Amino acid-mediated EPSPs at primary afferent synapses with substantia gelatinosa neurones in the rat spinal cord

Functional remodeling of presynaptic voltage-gated calcium channels in superficial layers of the dorsal horn during neuropathic pain

N- and P/Q-type voltage-gated Ca2+ channels are critical for synaptic transmission. While their expression is increased in the dorsal root ganglion (DRG) neuron cell bodies during neuropathic pain conditions, less is known about their synaptic remodeling. Here, we combined genetic tools with 2-photon Ca2+ imaging to explore the functional remodeling that occurs in central presynaptic terminals of DRG neurons during neuropathic pain. We imaged GCaMP6s fluorescence responses in an ex vivo spinal cord preparation from mice expressing GCaMP6s in Trpv1-Cre lineage nociceptors. We show that Ca2+ transient amplitude is increased in central terminals of these neurons after spared nerve injury, and that this increase is mediated by both N- and P/Q-type channels. We found that GABA-B receptor-dependent inhibition of Ca2+ transients was potentiated in the superficial layer of the dorsal horn. Our results provide direct evidence toward nerve injury-induced functional remodeling of presynaptic Ca2+ channels in Trpv1-lineage nociceptor terminals.

A review of dorsal root ganglia and primary sensory neuron plasticity mediating inflammatory and chronic neuropathic pain

Pain is a sensory state resulting from complex integration of peripheral nociceptive inputs and central processing. Pain consists of adaptive pain that is acute and beneficial for healing and maladaptive pain that is often persistent and pathological. Pain is indeed heterogeneous, and can be expressed as nociceptive, inflammatory, or neuropathic in nature. Neuropathic pain is an example of maladaptive pain that occurs after spinal cord injury (SCI), which triggers a wide range of neural plasticity. The nociceptive processing that underlies pain hypersensitivity is well-studied in the spinal cord. However, recent investigations show maladaptive plasticity that leads to pain, including neuropathic pain after SCI, also exists at peripheral sites, such as the dorsal root ganglia (DRG), which contains the cell bodies of sensory neurons. This review discusses the important role DRGs play in nociceptive processing that underlies inflammatory and neuropathic pain. Specifically, it highlights nociceptor hyperexcitability as critical to increased pain states. Furthermore, it reviews prior literature on glutamate and glutamate receptors, voltage-gated sodium channels (VGSC), and brain-derived neurotrophic factor (BDNF) signaling in the DRG as important contributors to inflammatory and neuropathic pain. We previously reviewed BDNF's role as a bidirectional neuromodulator of spinal plasticity. Here, we shift focus to the periphery and discuss BDNF-TrkB expression on nociceptors, non-nociceptor sensory neurons, and non-neuronal cells in the periphery as a potential contributor to induction and persistence of pain after SCI. Overall, this review presents a comprehensive evaluation of large bodies of work that individually focus on pain, DRG, BDNF, and SCI, to understand their interaction in nociceptive processing.

Presynaptic glutamate receptors in nociception

Chronic pain is a frequent, distressing and poorly understood health problem. Plasticity of synaptic transmission in the nociceptive pathways after inflammation or injury is assumed to be an important cellular basis for chronic, pathological pain. Glutamate serves as the main excitatory neurotransmitter at key synapses in the somatosensory nociceptive pathways, in which it acts on both ionotropic and metabotropic glutamate receptors. Although conventionally postsynaptic, compelling anatomical and physiological evidence demonstrates the presence of presynaptic glutamate receptors in the nociceptive pathways. Presynaptic glutamate receptors play crucial roles in nociceptive synaptic transmission and plasticity. They modulate presynaptic neurotransmitter release and synaptic plasticity, which in turn regulates pain sensitization. In this review, we summarize the latest understanding of the expression of presynaptic glutamate receptors in the nociceptive pathways, and how they contribute to nociceptive information processing and pain hypersensitivity associated with inflammation / injury. We uncover the cellular and molecular mechanisms of presynaptic glutamate receptors in shaping synaptic transmission and plasticity to mediate pain chronicity, which may provide therapeutic approaches for treatment of chronic pain.

Capsaicin-Induced Endocytosis of Endogenous Presynaptic CaV2.2 in DRG-Spinal Cord Co-Cultures Inhibits Presynaptic Function

The N-type calcium channel, CaV2.2 is key to neurotransmission from the primary afferent terminals of dorsal root ganglion (DRG) neurons to their postsynaptic targets in the spinal cord. In this study, we have utilized CaV2.2_HA knock-in mice, because the exofacial epitope tag in CaV2.2_HA enables accurate detection and localization of endogenous CaV2.2. CaV2.2_HA knock-in mice were used as a source of DRGs to exclusively study the presynaptic expression of N-type calcium channels in co-cultures between DRG neurons and wild-type spinal cord neurons. CaV2.2_HA is strongly expressed on the cell surface, particularly in TRPV1-positive small and medium DRG neurons. Super-resolution images of the presynaptic terminals revealed an increase in CaV2.2_HA expression and increased association with the postsynaptic marker Homer over time in vitro. Brief application of the TRPV1 agonist, capsaicin, resulted in a significant down-regulation of cell surface CaV2.2_HA expression in DRG neuron somata. At their presynaptic terminals, capsaicin caused a reduction in CaV2.2_HA proximity to and co-localization with the active zone marker RIM 1/2, as well as a lower contribution of N-type channels to single action potential-mediated Ca2+ influx. The mechanism of this down-regulation of CaV2.2_HA involves a Rab11a-dependent trafficking process, since dominant-negative Rab11a (S25N) occludes the effect of capsaicin on presynaptic CaV2.2_HA expression, and also prevents the effect of capsaicin on action potential-induced Ca2+ influx. Taken together, these data suggest that capsaicin causes a decrease in cell surface CaV2.2_HA expression in DRG terminals via a Rab11a-dependent endosomal trafficking pathway.

GABAB receptors constrain glutamate presynaptic release and postsynaptic actions in substantia gelatinosa of rat spinal cord

The substantia gelatinosa (SG, lamina II of spinal cord gray matter) is pivotal for modulating nociceptive information from the peripheral to the central nervous system. γ-Aminobutyric acid type B receptors (GABA_BRs), the metabotropic GABA receptor subtype, are widely expressed in pre- and postsynaptic structures of the SG. Activation of GABA_BRs by exogenous agonists induces both pre- and postsynaptic inhibition. However, the actions of endogenous GABA via presynaptic GABA_BRs on glutamatergic synapses, and the postsynaptic GABA_BRs interaction with glutamate, remain elusive. In the present study, first, using in vitro whole-cell recordings and taking minimal stimulation strategies, we found that in rat spinal cord glutamatergic synapses, blockade of presynaptic GABA_BRs switched "silent" synapses into active ones and increased the probability of glutamate release onto SG neurons; increasing ambient GABA concentration mimicked GABA_BRs activation on glutamatergic terminals. Next, using holographic photostimulation to uncage glutamate on postsynaptic SG neurons, we found that postsynaptic GABA_BRs modified glutamate-induced postsynaptic potentials. Taken together, our data identify that endogenous GABA heterosynaptically constrains glutamate release via persistently activating presynaptic GABA_BRs; and postsynaptically, GABA_BRs modulate glutamate responses. The results give new clues for endogenous GABA in modulating the nociception circuit of the spinal dorsal horn and shed fresh light on the postsynaptic interaction of glutamate and GABA.

