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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Pain. Author manuscript; available in PMC Oct 1, 2011.
Published in final edited form as:
PMCID: PMC2943935

Changes in expression of NMDA-NR1 receptor subunits in the rostral ventromedial medulla modulates pain behaviors


Increasing the NR1 subunit of the NMDA receptor in the RVM produces hyperalgesia, while decreasing NR1 in the RVM produces hypoalgesia and prevents development of chronic muscle hyperalgesia.

Keywords: NR1 subunit, NMDA receptors, rostral ventromedial medulla, muscle pain, hyperalgesia, FIV virus

1. Introduction

Glutamate and its receptors in the brainstem play an important role in both inhibitory and facilitatory pathways of pain modulation. Injection of low doses of glutamate into the rostral ventromedial medulla (RVM) facilitates behavioral responses to noxious stimuli while high doses of glutamate inhibit behavioral responses to noxious stimuli 48,49. After injury, changes in the balance between facilitation and inhibition occur with enhanced facilitation thought to depend on glutamate binding to NMDA receptors. In animals with tissue injury, there is an enhanced release of glutamate in the RVM and enhanced excitatory transmission of RVM neurons 27,47, which can be reversed by pharmacological blockade of NMDA receptors 47. Pharmacological blockade of NMDA receptors in the RVM also reverses hyperalgesia produced by colon inflammation, nerve injury, or muscle pain induced by repeated acid injections 8,9,40,43. Thus, current data support a role for NMDA receptors in the facilitation of nociception within the RVM.

NMDA receptors are ionotropic channels, heteromers composed of NR1 and one or more of the 4 NMDA2 (NR2A-D) or 2 NMDA3 (NR3A and B) subunits 41. The NR1 subunit is essential for function of the NMDA receptors and phosphorylation of NR1 is a major mechanism of modulating channel activity and trafficking to the neuronal surface 5,6. In the spinal cord, downregulation of NR1 subunits in the dorsal horn has no effect on baseline nociceptive responses in uninjured animals but prevents formalin or complete Freund’s adjuvant (CFA)-induced nociceptive behaviors in inflammatory pain models 7,14,30,35. In the RVM, gene expression of NR1 is increased for 7 days after CFA-induced hindpaw inflammation 23. Together, these data suggest an important role for the NR1 subunit in nociceptive processing particularly in the brain and spinal cord.

Our recent studies of muscle pain suggest the RVM is a critical site to modulate both cutaneous and muscle hypersensitivity induced by repeated intramuscular acid injections. Anesthetic blockade in the RVM during the intramuscular injections prevents the development of both cutaneous and muscle hypersensitivity while both anesthetic blockade and NMDA receptor blockade in the RVM reverses the hypersensitivity once developed 9,37. Our model of deep tissue muscle pain is unique in that once developed, central mechanisms, not peripheral mechanisms, maintain the hyperalgesia 34,37. We propose that the NR1 subunit in the RVM is critical for full development of the hypersensitivity after muscle insult. To test this hypothesis we used recombinant feline immunodeficiency virus (FIV) based vectors to overexpress the NR1 subunit in the RVM and measured nociceptive sensitivity to cutaneous and muscle stimuli. We also down-regulated expression of NR1 in the RVM and measured the hyperalgesia produced by repeated acid injections.

2. Materials and Methods

All experiments with laboratory animals were approved by the Animal Care and Use Committee at the University of Iowa and were conducted in accordance with National Institute of Health guidelines. Adult male Sprague-Dawley rats (250–350 g, Harlan, Indianapolis, IN) were used for this study. The animals were housed in a 12-h dark/light cycle, and the testing was done only during the light cycle. Food and water were available to the animals ad libitum.

2.1. Placement of guide cannula

An intracerebral guide cannula was stereotaxically implanted in the RVM. The rats were anesthetized with pentobarbital sodium (Nembutal, 50mg/kg, i.p.), secured in a stereotactic head holder and implanted with a guide cannula (18.0 mm in length, 26 gauge; Plastics One, Roanoke, VA). After the midline incision, the skull was exposed, and a small hole drilled for placement of the guide cannula. The cannula was placed 3.0 mm dorsal to the RVM using the following coordinates: intra-aural: −2.0 mm; mediolateral: 0.0 mm; and dorsoventral: −7.5 mm 25. The cannula was secured to the skull by two stainless-steel screws and dental acrylic cement. An obturator (33 gauge, Plastics One) was inserted into the guide cannula to maintain its patency. All rats were allowed 3 days to recover before testing began.

