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Proc Natl Acad Sci U S A. 2000 May 23; 97(11): 6132–6137.
Published online 2000 May 9. doi: 10.1073/pnas.100124197
PMCID: PMC18570
PMID: 10801976

Altered pain responses in mice lacking α1E subunit of the voltage-dependent Ca2+ channel

Hironao Saegusa,*† Takashi Kurihara,*† Shuqin Zong,* Osamu Minowa,‡ An-a Kazuno,*† Wenhua Han,*† Yoshihiro Matsuda,*† Hitomi Yamanaka,‡ Makoto Osanai,*† Tetsuo Noda,‡§ and Tsutomu Tanabe*†
Author information Article notes Copyright and License information PMC Disclaimer

Abstract

α1 subunit of the voltage-dependent Ca2+ channel is essential for channel function and determines the functional specificity of various channel types. α1E subunit was originally identified as a neuron-specific one, but the physiological function of the Ca2+ channel containing this subunit (α1E Ca2+ channel) was not clear compared with other types of Ca2+ channels because of the limited availability of specific blockers. To clarify the physiological roles of the α1E Ca2+ channel, we have generated α1E mutant (α1E−/−) mice by gene targeting. The lacZ gene was inserted in-frame and used as a marker for α1E subunit expression. α1E−/− mice showed reduced spontaneous locomotor activities and signs of timidness, but other general behaviors were apparently normal. As involvement of α1E in pain transmission was suggested by localization analyses with 5-bromo-4-chloro-3-indolyl β-d-galactopyranoside staining, we conducted several pain-related behavioral tests using the mutant mice. Although α1E+/− and α1E−/− mice exhibited normal pain behaviors against acute mechanical, thermal, and chemical stimuli, they both showed reduced responses to somatic inflammatory pain. α1E+/− mice showed reduced response to visceral inflammatory pain, whereas α1E−/− mice showed apparently normal response compared with that of wild-type mice. Furthermore, α1E−/− mice that had been presensitized with a visceral noxious conditioning stimulus showed increased responses to a somatic inflammatory pain, in marked contrast with the wild-type mice in which long-lasting effects of descending antinociceptive pathway were predominant. These results suggest that the α1E Ca2 + channel controls pain behaviors by both spinal and supraspinal mechanisms.

Voltage-dependent calcium channels (VDCCs) are classified into several distinct groups termed L-, N-, P-, Q-, R-, and T-types (1, 2). These types of VDCCs play important roles in various neuronal activities, including the control of neurotransmitter release, membrane excitability, and gene expression (3), but exact roles of each channel type are not necessarily clarified. In particular, functions of the R-type Ca2+ channel are least understood. The R-type Ca2+ channel was originally defined as a channel “Resistant” to blockers for L-, N-, P-, and Q-type Ca2+ channels (4); therefore, it is possible that the R-type current is a mixture of several different drug-resistant Ca2+ currents. Although the R-type Ca2+ channel is suggested to play a critical role in the release of neurotransmitters and somatodendritic excitability in a certain set of neurons (4–6), the physiological functions of this channel remain to be clarified.

VDCCs are heteromultimers composed of α1, α2-δ, β, and γ subunits. α1 subunit is essential for channel function and determines the type of each Ca2+ channel. So far, 10 different α1 cDNAs (α1A-I and α1S) have been cloned from a variety of tissues, and extensive studies have been made to clarify the relationship between each cloned α1 subunit and native Ca2+ channels (2). Most of the α1 subunits are known to have some molecular forms resulting from the alternative splicing, and, in most cases, the functional properties of each α1 isoform have been confirmed to be analogous to those of the corresponding native channel. However, in the case of α1E subunit, it is unclear whether Ca2+ channel containing the α1E subunit (α1E Ca2+ channel) represents only a single type of channel. There are several lines of evidence showing that the α1E subunit, when expressed in heterologous systems, defines a mid-voltage activated channel where the voltage range of activation is between those of high-voltage and low-voltage activated channels (7, 8) with permeation properties similar to those of the T-type channels (9). Furthermore, expression of the α1E gene corresponds to the presence of T-type channels in mouse spermatogenic cells and rat cardiac myocytes (10, 11). These results suggest that α1E subunit underlies at least certain aspects of T-type Ca2+ channel functions. Nonetheless, expression of other members of the α1E gene family produces high-voltage activated channels (12–15), and correlation between the neuronal rat α1E gene expression and the neuronal R-type channel has been demonstrated (16), suggesting the involvement of α1E subunit in high-voltage activated R-type channel.

