Blocking NMDA receptors in the hippocampal dentate gyrus with AP5 produces analgesia in the formalin pain test

N-methyl-D-aspartate receptors and thymoquinone induce apoptosis and alteration in mitochondria in colorectal cancer cells

Colon cancer is on the rise in both men and women. In addition to traditional treatment methods, herbal treatments from complementary and alternative medicine are actively followed. Naturally derived from plants, thymoquinone (TQ) has drawn a lot of attention in the field of cancer treatment. MK-801, an N-methyl-D-aspartate agonist, is used to improve memory and plasticity, but it has also lately been explored as a potential cancer treatment. This study aimed to determine the roles of N-Methyl-D-Aspartate agonists and Thymoquinone on mitochondria and apoptosis. HT-29 cells were treated with different TQ and MK-801 concentrations. We analyzed cell viability, apoptosis, and alteration of mitochondria. Cell viability significantly decreased depending on doses of TQ and MK-801. Apoptosis and mitochondrial dysfunctions induced by low and high doses of TQ and MK-801. Our study emphasizes the need for further safety evaluation of MK-801 due to the potential toxicity risk of TQ and MK-801. Optimal and toxic doses of TQ and MK-801 were determined for the treatment of colon cancer. It should be considered as a possibility that colon cancer can be treated with TQ and MK-801.

Blocking the dopaminergic receptors within the hippocampal dentate gyrus reduced analgesic responses induced by restraint stress in the formalin test

Previous studies have shown that various receptors, including dopamine receptors, are expressed in the hippocampal dentate gyrus (DG). Besides, indicatively, dopamine receptors play an essential role in the modulation of pain perception. On the other hand, stressful experiences can produce analgesia, termed stress-induced analgesia (SIA). The current study examined the probable role of dopamine receptors within the DG in antinociception induced by restraint stress (RS). Ninety-seven male albino Wistar rats were unilaterally implanted with a cannula in the DG. Animals received intra-DG microinjections of SCH23390 or Sulpiride (0.25, 1, and 4 μg/rat) as D1-and D2-like dopamine receptor antagonists, respectively, five minutes before RS. Ten minutes after the end of the induction of RS for three hours, 50 μl 2.5% formalin was injected subcutaneously into the plantar surface of the hind paw to induce persistent inflammatory pain. Pain scores were evaluated at 5-minute intervals for 60 minutes. These findings showed that; exposure to RS for three hours produced SIA in both phases of the formalin test, while this RS-induced analgesia was attenuated in the early and late phases of the formalin test by intra-DG microinjection of SCH23390 and Sulpiride. The results of the present study suggested that both D1- and D2-like dopamine receptors in the DG have a considerable role in the induced analgesia by RS.

Orexin receptors in the hippocampal dentate gyrus modulated the restraint stress-induced analgesia in the animal model of chronic pain

Previous studies have shown that stressful stimuli induced an adaptive response of reduced nociception, known as stress-induced analgesia (SIA). Since orexin neuropeptides are involved in pain modulation, and orexin neurons, primarily located in the lateral hypothalamus (LH), project to various hippocampal regions, such as the dentate gyrus (DG), the current study aimed to examine the role of orexin receptors within the DG region in the restraint SIA in the animal model of chronic pain. One hundred-thirty adult male Wistar rats (230-250 g) were unilaterally implanted with a cannula above the DG region. Animals were given SB334867 or TCS OX2 29 (1, 3, 10, and 30 nmol, 0.5 µl/rat) into the DG region as orexin-1 receptor (OX1r) and orexin-2 receptor (OX2r) antagonists, respectively, five min before exposure to a 3-hour restraint stress (RS) period. Animals were then undergone the formalin test to assess pain-related behaviors as the animal model of chronic pain. The results showed that RS produces an analgesic response during the early and late phases of the formalin test. However, intra-DG microinjection of OX1r and OX2r antagonists attenuated the restraint SIA. OX2r antagonist was more potent than OX1r antagonist in the early phase of the formalin test, while OX1r antagonist was little more effective in the late phase. Predominantly, it could be concluded that the orexinergic system in the DG region might act as a potential endogenous pain control system and a novel target for treating stress-related disorders.

The integrative role of orexin-1 and orexin-2 receptors within the hippocampal dentate gyrus in the modulation of the stress-induced antinociception in the formalin pain test in the rat

The stressful experiences, by triggering a cascade of hormonal and neural changes, can produce antinociception commonly referred to as stress-induced antinociception (SIA). Orexin neuropeptides have an essential role in stress responses and pain modulation. The dentate gyrus receives orexinergic projections and has been shown to be involved in pain processing. The current study investigated the possible role of orexin-1 and orexin-2 receptors (OX1r and OX2r, respectively) within the dentate gyrus in SIA in a rat model of formalin-induced pain behavior in one hind paw. Male Wistar rats weighing 230-250 g underwent stereotaxic surgery and a cannula was implanted in their brains, above the dentate gyrus region. Either SB334867 or TCS OX2 29 (OX1r and OX2r antagonists, respectively) was microinjected into the dentate gyrus region at a range of doses at 1, 3, 10, and 30 nmol (control group received DMSO 12% as vehicle), 5 min before the forced swim stress (FSS) exposure. The formalin test was performed to assess pain-related behaviors. The results indicated that FSS exposure relieves pain-related behavior in the early and late phases of the formalin test. Blockade of intra-dentate gyrus OX1 or OX2 receptors reduced the antinociceptive responses induced by FSS in the formalin test, with more impact during the late phase. Our findings support the potential role of intra-dentate gyrus orexin receptors as target sites of orexin neurons in painful and stressful situations. Therefore, understanding the exact mechanisms of SIA and the role of the orexinergic system in this phenomenon can lead to identifying the strategies to guide future research and offer a new approach to discovering new pain therapeutic agents.

Similar functional roles of the Orexin-1 and Orexin-2 receptors within the dentate gyrus area of the hippocampus in the stress-induced antinociceptive responses in the acute pain model in the rat

Studies establish that the brain's Orexin system is involved in pain modulation. Orexin-1 and orexin-2 receptors (OX1 and OX2r, respectively) are essential in responsiveness to stressful stimuli. Some evidence indicates that the hippocampus's dentate gyrus (DG) potentially modulates pain and stress. The present study examined the involvement of OX1 and OX2 receptors within the DG in response to acute pain after exposure to forced swim stress (FSS). Five to seven days post-stereotaxic surgery, the baseline tail-flick latency (TFL) was taken from the animal, then rats unilaterally received through an implanted cannula either different doses of OX1r antagonist (SB334867; 1, 3, 10, and 30 nmol), OX2r antagonist (TCS OX2 29; 1, 3, 10 and 30 nmol), or vehicle (0.5 μl solution of 12% DMSO). After 5 min, rats were exposed to the FSS for six minutes. Subsequently, the tail-flick test was conducted, and the TFLs were measured at the 60-min time set intervals. Results indicated that FSS produces antinociceptive responses in the tail-flick test. Two-way ANOVA analysis showed that Microinjection of OX1r and OX2r antagonists into the DG region of the brain reduced FSS-induced analgesia in the tail-flick test. The decrement effects of these two antagonists were almost the same. Additionally, results showed that the role of both receptors was the same in modulating stress-induced analgesia (SIA). These findings show that the orexin system in the hippocampal DG region might be partially involved in the SIA in acute pain.

