Mechanisms for ovariectomy-induced hyperalgesia and its relief by calcitonin: participation of 5-HT1A-like receptor on C-afferent terminals in substantia gelatinosa of the rat spinal cord

Comparison of the effects of elcatonin and risedronate on back and knee pain by electroalgometry using fall of skin impedance and quality of life assessment using SF-36

Back and knee pain is a widespread health problem and a serious threat to the quality of life (QOL) in middle-aged and older adults, as it frequently accompanies osteoporosis and osteoarthritis. In order to compare the effects of elcatonin and risedronate on such pain, 20 units of elcatonin was intramuscularly injected to 18 patients, and 5 mg of risedronate was orally administered daily to 20 others with similar backgrounds. Exercise-induced pain was analyzed by measuring the fall of skin impedance by electroalgometry (EAM), and subjective pain was recorded by a visual rating system (VRS) on a scale of 0 (no pain) to 100 (unbearable pain). In patients treated with elcatonin, the mean EAM-estimated pain was significantly reduced after 4, 5 and 6 months of treatment, and the VRS score after 3, 5 and 6 months, indicating a significant analgesic effect. In the risedronate group, however, improvement was less remarkable. Two-way analysis of variance using pain as a dependent variable and treatment group and time as independent variables revealed a significantly greater effect of elcatonin over risedronate on both the EAM and VRS scores, and the influence of treatment time on pain was indistinguishable between the two treatment groups. Effect of exercise load on pain was less on knee load than knee and spine load and spine load, but indistinguishable between the two groups. Changes in QOL were evaluated by the SF-36 system. Norm-based scoring showed significant improvements in 3 of 4 categories for elcatonin and in 2 of 4 for risedronate, suggesting comparable effects on the physical aspects of QOL, whereas responses to emotionally and socially directed questions indicated significant improvements in all 4 categories for risedronate, but none for elcatonin, suggesting a more physical than emotional component in elcatonin effects compared to risedronate.

Blocking TNF-α with infliximab alleviates ovariectomy induced mechanical and thermal hyperalgesia in rats

Studies have proved an increased expression of tumor necrosis factor alpha (TNF-α) in estrogen deficiency animals, and TNF-α also plays a role in inflammation and neuropathic pain. This study aimed to explore the relationship between TNF-α and ovariectomy induced hyperalgesia. 36 female Sparague-Dawley were included, estrogen depletion models were established by ovariectomy. Then infliximab (a TNF-α blocker) was administrated to the ovariectomized rats for 8 weeks. Pain behavioral tests were performed once a week. The bone mineral density (BMD), serum estradiol and TNF-α level were determined at the 8th week after ovariectomy. The expression of TNF-α in lumbar 5 dorsal root ganglions (L5 DRGs) was examined by immunofluorescence method. Significant hyperalgesia to mechanical and thermal stimuli in groups Ovx-1 and Ovx-2 was observed 1 week after the operation. After treated with infliximab, the pain threshold of Ovx-2 was partially restored, although still lower than the Sham group. The serum TNF-α level of Ovx-1 was significantly higher than Sham and Ovx-2. TNF-α immunofluorescence indicated a significant increase in the expression of TNF-α at L5 DRGs in group Ovx-1 when compared with groups Sham and Ovx-2. The BMD of group Ovx-2 was significantly higher than group Ovx-1 and lower than group Sham. In conclusion, TNF-α plays an important role in estrogen deficiency induced mechanical and thermal hyperalgesia, and DRG may be one site on which TNF-α acts to cause hyperalgesia. Blocking the effect of TNF-α could partially alleviate the estrogen deficiency induced hyperalgesia in rats. Thus, TNF-α may contribute to chronic pain in postmenopausal women.

Synaptic connections of the neurokinin 1 receptor-like immunoreactive neurons in the rat medullary dorsal horn

The synaptic connections between neurokinin 1 (NK1) receptor-like immunoreactive (LI) neurons and γ-aminobutyric acid (GABA)-, glycine (Gly)-, serotonin (5-HT)- or dopamine-β-hydroxylase (DBH, a specific marker for norepinephrinergic neuronal structures)-LI axon terminals in the rat medullary dorsal horn (MDH) were examined under electron microscope by using a pre-embedding immunohistochemical double-staining technique. NK1 receptor-LI neurons were observed principally in laminae I and III, only a few of them were found in lamina II of the MDH. GABA-, Gly-, 5-HT-, or DBH-LI axon terminals were densely encountered in laminae I and II, and sparsely in lamina III of the MDH. Some of these GABA-, Gly-, 5-HT-, or DBH-LI axon terminals were observed to make principally symmetric synapses with NK1 receptor-LI neuronal cell bodies and dendritic processes in laminae I, II and III of the MDH. The present results suggest that neurons expressing NK1 receptor within the MDH might be modulated by GABAergic and glycinergic inhibitory intrinsic neurons located in the MDH and 5-HT- or norepinephrine (NE)-containing descending fibers originated from structures in the brainstem.

Perception of thermal pain and the thermal grill illusion is associated with polymorphisms in the serotonin transporter gene

Aim

The main aim of this study was to assess if the perception of thermal pain thresholds is associated with genetically inferred levels of expression of the 5-HT transporter (5-HTT). Additionally, the perception of the so-called thermal grill illusion (TGI) was assessed. Forty-four healthy individuals (27 females, 17 males) were selected a-priori based on their 5-HTTLPR/rs25531 (‘tri-allelic 5-HTTLPR’) genotype, with inferred high or low 5-HTT expression. Thresholds for heat- and cold-pain were determined along with the sensory and affective dimensions of the TGI.

Results

Thresholds to heat- and cold-pain correlated strongly (rho  = −0.58, p<0.001). Individuals in the low 5-HTT-expressing group were significantly less sensitive to heat-pain (p = 0.02) and cold-pain (p = 0.03), compared to the high-expressing group. A significant gender-by-genotype interaction also emerged for cold-pain perception (p = 0.02); low 5-HTT-expressing females were less sensitive. The TGI was rated as significantly more unpleasant (affective-motivational dimension) than painful (sensory-discriminatory dimension), (p<0.001). Females in the low 5-HTT expressing group rated the TGI as significantly less unpleasant than high 5-HTT expressing females (p<0.05), with no such differences among men.

Conclusion/Significance

We demonstrate an association between inferred low 5-HTT expression and elevated thresholds to thermal pain in healthy non-depressed individuals. Despite the fact that reduced 5-HTT expression is a risk factor for chronic pain we found it to be related to hypoalgesia for threshold thermal pain. Low 5-HTT expression is, however, also a risk factor for depression where thermal insensitivity is often seen. Our results may thus contribute to a better understanding of the molecular underpinnings of such paradoxical hypoalgesia. The results point to a differential regulation of thermoafferent-information along the neuraxis on the basis of 5-HTT expression and gender. The TGI, suggested to rely on the central integration of thermoafferent-information, may prove a valuable tool in probing the affective-motivational dimension of these putative mechanisms.

Modulation of synaptic transmission from primary afferents to spinal substantia gelatinosa neurons by group III mGluRs in GAD65-EGFP transgenic mice

Group III metabotropic glutamate receptors (mGluRs) are involved in nociceptive transmission in the spinal cord. However, the cellular mechanism underlying the modulation of synaptic transmission from nociceptive primary afferents to dorsal horn neurons by group III mGluRs has yet to be explored. In this study, we used transgenic mice expressing enhanced green fluorescent protein (EGFP) under the control of the glutamate decarboxylase (GAD) 65 promoter to identify specific subpopulations of GABAergic inhibitory interneurons. By GABA immunolabeling, we confirmed the majority of GAD65-EGFP-expressing neurons were GABAergic. Because GAD65-EGFP-expressing neurons have not been examined in detail before, we first investigated the physiological properties of GAD65-EGFP- and non-EGFP-expressing neurons in substantia gelatinosa (SG) of the spinal dorsal horn. Membrane properties, such as the resting membrane potential, membrane capacitance, action potential threshold, and action potential height, differed significantly between these two groups of neurons. Most EGFP-expressing neurons displayed a tonic firing pattern (73% of recorded neurons) and received monosynaptic Aδ and/or C primary afferent inputs (85% of recorded neurons). In contrast, we observed a delayed firing pattern in 53% of non-EGFP-expressing neurons. After identifying the physiological properties of EGFP-expressing neurons, we tested the effects of group III mGluRs on synaptic transmission pharmacologically. A group III mGluR agonist, L-AP4, attenuated Aδ fiber-evoked synaptic transmission but did not affect C fiber-evoked synaptic transmission to EGFP-expressing neurons. Similar primary afferent-specific inhibition by L-AP4 was also observed in non-EGFP-expressing neurons. Moreover, Aδ fiber-evoked synaptic transmission was suppressed by a selective mGluR7 agonist, AMN082. These results suggest that modulation of the synaptic transmission from primary afferents to SG neurons by group III mGluR agonist is specific to the type of nociceptive primary afferents but not to the type of target neurons.

