Subpopulations of GABAergic and non-GABAergic rat dorsal horn neurons express Ca2+-permeable AMPA receptors

Antidepressant-induced increase in GluA2 expression does not translate in changes of AMPA receptor-mediated synaptic transmission at CA3/CA1 synapses in rats

Chronic treatment with serotonin selective reuptake inhibitors or tryciclic antidepressant drugs in rodents has been shown to increase the expression of GluA1 and/or GluA2 AMPA receptor (AMPAR) subunits in several brain areas, including the hippocampus. These changes in AMPAR composition have been suggested to result in increased glutamatergic neurotransmission and possibly underlie enhanced hippocampal synaptic plasticity through the increased availability of calcium-permeable AMPARs, specifically at CA3/CA1 synapses. However, the possibility that chronic treatment with antidepressants actually results in strengthened glutamatergic neurotransmission in CA1 has poorly been investigated. Here, we studied whether chronic treatment with the multimodal antidepressant drug trazodone mimicked the effect of paroxetine on the expression of AMPAR subunits in male wistar rat hippocampus and whether these drugs produced a parallel facilitation of field excitatory postsynaptic potentials (fEPSP) responses evoked by activation of CA3/CA1 synapses in dorsal hippocampal slices. In addition, we investigated whether the quality of glutamatergic AMPARs involved in basal neurotransmission was changed by altered subunit expression, e.g. leading to appearance of calcium-permeable AMPARs. We found a significant increase in GluA2 subunit expression following treatment with trazodone or paroxetine for twenty-one days, but not after seven-days treatment. In contrast, we did not find any significant changes in fEPSP responses supporting either a facilitation of glutamatergic neurotransmission in basal conditions or the appearance of functional calcium-permeable AMPARs at CA3/CA1 pyramidal neuron synapses. Thus, neurochemically-detected increases in the expression of AMPAR subunits cannot directly be extrapolated in increased number of functioning receptors and/or facilitated basal neurotransmission.

Ca2+-Permeable AMPA Receptors Contribute to Changed Dorsal Horn Neuronal Firing and Inflammatory Pain

The dorsal horn (DH) neurons of the spinal cord play a critical role in nociceptive input integration and processing in the central nervous system. Engaged neuronal classes and cell-specific excitability shape nociceptive computation within the DH. The DH hyperexcitability (central sensitisation) has been considered a fundamental mechanism in mediating nociceptive hypersensitivity, with the proven role of Ca²⁺-permeable AMPA receptors (AMPARs). However, whether and how the DH hyperexcitability relates to changes in action potential (AP) parameters in DH neurons and if Ca²⁺-permeable AMPARs contribute to these changes remain unknown. We examined the cell-class heterogeneity of APs generated by DH neurons in inflammatory pain conditions to address these. Inflammatory-induced peripheral hypersensitivity increased DH neuronal excitability. We found changes in the AP threshold and amplitude but not kinetics (spike waveform) in DH neurons generating sustained or initial bursts of firing patterns. In contrast, there were no changes in AP parameters in the DH neurons displaying a single spike firing pattern. Genetic knockdown of the molecular mechanism responsible for the upregulation of Ca²⁺-permeable AMPARs allowed the recovery of cell-specific AP changes in peripheral inflammation. Selective inhibition of Ca²⁺-permeable AMPARs in the spinal cord alleviated nociceptive hypersensitivity, both thermal and mechanical modalities, in animals with peripheral inflammation. Thus, Ca²⁺-permeable AMPARs contribute to shaping APs in DH neurons and nociceptive hypersensitivity. This may represent a neuropathological mechanism in the DH circuits, leading to aberrant signal transfer to other nociceptive pathways.

A neural tract tracing study on synaptic connections for cortical glutamatergic terminals and cervical spinal calretinin neurons in rats

The cerebral cortex innervates motor neurons in the anterior horn of the spinal cord by regulating of interneurons. At present, nerve tracing, immunohistochemistry, and immunoelectron microscopy are used to explore and confirm the characteristics of synaptic connections between the corticospinal tract (CST) and cervical spinal calretinin (Cr) interneurons. Our morphological results revealed that (1) biotinylated dextran amine labeled (BDA+) fibers from the cerebral cortex primarily presented a contralateral spinal distribution, with a denser distribution in the ventral horn (VH) than in the dorsal horn (DH). An electron microscope (EM) showed that BDA+ terminals formed asymmetric synapses with spinal neurons, and their mean labeling rate was not different between the DH and VH. (2) Cr-immunoreactive (Cr+) neurons were unevenly distributed throughout the spinal gray matter, and were denser and larger in the VH than in the DH. At the single labeling electron microscope (EM) level, the labeling rate of Cr+ dendrites was higher in the VH than in the DH, in which Cr+ dendrites mainly received asymmetric synaptic inputs, and between the VH and DH. (3) Immunofluorescence triple labeling showed obvious apposition points among BDA+ terminals, synaptophysin and Cr+ dendrites, with a higher density in the VH than in the DH. (4) Double labeling in EM, BDA+ terminals and Cr+ dendrites presented the same pattern, BDA+ terminals formed asymmetric synapses either with Cr+ dendrites or Cr negative (Cr-) dendrites, and Cr+ dendrites received either BDA+ terminals or BDA- synaptic inputs. The average percentage of BDA+ terminals targeting Cr+ dendrites was higher in the VH than in the DH, but the percentage of BDA+ terminals targeting Cr- dendrites was prominently higher than that targeting Cr+ dendrites. There was no difference in BDA+ terminal size. The percentage rate for Cr+ dendrites receiving BDA+ terminal inputs was lower than that receiving BDA- terminal inputs, and the BDA+ terminal size was larger than the BDA- terminal size received by Cr+ dendrites. The present morphological results suggested that spinal Cr+ interneurons are involved in the regulatory process of the cortico-spinal pathway.

Spinal AMPA receptors: Amenable players in central sensitization for chronic pain therapy?

The activity-dependent trafficking of AMPA receptors (AMPAR) mediates synaptic strength and plasticity, while the perturbed trafficking of the receptors of different subunit compositions has been linked to memory impairment and to causing neuropathology. In the spinal cord, nociceptive-induced changes in AMPAR trafficking determine the central sensitization of the dorsal horn (DH): changes in AMPAR subunit composition compromise the balance between synaptic excitation and inhibition, rendering interneurons hyperexcitable to afferent inputs, and promoting Ca2+ influx into the DH neurons, thereby amplifying neuronal hyperexcitability. The DH circuits become over-excitable and carry out aberrant sensory processing; this causes an increase in pain sensation in central sensory pathways, giving rise to chronic pain syndrome. Current knowledge of the contribution of spinal AMPAR to the cellular mechanisms relating to chronic pain provides opportunities for developing target-based therapies for chronic pain intervention.

Diverse spinal commissural neuron populations revealed by fate mapping and molecular profiling using a novel Robo3Cre mouse

The two sides of the nervous system coordinate and integrate information via commissural neurons, which project axons across the midline. Commissural neurons in the spinal cord are a highly heterogeneous population of cells with respect to their birthplace, final cell body position, axonal trajectory, and neurotransmitter phenotype. Although commissural axon guidance during development has been studied in great detail, neither the developmental origins nor the mature phenotypes of commissural neurons have been characterized comprehensively, largely due to lack of selective genetic access to these neurons. Here, we generated mice expressing Cre recombinase from the Robo3 locus specifically in commissural neurons. We used Robo3 ^Cre mice to characterize the transcriptome and various origins of developing commissural neurons, revealing new details about their extensive heterogeneity in molecular makeup and developmental lineage. Further, we followed the fate of commissural neurons into adulthood, thereby elucidating their settling positions and molecular diversity and providing evidence for possible functions in various spinal cord circuits. Our studies establish an important genetic entry point for further analyses of commissural neuron development, connectivity, and function.

