OPIOIDS (Introduction, Receptors and Classification of opioids)

INTRODUCTION:-

Opioid is the broad term which is basically used to describe all the compounds that work on the opioid receptors. In opioids, natural opiates, semi-synthetic and synthetic compounds are included which produce morphine-like actions. Morphine and other different related alkaloids are derived from the opium which is the dark brown and resinous material of the poppy Papaver somniferum. By binding with the different and specific type of opioid receptors, these opioids produce wide range of effects but they are primarily used to relieve the severe pain (unpleasant sensation which is done by an internal or external harmful stimulus) that comes from chronic disease, injury or surgery etc. And also due to the euphoric properties of opioids, these agents are good choice for abuse. In case of opioid overdose, their antagonists are also available to reverse the opioids actions.

Opioid receptors are found within the CNS and throughout the peripheral tissues. These receptors are generally stimulated by the endogenous peptides (enkephalins, endorphins and dynorphins) which are released in response to harmful stimulation. These opioid receptors have Greek letters names which depend on their prototype agonists.

OPIOID RECEPTORS:-

1. Mu receptor:-

·         Mu receptor agonist is morphine. ·         They are found chiefly in brain stem and also in medial thalamus. ·         These receptors cause respiratory depression, sedation, supraspinal analgesia, euphoria, decreased GI motility and also physical dependence. Their subtypes are Mu1 and Mu2, Mu1 receptor is related to euphoria, analgesia and serenity and with Mu2, prolactin release, pruritus, sedation, anorexia, dependence and respiratory depression are related.

These receptors are also known as OP3 or Morphine opioid receptors (MOR).

2. Kappa receptor:-

·         Its agonist is ketocyclazocine. ·         These kappa receptors are present in limbic & other diencephalic areas, spinal cord and brain stem. ·         These receptors are responsible for dyspnea, dsyphoria, sedation, spinal analgesia, respiratory depression and dependence etc.

These receptors are also called as OP2 or Kappa opioid receptors (KOR).

3. Delta receptor:-

·         Delta receptor agonist is (delta-alanine 2-delta-leucine 5-enkephalin). ·         These receptors are found majorly in brain. ·         These delta receptors play important role in analgesia and gastric motility.

They are also known as OP1 and delta opioid receptors (DOR). 

CLASSIFICATION OF OPIOIDS:-

1)      Classification of opioids on the basis of analgesic potency (traditional).

TRADITIONAL
STRONG	INTERMEDIATE	WEAK
Morphine	Buprenorphine	Codeine
Pethidine	Pentazocine
Fentanyl	Butrophanol
Alfentanil	Nalbuphine
Remifentanil
Sufentanil

 
2)      Classification of opioids on the basis of their origin (like naturally occurring or manufactured).

ORIGIN
NATURALLY OCCURING	SEMI-SYNTHETIC	SYNTHETIC
Morphine	Diamorphine	Phenylperidines:- (pethidine, fentanyl, alfentanil, sufentanil)
Codeine	Dihydrocodeine	Diphenylpropylamines:- (methadone, dextropropoxyphene)
Papavarine	Buprenorphine	Morphinans:- (butorphanol, levorphanol)
Thebaine		Benzomorphans:- (pentazocine)

 
3)       Classification of opioids on the basis of their actions on opioid receptors.

FUNCTIONS
PURE AGONISTS	PARTIAL AGONISTS	AGONISTS-ANTAGONISTS	PURE ANTAGONISTS
Morphine	Buprenorphine	Pentazocine	Naloxone
Fentanyl		Nalbuphine	Naltrexone
Alfentanil		Nalorphine
Remifentanil
Sufentanil