To understand the physiological function of α1E Ca2+ channel, a genetic approach instead of a pharmacological one seems quite useful. We have generated α1E mutant mice by using homologous recombination in embryonic stem (ES) cells. Our strategy is to insert lacZ gene encoding β-galactosidase (β-gal) into the first exon of cacna1e encoding α1E subunit to disrupt the gene and, at the same time, label the α1E-expressing cells with the β-gal activity. Using this reporter gene, we have found that α1E subunit is expressed in various regions involved in the control of pain transmission, such as dorsal root ganglia (DRGs) and dorsal horn of the spinal cord (SC). Recently, the modulation of Ca2+ channel function directly by channel blockers or indirectly through receptor/G-protein pathways (17) has attracted attention as a therapeutic means for controlling nociceptive transmission and soothing pain symptoms (18). Therefore, in this respect, to further investigate the physiological relevance of above-mentioned expression of α1E Ca2+ channel in the pain-control system, we have also conducted several pain-related behavioral tests and found that the α1E mutant mice show abnormalities in pain responses and that this mutant is useful for studying the mechanisms of pain transmission.

Materials and Methods

Gene Targeting.

Genomic clones containing the mouse cacna1e were screened from the 129/Sv mouse genomic library (Stratagene) with 184 bp (nucleotides 1–184) fragment from rabbit α1E cDNA (19). Lambda phage clones containing the first exon were isolated, and the inserts were subcloned into pBluescriptII KS(+) (Stratagene).

Targeting vector BIIZneo was constructed by deleting a 2.3-kb fragment, located from the NotI site in the first exon to an SstI site in the first intron, and inserting nlacZ (gene for Escherichia coli β-gal with a nuclear localization signal at its amino terminus) in-frame with the cacna1e reading frame and neomycin resistance gene driven by the phosphoglycerate kinase promoter (PGK-neo cassette) in its place. Thus, nlacZ and PGK-neo cassette is flanked by a 1.3-kb SalI–NotI fragment (SalI site is from the vector) and a 7-kb SstI–SstI fragment as 5′- and 3′-homologous regions, respectively. The diphtheria toxin A fragment gene was used as a negative selection marker (20).

Linearized BIIZneo was electroporated into J1 ES cells (derived from 129/Sv strain) (21), and homologous recombinant ES cells were screened by Southern blot analysis. Mutant mice were generated by using standard techniques (22). Mice with hybrid background of C57BL/6 (B6) and 129/Sv were used in all of the experiments.

Southern and Northern Hybridization.

Procedures were essentially the same as those reported previously (23), except that the probes were labeled with digoxigenin (DIG) by using DIG-High Prime (Roche Molecular Biochemicals) and that the labeled probe was detected by using alkaline phosphatase (AP)-labeled anti-DIG antibody and CSPD (disodium 3-(4-methoxyspiro{1,2-dioxetane-3, 2′-(5′-chloro)tricyclo[3.3.1.13.7]decan}-4-yl) phenyl phosphate, Roche Molecular Biochemicals) as a substrate for AP.

Reverse Transcription (RT)–PCR to Detect cacna1e Expression.

Total RNA was prepared from mouse brains by the acid guanidinium thiocyanate-phenol-chloroform (AGPC) method (24) and used for the first-strand DNA synthesis with random hexamers and SuperscriptII (GIBCO/BRL). This cDNA preparation was treated with RNase H and used for the template for PCR (0.1 μg RNA equivalent was used in one reaction). PCR primers used were mA1E-F1(5′-AGCAGGAACCGACAAGGAACC-3′, corresponding to upstream region from the nlacZ insertion site in exon 1 of cacna1e) and mA1E-R1 (5′-GGTGGCCAGGATCATGTACTC-3′, possibly located in exon 2).

Immunoblotting.

