Tatsuya Oyama

Dual effect of serotonin on formalin-induced nociception in the rat spinal cord.

To examine the role of the descending serotonergic system in the regulation of spinal nociceptive processing, the effects of serotonin (5-HT) and selective ligands for 5-HT receptor subtypes on persistent nociception were investigated. Formalin (5% formaldehyde) injected into the plantar region of the rat hindpaw induced two phases of aversive responses such as licking and biting. Intrathecal administration of selective 5-HT3 receptor antagonists, granisetron (0.1-100 pmol/rat) and ondansetron (1-1000 pmol/rat), reduced the second phase of the formalin-induced aversive responses without affecting the first one. The antinociceptive effect of granisetron (100 pmol/rat) was abolished when 5-HT was depleted from the lumbar cord by pretreatment with 5,7-dihydroxytryptamine (5,7-DHT). In the 5,7-DHT-treated rats, intrathecal administration of 1-(m-chlorophenyl)-biguanide, a selective 5-HT3 receptor agonist, facilitated the aversive responses in the second phase whereas that of 8-OH-DPAT, a selective 5-HT1A receptor agonist, suppressed them. Intrathecal administration of 5-HT showed a dual effect on the second phase of the aversive responses in the 5,7-DHT-treated rats; 5-HT inhibited the aversive responses when administered at a low dose (0.1 nmol/rat) but facilitated them at a high dose (1 nmol/rat). In addition, the inhibitory and facilitatory effects of intrathecal 5-HT were blocked by its co-administration with NAN190, a 5-HT1A receptor antagonist, and granisetron, respectively. These results suggest that 5-HT suppresses formalin-induced nociception in the spinal cord via the 5-HT1A receptor and facilitates it via the 5-HT3 receptor.

Opioidergic inhibition of capsaicin-evoked release of glutamate from rat spinal dorsal horn slices

We investigated the effects of opioid agonists on the capsaicin-evoked release of glutamate from nociceptive primary afferent fibers of the rat (6-8 weeks) using a fluorometric on-line continuous monitoring system for glutamate. In the presence of 0.3 microM tetrodotoxin, the application of 3 microM capsaicin to spinal dorsal horn slices produced an evoked glutamate release (55.9 +/- 4.02 pmol.mg-1 protein, n = 15). DAMGO ([D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin; 0.3-10 microM) and morphine (1-30 microM), mu-opioid agonists, produced a concentration-dependent reduction (approximately 85 and approximately 77% reduction, respectively) in the capsaicin (3 microM)-evoked release of glutamate. These inhibitory effects were significantly antagonized by naloxone (1 microM). DPDPE ([D-Pen2,5]enkephalin; 1-10 microM), a delta-opioid agonist, also reduced the capsaicin-evoked release in a concentration-dependent manner (approximately 59% reduction). Naltrindole (1 microM), a selective delta-antagonist, significantly antagonized the inhibitory effect of DPDPE (10 microM). In contrast, neither U-50,488H (1-10 microM) nor U-69,593 (10 microM), kappa-opioid agonists, had any effects on the evoked release of glutamate. These results suggest that mu-, and delta-opioid agonists modulate pain transmission in the spinal dorsal horn, at least in part, by inhibiting the release of glutamate from capsaicin-sensitive primary afferents.

Alpha2-adrenoceptor-mediated inhibition of capsaicin-evoked release of glutamate from rat spinal dorsal horn slices

It is known that the descending noradrenergic system suppresses nociceptive transmission in the spinal dorsal horn. To determine whether noradrenaline-alpha-adrenoceptor systems exert inhibitory influence on the release of glutamate from the nociceptive primary afferents, the effects of alpha-adrenoceptor agonists on the capsaicin (3 microM)-evoked release of glutamate from rat spinal dorsal horn slices were examined using an on-line continuous monitoring system for glutamate. The application of alpha 2-agonists clonidine (0.1-10 microM) and ST-91 (1 and 10 microM), respectively, decreased the capsaicin-evoked release of glutamate in a concentration-dependent manner. The partial alpha 2-agonist oxymetazoline (1 and 10 microM) produced a slight inhibition in the evoked release of glutamate. In contrast, the alpha 1-agonist phenylephrine (1 and 10 microM) did not show any significant effects on the evoked release of glutamate. The inhibitory action of 10 microM clonidine on the evoked glutamate release was antagonized by the alpha 2-antagonist yohimbine (1 microM) but not by the opioid antagonist naloxone (10 microM). These findings suggest that noradrenaline, probably released from the descending inhibitory system, reduces the release of glutamate from the capsaicin-sensitive primary afferent terminals through alpha 2-adrenoceptors.

