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Opioid analgesics

Created: 1/6/2004
The terms “opioid” and “opiate” are often used interchangeably. However, their meaning is slightly different. “Opiate” means that a substance/drug is extracted from opium or is similar in structure to such substances. This is an older term, which refers mainly to morphine-like compounds, which have a non-peptide structure. Opium is a dried exudate from unripe seed pods of the poppy Papaver somniferum, and it contains morphine, codeine and various other alkaloids, some not related to morphine. The opiates available for use in the clinic are either natural or synthetic compounds. “Opioid” is a term that has been used mainly to designate substances that are not derived from opium, and in particular opioid peptides, i.e. natural substances that bind to opioid receptors and mimic the effect of morphine-like compounds.  However, the term “opioid” is now used increasingly to designate all agents that act on opioid receptors, irrespective of their nature (natural or synthetic, peptide or non-peptide).

Although morphine has been used for many centuries, it was only in 1973 that specialised receptors for this drug, the opioid receptors, were shown to be present in the central nervous system (and also in peripheral organs, like the gut). This was followed in 1975 by the discovery of the first endogenous opioid peptides, the enkephalins. The list of opioid peptides has become longer over the years, and the classification of these peptides is complex. The following are just a few examples:

 Dynorphin A
 Dynorphin B

Other peptides, more recently discovered, such as nociceptin and nocistatin, are related structurally to opioid peptides. Interestingly, these two peptides are derived from the same precursor, and appear to have mutually antagonistic actions in terms of analgesia/ hyperalgesia.

Opioid peptides bind to opioid receptors, which are G-protein coupled receptors. These receptors have been subdivided into three main categories:

1. Mu receptors   
2. Delta receptors                    
3. Kappa receptors

Each of these receptor categories can be further subdivided, defining various opioid receptor subtypes. Opioid peptides act as agonists at opioid receptors, and generally have limited selectivity for a given receptor type.

The activation of the opioid receptors is associated at the cellular level with inhibition of the cell. Opioid agonists reduce neuronal excitability (by decreasing potassium conductance), and inhibit neurotransmitter release (by decreasing calcium influx, required for exocytotic release). From the functional systemic point of view, opioid agonists induce a range of effects, including analgesia. One could associate each type of opioid receptor with certain predominant effects. Most of the opioid drugs presently used (in particular morphine, as a prototype drug) are agonists with significant affinity at mu opioid receptors. If used appropriately, the relief of pain can be significant, but is often accompanied by unwanted effects, some of which may become life-threatening, such as the significant respiratory depression seen at high doses of morphine.

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