Immunoblotting was performed in a standard method. Briefly, a 100,000 × g membrane fraction from mouse brains was dissolved in SDS/PAGE sample buffer (10 mM Tris⋅HCl, pH 6.8, containing 0.005% Coomassie brilliant blue-G, 1.5% SDS, 2 M urea, and 10 mM DTT), and the proteins were resolved on a 5% polyacrylamide gel. The proteins were transferred onto polyvinylidene difluoride membrane (Immobilon P; Millipore), and the blot was probed with a rabbit polyclonal anti-α1E antibody (Alomone Laboratories, Jerusalem) by using ECL system (Amersham Pharmacia).

Double Staining with 5-bromo-4-chloro-3-indolyl β-d-galactopyranoside (X-Gal) and IB4.

DRGs and the SC were dissected out and immersion-fixed (or perfusion-fixed) in 4% paraformaldehyde (PFA)/PBS for 1 h, rinsed with PBS, and then stained with X-Gal overnight (25). They were then postfixed with 4% PFA in PBS. Paraffin or frozen sections (7 μm) were stained with peroxidase-labeled IB4 (Sigma) as described (26). As a substrate for peroxidase, 3,3′-diaminobenzidine was used.

X-Gal Staining Followed by RNA in Situ Hybridization.

Frozen sections (7 μm) of DRGs stained with X-Gal were treated for in situ hybridization as described (27). A partial cDNA fragment (0.8 kb) of mouse preprotachykinin A (PPT-A), amplified by RT-PCR, was cloned into pCRII (Invitrogen), and the resultant plasmid was used as a template for synthesis of DIG-labeled riboprobes. Hybridized probe was detected by using AP-labeled anti-DIG antibody (Roche Molecular Biochemicals) with nitroblue tetrazolium chloride and 5-bromo-4-chloro-3-indolyl phosphate as substrates for AP. For cloning of the PPT-A cDNA fragment, primers SP-F1(5′-GTCTGACCGCAAAATCGAAC-3′, nucleotides 81–100, GenBank accession no. D17584) and SP-R1(5′-CAGGAAACATGCTGCTAGGA-3′ nucleotides 902–921) were used.

Behavioral Studies.

The experiments were performed in a blind manner. The data were expressed as mean ± SEM and analyzed by Tukey test for multiple comparisons or by Student's t test for comparison between groups.

Anxiety-related tests.

Mice of both sexes were used at the age of 6–9 wk (at the beginning of a series of experiments). All of the mice were tested sequentially in the three behavioral paradigms (see below). Mice were housed independently 1 wk before the start of the behavioral tests and were handled every day. Before starting each session, the test apparatus was cleaned with 1% acetic acid.

Open-field test. The open field was made of polyvinyl chloride (PVC) plates and was 50 cm × 50 cm × 40 cm in size. Each mouse was transferred to the center of the field, and its locomotor activity was measured for 5 min using a color tracking system (CompACT VAS; Muromachi Kikai, Tokyo).

Elevated plus-maze. The plus-maze consisted of two open arms (25 cm × 8 cm) and two closed arms with translucent plastic walls (15 cm high). The arms and the center square were made of white plastic (PVC) plates and placed at the 50-cm height from the floor. The open arms were surrounded by Plexiglas edges (3 mm high) to avoid animals' falling from the maze. Each mouse was transferred to the center square, and its behavior was videotaped for 5 min. The video images were captured as TIFF format data at 1 frame/s and analyzed on a Macintosh computer with NIH image EP 2.10 (O'Hara, Tokyo), a software modified from the NIH image program (developed at the U.S. National Institutes of Health and available at http://rsb.info.nih.gov/nih-image/). Total time spent on the open and closed arms was each calculated.

Startle response. Acoustic startle response was measured by using a startle chamber (SR-LAB System, San Diego). Briefly, each mouse was put into a Plexiglas cylinder, beneath which a piezoelectric accelerometer was attached to monitor the movement of the mouse. The mouse was exposed to a background noise (about 65 dB) for 5 min at the beginning of the session, and then acoustic stimuli (pulses of white noise with 105, 115, or 117 dB in a randomized order, each with 1 ms duration) were given from a speaker located 25 cm above the cylinder. The interval of the sound pulses was 30 s, and total of 60 pulses were given.

Pain-related tests.

