Metabotropic glutamate receptor 5 regulates excitability and Kv4.2-containing K⁺ channels primarily in excitatory neurons of the spinal dorsal horn

Spinal Glycine Receptor Alpha 3 Cells Communicate Sensations of Chemical Itch in Hairy Skin

Glycinergic neurons regulate nociceptive and pruriceptive signaling in the spinal cord, but the identity and role of the glycine-regulated neurons are not fully known. Herein, we have characterized spinal glycine receptor alpha 3 (Glra3) subunit-expressing neurons in Glra3-Cre female and male mice. Glra3-Cre(+) neurons express Glra3, are located mainly in laminae III-VI, and respond to glycine. Chemogenetic activation of spinal Glra3-Cre(+) neurons induced biting/licking, stomping, and guarding behaviors, indicative of both a nociceptive and pruriceptive role for this population. Chemogenetic inhibition did not affect mechanical or thermal responses but reduced behaviors evoked by compound 48/80 and chloroquine, revealing a pruriceptive role for these neurons. Spinal cells activated by compound 48/80 or chloroquine express Glra3, further supporting the phenotype. Retrograde tracing revealed that spinal Glra3-Cre(+) neurons receive input from afferents associated with pain and itch, and dorsal root stimulation validated the monosynaptic input. In conclusion, these results show that spinal Glra3(+) neurons contribute to acute communication of compound 48/80- and chloroquine-induced itch in hairy skin.

Cortical surface plasticity promotes map remodeling and alleviates tinnitus in adult mice

Tinnitus induced by hearing loss is caused primarily by irreversible damage to the peripheral auditory system, which results in abnormal neural responses and frequency map disruption in the central auditory system. It remains unclear whether and how electrical rehabilitation of the auditory cortex can alleviate tinnitus. We hypothesize that stimulation of the cortical surface can alleviate tinnitus by enhancing neural responses and promoting frequency map reorganization. To test this hypothesis, we assessed and activated cortical maps using our newly designed graphene-based electrode array with a noise-induced tinnitus animal model. We found that cortical surface stimulation increased cortical activity, reshaped sensory maps, and alleviated hearing loss-induced tinnitus behavior in adult mice. These effects were likely due to retained long-term synaptic potentiation capabilities, as shown in cortical slices from the mice model. These findings suggest that cortical surface activation can be used to facilitate practical functional recovery from phantom percepts induced by sensory deprivation. They also provide a working principle for various treatment methods that involve electrical rehabilitation of the cortex.

Intrinsic and synaptic properties of adult mouse spinoperiaqueductal gray neurons and the influence of neonatal tissue damage

ABSTRACT

The periaqueductal gray (PAG) represents a key target of projection neurons residing in the spinal dorsal horn. In comparison to lamina I spinoparabrachial neurons, little is known about the intrinsic and synaptic properties governing the firing of spino-PAG neurons, or whether such activity is modulated by neonatal injury. In this study, this issue was addressed using ex vivo whole-cell patch clamp recordings from lamina I spino-PAG neurons in adult male and female FVB mice after hindpaw incision at postnatal day (P)3. Spino-PAG neurons were classified as high output, medium output, or low output based on their action potential discharge after dorsal root stimulation. The high-output subgroup exhibited prevalent spontaneous burst firing and displayed initial burst or tonic patterns of intrinsic firing, whereas low-output neurons showed little spontaneous activity. Interestingly, the level of dorsal root-evoked firing significantly correlated with the resting potential and membrane resistance but not with the strength of primary afferent-mediated glutamatergic drive. Neonatal incision failed to alter the pattern of monosynaptic sensory input, with most spino-PAG neurons receiving direct connections from low-threshold C-fibers. Furthermore, primary afferent-evoked glutamatergic input and action potential discharge in adult spino-PAG neurons were unaltered by neonatal surgical injury. Finally, Hebbian long-term potentiation at sensory synapses, which significantly increased afferent-evoked firing, was similar between P3-incised and naive littermates. Collectively, these data suggest that the functional response of lamina I spino-PAG neurons to sensory input is largely governed by their intrinsic membrane properties and appears resistant to the persistent influence of neonatal tissue damage.

Identification of Kv4.2 protein complex and modifications by tandem affinity purification-mass spectrometry in primary neurons

Proteins usually form complexes to fulfill variable physiological functions. In neurons, communication relies on synapses where receptors, channels, and anchoring proteins form complexes to precisely control signal transduction, synaptic integration, and action potential firing. Although there are many published protocols to isolate protein complexes in cell lines, isolation in neurons has not been well established. Here we introduce a method that combines lentiviral protein expression with tandem affinity purification followed by mass-spectrometry (TAP-MS) to identify protein complexes in neurons. This protocol can also be used to identify post-translational modifications (PTMs) of synaptic proteins. We used the A-type voltage-gated K⁺ channel subunit Kv4.2 as the target protein. Kv4.2 is highly expressed in the hippocampus where it contributes to learning and memory through its regulation of neuronal excitability and synaptic plasticity. We tagged Kv4.2 with the calmodulin-binding-peptide (CBP) and streptavidin-binding-peptide (SBP) at its C-terminus and expressed it in neurons via lentivirus. Kv4.2 was purified by two-step TAP and samples were analyzed by MS. MS identified two prominently known Kv4.2 interacting proteins [dipeptidyl peptidase like (DPPs) and Kv channel-interacting proteins (KChIPs)] in addition to novel synaptic proteins including glutamate receptors, a calcium channel, and anchoring proteins. Co-immunoprecipitation and colocalization experiments validated the association of Kv4.2 with glutamate receptors. In addition to protein complex identification, we used TAP-MS to identify Kv4.2 phosphorylation sites. Several known and unknown phosphorylation sites were identified. These findings provide a novel path to identify protein-protein interactions and PTMs in neurons and shed light on mechanisms of neuronal signaling potentially involved in the pathology of neurological diseases.

