Peripheral neuropathy induces cutaneous hypersensitivity in chronically spinalized rats.

BACKGROUND/OBJECTIVES: The present study was aimed at the issue of whether peripheral nerve injury-induced chronic pain is maintained by supraspinal structures governing descending facilitation to the spinal dorsal horn, or whether altered peripheral nociceptive mechanisms sustain central hyperexcitability and, in turn, neuropathic pain. We examined this question by determining the contribution of peripheral/spinal mechanisms, isolated from supraspinal influence(s), in cutaneous hypersensitivity in an animal model of peripheral neuropathy. METHODS: Adult rats were spinalized at T8-T9; 8 days later, peripheral neuropathy was induced by implanting a 2-mm polyethylene cuff around the left sciatic nerve. Hind paw withdrawal responses to mechanical or thermal plantar stimulation were evaluated using von Frey filaments or a heat lamp, respectively. RESULTS: Spinalized rats without cuff implantation exhibited a moderate decrease in mechanical withdrawal threshold on ~day 10 (P < 0.05) and in thermal withdrawal threshold on ~day 18 (P < 0.05). However, cuff-implanted spinalized rats developed a more rapid and significant decrease in mechanical (~day 4; P < 0.001) and thermal (~day 10; P < 0.05) withdrawal thresholds that remained significantly decreased through the duration of the study. CONCLUSIONS: Our findings demonstrate an aberrant peripheral/spinal mechanism that induces and maintains thermal and to a greater degree tactile cutaneous hypersensitivity in the cuff model of neuropathic pain, and raise the prospect that altered peripheral/spinal nociceptive mechanisms in humans with peripheral neuropathy may have a pathologically relevant role in both inducing and sustaining neuropathic pain.

Synapse-specific diversity of distinct postsynaptic GluN2 subtypes defines transmission strength in spinal lamina I.

The unitary postsynaptic response to presynaptic quantal glutamate release is the fundamental basis of excitatory information transfer between neurons. The view, however, of individual glutamatergic synaptic connections in a population as homogenous, fixed-strength units of neural communication is becoming increasingly scrutinized. Here, we used minimal stimulation of individual glutamatergic afferent axons to evoke single synapse resolution postsynaptic responses from central sensory lamina I neurons in an ex vivo adult rat spinal slice preparation. We detected unitary events exhibiting a NMDA receptor component with distinct kinetic properties across synapses conferred by specific GluN2 subunit composition, indicative of GluN2 subtype-based postsynaptic heterogeneity. GluN2A, 2A and 2B, or 2B and 2D synaptic predominance functioned on distinct lamina I neuron types to narrowly, intermediately, or widely tune, respectively, the duration of evoked unitary depolarization events from resting membrane potential, which enabled individual synapses to grade differentially depolarizing steps during temporally patterned afferent input. Our results lead to a model wherein a core locus of proteomic complexity prevails at this central glutamatergic sensory synapse that involves distinct GluN2 subtype configurations. These findings have major implications for subthreshold integrative capacity and transmission strength in spinal lamina I and other CNS regions.

Conserved transcriptional programming across sex and species after peripheral nerve injury predicts treatments for neuropathic pain.

BACKGROUND AND PURPOSE: Chronic pain is a devastating problem affecting one in five individuals around the globe, with neuropathic pain the most debilitating and poorly treated type of chronic pain. Advances in transcriptomics have contributed to cataloguing diverse cellular pathways and transcriptomic alterations in response to peripheral nerve injury but have focused on phenomenology and classifying transcriptomic responses. EXPERIMENTAL APPROACH: To identifying new types of pain-relieving agents, we compared transcriptional reprogramming changes in the dorsal spinal cord after peripheral nerve injury cross-sex and cross-species, and imputed commonalities, as well as differences in cellular pathways and gene regulation. KEY RESULTS: We identified 93 transcripts in the dorsal horn that were increased by peripheral nerve injury in male and female mice and rats. Following gene ontology and transcription factor analyses, we constructed a pain interactome for the proteins encoded by the differentially expressed genes, discovering new, conserved signalling nodes. We investigated the interactome with the Drug-Gene database to predict FDA-approved medications that may modulate key nodes within the network. The top hit from the analysis was fostamatinib, the molecular target of which is the non-receptor spleen associated tyrosine kinase (Syk), which our analysis had identified as a key node in the interactome. We found that intrathecally administrating the active metabolite of fostamatinib, R406 and another Syk inhibitor P505-15, significantly reversed pain hypersensitivity in both sexes. CONCLUSIONS AND IMPLICATIONS: Thus, we have identified and shown the efficacy of an agent that could not have been previously predicted to have analgesic properties.

