Opioid receptor mechanism

The binding site is called the ‘receptor’ and the mechanism as opioid receptor mechanism . Any pharmacological activity requires an agent, a target organ and a specific binding site in that organ.

The receptor due to its affinity to the drug and opioid receptor mechanism , permits attachment of the drug to itself, which then modifies the functioning of the tissue or organ, in which the receptor is situated.

Peculiarity of binding sites in opioid receptor mechanism for opioid drugs, as they are called “opioid receptors” is that:

1. These receptors, as is common place with other types of receptors, get activated after administration of specific agonists.

2. There are various endogenous ligands or endogenous neurotransmitters which are also involved in modulatory effect on these receptors.

3. Opioid receptor mechanism is one of the various other mechanisms and processes that are also known to cause stimulation of these receptors in modulation of nociception.

4. Due to subjective variation in the effects produced by stimulation of these receptors, the drugs acting in these individual subsets of receptors also have variations in their spectrum of clinical activity. Thus withdrawal syndrome produced by morphine abstinence is qualitatively different from that produced by nalorphine.

5. There are specific antagonists, which are pharmacologically related to the agonists, as such, which have the capability of reversing the actions of agonists.

6. Due to the variations in opioid receptor mechanism produced on each individual sets and subsets of receptors, it is essential to identify the drugs as pure, partial, and combination of agonist and antagonist in that manner.

All the receptors are guanine nucleotide binding proteins, dependent for their actions. The opioid receptor mechanism is mediated by affecting various ionic channels. The net effect is inhibitory in terms of modulation of nociception.

Special mention about K and σ receptors is necessary. The effects such as psychotomimetic effects, dysphoria which were thought to be mediated by a stimulation now have to be attributed exclusively to K receptors especially K 3 subtype.

Opioid receptor mechanism is this property of highly selective K agonists, for example, benzomorphans (pentazocine) leading to severe dysphoria, and other psychomimetic effects. Another important difference about K receptors is that most of the K agonists cause release of dynorphins.

Dynorphins are stored in neurohypophysis along with vasopressin. The released dynorphin suppresses release of vasopressin thus causing diuresis. Many of the K agonists have tendency to produce diuresis rather than urinary retention, as is the case with pure opioid receptor mechanism of agonists.

σ-receptors:  These are mysterious set of opioid receptors. Opioid receptor mechanism and opioid existence is beyond any dispute. However, they do not get activated by the naturally occurring levo isomers.

Many synthetic opioids are racemic mixtures (mixtures of both levo and dextro rotatory isomers/enantiomers). Rather than a receptors getstimulated by the dextro isomers, which do not have opioid activity.

Thus as was originally believed that σ-receptor stimulation causes psycho mimetic effects has been disproved and the opioid receptor mechanism has to be attributed mainly to K stimulation.

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