Blocking the dopaminergic receptors in the hippocampal dentate gyrus reduced the stress-induced analgesia in persistent inflammatory pain in the rat

Although the dentate gyrus (DG) as a component of the hippocampal formation has been well known for its role in memory, various studies showed a diverse population of unique cell types and various inputs and outputs in this region. Besides, brain dopamine is known for its roles in reward, motivation, pleasure, and being involved in the pain process. Further, previous studies demonstrated the participation of DG dopaminergic receptors in antinociception induced by lateral hypothalamus stimulation. This study aimed to investigate the role of DG dopaminergic receptors (D1- and D2-like dopamine receptors) in stress-induced analgesia (SIA) using the formalin test as a persistent inflammatory pain model. One hundred two male Wistar rats were unilaterally implanted with a cannula into the DG. Animals received an intra-DG infusion of SCH23390 (0.25, 1, and 4 μg/rat), or Sulpiride (0.25, 1, and 4 μg/rat) as D1- and D2-like dopamine receptor antagonists, respectively, five min before exposure to forced swim stress (FSS). Ten minutes after FSS termination, 2.5% formalin solution as an inflammatory agent was subcutaneously injected into the plantar surface of the hind paw, and the pain score was quantified for one hour. The findings revealed that exposure to FSS produced SIA, though this FSS-induced analgesia was attenuated in the early and late phase of the formalin test by intra-DG microinjection of SCH23390 or Sulpiride. These results suggested that both D1- and D2-like dopamine receptors in the DG have a considerable role in analgesia induced by FSS.

[Hippocampus is involved in 17β-estradiol exacerbating experimental occlusal inter- ference-induced chronic masseter hyperalgesia in ovariectomized rats]

OBJECTIVE

To investigate the influence of chronic masseter hyperalgesia induced by 17β-estradiol (E2) and experimental occlusal interference (EOI) on underlying mechanism in hippocampus of ovariectomized (OVX) rats.

METHODS

In the study, 32 OVX rats were randomly divided into 4 groups (8 rats/group): The control group was OVX group, and 0 μg/d E2 (vehicle) injection was started 7 d after OVX without EOI; in the experimental group (1) OVX + E2 group, 80 μg/d E2 injection was started 7 d after OVX without EOI; in the experimental group (2) OVX + EOI group, vehicle injection was started 7 d after OVX and EOI was applied 17 d after OVX; in the experimental group (3) OVX + E2 + EOI group, 80 μg/d E2 injection was started 7 d after OVX and EOI was applied 17 d after OVX. Bilateral masseter muscle mechanical withdrawal thresholds were measured before OVX, 7 days after OVX (before E2 injection), 17 days after OVX (10 days after E2 injection and before EOI) and 24 days after OVX (7 days after EOI). Immunofluorescence staining was used to reveal phospho-extracellular signal regulated kinase 1/2 (p-ERK1/2)-positive neurons in CA3 of hippocampus. The protein expression of p-ERK1/2 in hippocampus was detected using Western Blot.

RESULTS

Compared with the control group [left side: (135.3±8.5) g, right side: (135.4±10.8) g], bilateral masseter muscle mechanical withdrawal thresholds of OVX+E2 group [left side: (113.3±5.6) g, right side: (112.5 ± 5.6) g] and OVX+EOI group [left side: (93.3±5.4) g, right side: 90.8±5.5) g] were decreased (P < 0.01). Bilateral masseter muscle mechanical withdrawal thresholds were significantly lower in OVX+E2+EOI group [left side: (81.2±6.2) g, right side: 79.8±7.7) g] than in the control, OVX+E2 and OVX+EOI groups (P < 0.05). The proportion of p-ERK1/2 positive neurons in the CA3 region of the hippocampus was increased in the control, OVX+E2, OVX+EOI and OVX+E2+EOI groups in turn, and the difference between the groups was statistically significant (P < 0.05). p-ERK1/2 protein expression was increased in the control, OVX+E2 and OVX+EOI groups in turn, but the difference was not statistically significant (P>0.05). p-ERK1/2 expression was significantly higher in OVX+E2+EOI group than in the other three groups (P < 0.05).

CONCLUSION

High concentration of E2 could exacerbated EOI-induced chronic masseter hyperalgesia in ovariectomized rats, and its central mechanism may be related to the upregulation of the phosphorylation of ERK1/2 in hippocampus.

Altered nociception in Alzheimer disease is associated with striatal-enriched protein tyrosine phosphatase signaling

ABSTRACT

Alzheimer disease (AD) is the most common form of dementia, accounting for approximately 60% of cases. In addition to memory loss, changes in pain sensitivity are found in a substantial proportion of patients with AD. However, the mechanism of nociception deficits in AD is still unclear. Here, we hypothesize that the nociception abnormality in AD is due to the aberrant activation of striatal-enriched protein tyrosine phosphatase (STEP) signaling, which modulates proteins related to nociception transduction. Our results indicated that the transgenic mice carrying human amyloid precursor protein (APP) gene had lower sensitivity to mechanical and thermal stimulation than the wild-type group at the ages of 6, 9, and 12 months. These APP mice exhibited elevated STEP activity and decreased phosphorylation of proteins involved in nociception transduction in hippocampi. The pharmacological inhibition of STEP activity using TC-2153 further reversed nociception and cognitive deficits in the APP mice. Moreover, the phosphorylation of nociception-related proteins in the APP mice was also rescued after STEP inhibitor treatment, indicating the key role of STEP in nociception alteration. In summary, this study identifies a mechanism for the reduced nociceptive sensitivity in an AD mouse model that could serve as a therapeutic target to improve the quality of life for patients with AD.

2-Pentadecyl-2-oxazoline ameliorates memory impairment and depression-like behaviour in neuropathic mice: possible role of adrenergic alpha2- and H3 histamine autoreceptors

Neuropathic pain (NP) remains an untreatable disease due to the complex pathophysiology that involves the whole pain neuraxis including the forebrain. Sensory dysfunctions such as allodynia and hyperalgesia are only part of the symptoms associated with neuropathic pain that extend to memory and affectivity deficits. The development of multi-target molecules might be a promising therapeutic strategy against the symptoms associated with NP. 2-pentadecyl-2-oxazoline (PEA-OXA) is a plant-derived agent, which has shown effectiveness against chronic pain and associated neuropsychiatric disorders. The molecular mechanisms by which PEA-OXA exerts its effects are, however, only partially known. In the current study, we show that PEA-OXA, besides being an alpha2 adrenergic receptor antagonist, also acts as a modulator at histamine H3 receptors, and report data on its effects on sensory, affective and cognitive symptoms associated with the spared nerve injury (SNI) model of neuropathic pain in mice. Treatment for 14 days with PEA-OXA after the onset of the symptoms associated with neuropathic pain resulted in the following effects: (i) allodynia was decreased; (ii) affective/cognitive impairment associated with SNI (depression, spatial, and working memories) was counteracted; (iii) long-term potentiation in vivo in the lateral entorhinal cortex-dentate gyrus (perforant pathway, LPP) was ameliorated, (iv) hippocampal glutamate, GABA, histamine, norepinephrine and dopamine level alterations after peripheral nerve injury were reversed, (v) expression level of the TH positive neurons in the Locus Coeruleus were normalized. Thus, a 16-day treatment with PEA-OXA alleviates the sensory, emotional, cognitive, electrophysiological and neurochemical alterations associated with SNI-induced neuropathic pain.

