Involvement of glutamate receptors on hyperexcitability of wide dynamic range neurons in the gracile nucleus of the rats with experimental mononeuropathy

Inhibiting spinal secretory phospholipase A2 after painful nerve root injury attenuates established pain and spinal neuronal hyperexcitability by altering spinal glutamatergic signaling

Neuropathic injury is accompanied by chronic inflammation contributing to the onset and maintenance of pain after an initial insult. In addition to their roles in promoting immune cell activation, inflammatory mediators like secretory phospholipase A₂ (sPLA₂) modulate nociceptive and excitatory neuronal signaling during the initiation of pain through hydrolytic activity. Despite having a known role in glial activation and cytokine release, it is unknown if sPLA₂ contributes to the maintenance of painful neuropathy and spinal hyperexcitability later after neural injury. Using a well-established model of painful nerve root compression, this study investigated if inhibiting spinal sPLA₂ 7 days after painful injury modulates the behavioral sensitivity and/or spinal dorsal horn excitability that is typically evident. The effects of sPLA₂ inhibition on altered spinal glutamatergic signaling was also probed by measuring spinal intracellular glutamate levels and spinal glutamate transporter (GLAST and GLT1) and receptor (mGluR5, GluR1, and NR1) expression. Spinal sPLA₂ inhibition at day 7 abolishes behavioral sensitivity, reduces both evoked and spontaneous neuronal firing in the spinal cord, and restores the distribution of neuronal phenotypes to those of control conditions. Inhibiting spinal sPLA₂ also increases intracellular glutamate concentrations and restores spinal expression of GLAST, GLT1, mGluR5, and GluR1 to uninjured expression with no effect on NR1. These findings establish a role for spinal sPLA₂ in maintaining pain and central sensitization after neural injury and suggest this may be via exacerbating glutamate excitotoxicity in the spinal cord.

Enhancement in Tonically Active Glutamatergic Inputs to the Rostral Ventrolateral Medulla Contributes to Neuropathic Pain-Induced High Blood Pressure

Neuropathic pain increases the risk of cardiovascular diseases including hypertension with the characteristic of sympathetic overactivity. The enhanced tonically active glutamatergic input to the rostral ventrolateral medulla (RVLM) contributes to sympathetic overactivity and blood pressure (BP) in cardiovascular diseases. We hypothesize that neuropathic pain enhances tonically active glutamatergic inputs to the RVLM, which contributes to high level of BP and sympathetic outflow. Animal model with the trigeminal neuropathic pain was induced by the infraorbital nerve-chronic constriction injury (ION-CCI). A significant increase in BP and renal sympathetic nerve activity (RSNA) was found in rats with ION-CCI (BP, n = 5, RSNA, n = 7, p < 0.05). The concentration of glutamate in the RVLM was significantly increased in the ION-CCI group (n = 4, p < 0.05). Blockade of glutamate receptors by injection of kynurenic acid into the RVLM significantly decreased BP and RSNA in the ION-CCI group (n = 5, p < 0.05). In two major sources (the paraventricular nucleus and periaqueductal gray) for glutamatergic inputs to the RVLM, the ION-CCI group (n = 5, p < 0.05) showed an increase in glutamate content and expression of glutaminase 2, vesicular glutamate transporter 2 proteins, and c-fos. Our results suggest that enhancement in tonically active glutamatergic inputs to the RVLM contributes to neuropathic pain-induced high blood pressure.

Role of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunit GluR1 in spinal dorsal horn in inflammatory nociception and neuropathic nociception in rat

The present study aims to investigate changes of spinal cord AMPA receptor GluR1 and its phosphorylation in inflammatory and neuropathic pain. Complete Freund's adjuvant (CFA) injection into the hind paw produced inflammatory thermal hyperalgesia that was assessed by decreased response latency to radiant heat; spinal nerve ligation (SNL) was used to induce mechanical allodynia that was evaluated with von Frey hairs. By method of Western blot, expression of GluR1 (the main subunit of the AMPA receptor) and its phosphorylated forms at serine 845 (pGluR1-Ser845) and at serine 831 (pGluR1-Ser831) in the spinal dorsal horn was observed. It was found that the expression of pGluR1-Ser845 and pGluR1-Ser831 increased significantly at 1 h after CFA injection, reached peak at 4 h and returned to the normal control level at 24 h, while no significant change was detected in GluR1 itself. In contrast, neither GluR1 nor pGluR1 showed any significant change in rats following SNL. These results suggest that phosphorylated GluR1 (pGluR1-Ser845 and pGluR1-Ser831) might play a role in the induction of inflammatory but not neuropathic pain.