Differential Modulation of Dorsal Horn Neurons by Various Spinal Cord Stimulation Strategies

New strategies for spinal cord stimulation (SCS) for chronic pain have emerged in recent years, which may work better via different analgesic mechanisms than traditional low-frequency (e.g., 50 Hz) paresthesia-based SCS. To determine if 10 kHz and burst SCS waveforms might have a similar mechanistic basis, we examined whether these SCS strategies at intensities ostensibly below sensory thresholds would modulate spinal dorsal horn (DH) neuronal function in a neuron type-dependent manner. By using an in vivo electrophysiological approach in rodents, we found that low-intensity 10 kHz SCS, but not burst SCS, selectively activates inhibitory interneurons in the spinal DH. This study suggests that low-intensity 10 kHz SCS may inhibit pain-sensory processing in the spinal DH by activating inhibitory interneurons without activating DC fibers, resulting in paresthesia-free pain relief, whereas burst SCS likely operates via other mechanisms.

Communication consumes 35 times more energy than computation in the human cortex, but both costs are needed to predict synapse number

Darwinian evolution tends to produce energy-efficient outcomes. On the other hand, energy limits computation, be it neural and probabilistic or digital and logical. Taking a particular energy-efficient viewpoint, we define neural computation and make use of an energy-constrained computational function. This function can be optimized over a variable that is proportional to the number of synapses per neuron. This function also implies a specific distinction between adenosine triphosphate (ATP)-consuming processes, especially computation per se vs. the communication processes of action potentials and transmitter release. Thus, to apply this mathematical function requires an energy audit with a particular partitioning of energy consumption that differs from earlier work. The audit points out that, rather than the oft-quoted 20 W of glucose available to the human brain, the fraction partitioned to cortical computation is only 0.1 W of ATP [L. Sokoloff, Handb. Physiol. Sect. I Neurophysiol. 3, 1843-1864 (1960)] and [J. Sawada, D. S. Modha, "Synapse: Scalable energy-efficient neurosynaptic computing" in Application of Concurrency to System Design (ACSD) (2013), pp. 14-15]. On the other hand, long-distance communication costs are 35-fold greater, 3.5 W. Other findings include 1) a [Formula: see text]-fold discrepancy between biological and lowest possible values of a neuron's computational efficiency and 2) two predictions of N, the number of synaptic transmissions needed to fire a neuron (2,500 vs. 2,000).

Menthol facilitates excitatory and inhibitory synaptic transmission in rat medullary dorsal horn neurons

Menthol, which acts as an agonist for transient receptor potential melastatin 8 (TRPM8), has complex effects on nociceptive transmission, including pain relief and hyperalgesia. Here, we addressed the effects of menthol on spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs, respectively) in medullary dorsal horn neurons, using a whole-cell patch-clamp technique. Menthol significantly increased sEPSC frequency, in a concentration-dependent manner, without affecting current amplitudes. The menthol-induced increase in sEPSC frequency could be completely blocked by AMTB, a TRPM8 antagonist, but was not blocked by HC-030031, a transient receptor potential ankyrin 1 (TRPA1) antagonist. Menthol still increased sEPSC frequency in the presence of Cd²⁺, a general voltage-gated Ca²⁺ channel blocker, suggesting that voltage-gated Ca²⁺ channels are not involved in the menthol-induced increase in sEPSC frequency. However, menthol failed to increase sEPSC frequency in the absence of extracellular Ca²⁺, suggesting that TRPM8 on primary afferent terminals is Ca²⁺ permeable. On the other hand, menthol also increased sIPSC frequency, without affecting current amplitudes. The menthol-induced increase in sIPSC frequency could be completely blocked by either AMTB or CNQX, an AMPA/KA receptor antagonist, suggesting that the indirect increase in excitability of inhibitory interneurons may lead to the facilitation of spontaneous GABA and/or glycine release. The present results suggested that menthol exerts analgesic effects, via the enhancement of inhibitory synaptic transmission, through central feed-forward neural circuits within the medullary dorsal horn region.

(-)-menthol increases excitatory transmission by activating both TRPM8 and TRPA1 channels in mouse spinal lamina II layer

(-)-menthol, a major form of menthol, is one of the most commonly used chemicals. Many studies have demonstrated that (-)-menthol produces analgesic action through peripheral mechanisms which are mainly mediated by activation of TRPM8. Moreover, intrathecal injection of menthol induces analgesia as well. However, the central actions and mechanisms of (-)-menthol remain unclear. Here, we have investigated the action of (-)-menthol on excitatory synaptic transmission in spinal lamina II layer which plays a pivotal role in modulating nociceptive transmission from the periphery by using patch-clamp technique in mice spinal cord. We found that (-)-menthol increased miniature excitatory postsynaptic current frequency. The frequency increases which (-)-menthol induced were in a dose-dependent manner (EC50: 0.1079 mM). However, neither genetic knockout nor pharmacological inhibition of TRPM8 could block (-)-menthol-induced effects entirely. Furthermore, this increase was also impaired by TRPA1 antagonist HC030031, but abolished utterly by co-application of TRPM8 and TRPA1 antagonist. Our results indicate that (-)-menthol increases the excitatory synaptic transmission by activating either TRPA1 or TRPM8 channels in spinal lamina II layer.

Molecular Mechanisms of the Sense of Touch: An Overview of Mechanical Transduction and Transmission in Merkel Discs of Whisker Hair Follicles and Some Clinical Perspectives

The Merkel disc is a main type of tactile end organs for sensing gentle touch and is essential for sophisticated sensory tasks including social interaction, environmental exploration, and tactile discrimination. Recent studies have shown that Merkel cells are primary sites of mechanotransduction using Piezo2 channels as a molecular transducer in Merkel discs. Furthermore, tactile stimuli trigger serotonin release from Merkel cells to excite their associated whisker Aβ-afferent endings and transmit tactile signals. The tactile transduction and transmission at Merkel discs may have important clinical implications in sensory dysfunctions such as the loss of tactile sensitivity and tactile allodynia seen in patients who have diabetes and inflammatory diseases and undergo chemotherapy.

