mk- (mu) and δ — (delta) receptors are the two main types of opioid receptors in the human body that are part of the complex endogenous opioid system. This system plays a key role in regulating pain, reward, and many other physiological processes.
Both receptors belong to the class of G-protein coupled receptors (GPCRs), and their activation leads mainly to inhibitory effects in the nervous system.
For clarity, here is a comparative table of their main characteristics:
| Characteristic | μ-receptor (Mu receptor) | δ-receptor (Delta receptor) |
|---|---|---|
| Primary functions | • Analgesia (pain relief) • Euphoria, sense of well-being • Respiratory depression • Constipation • Physical dependence • Sedative effect | • Analgesia (especially in chronic pain) • Mood regulation (antidepressant effect) • Neuroprotection • Modulation of μ-receptor-mediated effects |
| Endogenous ligands (natural body agents) | β-endorphin (high affinity) Enkephalins (moderate affinity) | Enkephalins (high affinity) β-endorphin (moderate affinity) |
| Exogenous agonists (external activators) | Morphine, heroin, fentanyl, methadone, oxycodone | Leu-enkephalin, Met-enkephalin (unstable) SNC80 (experimental compound) |
| Antagonists (blockers) | Naloxone, naltrexone | Naltrindole (experimental) |
| Key effects upon activation | Potent analgesia, euphoria, respiratory depression (main cause of death in overdose), miosis (pupil constriction), reduced gastrointestinal motility. | Analgesia (less potent but with lower risk of respiratory depression), antidepressant effect, modulation of μ-receptor-mediated responses. |
| CNS localization | High density in: • Thalamus (pain relay center) • Periaqueductal gray matter (PAG) • Brainstem (respiratory center) | More restricted distribution: • Olfactory bulbs • Neocortex • Limbic system (emotion-related) |
| Addiction potential | Very high. Primary mediator of the rewarding effects (pleasure/reinforcement) of opioids. | Low. δ-receptor activation alone does not produce significant euphoria or physical dependence. |
Learn more about the mk-receptor (Mu-receptor)
This is the most well-known and clinically significant receptor. It is associated with both the powerful therapeutic effects of opioid analgesics and their main dangerous side effects.
- Role in medicine: It is the main target for morphine and other powerful painkillers used to relieve acute and severe chronic pain (for example, in cancer).
- Side effects:
- Respiratory depression: The most dangerous effect. mk-agonists reduce the sensitivity of the respiratory center in the brain stem to CO₂. If you overdose, your breathing simply stops.
- Constipation: Activation of m-receptors in the gut drastically reduces intestinal motility, which is a common problem for patients taking long-term opioids.
- Euphoria and addiction: A strong sense of pleasure and well-being caused by the activation of m-receptors in the brain’s mesolimbic pathway is a major cause of opioid drug addiction and abuse.
Learn more about the Delta receptor
The role of δ-receptors is less well understood, but no less important. They often work in tandem with m-receptors.
- Prospects for pharmacology: Scientists are actively looking for delta-receptor agonists, as there is evidence that they can provide pain relief with a lower risk of respiratory depression and addiction development. . This could be a breakthrough in creating safe analgesics.
- Influence on mood: Activation of δ-receptors demonstrates antidepressant and anxiolytic (anti-anxiety) effects in preclinical studies.
- Modulation: It is believed that δ-receptors modulate the activity of μ-receptors. For example, blockade of δ-receptors may enhance some of the effects of μ-agonists.
Interaction and meaning
Opioid receptors rarely work in isolation. Their interaction creates a complex regulatory network. For example, some drugs aim to be bifunctional agonists (for example, activate both m-and d-receptors) or agonist-antagonists (block one type and activate the other) to try to separate desirable effects (pain relief) from undesirable ones (respiratory depression, addiction).
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