Intrathecal midazolam regulates spinal AMPA receptor expression and function after nerve injury in rats

Spinal gamma-aminobutyric acid (GABA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors have been implicated in the mechanisms of neuropathic pain after nerve injury; however, how these two receptors interact at the spinal level remains unclear. Here we show that intrathecal midazolam through activation of spinal GABAA receptors attenuated the expression and function of spinal AMPA receptors in rats following peripheral nerve injury. Thermal hyperalgesia and mechanical allodynia induced by chronic constriction nerve injury (CCI) in rats were attenuated by the short-acting benzodiazepine midazolam (20=10>5 mug>vehicle) administered intrathecally once daily for 7 postoperative days. CCI-induced upregulation of AMPA receptors within the spinal cord dorsal horn was also significantly reduced by the intrathecal midazolam (10, 20 mug) treatment. The inhibitory effects of midazolam (10, 20 mug) on neuropathic pain behaviors and AMPA receptor expression were prevented by co-administration of midazolam with the GABAA receptor antagonist bicuculline (3 mug), whereas intrathecal treatment with bicuculline (1 or 3 mug) alone in naive rats induced the upregulation of spinal AMPA receptor expression and nociceptive responses, indicating a tonic regulatory effect from endogenous GABAergic activity on the AMPA receptor expression and spinal nociceptive processing. These results indicate that modulation of spinal AMPA receptor expression and function by the GABAergic activity may serve as a mechanism contributory to the spinal nociceptive processing.

Alteration in sensitivity of ionotropic glutamate receptors and tachykinin receptors in spinal cord contribute to development and maintenance of nerve injury-evoked neuropathic pain

Allodynia or hyperalgesia induced by peripheral nerve injury may be involved in changes in the sensitivity of neurotransmitters at the spinal cord level. In order to clarify the functional role of neurotransmitters in peripheral nerve injury, we used rats with nerve injury induced by chronic constriction of the sciatic nerve (CCI rat model) and estimated the effects of the intrathecal injection of drugs known to affect glutamate and tachykinin receptors. In sham-operated rats, the NMDA receptor agonist NMDA and AMPA-kinate receptor agonist RS-(5)-bromowillardin reduced withdrawal latency. The non-competitive NMDA receptor antagonist MK-801, competitive NMDA receptor antagonist AP-5 and AMPA-kinate receptor antagonist NBQX increased withdrawal latency. Substance P (SP) increased the withdrawal latency but only transitorily. The NK1 receptor antagonist RP67580 increased withdrawal latency, but the NK2 receptor antagonist SR48968 did not show an effect. In CCI rats, RS-(5)-bromowillardin reduced withdrawal latency, but NMDA did not show an effect. NBQX increased withdrawal latency, while MK-801 and AP-5 showed little or no effect. SP reduced withdrawal latency, and both RP67580 and SR48968 increased it. These results indicate that the alteration in sensitivity of ionotropic glutamate receptors and tachykinin receptors in the spinal cord contribute to development and maintenance of nerve injury-evoked neuropathic pain.

Involvement of Dorsal Column Nucleus Neurons in Nociceptive Transmission in Aged Rats

To clarify the functional role of the dorsal column nucleus (DCN) in nociception in rats with advancing age, single neuronal activity and substance P–like immunoreactivity (SP-LI) of the gracile nucleus (GN) were studied in aged rats (29 to 34 mo old) and adult rats (9 to 12 mo old). A total of 122 neurons [aged: 34 wide-dynamic-range (WDR), two nociceptive-specific (NS), and 32 low-threshold mechanical (LTM) neurons; adult: 22 WDR and 32 LTM neurons] were recorded from GN. For WDR neurons, the latency to antidromic activation of the ventral posterior lateral nucleus of the thalamus showed no difference between the aged and adult rats. Sciatic nerve stimulation with C-fiber intensity induced responses of GN with significantly longer latency in aged rats than in adults, whereas there was no difference in the response latency to A-fiber intensity stimulation. Background activity and afterdischarges were significantly higher in the aged rats than those in the adult rats. Responses to noxious mechanical and thermal stimuli were significantly greater in the aged rats during application of graded stimuli. There were no significant differences in responses to nonnoxious mechanical stimulus, mechanical response threshold, and the size of the receptive fields between neurons in the aged and adult rats. The area occupied by SP-LI fibers in the GN and the size of SP-LI dorsal root ganglia neurons were significantly larger in aged rats than in adults. The present findings suggest that the hyperexcitability of GN neurons could be involved in abnormal noxious pain sensations with advancing age.

Enhanced rat sciatic nerve regeneration through silicon tubes filled with pyrroloquinoline quinone

Pyrroloquinoline quinone (PQQ) is an antioxidant that also stimulates nerve growth factor (NGF) synthesis and secretion. In an earlier pilot study in our laboratory, Schwann cell growth was accelerated, and NGF mRNA expression and NGF secretion were promoted. The present study was designed to explore the possible nerve-inducing effect of PQQ on a nerve tube model over a 1-cm segmental deficit. An 8-mm sciatic nerve deficit was created in a rat model and bridged by a 1-cm silicone tube. Then,10 mul of 0.03 mmol/l PQQ were perfused into the silicone chamber in the PQQ group. The same volume of normal saline was delivered in the control group. Each animal underwent functional observation (SFI) at 2-week intervals and electrophysiological studies at 4-week intervals for 12 weeks. Histological and morphometrical analyses were performed at the end of the experiment, 12 weeks after tube implantation. Using a digital image-analysis system, thickness of the myelin sheath was measured, and total numbers of regenerated axons were counted. There was a significant difference in SFI, electrophysiological index (motor-nerve conduct velocity and amplitude of activity potential), and morphometrical results (regenerated axon number and thickness of myelin sheath) in nerve regeneration between the PQQ group and controls (P < 0.05). More mature, high-density, newly regenerated nerve was observed in the PQQ group. We conclude that PQQ is a potent enhancer for the regeneration of peripheral nerves.