At the end of the experiment the rats were euthanized and transcardially perfused with 4% paraformaldehyde. The brain was removed and stored in 30% sucrose until processing for immunofluorescence for NR1, described below. The placement of the guide cannula was determined during immunostaining analysis.

2.2. Virus construction

FIV expressing NR1 (FIV-NR1): The pRK5 plasmid containing the cDNA for rat NR1-1a was generously supplied by Dr. Ehlers (Duke University). The full-length cDNA was inserted into the FIV cloning vector, pVETLC SKh10, and verified by restriction mapping. The FIV shuttle vector expressing GFP (FIV-GFP; control), pVETLeGFP, was obtained from the Gene Transfer Vector Core at the University of Iowa. FIV expressing miRNA to NR1: Five potential miRNA sequences against rat NR1 (NR1)(GenBank Accession NM_017010) were tested. N1540 miRNA, which matches the target sequence, AGUCAAUGGUGACCCAGUGA in rat NR1 at position 1540–1549, showed the greatest percentage knockdown of NR1 expression (68%) of the 5 miRNA sequences in an in vitro HEK293T cell assay (unpublished results). N1540 miRNA was ligated into pVETLeGFP at the MfeI site, and sequenced to verify the cloned insert. The N1540 miRNA target sequence is found in all of the 8 known splice variants of NR1, and can inhibit expression of all NR1 subunits. FIV expressing GFP: FIV expressing GFP was used as a control (FIV-GFP).

High titer stocks of FIV viruses were generated by transfecting the FIV cloning vectors with the packaging vector, and the envelope (VSV-G) plasmid at the Gene Transfer Vector Core at the University of Iowa. The VSV-G envelope mediates gene transfer of the FIV viruses predominantly to neurons, rather than to non-neuronal cells 2,31. FIV viruses infect local and not distant neurons 31.

2.3. Virus administration

One of three recombinant FIV viruses was microinjected into the RVM 3 days after cannulae implantation surgery: FIV expressing GFP (FIV-GFP; control), FIV expressing NR1 (FIV-NR1), or FIV expressing miRNA to NR1 and GFP (FIV-miRNA). Microinjection (0.5 μl,1010TU/ml) was performed over a 2-min period.

2.4. Muscle-induced hypersensitivity

Two weeks after microinjection of FIV recombinant viruses, rats were anesthetized with isofluorane (2–5%) and received the first injection of pH 4.0 sterile saline (100 μ) into the left gastrocnemius muscle. This was repeated 5 days later for a total of two acidic saline intramuscular injections. This procedure produces bilateral mechanical hypersensitivity of the muscle and paw that lasts up to 4 weeks 34,46.

2.5. Behavioral measurements

The paw withdrawal threshold to mechanical stimuli (PWT) and the muscle withdrawal threshold (MWT) were tested for all groups of rats, and both tests were performed in the same animals. Both PWT and MWT were performed before and 2 weeks after the virus microinjections. Testing was also performed before (2 weeks after virus) and 5 days after the first injection of acidic saline and 24h after the 2nd injection of acidic saline. Rats were tested for PWT with von Frey filaments applied to the paw. Filaments with increasing bending forces (9.4 to 211 mN) were applied twice on the plantar surface of the hind paw until the rat withdrew from the stimulus. PWT is defined as the lowest force at which the withdrawal response from one of two applications was obtained. A decrease in PWT was interpreted as cutaneous hypersensitivity. This testing method has shown significance in test-retest reliability 33. We were unable to test if there was an increase in cutaneous mechanical sensitivity due to the sensitivity of the assay at higher forces. The maximal force presented to the animal is generally the baseline values (211mN). The next von Frey filament above this force is 427 mN making us unable to detect accurately increases in force.