Diverse roles and modulations of IA in spinal cord pain circuits

This review highlights recent findings of different amplitude ranges, roles, and modulations of A-type K⁺ currents (I_A) in excitatory (GAD67-GFP^-) and inhibitory (GAD67-GFP⁺) interneurons in mouse spinal cord pain pathways. Endogenous neuropeptides, such as TAFA4, oxytocin, and dynorphin in particular, have been reported to modulate I_A in these pain pathways, but only TAFA4 has been shown to fully reverse the opposing modulations that occur selectively in LIIo GAD67-GFP^- and LIIi GAD67-GFP⁺ interneurons following both neuropathic and inflammatory pain. If, as hypothesized here, Kv4 subunits underlie I_A in both GAD67-GFP^- and GAD67-GFP⁺ interneurons, then I_A diversity in spinal cord pain pathways may depend on the interneuron-subtype-selective expression of Kv4 auxiliary subunits with functionally different N-terminal variants. Thus, I_A emerges as a good candidate for explaining the mechanisms underlying injury-induced mechanical hypersensitivity.

Fast A-type currents shape a rapidly adapting form of delayed short latency firing of excitatory superficial dorsal horn neurons that express the neuropeptide Y Y1 receptor

KEY POINTS

Neuropeptide Y Y1 receptor-expressing neurons in the dorsal horn of the spinal cord contribute to chronic pain. For the first time, we characterized the firing patterns of Y1-expressing neurons in Y1eGFP reporter mice. Under hyperpolarized conditions, most Y1eGFP neurons exhibited fast A-type potassium currents and delayed, short-latency firing (DSLF). Y1eGFP DSLF neurons were almost always rapidly adapting and often exhibited rebound spiking, characteristics of spinal pain neurons under the control of T-type calcium channels. These results will inspire future studies to determine whether tissue or nerve injury downregulates the channels that underlie A-currents, thus unmasking membrane hyperexcitability in Y1-expressing dorsal horn neurons, leading to persistent pain.

ABSTRACT

Neuroanatomical and behavioural evidence indicates that neuropeptide Y Y1 receptor-expressing interneurons (Y1-INs) in the superficial dorsal horn (SDH) are predominantly excitatory and contribute to chronic pain. Using an adult ex vivo spinal cord slice preparation from Y1eGFP reporter mice, we characterized firing patterns in response to steady state depolarizing current injection of GFP-positive cells in lamina II, the great majority of which expressed Y1 mRNA (88%). Randomly sampled (RS) and Y1eGFP neurons exhibited five firing patterns: tonic, initial burst, phasic, delayed short-latency <180 ms (DSLF) and delayed long-latency >180 ms (DLLF). When studied at resting membrane potential, most RS neurons exhibited delayed firing, while most Y1eGFP neurons exhibited phasic firing. A preconditioning membrane hyperpolarization produced only subtle changes in the firing patterns of RS neurons, but dramatically shifted Y1eGFP neurons to DSLF (46%) and DLLF (24%). In contrast to RS DSLF neurons, which rarely exhibited spike frequency adaptation, Y1eGFP DSLF neurons were almost always rapidly adapting, a characteristic of nociceptive-responsive SDH neurons. Rebound spiking was more prevalent in Y1eGFP neurons (6% RS vs. 32% Y1eGFP), indicating enrichment of T-type calcium currents. Y1eGFP DSLF neurons exhibited fast A-type potassium currents that are known to delay or limit action potential firing and exhibited smaller current density as compared to RS DSLF neurons. Our results will inspire future studies to determine whether tissue or nerve injury downregulates channels that contribute to A-currents, thus potentially unmasking T-type calcium channel activity and membrane hyperexcitability in Y1-INs, leading to persistent pain.

N-Acetylcysteine causes analgesia in a mouse model of painful diabetic neuropathy

N-Acetylcysteine, one of the most prescribed antioxidant drugs, enhances pain threshold in rodents and humans by activating mGlu2 metabotropic glutamate receptors. Here, we assessed the analgesic activity of N-acetylcysteine in the streptozotocin model of painful diabetic neuropathy and examined the effect of N-acetylcysteine on proteins that are involved in mechanisms of nociceptive sensitization. Mice with blood glucose levels ≥250 mg/dl in response to a single intraperitoneal (i.p.) injection of streptozotocin (200 mg/kg) were used for the assessment of mechanical pain thresholds. Systemic treatment with N-acetylcysteine (100 mg/kg, i.p., either single injection or daily injections for seven days) caused analgesia in diabetic mice. N-acetylcysteine-induced analgesia was abrogated by the Sxc− inhibitors, sulfasalazine (8 mg/kg, i.p.), erastin (30 mg/kg, i.p.), and sorafenib (10 mg/kg, i.p.), or by the mGlu2/3 receptor antagonist, LY341495 (1 mg/kg, i.p.). Repeated administrations of N-acetylcysteine in diabetic mice reduced ERK1/2 phosphorylation in the dorsal region of the lumbar spinal cord. The analgesic activity of N-acetylcysteine was occluded by the MEK inhibitor, PD0325901 (25 mg/kg, i.p.), the TRPV1 channel blocker, capsazepine (40 mg/kg, i.p.), or by a cocktail of NMDA and mGlu5 metabotropic glutamate receptor antagonists (memantine, 25 mg/kg, plus MTEP, 5 mg/kg, both i.p.). These findings offer the first demonstration that N-acetylcysteine relieves pain associated with diabetic neuropathy and holds promise for the use of N-acetylcysteine as an add-on drug in diabetic patients.