Effects of 5-hydroxytryptamine on substantia gelatinosa neurons of the trigeminal subnucleus caudalis in immature mice

Serotonin (5-hydroxytryptamine, 5-HT) is involved in the descending modulation of nociceptive transmission in the spinal dorsal horn. The trigeminal subnucleus caudalis (Vc; medullary dorsal horn) processes nociceptive input from the orofacial region, and 5-HT-containing axons are numerous in the superficial layers of the Vc. This study examined the actions of 5-HT on the substantia gelatinosa (SG) neurons of the Vc, using gramicidin-perforated patch-clamp recording in brainstem slice preparations from immature mice. In order to clarify the possible mechanisms underlying 5-HT actions in the SG of the Vc, the direct membrane effects of 5-HT and effects of 5-HT receptor subtype agonists were examined. 5-HT induced a hyperpolarization in the majority (64/115, 56%) of the SG neurons tested. Thirty nine (34%) SG neurons showed no response, and 12 (10%) neurons responded with depolarization. The hyperpolarizing response to 5-HT was concentration-dependent (0.1-30 μM; n=7), not desensitized by repeated application (n=22), and significantly attenuated by Ba(2+) (K(+) channel blocker; n=8). The 5-HT-induced hyperpolarization was maintained in the presence of TTX (Na(+) channel blocker), CNQX (non-NMDA glutamate receptor antagonist), AP5 (NMDA glutamate receptor antagonist), picrotoxin (GABA(A) receptor antagonist), and strychnine (glycine receptor antagonist), indicating direct postsynaptic action of 5-HT on SG neurons (n=7). The 5-HT-induced hyperpolarizing effects were mimicked by 8-OH-DPAT (5-HT(1A) receptor agonist) and α-methyl-5-HT (5-HT(2) receptor agonist) and blocked by WAY-100635 (5-HT(1A) receptor antagonist) and ketanserin (5-HT(2) receptor antagonist). Single-cell RT-PCR also revealed the presence of mRNA for 5-HT(1A) and 5-HT(2C) subtypes in the SG neurons. These results suggest that 5-HT acts directly on SG neurons and 5-HT-induced hyperpolarization is mediated, in part, by 5-HT(1A) receptors and 5-HT(2) receptors, as well as by the activation of K(+) channels, indicating an important role for 5-HT in the modulation of orofacial nociceptive processing at the level of the SG of the Vc in mice.

The effects of risedronate and exercise on osteoporotic lumbar rat vertebrae and their sensory innervation

STUDY DESIGN

Investigation of sensory innervation of rat osteoporotic lumbar vertebrae using in vitro and in vivo models.

OBJECTIVE

To investigate (1) sensory innervation of osteoporotic rat vertebrae, (2) effects of risedronate on sensory neurons, (3) effects of osteoporosis treatment on bone mineral densities (BMDs) and the sensory innervation.

SUMMARY OF BACKGROUND DATA

Osteoporotic patients without fractures sometimes experience vague low back pain of unknown origin. The mechanisms of osteoporosis treatments against the pain are unclear.

METHODS

(1) The expression of calcitonin gene-related peptide (CGRP) immunoreactive (-ir) or transient receptor potential vanilloid 1 (TRPV1)-ir nerve fibers in vertebrae and dorsal root ganglions (DRG) innervating L3 vertebrae of Sprague Dawley rats labeled with neurotracer were examined in control, sham, and ovariectomized (OVX) rats. (2) Cultured rat neonate DRG neurons in media containing different concentrations of risedronate were immunostained for CGRP, and we measured its activity using axonal length and proportion of CGRP-ir neurons. (3) BMDs and CGRP expression in DRG neurons innervating L3 vertebrae were examined in the following 5 groups: sham (treated with saline), OVX (saline), OVX+EXE (treadmill exercise), OVX+RIS (risedronate), and OVX+RIS+EXE (risedronate and exercise).

RESULTS

(1) A few CGRP-ir or TRPV1-ir nerve fibers were observed in the bone marrow. CGRP or TRPV1 expression in DRG was elevated in the OVX group (P < 0.05). (2) The axonal length and proportion of CGRP-ir neurons were dose-dependently suppressed (P < 0.05). (3) BMDs improved and the CGRP expression decreased in the risedronate-treated groups (P < 0.05), especially in the OVX+RIS+EXE group.

CONCLUSION

Sensory innervation of osteoporotic rat vertebrae showed increased expression of CGRP and TRPV1 in DRG neurons. Risedronate suppressed activity of CGRP-ir neurons in vitro, improved BMD, and decreased CGRP expression, especially together with exercise in vivo.

Molecular assays for characterization of alternatively spliced isoforms of the u opioid receptor (MOR)

Mu-opioid receptor (MOR) belongs to a family of heptahelical G-protein-coupled receptors (GPCRs). Studies in humans and rodents demonstrated that the OPRM1 gene coding for MOR undergoes extensive alternative splicing afforded by the genetic complexity of OPRM1. Evidence from rodent studies also demonstrates an important role of these alternatively spliced forms in mediating opiate analgesia via their differential signaling properties. MOR signaling is predominantly G(ia) coupled. Release of the alpha subunit from G-protein complex results in the inhibition of adenylyl cyclase/cAMP pathway, whereas release of the betagamma subunits activates G-protein-activated inwardly rectifying potassium channels and inhibits voltage-dependent calcium channels. These molecular events result in the suppression of cellular activities that diminish pain sensations. Recently, a new isoform of OPRM1, MOR3, has been identified that shows an increase in the production of nitric oxide (NO) upon stimulation with morphine. Hence, there is a need to describe molecular techniques that enable the functional characterization of MOR isoforms. In this review, we describe the methodologies used to assay key mediators of MOR activation including cellular assays for cAMP, free Ca(2+), and NO, all of which have been implicated in the pharmacological effects of MOR agonists.

TRPA1-expressing primary afferents synapse with a morphologically identified subclass of substantia gelatinosa neurons in the adult rat spinal cord

The TRPA1 channel has been proposed to be a molecular transducer of cold and inflammatory nociceptive signals. It is expressed on a subset of small primary afferent neurons both in the peripheral terminals, where it serves as a sensor, and on the central nerve endings in the dorsal horn. The substantia gelatinosa (SG) of the spinal cord is a key site for integration of noxious inputs. The SG neurons are morphologically and functionally heterogeneous and the precise synaptic circuits of the SG are poorly understood. We examined how activation of TRPA1 channels affects synaptic transmission onto SG neurons using whole-cell patch-clamp recordings and morphological analyses in adult rat spinal cord slices. Cinnamaldehyde (TRPA1 agonist) elicited a barrage of excitatory postsynaptic currents (EPSCs) in a subset of the SG neurons that responded to allyl isothiocyanate (less specific TRPA1 agonist) and capsaicin (TRPV1 agonist). Cinnamaldehyde evoked EPSCs in vertical and radial but not islet or central SG cells. Notably, cinnamaldehyde produced no change in inhibitory postsynaptic currents and nor did it produce direct postsynaptic effects. In the presence of tetrodotoxin, cinnamaldehyde increased the frequency but not amplitude of miniature EPSCs. Intriguingly, cinnamaldehyde had a selective inhibitory action on monosynaptic C- (but not Adelta-) fiber-evoked EPSCs. These results indicate that activation of spinal TRPA1 presynaptically facilitates miniature excitatory synaptic transmission from primary afferents onto vertical and radial cells to initiate action potentials. The presence of TRPA1 channels on the central terminals raises the possibility of bidirectional modulatory action in morphologically identified subclasses of SG neurons.