Rats were tested bilaterally for MWT with a pair of forceps applied to the gastrocnemius muscles as previously described 32. Rats were acclimated for 5 min per session in a restraining device, 3 times a day 1 h apart for 2 days (two days immediately before the first intramuscular injection of acidic saline). The forceps were equipped with two strain gauges to measure force. To measure the MWT, animals were placed in the restrainer, the hindlimb was extended, and the gastrocnemius muscle was compressed with the tip of the forceps until the animal withdrew the leg. The MWT is defined as the maximum force applied during compression to elicit muscle withdrawal. Three trials five minutes apart at each time period were performed and averaged to obtain one reading per time period. A decrease in withdrawal threshold of the muscle was interpreted as muscle hypersensitivity, and an increase in withdrawal thresholds interpreted as muscle hypoalgesia.

2.6. Immunofluorescence staining of NR1

Three weeks after virus microinjection into the RVM, rats were transcardially perfused with 4% paraformaldehyde. Brain tissue was removed, cryoprotected in 30% sucrose, sectioned at 20 μm thickness, and placed on slides for subsequent analysis. Tissue sections were stained simultaneously with anti- NR1 (1:300, MAB1570, Millipore Corporate Headquarters, Billerica, MA) followed by anti-mouse-IgG2b conjugated to Alexa568, (1:500, Invitrogen). Sections were coverslipped with Vectashield Images were collected on an Olympus BX51 fluorescent microscope. Importantly, all images stained and analyzed at the same time were taken under the same conditions. Sections in which GFP was expressed (miRNA or control) or with visible cannula placement (NR1) were assessed for the number of cells stained for NR1. Cells were counted in 24,549 μm2 area (167 × 147μm), encompassing the nucleus raphe magnus, in 5 sections per animal. This area was kept constant across slides and between animals. Cells from all experimental groups were stained simultaneously, the number of cells was counted, and percent of control calculated. Only cells with a visible nucleus were counted.

2.7. Statistical analysis

The changes in expression of NR1 were presented as percentage of change from controls immunostained simultaneously. Comparisons were made with a one-way analysis of variance (ANOVA) followed by a Tukey’s pos-hoc test. As mechanical withdrawal thresholds of the paw were not normally distributed, a non-parametric Kruskal–Wallis ANOVA at each time period followed by a Mann-Whitney post-hoc test for testing differences between groups (vehicle and drugs) was done. Values of MWT and PWT after acid injection were represented as percentage of the first baseline (Day 0, two weeks after virus injection) and tested with a two-way analysis of variance (two-way ANOVA) with repeated measures, for time (repeated factor) and for treatment as variables, followed by a Tukey’s pos-hoc test for differences between groups. P<0.05 was considered statistically significant. Data for the MWT and PWT are represented as the mean ± S.E.M.

3. Results

3.1. Viral delivery of vectors to modulate NR1 in the RVM alters expression of NR1

Microinjection of either FIV-NR1 or FIV-miNR1 into the RVM induced changes inexpression of the NR1 subunit after microinjection (Figure 1)(F2,13=60.7, p=0.0001). Quantification of the immunofluorescence following immunohistochemical staining for NR1 showed an average of 27 ± 1.5 cells. The number of cells significantly decreased to 16 ± 1.9 cells in those treated with FIV-miNR1 (p=0.03) and increased to 70 ± 8 cells in those treated with FIV-NR1 (p =0.0001). Fluorescence microscopy of brain tissue sections for GFP expression revealed that these viruses predominantly infected neuronal cells, and were expressed in the RVM and not in distal neurons.

Figure 1
Fluorescent photomicrographs of neurons in RVM immunostained for NR1 in animals microinjected with (A,B) FIV-GFP, (C,D) FIV-miRNA to NR1 or (E,F) FIV-NR1. The white box in A, C and E represents the representative area that is shown at higher magnification ...