High-frequency spinal cord stimulation treatment attenuates the increase in spinal glutamate release and spinal miniature excitatory postsynaptic currents in rats with spared nerve injury-induced neuropathic pain

High-frequency spinal cord stimulation (HFSCS) at 10 kHz provides paresthesia-free treatment for chronic pain. However, the underlying mechanisms of its action have not been fully elucidated. The aim of the present study was to investigate the effect of HFSCS treatment on spinal glutamate release and uptake in spared nerve injury (SNI) rats. HFSCS was applied to the T10/T11 spinal cord 3 days after SNI. The concentration of spinal glutamate, glutamate transporter activity and miniature excitatory postsynaptic currents (mEPSCs) from neurons in lamina II were evaluated. HFSCS treatment alleviated SNI pain induced by mechanical and cold allodynia. HFSCS treatment also partially restored altered spinal glutamate uptake activity, the levels of spinal glutamate, and the frequency of mEPSCs following SNI. In conclusion, HFSCS treatment attenuated SNI-induced neuropathic pain and partially restored the altered glutamate uptake after SNI.

PKCγ interneurons, a gateway to pathological pain in the dorsal horn

Chronic pain is a frequent and disabling condition that is significantly maintained by central sensitization, which results in pathological amplification of responses to noxious and innocuous stimuli. As such, mechanical allodynia, or pain in response to a tactile stimulus that does not normally provoke pain, is a cardinal feature of chronic pain. Recent evidence suggests that the dorsal horn excitatory interneurons that express the γ isoform of protein kinase C (PKCγ) play a critical role in the mechanism of mechanical allodynia during chronic pain. Here, we review this evidence as well as the main aspects of the development, anatomy, electrophysiology, inputs, outputs, and pathophysiology of dorsal horn PKCγ neurons. Primary afferent high-threshold neurons transmit the nociceptive message to the dorsal horn of the spinal cord and trigeminal system where it activates second-order nociceptive neurons relaying the information to the brain. In physiological conditions, low-threshold mechanoreceptor inputs activate inhibitory interneurons in the dorsal horn, which may control activation of second-order nociceptive neurons. During chronic pain, low-threshold mechanoreceptor inputs now activate PKCγ neurons that forward the message to second-order nociceptive neurons, turning thus tactile inputs into pain. Several mechanisms may contribute to opening this gate, including disinhibition, activation of local astrocytes, release of diffusible factors such as reactive oxygen species, and alteration of the descending serotoninergic control on PKCγ neurons through 5-HT_2A serotonin receptors. Dorsal horn PKCγ neurons, therefore, appear as a relevant therapeutic target to alleviate mechanical allodynia during chronic pain.

Excitatory neurons are more disinhibited than inhibitory neurons by chloride dysregulation in the spinal dorsal horn

Neuropathic pain is a debilitating condition caused by the abnormal processing of somatosensory input. Synaptic inhibition in the spinal dorsal horn plays a key role in that processing. Mechanical allodynia - the misperception of light touch as painful - occurs when inhibition is compromised. Disinhibition is due primarily to chloride dysregulation caused by hypofunction of the potassium-chloride co-transporter KCC2. Here we show, in rats, that excitatory neurons are disproportionately affected. This is not because chloride is differentially dysregulated in excitatory and inhibitory neurons, but, rather, because excitatory neurons rely more heavily on inhibition to counterbalance strong excitation. Receptive fields in both cell types have a center-surround organization but disinhibition unmasks more excitatory input to excitatory neurons. Differences in intrinsic excitability also affect how chloride dysregulation affects spiking. These results deepen understanding of how excitation and inhibition are normally balanced in the spinal dorsal horn, and how their imbalance disrupts somatosensory processing.

Neonatal vincristine administration modulates intrinsic neuronal excitability in the rat dorsal root ganglion and spinal dorsal horn during adolescence

Our recent work has shown that the early-life administration of vincristine (VNC), commonly used to treat pediatric cancers, evokes mechanical pain hypersensitivity in rats that emerges during adolescence and persists into adulthood. However, the underlying mechanisms remain unclear, as nothing is known about how neonatal VNC treatment influences peripheral and central nociceptive processing at the cellular level. Here, we used in vitro intracellular microelectrode and whole-cell patch-clamp recordings to evaluate the consequences of early-life VNC administration on the intrinsic membrane properties of adolescent dorsal root ganglion and spinal superficial dorsal horn neurons. The results demonstrate that VNC treatment increased the prevalence and rate of repetitive firing in both large- and medium-diameter sensory neurons, while reducing repetitive firing in small-diameter neurons, in comparison with vehicle-treated littermate controls. By contrast, passive membrane properties and peripheral conduction velocities were similar between experimental groups across all classes of primary afferents. Within the adolescent superficial dorsal horn, neonatal VNC exposure significantly enhanced the intrinsic membrane excitability of lamina I spinoparabrachial neurons, as evidenced by a decrease in rheobase and elevation of repetitive firing frequency compared with controls. Meanwhile, putative interneurons within lamina I exhibited a reduction in repetitive action potential discharge after early-life chemotherapy. Collectively, these findings suggest that neonatal VNC treatment evokes cell type-specific changes in intrinsic excitability at multiple levels of the ascending pain pathway. Overall, this work lays an essential foundation for the future exploration of the ionic mechanisms that drive chemotherapy-induced chronic pain in children and adolescents.

