Activity In Dorsal Horn Research Articles

Spinal astrocytic activation contributes to mechanical allodynia in a mouse model of type 2 diabetes

Diabetic neuropathic pain (DNP) plays a major role in decreased life quality of type 2 diabetes patients, however, the molecular mechanisms underlying DNP remain unclear. Emerging research implicates the participation of spinal glial cells in some neuropathic pain models. However, it remains unknown whether spinal glial cells are activated under type 2 diabetic conditions and whether they contribute to diabetes-induced neuropathic pain. In the present study, using a db/db type 2 diabetes mouse model that displayed obvious mechanical allodynia, we found that spinal astrocyte but not microglia was dramatically activated. The mechanical allodynia was significantly attenuated by intrathecally administrated l-α-aminoadipate (astrocytic specific inhibitor) whereas minocycline (microglial specific inhibitor) did not have any effect on mechanical allodynia, which indicated that spinal astrocytic activation contributed to allodynia in db/db mice. Further study aimed to identify the detailed mechanism of astrocyte-incudced allodynia in db/db mice. Results showed that spinal activated astrocytes dramatically increased interleukin (IL)-1β expression which may induce N-methyl- d-aspartic acid receptor (NMDAR) phosphorylation in spinal dorsal horn neurons to enhance pain transmission. Together, these results suggest that spinal activated astrocytes may be a crucial component of mechanical allodynia in type 2 diabetes and “Astrocyte-IL-1β-NMDAR-Neuron” pathway may be the detailed mechanism of astrocyte-incudced allodynia. Thus, inhibiting astrocytic activation in the spinal dorsal horn may represent a novel therapeutic strategy for treating DNP.

Evidence for suppression of spinal glial activation by dexmedetomidine in a rat model of monoarthritis

1. Spinal glial cells play a key role in developing and maintaining allodynia and hyperalgesia following tissue inflammation. Dexmedetomidine, a highly selective α(2) -adrenoceptor (α(2) -AR) agonist, has exhibited a significant analgesic effect in various rodent models of chronic pain. However, whether spinal glial activation is involved in the analgesic effect of dexmedetomidine remains unknown. The present study investigated whether spinal administration of dexmedetomidine could antagonize glial activation in the spinal dorsal horn and attenuate thermal hyperalgesia in complete Freund's adjuvant (CFA)-induced ankle joint monoarthritic (MA) rats. 2. Unilateral intra-articular injection of CFA produced a robust activation of microglia and astrocytes in the spinal cord, which was associated with the development and maintenance of thermal hyperalgesia. Repeated lumbar puncture (LP) administration of dexmedetomidine (10 μg) significantly attenuated MA-induced thermal hyperalgesia in a cumulative manner. Monoarthritis-induced spinal glial activation was also suppressed following dexmedetomidine application. The α(2A) -AR, essential for the antinociceptive effects of α(2) -AR agonists, was detected in spinal neurons and glia, as well as in dorsal root ganglion primary afferent neurons, which may be implicated in dexmedetomidine-induced suppression of spinal glial activation and antihyperalgesic effects. 3. These data provide the first evidence that blocking spinal glial activation is involved in the analgesic action of dexmedetomidine.

A novel cell–cell signaling by microglial transmembrane TNFα with implications for neuropathic pain

Neuropathic pain is accompanied by neuroimmune activation in dorsal horn of spinal cord. We have observed that in animal models this activation is characterized by an increased expression of transmembrane tumor necrosis factor α (mTNFα) without the release of soluble tumor necrosis factor α (sTNFα). Herein we report that the pain-related neurotransmitter peptide substance P (SP) increases the expression of mTNFα without the release of sTNFα from primary microglial cells. We modeled this interaction using an immortalized microglial cell line; exposure of these cells to SP also resulted in the increased expression of mTNFα but without any increase in the expression of the TNF-cleaving enzyme (TACE) and no release of sTNFα. In order to evaluate the biological function of uncleaved mTNFα, we transfected COS-7 cells with a mutant full-length TNFα construct resistant to cleavage by TACE. Coculture of COS-7 cells expressing the mutant TNFα with microglial cells led to microglial cell activation indicated by increased OX42 immunoreactivity and release of macrophage chemoattractant peptide 1 (CCL2) by direct cell–cell contact. These results suggest a novel pathway through which the release of SP by primary afferents activates microglial expression of mTNFα, establishing a feed-forward loop that may contribute to the establishment of chronic pain.