HO-CO pathway activation may be associated with hippocampal μ and δ opioid receptors in inhibiting inflammatory pain aversiveness and nociception in WT but not NOS2-KO mice

Carbon monoxide (CO) and nitric oxide (NO) modulate inflammatory nociception and anxiety. We evaluate whether treatments with a heme oxygenase-1 (HO-1) inducer (CoPP) or a carbon monoxide-releasing molecule (CORM-2) are capable of inhibiting inflammatory pain aversiveness in wild type (WT) and inducible nitric oxide synthase-knock out (NOS2-KO) mice with persistent inflammation and its relationship with μ- (MOR) and δ- (DOR) opioid receptors. WT and NOS2-KO male mice with complete Freund's adjuvant (CFA) injected into the hind paw were evaluated in the von Frey and the escape-avoidance paradigm (PEAP) tests, at 10 days, before and after the treatment with CORM-2 (5 mg/kg) or CoPP (2.5 mg/kg). WT mice groups treated with CORM-2 or CoPP also received naloxone (NLX, a non-specific opioid receptor antagonist). The HO-1, neuronal nitric oxide synthase, NOS2, MOR, and DOR expression in the dorsal hippocampus were evaluated by western blot. CFA reduced mechanical threshold in WT and NOS2-KO mice but only increased the percentage of time in the light compartment in the PEAP in WT mice. CORM-2 and CoPP inhibited these effects in both strains. Pre-treatment with NLX reverses the anti-allodynic and anti-aversive effects of CORM-2 or CoPP in WT mice. CORM-2 and CoPP increases the protein levels of HO-1, MOR and DOR in the dorsal hippocampus of WT mice but not in NOS2-KO animals. Results showed that HOCO pathway activation promotes anti-allodynic effects and reduced pain aversiveness caused by peripheral inflammation by increasing the expression of MOR and DOR activated by HO-1 in the dorsal hippocampus.

Role of orexin receptors within the dentate gyrus in antinociception induced by chemical stimulation of the lateral hypothalamus in an animal model of inflammatory pain

Pain is a complex experience consisting of sensory, affective-motivational, and cognitive dimensions. Hence, identifying the multiple neural pathways subserving these functional aspects is a valuable task. The role of dentate gyrus (DG) as a relay station of neocortical afferents in the hippocampal formation (HF) in persistent pain is still controversial. The lateral hypothalamus (LH)-HF neural circuits are involved in numerous situations such as anxiety-like behavior, reward processing, feeding, orofacial as well as acute pain. Nonetheless, to our knowledge, the involvement of the LH-DG neural circuit in persistent pain has already remained unexplored. Adult male Wistar rats weighing 220-250 g were undergone stereotaxic surgery for unilateral implantation of two separate cannulae into the LH and DG. Intra-DG administration of the orexin-1 (OX1) and orexin-2 (OX2) receptor antagonists, SB334867 and TCS OX2 29, respectively, was performed 5 min before intra-LH microinjection of carbachol. Animals were then undergone the formalin test using 50 μl formalin injection (2.5%) into the plantar surface of the hind paw. Microinjection of SB334867 or TCS OX2 29 into the DG region attenuated the antinociceptive effect produced by carbachol microinjection into the LH. The preventive effect of SB334867 and TCS OX2 29 on intra-LH carbachol-induced antinociception was approximately equal in both early and late phases of formalin nociception. The results suggest a neural pathway from the LH to the DG, which contributes to the modulation of formalin-induced inflammatory pain through the recruitment of OX1 and OX2 receptors within the DG.

CaMKII binding to GluN2B at S1303 has no role in acute or inflammatory pain

Activation of Ca²⁺/calmodulin kinase II (CaMKII) and the N-Methyl D-aspartate receptor (NMDAR), particularly its GluN2B subunit, contribute to the central sensitization of nociceptive pathways and persistent pain. Using mutant mice wherein the activity-driven binding of CaMKII to S1303 in GluN2B is abrogated (GluN2BKI), this study investigated the importance of this interaction for acute and persistent inflammatory nociception. GluN2BKI, wild type and heterozygote mice did not differ in responses to acute noxious heat stimuli as measured with tail flick, paw flick, or hot plate assays, nor did they differ in their responses to mechanical stimulation with von Frey filaments. Surprisingly, the three genotypes exhibited similar spontaneous pain behaviors and hypersensitivity to heat or mechanical stimuli induced by intraplantar injection of capsaicin; however, GluN2BKI mice did not immediately attend to the paw. WT and GluN2BKI mice also did not differ in the nociceptive behaviors elicited by intraplantar injection of formalin, even though MK801 greatly reduced these behaviors in both genotypes concordant with NMDAR dependence. CaMKII binding to GluN2B at S1303 therefore does not appear to be critical for the development of inflammatory nociception. Finally, intrathecal KN93 reduced formalin-induced nociceptive behaviors in GluN2BKI mice. KN93 does not inhibit CaKMII, but rather binds Ca²⁺/calmodulin. It has multiple other targets including Ca²⁺-, Na⁺- and K⁺-channels, as well as various kinases. Therefore, the use of GluN2BKI mice provided genetic specificity in assessing the role of CaMKII in inflammatory pain signaling cascades. These results challenge current thinking on the involvement of the CaMKII-NMDAR interaction in inflammatory pain.

MgO and ZnO nanoparticles anti-nociceptive effect modulated by glutamate level and NMDA receptor expression in the hippocampus of stressed and non-stressed rats

The anti-nociceptive mechanisms of MgO and ZnO nanoparticles have not been thoroughly investigated; in this study, we evaluated the effects of anti-nociceptive dose of MgO and ZnO NPs on glutamate level and NMDA receptor subunits expression (NR1, NR2 and NR2B) in the rat whole hippocampus with and without acute restraint stress. Adult rats were divided into control, MgO and ZnO NPs 5 mg/kg, the stress of 90 min alone and with MgO or ZnO NPs 5 mg/kg groups. All components injected intraperitoneally and the nociceptive response was measured with hot plate apparatus 90 min after injections or stress induction. Magnesium, zinc, glutamate levels and NMDA receptor subunits expression were measured in the animal hippocampus. MgO NPs, ZnO NPs and acute stress induced anti-nociceptive effect. MgO NPs observably decreased glutamate and increased magnesium levels and NR2B subunit expression. ZnO NPs decreased glutamate level. Stress elevated endogenous magnesium and zinc levels and also the NR2B expression, but did not change glutamate level. MgO and ZnO NPs in the presence of stress increased the glutamate level and ZnO NPs increased the zinc and the NR2A expression. Stress decreased endogenous magnesium in the hippocampus. MgO and ZnO NPs could affect pain perception by changing glutamate level in the whole hippocampus tissue, while ion level changes followed by injection could probably affect the gene expression in the presence and the absence of stress. It seems that stress indirectly could adverse nanoparticles effects on glutamate level and increase zinc ion releasing from ZnO NPs by activating the gene expression without affecting pain perception.

