20.03.2 Introduction to Opioids

Opium is the juice of the opium poppy. Opiates (also known as opioids) are the drugs derived from opium, and these are morphine and codeine.

The effects of taking opioids, and even that they were receptor-mediated, was known before the endogenous opioid system was discovered. Thus, we knew that the opioids were effective at promoting pain relief, and that the effect was mediated by a receptor in the body for opioids, before we knew what the receptors were doing in the body.

There are opioid receptors in the body to mediate the effects of the endogenous opioid peptides. The endogenous opioids include enkephalins, endorphins, and dynorphins. There are at least three types of opioid receptor for these endogenous opioids, and they are known as μ (mu), δ (delta), and κ (kappa) receptors.

The analgesic effect of opioids is mediate by the μ receptor. The μ receptors are found in the spine (spinal) and in the brain (supraspinal, above the brain).

The opioids discussed in this chapter include morphine and fentanyl. Both morphine and fentanyl are full agonists at μ receptors. Codeine is a weak opioid. Codeine itself is not an opioid, but 10% of it is metabolised to morphine, which is an opioid. Buprenorphine is a partial agonist at μ receptor, which means it gives a lesser maximal response that morphine does at μ receptors. Tramadol is a weak agonist at μ receptor. Tramadol also inhibits the SERT (transport of 5-HT) and the NET (transport of noradrenaline) to increase the levels of 5-HT and noradrenaline in the brain.