The histology, physiology, neurochemistry and circuitry of the substantia gelatinosa Rolandi (lamina II) in mammalian spinal cord

The substantia gelatinosa Rolandi (SGR) was first described about two centuries ago. In the following decades an enormous amount of information has permitted us to understand - at least in part - its role in the initial processing of pain and itch. Here, I will first provide a comprehensive picture of the histology, physiology, and neurochemistry of the normal SGR. Then, I will analytically discuss the SGR circuits that have been directly demonstrated or deductively envisaged in the course of the intensive research on this area of the spinal cord, with particular emphasis on the pathways connecting the primary afferent fibers and the intrinsic neurons. The perspective existence of neurochemically-defined sets of primary afferent neurons giving rise to these circuits will be also discussed, with the proposition that a cross-talk between different subsets of peptidergic fibers may be the structural and functional substrate of additional gating mechanisms in SGR. Finally, I highlight the role played by slow acting high molecular weight modulators in these gating mechanisms.

Injured adult motor and sensory axons regenerate into appropriate organotypic domains of neural progenitor grafts

Neural progenitor cell (NPC) transplantation has high therapeutic potential in neurological disorders. Functional restoration may depend on the formation of reciprocal connections between host and graft. While it has been reported that axons extending out of neural grafts in the brain form contacts onto phenotypically appropriate host target regions, it is not known whether adult, injured host axons regenerating into NPC grafts also form appropriate connections. We report that spinal cord NPCs grafted into the injured adult rat spinal cord self-assemble organotypic, dorsal horn-like domains. These clusters are extensively innervated by regenerating adult host sensory axons and are avoided by corticospinal axons. Moreover, host axon regeneration into grafts increases significantly after enrichment with appropriate neuronal targets. Together, these findings demonstrate that injured adult axons retain the ability to recognize appropriate targets and avoid inappropriate targets within neural progenitor grafts, suggesting that restoration of complex circuitry after SCI may be achievable.

Non-canonical heterogeneous cellular distribution and co-localization of CaMKIIα and CaMKIIβ in the spinal superficial dorsal horn

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is a key enzyme in long-term plasticity in many neurons, including in the nociceptive circuitry of the spinal dorsal horn. However, although the role of CaMKII heterooligomers in neuronal plasticity is isoform-dependent, the distribution and co-localization of CaMKII isoforms in the dorsal horn have not been comprehensively investigated. Here, quantitative immunofluorescence analysis was used to examine the distribution of the two major neuronal CaMKII isoforms, α and β, in laminae I–III of the rat dorsal horn, with reference to inhibitory interneurons and neuronal populations defined by expression of parvalbumin, calretinin, and calbindin D28k. Unexpectedly, all or nearly all inhibitory and excitatory neurons showed both CaMKIIα and CaMKIIβ immunoreactivity, although at highly variable levels. Lamina III neurons showed less CaMKIIα immunoreactivity than laminae I–II neurons. Whereas CaMKIIα immunoreactivity was found at nearly similar levels in inhibitory and excitatory neurons, CaMKIIβ generally showed considerably lower immunoreactivity in inhibitory neurons. Distinct populations of inhibitory calretinin neurons and excitatory parvalbumin neurons exhibited high CaMKIIα-to-CaMKIIβ immunoreactivity ratios. CaMKIIα and CaMKIIβ immunoreactivity showed positive correlation at GluA2⁺ puncta in pepsin-treated tissue. These results suggest that, unlike the forebrain, the dorsal horn is characterized by similar expression of CaMKIIα in excitatory and inhibitory neurons, whereas CaMKIIβ is less expressed in inhibitory neurons. Moreover, CaMKII isoform expression varies considerably within and between neuronal populations defined by laminar location, calcium-binding protein expression, and transmitter phenotype, suggesting differences in CaMKII function both between and within neuronal populations in the superficial dorsal horn.

Differential Neuroprotective Effects of Interleukin-1 Receptor Antagonist on Spinal Cord Neurons after Excitotoxic Injury

Secondary damage following spinal cord injury (SCI) induces neuronal damage through inflammatory and excitotoxic pathways. We hypothesized that the interleukin-1 receptor antagonist (IL1RA) protects neuronal populations and suppresses apoptosis and gliosis after injury. Spinal cord slice cultures (SCSCs) were subjected to excitotoxic injury with N-methyl-D-aspartate (NMDA) and treated with IL1RA. Immunohistochemistry for neuronal nuclei (NeuN), MacII, glial fibrillary acidic protein, and TdT-mediated dUTP nick end labelling stains were used to evaluate neuronal survival, glial activation, and apoptosis. Treatment with IL1RA significantly reduced the number of apoptotic cells in both NMDA-lesioned and unlesioned cultures. Experimental injury with NMDA reduced the number of NeuN-positive ventral horn neurons, and IL1RA treatment counteracted this loss 1 day after injury. However, IL1RA had no effect on the number of presumable Renshaw cells, identified by their selective expression of the cholinergic nicotinic α2-receptor subunit (Chrna2). Activated microglial cells were more numerous in NMDA-lesioned cultures 1 day after injury, and IL1RA significantly reduced their numbers. We conclude that IL1RA modulates neuronal apoptosis and microglial activation in excitotoxically injured SCSCs. Renshaw cells were more susceptible to excitotoxic injury than other neurons and were not rescued by IL1RA treatment. Modulation of IL-1-mediated pathways may thus be effective in reducing excitotoxically induced neuronal damage after SCI, however only in specific neuronal populations, such as ventral horn neurons. These findings motivate further investigations of the possibility to antagonize inflammatory pathways after SCI.

A quantitative study of neurochemically defined excitatory interneuron populations in laminae I-III of the mouse spinal cord

BACKGROUND

Excitatory interneurons account for the majority of neurons in laminae I-III, but their functions are poorly understood. Several neurochemical markers are largely restricted to excitatory interneuron populations, but we have limited knowledge about the size of these populations or their overlap. The present study was designed to investigate this issue by quantifying the neuronal populations that express somatostatin (SST), neurokinin B (NKB), neurotensin, gastrin-releasing peptide (GRP) and the γ isoform of protein kinase C (PKCγ), and assessing the extent to which they overlapped. Since it has been reported that calretinin- and SST-expressing cells have different functions, we also looked for co-localisation of calretinin and SST.

RESULTS

SST, preprotachykinin B (PPTB, the precursor of NKB), neurotensin, PKCγ or calretinin were detected with antibodies, while cells expressing GRP were identified in a mouse line (GRP-EGFP) in which enhanced green fluorescent protein (EGFP) was expressed under control of the GRP promoter. We found that SST-, neurotensin-, PPTB- and PKCγ-expressing cells accounted for 44%, 7%, 12% and 21% of the neurons in laminae I-II, and 16%, 8%, 4% and 14% of those in lamina III, respectively. GRP-EGFP cells made up 11% of the neuronal population in laminae I-II. The neurotensin, PPTB and GRP-EGFP populations showed very limited overlap, and we estimate that between them they account for ~40% of the excitatory interneurons in laminae I-II. SST which is expressed by ~60% of excitatory interneurons in this region, was found in each of these populations, as well as in cells that did not express any of the other peptides. Neurotensin and PPTB were often found in cells with PKCγ, and between them, constituted around 60% of the PKCγ cells. Surprisingly, we found extensive co-localisation of SST and calretinin.

CONCLUSIONS

These results suggest that cells expressing neurotensin, NKB or GRP form largely non-overlapping sets that are likely to correspond to functional populations. In contrast, SST is widely expressed by excitatory interneurons that are likely to be functionally heterogeneous.

