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Opioid Receptors

Receptores opioides

For medical students2 min readUpdated 2026-10-10

Opioid receptors are key nervous system structures whose interaction triggers a cascade of intracellular reactions via G-proteins. They provide spinal analgesia, regulation of pain sensitivity, and alteration of the psychoemotional state.

Kappa receptor ligandsDynorphins are endogenous ligands
Ocular effectMiosis — pupil constriction induced by agonists
Molecular pathwayInhibition of adenylate cyclase and decrease in cAMP levels
LocalizationPredominantly at the spinal cord level

Characteristics of Kappa-Opioid Receptors

Kappa-opioid receptors play an important role in regulating pain sensitivity. Their natural endogenous ligands are dynorphins. The primary anatomical localization of their effect is concentrated at the spinal cord level, which mediates spinal analgesia. Unlike mu-receptors, which are characterized by euphoria, activation of kappa-receptors causes specific psychotropic features—dysphoria, expressed as a negative emotional state and discomfort. In addition, the risk of developing physical dependence remains.

Pharmacological Effects

The pharmacological action upon stimulation of these receptors affects several body systems:

Molecular Mechanism and Signal Transduction

The interaction of opioids with the receptor apparatus occurs via G-proteins, triggering a sequential cascade of intracellular changes:

  1. Inhibition of the enzyme adenylate cyclase, leading to a decrease in the intracellular concentration of cAMP.
  2. Postsynaptic effect: opening of G-protein-coupled potassium channels. The outward efflux of $K^+$ ions causes hyperpolarization of the cell membrane and a drop in neuronal excitability.
  3. Presynaptic effect: blockade of N-type calcium channels. This limits the influx of $Ca^{2+}$ ions into the presynaptic terminal and disrupts neurotransmitter release into the synaptic cleft.

Mnemonic

Kappa receptors: Dynorphins cause Dysphoria and Potassium efflux out (hyperpolarization).

Frequently asked questions

What are the main types of opioid receptors?

There are three main types of opioid receptors: mu ($\mu$), delta ($\delta$), and kappa ($\kappa$).

Features of the known receptor types:

  • Mu-receptors — characterized by the development of euphoria upon stimulation.
  • Kappa-receptors — their endogenous ligands are dynorphins. The effect is localized predominantly at the spinal cord level. Main pharmacological effects include inhibition of pain transmission, sedation, miosis, and minor reduction in GI motility. A psychotropic feature of agonists is dysphoria.
  • Delta-receptors — along with mu- and kappa-receptors, they are blocked by full competitive antagonists (naloxone, naltrexone).
Which endogenous ligands correspond to kappa-opioid receptors?

The endogenous ligands for kappa-receptors are dynorphins.

What psychotropic effect is characteristic of kappa-receptor agonists as opposed to mu-receptors?

Kappa-receptors are characterized by dysphoria — a negative emotional background and discomfort, whereas euphoria is typical for mu-receptors.

How does opioid receptor stimulation affect potassium and calcium channels?

At the postsynaptic level, potassium channels open with the efflux of potassium ions and membrane hyperpolarization, while at the presynaptic level, N-type calcium channels are blocked, reducing calcium ion influx.

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