All of the experiments were conducted under the ethical guidelines for the study of experimental pain in conscious animals (28), and the protocol of the pain behavioral studies described in this paper has been approved by the Animal Care Committee of Tokyo Medical and Dental University. Mice of both sexes were used at the age of 15–20 wk. All of the mice had been used for the above-mentioned behavioral tests before the pain-related tests were performed. Mice were acclimatized to the experimental room, which is sound-proof, for at least 1 h before the experiments. The experiments were performed in the light phase (L:D = 12:12).

Von Frey test. Fifty percent hindpaw withdrawal threshold to mechanical stimulation was determined with calibrated von Frey hairs using the up–down paradigm (29).

Paw flick test. Hindpaw withdrawal latency was measured by the method of Hargreaves (30) using a Ugo Basile plantar test apparatus. The tests were performed at low (infrared intensity 10) and high (infrared intensity 40) intensities. The cut-off time was 23 s for both intensities.

Tail flick test. Two-thirds of the tail was immersed in heated water (48–49°C), with the mouse lightly restrained, and the latency to flick the tail was recorded. The cut-off time was 25 s.

Hot plate test. Hot plate tests were performed at three different temperatures (50, 52, and 55°C). Latency to lick the hindpaw was recorded. The cut-off time was 60 s for 50°C, 40 s for 52°C, and 30 s for 55°C.

Formalin test. Under light halothane anesthesia, formalin (10 μl of 0.5% PFA in saline) was injected s.c. into the dorsal surface of a hindpaw. Then the mouse was transferred to an observation chamber. The time spent in licking or biting the injected paw was recorded at 1–3 min and 5–7 min after injection (phase 1) and then for 2 min every 5 min during 10–47 min after injection (phase 2).

Writhing test. Acetic acid (0.6%) was injected i.p. (0.1 ml/10 g body weight), and the number of writhes was counted for 20 min.

Peripheral inflammatory response. The extent of peripheral inflammation was assessed by measuring the volume of the right PFA-injected (Vr) and the left control (Vl) hindpaws with plethysmometer (Unicom TK101; Unicom, Chiba, Japan). Percent peripheral inflammation was calculated as follows: % peripheral inflammation = (Vr − Vl)/Vl × 100.

Results and Discussion

Generation of α1E Ca2+ Channel Mutant Mice.

In the targeting construct (BIIZneo), nlacZ was fused in-frame to the coding sequence of cacna1e gene (Fig. ​(Fig.11A). Thus, the nlacZ insertion is expected to disrupt the cacna1e gene and to make it possible to mark the α1E-expressing cells by the β-gal activity. We introduced linearized BIIZneo into J1 ES cells and screened for targeted ES cells. A total of 3 of 127 clones were found to be correctly targeted, and one of them yielded germ-line chimeras (Fig. ​(Fig.11B). Homozygous mutants (α1E−/−), obtained by intercrossing heterozygotes (α1E+/−), were viable and fertile. In both RT-PCR and Northern blot analyses of brain RNA, no positive signals were observed in the samples from α1E−/− (Fig. ​(Fig.11 C and D), nor was detected the α1E protein by the immunoblot analysis of brain membrane proteins from the α1E−/− mice (Fig. ​(Fig.11E). Thus, we conclude that this targeted nlacZ insertion resulted in a null mutation for cacna1e.

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Generation of α1E-deficient mice. (A) Simplified restriction map around exon 1 of cacna1e gene and structure of the targeting vector. Coding region of exon 1 is boxed. neo, PGK-neo cassette; DT-A, diphtheria toxin-A fragment gene; E, EcoRI; N, NotI; S, SstI; X, XbaI. (B) Southern blot analysis of tail DNA. DNA was digested with SstI, and the blot was hybridized with a probe shown in A. The 3.5-kb band is derived from the wild-type allele (WT) and the 4.6-kb band from the targeted allele (Mut). +/+, wild-type; +/−, heterozygote; −/−, homozygous mutant. (C) RT-PCR analysis. cDNA derived from brain total RNA was used as a template. A fragment of 231 bp is diagnostic of cacna1e expression. M, 100 bp ladder (GIBCO/BRL). (D) Northern blot analysis. Poly(A)+ RNA (2.5 μg) from mouse brains was loaded in each lane. The blot was probed with a cacna1e cDNA fragment (about 1 kb) corresponding to cytoplasmic loop between the repeat II and III of α1E. GAPDH probe was used for loading control (35). (E) Immunoblot analysis. Brain membrane proteins (100 μg/lane) were probed with a rabbit polyclonal anti-α1E antibody. This antibody detects a single band with molecular mass of ca. 250 kDa. Lane 1, wild-type; lane 2, heterozygote; lane 3, homozygous mutant in C, D, and E.