NALCN channels enhance the intrinsic excitability of spinal projection neurons

Spinal projection neurons convey nociceptive signals to multiple brain regions including the parabrachial (PB) nucleus, which contributes to the emotional valence of pain perception. Despite the clear importance of projection neurons to pain processing, our understanding of the factors that shape their intrinsic membrane excitability remains limited. Here, we investigate a potential role for the Na leak channel NALCN in regulating the activity of spino-PB neurons in the developing rodent. Pharmacological reduction of NALCN current (INALCN), or the genetic deletion of NALCN channels, significantly reduced the intrinsic excitability of lamina I spino-PB neurons. In addition, substance P (SP) activated INALCN in ascending projection neurons through downstream Src kinase signaling, and the knockout of NALCN prevented SP-evoked action potential discharge in this neuronal population. These results identify, for the first time, NALCN as a strong regulator of neuronal activity within central pain circuits and also elucidate an additional ionic mechanism by which SP can modulate spinal nociceptive processing. Collectively, these findings indicate that the level of NALCN conductance within spino-PB neurons tightly governs ascending nociceptive transmission to the brain and thereby potentially influences pain perception.

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.

Orai1 Plays a Crucial Role in Central Sensitization by Modulating Neuronal Excitability

Pathological pain is a common and debilitating condition that is often poorly managed. Central sensitization is an important mechanism underlying pathological pain. However, candidate molecules involved in central sensitization remain unclear. Store-operated calcium channels (SOCs) mediate important calcium signals in nonexcitable and excitable cells. SOCs have been implicated in a wide variety of human pathophysiological conditions, including immunodeficiency, occlusive vascular diseases, and cancer. However, the role of SOCs in CNS disorders has been relatively unexplored. Orai1, a key component of SOCs, is expressed in the human and rodent spinal cord dorsal horn, but its functional significance in dorsal horn neurons is poorly understood. Here we sought to explore a potential role of Orai1 in the modulation of neuronal excitability and A-type potassium channels involved in pain plasticity. Using both male and female Orai1 knock-out mice, we found that activation of Orai1 increased neuronal excitability and reduced A-type potassium channels via the protein kinase C-extracellular signal-regulated protein kinase (PKC-ERK) pathway in dorsal horn neurons. Orai1 deficiency significantly decreased acute pain induced by noxious stimuli, nearly eliminated the second phase of formalin-induced nociceptive response, markedly attenuated carrageenan-induced ipsilateral pain hypersensitivity and abolished carrageenan-induced contralateral mechanical allodynia. Consistently, carrageenan-induced increase in neuronal excitability was abolished in the dorsal horn from Orai1 mutant mice. These findings uncover a novel signaling pathway involved in the pain process and central sensitization. Our study also reveals a novel link among Orai1, ERK, A-type potassium channels, and neuronal excitability.SIGNIFICANCE STATEMENT Orai1 is a key component of store-operated calcium channels (SOCs) in many cell types. It has been implicated in such pathological conditions as immunodeficiency, autoimmunity, and cancer. However, the role of Orai1 in CNS disorders remains poorly understood. The functional significance of Orai1 in neurons is elusive. Here we demonstrate that activation of Orai1 modulates neuronal excitability and Kv4-containing A-type potassium channels via the protein kinase C-extracellular signal-regulated protein kinase (PKC-ERK) pathway. Genetic knock-out of Orai1 nearly eliminates the second phase of formalin-induced pain and markedly attenuates carrageenan-induced pain hypersensitivity and neuronal excitability. These findings reveal a novel link between Orai1 and neuronal excitability and advance our understanding of central sensitization.

Optical control of pain in vivo with a photoactive mGlu5 receptor negative allosteric modulator

Light-operated drugs constitute a major target in drug discovery, since they may provide spatiotemporal resolution for the treatment of complex diseases (i.e. chronic pain). JF-NP-26 is an inactive photocaged derivative of the metabotropic glutamate type 5 (mGlu5) receptor negative allosteric modulator raseglurant. Violet light illumination of JF-NP-26 induces a photochemical reaction prompting the active-drug's release, which effectively controls mGlu5 receptor activity both in ectopic expressing systems and in striatal primary neurons. Systemic administration in mice followed by local light-emitting diode (LED)-based illumination, either of the thalamus or the peripheral tissues, induced JF-NP-26-mediated light-dependent analgesia both in neuropathic and in acute/tonic inflammatory pain models. These data offer the first example of optical control of analgesia in vivo using a photocaged mGlu5 receptor negative allosteric modulator. This approach shows potential for precisely targeting, in time and space, endogenous receptors, which may allow a better management of difficult-to-treat disorders.

Inward-rectifying K+ (Kir2) leak conductance dampens the excitability of lamina I projection neurons in the neonatal rat

Spinal lamina I projection neurons serve as a major conduit by which noxious stimuli detected in the periphery are transmitted to nociceptive circuits in the brain, including the parabrachial nucleus (PB) and the periaqueductal gray (PAG). While neonatal spino-PB neurons are more than twice as likely to exhibit spontaneous activity compared to spino-PAG neurons, the underlying mechanisms remain unclear since nothing is known about the voltage-independent (i.e. 'leak') ion channels expressed by these distinct populations during early life. To begin identifying these key leak conductances, the present study investigated the role of classical inward-rectifying K⁺ (K_ir2) channels in the regulation of intrinsic excitability in neonatal rat spino-PB and spino-PAG neurons. The data demonstrate that a reduction in K_ir2-mediated conductance by external BaCl₂ significantly enhanced intrinsic membrane excitability in both groups. Similar results were observed in spino-PB neurons following K_ir2 channel block with the selective antagonist ML133. In addition, voltage-clamp experiments showed that spino-PB and spino-PAG neurons express similar amounts of K_ir2 current during the early postnatal period, suggesting that the differences in the prevalence of spontaneous activity between the two populations are not explained by differential expression of K_ir2 channels. Overall, the results indicate that K_ir2-mediated conductance tonically dampens the firing of multiple subpopulations of lamina I projection neurons during early life. Therefore, K_ir2 channels are positioned to tightly shape the output of the immature spinal nociceptive circuit and thus regulate the ascending flow of nociceptive information to the developing brain, which has important functional implications for pediatric pain.