Loss of Ca(2+)-permeable AMPA receptors in synapses of tonic firing substantia gelatinosa neurons in the chronic constriction injury model of neuropathic pain

Synapses transmitting nociceptive information in the spinal dorsal horn undergo enduring changes following peripheral nerve injury. Indeed, such injury alters the expression of the GluA2 subunit of glutamatergic AMPA receptors (AMPARs) in the substantia gelatinosa and this predicts altered channel conductance and calcium permeability, leading to an altered function of excitatory synapses. We therefore investigated the functional properties of synaptic AMPA receptors in rat substantia gelatinosa neurons following 10-20d chronic constriction injury (CCI) of the sciatic nerve; a model of neuropathic pain. We measured their single-channel conductance and sensitivity to a blocker of calcium permeable AMPA receptors (CP-AMPARs), IEM1460 (50μM). In putative inhibitory, tonic firing neurons, CCI reduced the average single-channel conductance of synaptic AMPAR from 14.4±3.5pS (n=12) to 9.2±1.0pS (n=10, p<0.05). IEM1460 also more effectively antagonized evoked, spontaneous and miniature EPSCs in tonic neurons from sham operated animals than in those from animals that had been subjected to CCI. By contrast, CCI did not change the effectiveness of IEM1460 in delay firing neurons although average single channel conductance was increased from 7.6±1.2pS (n=11) to 12.2±1.5pS (n=10, p<0.01). CCI thus elicits plastic changes in a specific set of glutamatergic synapses of substantia gelatinosa due to subunit recomposition and loss of GluA2-lacking CP-AMPAR. These insights reveal a molecular mechanism of nerve injury acting at synapses of inhibitory neurons to reduce their drive and therefore inhibitory tone in the spinal cord, therefore contributing to the central sensitization associated with neuropathic pain.

Liver X Receptor β Is Involved in Formalin-Induced Spontaneous Pain

Increasing evidence indicates that the liver X receptor(LXR) β modulates inflammatory pain. However, the molecular mechanisms through which LXRβ modulates pain are unclear. Here, we found that LXRβ-null mice responded more strongly to acute noxious stimuli than wild-type (WT) littermates (in the hot plate and Hargreaves tests) and had augmented tonic inflammatory pain (in the formalin test). This increased reactivity to inflammatory pain was accompanied by enhanced formalin-evoked Fos and pERK staining of second-order nociceptive neurons. Immunohistochemistry showed that the expression of CGRP, SP, and IB4 was increased in the lamina I-II of the lumbar dorsal horns in formalin-injected LXRβ knockout (KO) mice compared with the WT controls. In addition, LXRβ deletion in the mice enhanced the formalin-induced inflammation with more activated microglia and astrocytes in the spinal cord. Furthermore, the levels of pro-inflammatory cytokines (IL-1β ,TNF-α) as well as NFκB in the formalin-injected paw were elevated by the loss of LXRβ. Taken together, these data indicate that LXRβ is involved in acute as well as inflammatory pain, and thus, it may be considered as a new target for the development of analgesics.

Functional heterogeneity of calretinin-expressing neurons in the mouse superficial dorsal horn: implications for spinal pain processing

KEY POINTS

The superficial spinal dorsal horn contains a heterogeneous population of neurons that process sensory inputs. Information on the properties of excitatory interneurons in this region is limited. As calretinin is a protein thought to be restricted to an excitatory population in this region, the aim of this study was to characterize calretinin-expressing neurons. Most calretinin cells (85%) exhibited large A-type potassium currents and delayed firing action potential discharge, and received strong excitatory synaptic input, whereas the remainder exhibited hyperpolarization-activated cation currents and low threshold T-type calcium currents, and tonic- or initial bursting firing patterns, and received weak excitatory synaptic input. These respective features are consistent with properties of excitatory and inhibitory interneuron populations in this region of the spinal cord. Our findings have resolved a previously unidentified population of inhibitory interneurons. Furthermore, the contrasting excitability patterns of excitatory and inhibitory calretinin-expressing neurons suggest that they play distinct roles in spinal sensory processing circuits.

ABSTRACT

Neurons in the superficial dorsal horn (SDH) of the spinal cord play an important role in nociceptive, thermal, itch and light touch sensations. Excitatory interneurons comprise ∼65% of all SDH neurons but surprisingly few studies have investigated their role in spinal sensory processing. Here we use a transgenic mouse to study putative excitatory SDH neurons that express the calcium binding protein calretinin (CR). Our immunocytochemical, morphological and electrophysiological analysis identified two distinct populations of CR-expressing neurons, which we termed 'Typical' and 'Atypical'. Typical CR-expressing neurons comprised ∼85% of the population and exhibited characteristic excitatory interneuron properties including delayed firing discharge, large rapid A-type potassium currents, and central, radial or vertical cell morphologies. Atypical neurons exhibited properties consistent with inhibitory interneurons, including tonic firing or initial bursting discharge, Ih currents, and islet cell morphology. Although both Typical and Atypical CR-expressing neurons responded to noxious peripheral stimulation, the excitatory drive onto Typical CR-expressing neurons was much stronger. Furthermore, Atypical CR-expressing cells comprise at least two functionally distinct subpopulations based on their responsiveness to noxious peripheral stimulation and neurochemical profile. Together our data suggest CR expression is not restricted to excitatory neurons in the SDH. Under normal conditions, the contribution of 'Typical' excitatory CR-expressing neurons to overall SDH excitability may be limited by the presence of A-type potassium currents, which limit the effectiveness of their strong excitatory input. Their contribution may, however, be increased in pathological situations where A-type potassium currents are decreased. By contrast, 'Atypical' inhibitory neurons with their excitable phenotype but weak excitatory input may be more easily recruited during increased peripheral stimulation.

Pharmacological Investigation of Fluoro-Gold Entry into Spinal Neurons

The fluorescent tracer Fluoro-Gold has been widely used to label neurons retrogradely. Here we show that Fluoro-Gold can also enter neurons through AMPA receptor endocytosis. We found that a 30 minute application of Fluoro-Gold to the isolated spinal cord labeled neurons under control conditions and in the presence of glutamatergic agonists including NMDA and AMPA. The labeling was abolished or greatly reduced by glutamatergic antagonists and the endocytic inhibitors Dynasore and dynamin inhibitory peptide. Whole cell recordings from spinal neurons exposed to extracellular AMPA revealed large inward currents that spontaneously decayed in the presence of the agonist but were maintained when a dynamin inhibitory peptide was included in the electrode. These findings suggest that Fluoro-Gold enters spinal neurons through AMPA-mediated receptor internalization. Drugs used to induce locomotor-like activity in the spinal cord also increased and decreased Fluoro-Gold labeling in a drug and lamina specific manner, indicating that AMPAR endocytosis is altered in the presence of the locomotor cocktail. Our findings suggest that endocytosis of Fluoro-Gold could potentially complicate the interpretation of experiments in which the tracer is used to label neurons retrogradely. Moreover, they also demonstrate that many drugs, including the locomotor cocktail, can modulate the number and/or the composition of AMPA receptors on spinal neurons and thereby affect network excitability.

Presynaptic Ca²⁺-permeable AMPA-receptors modulate paired-pulse depression in nociceptive sensory synapses

The role of Ca(2+)-permeable AMPA-receptors (CP-AMPARs) in the induction of paired-pulse depression (PPD) at glutamatergic nociceptive sensory synapses was examined in co-culture of rat's dorsal root ganglion and dorsal horn neurons. CP-AMPARs make a considerable contribution to excitatory postsynaptic currents recorded in DH neurons following action potential generation in nociceptive DRG neurons. Activation of CP-AMPARs during fast synaptic transmission induces strong PPD (with a 200 ms inter-pulse interval). Blockage of CP-AMPARs with Naspm (100 μM) results in most cases in a significant reduction of the PPD magnitude, also reversing to paired-pulse facilitation in some cases. However, loading of postsynaptic DH neurons with BAPTA (10 mM) did not alter the effect of Naspm (100 μM) on the paired-pulse ratio. Our data provide evidence that presynaptic CP-AMPARs in nociceptive sensory synapses regulate the magnitude of PPD and that postsynaptic Ca(2+)-permeable glutamatergic receptors are not required for PPD induction.

