Opioids Flashcards
(84 cards)
what are the three types of opioid receptors?
mu receptor
kappa receptor
delta receptor
*all are GPCRs
what are the precursor proteins that encode for opioid peptides?
Proenkephalin → enkephalins
Prodynorphin → dynorphins
Proopiomelanocortin (POMC) → beta-endorphin
where do Enkephalins bind?
bind to delta and mu receptors in the brain, spinal cord, adrenal medulla and GI tract
what are the two forms of Enkephalin’s?
Met-enkephalin = Tyr-Gly-Gly-Phe-Met
Leu-enkephalin = Tyr-Gly-Gly-Phe-Leu
where do Dynorphins bind?
bind kappa receptors in the brain and spinal cord (dynorphin A 1-17)
what are beta-endorphins?
neuromodulator/neurohormones released in response to stress
where do beta-endorphins bind?
mu and delta receptors in cells bodies of the intermediate lobe of the pituitary and hypothalamus
what is a similarity between the structure of all the opioid peptides?
their amino acid chain all start with Tyr-Gly-Gly-Phe
what are the Cellular Mechanisms of Action for Opioid Receptors?
ALL are inhibitory G-proteins (Gi/o)
1. Inhibit Adenylyl Cyclase
2. Close Voltage-Gated Calcium (Ca²⁺) Channels (Presynaptic)
3. Open Potassium (K⁺) Channels (Postsynaptic)
what are the ways opioids can cause analgesia?
- BRAIN: central or supraspinal by activating descending pain-inhibitory pathways and modulation of the affective (emotional) component of pain
- SPINAL: Inhibition of dorsal horn pain transmission neurons
- PERIPHERAL: Activation of opioid receptors on peripheral nerve fibers (Aδ and C fibers)
what is the MOA for opioids at peripheral sites?
mu receptors found on primary afferent neurons are activated and reduce the firing of pain-sensing nerves
what is the MOA for opioids at the level of the spinal cord (dorsal horn)?
- presynaptic: block calcium channels (↓Ca²⁺) → prevents release of neurotransmitters (glutamate)
- postsynaptic: open potassium channels (↑K⁺) → hyperpolarization
what is the MOA for opioids at brainstem sites (PAG and RVM)?
inhibition of GABAergic neurons → “disinhibits” the inhibitory pain neuron (activates descending pain control)
how do opioids cause respiratory depression?
- ↓ Sensitivity to CO₂ in the brainstem
- Depress Pontine and Medullary Regions
- ↓ Tidal Volume and Rate
- Dose Dependent Effects (tolerance can occur)
how do opioids cause cough suppression?
depress the cough reflex area in the brainstem
how do opioids cause Miosis?
activate parasympathetic pathways via the Edinger-Westphal nucleus (part of the oculomotor nerve circuit)
how do opioids cause Euphoria?
stimulate the brain’s reward system, especially the ventral tegmental area (VTA) and nucleus accumbens by enhancing dopamine release
how do opioids cause Nausea and Vomitting?
initially stimulate the chemoreceptor trigger zone (CTZ) in the medulla that causes nausea and vomiting but later suppress the vomiting center due to tolerance
how do opioids cause constipation?
Increased smooth muscle tone
Decreased propulsive motility
Reduced secretions
Biliary tract spasm
No tolerance develops
how do opioids effect the bladder, uterus and bronchioles?
- cause urinary retention
- could affect uterine contractions
- slightly reduce airway tone
- tolerance develops
how do opioids cause Pruritus?
Opioids trigger mast cells to release histamine, which causes itching, redness, and warmth and stimulate neural circuits involved in the sensation of itch
what are the cardiovascular effects of opioids?
- Minor Bradycardia
- Vasodilation from histamine release may trigger orthostatic hypotension
- Cerebral vasodilation can cause increased intracranial pressure
what is opioid use disorder?
Addiction associated with significant
psychological dependence and
accompanied by compulsive drug seeking
and drug taking behaviors.
what is acute opioid toxicity (overdose)?
excessive opioid use leading to severe respiratory depression