PubMed · 7793559
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A F Smith. 1995. Postoperative shivering.. https://doi.org/10.1111/j.1365-2044.1995.tb06008.x
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The mu-opioid receptor mediates not only the beneficial painkilling effects of opiates like morphine but also the detrimental effects of chronic exposure such as tolerance and dependence. Different studies have linked tolerance to opioid receptor desensitization. Agonist activation of the mu-opioid receptor stimulates a mitogen-activated protein kinase (MAPK) activity, but the functional significance of this pathway remains unclear. We have focused on the MAPK signaling cascade to study mu-opioid receptor desensitization. We report that inhibition of the MAPK pathway blocks desensitization of mu-opioid receptor signaling as well as the loss of receptor density due to internalization. Our results suggest that a feedback signal emanating from the MAPK cascade is required for mu-opioid receptor desensitization.
Oscillations in neuronal population activity within the gamma frequency band (>25 Hz) have been correlated with cognition: Gamma oscillations could bind together features of a sensory stimulus by generating synchrony between discrete cortical areas [Eckhorn, R., Bauer, R., Jordan, W., Brosch, M., Kruse, W., Munk, M. & Reitboeck, H. J. (1989) Biol. Cybern. 60, 121-130; Singer, W. & Gray, C. M. (1995) Annu. Rev. Neurosci. 18, 555-556]. Herein we demonstrate that morphine and beta-endorphin disrupt this long-range synchrony of gamma oscillations while leaving the synchrony of local oscillations relatively intact. The effect is caused by a decrease in type A gamma-aminobutyric acid receptor-mediated inhibition of both excitatory pyramidal cells and inhibitory interneurons. The effects of morphine on gamma oscillations were blocked by mu-opioid receptor antagonists but not by antagonists of delta or kappa receptors. Morphine also produced burst firing in interneurons, because synaptic excitation from pyramidal cells was no longer balanced by synchronous inhibitory postsynaptic potentials. The loss of synchrony of gamma oscillations induced by morphine may constitute one mechanism involved in producing the cognitive deficits that this drug causes clinically.
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