Presynaptic Release-Regulating mGlu1 Receptors in Central Nervous System

Group I metabotropic glutamate (mGlu) receptors consists of mGlu1 and mGlu5 receptor subtypes. These receptors are widely distributed in the central nervous system (CNS), where they preferentially mediate facilitatory signaling in neurones and glial cells, mainly by favoring phospholipase (PLC) translocation. Based on the literature so far available, group I Metabotropic glutamate receptors (mGluRs) are preferentially expressed at the postsynaptic side of chemical synapsis, where they participate in the progression of the chemical stimulus. Studies, however, have shown the presence of these receptors also at the presynaptic level, where they exert several functions, including the modulation of transmitter exocytosis. Presynaptic Group I mGluRs can be both autoreceptors regulating release of glutamate and heteroreceptors regulating the release of various transmitters, including GABA, dopamine, noradrenaline, and acetylcholine. While the existence of presynaptic release-regulating mGlu5 receptors is largely recognized, the possibility that mGlu1 receptors also are present at this level has been a matter of discussion for a long time. A large body of evidence published in the last decade, however, supports this notion. This review aims at revisiting the data from in vitro studies concerning the existence and the role of release-regulating mGlu1 receptors presynaptically located in nerve terminals isolated from selected regions of the CNS. The functional interaction linking mGlu5 and mGlu1 receptor subtypes at nerve terminals and their relative contributions as modulators of central transmission will also be discussed. We apologize in advance for omission in our coverage of the existing literature.

Relationship of membrane properties, spike burst responses, laminar location, and functional class of dorsal horn neurons recorded in vitro

The input-output and discharge properties of neurons are shaped by both passive and active electrophysiological membrane properties. Whole cell patch-clamp recordings in lamina I-III neurons in an isolated preparation of the whole spinal cord of juvenile rats with attached dorsal roots and dorsal root ganglia were used to further define which of these properties provides the most impactful classification strategy. A total of 95 neurons were recorded in segment L5 and were classified based on the responses to L4 dorsal root stimulation. The results showed that high-threshold and silent neurons had higher membrane resistance and more negative resting membrane potential than low-threshold or wide-dynamic-range neurons. Rheobase in low-threshold and wide-dynamic-range neurons was significantly lower than that of high-threshold or silent neurons. Four types of firing patterns were identified in response to depolarizing current injections. Low-threshold cells most frequently showed a phasic firing pattern characterized by a short initial burst of action potentials, single spiking or irregular firing bursts at the onset of a depolarizing pulse. High-threshold and wide-dynamic-range neurons were characterized by tonic firing with trains of spikes occurring at regular intervals throughout the current pulse. The majority of silent neurons displayed a delayed onset of firing in response to current injection. These results indicate that the passive membrane properties of spinal neurons are tuned to optimize the responses to particular subsets of afferent stimuli.

Isolation and characterization of Ts19 Fragment II, a new long-chain potassium channel toxin from Tityus serrulatus venom

Ts19 Fragment II (Ts19 Frag-II) was first isolated from the venom of the scorpion Tityus serrulatus (Ts). It is a protein presenting 49 amino acid residues, three disulfide bridges, Mr 5534Da and was classified as a new member of class (subfamily) 2 of the β-KTxs, the second one described for Ts scorpion. The β-KTx family is composed by two-domain peptides: N-terminal helical domain (NHD), with cytolytic activity, and a C-terminal CSαβ domain (CCD), with Kv blocking activity. The extensive electrophysiological screening (16 Kv channels and 5 Nav channels) showed that Ts19 Frag-II presents a specific and significant blocking effect on Kv1.2 (IC50 value of 544±32nM). However, no cytolytic activity was observed with this toxin. We conclude that the absence of 9 amino acid residues from the N-terminal sequence (compared to Ts19 Frag-I) is responsible for the absence of cytolytic activity. In order to prove this hypothesis, we synthesized the peptide with these 9 amino acid residues, called Ts19 Frag-III. As expected, Ts19 Frag-III showed to be cytolytic and did not block the Kv1.2 channel. The post-translational modifications of Ts19 and its fragments (I-III) are also discussed here. A mechanism of post-translational processing (post-splitting) is suggested to explain Ts19 fragments production. In addition to the discovery of this new toxin, this report provides further evidence for the existence of several compounds in the scorpion venom contributing to the diversity of the venom arsenal.

Subpopulations of PKCγ interneurons within the medullary dorsal horn revealed by electrophysiologic and morphologic approach

Mechanical allodynia, a cardinal symptom of persistent pain, is associated with the unmasking of usually blocked local circuits within the superficial spinal or medullary dorsal horn (MDH) through which low-threshold mechanical inputs can gain access to the lamina I nociceptive output neurons. Specific interneurons located within inner lamina II (IIi) and expressing the gamma isoform of protein kinase C (PKCγ⁺) have been shown to be key elements for such circuits. However, their morphologic and electrophysiologic features are still unknown. Using whole-cell patch-clamp recordings and immunohistochemical techniques in slices of adult rat MDH, we characterized such lamina IIi PKCγ⁺ interneurons and compared them with neighboring PKCγ⁻ interneurons. Our results reveal that PKCγ⁺ interneurons display very specific activity and response properties. Compared with PKCγ⁻ interneurons, they exhibit a smaller membrane input resistance and rheobase, leading to a lower threshold for action potentials. Consistently, more than half of PKCγ⁺ interneurons respond with tonic firing to step current. They also receive a weaker excitatory synaptic drive. Most PKCγ⁺ interneurons express Ih currents. The neurites of PKCγ⁺ interneurons arborize extensively within lamina IIi, can spread dorsally into lamina IIo, but never reach lamina I. In addition, at least 2 morphologically and functionally different subpopulations of PKCγ⁺ interneurons can be identified: central and radial PKCγ⁺ interneurons. The former exhibit a lower membrane input resistance, rheobase and, thus, action potential threshold, and less PKCγ⁺ immunoreactivity than the latter. These 2 subpopulations might thus differently contribute to the gating of dorsally directed circuits within the MDH underlying mechanical allodynia.