Decrease in the descending inhibitory 5-HT system in rats with spinal nerve ligation

The descending serotonergic (5-HT) system is shown to be plastically altered under pathological conditions such as inflammation or peripheral nerve lesion. Although much evidence indicates that the potentiation of descending facilitatory 5-HT pathways may contribute to the development of chronic pain, the inhibition of descending inhibitory 5-HT system may be functionally more important to the development of central sensitization and neuropathic pain. In the present study, we observed that the inhibitory effects of 5-HT and its receptor agonists including 1A, 1B, 3, 4, and probably 2C receptor agonists, on the C-fiber responses of dorsal horn wide dynamic range (WDR) neurons in the spinal cord decreased significantly following spinal nerve ligation (SNL). Furthermore, we found that the antagonistic effects of 5-HT 1B, 2C, 3, and 4 receptor antagonists on the 5-HT-induced inhibition of C-fiber responses of WDR neurons were also attenuated after SNL. In consistent with these observations, we also found an obvious decrease in the content of 5-HT and 5-HIAA, and a marked increase in the turnover rate of 5-HT (5-HIAA/5-HT) in the ipsilateral dorsal half of the lumbar spinal cord after SNL. These data indicate that a loss or decrease in the descending inhibitory 5-HT system upon the spinal processing of nociceptive information appears to occur following spinal nerve injury, and this kind of decrease in the descending inhibitory 5-HT system is proposed to be involved in the development of central sensitization and ultimately to the nerve injury-induced neuropathic pain.

Anti-nociceptive effects of elcatonin injection for postmenopausal women with back pain: a randomized controlled trial

BACKGROUND

Eel calcitonin (elcatonin) injection is widely used for elderly patients suffering from somatic pain in Japan. However, there have been few reports on the analgesic effects of elcatonin injection. The purpose of this study was to examine the analgesic effects of elcatonin injection in postmenopausal women with lower back pain.

METHODS

This study was designed as a double-blind, randomized, placebo-controlled study. Thirty-six women aged >/=50 years with acute lower back pain participated in this study. They were randomly divided into two treatment groups according to whether they received a placebo or a weekly trigger point injection of elcatonin (20 units). They were observed for 5 weeks and the extent of pain at motion and at rest according to the visual analog scale (VAS) was evaluated. The mean VAS scores for the elcatonin group were then compared with those of the placebo group.

RESULTS

There were no statistically significant differences in the mean VAS scores for pain at rest between the two groups during the 5-week treatment course. However, the mean VAS scores for motion pain in the elcatonin group were significantly lower than those in the placebo group at the third, fifth and sixth weeks.

CONCLUSIONS

Elcatonin injection (20 units) significantly relieved motion pain in the lower back in postmenopausal women after three weeks of treatment. This analgesic effect continued for the subsequent 3 weeks.

Use of calcitonin to prevent complex regional pain syndrome type I in severe hemiplegic patients after stroke

PURPOSE

To evaluate the effects of calcitonin in preventing complex regional pain syndrome type I (CRPS) in patients with severe hemiplegia following stroke.

METHODS

In this observer-blinded, controlled study comparison with historical controls between April 2003 and May 2004, subjects comprised consecutive patients with post-stroke hemiplegia admitted between June 2004 and September 2005, with any upper limb or finger graded as Brunnstrom stage (BrST) III or below. One group was administered intramuscular injection of 20 units of elcatonin (EL) (Asu(1-7) eel calcitonin) weekly from immediately after admission to discharge, together with rehabilitation therapy. The control group received rehabilitation therapy alone. Patients were observed during the in-hospital rehabilitation period. The main outcome measure was onset of CRPS.

RESULTS

Incidence of CRPS in all patients with post-stroke hemiplegia during the control period was about 8.2%, similar to recent studies. Limited to serious hemiplegic patients graded as BrST III or below, incidence of CRPS was significantly lower in the EL group (12.5%) than in controls (57.1%). No significant differences in patient background were seen between groups. CRPS was completely prevented when EL injection was started <or=4 weeks after stroke, but prophylactic effects were weak when EL was started >6 weeks after stroke.

CONCLUSION

Intramuscular calcitonin appears to suppress onset of CRPS after stroke, particularly when started early after stroke.

Optimal interaction of respiratory and thermal regulation at rest and during exercise: role of a serotonin-gated spinoparabrachial thermoafferent pathway

Recent evidence indicates that the lateral parabrachial nucleus (LPBN) in dorsolateral pons is pivotal in mediating the feedback control of inspiratory drive by central chemoreceptor input and feedforward control of body temperature by cutaneous thermoreceptor input. The latter is subject to descending serotonergic inhibition which gates the transmission of ascending thermoafferent information from spinal dorsal horn to the LPBN. Here, a model is proposed which suggests that the LPBN may be important in balancing respiratory and thermal homeostasis, two conflicting goals that are heightened by environmental heat/cold stress or exercise where the effects of respiratory thermolysis become prominent. This optimization model of respiratory-thermoregulatory interaction is supported by a host of recent studies which demonstrate that animals with serotonin (5-HT) dysfunction at the spinal dorsal horn--due to 5-HT antagonism, genetic 5-HT defects or spinal cord injury--all display similar respiratory abnormalities that are consistent with hyperactivity of the spinoparabrachial thermoafferent (and pain) pathway.

Serotonin depletion increases nociception-evoked trigeminal NMDA receptor phosphorylation

OBJECTIVE

To investigate the effect of serotonin depletion on phosphorylation and expression of NR1 subunit of N-methyl-D-aspartate (NMDA) receptor in trigeminal nucleus caudalis (TNC), and on trigeminal nociception evoked by dural inflammation.

BACKGROUND

Migraine is associated with low serotonin condition and an increased neuronal excitability. NMDA receptor is implicated in central plasticity change that leads to neural sensitization. Alteration in NMDA receptor phosphorylation or expression in TNC may be responsible for increased trigeminal nociception in serotonin-depleted state.

METHODS

Adult male Wistar rats were separated into normal and low serotonin groups. Serotonin was depleted by intraperitoneal injection with para-chlorophenylalanine 3 days before the experiment. Trigeminal nociception was induced by applying inflammatory soup on exposed dura. Two hours after induction, phosphorylated NR1, NR1, and Fos expressions were studied in TNC by immunohistochemistry.

RESULTS

Dural application of inflammatory soup led to the activation of trigeminal nociceptive system as well as the phosphorylation of NR1, which were further enhanced in the low serotonin condition. There was a strong relationship between NR1 phosphorylation and trigeminal nociception. However, neither meningeal inflammation nor serotonin depletion altered NR1 expression.

CONCLUSIONS

Low serotonin condition facilitates dural inflammation-induced NR1 phosphorylation and trigeminal nociception. It is suggested that the mechanism of nociceptive facilitation in serotonin-depleted state may involve the increase in NR1 phosphorylation rather than the upregulation of NR1 subunit of NMDA receptor.

Responses to 5-HT in morphologically identified neurons in the rat substantia gelatinosa in vitro

Bath application of 5-HT (1-1000 muM) induced a tetrodotoxin (TTX)-resistant outward current at the holding membrane potential (V(H)) of -50 mV in 104/162 (64.2%) of substantia gelatinosa (SG) neurons from the rat spinal cord in vitro. The 5-HT-induced outward current was suppressed by an external solution containing Ba(2+), or a pipette solution containing Cs(2)SO(4) and tetraethylammonium. It was reversed near the equilibrium potential of the K(+) channel. The response to 5-HT was abolished 30 min after patch formation with a pipette solution containing guanosine-5-O-(2-thiodiphosphate)-S. The 5-HT-induced outward current was mimicked by a 5-HT(1A) agonist, (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide, and suppressed by a 5-HT(1A) antagonist, WAY100635, suggesting the 5HT(1A) receptor-mediated activation of K(+) channels in the outward current. In 11/162 (6.8%) SG neurons, 5-HT produced an inward current, which was mimicked by a 5-HT(3) agonist, 1-(m-chlorophenyl)-biguanide (mCPBG). The 5-HT-induced outward currents were observed in vertical cells (21/34) and small islet cells (11/34), while inward currents were induced in islet cells (1/5) and small islet (4/5) cells, but not in vertical cells. It is known that most vertical cells and islet cells in the SG are excitatory (glutamatergic) and inhibitory interneurons, respectively, while small islet cells consist of both excitatory and inhibitory neurons. Bath application of 5-HT or mCPBG increased the amplitude and the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs), but no neurons showed a decrease in sIPSC. Furthermore, frequency, but not amplitude, of miniature IPSCs increased with perfusion with 5-HT in the presence of TTX. These findings, taken together, suggest that 5-HT induces outward currents through 5-HT(1A) receptors in excitatory SG neurons. These findings also suggest that the inward currents are post- and presynaptically evoked through 5-HT(3) receptors, probably in inhibitory neurons.

