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Opioids are the class of analgesics which are either endogenous or synthetic. They are obtained from the opium which is a extract of the juice of poppy Papaver somniferum of family papaveraceae. Opioids are generally the morphine and its analogues or some synthetic derivatives ( phenylpiperidine , methadone, benzomorphan series and thebaine derivatives ).
Morphine analogues have same structure to that of morphine. There action is mediated by three G-protein coupled receptors namely m, k, and d and mainly by m receptors.
Mechanism Of Action - They act either at the cellular level or on the nociceptive pathway. At the cellular level they inhibit the adenylyl cyclase and decrease the intracellular cAMP content. They promote the opening of the potassium channel and inhibiting the voltage gated calcium channels thus inhibiting the neuronal excitability and transmitter release. So the maintain a inhibitory effect at the cellular level. At the spinal level they suppresses the nociceptive spinal reflexes and release of the substance P thus preventing the pain. Opioids act on the PAG ( periaqueductal grey matter ) which stimulate the NRM ( Nucleus raphe magnus ).This causes the release of the 5-HT and enkephalin that inhibit the transmission of the pain impulse in the nociceptive afferent neurons.
Opioids according to their receptor specificity and efficacy are classified as pure agonist (codeine, methadone ) ; mixed agonist and antagonist ( pentazocine, nalorphine ) ; antagonist (naloxone and naltrexone ). These opioids depending on there receptors presence develop various effects on the CNS like analgesia, euphoria , respiratory depression, activation of chemoreceptor trigger zone, pupillary constriction and depression of cough reflex. They also cause the decrease in the motility of GIT and thus causing constipation. They are used continuously to treat the chronic pain in case of high pain of cancers and its analogue codeine (3- methylmorphine ) found to have a marked antitussive action.
Pharmacokinetics - Morphine undergoes first pass metabolism so is mainly administered as I.V.or I.M. injections. They have a plasma half life of nearly 3 - 6 hours. They are metabolized in liver mainly by conjugation with glucuronides. Morphine-6-glucuronide is found to be more active than the morphine itself. They are excreted via urine.
New Approaches - Some new approaches have been developed like
* Non-peptide antagonists of substance P: They modulates transmission through the dorsal horn and prove to be useful analgesic drugs.
* Enkephalinase inhibitors : This include some experimental drug RB120 which act by inhibiting the metabolic degradation of endogenous opioid peptides, and have been shown to produce analgesia, together with other morphine-like effects, without causing dependence.
Morphine and its analogues have been found to develop various types of side effects on overdosing. They result in coma and respiratory depression. This leads to the constriction of pupil. They cause a strong physical dependence and there withdraw leads to the restlessness, diarrhea, shivering, piloerection. These effects can be overcome by the use of antagonists like nalorphine , naloxone, naltrexone. Thus their dose and usage should be checked to avoid the addiction and the related effects for the healthy people.
1. Rang and Dale's Pharmacology, 6th edition,(H.P.Rang, M.M.Dale, J.M.Ritter, R.J.Flower). Churchill Livingstone publisher, pp 596-605
2. Essential of Medicinal Pharmacology, 6th edition (KD Tripathi) Jaypee publisher, pp 453-468.
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