Abnormal Fear in the Homozygous Mutant Mice.

α1E−/− mice often seemed to struggle for escape more vigorously than wild-type (α1E+/+) mice, when an experimenter tried to pick them up. This might suggest the animals' abnormal emotional state, and, therefore, we tested them in several anxiety-related behavioral paradigms. Although it seemed that the behaviors of α1E−/− mice were grossly normal, they showed a significantly reduced level of spontaneous locomotor activities compared with α1E+/+ mice as revealed by an open-field test (Fig. ​(Fig.22 A and B). The percentage of the time spent in the center region of the field was also significantly lower in the α1E−/− mice (Fig. ​(Fig.22 C and D). These observations suggest that α1E−/− mice have an increased level of anxiety. We further examined the level of the fear that is assessed by the elevated plus-maze and startle response tests. In both tests, however, no significant differences were observed among the three genotypes (Fig. ​(Fig.22 E and F). These results suggest that the α1E−/− mice show increased level of fear for some kind of stimuli such as exposure to a novel environment.

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Anxiety-related behavioral tests of wild-type (+/+), heterozygote (+/−), and homozygous mutant mice (−/−). (A and B) An open-field test for a total of 5 min shows significant differences in path length (A) and locomotion time (B) in −/−mice (P < 0.05 and P < 0.01, respectively). +/+, n = 14; +/−, n = 22; −/−, n = 20. (C) Criteria for center vs. border. The center was defined as the inner 16 squares (C, Upper Left). An example of walking paths of a +/+ mouse (Upper Right), a +/− mouse (Bottom Left), and a −/− mouse (Bottom Right) in open-field tests. (D) Percentage of the time spent in the center for a total of 5 min in the open-field test shows a significant difference in −/− mice (P < 0.05). +/+, n = 14; +/−, n = 22; −/−, n = 20. (E) Elevated plus-maze test. Open columns, time spent on open arms; filled columns, time spent on closed arms. No statistically significant difference was observed among the genotypes. +/+, n = 15; +/−, n = 18; −/−, n = 19. (F) Startle responses against various intensities of sound pulses. Stimuli with 105 dB (open columns), 115 dB (gray columns), and 117 dB (filled columns) were given. No statistically significant difference was observed among the genotypes. +/+, n = 8; +/−, n = 10; −/−, n = 15.

α1E Ca2+ Channel Is Expressed in Various Regions Involved in the Control of Pain Transmission.

We examined the expression of α1E Ca2+ channel in the SC and DRGs by assessing the β-gal activity. When a whole SC from an α1E+/− mouse was stained with X-Gal, dense staining was observed in the dorsal horn along the entire length of the SC (Fig. ​(Fig.33 A and B). To examine the nature of the cells expressing β-gal, the X-Gal-stained SC was sectioned and double-labeled with an anti-substance P antibody (data not shown) or with a plant lectin IB4. It is generally accepted that primary afferent nociceptive neurons are roughly classified into two types: one produces peptide neurotransmitters such as substance P or calcitonin-gene-related peptide and the other expresses some enzymatic markers and binds IB4 (31). The results of double-staining show that α1E-expressing cells in the SC are located in the laminae I, II, and possibly III, because the X-Gal signals were observed both outside and inside the IB4 signals (Fig. ​(Fig.33C). Therefore, at least a part of the dorsal horn neurons expressing α1E are thought to be innervated by primary afferent nociceptive neurons.