Neto1 is a novel CUB-domain NMDA receptor-interacting protein required for synaptic plasticity and learning.

The N-methyl-D-aspartate receptor (NMDAR), a major excitatory ligand-gated ion channel in the central nervous system (CNS), is a principal mediator of synaptic plasticity. Here we report that neuropilin tolloid-like 1 (Neto1), a complement C1r/C1s, Uegf, Bmp1 (CUB) domain-containing transmembrane protein, is a novel component of the NMDAR complex critical for maintaining the abundance of NR2A-containing NMDARs in the postsynaptic density. Neto1-null mice have depressed long-term potentiation (LTP) at Schaffer collateral-CA1 synapses, with the subunit dependency of LTP induction switching from the normal predominance of NR2A- to NR2B-NMDARs. NMDAR-dependent spatial learning and memory is depressed in Neto1-null mice, indicating that Neto1 regulates NMDA receptor-dependent synaptic plasticity and cognition. Remarkably, we also found that the deficits in LTP, learning, and memory in Neto1-null mice were rescued by the ampakine CX546 at doses without effect in wild-type. Together, our results establish the principle that auxiliary proteins are required for the normal abundance of NMDAR subunits at synapses, and demonstrate that an inherited learning defect can be rescued pharmacologically, a finding with therapeutic implications for humans.

Tyrosine phosphatase STEP is a tonic brake on induction of long-term potentiation.

The functional roles of protein tyrosine phosphatases (PTPs) in the developed CNS have been enigmatic. Here we show that striatal enriched tyrosine phosphatase (STEP) is a component of the N-methyl-D-aspartate receptor (NMDAR) complex. Functionally, exogenous STEP depressed NMDAR single-channel activity in excised membrane patches. STEP also depressed NMDAR-mediated synaptic currents whereas inhibiting endogenous STEP enhanced these currents. In hippocampal slices, administering STEP into CA1 neurons did not affect basal glutamatergic transmission evoked by Schaffer collateral stimulation but prevented tetanus-induced long-term potentiation (LTP). Conversely, inhibiting STEP in CA1 neurons enhanced transmission and occluded LTP induction through an NMDAR-, Src-, and Ca(2+)-dependent mechanism. Thus, STEP acts as a tonic brake on synaptic transmission by opposing Src-dependent upregulation of NMDARs.

ErbB4 is a suppressor of long-term potentiation in the adult hippocampus.

ErbB4 has emerged as a leading susceptibility gene for schizophrenia but the function of the ErbB4 receptor in the adult brain is unknown. Here, we show in the adult hippocampus that long-term potentiation (LTP) of transmission at Schaffer collateral CA1 synapses was markedly enhanced in mutant mice lacking ErbB4. Concordantly, LTP was enhanced by acutely blocking ErbB4 in wild-type animals, indicating that ErbB4 activity constitutively suppresses LTP. Moreover, increasing ErbB4 signaling further suppressed LTP. By contrast, altering ErbB4 activity did not affect basal synaptic transmission or short-term facilitation. Our findings suggest that cognitive deficits in schizophrenia may be a consequence of hyperfunction of ErbB4 signaling leading to suppressed glutamatergic synaptic plasticity, thus opening new approaches for the treatment of this disorder.

