stress-induced chronic widespread pain.

Pharmacological switch in Abeta-fiber stimulation-induced spinal transmission in mice with partial sciatic nerve injury

BACKGROUND

We have previously demonstrated that different spinal transmissions are involved in the nociceptive behavior caused by electrical stimulation of Abeta-, Adelta- or C-fibers using a Neurometer in naïve mice. In this study, we attempted to pharmacologically characterize the alteration in spinal transmission induced by partial sciatic nerve injury in terms of nociceptive behavior and phosphorylation of extracellular signal-regulated kinase (pERK) in the spinal dorsal horn.

RESULTS

Abeta-fiber responses (2000-Hz), which were selectively blocked by the AMPA/kainate antagonist CNQX in naïve mice, were hypersensitized but blocked by the NMDA receptor antagonists MK-801 and AP-5 in injured mice in an electrical stimulation-induced paw withdrawal (EPW) test. Although Adelta-fiber responses (250-Hz) were also hypersensitized by nerve injury, there was no change in the pharmacological characteristics of Adelta-fiber responses through NMDA receptors. On the contrary, C-fiber responses (5-Hz) were hyposensitized by nerve injury. Moreover, Adelta- and C-, but not Abeta-fiber stimulations significantly increased the number of pERK-positive neurons in the superficial spinal dorsal horns of naïve mice, and corresponding antagonists used in the EPW test inhibited this increase. In mice with nerve injury, Abeta- as well as Adelta-fiber stimulations significantly increased the number of pERK-positive neurons in the superficial spinal dorsal horn, whereas C-fiber stimulation decreased this number. The nerve injury-specific pERK increase induced by Abeta-stimulation was inhibited by MK-801 and AP-5, but not by CNQX. However, Abeta- and Adelta-stimulations did not affect the number or size of pERK-positive neurons in the dorsal root ganglion, whereas C-fiber-stimulation selectively decreased the number of pERK-positive neurons.

CONCLUSION

These results suggest that Abeta-fiber perception is newly transmitted to spinal neurons, which originally receive only Adelta- and C-fiber-mediated pain transmission, through NMDA receptor-mediated mechanisms, in animals with nerve injury. This pharmacological switch in Abeta-fiber spinal transmission could be a mechanism underlying neuropathic allodynia.

Peripheral mechanisms of neuropathic pain - involvement of lysophosphatidic acid receptor-mediated demyelination

Recent advances in pain research provide a clear picture for the molecular mechanisms of acute pain; substantial information concerning plasticity that occurs during neuropathic pain has also become available. The peripheral mechanisms responsible for neuropathic pain are found in the altered gene/protein expression of primary sensory neurons. With damage to peripheral sensory fibers, a variety of changes in pain-related gene expression take place in dorsal root ganglion neurons. These changes, or plasticity, might underlie unique neuropathic pain-specific phenotype modifications - decreased unmyelinated-fiber functions, but increased myelinated A-fiber functions. Another characteristic change is observed in allodynia, the functional change of tactile to nociceptive perception. Throughout a series of studies, using novel nociceptive tests to characterize sensory-fiber or pain modality-specific nociceptive behaviors, it was demonstrated that communication between innocuous and noxious sensory fibers might play a role in allodynia mechanisms. Because neuropathic pain in peripheral and central demyelinating diseases develops as a result of aberrant myelination in experimental animals, demyelination seems to be a key mechanism of plasticity in neuropathic pain. More recently, we discovered that lysophosphatidic acid receptor activation initiates neuropathic pain, as well as possible peripheral mechanism of demyelination after nerve injury. These results lead to further hypotheses of physical communication between innocuous Abeta- and noxious C- or Adelta-fibers to influence the molecular mechanisms of allodynia.