Identification of a Spinal Circuit for Mechanical and Persistent Spontaneous Itch

Lightly stroking the lips or gently poking some skin regions can evoke mechanical itch in healthy human subjects. Sensitization of mechanical itch and persistent spontaneous itch are intractable symptoms in chronic itch patients. However, the underlying neural circuits are not well defined. We identified a subpopulation of excitatory interneurons expressing Urocortin 3::Cre (Ucn3⁺) in the dorsal spinal cord as a central node in the pathway that transmits acute mechanical itch and mechanical itch sensitization as well as persistent spontaneous itch under chronic itch conditions. This population receives peripheral inputs from Toll-like receptor 5-positive (TLR5⁺) Aβ low-threshold mechanoreceptors and is directly innervated by inhibitory interneurons expressing neuropeptide Y::Cre (NPY⁺) in the dorsal spinal cord. Reduced synaptic inhibition and increased intrinsic excitability of Ucn3⁺ neurons lead to chronic itch sensitization. Our study sheds new light on the neural basis of chronic itch and unveils novel avenues for developing mechanism-specific therapeutic advancements.

Action potentials and subthreshold potentials of dorsal horn neurons in a rat model of myositis: a study employing intracellular recordings in vivo

Intracellular in vivo recordings from rat dorsal horn neurons were made to study the contribution of microglia to the central sensitization of spinal synapses induced by a chronic muscle inflammation. To block microglia activation, minocycline was continuously administered intrathecally during development of the inflammation. The aim was to test whether an inflammation-induced sensitization of dorsal horn neurons is mediated by changes in synaptic strength or other synaptic changes and how activated microglia influence these processes. Intracellular recordings were used to measure subthreshold excitatory postsynaptic potentials (EPSPs) and suprathreshold action potentials (APs). The muscle inflammation significantly increased the proportion of dorsal horn neurons responding with APs or EPSPs to electrical stimulation of the muscle nerve from 27 to 56% (P < 0.01) and to noxious muscle stimulation (3 vs. 44%, P < 0.01). Neurons showing spontaneous ongoing AP or EPSP activity increased from 28 to 74% (P < 0.01). Generally, the increases in suprathreshold AP responses did not occur at the expense of subthreshold EPSPs, because EPSP-only responses also increased. Intrathecal minocycline prevented the inflammation-induced increase in responsiveness to electrical (24%, P < 0.02) and mechanical stimulation (14%, P < 0.02); the effect was stronger on suprathreshold APs than on subthreshold EPSPs. The increase in ongoing activity was only partly suppressed. These data suggest that the myositis-induced hypersensitivity of the dorsal horn neurons to peripheral input and its prevention by intrathecal minocycline treatment were due to both an increase in the number of active synapses and an increased synaptic strength.NEW & NOTEWORTHY During a chronic muscle inflammation (myositis), activated microglia controls both the increase in the number of active synapses and the increase in synaptic strength.

Itch and neuropathic itch

Neuropathic itch is a pathological condition that is due to damage within the nervous system. This type of itch can be severe and unrelenting, which has a very negative impact on quality of life. Neuropathic itch is more common than generally appreciated because most types of neuropathic pain have a neuropathic itch counterpart. Unfortunately, much like neuropathic pain, there is a lack of effective treatments for neuropathic itch. Here, we consider the neural basis of itch and then describe how injuries within these neural circuits can lead to neuropathic itch in both animal models and human disease states.

Ethanol-induced enhancement of inhibitory synaptic transmission in the rat spinal substantia gelatinosa

Recent studies have shown that ethanol produces a widespread modulation of neuronal activity in the central nervous system. It is not fully understood, however, how ethanol changes nociceptive transmission. We investigated acute effects of ethanol on synaptic transmission in the substantia gelatinosa (lamina II of the spinal dorsal horn) and mechanical responses in the spinal dorsal horn. In substantia gelatinosa neurons, bath application of ethanol at low concentration (10 mM) did not change the frequency and amplitude of spontaneous inhibitory postsynaptic currents. At medium to high concentrations (20–100 mM), however, ethanol elicited a barrage of large amplitude spontaneous inhibitory postsynaptic currents. In the presence of tetrodotoxin, such enhancement of spontaneous inhibitory postsynaptic currents was not detected. In addition, ethanol (20–100 mM) increased the frequency of spontaneous discharge of vesicular GABA transporter-Venus-labeled neurons and suppressed the mechanical nociceptive response in wide-dynamic range neurons in the spinal dorsal horn. The present results suggest that ethanol may reduce nociceptive information transfer in the spinal dorsal horn by enhancement of inhibitory GABAergic and glycinergic synaptic transmission.

Regulating nociceptive transmission by VGluT2-expressing spinal dorsal horn neurons

Vesicular glutamate transporter-2 (VGluT2) mediates the uptake of glutamate into synaptic vesicles in neurons. Spinal cord dorsal horn interneurons are highly heterogeneous and molecularly diverse. The functional significance of VGluT2-expressing dorsal horn neurons in physiological and pathological pain conditions has not been explicitly demonstrated. Designer receptors exclusively activated by designer drugs (DREADDs) are a powerful chemogenetic tool to reversibly control neuronal excitability and behavior. Here, we used transgenic mice with Cre recombinase expression driven by the VGluT2 promoter, combined with the chemogenetic approach, to determine the contribution of VGluT2-expressing dorsal horn neurons to nociceptive regulation. Adeno-associated viral vectors expressing double-floxed Cre-dependent Gαq-coupled human M3 muscarinic receptor DREADD (hM3D)-mCherry or Gαi-coupled κ-opioid receptor DREADD (KORD)-IRES-mCitrine were microinjected into the superficial spinal dorsal horn of VGluT2-Cre mice. Immunofluorescence labeling showed that VGluT2 was predominantly expressed in lamina II excitatory interneurons. Activation of excitatory hM3D in VGluT2-expressing neurons with clozapine N-oxide caused a profound increase in neuronal firing and synaptic glutamate release. Conversely, activation of inhibitory KORD in VGluT2-expressing neurons with salvinorin B markedly inhibited neuronal activity and synaptic glutamate release. In addition, chemogenetic stimulation of VGluT2-expressing neurons increased mechanical and thermal sensitivities in naive mice, whereas chemogenetic silencing of VGluT2-expressing neurons reversed pain hypersensitivity induced by tissue inflammation and peripheral nerve injury. These findings indicate that VGluT2-expressing excitatory neurons play a crucial role in mediating nociceptive transmission in the spinal dorsal horn. Targeting glutamatergic dorsal horn neurons with inhibitory DREADDs may be a new strategy for treating inflammatory pain and neuropathic pain.

Spinal dorsal horn astrocytes release GABA in response to synaptic activation

KEY POINTS

GABA is an essential molecule for sensory information processing. It is usually assumed to be released by neurons. Here we show that in the dorsal horn of the spinal cord, astrocytes respond to glutamate by releasing GABA. Our findings suggest a novel role for astrocytes in somatosensory information processing.