Minocycline Attenuates Mechanical Allodynia and Central Sensitization Following Peripheral Second-Degree Burn Injury

Burn injury induces severe pain that can be refractory to existing pharmacotherapies. The underlying mechanism of burn pain remains unclear. We previously established an animal model and reported that unilateral burn injury induces chronic and bilateral mechanical allodynia, which is associated with central sensitization and microglial activation in the spinal cord dorsal horn. Modulation of the activity of microglia and p38 mitogen-activated protein kinase (MAPK) has been shown to ameliorate neuropathic pain in several nerve-injury pain models. In the present study, we show in this rat model that daily treatment with the microglial inhibitor minocycline (10 mg/kg), administered at the time of burn injury and for 7 days thereafter, significantly attenuates ipsilateral and contralateral allodynia as assessed up to 1 month following burn injury. These sensory changes are paralleled by significant suppression of evoked hyperexcitability of dorsal-horn neurons and of the expression of phosphorylated p38 (phospho-p38) in OX42+ microglial cells within the dorsal horn. Our results suggest that modulation of inflammation at early times after burn injury may have long-lasting effects, attenuating central neuropathic mechanisms which contribute to pain after burn injury. We demonstrate, in a rodent model of burn-associated pain, that the microglial inhibitor minocycline, delivered at the time of burn injury and for 1 week thereafter, has long-lasting effects, attenuating microglial activation and neuronal hyperresponsiveness in the dorsal horns, and ameliorating allodynia for at least 1 month.

Chronic post-surgical pain

Chronic post-surgical pain (CPSP) is one of the most common and serious complications after surgery. There is no universally agreed definition of CPSP; however, the working definition proposed by Macrae and Davies is commonly used (Fig. 1). CPSP is associated with increased analgesic use, restriction of activities of daily living, significant effects on quality of life, and increased health-care utilization. More than 4 million people undergo surgery every year in the UK, so CPSP poses a significant economic and health-care burden. Not all studies are consistent about the incidence of CPSP; there are wide variations between different surgical procedures (Table 1). Some of this variability is likely to result from lack of clarity in the definition of CPSP, small sample sizes, poor questionnaire response rates, and selection bias. Nerve injury during surgery has been implicated in the development of CPSP; some (but not all) patients with CPSP have neuropathic pain. Inflammatory and immune reactions after damage to axons results in release of neurotransmitters that act locally and in the spinal cord to produce hypersensitivity and ectopic neural activity; this contributes to central sensitization (Fig. 2). Central sensitization occurs when repetitive nociceptive stimuli result in altered dorsal horn activity and amplification of sensory flow; this can lead to persistent nervous system changes, for example, death of inhibitory neurones, their replacement with excitatory afferent neurones, and microglial activation. These changes lead to evoked and spontaneous symptoms associated with neuropathic pain, for example, allodynia and hyperalgesia. CPSP is reported in more than 50% of patients who have surgery associated with nerve and tissue damage, for example, mastectomy, thoracotomy, and amputation. Despite this, there is no simple relationship between nerve injury during surgery and the development of CPSP. There is no association between intercostal nerve damage assessed at the time of thoracotomy by nerve conduction studies and the development of chronic pain 3 months later. Paradoxically, rib resection, which results in more intercostal nerve damage, is associated with a reduced incidence of post-thoracotomy neuralgia. Similarly, although damage to the intercostobrachial nerve has been implicated in the development of CPSP after mastectomy, many patients with objective signs of nerve injury (such as numbness) do not develop chronic pain. In addition, in other surgical procedures associated with nerve damage, not all patients with CPSP have neuropathic pain, for example, among thoracic surgery patients with CPSP, only half had significant neuropathic symptoms identified by a validated questionnaire. Patients with chronic pain after surgical procedures such as hip arthroplasty and hysterectomy do not demonstrate sensory loss, suggesting that mechanisms other than nerve injury are responsible for ongoing pain in these patients. Therefore, any link between nerve damage during surgery and the development of CPSP is complicated. Not all patients with nerve damage develop CPSP, and those who do develop CPSP do not necessarily have neuropathic pain. Some operations not associated with nerve damage can result in CPSP. Although the mechanisms behind the development of CPSP have yet to be fully elucidated, a number of risk factors have been identified. As our understanding of the pathophysiology, risk factors, and prevention of CPSP expands, the traditional focus of the anaesthetist on managing acute perioperative pain should include identifying and managing patients at risk of developing CPSP.