Convergent phosphomodulation of the major neuronal dendritic potassium channel Kv4.2 by pituitary adenylate cyclase-activating polypeptide

The endogenous neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) is secreted by both neuronal and non-neuronal cells in the brain and spinal cord, in response to pathological conditions such as stroke, seizures, chronic inflammatory and neuropathic pain. PACAP has been shown to exert various neuromodulatory and neuroprotective effects. However, direct influence of PACAP on the function of intrinsically excitable ion channels that are critical to both hyperexcitation as well as cell death, remain largely unexplored. The major dendritic K(+) channel Kv4.2 is a critical regulator of neuronal excitability, back-propagating action potentials in the dendrites, and modulation of synaptic inputs. We identified, cloned and characterized the downstream signaling originating from the activation of three PACAP receptor (PAC1) isoforms that are expressed in rodent hippocampal neurons that also exhibit abundant expression of Kv4.2 protein. Activation of PAC1 by PACAP leads to phosphorylation of Kv4.2 and downregulation of channel currents, which can be attenuated by inhibition of either PKA or ERK1/2 activity. Mechanistically, this dynamic downregulation of Kv4.2 function is a consequence of reduction in the density of surface channels, without any influence on the voltage-dependence of channel activation. Interestingly, PKA-induced effects on Kv4.2 were mediated by ERK1/2 phosphorylation of the channel at two critical residues, but not by direct channel phosphorylation by PKA, suggesting a convergent phosphomodulatory signaling cascade. Altogether, our findings suggest a novel GPCR-channel signaling crosstalk between PACAP/PAC1 and Kv4.2 channel in a manner that could lead to neuronal hyperexcitability.

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.

Morphological, biophysical and synaptic properties of glutamatergic neurons of the mouse spinal dorsal horn

Interneurons of the spinal dorsal horn are central to somatosensory and nociceptive processing. A mechanistic understanding of their function depends on profound knowledge of their intrinsic properties and their integration into dorsal horn circuits. Here, we have used BAC transgenic mice expressing enhanced green fluorescent protein (eGFP) under the control of the vesicular glutamate transporter (vGluT2) gene (vGluT2::eGFP mice) to perform a detailed electrophysiological and morphological characterisation of excitatory dorsal horn neurons, and to compare their properties to those of GABAergic (Gad67::eGFP tagged) and glycinergic (GlyT2::eGFP tagged) neurons. vGluT2::eGFP was detected in about one-third of all excitatory dorsal horn neurons and, as demonstrated by the co-expression of vGluT2::eGFP with different markers of subtypes of glutamatergic neurons, probably labelled a representative fraction of these neurons. Three types of dendritic tree morphologies (vertical, central, and radial), but no islet cell-type morphology, were identified in vGluT2::eGFP neurons. vGluT2::eGFP neurons had more depolarised action potential thresholds and longer action potential durations than inhibitory neurons, while no significant differences were found for the resting membrane potential, input resistance, cell capacitance and after-hyperpolarisation. Delayed firing and single action potential firing were the single most prevalent firing patterns in vGluT2::eGFP neurons of the superficial and deep dorsal horn, respectively. By contrast, tonic firing prevailed in inhibitory interneurons of the dorsal horn. Capsaicin-induced synaptic inputs were detected in about half of the excitatory and inhibitory neurons, and occurred more frequently in superficial than in deep dorsal horn neurons. Primary afferent-evoked (polysynaptic) inhibitory inputs were found in the majority of glutamatergic and glycinergic neurons, but only in less than half of the GABAergic population. Excitatory dorsal horn neurons thus differ from their inhibitory counterparts in several biophysical properties and possibly also in their integration into the local neuronal circuitry.

IA channels: diverse regulatory mechanisms

In many peripheral and central neurons, A-type K(+) currents, IA, have been identified and shown to be key determinants in shaping action potential waveforms and repetitive firing properties, as well as in the regulation of synaptic transmission and synaptic plasticity. The functional properties and physiological roles of native neuronal IA, however, have been shown to be quite diverse in different types of neurons. Accumulating evidence suggests that this functional diversity is generated by multiple mechanisms, including the expression and subcellular distributions of IA channels encoded by different voltage-gated K(+) (Kv) channel pore-forming (α) subunits, interactions of Kv α subunits with cytosolic and/or transmembrane accessory subunits and regulatory proteins and post-translational modifications of channel subunits. Several recent reports further suggest that local protein translation in the dendrites of neurons and interactions between IA channels with other types of voltage-gated ion channels further expands the functional diversity of native neuronal IA channels. Here, we review the diverse molecular mechanisms that have been shown or proposed to underlie the functional diversity of native neuronal IA channels.

Neurotrophins in bladder function: what do we know and where do we go from here?

AIMS

Neurotrophins (NTs) have attracted considerable attention in the urologic community. The reason for this resides in the recognition of their ability to induce plastic changes of the neuronal circuits that govern bladder function. In many pathologic states, urinary symptoms, including urgency and urinary frequency, reflect abnormal activity of bladder sensory afferents that results from neuroplastic changes. Accordingly, in pathologies associated with increased sensory input, such as the overactive bladder syndrome (OAB) or bladder pain syndrome/interstitial cystitis (BPS/IC), significant amounts of NTs have been found in the bladder wall.