ABSTRACT

Astrocytes participate in neuronal signalling by releasing gliotransmitters in response to neurotransmitters. We investigated if astrocytes from the dorsal horn of the spinal cord of adult red-eared turtles (Trachemys scripta elegans) release GABA in response to glutamatergic receptor activation. For this, we developed a GABA sensor consisting of HEK cells expressing GABA_A receptors. By positioning the sensor recorded in the whole-cell patch-clamp configuration within the dorsal horn of a spinal cord slice, we could detect GABA in the extracellular space. Puff application of glutamate induced GABA release events with time courses that exceeded the duration of inhibitory postsynaptic currents by one order of magnitude. Because the events were neither affected by extracellular addition of nickel, cadmium and tetrodotoxin nor by removal of Ca²⁺ , we concluded that they originated from non-neuronal cells. Immunohistochemical staining allowed the detection of GABA in a fraction of dorsal horn astrocytes. The selective stimulation of A∂ and C fibres in a dorsal root filament induced a Ca²⁺ increase in astrocytes loaded with Oregon Green BAPTA. Finally, chelating Ca²⁺ in a single astrocyte was sufficient to prevent the GABA release evoked by glutamate. Our results indicate that glutamate triggers the release of GABA from dorsal horn astrocytes with a time course compatible with the integration of sensory inputs.

Timing Mechanisms Underlying Gate Control by Feedforward Inhibition

The gate control theory proposes that Aβ mechanoreceptor inputs to spinal pain transmission T neurons are gated via feedforward inhibition, but it remains unclear how monosynaptic excitation is gated by disynaptic inhibitory inputs that arrive later. Here we report that Aβ-evoked, non-NMDAR-dependent EPSPs in T neurons are subthreshold, allowing time for inhibitory inputs to prevent action potential firing that requires slow-onset NMDAR activation. Potassium channel activities-including I_A, whose sizes are established constitutively by Preprodynorphin^Cre-derived inhibitory neurons-either completely filter away Aβ inputs or make them subthreshold, thereby creating a permissive condition to achieve gate control. Capsaicin-activated nociceptor inputs reduce I_A and sensitize the T neurons, allowing Aβ inputs to cause firing before inhibitory inputs arrive. Thus, distinct kinetics of glutamate receptors and electric filtering by potassium channels solve the timing problem underlying the gating by feedforward inhibition, and their modulation offers a way to bypass the gate control.

NMDA Receptor Dependent Long-term Potentiation in Chronic Pain

Since the discovery of NMDA receptor (NMDAR) dependent long-term potentiation (LTP) in the hippocampus, many studies have demonstrated that NMDAR dependent LTP exists throughout central synapses, including those involved in sensory transmission and perception. NMDAR LTP has been reported in spinal cord dorsal horn synapses, anterior cingulate cortex and insular cortex. Behavioral, genetic and pharmacological studies show that inhibiting or reducing NMDAR LTP produced analgesic effects in animal models of chronic pain. Investigation of signalling mechanisms for NMDAR LTP may provide novel targets for future treatment of chronic pain.

Mechanisms of the analgesic effect of calcitonin on chronic pain by alteration of receptor or channel expression

The polypeptide hormone calcitonin is well known clinically for its ability to relieve osteoporotic back pain and neuropathic pain such as spinal canal stenosis, diabetic neuropathy, chemotherapy-induced neuropathy, and complex regional pain syndrome. Because the analgesic effects of calcitonin have a broad range, the underlying mechanisms of pain relief by calcitonin are largely unknown. However, recent studies using several types of chronic pain models combined with various methods have been gradually clarifying the mechanism. Here, we review the mechanisms of the analgesic action of calcitonin on ovariectomy-induced osteoporotic and neuropathic pain. The analgesic action of calcitonin may be mediated by restoration of serotonin receptors that control selective glutamate release from C-afferent fibers in ovariectomized rats and by normalization of sodium channel expression in damaged peripheral nerves. Serotonin receptors are reduced or eliminated by the relatively rapid reduction in estrogen during the postmenopausal period, and damaged nerves exhibit hyperexcitability due to abnormal expression of Na⁺ channel subtypes. In addition, in chemotherapy-induced peripheral neuropathy, inhibition of signals related to transient receptor potential ankyrin-1 and melastatin-8 is proposed to participate in the anti-allodynic action of calcitonin. Further, an unknown calcitonin-dependent signal appears to be present in peripheral nervous tissues and may be activated by nerve injury, resulting in regulation of the excitability of primary afferents by control of sodium channel transcription in dorsal root ganglion neurons. The calcitonin signal in normal conditions may be non-functional because no target is present, and ovariectomy or nerve injury may induce a target. Moreover, it has been reported that calcitonin reduces serotonin transporter but increases serotonin receptor expression in the thalamus in ovariectomized rats. These data suggest that calcitonin could alleviate lower back pain in patients with osteoporosis or neuropathic pain by the alteration in receptor or channel expression.

Presynaptic mGluR5 receptor controls glutamatergic input through protein kinase C-NMDA receptors in paclitaxel-induced neuropathic pain

Chemotherapeutic drugs such as paclitaxel cause painful peripheral neuropathy in many cancer patients and survivors. Although NMDA receptors (NMDARs) at primary afferent terminals are known to be critically involved in chemotherapy-induced chronic pain, the upstream signaling mechanism that leads to presynaptic NMDAR activation is unclear. Group I metabotropic glutamate receptors (mGluRs) play a role in synaptic plasticity and NMDAR regulation. Here we report that the Group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) significantly increased the frequency of miniature excitatory postsynaptic currents (EPSCs) and the amplitude of monosynaptic EPSCs evoked from the dorsal root. DHPG also reduced the paired-pulse ratio of evoked EPSCs in spinal dorsal horn neurons. These effects were blocked by the selective mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP), but not by an mGluR1 antagonist. MPEP normalized the frequency of miniature EPSCs and the amplitude of evoked EPSCs in paclitaxel-treated rats but had no effect in vehicle-treated rats. Furthermore, mGluR5 protein levels in the dorsal root ganglion and spinal cord synaptosomes were significantly higher in paclitaxel- than in vehicle-treated rats. Inhibiting protein kinase C (PKC) or blocking NMDARs abolished DHPG-induced increases in the miniature EPSC frequency of spinal dorsal horn neurons in vehicle- and paclitaxel-treated rats. Moreover, intrathecal administration of MPEP reversed pain hypersensitivity caused by paclitaxel treatment. Our findings suggest that paclitaxel-induced painful neuropathy is associated with increased presynaptic mGluR5 activity at the spinal cord level, which serves as upstream signaling for PKC-mediated tonic activation of NMDARs. mGluR5 is therefore a promising target for reducing chemotherapy-induced neuropathic pain.