Changes in metabolites in the brain of patients with fibromyalgia after treatment with an NMDA receptor antagonist

The aims of this work were to evaluate whether the treatment of patients with fibromyalgia with memantine is associated with significant changes in metabolite concentrations in the brain, and to explore any changes in clinical outcome measures. Magnetic resonance spectroscopy was performed of the right anterior and posterior insula, both hippocampi and the posterior cingulate cortex. Questionnaires on pain, anxiety, depression, global function, quality of life and cognitive impairment were used. Ten patients were studied at baseline and after three months of treatment with memantine. Significant increases were observed in the following areas: N-acetylaspartate (4.47 at baseline vs. 4.71 at three months, p = 0.02) and N-acetylaspartate+N-acetylaspartate glutamate in the left hippocampus (5.89 vs. 5.98; p = 0.007); N-acetylaspartate+N-acetylaspartate glutamate in the right hippocampus (5.31 vs 5.79; p = 0.01) and the anterior insula (7.56 vs. 7.70; p = 0.033); glutamate+glutamine/creatine ratio in the anterior insula (2.03 vs. 2.17; p = 0.022) and the posterior insula (1.77 vs. 2.00; p = 0.004); choline/creatine ratio in the posterior cingulate (0.18 vs. 0.19; p = 0.023); and creatine in the right hippocampus (3.60 vs. 3.85; p = 0.007). At the three-month follow-up, memantine improved cognitive function assessed by the Cognition Mini-Exam (31.50, SD = 2.95 vs. 34.40, SD = 0.6; p = 0.005), depression measured by the Hamilton Depression Scale (7.70, SD = 0.81 vs. 7.56, SD = 0.68; p = 0.042) and severity of illness measured by the Clinical Global Impression severity scale (5.79, SD = 0.96 vs. 5.31, SD = 1.12; p = 0.007). Depression, clinical global impression and cognitive function showed improvement with memantine. Magnetic resonance spectroscopy could be useful in monitoring response to the pharmacological treatment of fibromyalgia.

Activation of hippocampal microglia in a murine model of cancer-induced pain

Introduction

Pain is a common and debilitating comorbidity of metastatic breast cancer. The hippocampus has been implicated in nociceptive processing, particularly relating to the subjective aspect of pain. Here, a syngeneic mouse model was used to characterize the effects of peripheral tumors on hippocampal microglial activation in relation to cancer-induced pain (CIP).

Materials and methods

Mice were systemically treated with the colony-stimulating factor 1 receptor inhibitor Pexidartinib prior to intrafemoral (IF) or subcutaneous 4T1 carcinoma cell inoculation. Spontaneous and evoked nociceptive responses were quantitated throughout tumor development, and contralateral hippocampi were collected via endpoint microdissection for RNA analysis. Additionally, IF tumor-bearing animals were sacrificed on days 5, 10, 15, and 20 post 4T1 cell inoculation, and brain sections were immunofluorescently stained for Iba1, a marker of activated microglia.

Results

Ablation of these neuroimmune cells with the CSF1R inhibitor Pexidartinib delayed the onset and severity of cancer-induced nociceptive behaviors in IF tumor-bearing animals, adding to the body of literature that demonstrates microglial contribution to the development and maintenance of CIP. Furthermore, in untreated IF tumor-bearing mice, nociceptive behaviors appeared to progress in parallel with microglial activation in hippocampal regions. Immunofluorescent Iba1+ microglia increased in the dentate gyrus and cornu ammonis 1 hippocampal regions in IF tumor-bearing animals over time, which was confirmed at the mRNA level using relevant microglial markers.

Conclusion

This is the first experimental evidence to demonstrate the effects of peripheral tumor-induced nociception on hippocampal microglial activation. The increase in hippocampal microglia observed in the present study may reflect the emotional and cognitive deficits reported by patients with CIP.

Effects of glial glutamate transporter activator in formalin-induced pain behaviour in mice

BACKGROUND

Nociceptive pain remains a prevalent clinical problem and often poorly responsive to the currently available analgesics. Previous studies have shown that astroglial glutamate transporter-1 (GLT-1) in the hippocampus and anterior cingulate cortex (ACC) is critically involved in pain processing and modulation. However, the role of astroglial GLT-1 in nociceptive pain involving the hippocampus and ACC remains unknown. We investigated the role of 3-[[(2-Methylphenyl) methyl]thio]-6-(2-pyridinyl)-pyridazine (LDN-212320), a GLT-1 activator, in nociceptive pain model and hippocampal-dependent behavioural tasks in mice.

METHODS

We evaluated the effects of LDN-212320 in formalin-induced nociceptive pain model. In addition, formalin-induced impaired hippocampal-dependent behaviours were measured using Y-maze and object recognition test. Furthermore, GLT-1 expression and extracellular signal-regulated kinase phosphorylation (pERK1/2) were measured in the hippocampus and ACC using Western blot analysis and immunohistochemistry.

RESULTS

The LDN-212320 (10 or 20 mg/kg, i.p) significantly attenuated formalin-evoked nociceptive behaviour. The antinociceptive effects of LDN-212320 were reversed by systemic administration of DHK (10 mg/kg, i.p), a GLT-1 antagonist. Moreover, LDN-212320 (10 or 20 mg/kg, i.p) significantly reversed formalin-induced impaired hippocampal-dependent behaviour. In addition, LDN-212320 (10 or 20 mg/kg, i.p) increased GLT-1 expressions in the hippocampus and ACC. On the other hand, LDN-212320 (20 mg/kg, i.p) significantly reduced formalin induced-ERK phosphorylation, a marker of nociception, in the hippocampus and ACC.

CONCLUSION

These results suggest that the GLT-1 activator LDN-212320 prevents nociceptive pain by upregulating astroglial GLT-1 expression in the hippocampus and ACC. Therefore, GLT-1 activator could be a novel drug candidate for nociceptive pain.

SIGNIFICANCE

The present study provides new insights and evaluates the role of GLT-1 activator in the modulation of nociceptive pain involving hippocampus and ACC. Here, we provide evidence that GLT-1 activator could be a potential therapeutic utility for the treatment of nociceptive pain.