3.2. Modulation of NR1 in the RVM alters nociceptive sensitivity

To test if overexpression of NR1 affected nociceptive behaviors we tested withdrawal thresholds of the muscle and paw 1 and 2 weeks after injection of the virus into the RVM. Microinjection of FIV-NR1 reduced the thresholds to mechanical stimulation of the paw both 1 and 2 weeks after injection (P<0.05) when compared to injection of FIV-GFP (Figure 2A). The muscle withdrawal threshold was significantly decreased 2 weeks after microinjection of FIV-NR1 when compared to FIV-GFP controls (P<0.05) (Figure 2B).

Figure 2
Graphs representing cutaneous (A) and muscle (B) withdrawal thresholds before and after microinjection of FIV-NR1 (n=8) compared to controls injected with FIV-GFP (n=6) into the RVM. Microinjection of FIV-NR1 significantly reduced both (A) cutaneous and ...

To test if downregulation of NR1 affected baseline withdrawal thresholds, we tested MWT at 1 and 2 weeks after injection of the virus. Microinjection of FIV-miNR1 into the RVM significantly increased the MWT when compared to microinjection of FIV-GFP both 1 and 2 weeks after injection (p<0.05, Figure 3).

Figure 3
Graphs representing muscle withdrawal thresholds before and after microinjection of FIV-miNR1 (n=7) compared to controls injected with FIV-GFP (n=6) into the RVM. Microinjection of FIV-miNR1 significantly increased muscle withdrawal threshold. Plot represents ...

3.3. Downregulation of NR1 in the RVM prevents the development of injury-induced hypersensitivity

To test if NR1 contributes to the development of hypersensitivity to muscle insult, we tested if downregulation of NR1 in the RVM contributes to the development of cutaneous and muscle hyperalgesia after repeated intramuscular acid injections. Because we show (see Figures 2 and and3)3) there is a difference in withdrawal thresholds 2 weeks after microinjection of the FIV-miNR1, we calculated a percent change on Day 5 before the second injection, and again on Day 6 24h after the second injection, in comparison to values immediately before the first injection and 2 weeks after FIV-miRNA. In agreement with previous studies 34,45 repeated intramuscular injections of acidic saline into the left gastrocnemius muscle reduced the cutaneous and muscle withdrawal thresholds in both the ipsilateral and contralateral hindlimbs of rats 24h after the second acidic saline injection in control animals injected with FIV-GFP in the RVM (Figure 4). These decreases in withdrawal thresholds for the paw and muscle 24h after the second acidic saline injection did not occur in the group that was injected with FIV-miNR1. The FIV-miNR1 injected animals displayed PWT and MWT that are significantly increased from controls injected with FIV-GFP 24h after the second intramuscular acidic saline injection (Figure 4). The animals microinjected with FIV-NR1 into the RVM showed decreases in thresholds at 2 weeks after injection of the virus (Figure 2A and B) and showed no further reductions in PWT and MWT after repeated intramuscular acid injections (data not shown).

Figure 4
Graphs representing the percent change (from 2 weeks post-FIV injection) in (A) paw and muscle (B) withdrawal thresholds after acidic saline injections on Day 5 before the second acid injection and again on Day 6 24h after the second acid injection in ...

4. Discussion

In the current study we show for the first time a functional role for the NR1 subunit of the NMDA receptor in the RVM in modulation of pain. Increasing expression of NR1 in the RVM enhances behavioral responses to noxious stimuli, and decreasing NR1 in the RVM reduces behavioral responses to noxious stimuli and prevents development of hyperalgesia in an animal model of muscle pain. These data suggest that regulation of NR1 in the RVM can regulate sensitivity to noxious stimuli and development of hyperalgesia.

There is substantial evidence that NMDA receptors are important for neural plasticity, including that associated with pain12,16,23,50. Pharmacological blockade of NMDA receptors in the spinal cord or RVM, reverses hyperalgesia associated with a variety of noxious stimuli. NMDA receptors are heteromers of 4 subunits that contain at least one NR1 subunit, and NR2 or NR3 subunits. The NR1 subunit is expressed as several splice variants that regulate trafficking and function of NMDA receptors5,6. In the current study we overexpressed NR1-1a, a splice variant that is located predominantly in NMDA receptors of synaptic cell membranes10. The experimental design of the NR1 overexpression virus (FIV-NR1) was aimed at increasing the cell-surface expression of NR1-1a containing NMDA receptors at the synapse to enhance neuronal activity. The FIV-miRNA virus expresses a miRNA targeted to a conserved sequence for all of the known splice variants of NR1, and could potentially inhibit the expression of all NMDA receptors within the RVM.