Three-dimensional distribution of sensory stimulation-evoked neuronal activity of spinal dorsal horn neurons analyzed by in vivo calcium imaging

The spinal dorsal horn comprises heterogeneous populations of interneurons and projection neurons, which form neuronal circuits crucial for processing of primary sensory information. Although electrophysiological analyses have uncovered sensory stimulation-evoked neuronal activity of various spinal dorsal horn neurons, monitoring these activities from large ensembles of neurons is needed to obtain a comprehensive view of the spinal dorsal horn circuitry. In the present study, we established in vivo calcium imaging of multiple spinal dorsal horn neurons by using a two-photon microscope and extracted three-dimensional neuronal activity maps of these neurons in response to cutaneous sensory stimulation. For calcium imaging, a fluorescence resonance energy transfer (FRET)-based calcium indicator protein, Yellow Cameleon, which is insensitive to motion artifacts of living animals was introduced into spinal dorsal horn neurons by in utero electroporation. In vivo calcium imaging following pinch, brush, and heat stimulation suggests that laminar distribution of sensory stimulation-evoked neuronal activity in the spinal dorsal horn largely corresponds to that of primary afferent inputs. In addition, cutaneous pinch stimulation elicited activities of neurons in the spinal cord at least until 2 spinal segments away from the central projection field of primary sensory neurons responsible for the stimulated skin point. These results provide a clue to understand neuronal processing of sensory information in the spinal dorsal horn.

Transmitting pain and itch messages: a contemporary view of the spinal cord circuits that generate gate control

The original formulation of Gate Control Theory (GCT) proposed that the perception of pain produced by spinal cord signaling to the brain depends on a balance of activity generated in large (nonnociceptive)- and small (nociceptive)-diameter primary afferent fibers. The theory proposed that activation of the large-diameter afferent "closes" the gate by engaging a superficial dorsal horn interneuron that inhibits the firing of projection neurons. Activation of the nociceptors "opens" the gate through concomitant excitation of projection neurons and inhibition of the inhibitory interneurons. Sixty years after publication of the GCT, we are faced with an ever-growing list of morphologically and neurochemically distinct spinal cord interneurons. The present Review highlights the complexity of superficial dorsal horn circuitry and addresses the question whether the premises outlined in GCT still have relevance today. By examining the dorsal horn circuits that underlie the transmission of "pain" and "itch" messages, we also address the extent to which labeled lines can be incorporated into a contemporary view of GCT.

Neuronal calcium-binding proteins 1/2 localize to dorsal root ganglia and excitatory spinal neurons and are regulated by nerve injury

Neuronal calcium (Ca(2+))-binding proteins 1 and 2 (NECAB1/2) are members of the phylogenetically conserved EF-hand Ca(2+)-binding protein superfamily. To date, NECABs have been explored only to a limited extent and, so far, not at all at the spinal level. Here, we describe the distribution, phenotype, and nerve injury-induced regulation of NECAB1/NECAB2 in mouse dorsal root ganglia (DRGs) and spinal cord. In DRGs, NECAB1/2 are expressed in around 70% of mainly small- and medium-sized neurons. Many colocalize with calcitonin gene-related peptide and isolectin B4, and thus represent nociceptors. NECAB1/2 neurons are much more abundant in DRGs than the Ca(2+)-binding proteins (parvalbumin, calbindin, calretinin, and secretagogin) studied to date. In the spinal cord, the NECAB1/2 distribution is mainly complementary. NECAB1 labels interneurons and a plexus of processes in superficial layers of the dorsal horn, commissural neurons in the intermediate area, and motor neurons in the ventral horn. Using CLARITY, a novel, bilaterally connected neuronal system with dendrites that embrace the dorsal columns like palisades is observed. NECAB2 is present in cell bodies and presynaptic boutons across the spinal cord. In the dorsal horn, most NECAB1/2 neurons are glutamatergic. Both NECAB1/2 are transported into dorsal roots and peripheral nerves. Peripheral nerve injury reduces NECAB2, but not NECAB1, expression in DRG neurons. Our study identifies NECAB1/2 as abundant Ca(2+)-binding proteins in pain-related DRG neurons and a variety of spinal systems, providing molecular markers for known and unknown neuron populations of mechanosensory and pain circuits in the spinal cord.

The sensory neurons of touch

The somatosensory system decodes a wide range of tactile stimuli and thus endows us with a remarkable capacity for object recognition, texture discrimination, sensory-motor feedback and social exchange. The first step leading to perception of innocuous touch is activation of cutaneous sensory neurons called low-threshold mechanoreceptors (LTMRs). Here, we review the properties and functions of LTMRs, emphasizing the unique tuning properties of LTMR subtypes and the organizational logic of their peripheral and central axonal projections. We discuss the spinal cord neurophysiological representation of complex mechanical forces acting upon the skin and current views of how tactile information is processed and conveyed from the spinal cord to the brain. An integrative model in which ensembles of impulses arising from physiologically distinct LTMRs are integrated and processed in somatotopically aligned mechanosensory columns of the spinal cord dorsal horn underlies the nervous system's enormous capacity for perceiving the richness of the tactile world.

Localization of neurones expressing the gap junction protein Connexin45 within the adult spinal dorsal horn: a study using Cx45-eGFP reporter mice

Connexin (Cx) proteins localized to neuronal and glial syncytia provide the ultrastructural components for intercellular communication via gap junctions. In this study, a Cx45 reporter mouse model in which the Cx45 coding sequence is substituted for enhanced green fluorescent protein (eGFP) was used to characterize Cx45 expressing neurones within adult mouse spinal cord. eGFP-immunoreactive (eGFP-IR) cells were localized at all rostro-caudal levels to laminae I-III of the dorsal horn (DH), areas associated with nociception. The neuronal rather than glial phenotype of these cells in DH was confirmed by co-localisation of eGFP-IR with the neuronal marker NeuN. Further immunohistochemical studies revealed that eGFP-IR interneurones co-express the calcium-binding protein calbindin, and to a lesser extent calretinin. In contrast, eGFP-IR profiles did not co-localize with either parvalbumin or GAD-67, both of which are linked to inhibitory interneurones. Staining with the primary afferent markers isolectin-B4 (IB4) and calcitonin gene-related peptide revealed that eGFP-IR somata within laminae I-III receive close appositions from the former, presumed non-peptidergic nociceptive afferents of peripheral origin. The presence of 5-HT terminals in close apposition to eGFP-IR interneuronal somata suggests modulation via descending pathways. These data demonstrate a highly localized expression of Cx45 in a population of interneurones within the mouse superficial dorsal horn. The implications of these data in the context of the putative role of Cx45 and gap junctions in spinal somatosensory processing and pain are discussed.

Populations of inhibitory and excitatory interneurons in lamina II of the adult rat spinal dorsal horn revealed by a combined electrophysiological and anatomical approach

Lamina II contains a large number of interneurons involved in modulation and transmission of somatosensory (including nociceptive) information. However, its neuronal circuitry is poorly understood due to the difficulty of identifying functional populations of interneurons. This information is important for understanding nociceptive processing and for identifying changes that underlie chronic pain. In this study, we compared morphology, neurotransmitter content, electrophysiological and pharmacological properties for 61 lamina II neurons recorded in slices from adult rat spinal cord. Morphology was related to transmitter content, since islet cells were GABAergic, while radial and most vertical cells were glutamatergic. However, there was considerable diversity among the remaining cells, some of which could not be classified morphologically. Transmitter phenotype was related to firing pattern, since most (18/22) excitatory cells, but few (2/23) inhibitory cells had delayed, gap or reluctant patterns, which are associated with A-type potassium (I_A) currents. Somatostatin was identified in axons of 14/24 excitatory neurons. These had variable morphology, but most of those tested showed delayed-firing. Excitatory interneurons are therefore likely to contribute to pain states associated with synaptic plasticity involving I_A currents. Although noradrenaline and serotonin evoked outward currents in both inhibitory and excitatory cells, somatostatin produced these currents only in inhibitory neurons, suggesting that its pro-nociceptive effects are mediated by disinhibition. Our results demonstrate that certain distinctive populations of inhibitory and excitatory interneuron can be recognised in lamina II. Combining this approach with identification of other neurochemical markers should allow further clarification of neuronal circuitry in the superficial dorsal horn.