Mechanically-evoked C-fiber activity in painful alcohol and AIDS therapy neuropathy in the rat

While altered activities in sensory neurons were noticed in neuropathic pain, caused by highly diverse insults to the peripheral nervous system, such as diabetes, alcohol ingestion, cancer chemotherapy and drugs used to treat AIDS, other infections and autoimmune diseases, as well as trauma, our understanding of how these various peripheral neuropathies manifest as altered neuronal activity is still rudimentary. The recent development of models of several of those neuropathies has, however, now made it possible to address their impact on primary afferent nociceptor function. We compared changes in mechanically-evoked C-fiber activity, in models of painful peripheral neuropathy induced by drinking ethanol (alcohol) or administering 2',3'-dideoxycytidine (ddC), a nucleoside reverse transcriptase inhibitor for AIDS therapy, two co-morbid conditions in which pain is thought to be mediated by different second messenger signaling pathways. In C-fiber afferents, ddC decreased conduction velocity. In contrast, alcohol but not ddC caused enhanced response to mechanical stimulation (i.e., decrease in threshold and increase in response to sustained threshold and supra-threshold stimulation) and changes in pattern of evoked activity (interspike interval and action potential variability analyses). These marked differences in primary afferent nociceptor function, in two different forms of neuropathy that produce mechanical hyperalgesia of similar magnitude, suggest that optimal treatment of neuropathic pain may differ depending on the nature of the causative insult to the peripheral nervous system, and emphasize the value of studying co-morbid conditions that produce painful peripheral neuropathy by different mechanisms.

Subthalamic stimulation evokes complex EPSCs in the rat substantia nigra pars reticulata in vitro

The subthalamic nucleus (STN) plays an important role in movement control by exerting its excitatory influence on the substantia nigra pars reticulata (SNR), a major output structure of the basal ganglia. Moreover, excessive burst firing of SNR neurons seen in Parkinson's disease has been attributed to excessive transmission in the subthalamonigral pathway. Using the 'blind' whole-cell patch clamp recording technique in rat brain slices, we found that focal electrical stimulation of the STN evoked complex, long-duration excitatory postsynaptic currents (EPSCs) in SNR neurons. Complex EPSCs lasted 200-500 ms and consisted of an initial monosynaptic EPSC followed by a series of late EPSCs superimposed on a slow inward shift in holding current. Focal stimulation of regions outside the STN failed to evoke complex EPSCs. The late component of complex EPSCs was markedly reduced by ionotropic glutamate receptor antagonists (2-amino-5-phosphonopentanoic acid and 6-cyano-7-nitro-quinoxalone) and by a GABAA receptor agonist (isoguvacine) when these agents were applied directly to the STN using a fast-flow microapplicator. Moreover, the complex EPSC was greatly enhanced by bath application of the GABAA receptor antagonists picrotoxin or bicuculline. These data suggest that recurrent glutamate synapses in the STN generate polysynaptic, complex EPSCs that are under tonic inhibition by GABA. Because complex EPSCs are expected to generate bursts of action potentials in SNR neurons, we suggest that complex EPSCs may contribute to the pathological burst firing that is associated with the symptoms of Parkinson's disease.

Novel mechanism of enhanced nociception in a model of AIDS therapy-induced painful peripheral neuropathy in the rat

To elucidate the underlying mechanisms involved in AIDS therapy-induced peripheral neuropathy, we have developed a model of nucleoside analog reverse transcriptase inhibitor-induced painful peripheral neuropathy in the rat, using 2',3'-dideoxycytidine (ddC), 2',3'-dideoxyinosine (ddI) and 2',3'-didehydro-3'-deoxythymidine (d4T), AIDS chemotherapeutic drugs that are also components of AIDS highly active anti-retroviral therapy. Administration of ddC, ddI and d4T produced dose-dependent mechanical hypersensitivity and allodynia. Peripheral administration of inhibitors of protein kinase A, protein kinase C, protein kinase G, p42/p44-mitogen-activated protein kinase (ERK1/2) and nitric oxide synthase, which have demonstrated anti-hyperalgesic effects in other models of metabolic and toxic painful peripheral neuropathies, had no effect on ddC-, ddI- and d4T-induced hypersensitivity. Since suramin, an anti-parasitic and anti-cancer drug, which shares with the anti-retroviral nucleoside analogs, mitochondrial toxicity, altered regulation of intracellular calcium, and a sensory neuropathy in humans, also produced mechanical hypersensitivity that was not sensitive to the above second messenger inhibitors we evaluated the role of intracellular calcium. Intradermal or spinal injection of intracellular calcium modulators (TMB-8 and Quin-2), which had no effect on nociception in control rats, significantly attenuated and together eliminated ddC and suramin-induced mechanical hypersensitivity. In electrophysiology experiments in ddC-treated rats, C-fibers demonstrated alterations in pattern of firing as indicated by changes in the distribution of interspike intervals to sustained suprathreshold stimuli without change in mechanical activation thresholds or in number of action potentials in response to threshold and suprathreshold stimulation. This study provides evidence for a novel, calcium-dependent, mechanism for neuropathic pain in a model of AIDS therapy-induced painful peripheral neuropathy.