Ligand-dependent recruitment of the ErbB4 signaling complex into neuronal lipid rafts.

Neuregulin (NRG) regulates synapse formation and synaptic plasticity, but little is known about the regulation of NRG signaling at synapses. Here we show that the NRG receptor ErbB4 was localized in anatomically defined postsynaptic densities in the brain. In cultured cortical neurons, ErbB4 was recruited to the neuronal lipid raft fraction after stimulation by NRG. Along with ErbB4, adaptor proteins Grb2 and Shc were translocated to lipid rafts by NRG stimulation. In transfected human embryonic kidney 293 cells, the partitioning of ErbB4 into a detergent-insoluble fraction that includes lipid rafts was increased by PSD-95 (postsynaptic density-95), through interaction of the ErbB4 C terminus with the PDZ [PSD-95/Discs large/zona occludens-1] domains of PSD-95. Disruption of lipid rafts inhibited NRG-induced activation of Erk and prevented NRG-induced blockade of induction of long-term potentiation at hippocampal CA1 synapses. Thus, our results indicate that NRG stimulation causes translocation of ErbB4 into lipid rafts and that lipid rafts are necessary for signaling by ErbB4.

NSAID-induced cyclooxygenase inhibition differentially depresses long-lasting versus brief synaptically-elicited responses of rat spinal dorsal horn neurons in vivo.

This electrophysiological study examined the effects of NSAID administration on synaptically-elicited responses of rat single spinal dorsal horn neurons to natural stimulation of peripheral receptive fields. Nociceptive responses consisted of a fast initial discharge during the stimulus followed by a slowly-decaying afterdischarge. The cyclooxygenase inhibitor, indomethacin (2.0-8.0 mg/kg, i.v.), was without effect on the on-going rate of discharge but dose-dependently inhibited synaptically-elicited responses to noxious cutaneous mechanical stimulation (fast initial discharge: n = 3/3 with 2 mg/kg, 5/8 with 4 mg/kg, 5/6 with 8 mg/kg; slowly-decaying afterdischarge: n = 3/3 with 2 mg/kg, 6/8 with 4 mg/kg, 6/6 with 8 mg/kg) and thermal (fast initial discharge: n = 7/9 with 8 mg/kg; slowly-decaying afterdischarge: n = 3/4 with 4 mg/kg, n = 7/9 with 8 mg/kg). The inhibitory effect of indomethacin started within 2-4 min and lasted up to 120 min. To eliminate any effect of indomethacin via cutaneous sensory receptors it was tested on the responses of some neurons to high intensity electrical stimulation of the sciatic nerve; indomethacin depressed these evoked responses (fast initial discharge: n = 5/6 with 2 mg/kg, n = 7/7 with 4 mg/kg; slowly-decaying afterdischarge: n = 6/6 with 2 mg/kg, n = 7/7 with 4 mg/kg). The brief excitatory responses to innocuous pressure (fast initial discharge: n = 2/3 with 2 mg/kg, n = 6/8 with 4 mg/kg, n = 4/6 with 8 mg/kg) and hair (n = 2/7 with 2 and 4 mg/kg, respectively) stimulation in both non-nociceptive and wide dynamic range neurons were also depressed but to a lesser extent. However, the prolonged excitation of three wide dynamic range neurons to continuous hair stimulation was almost entirely inhibited by indomethacin. Overall, inhibition of the afterdischarge and the excitatory effect of long-lasting synaptic input were greater than inhibition of the fast synaptic input-evoked initial discharge. The evidence supports the suggestion that systemically-administered indomethacin has an effect in the spinal cord and demonstrates an action specifically in the dorsal horn. The data are interpreted to suggest that sensory inputs are more involved than input-independent excitation of dorsal horn neurons in leading to de novo synthesis of eicosanoids and that the time course of this synthesis brings the levels to a point where COX inhibition can have an observable effect during prolonged excitation. Although the data suggest that COX inhibition differentially inhibits nociceptive versus non-nociceptive mechanisms at the cellular level, irrespective of the modality of the stimulus, this is the first direct demonstration that prolonged activation of synaptic mechanisms are preferentially inhibited. According to this it would be predictable that NSAIDs would be more effective on nociceptive types of pain characterized by time or prolonged inputs of primary afferents.