Inflammation alters trafficking of extrasynaptic AMPA receptors in tonically firing lamina II neurons of the rat spinal dorsal horn

Peripheral inflammation alters AMPA receptor (AMPAR) subunit trafficking and increases AMPAR Ca(2+) permeability at synapses of spinal dorsal horn neurons. However, it is unclear whether AMPAR trafficking at extrasynaptic sites of these neurons also changes under persistent inflammatory pain conditions. Using patch-clamp recording combined with Ca(2+) imaging and cobalt staining, we found that, under normal conditions, an extrasynaptic pool of AMPARs in rat substantia gelatinosa (SG) neurons of spinal dorsal horn predominantly consists of GluR2-containing Ca(2+)-impermeable receptors. Maintenance of complete Freund's adjuvant (CFA)-induced inflammation was associated with a marked enhancement of AMPA-induced currents and [Ca(2+)](i) transients in SG neurons, while, as we previously showed, the amplitude of synaptically evoked AMPAR-mediated currents was not changed 24 h after CFA. These findings indicate that extrasynaptic AMPARs are upregulated and their Ca(2+) permeability increases dramatically. This increase occurred in SG neurons characterized by intrinsic tonic firing properties, but not in those exhibited strong adaptation. This increase was also accompanied by an inward rectification of AMPA-induced currents and enhancement of sensitivity to a highly selective Ca(2+)-permeable AMPAR blocker, IEM-1460. Electron microcopy and biochemical assays additionally showed an increase in the amount of GluR1 at extrasynaptic membranes in dorsal horn neurons 24h post-CFA. Taken together, our findings indicate that CFA-induced inflammation increases functional expression and proportion of extrasynaptic GluR1-containing Ca(2+)-permeable AMPARs in tonically firing excitatory dorsal horn neurons, suggesting that the altered extrasynaptic AMPAR trafficking might participate in the maintenance of persistent inflammatory pain.

Neuronal circuitry for pain processing in the dorsal horn

Neurons in the spinal dorsal horn process sensory information, which is then transmitted to several brain regions, including those responsible for pain perception. The dorsal horn provides numerous potential targets for the development of novel analgesics and is thought to undergo changes that contribute to the exaggerated pain felt after nerve injury and inflammation. Despite its obvious importance, we still know little about the neuronal circuits that process sensory information, mainly because of the heterogeneity of the various neuronal components that make up these circuits. Recent studies have begun to shed light on the neuronal organization and circuitry of this complex region.

An in vitro assay system for studying synapse formation between nociceptive dorsal root ganglion and dorsal horn neurons

Synapses between nociceptive dorsal root ganglion (DRG) neurons and spinal cord dorsal horn neurons represent the first loci for transmission of painful stimuli. Our knowledge of the molecular organization and development of these synapses is sparse due, partly, to a lack of a reliable model system that reconstitutes synaptogenesis between these two neuronal populations. To address this issue, we have established an in vitro assay system consisting of separately purified DRG neurons and dorsal horn neurons on astrocyte microislands. Using immunocytochemistry, we have found that 97%, 93%, 98%, 96%, and 94% of DRG neurons on these microislands express markers often associated with nociceptive neurons including Substance P, TRPV1, calcitonin-gene related peptide (CGRP), TrKA, and peripherin, respectively. Triple labeling with these nociceptive-like markers, synaptic vesicle marker Vglut2 and using MAP2 as a dendritic marker revealed the presence of nociceptive-like markers at synaptic terminals. Using this immunocytochemical approach, we counted contact points as overlapping MAP2/Vglut2 puncta and showed that they increased with time in culture. Single and dual patch-clamp recordings showed that overlapping Vglut2/MAP2 puncta observed after a few days in culture are likely to be functional synapses between DRG and dorsal horn neurons in our in vitro assay system. Taken together, these data suggest our co-culture microisland model system consists of mostly nociceptive-like DRG neurons that express presynaptic markers and form functional synapses with their dorsal horn partners. Thus, this model system may have direct application for studies on factors regulating development of nociceptive DRG/dorsal horn synapses.

Neurochemical properties of enkephalinergic neurons in lumbar spinal dorsal horn revealed by preproenkephalin-green fluorescent protein transgenic mice

Enkephalin (ENK) has been implicated in nociceptive transmission in the spinal cord while its functional role is not clear because of difficulties in ideally visualizing ENKergic neurons. We thus developed preproenkephalin-green fluorescent protein transgenic mice to better identify ENKergic neurons. Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) together with immunohistochemistry and in situ hybridization were first employed to confirm the successful transgenic manipulation and its application in showing spinal ENKergic neurons. The proportions of ENKergic neurons in the spinal cord laminae I, II, III and IV-VI among dorsal horn neurons were 15.8 +/- 3.1%, 39.5 +/- 3.3%, 11.8 +/- 1.9% and 10.7 +/- 2.1%, respectively. Double labeling with other molecules was then performed to further clarify the neurochemical properties of spinal ENKergic neurons. GABA was found to exist in 42.9 +/- 2.8% of ENKergic neurons that were mainly located in lamina I-III. The proportions of parvalbumin-, calretinin-, calbindin- and neuronal nitric oxide synthase-positive cells among the ENKergic neurons were 5.2 +/- 0.7%, 42.6 +/- 2.3%, 25.8 +/- 2.2% and 11.1 +/- 1.6%, respectively. Compared with previously findings obtained with ENK antibody labeling, this line of newly generated mice can be a reliable tool for the study of specific spinal ENKergic neuronal population.

Modulation of AMPA excitatory postsynaptic currents in the spinal cord dorsal horn neurons by insulin

Glutamate AMPA receptors are critical for sensory transmission at the spinal cord dorsal horn (DH). Plasma membrane AMPA receptor endocytosis that can be induced by insulin may underlie long term modulation of synaptic transmission. Insulin receptors (IRs) are known to be expressed on spinal cord DH neurons, but their possible role in sensory transmission has not been studied. In this work the effect of insulin application on fast excitatory postsynaptic currents (EPSCs) mediated by AMPA receptors evoked in DH neurons was evaluated. Acute spinal cord slices from 6 to 10 day old mice were used to record EPSCs evoked in visually identified superficial DH neurons by dorsal root primary afferent stimulation. AMPA EPSCs could be evoked in all of the tested neurons. In 75% of the neurons the size of the AMPA EPSCs was reduced to 62.1% and to 68.9% of the control values when 0.5 or 10 microM insulin was applied. There was no significant change in the size of the AMPA EPSCs in the remaining 25% of DH neurons. The membrane permeable protein tyrosine kinase inhibitor, lavendustin A (10 microM), prevented the insulin induced AMPA EPSC depression. Our results suggest a possible role of the insulin pathway in modulation of sensory and nociceptive synaptic transmission in the spinal cord.

Activity-dependent codevelopment of the corticospinal system and target interneurons in the cervical spinal cord

Corticospinal tract (CST) connections to spinal interneurons are conserved across species. We identified spinal interneuronal populations targeted by the CST in the cervical enlargement of the cat during development. We focused on the periods before and after laminar refinement of the CST terminations, between weeks 5 and 7. We used immunohistochemistry of choline acetyltransferase (ChAT), calbindin, calretinin, and parvalbumin to mark interneurons. We first compared interneuron marker distribution before and after CST refinement. ChAT interneurons increased, while calbindin interneurons decreased during this period. No significant changes were noted in parvalbumin and calretinin. We next used anterograde labeling to determine whether the CST targets different interneuron populations before and after the refinement period. Before refinement, the CST terminated sparsely where calbindin interneurons were located and spared ChAT interneurons. After refinement, the CST no longer terminated in calbindin-expressing areas but did so where ChAT interneurons were located. Remarkably, early CST terminations were dense where ChAT interneurons later increased in numbers. Finally, we determined whether corticospinal system activity was necessary for the ChAT and calbindin changes. We unilaterally inactivated M1 between weeks 5 and 7 by muscimol infusion. Inactivation resulted in a distribution of calbindin and ChAT in spinal gray matter regions where the CST terminates that resembled the immature more than mature pattern. Our results show that the CST plays a crucial role in restructuring spinal motor circuits during development, possibly through trophic support, and provide strong evidence for the importance of connections with key spinal interneuron populations in development of motor control functions.