The direction of synaptic plasticity mediated by C-fibers in spinal dorsal horn is decided by Src-family kinases in microglia: The role of tumor necrosis factor-α

Previous studies have shown that Src-family kinases (SFKs) are selectively activated in spinal microglia following peripheral nerve injury and the activated SFKs play a key role for the development of neuropathic pain. To investigate the underlying mechanism, in the present study the effect of SFKs on long-term potentiation (LTP) at C-fiber synapses in spinal dorsal horn, which is believed as central mechanism of neuropathic pain, was investigated in adult rats. Electrophysiological data revealed that pretreatment with either microglia inhibitor (minocycline, 200μM) or SFKs inhibitors (PP2, 100μM and SU6656, 200μM) reversed the effect of high frequency stimulation (HFS), that is, HFS, which induces long-term potentiation (LTP) normally, induced long-term depression (LTD) after inhibition of either microglia or SFKs. Western blotting analysis showed that the level of phosphorylated SFKs (p-SFKs) in ipsilateral spinal dorsal horn was transiently increased after LTP induced by HFS, starting at 15min and returning to control level at 60min after HFS. Double-labeled immunofluorescence staining demonstrated that p-SFKs were highly restricted to microglia. Furthermore, we found that the inhibitory effects of minocycline or SU6656 on spinal LTP were reversed by spinal application of rat recombinant tumor necrosis factor-α (TNF-α 0.5ng/ml, 200μl). HFS failed to induce LTP of C-fiber evoked field potentials in TNF receptor-1 knockout mice and in rats pretreated with TNF-α neutralization antibody (0.6μg/ml, 200μl). The results suggested that in spinal dorsal horn activation of SFKs in microglia might control the direction of plastic changes at C-fiber synapses and TNF-α might be involved in the process.

Involvement of medullary dorsal horn glial cell activation in mediation of masseter mechanical allodynia induced by experimental tooth movement

Objective To investigate the involvement of microglial and astrocytic activation in the medullary dorsal horn (MDH) during the mediation of masseter area allodynia induced by experimental tooth movement (ETM). Design Five groups of adult Sprague–Dawley rats ( n = 60) were divided into control (CON), minocycline (MIN), ETM, and 10 mg/kg or 30 mg/kg MIN plus ETM (METM) groups. The upper-first-molar was moved mesially for rats in ETM and METM groups. Rats were pre-injected with minocycline in the MIN (30 mg/kg) and METM (10 mg/kg or 30 mg/kg) groups. Pressure pain threshold (PPT) in masseter area was tested from day 0 to 14 for all 5 groups. Immunohistochemistry against OX42 (microglial marker) or GFAP (astrocytic maker) in the MDH was examined at days 1, 3, 7 and 14 for CON, MIN and 30 mg/kg METM groups. Results Baseline PPT was expectedly seen in either CON or MIN groups, masseter mechanical allodynia was detected in the ETM group from day 4 to 13 ( P < 0.05). OX42 expression level at days 1, 3 and 7, and GFAP expression level at days 3, 7 and 14 were higher in ETM ( P < 0.05), but not in 30 mg/kg METM, than in CON group. Minocycline reduced activation of microglia and astrocytes, and significantly attenuated the development of masseter mechanical allodynia in this model. Conclusions These results indicate that mechanical allodynia in the masseter area induced by ETM can be attenuated by minocycline. Activation of microglia, possibly together with subsequent activation of astrocytes, seems to contribute to masseter mechanical allodynia.