Physiological evidence that the 'interphase' in the formalin test is due to active inhibition.

Injection of a dilute solution of formalin into a rat hindpaw produces a biphasic nociceptive response consisting of an early phase during the first 5 min after formalin injection and a later phase starting after 15 min and lasting for 40-50 min. The period between the two phases of nociceptive responding is generally considered to be a phase of inactivity. We compared the nociceptive behaviors produced by a single hindpaw injection of 50 microl of formalin with those produced by two formalin injections given 20 min apart. A single formalin injection at concentrations of either 1 or 2.5%, produced the typical biphasic nociceptive responses. In rats given a second injection of either 1 or 2.5% formalin 20 min after the first, a triphasic response occurred, with a second diminution of nociceptive behavior observed between 10 and 15 min after the second injection. When a second injection of 2.5% formalin was given 5 min after the first, there was no difference from the scores in the group given only one injection. In electrophysiological experiments on single dorsal horn nociceptive neurons, a second injection of 2.5% formalin into the peripheral cutaneous receptive field, 40 min after the first and at the same site of injection as the first formalin injection, depressed neuronal activity for approximately 15-20 min. From the data it can be concluded that the interphase period in the formalin test is due to active inhibition. Furthermore, the inhibition which we are reporting here is independent of the concentration of formalin used, and thus of any so-called inflammatory component. The lack of additive nociceptive effects when the inter-injection interval was only 5 min, suggests that a maximum inhibition was provoked by 2.5% formalin; it can also be concluded that the active inhibition is of overriding importance physiologically, compared with the nociceptive activity. Data from parallel electrophysiological experiments on spinal dorsal horn neurons demonstrated a diminution in excitability after a second formalin injection into the cutaneous receptive field. As these data were obtained from pentobarbital-anesthetized, spinalized rats, the data suggest further that the two excitatory phases and the active inhibition are mediated by spinal mechanisms and that the inhibition is not under regulation of a GABAergic mechanism. The implication of the results is not only further evidence of physiological control mechanisms interacting to regulate pain, but they also indicate the overriding priority of intrinsic inhibitory mechanisms. This, in turn, suggests that the clinical management of pain may be enhanced by efforts to potentiate mechanisms of inhibition.

GluN2B and GluN2D NMDARs dominate synaptic responses in the adult spinal cord.

The composition of the postsynaptic ionotropic receptors that receive presynaptically released transmitter is critical not only for transducing and integrating electrical signals but also for coordinating downstream biochemical signaling pathways. At glutamatergic synapses in the adult CNS an overwhelming body of evidence indicates that the NMDA receptor (NMDAR) component of synaptic responses is dominated by NMDARs containing the GluN2A subunit, while NMDARs containing GluN2B, GluN2C, or GluN2D play minor roles in synaptic transmission. Here, we discovered NMDAR-mediated synaptic responses with characteristics not described elsewhere in the adult CNS. We found that GluN2A-containing receptors contribute little to synaptic NMDAR responses while GluN2B dominates at synapses of lamina I neurons in the adult spinal cord. In addition, we provide evidence for a GluN2D-mediated synaptic NMDAR component in adult lamina I neurons. Strikingly, the charge transfer mediated by GluN2D far exceeds that of GluN2A and is comparable to that of GluN2B. Lamina I forms a distinct output pathway from the spinal pain processing network to the pain networks in the brain. The GluN2D-mediated synaptic responses we have discovered in lamina I neurons provide the molecular underpinning for slow, prolonged and feedforward amplification that is a fundamental characteristic of pain.

Dysregulated Src upregulation of NMDA receptor activity: a common link in chronic pain and schizophrenia.