Endogenous nitric oxide inhibits spinal NMDA receptor activity and pain hypersensitivity induced by nerve injury

The role of nitric oxide (NO) in nociceptive transmission at the spinal cord level remains uncertain. Increased activity of spinal N-methyl-d-aspartate (NMDA) receptors contributes to development of chronic pain induced by peripheral nerve injury. In this study, we determined how endogenous NO affects NMDA receptor activity of spinal cord dorsal horn neurons in control and spinal nerve-ligated rats. Bath application of the NO precursor l-arginine or the NO donor S-nitroso-N-acetylpenicillamine (SNAP) significantly inhibited NMDA receptor currents of spinal dorsal horn neurons in both sham control and nerve-injured rats. Inhibition of neuronal nitric oxide synthase (nNOS) or blocking the S-nitrosylation reaction with N-ethylmaleimide abolished the inhibitory effects of l-arginine on NMDA receptor currents recorded from spinal dorsal horn neurons in sham control and nerve-injured rats. However, bath application of the cGMP analog 8-bromo-cGMP had no significant effects on spinal NMDA receptor currents. Inhibition of soluble guanylyl cyclase also did not alter the inhibitory effect of l-arginine on spinal NMDA receptor activity. Furthermore, knockdown of nNOS with siRNA abolished the inhibitory effects of l-arginine, but not SNAP, on spinal NMDA receptor activity in both groups of rats. Additionally, intrathecal injection of l-arginine significantly attenuated mechanical or thermal hyperalgesia induced by nerve injury, and the l-arginine effect was diminished in rats treated with a nNOS inhibitor or nNOS-specific siRNA. These findings suggest that endogenous NO inhibits spinal NMDA receptor activity through S-nitrosylation. NO derived from nNOS attenuates spinal nociceptive transmission and neuropathic pain induced by nerve injury.

Serotonergic transmission at Merkel discs: modulation by exogenously applied chemical messengers and involvement of Ih currents

The Merkel disc is a main type of tactile end organ consisting of Merkel cells and Aβ-afferent endings that responds to tactile stimulation with slowly adapting type 1 (SA1) afferent impulses. Our recent study has shown that Merkel discs in whisker hair follicles are serotonergic synapses using endogenous serotonin to transmit tactile signals from Merkel cells to Aβ-afferent endings. In this study, we hypothesize that tactile sensitivity of Merkel discs can be modulated by chemical messengers. We tested this hypothesis by determining whether and how SA1 responses of mouse whisker hair follicles may be affected by exogenously applied chemical messengers. We found that SA1 responses were potentiated by serotonin at low concentration (10 μM) but almost completely occluded by serotonin at high concentration (2 mM). In contrast, SA1 responses were not significantly affected by ATP and its metabolically stable analog α,β-methylene-ATP, glutamate, γ-aminobutyric acid (GABA), and histamine. SA1 responses were also not affected by antagonists for P2X receptors, ionotropic glutamate receptors, and ionotropic GABA and glycine receptors. Whole-cell patch-clamp recordings reconfirm the presence of both ionotropic and metabotropic 5-HT receptors on afferent neurons and their terminals innervating whisker hair follicles. All whisker afferent neurons expressed hyperpolarization-activated inward currents (I_h ), which are potentiated by serotonin through the activation of metabotropic 5-HT receptors. Taken together, the findings substantiate the serotonergic mechanism of tactile transmission at Merkel discs and identify the involvement of I_h currents in postsynaptic excitatory actions of serotonin. In addition, the findings do not favor any significant involvement of ATP, glutamate, histamine, GABA, or glycine in tactile transmission at the Merkel discs of whisker hair follicles.

P2X7 receptor-sensitivity of astrocytes and neurons in the substantia gelatinosa of organotypic spinal cord slices of the mouse depends on the length of the culture period

The whole-cell patch-clamp technique was used to record current responses to AMPA, N-methyl-d-aspartate (NMDA), muscimol and dibenzoyl-ATP (Bz-ATP) in superficial (reactive/gliotic) substantia gelatinosa (SG) astrocytes and neurons of spinal cord slices kept for different periods of time in organotypic culture. Currents induced by AMPA, NMDA and muscimol confirmed the existence of their specific receptors in 2-week-old neurons; astrocytes cultured for the same period of time responded to AMPA and muscimol, but not to NMDA. AMPA had a larger effect on 2-week-old astrocytes than on the 1-week-old ones, in spite of a similar sensitivity of the age-matched neurons to this amino acid. The effect of the prototypic P2X7 receptor agonist Bz-ATP on superficial astrocytes and neurons depended on the drug concentration applied and increased in parallel with the lengthening of the culture period. The amplitudes of Bz-ATP currents of deep (resting) astrocytes were age-independent. Neurons located in deep layers exhibited after 1week of culturing much larger Bz-ATP currents than the superficial ones of the same age. In conclusion, whereas resting astrocytes had culture period-independent P2X7 receptor-sensitivity, reactive/gliotic astrocytes exhibited P2X7 receptor-sensitivity increasing in parallel with the prolongation of the time spent in culture. The results with Bz-ATP agree with the facilitation of AMPA-induced currents in reactive astrocytes during development, and with the hypothesis that extracellular ATP is an ontogenetically early transmitter/signaling molecule in the CNS.

Merkel disc is a serotonergic synapse in the epidermis for transmitting tactile signals in mammals

The evolution of sensory systems has let mammals develop complicated tactile end organs to enable sophisticated sensory tasks, including social interaction, environmental exploration, and tactile discrimination. The Merkel disc, a main type of tactile end organ consisting of Merkel cells (MCs) and Aβ-afferent endings, are highly abundant in fingertips, touch domes, and whisker hair follicles of mammals. The Merkel disc has high tactile acuity for an object's physical features, such as texture, shape, and edges. Mechanisms underlying the tactile function of Merkel discs are obscured as to how MCs transmit tactile signals to Aβ-afferent endings leading to tactile sensations. Using mouse whisker hair follicles, we show herein that tactile stimuli are transduced by MCs into excitatory signals that trigger vesicular serotonin release from MCs. We identify that both ionotropic and metabotropic 5-hydroxytryptamine (5-HT) receptors are expressed on whisker Aβ-afferent endings and that their activation by serotonin released from MCs initiates Aβ-afferent impulses. Moreover, we demonstrate that these ionotropic and metabotropic 5-HT receptors have a synergistic effect that is critical to both electrophysiological and behavioral tactile responses. These findings elucidate that the Merkel disc is a unique serotonergic synapse located in the epidermis and plays a key role in tactile transmission. The epidermal serotonergic synapse may have important clinical implications in sensory dysfunctions, such as the loss of tactile sensitivity and tactile allodynia seen in patients who have diabetes, inflammatory diseases, and undergo chemotherapy. It may also have implications in the exaggerated tactile sensations induced by recreational drugs that act on serotoninergic synapses.

Modulation of afferent nerve activity by prostaglandin E2 upon urinary bladder distension in rats

NEW FINDINGS

What is the central question of this study? It has been widely assumed that C fibres innervating the bladder are mainly excited in overactive bladder syndrome. However, it remains unclear whether Aδ fibres are also activated in pathological conditions. What is the main finding and its importance? We found that a certain population of Aδ fibres, which become active specifically at a bladder pressure of more than 15 cmH2 O in normal conditions, showed increased excitability in conditions of prostaglandin E2 -induced overactive bladder. This result suggests that a certain population of Aδ fibres, together with C fibres, triggers pathophysiological activity. In overactive bladder syndrome, afferent C fibres innervating the bladder show an increased activity level. However, it remains unclear whether all C fibres are highly activated and whether Aδ fibres, the other type of bladder afferent fibre, are also involved in pathological conditions. To address these questions, we analysed the relationship between bladder pressure and single-unit firing patterns of afferent nerves in the left L6 dorsal roots in living rats. The recorded fibres were classified as Aδ fibres or C fibres based on the response to 0.3 μm tetrodotoxin. Certain populations of both Aδ fibres and C fibres were activated at bladder pressures below 15 cmH2 O (classified as low-threshold fibres), indicating their potential contribution to detection of normal bladder filling. Intravesical administration of prostaglandin E2 (PGE2 ) induced hyperexcitation in approximately half of such C fibres, whereas the activity patterns of low-threshold Aδ fibres were unchanged. All fibres, regardless of type, which were almost silent in control conditions (classified as high-threshold fibres), were activated by application of PGE2 . Notably, the firing patterns of Aδ fibres, rather than C fibres, were highly time locked to PGE2 -induced micro-oscillation of bladder pressure. These modulatory effects of PGE2 on Aδ fibres and C fibres might trigger pathophysiological activity together in overactive bladder syndrome.