Altered temporal pattern of mechanically evoked C-fiber activity in a model of diabetic neuropathy in the rat

While enhanced nociceptor activity has been demonstrated in models of painful peripheral neuropathy, analyses of activity pattern, which could play a role in the symptoms experienced as well as help elucidate underlying mechanism, are still limited. We evaluated the pattern of C-fiber activity, in response to mechanical and chemical stimuli, in a rat model of diabetes induced by a pancreatic beta-cell toxin, streptozotocin (STZ). In diabetic rats the number of action potentials produced by threshold and suprathreshold (10 g) sustained (60 s) mechanical stimuli was elevated in approximately half of C-fibers. These high-firing C-fibers demonstrated a disproportionate increase in interspike intervals (ISIs) between 100 and 199 ms, compared with low-firing diabetic and control C-fibers. The co-efficient of variability (CV2), a frequency independent measure of ISI variability, was also greater in high-firing fibers, compared with control fibers. Unexpectedly, instantaneous frequency of the initial burst of activity during the first second was lower in high-firing fibers, even though the average frequency over the last 59 s was significantly higher. The number of action potentials evoked by a noxious chemical stimulus, 300 and 600 mM KCl, injected adjacent to the mechanical receptive field was also significantly increased in C-fibers from diabetic rats and mechanically high-firing fibers had more action potentials in response to KCl than control fibers and a disproportionate increase in ISIs between 100 and 199 ms for responses to chemical stimuli appeared only in mechanically high-firing C-fibers, compared with the mechanically low-firing diabetic or control C-fibers. There was, however, no corresponding change in CV2 or instantaneous frequency plots for the response to chemical stimulation in mechanically high-firing fibers, as there was in the response to mechanical stimulation. Our data demonstrate specific changes in firing pattern of high-firing C-fibers in the rat model of painful neuropathy produced by STZ-diabetes that might contribute to the symptoms experienced by patients.

Altered temporal pattern of evoked afferent activity in a rat model of vincristine-induced painful peripheral neuropathy

It is known that the level of activity in nociceptive primary afferent nerve fibers increases in neuropathic conditions that produce pain, but changes in the temporal patterning of action potentials have not been analyzed in any detail. Because the patterning of action potentials in sensory nerve fibers might play a role in the development of pathological pain states, we studied patterning of mechanical stimulus-evoked action potential trains in nociceptive primary afferents in a rat model of vincristine-induced painful peripheral neuropathy. Systemic administration of vincristine (100 microg/kg) caused approximately half the C-fiber nociceptors to become markedly hyperresponsive to mechanical stimulation. Instantaneous frequency plots showed that vincristine induced an irregular pattern of action-potential firing in hyperresponsive C-fibers, characterized by interspersed occurrences of high- and low-frequency firing. This pattern was associated with an increase in the percentage of interspike intervals 100-199 ms in duration compared with that in C-fibers from control rats and vincristine-treated C-fibers that did not become hyperresponsive. Variability in the temporal pattern of action potential firing was quantified by determining the coefficient of variability (CV2) for adjacent interspike intervals. This analysis revealed that vincristine altered the pattern of action-potential timing, so that combinations of higher firing frequency and higher variability occurred that were not observed in control fibers. The abnormal temporal structure of nociceptor responses induced by vincristine in some C-fiber nociceptors could contribute to the pathogenesis of chemotherapy-induced neuropathic pain, perhaps by inducing activity-dependent post-synaptic effects in sensory pathways.