Ultra-micronized palmitoylethanolamide rescues the cognitive decline-associated loss of neural plasticity in the neuropathic mouse entorhinal cortex-dentate gyrus pathway

Chronic pain is associated with cognitive deficits. Palmitoylethanolamide (PEA) has been shown to ameliorate pain and pain-related cognitive impairments by restoring glutamatergic synapses functioning in the spared nerve injury (SNI) of the sciatic nerve in mice. SNI reduced mechanical and thermal threshold, spatial memory and LTP at the lateral entorhinal cortex (LEC)-dentate gyrus (DG) pathway. It decreased also postsynaptic density, volume and dendrite arborization of DG and increased the expression of metabotropic glutamate receptor 1 and 7 (mGluR1 and mGluR7), of the GluR1, GluR1s845 and GluR1s831 subunits of AMPA receptor and the levels of glutamate in the DG. The level of the endocannabinoid 2-arachidonoylglycerol (2-AG) was instead increased in the LEC. Chronic treatment with PEA, starting from when neuropathic pain was fully developed, was able to reverse mechanical allodynia and thermal hyperalgesia, memory deficit and LTP in SNI wild type, but not in PPARα null, mice. PEA also restored the level of glutamate and the expression of phosphorylated GluR1 subunits, postsynaptic density and neurogenesis. Altogether, these results suggest that neuropathic pain negatively affects cognitive behavior and related LTP, glutamatergic synapse and synaptogenesis in the DG. In these conditions PEA treatment alleviates pain and cognitive impairment by restoring LTP and synaptic maladaptative changes in the LEC-DG pathway. These outcomes open new perspectives for the use of the N-acylethanolamines, such as PEA, for the treatment of neuropathic pain and its central behavioural sequelae.

Forebrain medial septum sustains experimental neuropathic pain

The present study explored the role of the medial septal region (MS) in experimental neuropathic pain. For the first time, we found that the MS sustains nociceptive behaviors in rodent models of neuropathic pain, especially in the chronic constriction injury (CCI) model and the paclitaxel model of chemotherapy-induced neuropathic pain. For example, inactivation of the MS with intraseptal muscimol (2 μg/μl, 0.5 μl), a GABA mimetic, reversed peripheral hypersensitivity (PH) in the CCI model and induced place preference in a conditioned place preference task, a surrogate measure of spontaneous nociception. The effect of intraseptal muscimol on PH was comparable to that seen with microinjection of the local anesthetic, lidocaine, into rostral ventromedial medulla which is implicated in facilitating experimental chronic nociception. Cellular analysis in the CCI model showed that the MS region sustains nociceptive gain with CCI by facilitating basal nociceptive processing and the amplification of stimulus-evoked neural processing. Indeed, consistent with the idea that excitatory transmission through MS facilitates chronic experimental pain, intraseptal microinjection of antagonists acting at AMPA and NMDA glutamate receptors attenuated CCI-induced PH. We propose that the MS is a central monitor of bodily nociception which sustains molecular plasticity triggered by persistent noxious insult.

Activation of hippocampal MEK1 contributes to the cumulative antinociceptive effect of electroacupuncture in neuropathic pain rats

Background

Electroacupuncture (EA) intervention can relieve a variety of pain; however, optimal EA protocols have not been clearly determined. In addition, although central mitogen-activated protein kinase kinase (MEK) signaling has been shown to be involved in the antinociceptive effect of acupuncture stimulation, its characteristics at different time-points of EA intervention have not been fully elucidated. Therefore, the present study investigated the relationship between the effects of different numbers of EA intervention sessions and the activation of MEK1 in the hippocampus and hypothalamus in a rat model of neuropathic pain.

Methods

After ligation of the left sciatic nerve, which induces chronic constriction injury (CCI), the acupoints Zusanli (ST36) and Yanglingquan (GB34) were applied. The thermal withdrawal latency of the hind paw was used to evaluate the effect of EA on pain thresholds. Intra-hippocampus microinjection of PD98059, a MEK inhibitor, was performed to validate the involvement of MEK in EA analgesia. The hippocampus and hypothalamus were harvested to examine the phosphorylation levels of MEK (pMEK) by western blotting.

Results

In CCI rats, the thermal pain threshold of the affected hind paw decreased significantly relative to the control. Following subsequent daily EA interventions, CCI-induced ipsilateral hyperalgesia was markedly improved from day 4 and the analgesic effect of EA lasted 3 days after cessation of EA. Four sessions of EA markedly suppressed CCI-induced decrease of hippocampal pMEK1 (normalized to the total MEK level). In contrast, successive sessions of EA intervention gradually down-regulated the CCI-induced up-regulation of hypothalamic pMEK1 along with the increase numbers of EA intervention. However, EA did not exert the same analgesic effect after microinjection of PD98059 into the contralateral hippocampus during the first 3 days of EA intervention.

Conclusions

EA intervention can induce time-dependent cumulative analgesia in neuropathic pain rats after 4 successive sessions of daily EA intervention, which is at least in part related to the activation of hippocampal MEK1.

Amidated and Ibuprofen-Conjugated Kyotorphins Promote Neuronal Rescue and Memory Recovery in Cerebral Hypoperfusion Dementia Model

Chronic brain ischemia is a prominent risk factor for neurological dysfunction and progression for dementias, including Alzheimer’s disease (AD). In rats, permanent bilateral common carotid artery occlusion (2VO) causes a progressive neurodegeneration in the hippocampus, learning deficits and memory loss as it occurs in AD. Kyotorphin (KTP) is an endogenous antinociceptive dipeptide whose role as neuromodulator/neuroprotector has been suggested. Recently, we designed two analgesic KTP-derivatives, KTP-amide (KTP–NH₂) and KTP–NH₂ linked to ibuprofen (IbKTP–NH₂) to improve KTP brain targeting. This study investigated the effects of KTP-derivatives on cognitive/behavioral functions (motor/spatial memory/nociception) and hippocampal pathology of female rats in chronic cerebral hypoperfusion (2VO-rat model). 2VO-animals were treated with KTP–NH₂ or IbKTP–NH₂ for 7 days at weeks 2 and 5 post-surgery. After behavioral testing (week 6), coronal sections of hippocampus were H&E-stained or immunolabeled for the cellular markers GFAP (astrocytes) and NFL (neurons). Our findings show that KTP-derivatives, mainly IbKTP–NH₂, enhanced cognitive impairment of 2VO-animals and prevented neuronal damage in hippocampal CA1 subfield, suggesting their potential usefulness for the treatment of dementia.

GABAergic neurons of the medial septum play a nodal role in facilitation of nociception-induced affect

The present study explored the functional details of the influence of medial septal region (MSDB) on spectrum of nociceptive behaviours by manipulating intraseptal GABAergic mechanisms. Results showed that formalin-induced acute nociception was not affected by intraseptal microinjection of bicuculline, a GABAA receptor antagonist, or on selective lesion of septal GABAergic neurons. Indeed, the acute nociceptive responses were dissociated from the regulation of sensorimotor behaviour and generation of theta-rhythm by the GABAergic mechanisms in MSDB. The GABAergic lesion attenuated formalin-induced unconditioned cellular response in the anterior cingulate cortex (ACC) and blocked formalin-induced conditioned place avoidance (F-CPA), and as well as the contextual fear induced on conditioning with brief footshock. The effects of lesion on nociceptive-conditioned cellular responses were, however, variable. Interestingly, the lesion attenuated the conditioned representation of experimental context in dorsal hippocampus field CA1 in the F-CPA task. Collectively, the preceding suggests that the MSDB is a nodal centre wherein the GABAergic neurons mediate nociceptive affect-motivation by regulating cellular mechanisms in ACC that confer an aversive value to the noxious stimulus. Further, in conjunction with a modulatory influence on hippocampal contextual processing, MSDB may integrate affect with context as part of associative learning in the F-CPA task.