Prior studies show that site selective reduction in NR1 impairs neuronal function associated with neuronal plasticity. Antisense or siRNA knockdown of the NR1 subunit in the CA1 region of the hippocampus, NR1 conditional CA1-NR1 knockout mice, or conditional CA3-NR1 knockout mice have deficits in learning, impaired responses to spatial, temporal, olfactory, visual, emotional stimuli and reduced LTP 18,20,22,24,28,38. Further, siRNA-mediated knockdown of NR1 in the hippocampus delays the onset for seizures and fear memory20. Decreasing NR1 expression in a small number of hippocampal neurons causes learning deficits3. Interestingly, in CA1-NR1 knockout mice, NR2 expression on CA1 dendrites is absent, and NR2 is retained intracellularly within the endoplasmic reticulum, confirming a role for NR1 in cell surface expression of NMDA receptors11. Thus, in the hippocampus, selective reduction of NR1 reduces learning, long-term potentiation, neuronal responses to NMDA, and distribution of NR2 subunits.

For nociceptive behavior, localized deletion of NR1 in the dorsal horn of adult mice prevented injury-induced nociceptive responses to mechanical and thermal stimuli as well as spontaneous pain-like behaviors 13,14,30,35. However, baseline nociceptive latencies to heat are unchanged in cortex-specific NR1-knockout mice 14,26, or following blockade of NMDA receptors in the RVM in naïve, uninjured animals 15,39,44. In contrast, the present study shows that selective deletion of NR1 in the RVM increases baseline threshold to mechanical stimulation of the muscle. These differences between our data and previous data could result from the type of stimuli (thermal or cutaneous vs. mechanical or muscle) and the site of NR1 down regulation (cortex or spinal cord vs. RVM).

Neurons within the RVM both facilitate and inhibit nociceptive stimuli, and it is generally thought that there is a balance between facilitation and inhibition. Glutamate in the RVM modulates both facilitatory and inhibitory pain pathways 48,49. Prior work from our laboratory and others show that NMDA receptors in the RVM play a critical role in both facilitation and inhibition of nociception after tissue insult 9,17,42,44. In fact there is an increase in NR1 mRNA in the RVM after peripheral inflammation 23, and pharmacological blockade of NMDA receptors in the RVM prevents development of hyperalgesia after tissue injury 8,9,39,42. Further, in this model of acid-induced hyperalgesia the RVM is critical for both development and maintenance of hyperalgesia9,37.

In the spinal cord, conditional deletion of the NR1 subunit mRNA reduced the NMDA currents, but not AMPA currents, by 86–88% in lamina II neurons35. This earlier spinal cord data prompted us to hypothesize that downregulation of NR1 in the RVM would decrease NMDA activity and prevent development of muscle and cutaneous hypersensitivity. In agreement with this hypothesis, microinjection of NMDA receptor antagonists into the RVM blocks visceral hypersensitivity, mustard oil-induced hypersensitivity and neuropathic hypersensitivity 8,39,42; as well as muscle and cutaneous hypersensitivity after repeated-acid injections 9. The current study shows that knockdown of NR1 in the RVM prevents development of hyperalgesia to repeated intramuscular acid injections, a model of neuronal plasticity that requires activation of NMDA receptors in the RVM. However, since the downregulation of NR1 also affected baseline nociceptive responses, the lack of development of hyperalgesia could be from a separate hypoalgesic mechanism. In agreement with the notion that NR1 mediates hypoalgesia in the absence of tissue injury, inhibition of nociceptive responses by electrical stimulation of the NRM in naïve animals is increased by approximately 200% after NMDA receptor blockade in the RVM suggesting tonic NMDA receptor activity mediates hypoalgesia 36.

