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Opioids refer to a class of drugs that act via opioid receptors in the nervous system to relieve pain. The term “opioid” includes:
– ENDOGENOUS opioids occurring naturally in the human body such as endorphins,
– OPIATES found in the opium poppy plant such as morphine,
– synthetic (methadone, fentanyl) and semi-synthetic opioids (heroin).
The major function of endogenous opioids is to modulate pain signals. They are synthesized in response to pain stimuli and exert their effects by binding to opioid receptors in the brain, spinal cord and peripheral nerves. In the brain, they also increase DOPAMINE release, producing EUPHORIC effects.
Opioid analgesics such as morphine and fentanyl mimic the action of endogenous opioids. They are powerful painkillers and are commonly used to manage severe pain. Continuous use, however, MAY lead to tolerance and dependence. Opioids slow down BREATHING and overdose can be FATAL. Their psychoactive effects also make them common drugs of abuse, with morphine being PARTICULARLY susceptible to addiction. Heroin, a semi-synthetic product made from morphine, is another drug that is highly popular among recreational users. Once administered, it is metabolized into morphine and 6-mono-acetyl-morphine, both of which are psychoactive. Heroin is rarely used in medicine.
How do opioids produce euphoric effects?
Dopamine neurotransmitter is at the basis of the brain reward pathway. Engaging in enjoyable activities causes dopamine release from dopamine-producing neurons into the synaptic space where it binds to and stimulates dopamine-receptors on the receiving neuron. This stimulation is believed to produce the pleasurable feelings or rewarding effect. Normally, GABA, another neurotransmitter, INHIBITS dopamine release in the nucleus accumbens. By binding to receptors on GABA inhibitory neurons, opioids REDUCE GABA’s activity, ultimately INCREASE dopamine release and induce pleasurable feelings.
Addiction, Dependence and Tolerance
Continued use of opioids can result in dependence and addiction. As the body gets used to euphoric effects of the drug, it may become irritated if drug use is reduced or discontinued.
Tolerance develops following a typical sequence of events. A drug exerts its effect by INcreasing or DEcreasing a certain substance or activity in the brain to an ABNORMALLY high or low level. REPEATED exposure MAINTAINS this abnormal level for a period of time. The brain eventually ADAPTS by pulling it BACK to NORMAL level. This means the drug, at the current dosage, NO longer produces the desirable psychoactive effect; a higher dose is required to do so. This vicious cycle repeats itself and eventually leads to drug overdose.