The hippocampus and TNF: Common links between chronic pain and depression

Major depression and chronic pain are significant health problems that seriously impact the quality of life of affected individuals. These diseases that individually are difficult to treat often co-exist, thereby compounding the patient's disability and impairment as well as the challenge of successful treatment. The development of efficacious treatments for these comorbid disorders requires a more comprehensive understanding of their linked associations through common neuromodulators, such as tumor necrosis factor-α (TNFα), and various neurotransmitters, as well as common neuroanatomical pathways and structures, including the hippocampal brain region. This review discusses the interaction between depression and chronic pain, emphasizing the fundamental role of the hippocampus in the development and maintenance of both disorders. The focus of this review addresses the hypothesis that hippocampal expressed TNFα serves as a therapeutic target for management of chronic pain and major depressive disorder (MDD).

Hippocampal subfields differentially correlate with chronic pain in older adults

Although previous studies have demonstrated that the hippocampus plays a role in pain processing, the role of hippocampal subfields is uncertain. The goal of this study was to examine the relationship between hippocampal subfield volumes and chronic pain in nondemented older adults. The study sample included 86 community-residing adults age 70 or older who were free of dementia and recruited from the Einstein Aging Study. Chronic pain was defined as pain over the last 3 months, that was moderate or severe (minimum rating of 4 out of 10) most, or all of the time. Hippocampal subfield volumes were estimated using FreeSurfer software. We modeled the association between chronic pain and hippocampal and subfield volume using linear regression. The sample had a mean age of 80 and was 58% female. Chronic pain, present in 55% of the sample, was associated with smaller right and total hippocampal volumes, particularly in women, after adjusting for age, education, and intracranial volume (eTICV). In addition, in women, volume was significantly reduced in participants with chronic pain in right CA2-3 (β=-0.35, p=0.010), right CA4-DG (β=-0.35, p=0.011), left presubiculum (β=-0.29, p=0.030), and left fimbria (β=-0.30, p=0.023). In men, chronic pain was not associated with the volume of any of the hippocampal subfield volumes. Chronic pain in women is associated with a reduction in the volume of right hippocampus and also selected hippocampal subfields. Future studies should clarify the mechanisms underlying the association between regional hippocampal volumes and chronic pain, particularly in women.

Exacerbation of tonic but not phasic pain by entorhinal cortex lesions

The hippocampus is actively involved in pain modulation. Previous studies have shown that inhibition, resection or pharmacological interference of the hippocampus or its subcortical afferent sources such as the medial septum and amygdala produce anti-nociceptive effects. But how the cortical connections of the hippocampus modulate pain remains unexplored. The entorhinal cortex (EC) constitutes the major gateway between the hippocampus and the neocortex. In the present study, rats with medial (MEC), lateral (LEC) or sham EC lesions and received the hot plate and the intra-plantar formalin injection tests. Neither MEC nor LEC lesions affected the hot plate test and the first phase of the formalin test. In contrast, paw licking responses in the second phase of the formalin test significantly increased with both MEC and LEC lesions. These results suggested that that the hippocampal-cortical interactions channeled by the EC were involved in tonic but not phasic pain conditions, and that cortical and sub-cortical connections of the hippocampus played independent roles in pain modulation.

Noradrenergic mechanism involved in the nociceptive modulation of hippocampal CA3 region of normal rats

Norepinephrine (NE) is an important neurotransmitter in the brain, and regulates antinociception. However, the mechanism of action of NE on pain-related neurons in the hippocampal CA3 region is not clear. This study examines the effects of NE, phentolamine on the electrical activities of pain-excited neurons (PENs) and pain-inhibited neurons (PINs) in the hippocampal CA3 region of rats. Trains of electric impulses applied to the right sciatic nerve were used as noxious stimulation. The electrical activities of PENs or PINs in the hippocampal CA3 region were recorded by using a glass microelectrode. Our results revealed that, in the hippocampal CA3 region, the intra-CA3 region microinjection of NE decreased the pain-evoked discharged frequency and prolonged the discharged latency of PEN, and increased the pain-evoked discharged frequency and shortened discharged inhibitory duration (ID) of PIN, exhibiting the specific analgesic effect of NE. While intra-CA3 region microinjection of phentolamine produced the opposite response. It implies that phentolamine can block the effect of endogenous NE to cause the enhanced response of PEN and PIN to noxious stimulation. On the basis of above findings we can deduce that NE, phentolamine and alpha-adrenoceptor are involved in the modulation of nociceptive information transmission in the hippocampal CA3 region.

Indoor volatile organic compounds and chemical sensitivity reactions

Studies of unexplained symptoms observed in chemically sensitive subjects have increased the awareness of the relationship between neurological and immunological diseases due to exposure to volatile organic compounds (VOCs). However, there is no direct evidence that links exposure to low doses of VOCs and neurological and immunological dysfunction. We review animal model data to clarify the role of VOCs in neuroimmune interactions and discuss our recent studies that show a relationship between chronic exposure of C3H mice to low levels of formaldehyde and the induction of neural and immune dysfunction. We also consider the possible mechanisms by which VOC exposure can induce the symptoms presenting in patients with a multiple chemical sensitivity.

Unique hippocampal changes and allodynia in a model of chronic stress

Sustained stress can have numerous pathologic effects. There have been several animal models for chronic stress. We tried to identify the changes of pain threshold and hippocampus in a model of chronic stress. Male Sprague-Dawley rats were kept in a cage filled with 23°C water to a height of 2.2 cm for 7 days. Nociceptive thresholds, expressed in grams, were measured with a Dynamic Plantar Aesthesiometer. Golgi staining was used to identify hippocampal changes. To demonstrate how long allodynia was lasting, behavioral test was repeated daily on another experiment. Compared to control group, chronic stress group showed bilateral mechanical hyper-responsiveness on days 5 (P = 0.047) and 7 (P = 0.032). In general, dendrite atrophic changes within hippocampus of chronic stress model were much more prominent in comparison with control. Compared to control, decreased spine number (P < 0.001) and spine length (P < 0.001) on Golgi staining were seen in the hippocampus of animals with chronic stress. Bilateral mechanical hyperresponsiveness was recovered on day 19 in animals with chronic stress. Chronic stress may bring about central sensitization and hippocampal changes in rats.