The current study further supports a critical role of the NMDA receptors, and the NR1 subunit, in mediating this balance between inhibition and facilitation of pain since over-expression of NR1-1a in the RVM enhances nociceptive behaviors to cutaneous and muscle mechanical stimuli in animals without tissue injury. The NR1-1a splice variant of NR1 localizes to cell membranes in discrete patches with very little NR1-1a located in the interior regions of the cell 10. Thus, over-expression of the NR1-1a subunit, as in this study, would suggest more membrane-bound NMDA receptors resulting in an enhanced response to endogenous ligands. This would increase cell excitability and thus enhance facilitation. Enhanced facilitation would be manifested behaviorally as decreased thresholds to nociceptive stimuli, i.e. hyperalgesia. These studies are the first to overexpress NR1 in regions involved in nociception, and show that regulation of NR1 in the RVM may be a critical factor for sensitivity to nociceptive stimuli.

Prior studies have demonstrated the association between development of hypersensitivity and changes in expression of NR1 in the spinal cord where nociceptive afferents terminate. For example, injection of CFA in the temporomandibular joint induces an upregulation of NR1 within trigeminal subnucleus caudalis 42. Chronic constriction of the sciatic nerve increases the number of NR1-immunoreactive neurons in the dorsal horn of rats 29. We suggest that increasing NR1 expression in the RVM is sufficient to induce hyperalgesia since the animals developed hypersensitivity without tissue injury two weeks after injection with FIV-NR1.

In parallel to our studies on pain and NMDA receptor overexpression in the RVM, Kalev-Zylinska et al. overexpressed mouse NR1-1a in the rat hippocampus with an adeno-associated viral vector 1/2 19,20. Overexpression of NR1-1a in the rat hippocampus increased fear memory behavior, neurogenesis, facilitated learning and delayed the onset of kainate induced seizures 20. This suggests that the neuronal alterations of NMDA receptor expression in the hippocampus can be affected by NR1 expression. There are other studies, however, in which overexpression of NR1 in the rat hippocampus had no effect on learning and memory behaviors 1,3,4. In these studies NR1 subunits were over-expressed with HSV-1 vectors and examined the effects of overexpression after only 5 days after introduction of the virus. This short-term overexpression of NR1ocurred in a small number of neurons and the fact that a different splice variant of NR1 may have been overexpressed in the rat hippocampus, may explain the lack of effect on learning and memory in these other studies. The current study expressed NR1 or miNR1 using an FIV vector with a VSV-G envelope to specifically mediate stable gene transfer of the FIV viruses predominantly to neurons, rather than to non-neuronal cells 2,31. We show a significant increase in expression of NR1 when using this FIV vector expressing NR1-1a, and a significant decrease in expression of NR1 when using an FIV expressing a miRNA to NR1. Although our changes in expression are significantly different from control, it should be noted that we determined this increase and decrease using profile counting, not with stereology. This limitation using profile counting would likely result in overcounting large cells 21 and thus we can make conclusions about relative differences and not absolute differences. Behaviorally, we also show, in the current study, a decrease in withdrawal thresholds of the paw, but not the muscle 1 week after infection and decreases in both paw and muscle 2 weeks after infection with the FIV-NR1. It may be that there is less expression of NR1 at 1 week when compared to 2 weeks. Thus we might observe differences in some pain measures and not all pain measures at 1 week. In agreement, prior studies show stable expression between 2 and 8 weeks after infection with FIV-expression vectors 31.

In summary, we show that selective modulation of NR1 in the RVM modulates pain behaviors in rats. Specifically, increasing expression of NR1 produces hyperalgesia, an enhanced responsiveness to noxious stimuli, while decreasing expression of NR1 produces analgesia, a decreased responsiveness to noxious stimuli. Further decreasing expression of NR1 in the RVM prevents the development of hyperalgesia induced by tissue insult. We thus suggest that NR1 subunits in the RVM are critical for modulating NMDA receptor function, which in turn sets the ‘tone’ of the nervous system’s response to noxious stimuli and tissue injury.


Supported by National Institutes of Health AR052316. We thank Jessica F. Danielson and Sandra Kolker for technical assistance and the Central Microscopy Core at the University of Iowa for help with imaging.


Conflict of Interest Statement

There are no conflicts of interest.

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