An immunohistochemical study of the antinociceptive effect of calcitonin in ovariectomized rats

BACKGROUND

Calcitonin is used as a treatment to reduce the blood calcium concentration in hypercalcemia and to improve bone mass in osteoporosis. An analgesic effect of calcitonin has been observed and reported in clinical situations. Ovariectomized (OVX) rats exhibit the same hormonal changes as observed in humans with osteoporosis and are an animal model of postmenopausal osteoporosis. The aim of this study to investigate antinociceptive effect of calcitonin in OVX rats using the immunohistochemical study.

METHODS

We assessed the antinociceptive effects of calcitonin in an ovariectomized (OVX) rat model, which exhibit osteoporosis and hyperalgesia, using the immunohistochemical method. Fifteen rats were ovariectomized bilaterally, and ten rats were received the same surgery expected for ovariectomy as a sham model. We used five groups: the OVX-CT (n = 5), the sham-CT (n = 5), and the OVX-CT-pcpa (n = 5) groups received calcitonin (CT: 4 U/kg/day), while OVX-vehi (n = 5) and the sham-vehi (n = 5) groups received vehicle subcutaneously 5 times a week for 4 weeks. The OVX-CT-pcpa-group was given traperitoneal injection of p-chlorophenylalanine (pcpa; an inhibitor of serotonin biosynthesis) (100 mg/kg/day) in the last 3 days of calcitonin injection. Two hours after 5% formalin (0.05 ml) subcutaneously into the hind paw, the L5 spinal cord were removed and the number of Fos-immunoreactive (ir) neurons were evaluated using the Mann-Whitney-U test.

RESULTS

The numbers of Fos-ir neurons in the OVX-CT and sham-CT groups were significantly less than in the OVX-vehi and sham-vehi groups, respectively (p = 0.0090, p = 0.0090). The number of Fos-ir neurons in the OVX-CT-pcpa-group was significantly more than that of the OVX-CT-group (p = 0.0283), which means pcpa inhibits calcitonin induced reduction of c-Fos production.

CONCLUSION

The results in this study demonstrated that 1) the increase of c-Fos might be related to hyperalgesia in OVX-rats. 2) Calcitonin has an antinociceptive effect in both OVX and sham rats. 3) The central serotonergic system is involved in the antinociceptive properties of calcitonin.

Activation of GIRK channels in substantia gelatinosa neurones of the adult rat spinal cord: a possible involvement of somatostatin

Recent studies have suggested that spinal G-protein-coupled, inwardly rectifying K(+) (GIRK) channels play an important role in thermal nociception and the analgesic actions of morphine and other agents. In this study, we show that spinal GIRK channels are activated by an endogenous neurotransmitter using whole-cell patch-clamp recordings from substantia gelatinosa (SG) neurones in adult rat spinal cord slices. Although repetitive stimuli applied to the dorsal root did not induce any slow responses, ones focally applied to the spinal dorsal horn produced slow inhibitory postsynaptic currents (IPSCs) at a holding potential of -50 mV in about 30% of the SG neurones recorded. The amplitude and duration of slow IPSCs increased with the number of stimuli and decreased with removal of Ca(2+) from the external Krebs solution. Slow IPSCs were associated with an increase in membrane conductance; their polarity was reversed at a potential close to the equilibrium potential for K(+), calculated from the Nernst equation. Slow IPSCs were blocked by addition of GDP-beta-S into the patch-pipette solution, reduced in amplitude in the presence of Ba(2+), and significantly suppressed in the presence of an antagonist of GIRK channels, tertiapin-Q. Somatostatin produced an outward current in a subpopulation of SG neurones and the slow IPSC was occluded during the somatostatin-induced outward current. Moreover, slow IPSCs were significantly inhibited by the somatostatin receptor antagonist cyclo-somatostatin. These results suggest that endogenously released somatostatin may induce slow IPSCs through the activation of GIRK channels in SG neurones; this slow synaptic transmission might play an important role in spinal antinociception.

Selective action of noradrenaline and serotonin on neurones of the spinal superficial dorsal horn in the rat

The superficial dorsal horn of the spinal cord (SDH; laminae I and II) receives strong input from thin primary afferent fibres and is involved in nociception, pain, temperature sensing and other experiences. The SDH also is the target of serotonergic and adrenergic projections from the brain stem. The interaction between descending pathways that utilize particular mediators and the neurone population of the SDH is poorly understood. To explore this issue, in rat spinal cord slices during whole-cell recordings from identified SDH neurones, noradrenaline (NA) or serotonin (5HT) were briefly applied in the superfusing artificial cerebrospinal fluid. The action of these agents proved specifically related to the type of SDH neurone and its dorsal-root afferent input. Vertical, radial and tonic central lamina II cells consistently expressed outward current to both NA and 5HT, but transient central and Substance P (SP)-insensitive lamina I cells were unaffected directly by either NA or 5HT. Extended islet cells responded with outward current to NA and inward current to 5HT. Lamina I SP-sensitive cells expressed an outward current regularly to NA. 5HT had inhibitory effects on Adelta and C fibre input to all types of SDH neurones. NA inhibited C fibre input to transient central neurones. The present results support the idea that descending systems may have multiple functions, including but not limited to nociceptive modulation.

Cell-type-specific excitatory and inhibitory circuits involving primary afferents in the substantia gelatinosa of the rat spinal dorsal horn in vitro

The substantia gelatinosa (SG) of the spinal dorsal horn shows significant morphological heterogeneity and receives primary afferent input predominantly from A delta- and C-fibres. Despite numerous anatomical and physiological studies, correlation between morphology and functional connectivity, particularly in terms of inhibitory inputs, remains elusive. To compare excitatory and inhibitory synaptic inputs on individual SG neurones with morphology, we performed whole-cell recordings with Neurobiotin-filled-pipettes in horizontal slices from adult rat spinal cord with attached dorsal roots. Based on dendritic arborization patterns, four major cell types were confirmed: islet, central, radial and vertical cells. Dorsal root stimulation revealed that each class was associated with characteristic synaptic inputs. Islet and central cells had monosynaptic excitatory inputs exclusively from C-afferents. Islet cells received primary-afferent-evoked inhibitory inputs only from A delta-fibres, while those of central cells were mediated by both A delta- and C-fibres. In contrast, radial and vertical cells had monosynaptic excitatory inputs from both A delta- and C-fibres and inhibitory inputs mediated by both fibre types. We further characterized the neurochemical nature of these inhibitory synaptic inputs. The majority of islet, central and vertical cells exhibited GABAergic inhibitory inputs, while almost all radial cells also possessed glycinergic inputs. The present study demonstrates that SG neurones have distinct patterns of excitatory and inhibitory inputs that are related to their morphology. The neurotransmitters responsible for inhibitory inputs to individual SG neurones are also characteristic for different morphological classes. These results make it possible to identify primary afferent circuits associated with particular types of SG neurone.

Reduced density of functional 5-HT1A receptors in the brain, medulla and spinal cord of monoamine oxidase-A knockout mouse neonates

Abnormally high brain 5-HT levels in monoamine oxidase-A knockout (MAO-A KO) mouse neonates raise the question of whether the distribution and density of the 5-HT1A receptors (5-HT1AR) expressed in the brain by postnatal day P7 are affected and, if so, whether the 5-HT1A autoreceptors in the dorsal raphe are modified in the same way as the postsynaptic 5-HT1AR present in raphe target structures. [3H]8-OH-DPAT binding and quantitative autoradiography were performed to answer these questions. Binding specificity was first confirmed in adult wild-type mice and rat brain sections. 5-HT1AR binding was then analyzed in four MAO-A mutant vs. five wild-type neonatal brains, from olfactory bulb to cervical cord. Among 12 structures expressing postsynaptic 5-HT1AR in wild-type neonates, the highest densities involved the retrosplenial cortex, entorhinal cortex, and septum (52-46 fmol/mg tissue); low densities occurred in the hippocampus and spinal cord (24 fmol/mg tissue); in addition, the raphe autoreceptor density was only 20 fmol/mg tissue. In mutants, the distribution of postsynaptic 5-HT1AR was unchanged, but an overall decrease in density occurred (-32% to -63%); the raphe autoreceptors decreased in mutants by at least -79%. Data are discussed with reference to the ectopic 5-HT uptake and accumulation reported to occur during the first 10 postnatal days in wild-type and MAO-A KO mice. As previously suggested to explain the raphe autoreceptor loss in 2-month-old MAO-A KO mice, the overall 5-HT1AR down-regulation in mutant pups probably results from extracellular 5-HT excess in both raphe and target structures. The greater the 5-HT excess, the more the functional receptor density decreases.