Accumulated hippocampal formaldehyde induces age-dependent memory decline

Aging is an important factor in memory decline in aged animals and humans and in Alzheimer's disease and is associated with the impairment of hippocampal long-term potentiation (LTP) and down-regulation of NR1/NR2B expression. Gaseous formaldehyde exposure is known to induce animal memory loss and human cognitive decline; however, it is unclear whether the concentrations of endogenous formaldehyde are elevated in the hippocampus and how excess formaldehyde affects LTP and memory formation during the aging process. In the present study, we report that hippocampal formaldehyde accumulated in memory-deteriorating diseases such as age-related dementia. Spatial memory performance was gradually impaired in normal Sprague-Dawley rats by persistent intraperitoneal injection with formaldehyde. Furthermore, excess formaldehyde treatment suppressed the hippocampal LTP formation by blocking N-methyl-D-aspartate (NMDA) receptor. Chronic excess formaldehyde treatment over a period of 30 days markedly decreased the viability of the hippocampus and down-regulated the expression of the NR1 and NR2B subunits of the NMDA receptor. Our results indicate that excess endogenous formaldehyde is a critical factor in memory loss in age-related memory-deteriorating diseases.

Neuroanatomical and neuropharmacological approaches to postictal antinociception-related prosencephalic neurons: the role of muscarinic and nicotinic cholinergic receptors

Several studies have suggested the involvement of the hippocampus in the elaboration of epilepsy. There is evidence that suggests the hippocampus plays an important role in the affective and motivational components of nociceptive perception. However, the exact nature of this involvement remains unclear. Therefore, the aim of this study was to determine the role of muscarinic and nicotinic cholinergic receptors in the dorsal hippocampus (dH) in the organization of postictal analgesia. In a neuroanatomical study, afferent connections were found from the somatosensory cortex, the medial septal area, the lateral septal area, the diagonal band of Broca, and the dentate gyrus to the dH; all these areas have been suggested to modulate convulsive activity. Outputs to the dH were also identified from the linear raphe nucleus, the median raphe nucleus (MdRN), the dorsal raphe nucleus, and the locus coeruleus. All these structures comprise the endogenous pain modulatory system and may be involved either in postictal pronociception or antinociception that is commonly reported by epileptic patients. dH-pretreatment with cobalt chloride (1.0 mmol/L CoCl2/0.2 μL) to transiently inhibit local synapses decreased postictal analgesia 10 min after the end of seizures. Pretreatment of the dH with either atropine or mecamylamine (1.0 μg/0.2 μL) attenuated the postictal antinociception 30 min after seizures, while the higher dose (5.0 μg/0.2 μL) decreased postictal analgesia immediately after the end of seizures. These findings suggest that the dH exerts a critical role in the organization of postictal analgesia and that muscarinic and nicotinic cholinergic receptor-mediated mechanisms in the dH are involved in the elaboration of antinociceptive processes induced by generalized tonic-clonic seizures.

Brain dysfunction in fibromyalgia and somatization disorder using proton magnetic resonance spectroscopy: a controlled study

OBJECTIVE

To evaluate the brain metabolite patterns in patients with fibromyalgia (FM) and somatization disorder (STD) compared with healthy controls through spectroscopy techniques and correlate these patterns with psychological variables.

METHOD

Design. Controlled, cross-sectional study. Sample. Patients were recruited from primary care in Zaragoza, Spain. The control group was recruited from hospital staff. Patients were administered questionnaires on pain catastrophizing, anxiety, depression, pain, quality of life, and cognitive impairment. All patients underwent Magnetic Resonance Imaging and magnetic resonance spectroscopy (MRS).

RESULTS

A significant increase was found in the glutamate + glutamine (Glx) levels in the posterior cingulate cortex (PCC): 10.73 (SD: 0.49) for FM and 9.67 (SD: 1.10) for STD 9.54 (SD: 1.46) compared with controls (P = 0.043). In the FM + STD group, a correlation between Glx and pain catastrophizing in PCC (r = 0.397; P = 0.033) and between quality of life and the myo-inositol/creatine ratio in the left hippocampus (r = -0.500; P = 0.025) was found. To conclude Glutamate seems to be relevant in the molecular processes involved in FM and STD. It also opens the door for Proton MRS ((1) H-MRS) in STD and suggests that reducing glutamatergic activity through pharmacological treatment could improve the outcome of patients with FM and STD.

CONCLUSION

Glutamate seems to be relevant in the molecular processes involved in FM and STD. It also opens the door for Proton MRS ((1) H-MRS) in STD and suggests that reducing glutamatergic activity through pharmacological treatment could improve the outcome of patients with FM and STD.

Tetracyclines and pain

Tetracyclines are natural or semi-synthetic bacteriostatic agents which have been used since late 1940s against a wide range of gram-positive and gram-negative bacteria and atypical organisms such as chlamydia, mycoplasmas, rickettsia, and protozoan parasites. After the discovery of the first tetracyclines, a second generation of compounds was sought in order to improve water solubility for parenteral administration or to enhance bioavailability after oral administration. This approach resulted in the development of doxycycline and minocycline in the 1970s. Doxycycline was included in the World Health Organization Model List of Essential Medicines either as antibacterial or to prevent malaria or to treat patients with this disease. Additional development led to the third generation of tetracyclines, being tigecycline the only medicine of this class to date. Besides antibacterial activities, the anti-inflammatory, antihypernociceptive and neuroprotective activities of tetracyclines began to be widely studied in the late 1990s. Indeed, there has been an increasing interest in investigating the effects induced by minocycline as this liposoluble derivative is known to cross the blood-brain barrier to the greatest extent. Minocycline induces antihypernociceptive effects in a wide range of animal models of nociceptive, inflammatory and neuropathic pain. In this study, we discuss the antihypernociceptive activity of tetracyclines and summarise its underlying cellular and molecular mechanisms.

Selection for stress-induced analgesia affects the mouse hippocampal transcriptome

Stress responsiveness, including pain sensitivity and stress-induced analgesia (SIA), depends on genotype and, partially, is mediated by hippocampus. The present study examined differences in constitutive gene expression in hippocampus in lines of mice bred for high (HA) and low (LA) swim SIA. Between the lines, we found 1.5-fold or greater differences in expression of 205 genes in the hippocampus in nonstressed animals. The identity of these genes indicates that selective breeding for swim SIA affected many aspects of hippocampal neurons physiology, including metabolism, structural changes, and cellular signaling. Genes involved in calcium signaling pathway, including Slc8a1, Slc8a2, Prkcc, and Ptk2b, were upregulated in LA mice. In HA mice, robust upregulation of genes coding some transcription factors (Klf5) or receptors for neurotensin (Ntsr2) and GABA (Gabard) suggests the genetic basis for a novel mechanism of the non-opioid type of SIA in HA animals. Additional groups of differentially expressed genes represented functional networks involved in carbohydrate metabolism, gene expression regulation, and molecular transport. Our data indicate that selection for a single and very specific stress response trait, swim SIA, alters hippocampal gene expression. The results suggest that individual stress responsiveness may be associated with characteristics of the constitutive hippocampal transcriptome.