Numbers, densities, and colocalization of AMPA- and NMDA-type glutamate receptors at individual synapses in the superficial spinal dorsal horn of rats

Ionotropic glutamate receptors play important roles in spinal processing of nociceptive sensory signals and induction of central sensitization in chronic pain. Here we applied highly sensitive freeze-fracture replica labeling to laminae I-II of the spinal dorsal horn of rats and investigated the numbers, densities, and colocalization of AMPA- and NMDA-type glutamate receptors at individual postsynaptic membrane specializations with a high resolution. All glutamatergic postsynaptic membranes in laminae I-II expressed AMPA receptors, and most of them (96%) were also immunoreactive for the NR1 subunit of NMDA receptors. The numbers of gold particles for AMPA and NMDA receptors at individual postsynaptic membranes showed a linear correlation with the size of postsynaptic membrane specializations and varied in the range of 8-214 and 5-232 with median values of 37 and 28, whereas their densities varied in the range of 325-3365/microm(2) and 102-2263/microm(2) with median values of 1115/microm(2) and 777/microm(2), respectively. Virtually all (99%) glutamatergic postsynaptic membranes expressed GluR2, and most of them (87%) were also immunoreactive for GluR1. The numbers of gold particles for pan-AMPA, NR1, and GluR2 subunits showed a linear correlation with the size of postsynaptic surface areas. Concerning GluR1, there may be two populations of synapses with high and low GluR1 densities. In synapses larger than 0.1 microm(2), GluR1 subunits were recovered in very low numbers. Differential expression of GluR1 and GluR2 subunits suggests regulation of AMPA receptor subunit composition by presynaptic mechanism.

Calcium-fluxing glutamate receptors associated with primary gustatory afferent terminals in goldfish (Carassius auratus)

Presynaptic ionotropic glutamate receptors modulate transmission at primary afferent synapses in several glutamatergic systems. To test whether primary gustatory afferent fibers express Ca(2+)-permeable AMPA/kainate receptors, we utilized kainate-stimulated uptake of Co(2+) along with immunocytochemistry for the Ca(2+)-binding proteins (CaBPs) calbindin and calretinin to investigate the primary gustatory afferents in goldfish (Carassius auratus). In goldfish, the primary gustatory nucleus (equivalent to the gustatory portion of the nucleus of the solitary tract) includes the vagal lobe, which is a large, laminated structure protruding dorsally from the medulla. Kainate-stimulated uptake of Co(2+) (a measure of Ca(2+)-fluxing glutamate receptors) shows punctate staining distributed in the distinct laminar pattern matching the layers of termination of the primary gustatory afferent fibers. In addition, CaBP immunocytochemistry, which correlates highly with expression of Ca(2+)-permeable AMPA/kainate receptors, shows a laminar pattern of distribution similar to that found with kainate-stimulated cobalt uptake. Nearly all neurons of the vagal gustatory ganglion show Co(2+) uptake and are immunopositive for CaBPs. Transection of the vagus nerve proximal to the ganglion results in loss of such punctate Co(2+) uptake and of punctate CaBP staining as soon as 4 days postlesion. These results are consonant with the presence of Ca(2+)-fluxing glutamate receptors on the presynaptic terminals of primary gustatory terminals, providing an avenue for modulation of primary gustatory input.

Enhanced LTP of primary afferent neurotransmission in AMPA receptor GluR2-deficient mice

Ca(2+)-permeable-AMPA receptors (AMPARs) are expressed in the superficial dorsal horn (SDH, laminae I/II) of the spinal cord, the area involved in transmission and modulation of sensory information, including nociception. A possible role of Ca(2+)-permeable-AMPARs in synaptic strengthening has been suggested in postnatal DH cultures, but their role in the long-lasting activity-dependent synaptic plasticity of primary afferent neurotransmission in the adult mouse SDH has not been investigated. In the present study the role of Ca(2+)-permeable-AMPARs in the regulation of long-lasting synaptic plasticity, specifically long-term potentiation (LTP) and long-term depression (LTD) in the SDH, was investigated using mice deficient in AMPAR GluR2 subunit. We show here that the GluR2 mutants exhibited no changes in passive membrane properties, but a significant increase in rectification of excitatory postsynaptic currents, the finding suggesting increased expression of Ca(2+)-permeable-AMPARs. In the absence of GluR2, high-frequency stimulation (HFS) of small-diameter primary afferent fibers induced LTP that is enhanced and non-saturating in the SDH at both primary afferent Adelta- and/or C-fibers monosynaptic and polysynaptic pathways, whereas neuronal excitability and paired-pulse depression were normal. The LTP could be induced in the presence of the NMDA receptor antagonist d-AP5, and L-type Ca(2+) channel blockers, suggesting that Ca(2+)-permeable-AMPARs are sufficient to induce LTP in the SDH neurons of adult mouse spinal cord. In contrast, the induction of HFS-LTD is reduced in the SDH of GluR2 mutants. These results suggest an important role for AMPAR GluR2 subunit in regulating synaptic plasticity with potential relevance for long-lasting hypersensitivity in pathological states.

Erythropoietin is neuroprotective on GABAergic neurons against kainic acid-excitotoxicity in the rat spinal cell cultures

The aim of this study was to investigate whether erythropoietin (EPO) protect the spinal GABAergic neurons against kainic acid (KA)-excitotoxic damage in rat spinal cord cell cultures. We performed immunohistochemical staining and Western blotting of glutamate decarboxylase 67 (GAD67), one isoform of GABA-producing enzyme, which was considered to have involved in nonsynaptic functions, such as energy metabolism or trophic support. T exposure to KA significantly reduced the intensity of GAD67 expression in the GABAergic neurons and whole cell lysate, indicating that the excitotoxic damage on the GABAergic neurons may lead to reduction of the GAD67 production following KA-exposure. We found that post-treatment of EPO for 48 h after KA-injury remarkably enhanced the expression level of GAD67 and erythropoietin receptors (EpoR), which were deteriorated by KA. Our confocal images clearly demonstrated a remarkably enhanced expression of EpoR on the surface of the GABAergic neurons by post-treated EPO after KA-damage. This result suggests that the neuroprotective effect of post-treated EPO on the GABAergic neurons can be related to the EPO-mediated EpoR upregulation following KA-excitotoxicity. We observed that the post-applied EPO clearly increased expression of tyrosine Janus kinase 2 (JAK2), which is known to be the first step of EpoR-stimulation. In conclusion, the post-treated EPO is a potent protector of the spinal GABAergic neurons against KA-excitotoxicity and regulates production of GAD67 for the multiple trophic roles after KA-induced disturbance. We suggested that the protective effect of post-treated EPO on the GABAergic neurons is mediated by signal transduction involving EpoR-dependent JAK2 pathway.

Both Ca2+-permeable and -impermeable AMPA receptors contribute to primary synaptic drive onto rat dorsal horn neurons

Blockade of Ca2+-permeable AMPA receptors in the rat spinal cord diminishes the development of hyperalgesia and allodynia associated with peripheral injury. Cobalt uptake studies reveal that Ca2+-permeable AMPA receptors are expressed by some substance P receptor-expressing (NK1R+) neurons in lamina I, as well as other neurons throughout the superficial dorsal horn. Selective elimination of NK1R+ neurons in lamina I and lamina III/IV of the dorsal horn also suppresses development of hyperalgesia and allodynia. These observations raise the possibility that Ca2+-permeable AMPA receptors contribute to excitatory synaptic drive onto the NK1R+ neurons associated with allodynia and hyperalgesia. The first synapse in the pain pathway is the glutamatergic excitatory drive from the primary afferent fibres onto dorsal horn neurons. Therefore, we tested whether Ca2+-permeable AMPA receptors are located on lamina I and lamina III/IV NK1R+ neurons postsynaptic to primary afferent fibres, using inward rectification and polyamine toxins for receptor identification. We examined three different populations of dorsal horn neurons; lamina I NK1R+ neurons, including projection neurons, and non-NK1R+ (NK1R-) neurons including interneurons, and lamina III/IV NK1R+ neurons, believed to contribute to the low-threshold mechanosensory pathway. The majority of synapses in all three groups had rectification indices less than 1.0 and greater than 0.4, indicating that the AMPA receptors at these synapses are a mixture of Ca2+-permeable and -impermeable forms. Lamina III/IV NK1R+ neurons and lamina I NK1R- neurons have a significantly higher proportion of postsynaptic Ca2+-permeable AMPA receptors than lamina I NK1R+ neurons. Thus synaptically positioned Ca2+-permeable AMPA receptors directly contribute to low-threshold sensory afferent drive into the dorsal horn, and can mediate afferent input onto interneurons such as GABAergic neurons. These receptors also contribute to high-threshold primary afferent drive onto NK1R+ neurons in the superficial dorsal horn, but do so less consistently.