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cacna1e expression in the nervous system involved in pain transmission. (A and B) X-Gal staining (blue) of the whole SC from a heterozygous mutant. (A) Dorsal view. (B) Cross-section. (C) IB4 binding (brown signal) was assessed in an X-Gal-stained SC section from a heterozygous mutant. (D) Whole lumbar DRG was stained with X-Gal. (E and F) X-Gal-stained lumbar DRG neurons were further stained with molecular markers. (E) Staining with IB4. Some of the X-Gal-stained neurons are also stained with IB4 (arrows). (F) RNA in situ hybridization with an antisense PPT-A riboprobe. Some of the X-Gal-positive neurons show PPT-A signal (arrows). Neurons labeled with only X-Gal were shown by arrowheads in E and F. (G) X-Gal staining of RVM from a heterozygous mutant. Staining was not observed in the RM. (H) X-Gal staining of PAG from a heterozygous mutant. (Scale bars: 1 mm in A, B, and G; 0.5 mm in D and H; 100 μm in C; 50 μm in E and F.) In situ hybridization experiments of wild-type mouse brain sections using DIG-labeled cacna1e riboprobes were in good agreement with those obtained by X-Gal staining of the heterozygous mutant brain, suggesting the X-Gal staining reflects the expression of cacna1e gene (data not shown).

In the DRGs from α1E+/− mice, some neurons expressed β-gal (Fig. ​(Fig.33D). To determine what types of cells expressed α1E, X-Gal-stained lumbar DRG sections were labeled with IB4 or in situ hybridized with an antisense PPT-A (encoding substance P) riboprobe. The results show that some X-Gal-stained neurons were positive for both (Fig. ​(Fig.33 E and F). Thus, α1E is expressed in both categories of primary afferent neurons.

α1E Mutant Mice Behave Normally Against Acute Pain Stimuli.

The above-mentioned results raise a possibility that α1E Ca2+ channel is involved in pain transmission. We therefore analyzed pain-related behaviors of the α1E mutant mice. First, threshold for mechanical stimuli was determined by von Frey test, but animals of each genotype exhibited no significant difference in the threshold (Fig. ​(Fig.44A). Then, we examined responses to noxious thermal stimuli by paw flick, tail flick, and hot plate tests. The paw flick and tail flick tests were used to evaluate the spinal reflexes at the lumbar and sacral levels, respectively, and the hot plate test was to examine a supraspinal involvement in nociception (32). Again, no significant differences among genotypes were detected by these assays (Fig. ​(Fig.44 B–D). Thus, the responses to acute mechanical or noxious thermal stimuli are normal in the α1E mutant mice.

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Acute nociceptive responses of wild-type(+/+), heterozygote (+/−), and homozygous mutant mice (−/−). (A) Fifty percent hindpaw withdrawal thresholds to stimulation with von Frey hairs (+/+, n = 13; +/−, n = 21; −/−, n = 24). (B) Hindpaw withdrawal latencies to noxious thermal stimuli with a low intensity (open columns) and a high intensity (filled columns). +/+, n = 13, 12; +/−, n = 21, 16; −/−, n = 24, 15 at low and high intensities, respectively. (C) Tail flick latencies to noxious heat (48–49°C; +/+, n = 12; +/−, n = 17; −/−, n = 15). (D) Hindpaw licking latencies in the hot plate tests (+/+, n = 11; +/−, n = 17; −/−, n = 15) at 50 °C (open columns), 52 °C (filled columns), and 55 °C (gray columns). There are no significant differences in these four tests across the three genotypes.

Altered Responses Against Noxious Inflammatory Stimuli in α1E Mutant Mice.

We next examined the responses related to the inflammatory pain by the formalin test and the acetic acid writhing test. Injection of formalin into a mouse hindpaw elicits a biphasic pain-response. In phase 1, formalin directly stimulates nociceptors and induces the pain response, and, in phase 2, inflammation caused by formalin elicits the pain response (33). We injected formalin into a hindpaw and observed the mouse behavior. In phase 1, no significant difference was observed between the α1E+/+ and mutant mice. However, the phase 2 response was significantly lowered in the α1E+/− and α1E−/− mice (Fig. ​(Fig.55A). Thus, α1E Ca2+ channel in SC and/or DRG is responsible for transmitting the inflammatory pain sensation (Fig. ​(Fig.6),6), suggesting that blockers of this channel may be useful for antinociception.