Post-conditioning experience with acute or chronic inflammatory pain reduces contextual fear conditioning in the rat

There is evidence that pain can impact cognitive function in people. The present study evaluated whether Pavlovian fear conditioning in rats would be reduced if conditioning were followed by persistent inflammatory pain induced by a subcutaneous injection of dilute formalin or complete Freund's adjuvant (CFA) on the dorsal lumbar surface of the back. Formalin-induced pain specifically impaired contextual fear conditioning but not auditory cue conditioning (Experiment 1A). Moreover, formalin pain only impaired contextual fear conditioning if it was initiated within 1h of conditioning and did not have a significant effect if initiated 2, 8 or 32 h after (Experiments 1A and 1B). Experiment 2 showed that formalin pain initiated after a session of context pre-exposure reduced the ability of that pre-exposure to facilitate contextual fear when the rat was limited to a brief exposure to the context during conditioning. Similar impairments in context- but not CS-fear conditioning were also observed if the rats received an immediate post-conditioning injection with CFA (Experiment 3). Finally, we confirmed that formalin and CFA injected s.c. on the back induced pain-indicative behaviours, hyperalgesia and allodynia with a similar timecourse to intraplantar injections (Experiment 4). These results suggest that persistent pain impairs learning in a hippocampus-dependent task, and may disrupt processes that encode experiences into long-term memory.

A role for the ventral hippocampal endocannabinoid system in fear-conditioned analgesia and fear responding in the presence of nociceptive tone in rats

The endogenous cannabinoid (endocannabinoid) system plays an important role in fear-conditioned analgesia (FCA) and expression and extinction of conditioned fear. The hippocampus has an established role in both pain and conditioned fear and is a substrate for endocannabinoid activity. This study aimed to investigate the role of the endocannabinoid system in the ventral hippocampus (vHip) in FCA and in fear responding in the presence of nociceptive tone. Fear-conditioned rats displayed significantly increased freezing and 22-kHz ultrasonic vocalisation and a reduction in formalin-evoked nociceptive behaviour (ie, FCA) upon re-exposure to a context previously paired with footshock. Tissue levels of the endocannabinoids, anandamide, and 2-arachidonoylglycerol, as well as the fatty acid amide, palmitoylethanolamide, were significantly higher in the vHip of fear-conditioned rats compared with non-fear-conditioned controls. URB597 (inhibitor of fatty acid amide hydrolase [FAAH]), administered bilaterally into the vHip, significantly enhanced FCA during the entire trial and increased fear responding in formalin-treated rats early in the trial. The URB597-induced enhancement of FCA was blocked by intra-vHip administration of the cannabinoid(1) (CB(1)) receptor antagonist/inverse agonist rimonabant. Intra-vHip rimonabant alone had no effect on the expression of FCA, and URB597 did not significantly alter formalin-evoked nociceptive behaviour in non-fear-conditioned rats. These data suggest an important role for the endocannabinoid system in the vHip in FCA, whereby levels of 2-arachidonoylglycerol and the FAAH substrates palmitoylethanolamide and anandamide are increased in rats expressing FCA, and pharmacological inhibition of FAAH in the vHip enhances this form of endogenous analgesia via a CB(1) receptor-dependent mechanism.

Reduced hippocampal glutamate-glutamine levels in irritable bowel syndrome: preliminary findings using magnetic resonance spectroscopy

OBJECTIVES

Enhanced stress responsiveness is an important pathophysiological factor in irritable bowel syndrome (IBS), suggesting the presence of a dysregulated hypothalamic-pituitary-adrenal (HPA) axis. A possible mechanism involves maladaption of the feedback mechanism of the HPA axis. We hypothesized that hippocampus, a key brain region providing inhibitory feedback to the HPA axis, would exhibit reduced excitatory glutamatergic neurotransmission and reduced N-acetyl-aspartate (NAA; a marker of neuronal integrity) levels in IBS patients.

METHODS

In this preliminary study, proton magnetic resonance spectroscopy was used to quantify absolute concentrations of metabolites in bilateral hippocampi of 15 IBS patients without significant psychiatric comorbidity and 15 age-matched controls.

RESULTS

The main finding was a reduction in hippocampal glutamate-glutamine (Glx) in IBS patients. Furthermore, Glx concentrations were inversely related to emotional stress indicators in patients only. No difference was found between subject groups for other metabolite concentrations, including NAA. However, an elevated myo-inositol (mI)/NAA ratio was found in IBS patients.

CONCLUSIONS

Our results provide preliminary evidence for the presence of abnormal hypofunction of hippocampal glutamatergic neurotransmission in IBS patients without psychiatric comorbidity, possibly as a result of the chronic pain. This supports the notion of an imbalance in regulatory brain regions in this subgroup of IBS patients. The inverse relationship between Glx and emotional stress indicators is in agreement with the inhibitory role of hippocampus on the stress system and suggests a sensitization of the mechanism to emotional arousal. The elevated mI/NAA ratio in IBS patients further suggests the presence of hippocampal glial proliferation and remodeling.

Instability of spatial encoding by CA1 hippocampal place cells after peripheral nerve injury

Several authors have shown that the hippocampus responds to painful stimulation and suggested that prolonged painful conditions could lead to abnormal hippocampal functioning. The aim of the present study was to evaluate whether the induction of persistent peripheral neuropathic pain would affect basic hippocampal processing such as the spatial encoding performed by CA1 place cells. These place cells fire preferentially in a certain spatial position in the environment, and this spatial mapping remains stable across multiple experimental sessions even when the animal is removed from the testing environment. To address the effect of prolonged pain on the stability of place cell encoding, we chronically implanted arrays of electrodes in the CA1 hippocampal region of adult rats and recorded the multichannel neuronal activity during a simple food-reinforced alternation task in a U-shaped runway. The activity of place cells was followed over a 3-week period before and after the establishment of an animal model of neuropathy, spared nerve injury. Our results show that the nerve injury increased the number of place fields encoded per cell and the mapping size of the place fields. In addition, there was an increase in in-field coherence while the amount of spatial information content that a single spike conveyed about the animal location decreased over time. Other measures of spatial tuning (in-field firing rate, firing peak and number of spikes) were unchanged between the experimental groups. These results demonstrate that the functioning of spatial place cells is altered during neuropathic pain conditions.

The effect of acetylcholine on pain-related electric activities in the hippocampal CA3 of rats

Acetylcholine (ACh) regulates pain perception in the central nervous system. However, the mechanism of action of ACh on pain-related neurons in the hippocampal CA3 is not clear. The present study aimed to determine the effect of ACh, muscarinic ACh receptors (mAChRs) agonist pilocarpine and mAChRs antagonist atropine on the pain-evoked responses of pain-excited neuron (PEN) and pain-inhibited neuron (PIN) in the hippocampal CA3 of normal rats. The trains of electric impulses applied to the sciatic nerve were used as noxious stimulation. The electric activities of PEN or PIN in the hippocampal CA3 were recorded by using a glass microelectrode. Our results showed that, in the hippocampal CA3, the intra-CA3 microinjection of ACh (2 μg/1 μl) or pilocarpine (2 μg/1 μl) decreased the discharge frequency and prolonged firing latency of PEN, and increased the discharge frequency and shortened firing inhibitory duration (ID) of PIN, i.e. exhibiting the analgesic effect of ACh or pilocarpine. The intra-CA3 administration of atropine (0.5 μg/1 μl) produced an opposite effect. On the basis of the above-mentioned findings, we can deduce that ACh and mAChRs in the hippocampal CA3 are involved in the modulation of nociceptive response by regulating the electric activities of PEN and PIN.