Functional magnetic resonance imaging of the spinal cord during sensory stimulation in diabetic rats

To determine if differences exist between control and diabetic rats in functional magnetic resonance imaging (fMRI) activity of the spinal cord and if fMRI can provide a means of early detection of diabetic neuropathy. fMRI of the spinal cord, using noxious electrical stimulation (15 V ( approximately 8 mA), 0.3 msec, 3 Hz) of the hind paw, was performed in groups of control and streptozotocin (STZ)-induced type 1 diabetic rats. Diabetic rats were lighter, hyperglycemic, and had lower blood pH than controls. fMRI activity at the lumbar enlargement of the spinal cord was identified in the dorsal horn ipsilateral to stimulus of all animals. Signal intensity changes across the lumbar spinal cord during periods of activity were not significantly different between control and diabetic rats, with a trend toward greater signal changes in controls. When specific regions of the spinal cord were analyzed, control rats exhibited significantly increased blood-oxygen level-dependent (BOLD) fMRI activity in both ipsilateral and contralateral dorsal horn compared to diabetic rats. The results of this study are consistent with reports that primary afferent input to the spinal cord is diminished by diabetes, and suggest that BOLD fMRI may be useful in early detection of diabetic neuropathy.

The N-Methyl-D-Aspartate Receptor (NMDAR) and neuronal-glial-cytokine interactions that initiate and maintain inflammatory pain

Following injury, NMDAR activation in spinal cord dorsal horn (SCDH) neurons facilitates the generation of pain in response to low threshold inputs (allodynia). The NMDAR signals the phosphorylation of extracellular signal-regulated kinase (ERK1/2), a key player in injury-induced pain in different cell types. Glia and pro-inflammatory cytokines are implicated in the maintenance of inflammatory pain and the modulation of neuronal activity. However, the role of the NMDAR and neuronal-glial-cytokine interactions that initiate and maintain inflammatory pain is not well defined.

A human trial of HSV-mediated gene transfer for the treatment of chronic pain

Gene transfer to the dorsal root ganglion using replication defective herpes simplex virus (HSV)-based vectors reduces pain-related behaviors in rodent models having inflammatory pain, neuropathic pain and pain caused by cancer in bone. HSV vectors engineered to produce inhibitory neurotransmitters, including the delta opioid agonist peptide enkephalin, the mu opioid agonist peptide endomorphin-2 and glutamic acid decarboxylase (GAD), to effect the release of gamma amino butyric acid (GABA) act to inhibit nociceptive neurotransmission at the first synapse between primary nociceptive and second-order neuron in the dorsal horn of the spinal cord. HSV vectors engineered to release anti-inflammatory peptides, including interleukin (IL)-4, IL-10 and the p55 soluble tumor necrosis factor alpha (TNFalpha) receptor reduce neuroimmune activation in the spinal dorsal horn. The path leading from preclinical animal studies to the ongoing phase 1 human trial of the enkephalin-producing vector in patients with pain from cancer, and plans for an efficacy trial with an opioid-producing vector in inflammatory pain and an efficacy trial with a GAD-producing vector in diabetic neuropathic pain are outlined.

Spinal nerve ligation reduces nitric oxide synthase activity and expression: Effect of resveratrol