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Nociceptive responses to noxious chemical stimulation of cutaneous or visceral tissue. (A) Formalin-evoked hindpaw-licking behavior in wild-type (+/+), heterozygote (+/−), and homozygous mutant mice (−/−). Open columns represent phase 1 (1–7 min after injection); filled columns, phase 2 (10–47 min after injection). Phase 2 responses were significantly reduced in hetero- and homozygous mutant mice (P < 0.01 and P < 0.001, respectively). +/+, n = 8; +/−, n = 7; −/−, n = 9. We found no difference in the peripheral inflammatory response to formalin injection [% peripheral inflammation in +/+ and −/− mice were 30.0 ± 5.3 (n = 7) and 29.8 ± 3.9 (n = 5), respectively]. (B) Visceral nociceptive response (abdominal writhes) produced by i.p. injection of 0.6% acetic acid (+/+, n = 6; +/−, n = 8; −/−, n = 13). Only heterozygous mutant mice exhibited reduced responses (P < 0.05). (C) Effects of sensitization by a noxious visceral conditioning stimulus on the formalin-evoked somatic nociception. In wild-type mice, which had received a noxious visceral stimulus (0.6% acetic acid, AA) 18–20 days before, the phase 2 response (filled column) was considerably reduced compared with the control (gray column). In a separate set of experiments, we also observed significantly reduced phase 2 responses (P < 0.001) in the sensitized B6 mice compared with the naive counterpart (n = 26 and n = 25, respectively). The phase 2 response in homozygous mutant mice after sensitization was significantly facilitated (P < 0.01) compared with that of naive homozygous mutants. +/+, n = 4; +/−, n = 10; −/−, n = 10. Data of naive mice (columns with dotted lines) are presented for comparison.

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A model for explaining α1E mutant phenotypes. Inflammatory mediators produced as a result of a chemical irritant injection stimulate primary afferent fibers, leading to excitation of the dorsal horn neurons. This information is further conveyed to supraspinal structures (e.g., thalamus). α1E Ca2+ channel mediates either or both of these sensory transmissions in a gene-dosage-dependent manner. α1E Ca2+ channel also mediates the descending antinociceptive signal by increasing the excitability of PAG neurons and/or by eliciting the release of an excitatory transmitter(s) from the terminals, which activate RM neurons. Serotonin released by the RM neurons in turn exerts inhibitory control on the spinal pain transmission.

Intraperitoneal injection with acetic acid induces a typical behavior termed writhing, which is a model for a visceral pain with inflammation (33). Interestingly, the writhing response after administration of 0.6% acetic acid was significantly decreased only in the α1E+/− mice (Fig. ​(Fig.55B). This suggests that the decrease in the density of α1E Ca2+ channel (to about half of the normal level) is responsible for lowering pain sensation caused by inflammation. This also raises a possibility that other channels compensate for the loss of α1E in the α1E−/− mice. It may be worth noting here that the response to formalin of the α1E−/− mice was smaller than that in the α1E+/+, in contrast with the case of the acetic acid writhing test. This difference in the responses of α1E−/− mice in both tests may reflect the different routes of entry of the inflammatory nociception: pain signals produced by formalin enter the lumbar SC, whereas those produced by acetic acid enter the thoracic and lumbar SC. If a compensatory mechanism occurred in the α1E−/− mice, it would have been at the thoracic level. Taking this into consideration, we studied the expression of genes coding for α1A, α1B, α1C, α1D, and α1G subunits of the VDCCs in the thoracic and lumbar SC by semiquantitative RT-PCR. Expression of these genes, however, did not significantly differ in both regions among α1E+/+, α1E+/−, and α1E−/− mice (data not shown). So far, we have not obtained data indicating the presence of a compensatory mechanism. Although these lines of evidence do not automatically eliminate the possibility of the presence of compensatory mechanism, we rather prefer to propose that the writhing response in the α1E−/− mice was apparently maintained because of a blockade of the supraspinal antinociceptive pathway. In fact, the following findings support this possibility.

Effects of Visceral Noxious Conditioning Stimuli on the Following Somatic Nociceptive Response.