Characterization of sensory neuron subpopulations selectively expressing green fluorescent protein in phosphodiesterase 1C BAC transgenic mice

BACKGROUND

The complex neuronal circuitry of the dorsal horn of the spinal cord is as yet poorly understood. However, defining the circuits underlying the transmission of information from primary afferents to higher levels is critical to our understanding of sensory processing. In this study, we have examined phosphodiesterase 1C (Pde1c) BAC transgenic mice in which a green fluorescent protein (GFP) reporter gene reflects Pde1c expression in sensory neuron subpopulations in the dorsal root ganglia and spinal cord.

RESULTS

Using double labeling immunofluorescence, we demonstrate GFP expression in specific subpopulations of primary sensory neurons and a distinct neuronal expression pattern within the spinal cord dorsal horn. In the dorsal root ganglia, their distribution is restricted to those subpopulations of primary sensory neurons that give rise to unmyelinated C fibers (neurofilament 200 negative). A small proportion of both non-peptidergic (IB4-binding) and peptidergic (CGRP immunoreactive) subclasses expressed GFP. However, GFP expression was more common in the non-peptidergic than the peptidergic subclass. GFP was also expressed in a subpopulation of the primary sensory neurons immunoreactive for the vanilloid receptor TRPV1 and the ATP-gated ion channel P2X3. In the spinal cord dorsal horn, GFP positive neurons were largely restricted to lamina I and to a lesser extent lamina II, but surprisingly did not coexpress markers for key neuronal populations present in the superficial dorsal horn.

CONCLUSION

The expression of GFP in subclasses of nociceptors and also in dorsal horn regions densely innervated by nociceptors suggests that Pde1c marks a unique subpopulation of nociceptive sensory neurons.

Sciatic chronic constriction injury produces cell-type-specific changes in the electrophysiological properties of rat substantia gelatinosa neurons

Peripheral nerve injury increases spontaneous action potential discharge in spinal dorsal horn neurons and augments their response to peripheral stimulation. This "central hypersensitivity, " which relates to the onset and persistence of neuropathic pain, reflects spontaneous activity in primary afferent fibers as well as long-term changes in the intrinsic properties of the dorsal horn (centralization). To isolate and investigate cellular mechanisms underlying "centralization," sciatic nerves of 20-day-old rats were subjected to 13-25 days of chronic constriction injury (CCI; Mosconi-Kruger polyethylene cuff model). Spinal cord slices were then acutely prepared from sham-operated or CCI animals, and whole cell recording was used to compare the properties of five types of substantia gelatinosa neuron. These were defined as tonic, irregular, phasic, transient, or delay according to their discharge pattern in response to depolarizing current. CCI did not affect resting membrane potential, rheobase, or input resistance in any neuron type but increased the amplitude and frequency of spontaneous and miniature excitatory postsynaptic currents (EPSCs) in delay, transient, and irregular cells. These changes involved alterations in the action potential-independent neurotransmitter release machinery and possible increases in the postsynaptic effectiveness of glutamate. By contrast, in tonic cells, CCI reduced the amplitude and frequency of spontaneous and miniature EPSCs. Such changes may relate to the putative role of tonic cells as inhibitory GABAergic interneurons, whereas increased synaptic drive to delay cells may relate to their putative role as the excitatory output neurons of the substantia gelatinosa. Complementary changes in synaptic excitation of inhibitory and excitatory neurons may thus contribute to pain centralization.

Expression of A-type K channel alpha subunits Kv 4.2 and Kv 4.3 in rat spinal lamina II excitatory interneurons and colocalization with pain-modulating molecules

Voltage-gated K(+) channel alpha subunits Kv 4.2 and Kv 4.3 are the major contributors of somatodendritic A-type K(+) currents in many CNS neurons. A recent hypothesis suggests that Kv 4 subunits may be involved in pain modulation in dorsal horn neurons. However, whether Kv 4 subunits are expressed in dorsal horn neurons remains unknown. Using immunohistochemistry, we found that Kv 4.2 and Kv 4.3 immunoreactivity was concentrated in the superficial dorsal horn, mainly in lamina II. Both Kv 4.2 and Kv 4.3 appeared on many rostrocaudally orientated dendrites, whereas Kv 4.3 could be also detected from certain neuronal somata. Kv 4.3(+) neurons were a subset of excitatory inerneurons with calretinin(+)/calbindin(-)/PKCgamma(-) markers, and a fraction of them expressed micro-opioid receptors. Kv 4.3(+) neurons also expressed ERK 2 and mGluR 5, which are molecules related to the induction of central sensitization, a mechanism mediating nociceptive plasticity. Together with the expression of Kv 4.3 in VR 1(+) DRG neurons, our data suggest that Kv C4 subunits could be involved in pain modulation.

Activated PKA and PKC, but not CaMKIIα, are required for AMPA/Kainate-mediated pain behavior in the thermal stimulus model

Secondary mechanical allodynia resulting from a thermal stimulus (52.5 degrees C for 45s) is blocked by intrathecal (i.t.) pretreatment with calcium-permeable AMPA/KA receptor antagonists, but not NMDA receptor antagonists. Spinal sensitization is presumed to underlie thermal stimulus-evoked secondary mechanical allodynia. We investigated whether this spinal sensitization involves activation and phosphorylation of calcium-dependent protein kinases (PKA, PKC and CaMKIIalpha), and examined if the noxious stimulus increases phosphorylated AMPA GLUR1 (pGLUR1 Ser-845 and pGLUR1 Ser-831). Secondary mechanical allodynia after thermal stimulation was not altered by i.t. pretreatment with control vehicles (saline or 5% DMSO). Comparable allodynia was observed after pretreatment with a selective CaMKIIalpha inhibitor (17 and 34nmol KN-93). In marked contrast, pretreatment with either a PKA (10nmol H89) or PKC (30nmol chelerythrine) inhibitor blocked allodynia. Western immunoblot analyses supported behavioral findings and revealed a thermal stimulus-evoked increase in spinal phosphorylated PKA and PKC, but not CaMKIIalpha. There was no increase in any of the total protein kinases. Although thermal stimulation did not change either pGLUR1 Ser-845 or pGLUR1 Ser-831, it was associated with an increase in cytosolic total GLUR1. Pretreatment with a selective calcium-permeable AMPA/KA receptor antagonist (5nmol joro spider toxin), but not an NMDA receptor antagonist (25nmol d-2-amino-5-phosphonovalerate, AP-5), blocked thermal stimulus-evoked increases in phosphorylated PKA and PKC, in addition to increased cytosolic GLUR1. These findings indicate that spinal sensitization in the thermal stimulus model does not involve CaMKIIalpha activation or AMPA GLUR1 receptor phosphorylation, and differs from that occurring in NMDAr-dependent pain states.

The development of nociceptive circuits

The study of pain development has come into its own. Reaping the rewards of years of developmental and molecular biology, it has now become possible to translate fundamental knowledge of signalling pathways and synaptic physiology into a better understanding of infant pain. Research has cast new light on the physiological and pharmacological processes that shape the newborn pain response, which will help us to understand early pain behaviour and to design better treatments. Furthermore, it has shown how developing pain circuitry depends on non-noxious sensory activity in the healthy newborn, and how early injury can permanently alter pain processing.

