Mitsuaki Yamazaki

Chronic Pain Induces Anxiety with Concomitant Changes in Opioidergic Function in the Amygdala

Clinically, it has been reported that chronic pain induces depression, anxiety, and reduced quality of life. The endogenous opioid system has been implicated in nociception, anxiety, and stress. The present study was undertaken to investigate whether chronic pain could induce anxiogenic effects and changes in the opioidergic function in the amygdala in mice. We found that either injection of complete Freunds adjuvant (CFA) or neuropathic pain induced by sciatic nerve ligation produced a significant anxiogenic effect at 4 weeks after the injection or surgery. Under these conditions, the selective μ-opioid receptor agonist [D-Ala2,N-MePhe4,Gly5-ol]-enkephalin (DAMGO)- and the selective δ-opioid receptor agonist (+)-4-[(αR)-α-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80)-stimulated [35S]GTPγS binding in membranes of the amygdala was significantly suppressed by CFA injection or nerve ligation. CFA injection was associated with a significant increase in the κ-opioid receptor agonist 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-phenyl-2-(1-pyrrolidinyl)ethyl]acetamide hydrochloride (ICI199,441)-stimulated [35S]GTPγS binding in membranes of the amygdala. The intracerebroventricular administration and microinjection of a selective μ-opioid receptor antagonist, a selective δ-opioid receptor antagonist, and the endogenous κ-opioid receptor ligand dynorphin A caused a significant anxiogenic effect in mice. We also found that thermal hyperalgesia induced by sciatic nerve ligation was reversed at 8 weeks after surgery. In the light–dark test, the time spent in the lit compartment was not changed at 8 weeks after surgery. Collectively, the present data constitute the first evidence that chronic pain has an anxiogenic effect in mice. This phenomenon may be associated with changes in opioidergic function in the amygdala.

Usefulness of antidepressants for improving the neuropathic pain-like state and pain-induced anxiety through actions at different brain sites.

Clinically, it is well known that chronic pain induces depression, anxiety, and a reduced quality of life. There have been many reports on the relationship between pain and emotion. We previously reported that chronic pain induced anxiety with changes in opioidergic function in the central nervous system. In this study, we evaluated the anxiolytic-like effects of several types of antidepressants under a chronic neuropathic pain-like state and searched for the brain site of action where antidepressants show anxiolytic or antinociceptive effects. Sciatic nerve-ligated mice exhibited thermal hyperalgesia and tactile allodynia from days 7 to 28 after nerve ligation. At 4 weeks after ligation, these mice showed a significant anxiety-related behavior in the light–dark test and the elevated plus–maze test. Under these conditions, repeated administration of antidepressants, including the tricyclic antidepressant (TCA) imipramine, the serotonin noradrenaline reuptake inhibitor (SNRI) milnacipran, and the selective serotonin reuptake inhibitor (SSRI) paroxetine, significantly prevented the anxiety-related behaviors induced by chronic neuropathic pain. These antidepressants also produced a significant reduction in thermal hyperalgesia and tactile allodynia. Moreover, the microinjection of paroxetine into the basolateral amygdala or cingulate cortex reduced anxiety-related behavior, and microinjection into the primary somatosensory cortex significantly attenuated thermal hyperalgesia. These findings suggest that serotonergic antidepressants are effective for treating anxiety associated with chronic neuropathic pain and may be useful for treating neuropathic pain with emotional dysfunction such as anxiety. Furthermore, SSRIs show anxiolytic and antinociceptive effects by acting on different brain regions.

The opioid peptide analogue biphalin induces less physical dependence than morphine

We compared the physical dependence liability of biphalin, a dimeric enkephalin analogue that possesses high antinociceptive activity, with that of morphine in equipotent intravenous doses. Naloxone challenge produced severe withdrawal signs after a 5-day infusion of morphine but only minor withdrawal signs after a 5-day biphalin infusion. In a cross-dependence study, biphalin did not suppress body weight loss after morphine withdrawal, but successfully suppressed weight loss after pentazocine withdrawal. These data support consideration of biphalin as a new analgesic with a novel pharmacological profile and minimum dependence liability.

