Masayasu Takahashi

Neuroprotective effects of the selective type 1 metabotropic glutamate receptor antagonist YM-202074 in rat stroke models

We describe in vitro properties and in vivo neuroprotective effects of a newly synthesized, high-affinity, selective allosteric metabotropic glutamate receptor type 1 (mGluR(1)) antagonist, N-cyclohexyl-6-{[(2-methoxyethyl)(methyl)amino]methyl}-N-methylthiazolo[3,2-a]benzimidazole-2-carboxamide (YM-202074). YM-202074 bound an allosteric site of rat mGluR(1) with a K(i) value of 4.8+/-0.37 nM. YM-202074 also inhibited the mGluR(1)-mediated inositol phosphates production in rat cerebellar granule cells with an IC(50) value of 8.6+/-0.9 nM, while showing selectivity over mGluR(2-7). When YM-202074 was infused intravenously at an initial dose of 20 mg/kg/h for 0.5 h followed by a dose of 5 mg/kg/h for 7.5 h, the free concentration of YM-202074 in the brain rapidly (<12 min) reached approximately 0.3 microM, reaching a steady-state phase within 1.5 h. We first treated rats such that they developed transient middle cerebral artery (MCA) occlusion. Results clearly demonstrate a dose-dependent improvement of neurological deficit and reduction of the infarct volume in both the hemisphere and cortex when YM-202074 was infused intravenously immediately after occlusion at a dose of 10 or 20 mg/kg/h for 0.5 h followed by a dose of 2.5 or 5 mg/kg/h for 23.5 h, respectively. Significant neuroprotection was maintained even when the administration of drugs was delayed by up to 2 h following the onset of ischemia. Furthermore, the improvement of neurological deficit and the reduction of infarct volume were sustained for 1 week following the onset of ischemia. These results suggest that YM-202074 exhibits great potential as a novel neuroprotective agent for the treatment of stroke.

Emergence of immunoreactivities for phosphorylated tau and amyloid-β protein in chronic stage of fluid percussion injury in rat brain

HEAD injury is one of the potential environmental factors in Alzheimers disease (AD). To study the chronic stage of concussive brain injury, histological analyses were performed 2–6 months after right lateral fluid percussion (FP) brain injury (3.6–4.8 atm) in rats. Six months after injury, numerous normal-looking neurons in the telencephalon and brain stem were immunoreactive with either antibody to phosphorylated tau or with four antibodies to β-amyloid protein. Neuronal counts in the cortices were gradually decreased after injury, up to 42% loss at 6 months after injury. These neuropathological changes suggest that this animal model could serve as a good animal model of neurodegenerative diseases such as AD.

The Glutamate AMPA Receptor Antagonist, YM872, Attenuates Cortical Tissue Loss, Regional Cerebral Edema, and Neurological Motor Deficits after Experimental Brain Injury in Rats

A massive increase in extracellular glutamate is thought to contribute to brain damage after traumatic brain injury. We examined the neuroprotective effect of the AMPA receptor antagonist YM872 in a rat head injury model using the fluid-percussion procedure. Male Sprague-Dawley rats were subjected to right lateral (parasagittal) fluid-percussion brain injury or sham injury. At 15 min postinjury, they received either YM872 (20 mg/kg/h, 20 mg/3 mL) or normal saline (vehicle) intravenously for 4 h. The administration of YM872 significantly improved the composite neuroscore at 1 and 2 weeks postinjury (p < 0.05), and markedly reduced the volume of tissue loss in the injured cortex (p < 0.05). It also significantly reduced cerebral edema in the ipsilateral parietal cortex at 48 h postinjury (p < 0.01). These results indicate that the posttraumatic administration of YM872 may be neuroprotective by ameliorating cortical tissue loss and regional cerebral edema, and suggest the importance of AMPA receptors in traumatic brain damage involving secondary injury processes.

Different pathophysiology underlying animal models of fibromyalgia and neuropathic pain: Comparison of reserpine-induced myalgia and chronic constriction injury rats

The reserpine-induced myalgia (RIM) rat manifests fibromyalgia-like chronic pain symptoms. The present study explored the pathophysiology underlying the pain symptoms in the RIM rat and the chronic constriction injury (CCI) rat, an animal model of neuropathic pain as a reference. Nerve tissue samples were collected from the nociception-tested animals for pathological examinations. Additionally, the therapeutic efficacy of a sodium channel blocker mexiletine was assessed in both rats. A slight vacuolization in the substantia nigra (SN) occurred in some of the RIM rats without any other histopathological changes in the brain or peripheral neurons. All the RIM rats, with or without vacuolization, showed hypersensitivity to tactile, muscle pressure, and cold stimuli. In the CCI rat, neurodegenerative changes were apparent in the sciatic nerve and the spinal cord only. CCI rats displayed muscle hyperalgesia in addition to tactile and cold allodynia. Pharmacotherapy with mexiletine did not attenuate the pain in the RIM rat, although it was effective in the CCI rat. Taken together, it is not likely that pain symptoms in RIM rats are caused by degenerative changes at the level of primary afferents and spinal cord, as is the case for CCI rats. The significance of the vacuolization in the SN is less clear at present because of the minor extent of the change and the lack of correlation with nociceptive sensitivity. The pain symptoms in RIM rats could be associated with dysfunction of biogenic amines-mediated CNS pain control even without apparent pathologies in the nervous system.

