Brain research
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The effect of a single subcutaneous (s.c.) injection of the ultrapotent capsaicin analogue resiniferatoxin (RTX) on responses of adult rats to noxious thermal and mechanical stimulation was examined. The effects of RTX treatment on the nociceptive flexor reflex and activity-dependent increase in spinal excitability after conditioning C-fiber stimulation (CS) were also assessed. Finally, the expression of galanin message associated peptide (GMAP) mRNA in dorsal root ganglion (DRG) cells and the effects of the high affinity galanin receptor antagonist M35 on the flexor reflex in RTX-treated rats were evaluated. ⋯ The C-fiber mediated hyperexcitability was potentiated by the galanin receptor antagonist M35, more so in the non-recovered rats than in the partially recovered rats. The number of DRG cells expressing GMAP mRNA was significantly higher in non-recovered than in partially recovered rats. Thus, RTX produced marked and prolonged impairment of capsaicin-sensitive afferents and upregulation of the inhibitory neuropeptides GMAP and galanin in DRG neurons, which may underlie the prolonged effect of RTX.
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Ibogaine, an alkaloid isolated from the bark of the African shrub, Tabernanthe iboga, has been claimed to decrease the self-administration of drugs of abuse like morphine, cocaine and alcohol. To determine whether these effects are mediated via opioid receptor systems, the effects of ibogaine and its metabolite, noribogaine on the antinociceptive actions of morphine, U-50,488H and [D-Pen2,D-Pen5]enkephalin (DPDPE) which are mu- kappa- and delta-opioid receptor agonists, respectively, were determined in male Swiss-Webster mice. Administration of morphine (7 or 10 mg/kg, s.c.), U-50,488H (15 or 25 mg/kg, i.p.) or DPDPE (10 microg/mouse, i.c.v.) produced antinociception in mice as measured by the tail-flick test. ⋯ It is concluded that ibogaine, which has been suggested to decrease the self-administration of cocaine and opiates like heroin in humans, does not produce such an action by interacting directly with multiple opioid receptors. However, the metabolite of ibogaine enhances the antinociception of morphine but not of U-50,488H or DPDPE. Thus, in vivo evidence has been provided for the possible interaction of ibogaine with mu-opioid receptor following its metabolism to noribogaine.
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Peripheral nerve injury in a rat model (spinal nerve ligation) of neuropathic pain triggers sprouting of sympathetic fibers in the dorsal root ganglion (DRG). This sympathetic sprouting has been suggested as an important underlying mechanism for pain behaviors. ⋯ In addition, many vesicle-containing axonal enlargements (we will refer these as synaptic varicosities) were found in the interstitial space around DRG neurons, and some were enclosed within the satellite cell capsule which surrounded the DRG soma. The presence of sympathetic synaptic varicosities near or in apposition with either the DRG somata or their processes provides a structural basis for possible interactions between sensory neurons and sympathetic fibers in the DRG of neuropathic rats.
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In the present study we examined the distribution of chemically identified subpopulations of nonprincipal neurons in the rat hippocampus, focusing on the dorsoventral differences in their distributions. The subpopulations analyzed were those immunoreactive for parvalbumin, calretinin, nitric oxide synthase, somatostatin, calbindin D28K, vasoactive intestinal polypeptide and cholecystokinin. Using a confocal laser scanning light microscope, we could confirm that the penetration of each immunostaining, except that of calbindin D28K, was complete throughout 50 microns thick sections under our immunostaining conditions. ⋯ That of nitric oxide synthase positive neurons was significantly larger in ventral levels than in dorsal levels of the CA3 region as well as of the DG but not of the CA1 region. The numerical density of calretinin positive neurons was larger in ventral levels than in dorsal levels of all hippocampal subdivisions. The present study also revealed that dorsal and ventral levels of the hippocampus differ from each other in the composition of their nonprincipal neurons.
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The effects of inhalation anesthetics, nitrous oxide (N2O) and halothane, on the expression of c-Fos protein evoked by formalin injection were studied in the spinal cord in the rat. The expression of c-Fos protein was detected by immunocytochemistry following the injection of formalin (5%, 100 microliters) into the plantar surface of the left hindpaw. After 15 min of halothane (F) anesthesia, the anesthetics was switched to 40% or 70% of N2O, 0.5% or 1.5% of F or room air (for control) immediately following the formalin injection. ⋯ The current study indicates that inhalation anesthetics do not act equally on every kind of spinal neurons. Both N2O and halothane have effects on spinal neurons in the deeper layers but not on the neurons existed in laminae I-II, some of which directly receive noxious inputs. Pretreatment with 2 mg/kg of naloxone, which completely reversed the effects of morphine, did not alter the effect of 70%N2O, suggesting that the analgesic effect of N2O is not mediated by an intrinsic opioid mechanism at the spinal cord level.