The effect of resveratrol on activity and expression of nitric oxide synthase (NOS) in the spinal cord of neuropathic rats was assessed. Spinal nerve ligation produced tactile allodynia along with a reduction of catalytic activity of the constitutive Ca 2+-dependent NOS (eNOS and nNOS isoforms) in the ipsilateral dorsal horn, but not contralateral dorsal or ipsilateral or contralateral ventral, spinal cord at 1, 5, 10 and 15 days after surgery compared to naïve and sham-operated animals. Nerve ligation also induced a reduction of nNOS expression in the ipsilateral dorsal horn spinal cord at 10 and 15 days after surgery. Intrathecal resveratrol reduced allodynia and reversed the reduction of constitutive Ca 2+-dependent NOS activity in the ipsilateral dorsal spinal cord. Moreover, resveratrol significantly reversed the reduction of nNOS expression in the ipsilateral dorsal horn spinal cord. Results show that spinal nerve ligation leads to development of tactile allodynia along with a reduction in constitutive Ca 2+-dependent NOS activity and nNOS isoform expression in the ipsilateral dorsal horn. Data suggest that resveratrol may reduce tactile allodynia in neuropathic rats by restoring altered NOS activity and expression.

Volatile Anesthetic Effects on Midbrain-elicited Locomotion Suggest that the Locomotor Network in the Ventral Spinal Cord Is the Primary Site for Immobility

Volatile anesthetics produce immobility primarily by action in the spinal cord; however, anesthetic effects among different neuronal classes located in different spinal regions, and how they relate to immobility, are not understood. In decerebrated rats, effects of isoflurane and halothane on movement elicited by electrical microstimulation of the mesencephalic locomotor region (MLR) were assessed in relation to minimum alveolar concentration (MAC). Anesthetic effects on step frequency and isometric limb force were measured. The authors also examined effects of MLR stimulation on responses of nociceptive dorsal horn neurons and limb force responses to tail clamp. Mean isoflurane requirements to block MLR-elicited stepping were slightly but significantly greater than MAC by 10%. Mean halothane requirements to block MLR-elicited stepping were greater than those for isoflurane and exceeded MAC by 20%. From 0.4 to 1.3 MAC (but not 0.0 to 0.4 MAC), there was a dose-dependent reduction in the frequency and force of hind limb movements elicited by MLR stimulation during both anesthetics. MLR stimulation inhibited noxious stimulus evoked responses of dorsal horn neurons by approximately 80%. Aptly, MLR stimulation produced analgesia that outlasted the midbrain stimulus by at least 15 s, as indicated by an 81% reduction in hind limb force elicited noxious tail clamp. Because electrical stimulation of the MLR elicits movement independent of dorsal horn activation, the immobilizing properties of isoflurane and halothane are largely independent of action in the dorsal horn. The results suggest that volatile anesthetics produce immobility mainly by action on ventral spinal locomotor networks.

Galloping in Full Pursuit of the Mechanism of Anesthetic Immobility

Galloping in Full Pursuit of the Mechanism of Anesthetic Immobility

Long-lasting hyperalgesia and sympathetic dysregulation after formalin injection into the rat hind paw

Subcutaneous formalin injection has been used extensively to evaluate acute effects (over several hours) of chemical nociceptive stimulation on nociceptive reflexes. Also, a persistent hyperreflexia for mechanical and thermal stimulation, lasting 3 weeks after formalin injection, has been revealed and related to microglial activation in the spinal dorsal horn. The present study demonstrates more prolonged effects of formalin injection, lasting 6 weeks, on operant escape from nociceptive thermal stimulation. Operant escape requires cerebral processing of nociceptive input and can detect effects that are not limited to spinal or spinal–brain stem–spinal reflex circuits. Compared with rats injected with saline, escape responding to 44.5 °C and 47 °C stimulation was increased after bilateral s.c. injection of 5% formalin into the dorsal hind paws. The hyperalgesia outlasted visible signs of trauma (e.g. paw edema). Responses to 36 °C were not altered after formalin injection, providing a control for effects of the peripheral injury on activity levels or exploratory tendencies. Skin temperature recordings from the forepaws and contralateral hind paw during 44.5 °C stimulation of the left hind paw provided an indirect measure of cutaneous blood flow in formalin- and saline-injected animals. Normal reductions in skin temperature during thermal stimulation were attenuated (nearly eliminated) at 1 and 2 weeks after formalin injection and partially recovered by 10 weeks. Thus, formalin-induced tissue injury produced a long-term secondary hyperalgesia, accompanied by a reduced sympathetic responsivity. The similar time-course for these phenomena suggests that there are mechanistic linkages between focal injury, autonomic dysregulation and enhanced pain sensitivity.