Mechanisms for the Anti-nociceptive Actions of the Descending Noradrenergic and Serotonergic Systems in the Spinal Cord

The sensation of pain plays a critical role as an alert and as a protection system against tissue damage from mechanical, chemical, and thermal stimuli. Despite the protective role of pain, the severity of pain sensation is markedly attenuated by the endogenous pain inhibitory systems that predominantly originate at the brain stem. Both behavioral and in vivo extracellular recording studies have sought the loci producing analgesia and clarification of the anti-nociceptive actions. Among those loci, the main descending systems to the spinal dorsal horn are noradrenergic and serotonergic. Although, in vivo studies have provided basic knowledge of these systems, the precise synaptic mechanisms underlying the analgesic actions have not yet been elucidated until recently. The newly developed in vitro slice and in vivo patch-clamp recordings have disclosed the synaptic mechanisms of the noradrenergic and serotonergic effects at the level of spinal dorsal horn. This paper reviews the anti-nociceptive action of these systems, while particularly focusing on the electrophysiological aspects of the systems at the single neuron level in the spinal dorsal horn as well as their origins and responsible receptor subtypes.

Estrogen-dependent abdominal hyperalgesia induced by ovariectomy in adult mice: A model of functional abdominal pain

Some abdominal pain syndromes are characterized by persistent pain without demonstrable pathology. Many of them are prevalent in women and it is known that sex hormones are associated with differences in pain perception between males and females. To model a process of functional abdominal pain in females we studied the time course and estrogen dependency of a hyperalgesic state induced by ovariectomy in adult mice. Three groups of C57/BL6 mice were used: virgin mice, proven breeders (2 or 3 successful pregnancies) and retired breeders (more than three successful pregnancies). Within each group, a third of the mice were ovariectomized (OVX), a third received sham surgery and a third were controls. OVX mice, but not sham or controls, developed a robust mechanical hyperalgesia and allodynia in the abdomen, hindlimbs and proximal tail, but not in the forelimbs, that was established 4 weeks after OVX and lasted for the 7 weeks of the experiment. Increases in visceral sensitivity were also observed in OVX mice. Thermal pain thresholds (hot plate) remained unchanged. The reproductive history of the animals had no influence on the hyperalgesia. In another series of experiments a slow release pellet containing 17beta-estradiol was implanted in half of the OVX mice and a similar pellet without the hormone in the other half. Hormone replacement prevented the development of hyperalgesia in OVX animals but did not stop the involution of the internal reproductive organs. We conclude that OVX in mice provides a useful model for a hormonally dependent hyperalgesic state resembling functional pain.

In vivo and in vitro Patch-clamp Recording Analysis of the Process of Sensory Transmission in the Spinal Cord and Sensory Cortex

Following the integration and modification of the sensory inputs in the spinal cord, the information is transmitted to the primary sensory cortex where the integrated information is further processed and perceived. Processing of the sensory information in the spinal cord has been intensively investigated. However, the mechanisms of how the inputs are processed in the cortex are still unclear. To know the correlation of the sensory processing in the dorsal horn and cortex, in vivo and in vitro patch-clamp recordings were made from rat dorsal horn and sensory cortex. Although dorsal horn neurons showed spontaneous and evoked EPSCs by noxious and non-noxious stimuli, most somatosensory neurons located at 100 to 1000 microm from the surface of the cortex exhibited an oscillatory activity and received synaptic inputs from non-noxious but not noxious receptors. These observations suggest that the synaptic responses in cortical neurons are processed in a more complex manner; and this may be due to the reciprocal synaptic connection between thalamus and cortex.

Effects of combined elcatonin and alendronate treatment on the architecture and strength of bone in ovariectomized rats

We examined the combined effects of elcatonin (ECT) and alendronate (ALN) on bone mass, architecture, and strength in ovariectomized (OVX) rats. Fifty female Sprague Dawley rats, aged 13 weeks, were divided into Sham, OVX, OVX+ECT, OVX+ALN, and OVX+ECT+ALN groups (n = 10). Immediately after ovariectomy, ECT was administered at a dose of 15 units (U)/kg three times a week, and ALN was administered daily at a dose of 2.0 microg/kg, subcutaneously for 12 weeks. The three-dimensional architecture of the bone in the distal femoral metaphysis was analyzed using a microfocus X-ray computed tomography system (microCT), and bone strength was measured using a material-testing machine. Trabecular bone volume (BV/TV) and number (Tb.N) were significantly greater in the OVX+ECT and OVX+ALN groups than in the OVX group. In the OVX+ECT+ALN group, BV/TV and Tb.N were significantly greater when compared with those in the OVX+ECT and OVX+ALN groups. Trabecular thickness (Tb.Th) was significantly greater in the OVX+ECT+ALN group than in the OVX+ALN group. With regard to bone strength, the compression strength in the femoral metaphysis was significantly lower in the OVX group than in the Sham group. The reduction of compression strength was slightly lower in the OVX+ECT and OVX+ALN groups. In the OVX+ECT+ALN group, the compression strength in the femoral metaphysis significantly increased when compared with the OVX and OVX+ECT groups. These results suggest that the combined treatment of ECT and ALN does not alter the individual effects of each drug and that it exerts an additive effect on trabecular architecture and bone strength in OVX rats.

Administration of estrogen shortly after ovariectomy mimics the anti-nociceptive action and change in 5-HT1A-like receptor expression induced by calcitonin in ovariectomized rats

Although many clinical reviews are consistent with the view that hormone replacement therapy should be recommended for increasing bone mass of osteoporotic patients, calcitonin administration is preferable to hormone replacement therapy for the alleviation of pain accompanying osteoporosis, despite the fact that osteoporosis and the accompanying pain are accelerated by the reduction in estrogen levels. Distinct from the clinical view, animal studies have shown that estrogen treatment reduces ovariectomy-induced hyperalgesia, although the mechanism of this phenomenon is unknown. The discrepancy in clinical and animal study outcomes may be due to the timing of administration of estrogen after depletion of the hormone. To address this possibility, the anti-nociceptive effect of estrogen was compared with calcitonin using the tail withdrawal test in rats injected with estrogen or calcitonin at 3 weeks (short term) or 15 weeks (long term) after ovariectomy. Furthermore, we analyzed the change in [3H]8-OH-DPAT binding in the spinal cord, addressing whether estrogen exerts its anti-nociceptive effect by the expression of 5-HT receptors attributable to calcitonin-induced analgesia, as has been reported in our previous animal studies. The present study demonstrates that the administration of estrogen injected in the short term, but not long term, after ovariectomy mimicked the effects of calcitonin-induced anti-nociception and prevention of ovariectomy-induced decrease in 5-HT receptor expression in the spinal cord, although the effects of calcitonin were observed regardless of the timing of calcitonin injection. These results suggest that the estrogen receptor is downregulated gradually after ovariectomy. Disappearance of the estrogen receptor may be one of the reasons that estrogen is not recommended for the treatment for chronic pain associated with osteoporosis.

Electrophysiological mapping of the nociceptive inputs to the substantia gelatinosa in rat horizontal spinal cord slices

To study the functional projection patterns of the primary afferents in the spinal cord, the postsynaptic responses of substantia gelatinosa (SG) neurones evoked by L5 dorsal root stimulation (DRS) were examined from the neurones located at L2 to S1 in horizontal slices of the adult rat spinal cord using a blind whole-cell patch-clamp technique. In the voltage-clamp mode, the L5 DRS evoked the Adelta- and C-afferent-mediated excitatory postsynaptic currents (EPSCs) in more than 70% of the neurones tested at the L5 level. Both Adelta- and C-afferent EPSCs were also recorded in more than 50% of the neurones at L4. At L3 and L6, the number of neurones receiving the C-afferent EPSCs (> 40%) was significantly greater than that of Adelta-afferent EPSCs (< 20%). On the other hand, the Adelta- and C-afferent-mediated inhibitory postsynaptic currents (IPSCs) elicited by L5 DRS were almost equally observed from L2 to S1. In the current-clamp mode, L5 DRS evoked Adelta- and C-afferent-mediated EPSPs, some of which initiated action potentials (APs). Most of the Adelta-afferent-mediated APs were limited at the L5 level, while C-afferent-mediated APs were observed at L5 and L4. As the L2 DRS-evoked APs in the L2 SG neurones were suppressed by L5 DRS, the widespread distribution of the inhibitory inputs was considered to be functional. These findings suggest that the excitatory projection of the C afferents to the SG neurones was thus spread more rostrocaudally than that of the Adelta afferents, thereby contributing to more diffuse pain transmission. In addition, the widespread distribution of the inhibitory inputs may thus play a role as a lateral inhibitory network and thereby prevent the expansion of the excitatory inputs of noxious stimuli.