In vivo patch-clamp analysis of the antinociceptive actions of TRPA1 activation in the spinal dorsal horn

Background

Transient receptor potential (TRP) channels are nonselective cation channels expressed in a variety of sensory structures, and are important molecular mediators of thermal, mechanical, cellular and chemical signals. We investigated the function of one key member of the TRP superfamily, TRPA1, in the spinal dorsal horn using in vivo patch-clamp recordings.

Results

The application of allyl isothiocyanate (AITC), a TRPA1 agonist, significantly increased the frequency and amplitude of inhibitory postsynaptic currents (IPSCs; holding potential (V_H) = 0 mV) as well as excitatory postsynaptic currents (EPSCs; V_H = −70 mV) in substantia gelatinosa (SG) neurons. The AITC-induced increases in EPSC frequency and amplitude were resistant to the Na⁺ channel blocker tetrodotoxin (TTX). In the presence of the glutamate receptor antagonists CNQX and AP5, AITC did not generate any synaptic activity. The AITC-induced increases in IPSC frequency and amplitude were abolished by TTX or glutamate receptor antagonists. Moreover, the duration of IPSCs enhanced by TRPA1 activation were significantly longer than those of EPSCs enhanced by activation of this channel in the spinal dorsal horn. AITC induced hyperpolarization of the membrane potential of SG neurons in the spinal cord but depolarized the membrane potential in the presence of TTX. Furthermore, we examined the effects of mechanical stimuli to the skin during TRPA1 activation in the spinal dorsal horn in normal rats in both voltage-clamp and current-clamp modes. In the peripheral tissue stimuli test, AITC significantly suppressed EPSCs evoked by pinch or air puff stimulation of the skin. In current-clamp mode, AITC significantly suppressed excitatory postsynaptic potentials (EPSPs) evoked by pinch stimuli.

Conclusions

TRPA1 appears to be localized not only at presynaptic terminals on SG neurons, enhancing glutamate release, but also in the terminals of primary afferents innervating spinal inhibitory interneurons, which have synaptic interactions with SG neurons. This study offers further insight into the mechanisms underlying the possible antinociceptive actions of TRPA1 activation in the spinal dorsal horn. Our findings suggest that pharmacological activation of spinal TRPA1 channels may have therapeutic potential for the treatment of pain.

Blockade of spinal glutamate recycling produces paradoxical antinociception in rats with orofacial inflammatory pain

In our current study, we investigated the role of spinal glutamate recycling in the development of orofacial inflammatory pain. DL-threo-β-benzyloxyaspartate (TBOA) or methionine sulfoximine (MSO) was administered intracisternally to block spinal glutamate transporter and glutamine synthetase activity in astroglia. Intracisternal administration of high dose TBOA (10 μg) produced thermal hyperalgesia in naïve rats but significantly attenuated the thermal hyperalgesia in rats that had been pretreated with interleukin (IL)-1β or Complete Freund's Adjuvant (CFA). In contrast, intracisternal injection of MSO produced anti-hyperalgesic effects against thermal stimuli in CFA-treated rats only. To confirm the paradoxical antinociceptive effects of TBOA and MSO, we examined changes in c-Fos expression in the medullary dorsal horn produced by thermal stimulation in naïve, IL-1β-, or CFA-treated rats, after intracisternal injections of TBOA and MSO. Intracisternal administration of TBOA significantly increased c-Fos immunoreactivity in naïve rats. In contrast, intracisternal administration of TBOA significantly decreased the up-regulation of c-Fos immunoreactivity in the medullary dorsal horn of IL-1β- and CFA-treated rats. However, intracisternal injection of MSO blocked the up-regulation of c-Fos immunoreactivity in CFA-treated rats only. We also investigated the effects of botulinum toxin type A (BoNT-A) on TBOA-induced paradoxical antinociception in CFA-treated rats, as BoNT-A inhibits the release of neurotransmitters, including glutamate. BoNT-A treatment reversed behavioral responses produced by intracisternal administration of TBOA in CFA-treated rats. These results suggest that the paradoxical responses produced by blocking glutamate transporters under inflammatory pain conditions are mediated by the modulation of glutamate release from presynaptic terminals. Moreover, blockade of glutamate reuptake could represent a new therapeutic target for the treatment of chronic inflammatory pain conditions.

Attenuation of inflammatory and neuropathic pain behaviors in mice through activation of free fatty acid receptor GPR40

Background

The G-protein-coupled receptor 40 (GPR40) is suggested to function as a transmembrane receptor for medium- to long-chain free fatty acids and is implicated to play a role in free fatty acids-mediated enhancement of glucose-stimulated insulin secretion from pancreas. However, the functional role of GPR40 in nervous system including somatosensory pain signaling has not been fully examined yet.

Results

Intrathecal injection of GPR40 agonist (MEDICA16 or GW9508) dose-dependently reduced ipsilateral mechanical allodynia in CFA and SNL models and thermal hyperalgesia in carrageenan model. These anti-allodynic and anti-hyperalgesic effects were almost completely reversed by a GPR40 antagonist, GW1100. Immunohistochemical analysis revealed that GPR40 is expressed in spinal dorsal horn and dorsal root ganglion neurons, and immunoblot analysis showed that carrageenan or CFA inflammation or spinal nerve injury resulted in increased expression of GPR40 in these areas. Patch-clamp recordings from spinal cord slices exhibited that bath-application of either MEDICA16 or GW9508 significantly decreased the frequency of spontaneous excitatory postsynaptic currents in the substantia gelatinosa neurons of the three pain models.

Conclusions

Our results indicate that GPR40 signaling pathway plays an important suppressive role in spinal nociceptive processing after inflammation or nerve injury, and that GPR40 agonists might serve as a new class of analgesics for treating inflammatory and neuropathic pain.

Electronic supplementary material

The online version of this article (doi:10.1186/s12990-015-0003-8) contains supplementary material, which is available to authorized users.