The pattern of Fos expression in the spinal dorsal horn following acute noxious mechanical stimulation of bone

Little is known of the spinal mechanisms that mediate bone nociception. The aim of this study was to determine the pattern of neuronal activation in the spinal dorsal horn following acute noxious mechanical stimulation of bone. This was achieved by examining Fos expression in the spinal dorsal horn following acute, noxious mechanical stimulation of the rat tibia. Noxious mechanical stimuli were applied by bone drilling and raising tibial intra-osseous pressure. Control experiments consisted of surgery to expose the tibia. There was a significant increase in the number of Fos-like immunoreactive (Fos-LI) nuclei in the superficial, ipsilateral dorsal horn of animals in the bone drilling and pressure groups relative to animals of the control group at spinal cord segments L3 and L4 ( P < 0.05). The number of Fos-LI nuclei in the deep dorsal horn was always lower than the number in the superficial dorsal horn (significant at L3 but not L4; P < 0.05). Whilst there appeared to be a small increase in the number of Fos-LI nuclei in the ipsilateral deep dorsal horn of bone drilling and pressure groups relative to the ipsilateral deep dorsal horn control group at both L3 and L4 segments, no significant effect was observed ( P > 0.05). The present study implicates the superficial dorsal horn of the spinal cord as a region of interest in studies of acute bone pain, and highlights the notion that spinal mechanisms that mediate bone nociception may be different to those that mediate nociception of cutaneous and visceral origin.

Reorganization of sensory processing below the level of spinal cord injury as revealed by fMRI

The adult mammalian CNS undergoes plastic changes in response to injury. To investigate such changes in spinal cord, functional magnetic resonance imaging (fMRI) was applied in rats subjected to complete transection of the mid-thoracic spinal cord. Blood oxygenation level-dependent (BOLD) contrasts were recorded in the distal spinal cord different times after injury (3, 7, and 14 days, and 1, 3, and 6 months) in response to electrical hind limb stimulation. Functional MRI demonstrated a substantial increase of neuronal activation in the ipsilateral dorsal horn after injury. Notably, 0.5 mA, which did not evoke activation in the normal spinal cord and was considered a non-painful stimulus, induced significant BOLD responses in the dorsal horn after injury. Increased sensitivity was also seen in response to 1.0 mA stimulation. Our results suggest exaggerated responsiveness of spinal neurons after spinal cord injury. Reorganization in the injured spinal cord has been shown to involve the amplification of peripheral inputs and implicated as one underlying mechanism causing neuropathic pain and autonomic dysreflexia. Since BOLD signals can demonstrate such plastic changes in spinal cord parenchyma, we propose fMRI as a method to monitor functional reorganization in the spinal cord after injury. Combining brain and spinal cord fMRI allows the visualization of neuronal activities along the entire neuroaxis and thereby an evaluation of the different plastic responses to CNS injuries that occur in the brain and the spinal cord.

Functional MRI of the cervical spinal cord during noxious and innocuous thermal stimulation in the α-chloralose- and halothane-anesthetized rat

Patterns of neuronal activity in the spinal cord using functional magnetic resonance imaging during noxious (48 degrees C) and innocuous (40 degrees C) thermal stimulation of the rat forepaw were examined. The patterns of functional activity elicited by thermal stimuli were compared in alpha-chloralose- and halothane-anesthetized rats. Although the locations of active pixels were similar during both types of stimulation, the mean percentage signal change was higher during noxious stimulation in both anesthetic groups. Ipsilateral dorsal horn activity was evident during both noxious and innocuous stimulation in all animals. The greatest consistency of ipsilateral dorsal horn activity occurred at the C3 to C5 spinal cord segments in all groups. Consistent contralateral dorsal horn activity appeared in segments C6 to C8 in all groups. C-fos immunohistochemical staining confirmed the presence of neural activity in the spinal cords of all animals.