Regulation by equilibrative nucleoside transporter of adenosine outward currents in adult rat spinal dorsal horn neurons

A current response induced by superfusing adenosine was examined in substantia gelatinosa (SG) neurons of adult rat spinal cord slices by using the whole-cell patch-clamp technique. In 78% of the neurons examined, adenosine induced an outward current at -70 mV [18.8 +/- 1.1 pA (n = 98) at 1mM] in a dose-dependent manner (EC(50) = 177 microM). A similar current was induced by A(1) agonist N(6)-cyclopentyladenosine (1 microM), whereas A(1) antagonist 8-cyclopentyl-1,3-dipropylxanthine (1 microM) reversed the adenosine action. The adenosine current reversed its polarity at a potential being close to the equilibrium potential for K(+), and was attenuated by Ba(2+) (100 microM) and 4-aminopyridine (5mM) but not tetraethylammonium (5mM). The adenosine current was enhanced in duration by equilibrative nucleoside-transport (rENT1) inhibitor S-(4-nitrobenzyl)-6-thioinosine (1 microM) and adenosine deaminase (ADA) inhibitor erythro-9-(2-hydroxy-3-nonyl) adenine (1 microM), and slowed in falling phase by adenosine kinase (AK) inhibitor iodotubercidine (1 microM). We conclude that a Ba(2+)- and 4-aminopyridine-sensitive K(+) channel in SG neurons is opened via the activation of A(1) receptors by adenosine whose level is possibly regulated by rENT1, adenosine deaminase and adenosine kinase. Considering that intrathecally-administered adenosine analogues produce antinociception, the regulatory systems of adenosine may serve as targets for antinociceptive drugs.

Adenosine inhibits GABAergic and glycinergic transmission in adult rat substantia gelatinosa neurons

The effect of adenosine on inhibitory postsynaptic currents (IPSCs) was examined in substantia gelatinosa (SG) neurons of adult rat spinal cord slices by using the whole cell patch-clamp technique. Adenosine reversibly reduced the amplitude of GABAergic and glycinergic electrically evoked IPSCs (eIPSCs) in a dose-dependent manner (EC50 = 14.5 and 19.1 microM, respectively). The A1 adenosine-receptor agonist N6-cyclopentyladenosine also reduced the eIPSCs, whereas the A1 antagonist 8-cyclopentyl-1,3-dimethylxanthine reversed the inhibition produced by adenosine. In paired-pulse experiments, the ratio of the second to first GABAergic or glycinergic eIPSC amplitude was increased by adenosine, whereas the response of SG neurons to exogenous GABA or glycine was unaffected. Adenosine reduced the frequency of GABAergic and glycinergic spontaneous IPSCs without changing their amplitude. This reduction in frequency disappeared in the presence of a K+ -channel blocker (4-aminopyridine) but not in the absence of Ca2+. The inhibition by adenosine disappeared in the presence of cyclic-AMP analog (8-Br-cyclic AMP) and adenylate-cyclase activator (forskolin) but not protein-kinase C (PKC) activator (phorbol-12,13-dibutyrate). We conclude that adenosine suppresses inhibitory transmission in SG neurons by activating presynaptic A1 receptors and that this action is mediated by K+ channels and cyclic AMP but not by Ca2+ channels and PKC. This inhibitory action of adenosine probably contributes to the modulation of pain transmission in the SG.

Sensory processing and functional reorganization of sensory transmission under pathological conditions in the spinal dorsal horn

The superficial dorsal horn, particularly substantia gelatinosa (SG) in the spinal cord, receives inputs from small-diameter primary afferents that predominantly convey noxious sensation. This sensory information via the high-threshold Adelta and C afferents is modified and integrated in SG, and consequently regulates the outputs of projection neurons located in lamina I and laminae IV-V. Recent studies using slice and in vivo patch-clamp recordings indicate that the sensory inputs to SG are functionally reorganized during post-natal development. Even in the mature state, the synaptic connectivity and receptor expression in SG can be altered easily following peripheral tissue damage. In addition, the descending pain inhibitory system to SG is also modified under certain pathological conditions. Considering that the pain system is phylogenetically primitive, it is, therefore, not surprising that the system easily exhibits a plastic change in response to inflammation or nerve damage. Because such plastic changes in the neuronal circuit or receptor expression in SG are now generally accepted to be one of the explanations for the induction of pathological pain, SG is thought to be a primary therapeutic target for chronic pain. We review here recent results demonstrating plastic changes in SG under pathological conditions.

Enhancement of the releases of GABA and glycine during ischemia in rat spinal dorsal horn

The present study examined a change in spontaneous inhibitory postsynaptic currents (sIPSCs) following ischemia in substantia gelatinosa (SG) neurons of adult rat spinal cord slices by using the whole-cell patch-clamp technique. At about 10 min after superfusion of an oxygen- and glucose-free medium, sIPSCs were remarkably increased in amplitude and frequency when compared with those in the control. In a phase of the increase in sIPSC activities, GABAergic and glycinergic sIPSCs, which were observed in the presence of strychnine and bicuculline, respectively, with TTX, were increased greatly in frequency with a minimal change in their amplitudes. It is concluded that the in vitro ischemia increases the spontaneous quantal releases of GABA and glycine to SG neurons from nerve terminals; a part of this enhancement is possibly due to an increase in spontaneous activities of inhibitory interneurons. GABA released thus might serve to inhibit the release of l-glutamate from nerve terminals.

Modulation by adenosine of aδ and c primary-afferent glutamatergic transmission in adult rat substantia gelatinosa neurons

The present study examined the actions of adenosine on monosynaptic Adelta and C primary-afferent excitatory postsynaptic currents (EPSCs) recorded from substantia gelatinosa (SG) neurons of an adult rat spinal cord slice. In 67% of the neurons examined, adenosine reversibly decreased the amplitude of the Adelta-fiber EPSC, while in 13% of the neurons the amplitude was reduced or unaffected, which was followed by its increase persisting for several minutes after adenosine washout. The remaining neurons did not exhibit a change in the amplitude. The reduction in Adelta-fiber EPSC amplitude by adenosine was dose-dependent with an effective concentration for half-inhibition (EC50) value of 217 microM. When examined by using a paired-pulse stimulus, a ratio of the second to first Adelta-fiber EPSC amplitude under the reduction was larger than that of EPSC amplitude in the control, suggesting a presynaptic action of adenosine. In 69% of the neurons tested, the C-fiber EPSC was reversibly decreased in amplitude by adenosine (100 microM) by an extent comparable to that of Adelta-fiber EPSC; the remaining neurons were without adenosine actions. Similar inhibitory actions of adenosine were also seen in neurons where both Adelta-fiber and C-fiber EPSCs were elicited. Similar reduction in the Adelta-fiber or C-fiber EPSC amplitude was induced by an A1 adenosine-receptor agonist, N6-cyclopentyladenosine (1 microM), and the adenosine-induced reduction was not observed in the presence of an A1 antagonist, 8-cyclopentyl-1,3-dipropylxanthine (1 microM). An A2a agonist, CGS 21680 (1 microM), did not significantly affect the Adelta-fiber EPSC amplitude. It is concluded that adenosine presynaptically inhibits monosynaptic Adelta-fiber and C-fiber transmission by a similar extent through the activation of the A1 receptor in many but not all SG neurons; this could contribute to at least a part of antinociception by intrathecally administered adenosine analogues and probably by endogenous adenosine.

