Opioid receptors

Transmembrane receptors, which are mainly found in the CNS, but also in many other tissues (for example in the intestine or skin) and belong to the family of endorphin receptors(Dhaliwal A et al. 2019).

Opioid receptors can be classified into main groups and some subgroups:

• µ-receptors (Mü-receptors: (Mü1, Mü2, Mü3): Stimulation of these receptors causes supraspinal analgesia, respiratory depression, miosis, bradycardia and euphoria. The antitussive effects and spastic paralysis of the intestine mediated by µ-receptors are among the most common effects sought to be achieved by the therapeutic administration of opiate derivatives (codeine, loperamide). The regular stimulation of µ-receptors leads to tolerance development and dependence.
• κ-receptors (Kappa-receptors: (kappa1, kappa2, kappa3): Stimulation causes spinal analgesia, sedation, also miosis and possibly dysphoria.
• σ receptors (sigma receptors): stimulation causes circulatory stimulation and mydriasis. Tolerance and dysphoria/halucinations occur.
• δ-receptors (delta-receptors; delta1, delta2): Irritation causes stress-induced and spinal analgesia as well as respiratory depression, hypotension and tolerance development
• Opioid receptor like-1 (ORL1)

Opiates, opioids and endorphins exert their effects by binding to the specific opioid receptors, which are occupied by corresponding endogenous ligands (endorphins) under physiological conditions. All opioid receptors are coupled to and activate inhibitory G-proteins (Al-Hasani R et al. 2011; Del Vecchio G et al. 2017). The receptors form homo- and heterodimeric complexes and induce kinase cascades.

Opioid receptors are expressed in the CNS as well as on peripheral sensory neurons, but also in many peripheral organs such as the gastrointestinal tract and skin (Stein C 2016). In the CNS, opioid receptors are found in high density in spinal and supraspinal ssnaps of the ascending nociceptive and descending antinosiceptive systems. In the posterior horn of the spinal cord, the axons of the Aδ- and C-fibers form synaptic contacts with the glutamatergic neurons of the tractus spinothalmamicus and spinorecticularis. There, they mediate inhibition of transmission in the first synapse of the ascending nociceptive system. In the periphery, they mediate a reduction of the discharge frequency at the nociceptors and thus inhibit pain perception.
When the specific opioid receptors are activated in the central nervous system, this leads to an activation of potassium channels and to an inhibition of voltage-dependent calcium channels and thus to a hyperpolarization of the neurons and to an inhibition of depolarization. The result is a decrease in the excitability of neurons. The transmission of neuronal activity is inhibited.

It can be assumed that the endogenous opioid system is one of the most important pain-inhibiting systems of the body. Afferent stimuli are modulated so that they are not transmitted unimpeded to the CNS. This prevents neuronal overexcitation and exaggerated pain sensation from occurring.In the skin, opioid recurrent pores influence cell proliferation, migration, and adhesion. μ-Opioid receptors have been found in all layers of the epidermis, while δ-receptors are concentrated at cell contacts and may decrease desmoglein expression.

Inflammation: Under inflammatory conditions, opioid receptors are upregulated. Thus, lymphocytes and macrophages express opioid peptides during inflammatory or traumatic processes and then also express opioid receptors.

Opioids as ligands of opioid receptors have immunomodulatory and immunosuppressive effects. They may increase the risk of infection. Opioids differ in their clinical effects due to different affinities for each type of receptor. The effect of opioids at their receptors can be reversed by the administration of opioid receptor antagonists (e.g. by the drugs naloxone or naltrexone).

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