Upregulation of N-methyl-D-aspartate (NMDA) receptor function by the nonreceptor protein tyrosine kinase Src has been implicated in physiological plasticity at glutamatergic synapses. Here, we highlight recent findings suggesting that aberrant Src upregulation of NMDA receptors may also be key in pathophysiological conditions. Within the nociceptive processing network in the dorsal horn of the spinal cord, pathologically increased Src upregulation of NMDA receptors is critical for pain hypersensitivity in models of chronic inflammatory and neuropathic pain. On the other hand, in the hippocampus and prefrontal cortex, the physiological upregulation of NMDA receptors by Src is blocked by neuregulin 1-ErbB4 signaling, a pathway that is genetically implicated in the positive symptoms of schizophrenia. Thus, either over-upregulation or under-upregulation of NMDA receptors by Src may lead to pathological conditions in the central nervous system. Therefore, normalizing Src upregulation of NMDA receptors may be a novel therapeutic approach for central nervous system disorders, without the deleterious consequences of directly blocking NMDA receptors.

Schizophrenia susceptibility pathway neuregulin 1-ErbB4 suppresses Src upregulation of NMDA receptors.

Hypofunction of the N-methyl D-aspartate subtype of glutamate receptor (NMDAR) is hypothesized to be a mechanism underlying cognitive dysfunction in individuals with schizophrenia. For the schizophrenia-linked genes NRG1 and ERBB4, NMDAR hypofunction is thus considered a key detrimental consequence of the excessive NRG1-ErbB4 signaling found in people with schizophrenia. However, we show here that neuregulin 1ß-ErbB4 (NRG1ß-ErbB4) signaling does not cause general hypofunction of NMDARs. Rather, we find that, in the hippocampus and prefrontal cortex, NRG1ß-ErbB4 signaling suppresses the enhancement of synaptic NMDAR currents by the nonreceptor tyrosine kinase Src. NRG1ß-ErbB4 signaling prevented induction of long-term potentiation at hippocampal Schaffer collateral-CA1 synapses and suppressed Src-dependent enhancement of NMDAR responses during theta-burst stimulation. Moreover, NRG1ß-ErbB4 signaling prevented theta burst-induced phosphorylation of GluN2B by inhibiting Src kinase activity. We propose that NRG1-ErbB4 signaling participates in cognitive dysfunction in schizophrenia by aberrantly suppressing Src-mediated enhancement of synaptic NMDAR function.

Governing role of primary afferent drive in increased excitation of spinal nociceptive neurons in a model of sciatic neuropathy.

Previously we reported that the cuff model of peripheral neuropathy, in which a 2 mm polyethylene tube is implanted around the sciatic nerve, exhibits aspects of neuropathic pain behavior in rats similar to those in humans and causes robust hyperexcitation of spinal nociceptive dorsal horn neurons. The mechanisms mediating this increased excitation are not known and remain a key unresolved question in models of peripheral neuropathy. In anesthetized adult male Sprague-Dawley rats 2-6 weeks after cuff implantation we found that elevated discharge rate of single lumbar (L(3-4)) wide dynamic range (WDR) neurons persists despite acute spinal transection (T9) but is reversed by local conduction block of the cuff-implanted sciatic nerve; lidocaine applied distal to the cuff (i.e. between the cuff and the cutaneous receptive field) decreased spontaneous baseline discharge of WDR dorsal horn neurons approximately 40% (n=18) and when applied subsequently proximal to the cuff, i.e. between the cuff and the spinal cord, it further reduced spontaneous discharge by approximately 60% (n=19; P<0.05 proximal vs. distal) to a level that was not significantly different from that of naive rats. Furthermore, in cuff-implanted rats WDR neurons (n=5) responded to mechanical cutaneous stimulation with an exaggerated afterdischarge which was reversed entirely by proximal nerve conduction block. These results demonstrate that the hyperexcited state of spinal dorsal horn neurons observed in this model of peripheral neuropathy is not maintained by tonic descending facilitatory mechanisms. Rather, on-going afferent discharges originating from the sciatic nerve distal to, at, and proximal to the cuff maintain the synaptically-mediated gain in discharge of spinal dorsal horn WDR neurons and hyperresponsiveness of these neurons to cutaneous stimulation. Our findings reveal that ectopic afferent activity from multiple regions along peripheral nerves may drive CNS changes and the symptoms of pain associated with peripheral neuropathy.

