Characteristics of kappa opioid receptors
Kappa opioid receptors play an important role in the regulation of pain sensitivity. Their natural endogenous ligands are dynorphins. The primary anatomical localization of the effect is concentrated at the spinal cord level, which ensures the development of spinal analgesia. Unlike mu receptors, which are characterized by euphoria, activation of kappa receptors causes specific psychotropic features — dysphoria, expressed as a negative emotional background 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:
- Inhibition of pain impulse transmission due to spinal localization.
- Sedative effect, reducing overall activity.
- Miosis, i.e., pronounced pupil constriction.
- Minor decrease in gastrointestinal (GI) motility.
Molecular mechanism and signal transduction
The interaction of opioids with the receptor apparatus occurs via G proteins, triggering a sequential cascade of intracellular changes:
- Inhibition of the enzyme adenylyl cyclase, which leads to a decrease in the intracellular concentration of cAMP.
- 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.
- Presynaptic effect: blockade of N-type calcium channels. This limits the influx of $Ca^{2+}$ ions into the presynaptic terminal and disrupts the release of neurotransmitters into the synaptic cleft.
Briefly: answers to topical 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 known receptor types:
- Mu receptors — characterized by the development of euphoria upon stimulation.
- Kappa receptors — their endogenous ligands are dynorphins. Localization of the effect occurs predominantly at the spinal cord level. The main pharmacological effects include inhibition of pain impulse transmission, sedation, miosis, and a minor decrease 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).
Kappa receptors: Dynorphins cause Dysphoria and Outward potassium movement (hyperpolarization).
Class and exam questions
What endogenous ligands correspond to kappa opioid receptors?
The endogenous ligands for kappa receptors are dynorphins.
What psychotropic effect is characteristic of kappa receptor agonists in contrast to mu receptors?
Kappa receptors are characterized by dysphoria — a negative emotional background and discomfort, whereas mu receptors are typically characterized by euphoria.
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, which reduces the influx of calcium ions.
What else to study on this topic
- Comparison of pharmacological effects of kappa and mu opioid receptors
- The role of cAMP and G proteins in intracellular signal transduction
- Mechanisms of physical dependence development upon opioid stimulation
- Effects of opioids on gastrointestinal motility
- Presynaptic and postsynaptic inhibitory mechanisms in synapses
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