Neuropharmacology
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The objective of this study was to determine the neuroprotective role of tropisetron on retinal ganglion cells (RGCs) as well as to explore the possible mechanisms associated with alpha7 nAChR-induced neuroprotection. Adult pig RGCs were isolated from all other retinal tissue using a two-step panning technique. Once isolated, RGCs were cultured for 3 days under control untreated conditions, in the presence of 500 μM glutamate to induce excitotoxicity, and when tropisetron was applied before glutamate to induce neuroprotection. 500 μM glutamate decreased RGC survival by an average of 62% compared to control conditions. ⋯ Another mechanism shown to be associated with neuroprotection involves internalization of NMDA receptors. Double-labeled immunocytochemistry and electrophysiology studies provided further evidence that tropisetron caused internalization of NMDA receptor subunits. The findings of this study suggest that tropisetron could be an effective therapeutic agent for the treatment of degenerative disorders of the central nervous system that involves excitotoxicity.
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NMDA glutamate receptors (NMDARs) have critical functional roles in the nervous system but NMDAR over-activity can contribute to neuronal damage. The open channel NMDAR blocker, memantine is used to treat certain neurodegenerative diseases, including Parkinson's disease (PD) and is well tolerated clinically. We have investigated memantine block of NMDARs in substantia nigra pars compacta (SNc) dopamine neurones, which show severe pathology in PD. ⋯ Using immunohistochemistry, confocal imaging and stereological analysis, NMDA was found to reduce the density of cells expressing tyrosine hydroxylase, a marker of viable dopamine neurones; memantine prevented the NMDA-evoked decrease. In conclusion, memantine blocked NMDAR populations in different subcellular locations in SNc dopamine neurones but the degree of block depended on the intensity of agonist presentation at the NMDAR. This profile may contribute to the beneficial effects of memantine in PD, as glutamatergic activity is reported to increase, and memantine could preferentially reduce over-activity while leaving some physiological signalling intact.
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Patients with schizophrenia show marked deficits in processing sensory inputs including a reduction in the generation and synchronization of 40 Hz gamma oscillations in response to steady-state auditory stimulation. Such deficits are not readily demonstrable at other input frequencies. Acute administration of NMDA antagonists to healthy human subjects or laboratory animals is known to reproduce many sensory and cognitive deficits seen in schizophrenia patients. ⋯ Administered alone, nicotine augmented 40 Hz steady state power and phase-locking. Nicotine's effects were blocked by simultaneous administration of the α4β2 antagonist DHßE. Thus we report for the first time, a rodent model that mimics a core neurophysiological deficit seen in patients with schizophrenia and a pharmacological approach to alleviate it.
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The exacerbation of musculoskeletal pain by stress in humans is modeled by the musculoskeletal hyperalgesia in rodents following a forced swim. We hypothesized that stress-sensitive corticotropin releasing factor (CRF) receptors and transient receptor vanilloid 1 (TRPV1) receptors are responsible for the swim stress-induced musculoskeletal hyperalgesia. We confirmed that a cold swim (26 °C) caused a transient, morphine-sensitive decrease in grip force responses reflecting musculoskeletal hyperalgesia in mice. ⋯ Desensitizing the TRPV1 receptor centrally or peripherally using desensitizing doses of resiniferatoxin (RTX) failed to prevent the musculoskeletal hyperalgesia produced by cold swim. SB-366791, a TRPV1 antagonist, also failed to influence swim-induced hyperalgesia. Together these data indicate that swim stress-induced musculoskeletal hyperalgesia is mediated, in part, by CRF2 receptors but is independent of the TRPV1 receptor.
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Tapentadol is a novel centrally acting drug that combines mu-opioid receptor (MOR) agonism and noradrenaline reuptake inhibition (NRI), producing analgesic effects in various painful conditions. We investigated the acute effects of tapentadol in the locus coeruleus (LC), a central nucleus regulated by the noradrenergic and opioid systems that is critical in pain modulation. In single-unit extracellular recordings of LC neurons from anaesthetized male Sprague-Dawley rats, tapentadol clearly inhibited the spontaneous electrophysiological activity of LC neurons in a dose-dependent manner (ED50 = 0.8 mg/kg). ⋯ Furthermore, tapentadol inhibited the LC response to mechanical stimulation of the hindpaw in a dose-dependent manner. In summary, we demonstrate that acute administration of tapentadol inhibits LC neurons in vivo, mainly due to the activation of alpha2-adrenoceptors. These data suggest that both the noradrenergic and opioid systems participate in the inhibitory effect of tapentadol on LC neurons, albeit to different extents, which may account for its potent analgesic effect and mild opioidergic side-effects.