Somatostatin directly inhibits substantia gelatinosa neurons in adult rat spinal dorsal horn in vitro

Effects of somatostatin (SST) on the synaptic transmission to substantia gelatinosa (SG) neurons of adult spinal cord slices were investigated using intracellular recording and blind whole-cell patch-clamp technique. Bath application of SST (1 microM) induced the membrane hyperpolarization that was accompanied by a decrease in input resistance and had the reversal potential of -92 +/- 3 mV (n=5) in the intracellular recording experiment. In patch-clamp experiment, SST (1 microM) induced an outward current with amplitude of 14 +/- 2 pA (n=60) at the holding potential of -60 mV, and was not affected by TTX (n=3). The effect was dose-dependent with EC50 value of 0.82 microM (Hill coefficient: 0.89). The outward current was suppressed when the patch-pipette solution containing potassium channel blockers, Cs+ and tetraethylammonium (TEA), and was inhibited by Ba2+ (200 microM) to 15 +/- 6% of the control (n=3). In addition, the SST current reversed its polarity at potential close to the equilibrium potential of K+ channel calculated by the Nernst equation. No significant changes were found in amplitude and frequency of miniature excitatory postsynaptic currents (mEPSCs) and dorsal root evoked EPSC (eEPSC) by SST. Also, SST did not affect both of the miniature inhibitory postsynaptic currents (mIPSCs) and evoked inhibitory postsynaptic currents (eIPSCs), mediated by either the GABA or glycine receptor. We conclude that SST activates the K+ channel resulting in postsynaptic hyperpolarization in adult rat SG neurons without affecting presynaptic component of the transmission, which are considered to account, at least a part, for the analgesic effects of SST reported previously.

Serotonin receptors 5-HT1A and 5-HT3 reduce hyperexcitability of dorsal horn neurons after chronic spinal cord hemisection injury in rat

Spinal cord injury (SCI) results in abnormal pain syndromes in humans. In a rodent model of SCI, T13 spinal hemisection results in allodynia and hyperalgesia due in part to interruption of descending pathways, including serotonergic (5-HT) systems, that leads to hyperexcitability of dorsal horn neurons. To characterize further the role of 5-HT and 5-HT receptor subtypes 5-HT(1A) and 5-HT(3) in neuronal activation after hemisection, we have examined the responsiveness of dorsal horn neurons to a variety of innocuous and noxious peripheral stimuli. Male Sprague-Dawley rats, 150-175 g, were spinally hemisected (n=40) at T13 and allowed 4 weeks for development of mechanical allodynia and thermal hyperalgesia. Animals then underwent electrophysiologic recording and the results were compared with those from sham controls (n=15). Evoked responses of convergent dorsal horn neurons (n=224 total) at L3-L5 to innocuous and noxious peripheral stimuli were characterized after administration of vehicle, 5-HT (25, 50, 100, and 200 microg), 5-HT (100 microg) in conjunction with the selective 5-HT(1A) antagonist WAY 100135 (100 microg), the 5-HT(3) antagonist MDL 72222 (100 microg), the selective 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 150 microg), or the 5-HT(3) agonist 2-Me-5HT (75 microg), with or without pretreatment with antagonists; all treatments were delivered topically onto the cord adjacent to the recording electrode. In hemisected animals, increased responsiveness of convergent cells to all peripheral stimuli was observed bilaterally when compared to controls. No changes in ongoing background activity were present. In control animals, only the highest dose of 5-HT (200 microg) was sufficient to reduce evoked activity, whereas in hemisected animals a concentration-dependent decrease in response was observed. In hemisected animals, both 5-HT(1A) and 5-HT(3) receptor antagonism reduced the effectiveness of 5-HT, restoring elevated evoked activity by up to 70% at the doses tested. Administration of 5-HT(1A) and 5-HT(3) receptor agonists also decreased hyperexcitability, effects prevented by pretreatment with corresponding antagonists. These results demonstrate the development of denervation supersensitivity to 5-HT following SCI, corroborate behavioral studies showing the effectiveness of 5-HT in reducing allodynia and hyperalgesia after SCI, and contribute to a mechanistic understanding of the role of 5-HT receptor subtypes in chronic central pain.

Serotonin receptor mRNA expression in rat dorsal root ganglion neurons

In the present study, we have used in situ hybridization to examine the distribution of serotonin (5-HT) receptors in rat dorsal root ganglion (DRG) neurons. Within DRG neurons, mRNAs for 5-HT1B, 5-HT1D, 5-HT2A, 5-HT2B, 5-HT3B and 5-HT4 receptors were readily detected in small (<25 microm), medium (25-45 microm) and large (>45 microm) diameter neurons. In contrast mRNAs for 5-HT1A, 5-HT1E, 5-HT2C, 5-HT5A, 5-HT5B, 5-HT6 and 5-HT7 receptors were undetectable in these neurons. The present study provides an insight into the molecular profile of 5-HT receptor subtypes in neurons responsible for modulating sensory information.

GABA-mediated inhibition of glutamate release during ischemia in substantia gelatinosa of the adult rat

An ischemia-induced change in glutamatergic transmission was investigated in substantia gelatinosa (SG) neurons of adult rat spinal cord slices by use of the whole cell patch-clamp technique; the ischemia was simulated by superfusing an oxygen- and glucose-free medium (ISM). Following ISM superfusion, 21 of 37 SG neurons tested produced an outward current (23 +/- 4 pA at a holding potential of -70 mV), which was followed by a slow and subsequent rapid inward current; the remaining neurons had only inward currents. During such a change in holding currents, spontaneous excitatory postsynaptic currents (EPSCs) were remarkably decreased in a frequency with time (half-decay time of the frequency: about 65 s). The frequency of spontaneous EPSCs was reduced to 28 +/- 13% (n = 37) of the control level during the generation of the slow inward current (about 4 min after the beginning of ISM superfusion) without a change in the amplitude of spontaneous EPSCs. When ISM was superfused together with either bicuculline (10 microM) or CGP35348 (20 microM; GABA(A) and GABA(B) receptor antagonists, respectively), spontaneous EPSC frequency reduced by ISM recovered to the control level and then the frequency markedly increased [by 325 +/- 120% (n = 22) and 326 +/- 91% (n = 17), respectively, 4 min after ISM superfusion]; this alteration in the frequency was not accompanied by a change in spontaneous EPSC amplitude. Superfusing TTX (1 microM)-containing ISM resulted in a similar recovery of spontaneous EPSC frequency and following increase (by 328 +/- 26%, n = 12) in the frequency; strychnine (1 microM) did not affect ISM-induced changes in spontaneous EPSC frequency (n = 5). It is concluded that the ischemic simulation inhibits excitatory transmission to SG neurons, whose action is in part mediated by the activation of presynaptic GABA(A) and GABA(B) receptors, probably due to GABA released from interneurons as a result of an ischemia-induced increase in neuronal activities. This action might protect SG neurons from an excessive excitation mediated by L-glutamate during ischemia.

Anandamide inhibits excitatory transmission to rat substantia gelatinosa neurones in a manner different from that of capsaicin

Actions of anandamide (10 microM) were examined on monosynaptic glutamatergic transmission from the periphery to substantia gelatinosa (SG) neurones in adult rat spinal cord slices using the whole-cell patch-clamp technique. In 64% of neurones examined, Adelta-fibre-evoked excitatory postsynaptic currents (EPSCs) were depressed (in amplitude; by 32 +/- 4%, n=21) by anandamide. On the contrary, an inhibitory action on C-fibre-evoked EPSCs was observed in only 31% of neurones tested; this magnitude (17 +/- 3%, n=4) was less than that of Adelta-fibre EPSCs (P<0.05). A cannabinoid-receptor agonist, WIN 55,212-2 (5 microM), exhibited similar actions on the EPSCs. In a neurone with minimal effects of anandamide on C-fibre EPSCs, capsaicin (1 microM) largely depressed the EPSCs (n=3); Adelta-fibre EPSCs were little affected by capsaicin. It is concluded that unlike capsaicin, anandamide inhibits more effectively Adelta-fibre than C-fibre-mediated excitatory transmission in the SG, possibly through the activation of the cannabinoid receptor.