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.

Electrophysiological studies on the role of the NMDA receptor in nociception in the developing rat spinal cord

The present study investigated the effects of spinally applied N-methyl-D-aspartate (NMDA) antagonists 2-amino-5-phosphonovaleric acid (AP5) and ketamine on convergent neurones in the deep dorsal horn of rats, in vivo at different postnatal ages (P) 14, 21, 28, and 56 days. AP5 inhibited the primary afferent fibre input, the C fibre, post-discharge and windup evoked responses in a dose-dependent manner at each age, and was significantly more effective in the pups than adult rats (P<0.03 at 100-microg dose). AP5 100 microg abolished windup almost completely in the pups, whilst the adults required 10-fold higher doses. In contrast there was no difference in ketamine potency between age groups. Windup in the ketamine groups was reduced in a dose-dependent manner equally across all the age groups. The differential inhibitory effects of AP5 and ketamine may be due to postnatal changes in density, localisation and receptor subunit composition, altering receptor affinity and kinetics.

Pain: Molecular mechanisms

Our understanding of chronic inflammatory and neuropathic pain at the molecular and cellular level has developed at an extraordinary rate in recent years. Inflammatory, or neuropathic, neuronal plasticity describes the process by which the neurons involved in pain transmission are converted from a state of normosensitivity to one in which they are hypersensitive. Here we summarize current theories on somatosensory neuroplasticity in a molecular context, highlighting key receptors, ion channels, and signal molecules involved. We also suggest new possibilities for drug design, based on the rational targeting of these molecular players.

Primary afferent synaptic responses recorded from trigeminal caudal neurons in a mandibular nerve-brainstem preparation of neonatal rats

1. Whole-cell patch-clamp recordings were made from the neurons in the superficial trigeminal caudal nucleus (substantia gelatinosa) visually identified in a parasagittal brainstem slice of neonatal rat with the mandibular nerve attached. 2. Stimulation of the mandibular nerve at 0.03 Hz evoked compound excitatory postsynaptic potentials (EPSPs) or currents (EPSCs) in trigeminal caudal neurons. When stimulated at higher frequency (> 0.5 Hz), compound synaptic responses were largely attenuated and a small component remained. This component had a monosynaptic nature, following high-frequency stimulation (33-50 Hz) with a stable synaptic latency. 3. The N-methyl-D-aspartate (NMDA) receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP5, 50 microM) largely attenuated the slow polysynaptic EPSCs. The AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) largely attenuated monosynaptic EPSCs, but only weakly attenuated slow polysynaptic EPSCs. Simultaneous application of CNQX and D-AP5 completely abolished EPSCs. The monosynaptic EPSCs isolated by repetitive stimulation had both NMDA and non-NMDA components. 4. Monosynaptic EPSCs having high threshold had a relatively long latency. During repetitive stimulation (0.5-5.0 Hz), EPSCs having high threshold and long latency underwent a stepwise potentiation in an activity-dependent manner. The conduction velocity estimated for these EPSCs fell into the range of C-fibres. The activity-dependent potentiation was observed for both non-NMDA and NMDA EPSCs and was accompanied by a significant decrease in the coefficient of variation of EPSC amplitude. 5. We suggest that the activity-dependent potentiation of EPSCs is induced presynaptically and that it may underlie the wind-up phenomenon, an activity-dependent hyperexcitability of the primary afferent C-fibres.