We first challenged the mice with acetic acid to sensitize them. The pain responses were terminated about 1 h after acetic acid injection. Then, we returned the mice to the home cage. Behaviors of the mice were apparently normal, and they did not show any signs of pain. The sensitized mice were further housed for 18–20 days and then injected a hindpaw with formalin. The response to formalin in phase 1 is slightly reduced in the α1E+/+ mice, whereas it is slightly enhanced in the α1E+/− and the α1E−/− mice. On the other hand, a marked contrast was observed in the phase 2 response in the α1E+/+ and α1E−/− mice (Fig. ​(Fig.55C). In α1E−/− mice, the phase 2 response was significantly increased as compared with that of naive α1E−/− mice injected with formalin, whereas α1E+/+ mice exhibited an opposite response. The apparent hypoalgesic effect observed in the α1E+/+ mice can be interpreted as an example of descending antinociceptive regulatory mechanism. This descending antinociceptive pathway is somewhat similar but is different from the diffuse noxious inhibitory control pathway (34). We have recently identified that this suppression of pain response in sensitized B6 mice is opioid-independent but sensitive to serotonin receptor (5-HT2A/2C) antagonists,‖ suggesting serotonergic involvement in this descending antinociceptive pathway. The increased pain response observed in the α1E−/− mice would be explained by the attenuation of the inhibitory effect of descending antinociceptive pathway and unimpaired activation of the descending nociceptive facilitatory pathway. Indeed, similar kind of stimuli are known to activate both antinociceptive and nociceptive pathways at the supraspinal level (36).

Primary origin of the serotonergic neurons involved in the descending inhibitory pathway is thought to be the nucleus raphe magnus (RM) located in the rostral ventromedial medulla (RVM) (37). However, X-Gal staining of the α1E+/− brains did not reveal α1E expression in the RM (Fig. ​(Fig.33G), but was detected in the periaqueductal gray (PAG) (Fig. ​(Fig.33H), which is suggested to control the activity of RM neurons (37). Thus, it is interesting to speculate that α1E Ca2+ channel is responsible for the descending antinociception by controlling the excitability of PAG neurons and/or by releasing an excitatory transmitter(s) from them to activate RM neurons (Fig. ​(Fig.6),6), although further rigorous research would be necessary to prove this hypothesis.

Results of our recent study suggest that the prolonged activation of the antinociceptive pathway is not evoked by a pretreatment with formalin injection in contrast with the acetic acid injection.∥ This would be compatible with the lowered response in the formalin test (Fig. ​(Fig.55A) and enhanced response (compared with the α1E+/− mice) in the acetic acid writhing test in the naive α1E−/− mice (Fig. ​(Fig.55B). The activation of the antinociceptive pathway elicited by acetic acid lasts for a surprisingly long term (at least 3 wk, so far examined). It may be interesting to study whether this long-term effect is accompanied by neuronal plasticity such as long-term potentiation of synaptic transmission (38) at the PAG/RM axis. If it is, Ca2+ entering through α1E Ca2+ channel must be critical for the process.

To date, there have been several reports on genetically engineered mice that exhibit deficits in pain-related phenomena (reviewed in ref. 39). As far as we know, however, our α1E mutant mice are unique in that they show the possibility of the deficits in the descending antinociceptive pathway. Thus, they provide an intriguing clue to elucidate antinociceptive mechanism which is important to the animal's defensive system.

Acknowledgments

We thank S. Inada, M. Kondoh, T. Muno, M. Tamura, N. Yoneda, L. Gunsten, C. Le, and staff at Animal Research Center of Tokyo Medical and Dental University for assistance, Y. Kiyama and Prof. M. Mishina for helpful advice in behavioral tests, and Dr. T. Murakoshi for critically reading the manuscript. This work was supported by grants from the Ministry of Health and Welfare, the Ministry of Education, Science, Sports and Culture, Japan, Uehara Memorial Foundation, and Naito Memorial Foundation.

Abbreviations

VDCCvoltage-dependent Ca2+ channel
DRGdorsal root ganglion
ESembryonic stem
SCspinal cord
RMnucleus raphe magnus
PAGperiaqueductal gray
β-galβ-galactosidase
DIGdigoxigenin
RTreverse transcription
X-Gal5-bromo-4-chloro-3-indolyl β-d-galactopyranoside
PGKphosphoglycerate kinase
APalkaline phosphatase
PFAparaformaldehyde
PPT-Apreprotachykinin A

Footnotes

Article published online before print: Proc. Natl. Acad. Sci. USA, 10.1073/pnas.100124197.

Article and publication date are at www.pnas.org/cgi/doi/10.1073/pnas.100124197

‖Kurihara, T., Nonaka, T. & Tanabe, T. (2000) Jpn. J. Pharmacol. 82, Suppl. 1, 163 (abstr.).

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