Effects of YM872 on atrophy of substantia nigra reticulata after focal ischemia in rats.

MIDDLE cerebral artery (MCA) occlusion causes atrophy in the ipsilateral substantia nigra reticulata (SNR). The effects of glutamate AMPA receptor antagonism on SNR atrophy, which is supposed to inhibit excitatory inputs from the subthalamic nucleus to the SNR, was investigated in rats with permanent MCA occlusions. Histological examination revealed marked atrophy two weeks after MCA occlusion in the saline-treated control group. However, constant i.v. infusion of YM872, a selective AMPA receptor antagonist, for 2 weeks significantly reduced SNR atrophy; neurological deficits also decreased. These results suggest that the AMPA receptor may be involved in the pathogenesis of SNR atrophy during the subacute phase of focal cerebral ischemia.

Spontaneous and evoked pain-associated behaviors in a rat model of neuropathic pain respond differently to drugs with different mechanisms of action

Given that patients with neuropathic pain suffer a mixture of spontaneous and evoked pain symptoms, we assessed the effects of drugs with different mechanism of action on spontaneous and evoked pain-associated behaviors in a rat model of neuropathic pain induced by chronic constriction injury (CCI) of the sciatic nerve. Frequent aberrant limb movement on the operated side was measured to assess spontaneous pain-associated behavior, and mechanical allodynia and thermal hyperalgesia were evaluated to assess evoked pain-associated behaviors. These three types of behavior were assessed after administration of the following drugs: pregabalin (α2δ-subunit ligand), morphine (μ-opioid receptor agonist), perampanel (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid [AMPA] receptor antagonist), clonidine, dexmedetomidine (α2-adrenoceptor agonists), and diclofenac (non-steroidal anti-inflammatory drug [NSAID]). Pregabalin at an oral dose of 10 or 30mg/kg significantly alleviated frequent aberrant limb movement and mechanical allodynia, but not thermal hyperalgesia. Morphine at a subcutaneous dose of 1 or 3mg/kg significantly improved all three types of behavior. Perampanel at an oral dose of 1mg/kg attenuated only frequent aberrant limb movement. Intraperitoneal administration of clonidine (0.01 or 0.03mg/kg) and dexmedetomidine (0.03mg/kg) significantly improved all three types of behavior, while diclofenac did not relieve any of the behaviors. Pregabalin, clonidine, and dexmedetomidine significantly decreased motor performance at doses close to analgesic doses in the rotarod test. The present study demonstrates that responses to spontaneous and evoked pain symptoms in neuropathic pain condition differ depending on a drugs mechanism of action. The selection and application of drugs according to the specific symptoms would be considered for the medication of patients with neuropathic pain.

Functional MRI of the reserpine-induced putative rat model of fibromyalgia reveals discriminatory patterns of functional augmentation to acute nociceptive stimuli

Functional neuroimaging, applied to pre-clinical models of chronic pain, offers unique advantages in the drive to discover new treatments for this prevalent and oppressive condition. The high spatial and temporal resolution of fMRI affords detailed mapping of regional pharmacodynamics that underlie mechanisms of pain suppression by new analgesics. Despite evidence supporting the translational relevance of this approach, relatively few studies have investigated fMRI abnormalities in rodent models of chronic pain. In this study, we used fMRI to map the BOLD response in a recently developed putative rat model of fibromyalgia to innocuous and acute nociceptive stimuli by applying a step-wise graded electrical forepaw stimulation paradigm, with comparison to healthy controls. We observed discriminatory functional signatures (pu2009<u20090.001) to 2u2009mA electrical forepaw stimulation, found to be innocuous in the control group. As such, this translational approach provides sensitive and quantitative neural correlates of the underlying chronic disease. The regional patterns of functional augmentation were found to be concordant with previous studies of nociception in the anaesthetised rat brain, supporting the specificity of this approach in the study of altered central pain processing in reserpine induced myalgia. The methodology introduced in this work represents a novel platform for emerging treatment evaluation in highly experimentally controlled conditions.