Neuroscience letters
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Neuroscience letters · Oct 1995
Inhibition by spinal morphine of the tail-flick response is attenuated in rats with nerve ligation injury.
Nerve ligation injury in rats produces increased sensitivity and exaggerated responses to nociceptive stimuli (hyperalgesia) as well as nociceptive responses to normally innocuous stimuli (allodynia) analogous to clinical conditions of neuropathic pain. However, the effect of nerve injury on acute nociception has not been extensively studied. Nerve ligation injury was produced by unilateral ligation of the L5 and L6 spinal roots of the sciatic nerve of male Sprague-Dawley rats. ⋯ Antinociception was readily reversed by naloxone (5 mg/kg, i.p.) in both groups. These data indicate that nerve ligation injury reduces the potency and efficacy of i.th. morphine. While the reasons for this loss of morphine activity in nerve injured animals are unknown, it is possible to speculate that (a) degeneration of primary afferents subsequent to nerve ligation injury might result in a loss of presynaptic opioid (mu?) receptors in the dorsal horn, thereby reducing the antinociceptive activity of morphine at the spinal level; (b) changes in the efficiency of post-receptor transduction may occur following nerve injury which can reduce opioid efficacy; (c) changes in levels of spinal neurotransmitters (e.g., cholecystokinin) may act to diminish opioid action; or (d) sustained afferent input from the site of the injury may be important in limiting the activity of opioids.
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Neuroscience letters · Sep 1995
Expression of c-fos protein in substance P receptor-like immunoreactive neurons in response to noxious stimuli on the urinary bladder: an observation in the lumbosacral cord segments of the rat.
Chemical irritation of the urinary bladder with formalin in the rat induced c-fos protein-like immunoreactivity in more than 80% of substance P receptor-like immunoreactive (SPR-LI) neurons of the dorsal commissural nucleus, sacral parasympathetic nucleus and lamina I in the 6th lumbar and 1st sacral cord segments. These neurons with SPR-LI may receive noxious information from the urinary bladder through the primary afferent fibers with substance P.
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Neuroscience letters · Aug 1995
The effect of continuous morphine analgesia on chronic thermal hyperalgesia due to sciatic constriction injury in rats.
We employed hindfoot withdrawal latencies to radiant heat to assess the analgesic effect of prolonged morphine infusion on thermal hyperalgesia induced by chronic constriction injury (CCI) of the rat sciatic nerve. All CCI rats developed thermal hyperalgesia while sham-operated animals did not. Continuous systemic infusion of morphine dose-dependently reversed the thermal hyperalgesia in the CCI rats. ⋯ Tolerance to morphine's analgesic effect did not develop over a period of seven days of morphine infusion, which is considered long-term for animal models. These data suggest that morphine acts rapidly and effectively to reduce behavioral signs of hyperalgesia in rats with sciatic CCI, without the concomitant development of tolerance. Scheduled administration of morphine might be an appropriate treatment regimen for relief of neuropathic pain, and the infrequent use of opioids in equivalent human clinical pain syndromes due to fear of opioid unresponsiveness and tolerance might need to be re-evaluated.
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Neuroscience letters · Aug 1995
Evidence of decreased GABAergic influence on temporal integration in the inferior colliculus following acute noise exposure: a study of evoked potentials in the rat.
Many investigations have shown that modulation of sensory input, either by over stimulation or sensory deprivation, can cause a reorganization of structures located high in the central nervous system (CNS). Although most of these studies had focused on studying changes in the function and tonotopic organization of the sensory cortex, recent evidence has suggested that plastic changes in specific subcortical nuclei of sensory systems may also occur in response to modulation of sensory input, and may be partially responsible for changes reflected at the level of the cortex. In the present study we investigated the effects of noise exposure (4-kHz continuous tone at 104 dB sound pressure level (SPL) for 30 min duration) on the processing of auditory information at the level of the inferior colliculus (IC). ⋯ A microinjection of the GABAA antagonist, bicucullene, into the IC in the animals not exposed to the tone caused the amplitude of the peak to be less dependent on tone burst duration, which indicates that the decrease in the amplitude of this component of the response from the IC with increasing stimulus duration is a result of GABAA mediated inhibition on IC neurons. The tone exposure caused a similar decrease in amplitude of this component of the response from the IC, thus indicating that noise exposure reduced the GABAA mediated component of this function. This is supported by the finding that microinjections of bicucullene into the IC of noise-exposed animals did not significantly change the relationship between the amplitude of this peak and the stimulus duration.
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Neuroscience letters · Jul 1995
Clinical TrialResponse characteristics of cutaneous mechanoreceptors to vibratory stimuli in human glabrous skin.
We investigated the responsiveness of mechanoreceptors in the glabrous skin of the hand to triangular vibratory stimuli when the frequency and applied pressure to the skin were varied. Using a microneurographic technique, 13 single afferent activities were recorded from 4 kinds of mechanoreceptors. Both fast-adapting (FA) and slow-adapting (SA) units were sensitive to the vibratory stimuli. ⋯ FA-type II (FAII) sensitivity was augmented sharply over 60-100 Hz. SA-type I (SAI) and SA-type II (SAII) responsiveness was almost the same and characteristic sensitivity to the vibratory stimuli was revealed under 15 Hz. Functional roles of these units, especially type I units, are discussed in relation to active touch such as texture discrimination with a finger.