Ionotropic glutamate receptors are expressed in GABAergic terminals in the rat superficial dorsal horn

Ionotropic glutamate receptors (IGR), including NMDA, AMPA, and kainate receptors, are expressed in terminals with varied morphology in the superficial laminae (I-III) of the dorsal horn of the spinal cord. Some of these terminals can be identified as endings of primary afferents, whereas others establish symmetric synapses, suggesting that they may be gamma-aminobutyric acid (GABA)-ergic. In the present study, we used confocal and electron microscopy of double immunostaining for GAD65, a marker for GABAergic terminals, and for subunits of IGRs to test directly whether IGRs are expressed in GABAergic terminals in laminae I-III of the dorsal horn. Although colocalization is hard to detect with confocal microscopy, electron microscopy reveals a substantial number of terminals immunoreactive for GAD65 also stained for IGRs. Among all GAD65-immunoreactive terminals counted, 37% express the NMDA receptor subunit NR1; 28% are immunopositive using an antibody for the GluR2/4 subunits of the AMPA receptor; and 20-35% are immunopositive using antibodies for the kainate receptor subunits GluR5, GluR6/7, KA1, or KA2. Terminals immunoreactive for IGR subunits and GAD65 establish symmetric synapses onto dendrites and perikarya and can be presynaptic to primary afferent terminals within both type 1 and type 2 synaptic glomeruli. Activation of presynaptic IGR may reduce neurotransmitter release. As autoreceptors in terminals of Adelta and C afferent fibers in laminae I-III, presynaptic IGRs may play a role in inhibiting nociception. As heteroreceptors in GABAergic terminals in the same laminae, on the other hand, presynaptic IGRs may have an opposite role and even contribute to central sensitization and hyperalgesia.

Dopaminergic neurons in rat ventral midbrain cultures undergo selective apoptosis and form inclusions, but do not up-regulate iHSP70, following proteasomal inhibition

Dysfunction of the ubiquitin-dependent protein degradation system, either at the level of the proteasome itself, or at the level of ubiquitination, may play a role in the pathogenesis of Parkinson's disease (PD) and other related neurodegenerative disorders. We have employed a cellular model of this dysfunction in which lactacystin or epoxomicin, selective pharmacological inhibitors of the proteasome, are applied to primary cultures of embryonic rat ventral midbrain. Proteasomal inhibition with either agent led to apoptotic death specifically within phenotypically defined tyrosine hydroxylase (TH)-positive dopaminergic neurons, with little or no apoptotic death induced in GABAergic neurons. Inhibition of the proteasome also led to the formation of ubiquitin and alpha-synuclein-positive cytoplasmic inclusions in TH-positive and TH-negative neurons. Inclusions were observed in viable as well as apoptotic neurons, and required new or ongoing transcription. Tyrosine hydroxylase immunolabeling was often present within the inclusions. Such mislocalization may lead to dysfunction of dopamine biosynthesis. Interestingly, dopaminergic neurons, unlike other neurons within these cultures or cultured cortical neurons, failed to induce the chaperone Hsp70 in response to proteasomal inhibition. This failure may explain in part the increased sensitivity of these neurons to proteasomal inhibitors.

Characterisation of transverse slice culture preparations of postnatal rat spinal cord: preservation of defined neuronal populations

Spinal cord injury induces degenerative and regenerative processes and complex interactions of neurons with non-neuronal cells. In order to develop an in vitro tool for the investigation of such processes, we prepared and characterised spinal cord slice cultures (SCSC) from Wistar rats (p0-12). SCSC were sustained in vitro up to 12 days and characterised by immunohistochemistry. Calbindin+ neurons, distributed across the entire gray matter, were visible also after longer culture periods. NeuN+ neurons were best preserved in the dorsal horn whereas large NeuN+ and choline acetyltransferase+ motoneurons in the ventral horn vanished after 3 days in vitro. Nestin immunoreactivity was found in animals of all age groups, either in cells interspersed in the ependymal lining around the central canal or in cells resembling protoplasmic astrocytes. Glial fibrillary acidic protein+ astrocytes, initially restricted to the white matter, invaded the gray matter of SCSC early during the culture period. Microglial cells, stained by Griffonia simplicifolia isolectin B4, were rapidly activated in the dorsal tract and in the gray matter but declined in number with time. SCSC derived from p0 or p3 animals showed a better preservation of the cytoarchitecture than cultures derived from older animals. In summary, SCSC undergo degenerative changes, but they contain defined neuronal populations, the cytoarchitecture is partially preserved and the glial reaction is limited.

Functional similarities and differences of AMPA and kainate receptors expressed by cultured rat sensory neurons

Dorsal root ganglion neurons express functional AMPA and kainate receptors near their central terminals. Activation of these receptors causes a decrease in glutamate release during action potential evoked synaptic transmission. Due to differences in kinetic properties and expression patterns of these two families of glutamate receptors in subpopulations of sensory neurons, AMPA and kainate receptors are expected to function differently. We used embryonic dorsal root ganglion (DRG) neurons maintained in culture to compare functional properties of kainate and AMPA receptors. Most DRG neurons in culture expressed kainate receptors and about half also expressed AMPA receptors. Most AMPA and kainate receptor-expressing DRG neurons were sensitive to capsaicin, suggesting involvement of these glutamate receptors in nociception. When activated by kainate, AMPA receptors were capable of driving a sustained train of action potentials while kainate receptors tended to activate action potential firing more transiently. Glutamate elicited more action potentials and a larger steady-state depolarization in neurons expressing both AMPA and kainate receptors than in neurons expressing only kainate receptors. Adding to their more potent activation properties, AMPA receptors recovered from desensitization much more quickly than kainate receptors. Activation of presynaptic receptors by low concentrations of kainate, but not ATPA, caused a tetrodotoxin-sensitive increase in the frequency of spontaneous EPSCs recorded in dorsal horn neurons. By recording synaptic pairs of DRG and dorsal horn neurons, we found that activation of presynaptic kainate and AMPA receptors decreased evoked glutamate release from terminals of DRG neurons in culture. Our data suggest that the endogenous ligand, glutamate, will cause a different physiological impact when activating these two types of non-NMDA glutamate receptors at central or peripheral nerve endings of sensory neurons.

Neuroprotection of adult rat dorsal root ganglion neurons by combined hypothermia and alkalinization against prolonged ischemia

Ischemia and ischemia-induced secondary events, such as acidosis and excessive activation of receptors by amino acids, trigger neuron death. The isolation and dissociation of dorsal root ganglion (DRG) involves time during which the neurons are ischemic due to being densely packed within the intact DRG and surrounded by a connective tissue coat. Thus, the longer the time between killing the host animal and when the DRG are dissociated, the longer the neurons are ischemic and exposed to ischemia-induced secondary causes of neuron death. It is well established that hypothermia and alkalinization each separately protect neurons from ischemia and ischemia-induced secondary causes of neuron death, but there are no data on the neuroprotection provided by simultaneous hypothermia and alkalinization. The present experiments were designed to determine the combination of hypothermic and alkaline conditions that yield the largest number of viable neurons dissociated from intact DRG maintained ischemic for up to 4 h. Hypothermia (20 degrees C>15 degrees C>37 degrees C) and alkalinization (pH 9.3>pH 8.3>pH 7.4) increased the yield of viable neurons compared with the yield from DRG maintained under physiological conditions. Hypothermia and alkalinization combined (20 degrees C/pH 9.3) provided the greatest neuroprotection with a yield of viable neurons after 1 h of ischemia 2.5-fold larger than that from DRG maintained under physiological conditions (37 degrees C/pH 7.6). Over 4 h of ischemia, the yield of viable neurons from DRG maintained under both hypothermic/alkaline and physiological conditions decreased in a linear manner, but those at 20 degrees C/pH 9.3 had a 4.5-fold greater yield of viable neurons than those at 37 degrees C/pH 7.6. Thus, combined hypothermia and alkalinization provide significantly greater protection against ischemia and ischemia-induced secondary causes of neuron death than either alone.