Opioids various types

Various types of opioid receptors have been postulated solely for explaining the different actions of opioids. 

In recent years there has been a major research effort, so far without success, to produce potent, centrally acting analgesics that do not have an abuse potential. The discovery of various types of opioid receptor, which may have different effects on central neurotransmitter function, may ultimately lead to the development of such a drug. In the meantime, the most widely used opioids, for example morphine, heroin (also called diacetylmorphine) and codeine are therapeutically effective but are liable to be abused and produce dependence. The structure of some of the morphine-like analgesics and their antagonists are shown in Figure 15.2. 

Strong evidence indicates at least a familial pattern and perhaps a hereditary basis for some types of alcoholism ( 393). More recent data show that genotype accounts for approximately 33% of the overall variance in liability ( 394). In addition, specific neurocircuitry and neurochemical systems appear to be important in the etiology of alcoholism (395). Thus, positive reinforcement may be mediated by activation of g-aminobutyric acid (GABA) receptors, release of opioid peptides and dopamine, inhibition of glutamate receptors, and interactions with the 5-HT system. Furthermore, neurobehavioral effects of alcohol and their association with these various neurotransmitters serve as potential targets for novel drug therapies. 

Opioid receptors. Direct binding of highly radioactive opiates has permitted localization of specific opiate receptors of several types.863-866 The three major types (p, 8, k) are all 7-helix receptors coupled to adenylate cyclase, K+ and Ca2+ channels, and the MAP kinase cascade.866 The p receptors bind morphine most tightly.867 8673 These receptors are found in various cortical and subcortical regions of the brain. Most narcotics are polycyclic in nature and share the grouping indicated in Fig. 30-30. However, the flexible molecule methadone binds to the same receptors.868 Among antagonists that block the euphoric effects of opiates the most effective is naloxone (Fig. 30-30). 

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