Suppression of enriched environment-induced neurogenesis in a rodent model of neuropathic pain

Exposure to an enriched environment (EE) enhances neurogenesis and regulates emotionality. Previous reports have revealed that the rate of neurogenesis can be influenced by various environmental, endocrine, and pharmacologic stimuli. Chronic pain is a debilitating disease state characterized by complex alterations in both peripheral and central nociceptive pathways. In the present study, we evaluated the effect of chronic pain on environmental enrichment-induced hippocampal neurogenesis. Nerve-ligated mice were housed either in a standard environment or in the EE for 4 weeks. EE increased the immunoreactivity for doublecortin (DCX), a marker for immature neuron-positive cells, in the dentate gyrus (DG). Furthermore, the number of NeuroD (a neurogenic basic helix-loop-helix factor)-positive cells, in the DG was clearly increased by EE. Under these conditions, chronic pain suppressed enriched environment-mediated induction of both DCX- and NeuroD-labeled cells. These results suggest that chronic pain has stress-like damaging modulatory effects on hippocampal neurogenesis.

Effects of gabapentin on brain hyperactivity related to pain and sleep disturbance under a neuropathic pain-like state using fMRI and brain wave analysis.

Neuropathic pain is the most difficult pain to manage in the pain clinic, and sleep problems are common among patients with chronic pain including neuropathic pain. In the present study, we tried to visualize the intensity of pain by assessing neuronal activity and investigated sleep disturbance under a neuropathic pain‐like state in mice using functional magnetic resonance imaging (fMRI) and electroencephalogram (EEG)/electromyogram (EMG), respectively. Furthermore, we investigated the effect of gabapentin (GBP) on these phenomena. In a model of neuropathic pain, sciatic nerve ligation caused a marked decrease in the latency of paw withdrawal in response to a thermal stimulus only on the ipsilateral side. Under this condition, fMRI showed that sciatic nerve ligation produced a significant increase in the blood oxygenation level‐dependent (BOLD) signal intensity in the pain matrix, which was significantly decreased 2 h after the i.p. injection of GBP. Based on the results of an EEG/EMG analysis, sciatic nerve‐ligated animals showed a statistically significant increase in wakefulness and a decrease in nonrapid eye movement (NREM) sleep during the light phase, and the sleep disturbance was almost completely alleviated by a higher dose of GBP in nerve‐ligated mice. These findings suggest that neuropathic pain associated with sleep disturbance can be objectively assessed by fMRI and EEG/EMG analysis in animal models. Furthermore, GBP may improve the quality of sleep as well as control pain in patients with neuropathic pain. Synapse 2011.

The Effects of Sevoflurane and Propofol on Qt Interval and Heterologously Expressed Human Ether-a-go-go Related Gene Currents in xenopus Oocytes

Sevoflurane can induce prolongation of the cardiac QT interval by inhibiting the repolarization phase of the action potential. This may occur as a result of inhibition of the human ether-a-go-go related gene (HERG) channel. To clarify the mechanisms of anesthetics on HERG channels, we monitored the electrocardiogram and measured QT intervals in the guinea pig in the presence of sevoflurane and propofol. Sevoflurane (1%–4%) prolonged QTc dose-dependently (7.5%–21.2%), but propofol did not affect it. Furthermore, HERG channels were expressed in Xenopus oocytes and outward HERG currents were obtained on step depolarization from a holding potential of −70 mV. Repolarization to −70 mV from positive test potentials resulted in large outward tail currents. Sevoflurane (1%–4%), in a dose-dependent manner, inhibited the HERG outward tail currents (9.7%–26.6%), whereas steady-state currents were inhibited only at large concentrations. The time constant of the converging current was decreased in the presence of sevoflurane, but the inactivation and activation curves were not shifted. Propofol did not affect these currents within the clinically relevant concentration. In conclusion, compared with steady-state currents, sevoflurane was more potent in inhibiting the outward tail currents, suggesting that sevoflurane may modulate the HERG channel kinetics in its inactivated state.

Effects of the 5-HT3 receptor antagonist ondansetron on the ketamine- and dizocilpine-induced place preferences in mice

The effects of the 5-HT(3) receptor antagonist ondansetron on the ketamine- and dizocilpine-induced place preferences in mice were examined. The non-competitive NMDA receptor antagonists ketamine (1. 0-10 mg/kg, i.p.) and dizocilpine (0.1 and 0.2 mg/kg, i.p.) each produced a place preference in a dose-dependent manner. The ketamine (10 mg/kg)- and dizocilpine (0.2 mg/kg)-induced place preferences were dose-dependently blocked by pretreatment with ondansetron (0. 03-0.1 mg/kg, s.c.). These results suggest that 5-HT(3) receptor may be involved in the development of the place preferences produced by ketamine and dizocilpine.