METHODS

Here, current knowledge about the importance of NTs in bladder function will be reviewed, with a focus on the most well-studied NTs, nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF).

RESULTS

Both NTs are present in the bladder and regulate bladder sensory afferents and urothelial cells. Experimental models of bladder dysfunction show that upregulation of these NTs is strongly linked to bladder hyperactivity and, in some cases, pain. NT manipulation has been tested in animal models of bladder dysfunction, and recently, NGF downregulation, achieved by administration of a monoclonal antibody, has also been tested in patients with BPS/IC and chronic prostatitis/chronic pelvic pain syndrome (CP/CPPS). NTs have also been found in high quantities in the urine of OAB and BPS/IC patients, raising the possibility of NTs serving as biomarkers.

CONCLUSIONS

Available data show that our knowledge of NTs has greatly increased in recent years and that some results may have future clinical application.

HCN4 subunit expression in fast-spiking interneurons of the rat spinal cord and hippocampus

Hyperpolarisation-activated (I_h) currents are considered important for dendritic integration, synaptic transmission, setting membrane potential and rhythmic action potential (AP) discharge in neurons of the central nervous system. Hyperpolarisation-activated cyclic nucleotide-gated (HCN) channels underlie these currents and are composed of homo- and hetero-tetramers of HCN channel subunits (HCN1–4), which confer distinct biophysical properties on the channel. Despite understanding the structure–function relationships of HCN channels with different subunit stoichiometry, our knowledge of their expression in defined neuronal populations remains limited. Recently, we have shown that HCN subunit expression is a feature of a specific population of dorsal horn interneurons that exhibit high-frequency AP discharge. Here we expand on this observation and use neuroanatomical markers to first identify well-characterised neuronal populations in the lumbar spinal cord and hippocampus and subsequently determine whether HCN4 expression correlates with high-frequency AP discharge in these populations. In the spinal cord, HCN4 is expressed in several putative inhibitory interneuron populations including parvalbumin (PV)-expressing islet cells (84.1%; SD: ±2.87), in addition to all putative Renshaw cells and Ia inhibitory interneurons. Similarly, virtually all PV-expressing cells in the hippocampal CA1 subfield (93.5%; ±3.40) and the dentate gyrus (90.9%; ±6.38) also express HCN4. This HCN4 expression profile in inhibitory interneurons mirrors both the prevalence of I_h sub-threshold currents and high-frequency AP discharge. Our findings indicate that HCN4 subunits are expressed in several populations of spinal and hippocampal interneurons, which are known to express both I_h sub-threshold currents and exhibit high-frequency AP discharge. As HCN channel function plays a critical role in pain perception, learning and memory, and sleep as well as the pathogenesis of several neurological diseases, these findings provide important insights into the identity and neurochemical status of cells that could underlie such conditions.

Peripheral nerve injury produces a sustained shift in the balance between glutamate release and uptake in the dorsal horn of the spinal cord

Peripheral nerve injury provokes heightened excitability of primary sensory afferents including nociceptors, and elicits ectopic activity in lesioned and neighboring intact nerve fibers. The major transmitter released by sensory afferents in the superficial dorsal horn of the spinal cord is glutamate. Glutamate is critically involved in nociceptive signaling and the development of neuropathic pain. We recorded miniature excitatory postsynaptic currents (mEPSCs) from neurons in lamina II of the rat dorsal horn to assess spontaneous synaptic activity after spared nerve injury (SNI), a model of chronic neuropathic pain. Following SNI, the frequency of mEPSCs doubled, indicating heightened glutamate release from primary afferents or spinal interneurons. Consistent with this finding, glutamate concentrations in the cerebrospinal fluid were elevated at 1 and 4 weeks after SNI. Transmitter uptake was insufficient to prevent the rise in extracellular glutamate as the expression of glutamate transporters remained unchanged or decreased. 2-Methyl-6-(phenylethynyl)pyridine hydrochloride, an antagonist of metabotropic glutamate receptor 5 (mGluR5), reduced the frequency of mEPSCs to its preinjury level, suggesting a positive feedback mechanism that involves facilitation of transmitter release by mGluR5 activation in the presence of high extracellular glutamate. Treatment with the β-lactam antibiotic ceftriaxone increased the expression of glutamate transporter 1 (Glt1) in the dorsal horn after SNI, raised transmitter uptake, and lowered extracellular glutamate. Improving glutamate clearance prevented the facilitation of transmitter release by mGluR5 and attenuated neuropathic pain-like behavior. Balancing glutamate release and uptake after nerve injury should be an important target in the management of chronic neuropathic pain.

Duloxetine blocks cloned Kv4.3 potassium channels

The effects of duloxetine were examined on cloned Kv4.3 channels stably expressed in CHO cells using the whole-cell patch-clamp technique. Duloxetine decreased the peak amplitude of Kv4.3 currents with an acceleration of the decay rate of current inactivation in a concentration-dependent manner. The IC(50) values required for the blocking effects of duloxetine on the peak amplitude and the integral of currents were 8.4 and 2.1μM, respectively. Duloxetine accelerated the rate of inactivation of Kv4.3 currents and thereby decreased the time-to-peak in a concentration-dependent manner. Analysis of the time dependence of the drug block produced estimates of 21.9μM(-1)s(-1) and 165.9s(-1), for the respective association (k(+1)) and dissociation (k(-1)) rate constants. The K(d) value (k(-1)/k(+1)) yielded 7.5μM, which approximates the experimental IC(50) value obtained from the concentration-response curve. The block of Kv4.3 by duloxetine was voltage-dependent at a membrane potential coinciding with the activation of the channels. At a more positive potential, however, the block was relieved. Duloxetine produced a hyperpolarizing shift in the voltage dependence of the steady-state inactivation of Kv4.3, and accelerated the closed-state inactivation of Kv4.3 in the subthreshold voltage range. Duloxetine induced a significant use-dependent block at frequencies of 1 and 2Hz. In the presence of duloxetine, the recovery from inactivation was slower than under control conditions. These results demonstrate that duloxetine exerts a concentration-dependent block of Kv4.3 by binding to the channels in the open and inactivated states and these actions may contribute to its analgesic effect in neuropathic pain.

