Tolerance, drug opioids

All of the opioid agonists produce some degree of tolerance and physical dependence. The biochemical mechanisms underlying tolerance and physical dependence are unclear. It is known, however, that intracellular mechanisms of tolerance to opioids include increases in calcium levels in the cells, increased production of cAMP, decreased potassium efflux, alterations in the phosphorylation of intracellular and intranuclear proteins, and the resultant return to normal levels of release of most neurotransmitters and neuromodulators. Tolerance to the analgesic effects of opioids occurs rapidly, especially when large doses of the drugs are used at short intervals. However, tolerance to the respiratory depressant and emetic effects of the opioids occurs more slowly. The miotic and constipative effects of the opioids rarely show tolerance. 

Alcohol-dependent people with a physical tolerance are relatively tolerant of some other cerebral depressant drugs (hydrocarbon anaesthetics), but of course the synergism with these drugs still occurs. There is no significant acquired cross-tolerance with opioids. 

Multimodal therapy clonidine seems to have a synergistic effect with opioids. This provides for more intense analgesia with the minimization of some of the adverse effects of either class of drugs. There is some cross-tolerance with opioids however, for patients with high opioid tolerance clonidine is an excellent adjimct... 

Finally, there is little or no clinical evidence that morphine causes psychological dependence or drug-seeking behaviour, tolerance or problematic respiratory depression in patients. These events simply do not occur when opioids are used to control pain. The reason is likely to be that the actions of morphine and the context of its use in a person in pain are neurobiologically quite different from the effects of opioids in street use. These actions of opioids are described in more detail in Chapter 23. 

Tolerance and dependence are characteristic of repeated administration of opioid drugs, which limits their use in treatment. It has been proposed that tolerance results from desensitization of opioid receptors and altered... 

Elliott K, Kest B, Man A, Kao B, Inturrisi CE. (1995). N-methyl-D-aspartate (NMDA) receptors, mu and kappa opioid tolerance, and perspectives on new analgesic drug development. Neuropsychopharmacology. 13(4) 347-56. 

Use opioids with caution in patients with alcoholism or other drug dependencies because of the increased frequency of opioid tolerance, dependence, and the risk of addiction observed in these patient populations. Abuse of opioids in combination with other CNS depressants can result in serious risk to the patient. 

The treatment of opioid abuse and dependence aims also at preventing the social complications of abuse, especially infections linked to parenteral administration (HIV and HepB). It relies on the use of substimtive drugs that can be either pure agonists, or partial agonist-antagonists (methadone, buprenor-phine, naltrexone), with the objective of limiting receptor desensitization and the development of tolerance. Any success in the treatment of opiate dependence may stem as much from the re-establishment of healthcare contact and social reinsertion as from any treatment induced decrease in the abuse behaviour itself. 

It is beheved that phenomena such as sensitization, tolerance and drug-dependence might also involve synaptic plasticity. In fact, numerous studies indicate that NMDA receptor antagonists block sensitization to amphetamine and cocaine as well as tolerance and dependence to ethanol and opioids in animal models (Trujillo and Akil 1991 Pasternak and Inturrisi 1995 Trujillo and Akil 1995 Mao 1999). Recent studies indicate that the uncompetitive NMDA receptor antagonists dextromethorphan, memantine and neramexane not only prevent the development of morphine tolerance, but also reverse estabhshed tolerance in the continuing presence of this opioid, prevent the expression of withdrawal symptoms in rats (Popik and Skolnick 1996 Popik and Danysz 1997 Popik and Kozela 1999 Houghton et al. 2001) and attenuate the expres-... 

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