Effects of Ketamine on Voltage-Dependent Ca2+ Currents in Single Smooth Muscle Cells from Rabbit Portal Vein

The effects of ketamine on membrane potentials and long-lasting type (L-type) voltage-dependent Ca2+ currents were investigated in dispersed single smooth muscle cells from rabbit portal veins. The amplitude and duration of the action potentials evoked by intracellular stimulation were inhibited by ketamine and were completely blocked by 10(-3) mol/l ketamine. Ketamine, however, did not alter the resting membrane potential. Whole cell voltage clamp experiments revealed that ketamine at concentrations higher than 10(-4) mol/l reduced the peak L-type voltage-dependent Ca2+ currents. These effects of ketamine were concentration-dependent and reversible at concentrations from 10(-4) to 10(-3) mol/l. Moreover, the activation threshold of L-type Ca2+ current (approximately -30 mV) was slightly shifted to the positive potential side by ketamine. This effect can explain that the action potential was abolished by 10(-3) mol/l ketamine. It is concluded that relaxation of the vascular smooth muscle by ketamine may have to be attributed to the inhibition of L-type voltage-dependent Ca2+ current.

The novel κ-opioid receptor agonist TRK-820 has no affect on the development of antinociceptive tolerance to morphine in mice

The effects of the novel kappa-opioid receptor agonist 17-cyclopropylmethyl-3,14beta-dihydroxy-4, 5alpha-epoxy-6beta-[N-methyl-trans-3-(3-furyl)acrylamido+ ++]morphinan hydrochloride (TRK-820) on the development of antinociceptive tolerance to morphine were investigated in mice and compared with those of trans-3,4-dichloro-N-(2-(1-pyrrolidinyl)-cyclohexyl) benzenacetamide methane sulfonate hydrochloride (U-50,488H), a well-defined exogenous kappa-opioid receptor agonist. Morphine (1. 25-20 mg/kg, s.c.) produced a dose-related antinociceptive effect in the 51 degrees C warm-plate test. Daily treatment with morphine (10 mg/kg, s.c.) resulted in the development of antinociceptive tolerance. The development of antinociceptive tolerance to morphine was dose-dependently suppressed by the co-administration of U-50, 488H (1-10 mg/kg, s.c.) with morphine, but not TRK-820 (0.003-0.03 mg/kg, s.c.). These results suggest that TRK-820-sensitive kappa-opioid receptor subtypes may not be involved in modulating the development of antinociceptive tolerance to morphine.

Olprinone and colforsin daropate alleviate septic lung inflammation and apoptosis through CREB-independent activation of the Akt pathway

Olprinone, a specific phosphodiesterase III inhibitor, and corforsin daropate, a direct adenylate cyclase activator, are now being used in critical conditions. We investigated whether their therapeutic use provides protection against septic acute lung injury (ALI) and mortality. Polymicrobial sepsis was induced by cecal ligation and puncture (CLP) in BALB/c mice. Olprinone or colforsin daropate was continuously given through an osmotic pump that was implanted into the peritoneal cavity immediately following CLP. These treatments prevented the ALI development in CLP mice, as indicated by the findings that severe hypoxemia, increased pulmonary vascular permeability, and histological lung damage were strikingly remedied. Furthermore, continued administration of olprinone or colforsin daropate suppressed apoptosis induction in septic lungs and improved the survival of CLP mice. Olprinone and corforsin daropate enhanced Akt phosphorylation in septic lungs. Wortmannin, which inhibits the Akt upstream regulator phosphatidylinositol 3-kinase, abrogated the protective effects of olprinone and corforsin daropate on sepsis-associated lung inflammation and apoptosis. In vivo transfection of cyclic AMP response element binding protein (CREB) decoy oligodeoxynucleotide failed to negate the abilities of these agents to increase Akt phosphorylation and to inhibit IκBα degradation in septic lungs. These results demonstrate for the first time that CREB-independent Akt-mediated signaling is a critical mechanism contributing to the therapeutic effects of olprinone and corforsin daropate on septic ALI. Moreover, our data also suggest that these cyclic AMP-related agents, by blocking both nuclear factor-κB activation and apoptosis induction, may represent an effective therapeutic approach to the treatment of the septic syndrome.