Suppression of Bladder Oxidative Stress and Inflammation by a Phytotherapeutic Agent in a Rat Model of Partial Bladder Outlet Obstruction

PURPOSE Ischemia/reperfusion injury is a major etiological factor in the progression of bladder dysfunction after partial bladder outlet obstruction and it is partly mediated by the generation of free radicals. The phytotherapeutic agent Eviprostat, a popular treatment for benign prostatic hyperplasia in Japan and Germany, has antioxidant and anti-inflammatory activity. We investigated the effect of Eviprostat on oxidative stress and inflammation in bladder dysfunction in a bladder outlet obstruction rat model. MATERIALS AND METHODS Bladder outlet obstruction was surgically induced in male rats by placing a rubber ring around the urethra. Rats with bladder outlet obstruction were administered daily oral Eviprostat or vehicle, while sham operated animals were treated with vehicle. On day 6 after surgery bladder weight, oxidative stress markers and proinflammatory cytokine levels as a measure of bladder inflammation, were determined and histological alterations noted. Functional contractility studies were performed with longitudinal bladder strips. RESULTS Bladder outlet obstruction led to a significant increase in bladder weight, oxidative stress markers and proinflammatory cytokine levels. Eviprostat significantly suppressed these increases without affecting bladder weight. Histological analysis showed increased detrusor muscle hypertrophy and increased numbers of collagen fibers with accompanying inflammatory infiltration in the bladder of vehicle treated bladder outlet obstruction animals. Eviprostat treatment was associated with suppression of these changes. Decreased responses of obstructed bladder strips to electrical stimulation and KCl were ameliorated by Eviprostat treatment. CONCLUSIONS Eviprostat mediated decrease of the increased oxidative stress and bladder inflammation caused by bladder outlet obstruction may contribute to the protection of bladder function.

Suppression of bladder overactivity and oxidative stress by the phytotherapeutic agent, Eviprostat, in a rat model of atherosclerosis-induced chronic bladder ischemia.

Objectives:  To clarify the mechanism by which chronic bladder ischemia causes bladder functional changes, and to investigate the involvement of oxidative stress and pro‐inflammatory cytokines, and the effects of the phytotherapeutic drug, Eviprostat, on these biochemical marker levels and bladder function.

Effect of tramadol on pain-related behaviors and bladder overactivity in rodent cystitis models.

Tramadol is a widely used analgesic that stimulates the μ opioid receptor and inhibits serotonin and noradrenalin reuptake. There have been studies on the analgesic effects of tramadol based on the tail-flick test, the formalin test, and the induction of allodynia by sciatic-nerve ligation. However, the effects of tramadol on behaviors related to bladder pain and bladder overactivity induced by cystitis have not been reported. To investigate the usefulness of tramadol for patients with cystitis, we investigated these effects of tramadol in rodent cystitis models. Intraperitoneal injection of cyclophosphamide caused bladder-specific inflammation and increases in pain-related behaviors, the number of voids and bladder weight in mice. Tramadol suppressed the cyclophosphamide-induced pain-related behaviors but did not affect the number of voids or the bladder weight. During continuous-infusion cystometrograms in anesthetized rats, cyclophosphamide shortened the intercontraction interval, indicating bladder overactivity. Tramadol significantly prolonged the intercontraction interval, and the effect was partially blocked by the opioid antagonist naloxone. This finding indicates that μ opioid receptors may be involved in the action of tramadol. In conclusion, tramadol ameliorated cyclophosphamide-induced bladder-pain-related behaviors and bladder overactivity in rodents. These findings suggest that tramadol might be a treatment option for cystitis-induced bladder pain and bladder overactivity.

The analgesic effect of tramadol in animal models of neuropathic pain and fibromyalgia

(±)-Tramadol hydrochloride (tramadol) is a widely used analgesic for the treatment of cancer pain and chronic pain. Although many animal studies have shown antinociceptive effects of tramadol in both acute and chronic pain, little is known about the effect of tramadol in putative animal models of fibromyalgia. In this study, we compared the antiallodynic effects of oral administration of tramadol in two kinds of rat chronic pain models, neuropathic pain induced by partial sciatic nerve ligation (PSL) and reserpine-induced myalgia (RIM). In PSL rats, the threshold for responses induced by tactile stimulation with von Frey filaments was significantly decreased seven days after the operation, suggesting that the operation induced tactile allodynia. Orally administered tramadol showed a potent and dose-dependent antiallodynic effect on PSL-induced allodynia. In RIM rats, the threshold was significantly decreased five days after reserpine treatment. Orally administered tramadol also attenuated reserpine-induced tactile allodynia. To explore the mechanism of the antiallodynic effect of tramadol in RIM rats, we investigated the effect of the opioid antagonist naloxone on the tramadol-induced analgesic effect in these rats. The effect of tramadol was partially antagonized by naloxone, suggesting that the opioid receptor is involved at least in part in the antiallodynic effect of tramadol in RIM rats. These data indicate that orally administered tramadol produced improvement in both PSL rats and RIM rats at similar doses and provide evidence that the opioid system is partly involved in the analgesic effect of tramadol in RIM rats.