Rostral ventromedial medulla control of spinal sensory processing in normal and pathophysiological states

Complex networks of pathways project from various structures in the brain to modulate spinal processing of sensory input in a top-down fashion. The rostral ventromedial medulla (RVM) in the brainstem is one major final common output of this endogenous modulatory system and is involved in the relay of sensory information between the spinal cord and brain. The net output of descending neurons that exert inhibitory and facilitatory effects will determine whether neuronal activity in the spinal cord is increased or decreased. By pharmacologically blocking RVM activity with the local anesthetic lignocaine, and then measuring evoked responses of dorsal horn neurons to a range of applied peripheral stimuli, our aim was to determine the prevailing descending influence operating in normal anesthetized animals and animals with experimental neuropathic pain. The injection of 0.8 μl 2% lignocaine into the RVM caused a reduction in deep dorsal horn neuronal responses to electrical and natural stimuli in 64% of normal animals and in 81% of spinal-nerve-ligated (SNL) animals. In normal animals, responses to noxious input were predominantly reduced, while in SNL animals, reductions in spinal cord activity induced by intra-RVM lignocaine further included responses to non-noxious stimuli. This suggests that in terms of activity at least, if not number, descending facilitations are the predominant RVM influence that impacts the spinal cord in normal animals. Moreover, the increase in the proportion of neurons showing a post-lignocaine reduction in dorsal horn activity in SNL rats suggests that the strength of these facilitatory influences increases after neuropathy. This predominant inhibitory spinal effect following the injection of lignocaine into the RVM may be due to blockade of facilitatory On cells.

Dorsal root ganglion regulates the transient ERK activation in the dorsal horn of the spinal cord during development

In the dorsal horn of the chick embryo spinal cord, extracellular signal-regulated kinase (ERK) was phosphorylated transiently during embryonic days 6 and 9. Co-culture studies suggested that dorsal root ganglion (DRG) activated ERK in the dorsal horn. Brain-derived neurotrophic factor (BDNF) activated ERK in the dorsal horn, and anti-BDNF blocked the DRG-induced ERK activation. These results suggest roles of BDNF in the DRG-induced ERK activation in the embryonic dorsal horn.

The Activation of Nicotinic Acetylcholine Receptors Enhances the Inhibitory Synaptic Transmission in the Deep Dorsal Horn Neurons of the Adult Rat Spinal Cord

Somatosensory information can be modulated by nicotinic acetylcholine receptors (nAChRs) in the superficial dorsal horn of the spinal cord. Nonetheless, the functional significance of nAChRs in the deep dorsal horn of adult animals remains unclear. Using whole-cell patch-clamp recordings from lamina V neurons in the adult rat spinal cord, we investigated whether the activation of nAChRs could modulate the inhibitory synaptic transmission in the deep dorsal horn. In the presence of CNQX and APV to block excitatory glutamatergic synaptic transmission, bath applications of nicotine (100 μM) significantly increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in almost all neurons tested. The effect of nicotine was mimicked by N-methyl-4-(3-pyridinyl)-3-butene-1-amine (RJR-2403, 100 μM), an α4β2-nAChR agonist, and was also mimicked by choline (10 mM), an α7-nAChR agonist. The effect of nicotine was completely blocked by the nAChR antagonist mecamylamine (5 μM). In the presence of tetrodotoxin (0.5 μM), nicotine (100 μM) significantly increased the miniature IPSC frequency. On the other hand, RJR-2403 (100 μM) or choline (10 mM) did not affect miniature IPSCs. The application of nicotine (100 μM) also evoked a large inward current in all lamina V neurons tested when cells were held at -60 mV. Similarly, RJR-2403 (100 μM) induced inward currents in the majority of lamina V neurons examined. On the other hand, choline (10 mM) did not elicit any detectable whole-cell currents. These results suggest that several nAChR subtypes are expressed on the presynaptic terminals, preterminals, and neuronal cell bodies within lamina V and that these nAChRs are involved in the modulation of inhibitory synaptic activity in the deep dorsal horn of the spinal cord.