Pain processing by spinal microcircuits: afferent combinatorics

Pain, itch, heat, cold, and touch represent different percepts arising from somatosensory input. How stimuli give rise to these percepts has been debated for over a century. Recent work supports the view that primary afferents are highly specialized to transduce and encode specific stimulus modalities. However, cross-modal interactions (e.g. inhibition or exacerbation of pain by touch) support convergence rather than specificity in central circuits. We outline how peripheral specialization together with central convergence could enable spinal microcircuits to combine inputs from distinctly specialized, co-activated afferents and to modulate the output signals thus formed through computations like normalization. These issues will be discussed alongside recent advances in our understanding of microcircuitry in the superficial dorsal horn.

Deconstructing the neuropathic pain phenotype to reveal neural mechanisms

After nerve injury maladaptive changes can occur in injured sensory neurons and along the entire nociceptive pathway within the CNS, which may lead to spontaneous pain or pain hypersensitivity. The resulting neuropathic pain syndromes present as a complex combination of negative and positive symptoms, which vary enormously from individual to individual. This variation depends on a diversity of underlying pathophysiological changes resulting from the convergence of etiological, genotypic, and environmental factors. The pain phenotype can serve therefore, as a window on underlying pathophysiological neural mechanisms and as a guide for developing personalized pain medicine.

Are all spinal segments equal: intrinsic membrane properties of superficial dorsal horn neurons in the developing and mature mouse spinal cord

Neurons in the superficial dorsal horn (SDH; laminae I-II) of the spinal cord process nociceptive information from skin, muscle, joints and viscera. Most of what we know about the intrinsic properties of SDH neurons comes from studies in lumbar segments of the cord even though clinical evidence suggests nociceptive signals from viscera and head and neck tissues are processed differently. This ‘lumbar-centric' view of spinal pain processing mechanisms also applies to developing SDH neurons. Here we ask whether the intrinsic membrane properties of SDH neurons differ across spinal cord segments in both the developing and mature spinal cord. Whole cell recordings were made from SDH neurons in slices of upper cervical (C2-4), thoracic (T8-10) and lumbar (L3-5) segments in neonatal (P0-5) and adult (P24-45) mice. Neuronal input resistance (R(IN)), resting membrane potential, AP amplitude, half-width and AHP amplitude were similar across spinal cord regions in both neonates and adults (∼100 neurons for each region and age). In contrast, these intrinsic membrane properties differed dramatically between neonates and adults. Five types of AP discharge were observed during depolarizing current injection. In neonates, single spiking dominated (∼40%) and the proportions of each discharge category did not differ across spinal regions. In adults, initial bursting dominated in each spinal region, but was significantly more prevalent in rostral segments (49% of neurons in C2-4 vs. 29% in L3-5). During development the dominant AP discharge pattern changed from single spiking to initial bursting. The rapid A-type potassium current (I(Ar)) dominated in neonates and adults, but its prevalence decreased (∼80% vs. ∼50% of neurons) in all regions during development. I(Ar) steady state inactivation and activation also changed in upper cervical and lumbar regions during development. Together, our data show the intrinsic properties of SDH neurons are generally conserved in the three spinal cord regions examined in both neonate and adult mice. We propose the conserved intrinsic membrane properties of SDH neurons along the length of the spinal cord cannot explain the marked differences in pain experienced in the limbs, viscera, and head and neck.

Multiple T-type Ca2+ current subtypes in electrophysiologically characterized hamster dorsal horn neurons: possible role in spinal sensory integration

Whole cell patch-clamp recordings were used to investigate the contribution of transient, low-threshold calcium currents (I(T)) to firing properties of hamster spinal dorsal horn neurons. I(T) was widely, though not uniformly, expressed by cells in Rexed's laminae I-IV and correlated with the pattern of action potential discharge evoked under current-clamp conditions: I(T) in neurons responding to constant membrane depolarization with one or two action potentials was nearly threefold larger than I(T) in cells responding to the same activation with continuous firing. I(T) was evoked by depolarizing voltage ramps exceeding 46 mV/s and increased with ramp slope (240-2,400 mV/s). Bath application of 200 μM Ni(2+) depressed ramp-activated I(T). Phasic firing recorded in current clamp could only be activated by membrane depolarizations exceeding ∼43-46 mV/s and was blocked by Ni(2+) and mibefradil, suggesting I(T) as an underlying mechanism. Two components of I(T), "fast" and "slow," were isolated based on a difference in time constant of inactivation (12 ms and 177 ms, respectively). The amplitude of the fast subtype depended on the slope of membrane depolarization and was twice as great in burst-firing cells than in cells having a tonic discharge. Post hoc single-cell RT-PCR analyses suggested that the fast component is associated with the Ca(V)3.1 channel subtype. I(T) may enhance responses of phasic-firing dorsal horn neurons to rapid membrane depolarizations and contribute to an ability to discriminate between afferent sensory inputs that encode high- and low-frequency stimulus information.