Molecular and functional properties of synaptically activated NMDA receptors in neonatal motoneurons in rat spinal cord slices

The functional properties of N-methyl-D-aspartate (NMDA) receptor-mediated excitatory postsynaptic currents (EPSC) were studied in fluorescence-labelled motoneurons in thin spinal cord slices. The deactivation of NMDA receptor EPSCs in motoneurons voltage-clamped at +40 mV was independent of intensity or location of stimulation and of postnatal age [taufast = 28.5 +/- 4.6 ms (63.6 +/- 8.8%) and tauslow = 165.6 +/- 49.6 ms]. In the presence of 1 mM Mg2+ the amplitude of NMDA receptor EPSCs was voltage-dependent. Boltzmann analysis of the relationship between peak NMDA receptor EPSC amplitude and membrane potential suggested an apparent Kd of Mg2+ (at 0 mV) of 0.87 mM. Nonstationary variance analysis of NMDA receptor EPSCs gave an estimated single-channel conductance of 59 +/- 14 pS. Direct measurement of the NMDA receptor channel openings in outside-out patches isolated from motoneurons indicated the presence of single-channel conductance levels of 21.8 +/- 2.8 pS, 37. 1 +/- 3.2 pS, 49.6 +/- 5.1 pS and 69.6 +/- 4.2 pS. Single-cell RT-PCR analysis of mRNA revealed that NR1, NR2A-D and NR3A transcripts were expressed in motoneurons. These results suggest that specific assembly of NMDA receptor subunits in motoneurons determines the functional and pharmacological properties of the receptors in these cells.

Synaptic reorganization in the substantia gelatinosa after peripheral nerve neuroma formation: aberrant innervation of lamina II neurons by Abeta afferents

Intracellular recording and extracellular field potential (FP) recordings were obtained from spinal cord dorsal horn neurons (laminae I-IV) in a rat transverse slice preparation with attached dorsal roots. To study changes in synaptic inputs after neuroma formation, the sciatic nerve was sectioned and ligated 3 weeks before in vitro electrophysiological analysis. Horseradish peroxidase labeling of dorsal root axons indicated that Abeta fibers sprouted into laminae I-II from deeper laminae after sciatic nerve section. FP recordings from dorsal horns of normal spinal cord slices revealed long-latency synaptic responses in lamina II and short-latency responses in lamina III. The latencies of synaptic FPs recorded in lamina II of the dorsal horn after sciatic nerve section were reduced. The majority of monosynaptic EPSPs recorded with intracellular microelectrodes from lamina II neurons in control slices were elicited by high-threshold nerve stimulation, whereas the majority of monosynaptic EPSPs recorded in lamina III were elicited by low-threshold nerve stimulation. After sciatic nerve section, 31 of 57 (54%) EPSPs recorded in lamina II were elicited by low-threshold stimulation. The majority of low-threshold EPSPs in lamina II neurons after axotomy displayed properties similar to low-threshold EPSPs in lamina III of control slices. These results indicate that reoccupation of lamina II synapses by sprouting Abeta fibers normally terminating in lamina III occurs after sciatic nerve neuroma formation. Furthermore, these observations indicate that the lamina II neurons receive inappropriate sensory information from low-threshold mechanoreceptor after sciatic nerve neuroma formation.

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.

Single-channel activations and concentration jumps: comparison of recombinant NR1a/NR2A and NR1a/NR2D NMDA receptors

