What receptors does heroin effect?

Answer 1

Heroin (diacetyl-morphine) by itself has very little effect on receptors in the brain. It's main physiological effects are attributed to the conversion of heroin by the body to its metabolites morphine and/or 6-monoacetylmorphine (depending upon the route of administration of heroin). But for all intents and purposes, this is a just a pharmacological technicality, as most drugs (such as codeine) are not very active in their own right and depend upon metabolism by the liver or other organs to produce the specific effects of the drug.

Heroin is an opioidergic drug and thus affects endogenous opioid receptors, acting as an agonist (meaning it plugs into and activates these receptors). It has the highest affinity for the μ-opioid (or "mu-opioid") sub receptor. This specific opioid-receptor is mainly responsible for providing all the classical narcotic effects of most opiate/opioid painkillers, including: pain relief, relaxation of smooth muscle (in the gut), euphoria, and central nervous system depression (which causes reduction in anxiety and hypertension).

Like most other opiates, heroin also binds to two other opioid receptors as an agonist. First, it has moderate affinity for the δ- (or "delta-") opioid receptor. The physiological effects of δ opioid receptor agonists is not as fully understood as the other types; but it is thought to play a role in dependence and tolerance; while also providing similar effects to μ-opioid receptors agonists, primarily pain relief and central nervous system depression.

Finally, heroin has some-but very low-affinity for the κ-opioid (or "kappa-opioid") receptor. The effects of κ-opioidergic drugs are extremely different than μ- & δ- opioid agonists. Effects include hallucinations, dissociation, delirium, and anesthesia. Most of the effects associated with the μ-opioid receptor do not occur with kappa agonists, including: euphoria, CNS depression, and relaxation of the smooth-muscle in the gut. Opposite to heroin, a substance that strongly binds to κ-opioid receptors, with very little affinity for μ- & δ- receptors, is salvinorin a, the active chemical in Salvia. In the 1970s, κ-opioid agonist drugs were investigated as atypical anesthetics, but were quickly abandoned due to their severely adverse, unpleasant, and hallucinogenic effects. However, heroin's affinity for this opioid receptor is minimal, even at high doses, compared to other opiates such as meperidine, which have far less abuse potential because of kappa-opioid agonist activity at higher doses.

Additionally:

All drugs that act as μ-opioid agonists, including endogenous endorphins, similarly indirectly impact dopamine and serotonin and induce antidepressant effects. Furthermore, heroin like nearly every other opiate, induces a histamine reaction (it is not known if this is due to an indirect or direct effect at the H1, or histamine, receptors). This can create some-allergic reactions like itchiness. Although other typical allergic-effects like watering-eyes are usually counteracted by μ-opioid agonist effects, which induce dryness in mucus membranes (such as in the mouth and eyes).

Note:

Unlike other opiates & opioids, heroin has not yet been shown to have any known affinity for other non-opioidergic systems in the brain, such as acting as an antagonist (i.e., a "blocking" agent) at NMDA receptors as seen with methadone (NMDA antagonists reduce glutamate levels and decrease anxiety and in higher doses can cause anesthesia, Ketamine is a potent NMDA antagonist). Heroin also has not been demonstrated to act at nociceptin receptors, like the synthetic-opiate buprenorphine. Nociceptin is a opioid-class receptor, but has opposite physiological effects, acting as an anti-analgesic; however, its overall impact upon opiate pharmacology is not fully understood.