PSD-95 is a negative regulator of the tyrosine kinase Src in the NMDA receptor complex.

The tyrosine kinase Src upregulates the activity of the N-methyl-D-aspartate subtype of glutamate receptor (NMDAR) and tyrosine phosphorylation of this receptor is critical for induction of NMDAR-dependent plasticity of synaptic transmission. A binding partner for Src within the NMDAR complex is the protein PSD-95. Here we demonstrate an interaction of PSD-95 with Src that does not require the well-characterized domains of PSD-95. Rather, we show binding to Src through a 12-amino-acid sequence in the N-terminal region of PSD-95, a region not previously known to participate in protein-protein interactions. This region interacts directly with the Src SH2 domain. Contrary to typical SH2 domain binding, the PSD-95-Src SH2 domain interaction is phosphotyrosine-independent. Binding of the Src-interacting region of PSD-95 inhibits Src kinase activity and reduces NMDAR phosphorylation. Intracellularly administering a peptide matching the Src SH2 domain-interacting region of PSD-95 depresses NMDAR currents in cultured neurons and inhibits induction of long-term potentiation in hippocampus. Thus, the PSD-95-Src SH2 domain interaction suppresses Src-mediated NMDAR upregulation, a finding that may be of broad importance for synaptic transmission and plasticity.

Second phase of formalin-induced excitation of spinal dorsal horn neurons in spinalized rats is reversed by sciatic nerve block.

Considerable debate persists concerning peripheral vs. central mechanisms underlying the second phase of the nociceptive response in the formalin test in the rat. To gain insight into the neurophysiological basis of this pain, we investigated the effects of block of afferent nerve conduction during the second phase of formalin-evoked excitation of single nociceptive neurons recorded extracellularly from rat spinal dorsal horn segments (L(3-4)) in pentobarbital-anaesthetized, male Sprague-Dawley rats. Rats were spinally transected (T(9)) to examine exclusively peripheral and spinal nociceptive processing. In six control rats, hind paw intraplantar formalin injection (50 microL, 2.5%) induced the typical biphasic increase in the discharge rate of the six wide dynamic range neurons tested. This response consisted of a relatively brief immediate phase (approximately 5 min), followed by decreased firing. An ensuing second phase of elevated discharge began approximately 35 min after injection and persisted to at least 80 min. In this control group, 0.9% saline was applied to the exposed ipsilateral sciatic nerve after onset of the second phase (40 min after formalin injection). In a group of six test rats, application of 2% lidocaine instead of saline reversed the second phase of excitation in all six wide dynamic range neurons examined. When the firing rate was normalized to that at 40 min (100%), the time of saline or lidocaine administration, the rate at 50 min was 120 +/- 7.5% in the saline-treated group and 31 +/- 7.4% in the lidocaine-treated group; following lidocaine treatment firing rate remained markedly less than that before administration throughout the remainder of the recording. It is concluded that: (i) spinal mechanisms alone are not sufficient for induction and maintenance of second phase increased discharge of spinal nociceptive dorsal horn neurons; (ii) descending influences via supraspinal inputs are not causal in the development and maintenance of second phase increased discharge and (iii) tonic input from afferent neurons during the second phase plays a primary and essential role in generating and sustaining the second phase of elevated discharge of dorsal horn neurons and, thus, presumably the second phase of nociceptive scores in the formalin test. The data in this study reveal how much of an altered synaptically elicited response in the spinal dorsal horn can be attributed to postsynaptic plastic changes vs. how much can be simply due to increased synaptic input. The present results are important not only in the context of the formalin test but also in the context of other models related to inflammatory pain and neuropathic pain.

