Opiate receptors are a type of protein found in the brain, spinal cord and gastrointestinal tract. Opiates activate receptors once they reach the brain. They produce effects that directly correlate with the area of the brain involved. Opiates facilitate pain relief and stimulate the pleasure centers in the brain that signal reward. When a person injects, sniffs or orally ingests heroin or morphine, the drugs travel quickly through the bloodstream to the brain.
Once heroin gets to the brain, it converts rapidly to morphine, which activates the receptors. In the reward system, the drugs activate these areas of the brain: ventral tegmental area (VTA), cerebral cortex and nucleus accumbens. Research shows that stimulation of the opiate receptors by heroin, morphine and other opiates results in feelings of reward. It also activates pleasure circuits by causing a larger amount of dopamine to be released in the nucleus accumbens. This causes a rush, or intense feelings of euphoria, which subside quickly and are followed by relaxation and contentment. The calm typically lasts a few hours. Excessive release of dopamine and activation of the reward system can lead to addiction.
Types of Opiate Receptors
The four major subgroups of opiate receptors are delta, kappa, mu and Nociceptin, and each is involved in controlling different functions of the brain. As an example, opiates and endorphins block pain signals by binding to the mu receptor site. The delta receptor in the brain is involved in pain relief, antidepressant effects and physical dependence. Kappa receptors in the brain and spinal cord are linked with sedation, spinal analgesia and pupil constriction.
The functions of the mu receptors in the brain and spinal cord are physical dependence, respiratory depression, euphoria, pupil constriction and supraspinal analgesia. Nociceptin receptors in the brain and spinal cord are involved with appetite, depression, anxiety and the development of tolerance to mu agonists.
Opiates and Addiction
Research involving opiate drugs and opiate receptors is a high priority because of problems associated with them. Because of the euphoric state associated with opiates, they are a class of drug often misused and abused. They can also be highly addicting. In particular, prescription opiates can be problematic because they can be obtained legally from a doctor.
The availability of opiate painkillers – through prescription and sale on the black market – has resulted in unforeseen cases of dependence. Natural opiates are contained in the resin of the opium poppy plant and include morphine and codeine. Semi-synthetic opiates are created from natural opioids and include buprenorphine, hydrocodone, oxycodone, hydromorphone and oxymorphone. Fully synthetic opioids include Fentanyl, Tramadol and Methadone.
Endogenous opioid peptides are produced naturally by the body and include endorphins and dynorphins. Once the body’s opioid receptors have been desensitized because of prolonged use, users can build up a tolerance and require a higher dose to achieve the same effect. Physical and/or psychological addiction can occur with regular use.