Synergistic enhancement of glutamate-mediated responses by serotonin and forskolin in adult mouse spinal dorsal horn neurons

Glutamate is the major excitatory amino acid neurotransmitter in the CNS, including the neocortex, hippocampus, and spinal cord. Normal synaptic transmission is mainly mediated by glutamate AMPA and/or kainate receptors. Glutamate N-methyl-D-aspartate (NMDA) receptors are normally inactive and only activated when a sufficient postsynaptic depolarization is induced by the activity. Here we show that in sensory synapses of adult mouse, some synaptic responses (26.3% of a total of 38 experiments) between primary afferent fibers and dorsal horn neurons are almost completely mediated by NMDA receptors. Dorsal root stimulation did not elicit any detectable AMPA/kainate receptor-mediated responses in these synapses. Unlike young spinal cord, serotonin alone did not produce any long-lasting synaptic enhancement in adult spinal dorsal horn neurons. However, co-application of the adenylyl cyclase activator forskolin and serotonin (5-HT) produced long-lasting enhancement, including the recruitment of functional AMPA receptor-mediated responses. Calcium-sensitive, calmodulin-regulated adenylyl cyclases (AC1, AC8) are required for the enhancement. Furthermore the thresholds for generating action potential responses were decreased, and, in many cases, co-application of forskolin and 5-HT led to the generation of action potentials by previously subthreshold stimulation of primary afferent fibers in the presence of the NMDA receptor blocker 2-amino-5-phosphonovaleric acid. Our results suggest that pure NMDA synapses exist on sensory neurons in adult spinal cord and that they may contribute to functional sensory transmission. The synergistic recruitment of functional AMPA responses by 5-HT and forskolin provides a new cellular mechanism for glutamatergic synapses in mammalian spinal cord.

Nociceptin inhibits excitatory but not inhibitory transmission to substantia gelatinosa neurones of adult rat spinal cord

Although intrathecal administration of nociceptin, an endogenous ligand of the opioid receptor-like1 receptor, exhibits an antinociceptive effect in various pain models, cellular mechanisms underlying this action are still unknown. Here, we investigated the effects of nociceptin on excitatory and inhibitory synaptic transmission to substantia gelatinosa neurones of an adult rat spinal cord slice with an attached dorsal root by use of the blind whole-cell patch-clamp technique; this was done under the condition of a blockade of a hyperpolarising effect of nociceptin. In about 70% of the neurones examined, nociceptin (1 microM) reduced the amplitude of glutamatergic excitatory postsynaptic currents (EPSCs) which were monosynaptically evoked by stimulating Adelta- or C-afferent fibres; the inhibition of C-fibre EPSCs (50+/-6%, n=11) was larger than that of Adelta-fibre EPSCs (30+/-5%, n=23; P<0.05). Each of the nociceptin actions was dose-dependent in a concentration range of 0.1 to 1 microM, and was largely suppressed by a selective opioid receptor-like1 receptor antagonist, 1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one (3 microM). Nociceptin (1 microM) also decreased miniature EPSCs frequency by 22+/-6% (n=7) while not affecting their amplitude. Responses of substantia gelatinosa neurones to bath-applied alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (10 microM) were not changed by nociceptin. Both electrically evoked and miniature inhibitory postsynaptic currents, mediated by either the GABA(A) or glycine receptor, were unaffected by nociceptin. These results indicate that nociceptin suppresses excitatory but not inhibitory synaptic transmission to substantia gelatinosa neurones through the activation of the opioid receptor-like1 receptor; this action is pre-synaptic in origin. Considering that the substantia gelatinosa is the main part of termination of Adelta- and C-fibres transmitting nociceptive information, the present finding would account for at least a part of the inhibitory action of nociceptin on pain transmission. Nociceptin could inhibit more potently slow-conducting than fast-conducting pain transmission.

Adenosine inhibits excitatory transmission to substantia gelatinosa neurons of the adult rat spinal cord through the activation of presynaptic A(1) adenosine receptor

Although intrathecal administration of adenosine analogues or A(1) adenosine receptor agonists is known to result in antinociception, this has not been examined yet at the cellular level. In the present study, we examined in pharmacology an action of adenosine on glutamatergic miniature excitatory postsynaptic currents (mEPSCs) in substantia gelatinosa (SG) neurons of an adult rat spinal cord slice; this was done under the condition where a postsynaptic action of adenosine was blocked. In 65% of the neurons examined (n=72), adenosine at a concentration of 100 microM depressed the frequency of mEPSC in a reversible manner; the remaining neurons exhibited an inhibition followed by potentiation of the frequency. When examined quantitatively in extent in some cells (n=25), the inhibition was 40+/-3% (n=25) while the potentiation was 42+/-8% (n=6). These actions were not accompanied by a change in mEPSC amplitude. The inhibitory action on mEPSC frequency was dose-dependent in a range of 10-500 microM with an EC(50) value of 277 microM. The inhibitory action of adenosine was mimicked by a selective A(1) adenosine receptor agonist, CPA (1 microM; depression: 54+/-9%, n=4); this action of adenosine (100 microM) was not observed in the presence of a specific A(1) adenosine receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) (1 microM; 94+/-4% of control, n=3). The facilitatory action of adenosine (100 microM) was unaffected by an A(2a) antagonist, ZM 241385 (0.1 microM, n=3); an A(2a) agonist, CGS 21680 (0.1-10 microM; n=6), was without actions on mEPSC frequency. It is concluded that adenosine inhibits excitatory transmission to SG neurons through the activation of presynaptic A(1) adenosine receptor and that some of the inhibition is followed by a potentiation of the transmission. It remains to be examined which subtypes of adenosine receptors except for the A(1)- and A(2a)-subtypes are involved in the potentiating action. Considering that adenosine-like immunoreactivity and adenosine receptors are expressed at a high density in the SG, which is thought to play an important role in modulating nociceptive transmission from the periphery to the central nervous system, this inhibitory action of adenosine could contribute to a negative modulation of pain transmission.

Voltage-clamp recordings of postsynaptic currents in substantia gelatinosa neurons in vitro and its applications to assess synaptic transmission

We describe here procedures for recording postsynaptic currents in substantia gelatinosa neurons on a transverse spinal cord slice preparation with an attached dorsal root. At the holding potential of -70 mV, glutamatergic spontaneous excitatory postsynaptic currents (EPSCs) and dorsal root (A delta and/or C fiber) stimulation-evoked EPSCs could be observed. Whereas at the holding potential of 0 mV, spontaneous inhibitory postsynaptic currents (IPSCs) and dorsal root A delta fiber stimulation-evoked IPSCs could be encountered. The methods make it possible to evaluate synaptic transmission by analysing the postsynaptic currents on dorsal root attached spinal cord slice.

Nociceptin-induced outward current in substantia gelatinosa neurones of the adult rat spinal cord

Nociceptin (NOC), also known as orphanin FQ, is a newly discovered endogenous ligand for the opioid receptor-like1 (ORL1) receptor. Although NOC has been shown to modulate nociceptive transmission, mechanisms for this action are still unknown. In the present study, actions of NOC on substantia gelatinosa (SG) neurones were examined in adult rat spinal cord slice preparations by using the whole-cell patch-clamp technique. NOC at a concentration of 1 microM induced an outward current having an amplitude of 26+/-5 pA (n=68) at a holding potential of -70 mV; this action was dose-dependent with an EC(50) value of 0.23 microM (Hill coefficient: 1.5). The NOC current reversed its polarity at a potential which was close to the equilibrium potential for K(+), as calculated by the Nernst equation (n=4). The NOC current had slope conductances of 0.80+/-0.15 nS and 0.50+/-0.13 nS (n=4) in voltage ranges of -120 to -140 mV and of -60 to -90 mV, respectively. The NOC current was inhibited by Ba(2+) (100 microM; by 56+/-8%, n=4) but not by 4-aminopyridine (4-AP; 1 mM; n=4) and tetraethylammonium (TEA; 5 mM; n=4). The NOC current was not affected by tetrodotoxin (TTX; 1 microM; n=4) and also by a non-specific opioid receptor antagonist, naloxone (1 microM; n=4). When examined using some inhibitors with respect to the ORL1 receptor, the NOC (1 microM) current was depressed in amplitude by a putative NOC precursor product, nocistatin (1 microM; by 18+/-4%, n=6) and also by a non-peptidyl ORL1 receptor antagonist, CompB (1 microM; by 64+/-10%, n=7) without a change in holding currents. On the other hand, a putative ORL1 receptor antagonist, [Phe(1)psi(CH(2)-NH)Gly(2)]nociceptin-(1-13)-NH(2) (1 microM; which is a derivative of NOC), by itself induced an outward current (7+/-3 pA, n=8), during which the NOC current was suppressed in amplitude by 56+/-8% (n=8). We conclude that NOC activates in SG neurones a K(+) channel exhibiting a mild inwardly rectification through the activation of ORL1 receptor; this hyperpolarising action of NOC might contribute to at least a part of its antinociceptive effect.