Inflammatory sensitization of nociceptors depends on activation of NMDA receptors in DRG satellite cells

The present study evaluated the role of N-methyl-D-aspartate receptors (NMDARs) expressed in the dorsal root ganglia (DRG) in the inflammatory sensitization of peripheral nociceptor terminals to mechanical stimulation. Injection of NMDA into the fifth lumbar (L5)-DRG induced hyperalgesia in the rat hind paw with a profile similar to that of intraplantar injection of prostaglandin E2 (PGE2), which was significantly attenuated by injection of the NMDAR antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP-5) in the L5-DRG. Moreover, blockade of DRG AMPA receptors by the antagonist 6,7-dinitroquinoxaline-2,3-dione had no effect in the PGE2-induced hyperalgesia in the paw, showing specific involvement of NMDARs in this modulatory effect and suggesting that activation of NMDAR in the DRG plays an important role in the peripheral inflammatory hyperalgesia. In following experiments we observed attenuation of PGE2-induced hyperalgesia in the paw by the knockdown of NMDAR subunits NR1, NR2B, NR2D, and NR3A with antisense-oligodeoxynucleotide treatment in the DRG. Also, in vitro experiments showed that the NMDA-induced sensitization of cultured DRG neurons depends on satellite cell activation and on those same NMDAR subunits, suggesting their importance for the PGE2-induced hyperalgesia. In addition, fluorescent calcium imaging experiments in cultures of DRG cells showed induction of calcium transients by glutamate or NMDA only in satellite cells, but not in neurons. Together, the present results suggest that the mechanical inflammatory nociceptor sensitization is dependent on glutamate release at the DRG and subsequent NMDAR activation in satellite glial cells, supporting the idea that the peripheral hyperalgesia is an event modulated by a glutamatergic system in the DRG.

Canadian Association of Neuroscience Review: Cellular and Synaptic Insights into Physiological and Pathological Pain

Neurons and synapses in the central nervous system are plastic, undergoing long-term changes throughout life. Studies of molecular and cellular mechanisms of such changes not only provide important insight into how we learn and store new knowledge in our brains, but they also reveal the mechanisms of pathological changes that occur following injury. The author proposes that during induction, neuronal mechanisms underlying physiological functions, such as learning and memory, may share some common signaling molecules with abnormal or injury-related changes in the brain. Distinct synaptic and neuronal network mechanisms are involved in pathological pain as compared to cognitive learning and memory. Nociceptive information is transmitted and regulated at different levels of the brain, from the spinal cord to the forebrain. Furthermore, N-methyl-D-aspartate receptor-dependent and calcium-calmodulin activated adenylyl cyclases (AC1 and AC8) in the anterior cingulate cortex play important roles in the induction and expression of persistent inflammatory and neuropathic pain. Neuronal activity in the anterior cingulate cortex can also influence nociceptive transmission in the dorsal horn of the spinal cord by activating the endogenous facilitatory system. Our results provide important synaptic and molecular insights into physiological responses to injury.

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.

Curcumin protects against ischemic spinal cord injury: The pathway effect

Inducible nitric oxide synthase and N-methyl-D-aspartate receptors have been shown to participate in nerve cell injury during spinal cord ischemia. This study observed a protective effect of curcumin on ischemic spinal cord injury. Models of spinal cord ischemia were established by ligating the lumbar artery from the left renal artery to the bifurcation of the abdominal aorta. At 24 hours after model establishment, the rats were intraperitoneally injected with curcumin. Reverse transcription-polymerase chain reaction and immunohistochemical results demonstrated that after spinal cord ischemia, inducible nitric oxide synthase and N-methyl-D-aspartate receptor mRNA and protein expression significantly increased. However, curcumin significantly decreased inducible nitric oxide synthase and N-methyl-D-aspartate receptor mRNA and protein expression in the ischemic spinal cord. Tarlov scale results showed that curcumin significantly improved motor function of the rat hind limb after spinal cord ischemia. The results demonstrate that curcumin exerts a neuroprotective fect against ischemic spinal cord injury by decreasing inducible nitric oxide synthase and N-methyl-D-aspartate receptor expression.

TRP Channels Involved in Spontaneous L-Glutamate Release Enhancement in the Adult Rat Spinal Substantia Gelatinosa

The spinal substantia gelatinosa (SG) plays a pivotal role in modulating nociceptive transmission through dorsal root ganglion (DRG) neurons from the periphery. TRP channels such as TRPV1 and TRPA1 channels expressed in the SG are involved in the regulation of the nociceptive transmission. On the other hand, the TRP channels located in the peripheral terminals of the DRG neurons are activated by nociceptive stimuli given to the periphery and also by plant-derived chemicals, which generates a membrane depolarization. The chemicals also activate the TRP channels in the SG. In this review, we introduce how synaptic transmissions in the SG neurons are affected by various plant-derived chemicals and suggest that the peripheral and central TRP channels may differ in property from each other.

Differential regulation of primary afferent input to spinal cord by muscarinic receptor subtypes delineated using knockout mice

Stimulation of muscarinic acetylcholine receptors (mAChRs) inhibits nociceptive transmission at the spinal level. However, it is unclear how each mAChR subtype regulates excitatory synaptic input from primary afferents. Here we examined excitatory postsynaptic currents (EPSCs) of dorsal horn neurons evoked by dorsal root stimulation in spinal cord slices from wild-type and mAChR subtype knock-out (KO) mice. In wild-type mice, mAChR activation with oxotremorine-M decreased the amplitude of monosynaptic EPSCs in ∼67% of neurons but increased it in ∼10% of neurons. The inhibitory effect of oxotremorine-M was attenuated by the M2/M4 antagonist himbacine in the majority of neurons, and the remaining inhibition was abolished by group II/III metabotropic glutamate receptor (mGluR) antagonists in wild-type mice. In M2/M4 double-KO mice, oxotremorine-M inhibited monosynaptic EPSCs in significantly fewer neurons (∼26%) and increased EPSCs in significantly more neurons (33%) compared with wild-type mice. Blocking group II/III mGluRs eliminated the inhibitory effect of oxotremorine-M in M2/M4 double-KO mice. In M2 single-KO and M4 single-KO mice, himbacine still significantly reduced the inhibitory effect of oxotremorine-M. However, the inhibitory and potentiating effects of oxotremorine-M on EPSCs in M3 single-KO and M1/M3 double-KO mice were similar to those in wild-type mice. In M5 single-KO mice, oxotremorine-M failed to potentiate evoked EPSCs, and its inhibitory effect was abolished by himbacine. These findings indicate that activation of presynaptic M2 and M4 subtypes reduces glutamate release from primary afferents. Activation of the M5 subtype either directly increases primary afferent input or inhibits it through indirectly stimulating group II/III mGluRs.

Alleviation of behavioral hypersensitivity in mouse models of inflammatory pain with two structurally different casein kinase 1 (CK1) inhibitors

BACKGROUND

The phylogenetically highly conserved CK1 protein kinases consisting of at least seven isoforms form a distinct family within the eukaryotic protein kinases. CK1 family members play crucial roles in a wide range of signaling activities. However, the functional role of CK1 in somatosensory pain signaling has not yet been fully understood. The aim of this study was to clarify the role of CK1 in the regulation of inflammatory pain in mouse carrageenan and complete Freund's adjuvant (CFA) models.