Effect of a single treatment with tadalafil on blood flow in lower urinary tract tissues in rat models of bladder overdistension/emptying and abdominal aorta clamping/release.

Impaired blood flow in lower urinary tract (LUT) tissues is a pathophysiological cause of LUT symptoms. We investigated the effects of the phosphodiesterase 5 (PDE5) inhibitor tadalafil on the sustained decrease in bladder blood flow (BBF) and time-dependent changes in BBF and prostate blood flow (PBF) resulting from ischemia/reperfusion in two rat models. In a rat model of bladder overdistension/emptying (O/E), the bladder was overdistended by saline infusion and emptied after 2h. Tadalafil was administered intraduodenally immediately after emptying. In a rat model of clamping/release (C/R), the abdominal aorta was clamped for 2h after a single oral dose of tadalafil and then the clamp was released. BBF in O/E and C/R rats and PBF in C/R rats were measured by laser Doppler flow imaging. BBF decreased on overdistension and partially recovered after emptying. A progressive decrease in BBF was observed after O/E, and this was prevented by tadalafil treatment. Both BBF and PBF decreased during clamping of the abdominal aorta and partially recovered after clamp removal. Oral pretreatment with tadalafil partially or completely prevented the decreases in BBF and PBF not only after clamp removal but also during clamping. PDE5 mRNA was highly expressed in the bladder and the supporting vasculature. Tadalafil inhibited the O/E-induced decrease in BBF and the C/R-induced time-dependent decreases in BBF and PBF. PDE5 inhibition by tadalafil may improve both BBF and PBF.

Tramadol enhances urethral continence function through µ-opioid receptors in rats†‡

(±)‐Tramadol hydrochloride (tramadol) is a widely used analgesic that stimulates the µ‐opioid receptor and inhibits the reuptake of serotonin and noradrenalin. Although tramadol is also known to inhibit the micturition reflex in rats, its effects on urethral continence function have not been reported. We therefore examined whether intravenous tramadol (1, 3, and 10 mg/kg) affects intraurethral pressure, bladder leak point pressure, and leak volume in urethane‐anesthetized female rats.

Acamprosate {monocalcium bis(3-acetamidopropane-1-sulfonate)} reduces ethanol-drinking behavior in rats and glutamate-induced toxicity in ethanol-exposed primary rat cortical neuronal cultures

Acamprosate, the calcium salt of bis(3-acetamidopropane-1-sulfonate), contributes to the maintenance of abstinence in alcohol-dependent patients, but its mechanism of action in the central nervous system is unclear. Here, we report the effect of acamprosate on ethanol-drinking behavior in standard laboratory Wistar rats, including voluntary ethanol consumption and the ethanol-deprivation effect. After forced ethanol consumption arranged by the provision of only one drinking bottle containing 10% ethanol, the rats were given a choice between two drinking bottles, one containing water and the other containing 10% ethanol. In rats selected for high ethanol preference, repeated oral administration of acamprosate diminished voluntary ethanol drinking. After three months of continuous access to two bottles, rats were deprived of ethanol for three days and then presented with two bottles again. After ethanol deprivation, ethanol preference was increased, and the increase was largely abolished by acamprosate. After exposure of primary neuronal cultures of rat cerebral cortex to ethanol for four days, neurotoxicity, as measured by the extracellular leakage of lactate dehydrogenase (LDH), was induced by incubation with glutamate for 1h followed by incubation in the absence of ethanol for 24h. The N-methyl-D-aspartate receptor blocker 5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine, the metabotropic glutamate receptor subtype 5 antagonist 6-methyl-2-(phenylethynyl)pyridine and the voltage-gated calcium-channel blocker nifedipine all inhibited glutamate-induced LDH leakage from ethanol-exposed neurons. Acamprosate inhibited the glutamate-induced LDH leakage from ethanol-exposed neurons more strongly than that from intact neurons. In conclusion, acamprosate showed effective reduction of drinking behavior in rats and protected ethanol-exposed neurons by multiple blocking of glutamate signaling.