Nociceptive response to innocuous mechanical stimulation is mediated via myelinated afferents and NK-1 receptor activation in a rat model of neuropathic pain.

Peripheral nerve injury in humans can produce a persistent pain state characterized by spontaneous pain and painful responses to normally innocuous stimuli (allodynia). Here we attempt to identify some of the neurophysiological and neurochemical mechanisms underlying neuropathic pain using an animal model of peripheral neuropathy induced in male Sprague-Dawley rats by placing a 2-mm polyethylene cuff around the left sciatic nerve according to the method of Mosconi and Kruger. von Frey hair testing confirmed tactile allodynia in all cuff-implanted rats before electrophysiological testing. Rats were anesthetized and spinalized for extracellular recording from single spinal wide dynamic range neurons (L(3-4)). In neuropathic rats (days 11-14 and 42-52 after cuff implantation), ongoing discharge was greater and hind paw receptive field size was expanded compared to control rats. Activation of low-threshold sensory afferents by innocuous mechanical stimulation (0.2 N for 3 s) in the hind paw receptive field evoked the typical brief excitation in control rats. However, in neuropathic rats, innocuous stimulation also induced a nociceptive-like afterdischarge that persisted 2-3 min. This afterdischarge was never observed in control rats, and, in this model, is the distinguishing feature of the spinal neural correlate of tactile allodynia. Electrical stimulation of the sciatic nerve at 4 and at 20 Hz each produced an initial discharge that was identical in control and in neuropathic rats. This stimulation also produced an afterdischarge that was similar at the two frequencies in control rats. However, in neuropathic rats, the afterdischarge produced by 20-Hz stimulation was greater than that produced by 4-Hz stimulation. Given that acutely spinalized rats were studied, only peripheral and/or spinal mechanisms can account for the data obtained; as synaptic responses from C fibers begin to fail above approximately 5-Hz stimulation [Pain 46 (1991) 327], the afterdischarge in response to 20-Hz stimulation suggests a change mainly in myelinated afferents and a predominant role of these fibers in eliciting this afterdischarge. These data are consistent with the suggestion that peripheral neuropathy induces phenotypic changes predominantly in myelinated afferents, the sensory neurons that normally respond to mechanical stimulation. The NK-1 receptor antagonist, CP-99,994 (0.5 mg/kg, i.v.), depressed the innocuous pressure-evoked afterdischarge but not the brief initial discharge of wide dynamic range neurons, and decreased the elevated ongoing rate of discharge in neuropathic rats. These results support the concept that following peripheral neuropathy, myelinated afferents may now synthesize and release substance P. A result of this is that tonic release of substance P from the central terminals of these phenotypically altered neurons would lead to ongoing excitation of NK-1-expressing nociceptive spinal neurons. In addition, these spinal neurons would also exhibit exaggerated responses to innocuous pressure stimulation. The data in this study put forth a possible neurophysiological and neurochemical basis of neuropathic pain and identify substance P and the NK-1 receptor as potential neurochemical targets for its management.

DREAM is a critical transcriptional repressor for pain modulation.

Control and treatment of chronic pain remain major clinical challenges. Progress may be facilitated by a greater understanding of the mechanisms underlying pain processing. Here we show that the calcium-sensing protein DREAM is a transcriptional repressor involved in modulating pain. dream(-/-) mice displayed markedly reduced responses in models of acute thermal, mechanical, and visceral pain. dream(-/-) mice also exhibited reduced pain behaviors in models of chronic neuropathic and inflammatory pain. However, dream(-/-) mice showed no major defects in motor function or learning and memory. Mice lacking DREAM had elevated levels of prodynorphin mRNA and dynorphin A peptides in the spinal cord, and the reduction of pain behaviors in dream(-/-) mice was mediated through dynorphin-selective kappa (kappa)-opiate receptors. Thus, DREAM appears to be a critical transcriptional repressor in pain processing.