Enhancement of stimulation-induced ERK activation in the spinal dorsal horn and gracile nucleus neurons in rats with peripheral nerve injury

It has been suggested that low-threshold sensory pathways have an important role in the formation and maintenance of sensory abnormalities which are observed after peripheral nerve injury. In the present study, we examined the involvement of these pathways in the development of hyperexcitability after sciatic nerve injury (SNI) by detecting the intracellular signal molecule. The rats that received a transection of the sciatic nerve 7 days before were electrically stimulated at 0.1 mA and 3 mA in the proximal region of the nerve injury site. We found a small number of phosphorylated extracellular signal-regulated kinase (pERK)-labelled neurons in laminae I-II and III-IV of the spinal dorsal horn in the control rats after 0.1 mA stimulation. By contrast, there was a marked increased of pERK-labelled neurons both in the superficial laminae and laminae III-IV after the same stimulation in the SNI rats. Enhancement of ERK activation induced by 3 mA stimulation was also observed. Immunoreactivity of pERK in gracile nucleus neurons was also dramatically increased after 0.1 mA stimulation to the injured nerve. These data suggest that the rats with peripheral nerve injury had an increased responsiveness to the low- or high-threshold peripheral stimuli in I-II, III-IV and gracile nucleus neurons. Furthermore, SNI rats that received neonatal capsaicin treatment showed a decreased number of pERK neurons after 0.1 mA stimulation in the dorsal horn and gracile nucleus neurons compared to the control rats. Thus, C-fibres may contribute to the enhanced excitability of the low-threshold sensory neurons after peripheral nerve injury.

The influence of ovariectomy with or without estrogen replacement on responses of rat gracile nucleus neurons to stimulation of hindquarter skin and pelvic viscera

Responses of neurons in the gracile nucleus (NG) of female rats to tactile and visceral stimulation change across the estrous cycle [J. Neurosci. 20 (2000) 7722]. To investigate estrogen's role in these changes, responses of NG neurons to tactile and visceral stimuli were examined in three groups: ovariectomized (OVX), OVX with estrogen replacement (OVX+E2), or sham OVX (tested in diestrus; shamOVX-D). The stimuli were: gentle brushing of hindquarter skin, pressure on the cervix, and distention of the uterus, vagina, or colon. After OVX, the magnitude of multi-unit responses to brushing the perineum, hip and tail, but not the foot and leg, were significantly reduced relative to shamOVX-D. OVX+E2 restored this magnitude to the same level as shamOVX-D, but not, as expected, to levels as large as previously observed in proestrus. After OVX, responses of single neurons to stimulation of the uterus, cervix, and colon were more likely to be excitatory (versus inhibitory) than they had been in cycling rats in proestrus (uterus, cervix) or diestrus (colon); OVX+E2 did not restore the inhibitory responses. In contrast, whereas all responses to vaginal distention after OVX were also excitatory, OVX+E2 in this case significantly restored the inhibitory responses. These findings provide further support for the conclusion that response characteristics of NG neurons are influenced by the rat's hormonal milieu, but also indicate that the influences are not a simple reflection of estrogen levels. The findings further suggest that NG is a component of neural systems that contribute to both reproductive behaviors and vaginal nociception.

Sexual dimorphism for protein kinase c epsilon signaling in a rat model of vincristine-induced painful peripheral neuropathy

Painful peripheral neuropathy is a major dose-limiting adverse effect of many cancer chemotherapeutic agents, such as the vinca alkaloids and taxanes. Recent studies demonstrate sexual dimorphism in second-messenger signaling for primary afferent nociceptor sensitization, and a role of second messengers in the models of metabolic and toxic painful peripheral neuropathies. This study tested the hypothesis that sexual dimorphism alters the severity and second-messenger signaling pathways for enhanced nociception in an animal model of vincristine-induced painful peripheral neuropathy.I.V. injection of vincristine induced mechanical hyperalgesia that was greater in female rats. Gonadectomy in the females but not the males abolished the sex-dependent difference in mechanical hyperalgesia; this effect of gonadectomy in females was reversed by estrogen replacement. Inhibition of protein kinase C epsilon (PKC epsilon ) attenuated vincristine-induced hyperalgesia in males and ovariectomized females, but not in normal females or in estrogen-replaced ovariectomized females. Inhibitors of protein kinase A, protein kinase G, p42 / p44-mitogen activated protein kinase and nitric oxide synthase also attenuated vincristine-induced hyperalgesia, but to a similar degree in both sexes. These data demonstrate an estrogen-dependent sexual dimorphism in vincristine-induced hyperalgesia (female>male) and an unexpected opposite sexual dimorphism in the contribution of PKC epsilon to the severity of this hyperalgesia (male>female).