1. We have expressed recombinant NR1a/NR2A and NR1a/NR2D N-methyl-D-aspartate (NMDA) receptor channels in Xenopus oocytes and made recordings of single-channel and macroscopic currents in outside-out membrane patches. For each receptor type we measured (a) the individual single-channel activations evoked by low glutamate concentrations in steady-state recordings, and (b) the macroscopic responses elicited by brief concentration jumps with high agonist concentrations, and we explore the relationship between these two sorts of observation. 2. Low concentration (5-100 nM) steady-state recordings of NR1a/NR2A and NR1a/NR2D single-channel activity generated shut-time distributions that were best fitted with a mixture of five and six exponential components, respectively. Individual activations of either receptor type were resolved as bursts of openings, which we refer to as 'super-clusters'. 3. During a single activation, NR1a/NR2A receptors were open for 36 % of the time, but NR1a/NR2D receptors were open for only 4 % of the time. For both, distributions of super-cluster durations were best fitted with a mixture of six exponential components. Their overall mean durations were 35.8 and 1602 ms, respectively. 4. Steady-state super-clusters were aligned on their first openings and averaged. The average was well fitted by a sum of exponentials with time constants taken from fits to super-cluster length distributions. It is shown that this is what would be expected for a channel that shows simple Markovian behaviour. 5. The current through NR1a/NR2A channels following a concentration jump from zero to 1 mM glutamate for 1 ms was well fitted by three exponential components with time constants of 13 ms (rising phase), 70 ms and 350 ms (decaying phase). Similar concentration jumps on NR1a/NR2D channels were well fitted by two exponentials with means of 45 ms (rising phase) and 4408 ms (decaying phase) components. During prolonged exposure to glutamate, NR1a/NR2A channels desensitized with a time constant of 649 ms, while NR1a/NR2D channels exhibited no apparent desensitization. 6. We show that under certain conditions, the time constants for the macroscopic jump response should be the same as those for the distribution of super-cluster lengths, though the resolution of the latter is so much greater that it cannot be expected that all the components will be resolvable in a macroscopic current. Good agreement was found for jumps on NR1a/NR2D receptors, and for some jump experiments on NR1a/NR2A. However, the latter were rather variable and some were slower than predicted. Slow decays were associated with patches that had large currents.

Stimulation-induced responses of the trigeminal caudal neurons in the brainstem preparation isolated from newborn rats

The brainstem preparation with the trigeminal mandibular nerve attached was isolated from rats postnatal day 0-6 (P0-P6) to test if the potentiation could be induced in neonatal neurons in the trigeminal subnucleus caudalis by stimulation of the primary afferents. The stimulation-induced potentials in 92 neurons recorded extracellularly, and the synaptic potentials in 16 neurons recorded by the whole-cell patch clamp technique were examined. The extracellularly recorded neurons responded to stimulation (0.5 Hz) with either an increase, a decrease, or little change in spike numbers, and were classified as Type 1, Type 2, and Type 3, respectively. Type 1 neurons at P4 and older responded in a low Mg2+ solution with a progressive increase in spike number lasting for several minutes after the cessation of stimulation, i.e., short-term potentiation, STP. This potentiation was antagonized by 20 microM of (+)-MK-801 hydrogen maleate (MK-801) or 25 microM of 2-amino-5-phosphonovaleric acid. In contrast, Type 1 at P3 and younger did not exhibit STP. The age-related distinct response properties were observed between Type 1 neurons at P4-P6 and at P0-P3. The percentage of Type 1 in studied neurons increased from 24% at P0-P3 to 53% at P4-P6. In the intracellular experiment, the mean latency of excitatory postsynaptic potential (EPSP) of recorded neurons indicated that the conduction velocity of the convergent afferents was 0.37 m/s, in the range of C-fiber. Neurons were classified as Type E and Type I. Type E responded with EPSP only, or with both EPSP and inhibitory postsynaptic potentials (IPSP), while Type I responded with IPSP only. In Type E at P4 and older, a single stimulation produced a burst of spike discharges that lasted for several seconds. Stimulation at a hyperpolarized membrane potential showed that aggregated slow EPSPs lay under a burst of spike discharges, and that slow EPSPs, but not a short-latency EPSP, were completely blocked by MK-801. In contrast, Type E at P3 and younger did not evoke a burst of spikes. Morphological examination of recorded neurons showed that the formation of networks was sparse at P1 and rapidly developed up to P4. The results suggest that: (1) short-term potentiation is induced with the development of synaptic network formation in the caudal nucleus at P4 and older; (2) the summation of N-methyl-d-aspartate (NMDA)-mediated slow EPSPs build up a prolonged depolarization; and (3) the brainstem preparation is applicable for neurophysiological studies on the trigeminal pain system.

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.