RESULTS

We have used two structurally different CK1 inhibitors, TG003 and IC261. TG003, which was originally identified as a cdc2-like kinase inhibitor, had potent inhibitory effects on CK1 isoforms in vitro and in cultured cells. Intrathecal injection of either TG003 (1-100 pmol) or IC261 (0.1-1 nmol) dose-dependently decreased mechanical allodynia and thermal hyperalgesia induced by carrageenan or CFA. Bath-application of either TG003 (1 μM) or IC261 (1 μM) had only marginal effects on spontaneous excitatory postsynaptic currents (sEPSCs) recorded in the substantia gelatinosa neurons of control mice. However, both compounds decreased the frequency of sEPSCs in both inflammatory pain models.

CONCLUSIONS

These results suggest that CK1 plays an important pathophysiological role in spinal inflammatory pain transmission, and that inhibition of the CK1 activity may provide a novel strategy for the treatment of inflammatory pain.

Calcium-permeable ion channels in pain signaling

The detection and processing of painful stimuli in afferent sensory neurons is critically dependent on a wide range of different types of voltage- and ligand-gated ion channels, including sodium, calcium, and TRP channels, to name a few. The functions of these channels include the detection of mechanical and chemical insults, the generation of action potentials and regulation of neuronal firing patterns, the initiation of neurotransmitter release at dorsal horn synapses, and the ensuing activation of spinal cord neurons that project to pain centers in the brain. Long-term changes in ion channel expression and function are thought to contribute to chronic pain states. Many of the channels involved in the afferent pain pathway are permeable to calcium ions, suggesting a role in cell signaling beyond the mere generation of electrical activity. In this article, we provide a broad overview of different calcium-permeable ion channels in the afferent pain pathway and their role in pain pathophysiology.

Ionotropic glutamate receptors and voltage-gated Ca²⁺ channels in long-term potentiation of spinal dorsal horn synapses and pain hypersensitivity

Over the last twenty years of research on cellular mechanisms of pain hypersensitivity, long-term potentiation (LTP) of synaptic transmission in the spinal cord dorsal horn (DH) has emerged as an important contributor to pain pathology. Mechanisms that underlie LTP of spinal DH neurons include changes in the numbers, activity, and properties of ionotropic glutamate receptors (AMPA and NMDA receptors) and of voltage-gated Ca²⁺ channels. Here, we review the roles and mechanisms of these channels in the induction and expression of spinal DH LTP, and we present this within the framework of the anatomical organization and synaptic circuitry of the spinal DH. Moreover, we compare synaptic plasticity in the spinal DH with classical LTP described for hippocampal synapses.

Tlx3 and Runx1 act in combination to coordinate the development of a cohort of nociceptors, thermoceptors, and pruriceptors

Neurons in the mouse dorsal root ganglia (DRGs) are composed of a variety of sensory modalities, such as pain-related nociceptors, itch-related pruriceptors, and thermoceptors. All these neurons are derived from late-born neurons that are initially marked by the expression of the nerve growth factor receptor TrkA. During perinatal and postnatal development, these TrkA lineage neurons are globally segregated into Ret-expressing and TrkA-expressing subtypes, and start to express a variety of sensory receptors and ion channels. The runt domain transcription factor Runx1 plays a pivotal role in controlling these developmental processes, but it remains unclear how it works. Here we showed that the homeodomain transcription factor Tlx3, expressed broadly in DRG neurons, is required to establish most Runx1-dependent phenotypes, including the segregation of TrkA-expressing versus Ret-expressing neurons and the expression of a dozen of sensory channels and receptors implicated in sensing pain, itch and temperature. Expression of Runx1 and Tlx3 is independent of each other at prenatal stages when they first establish the expression of these channels and receptors. Moreover, overexpression of Runx1 plus Tlx3 was able to induce ectopic expression of sensory channels and receptors. Collectively, these studies suggest that genetically Tlx3 acts in combination with Runx1 to control the development of a cohort of nociceptors, thermoceptors, and pruriceptors in mice.

Presynaptic Cav3.2 channels regulate excitatory neurotransmission in nociceptive dorsal horn neurons

It is generally accepted that presynaptic transmitter release is mainly regulated by subtypes of neuronal high-voltage-activated Ca(2+) channels. Here for the first time, we examined the role of T-type Ca(2+) channels (T-channels) in synaptic transmission in the dorsal horn (DH) of the spinal cord using patch-clamp recordings from acute spinal cord preparations from both rat and mouse. We found that selective pharmacological antagonism of T-channels inhibited spontaneous synaptic release of glutamate in superficial laminae I-II of the DH, while GABA release was spared. We found similar effect in identified nociceptive projection neurons of lamina I of the DH, but not in inhibitory DH interneurons. In comparison, antagonism of T-channels did not affect excitatory transmission in deeper non-nociceptive DH laminae. Furthermore, we used isoform-specific agents, knock-out mice and immunohistochemistry to specifically implicate presynaptic Ca(V)3.2 channels. We also used an animal model of painful diabetic neuropathy to demonstrate that blocking T-channels in superficial DH neurons suppressed spontaneous excitatory synaptic transmission in diabetic rats in greater degree than in healthy age-matched animals. These studies provide previously unknown information regarding the role of presynaptic T-channels in nociceptive signaling in the spinal cord.

Tlx1/3 and Ptf1a control the expression of distinct sets of transmitter and peptide receptor genes in the developing dorsal spinal cord

Establishing the pattern of expression of transmitters and peptides as well as their receptors in different neuronal types is crucial for understanding the circuitry in various regions of the brain. Previous studies have demonstrated that the transmitter and peptide phenotypes in mouse dorsal spinal cord neurons are determined by the transcription factors Tlx1/3 and Ptf1a. Here we show that these transcription factors also determine the expression of two distinct sets of transmitter and peptide receptor genes in this region. We have screened the expression of 78 receptor genes in the spinal dorsal horn by in situ hybridization. We found that receptor genes Gabra1, Gabra5, Gabrb2, Gria3, Grin3a, Grin3b, Galr1, and Npy1r were preferentially expressed in Tlx3-expressing glutamatergic neurons and their derivatives, and deletion of Tlx1 and Tlx3 resulted in the loss of expression of these receptor genes. Furthermore, we obtained genetic evidence that Tlx3 uses distinct pathways to control the expression of receptor genes. We also found that receptor genes Grm3, Grm4, Grm5, Grik1, Grik2, Grik3, and Sstr2 were mainly expressed in Pax2-expressing GABAergic neurons in the spinal dorsal horn, and their expression in this region was abolished or markedly reduced in Ptf1a and Pax2 deletion mutant mice. Together, our studies indicate that Tlx1/3 and Ptf1a, the key transcription factors for fate determination of glutamatergic and GABAergic neurons in the dorsal spinal cord, are also responsible for controlling the expression of two distinct sets of transmitter and peptide receptor genes.